FABRICATION OF LIGHTWEIGHT SANDWICH 

PANELS OF THE AIRCRAFT TYPE 

Information Reviewed and Reaffirmed 

March 1954 

This Report is One of a Series 

lssued in Cooperation with 

AIR FORCE-NAVY-CIVIL SUBCOMMITTEE 

on 

AIRCRAFT DESIGN CRITERIA 

Under the Supervision of the 

AIRCRAFT COMMITTEE 

of the 

MUNITIONS BOARD 

No. 1574 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

Madison 5, Wisconsin 

ln Cooperation with the University of Wisconsin 
TABLE OF CONTENTS 

Preparation of Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Balsa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Cellular cellulose acetate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Cellular hard rubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Paper honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Glass-cloth honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Cotton-cloth honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Preparation of Face Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Plywood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Aluminum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Glass-cloth faces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Panel-assembly Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Plywood to balsa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aluminum to balsa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Glass cloth to balsa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Glass cloth to cellulose acetate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Plywood to cellulose acetate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Plywood to cellular hard rubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Aluminum to cellulose acetate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Aluminum to cellular hard rubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Glass cloth to cellular hard rubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Plywood to paper honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Plywood to glass-cloth honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Plywood to cotton-cloth honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Aluminum to paper honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Aluminum to glass-cloth honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Aluminum to cotton-cloth honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Glass cloth to paper honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Glass cloth to glass-cloth honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Glass cloth to cotton-cloth honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . 

Curved sandwich panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

APPENDIX I . Balsa lumber specification . . . . . . . . . . . . . . . . . . . . . . . . . . 

APPENDIX II . Preliminary experimental work . . . . . . . . . . . . . . . . . . . . . . 

Band sawing end-grain balsa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Planing end-grain balsa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Pressing end.-grain balsa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Spreading high-viscosity laminating resin (A) on glass cloth . . . 

Strength determinations to evaluate fabrication techniques . . . . . 

Compression of core materials at elevated temperatures . . . . . . . . . 

APPENDIX III - Description of resins andadhesives . . . . . . . . . . . . . . . 

Page 

1 

1 

3 

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6 

7 

7 

7 

7 

3 

9 

9 

12 

12 

14 

14 

14 

15 

15 

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29 

Report No. 1574 
- - - - 

FABRICATION OF LIGHTWEIGHT SANDWICH PANELS 

OF THE AIRCRAFT TYPE1 

By 

BRUCE G. HEEBINK, Engineer 

ALVIN A. MOHAUPT, Technologist 

and 

JOHN J. KUNZWEILER, Technologist 

Introduction 

The Army-Navy-Civil Subcommittee on Wood Aircraft Structures early in 

1944 requested the Forest Products Laboratory to fabricate and test the panels 

necessary to establish design criteria for sandwich constructions. In January 

1945, the ANC Subcommittee requested that the development of fabrication 

techniques for sandwich panels be considered a part of the sandwich research 

program at the Forest Products Laboratory and that all observations and 

techniques should be recorded. 

The purpose of this report is to describe the materials used, to 

record the fabricating procedures adopted, and to summarize the techniques 

that were tried and found impracticable or unworkable in the production of 

the required test panels. 

Preparation of Cores 

Balsa 

An attempt was made to select the balsa within a density range of 5 to 

8 pounds per cubic foot on a 12 percent equilibrium-moisture-content basis. 

Previously, balsa in a range of 6 to 9 pounds per cubic foot was used. The 

material was received full 3 inches thick, surfaced on two opposite sides, in 

random widths and lengths. The specifications used in its procurement are 

given in Appendix 1. If necessary, the material as received was first kiln 

dried to a moisture content approximately in equilibrium with the workrooms 

(5 to 7 percent when the work was done during the winter months). 

1This is one of a series of reports prepared by the Forest Products Laboratory relating to the use of wood in aircraft. Results here reported are 

preliminary and may be revised as additional data become available. 

Report No. 1574 -1-
Density determinations were made by cutting 1/2-inch cross sections 

from each end of every plank. If the two samples fell within the acceptable 

limits, the plank was used. If, however, one end was too heavy or too light, 

a 30-inch length was cut from that end and another density check made. If 

one end was too heavy and the other too light, sections were cut from each 

end and further density measurements were made. If both ends were too light 

or too heavy, the entire plank was rejected. 

These cross-sectional density checks merely established the average 

density of the plank at the point of cutting, but it was assumed that these 

average values were applicable to the whole plank. It was found, however, 

that the density often doubled from one growth ring to the next. Rejections 

due to unacceptable density were 20 to 50 percent of three shipments of 

balsa. 

Planks or portions of planks with obvious defects, such as checks or 

rot, were also rejected before processing further. Acceptable material was 

cut to 30-inch lengths and these were accurately jointed and planed to 

rectangular shape. They were then cut into end-grain slabs of the desired 

thickness on a 14-inch circular cross-cut saw running at 3,600 revolutions 

per minute and having about 5-1/2 teeth per inch. By keeping the saw in good 

condition and removing all end play from the arbor, it was possible to produce acceptable slabs by the method shown in figure 1. Slabs were inspected 

at the saw by calipering each corner for thickness. Tolerances of 0.003 

inch were permitted. Rejections for thickness variations were normally less 

than 10 percent on this basis. 

In spite of the carefully controlled technique used in sawing the 

balsa slabs, the end grain surfaces of the slabs were occasionally slightly 

wavy. The growth rings of higher density produced the ridges and the rings 

of lower density formed the valleys. Determinations made on one slab revealed a density of about 9.5 pounds per cubic foot in a high-density ring 

and 5.3 pounds per cubic foot in the adjacent low-density ring one-half inch 

away. Figure 2 shows a slab that had very noticeable ridges, and figure 3 

is a photomicrograph of the sawn surface of a similar slab showing the two 

types of surfaces. Further investigations of this condition have shown that 

balsa wood that produces a granular or pithlike break when a blunt instrument 

is drawn across the end surface in the radial direction so as to tear out a 

groove has a density of less than 5 pounds per cubic foot. Such balsa is 

sometimes termed "corcho." 2 

Further investigation showed that in densities under 5 pounds per 

cubic foot, balsa wood has very few long fibers and a large number of short 

fibers with large diameters and thin walls. Wood which does not have the 

typical "corcho" tear has large percentages of long fibers with thicker walls 

and smaller diameters, hence its greater specific gravity. 

2"Corcho" is a Spanish term meaning "corklike.'' 

Report No. 1574 -2-
The result of this investigation indicated that so-called "corcho" 

and low-density balsa (specific gravity under 0.080) are synonymous. Figure 

4 shows the effect of cutting balsa planks containing corcho with a circular 

saw. The saw cuts have a distinct indentation where they are made across 

wide bands of “corcho’’ or low-density material. 

Cores of the sizes desired were originally made by gluing these slabs 

(about 3 by 5 inches) together by hand and weight pressure with animal glue. 

Later they were assembled into panels with the use of a 

gluing machine, as shown in figure 5, with phenolic and 

After assembling, the cores were lightly sanded with fine 

wood block, to remove glue squeeze-out and other minor 

and were trimmed to size. A typical end-grain balsa core 

high-frequency edgeresorcinol adhesives. 

sandpaper on a 

surface imperfections, 

is shown in figure 

6. No attempt was made to match cores or to have all the slabs in one core 

from the same plank except in special cases. 

Cellular Cellulose Acetate3 

The cellular cellulose acetate was received in the form of continuously extruded bars, approximately 5/8 by 2-5/8 to 5-1/2 inches in cross 

section (fig. 7), and included about 3 percent of chopped-glass fiber. The 

density was between 6.0 and 6.8 pounds per cubic foot, after the skins were 

removed. 

The extruded bars, which were easily machined by ordinary woodworking methods, were dimensioned by jointing one side and edge before they 

were ripped to proper thickness on a circular saw. It was later found that 

a wood-cutting band saw running at 4,000 feet per minute and having five 

teeth per inch produced an equally smooth surface and resulted in less 

waste in saw kerf (fig. 8). 

Strips of the proper 

were at first glued together 

they were assembled on the 

or resorcinol adhesives. 

Cellular Hard Rubber4 

thickness (within tolerances of ±0.005 inch)

with animal glue to the desired size. Later 

high-frequency edge-gluing machine with phenolic 

The cellular hard rubber was received in the form of slabs, approximately 1-1/2 by 20 by 36 inches. It was an expanded, hard, synthetic, 

sponge rubber, black in color (fig. 9). Its density, with the outer skin 

and one-eighth inch of higher density material removed, was from 6.2 to 7.2 

pounds per cubic foot. Ordinary woodworking procedures were used in cutting 

this rubber to dimensions desired. It could be planed to a minimum thickness of one-fourth inch by feeding it on a back-up board through a planer if 

the feed-roll and chip-breaker pressures were reduced. 

3Appendix III Note 22. 

4Appendix III Note 21. 

Report No. 1574 -3-
Rubber cores were prepared by removing the outer one-eighth inch on 

a jointer and by planing to proper thickness after resawing on a band saw. 

One-sixteenth inch on each side was allowed for the planing to remove feed 

roll marks. Cores longer than the size of the slabs were first fabricated by edge gluing with animal glue. Later the cores were made using a 

high-frequency edge-gluing machine with phenolic or resorcinol adhesives. 

Paper Honeycomb 

All the paper honeycomb material 5used in this study was made at the 

Forest Products Laboratory. The material was produced by impregnating 

4.5-ail kraft paper of 12-inch width with about 10 percent of a phenolic

resin (T),6 thinned with alcohol and water. This preliminary treatment was 

done by passing the paper around a roller partly submerged in the resin, and 

then through a drier about 12 feet long at 140° C. at a speed of about 

3 feet per minute. The phenolic-treated paper was then put through a 

"B-flute" corrugating machine, cut to 40-inch lengths, and nested with other 

sheets. The nesting is important to produce sheets as similar as possible 

so that the corrugations line up when the sheets are assembled in a block. 

These nested sheets were cured in an oven at 125° C. for 5 to 6 hours. 

The individual corrugated sheets were impregnated with a low-viscosity, 

laminating resin (B),7 to a 55 percent resin content by laying the sheets on 

a piece of plate glass covered with a uniform film of the resin, The sheets 

were then placed on a jig that had heating elements on the side to cure the 

resin partially and thus reduce slippage when the corrugated sheets were laid 

node to node. It also prevented the resin from running to one side and 

thereby producing a block of nonuniform density. Sufficient sheets were laid. 

together to produce a block from 2-1/2 to 2-3/4 inches thick. The use of 

blocks of greater thicknesses resulted in greater variations than desired 

when cut into thin sections, and the thicker blocks could not be cut with a 

circular saw. The approximate dimensions of the blocks were 2-1/2 by 12 by 

40 inches with the cells running in the 12-inch direction. In this form, 

the blocks were given a final cure in an oven provided with forced-air 

circulation at 120° to 125° C. for 2-1/2 hours, 

The paper honeycomb blocks were trimmed, measured, and weighed and the 

density of each block determined. This was approximately 6 pounds per cubic 

foot, Two methods were used for sawing the paper honeycomb: one, a band saw 

with four teeth per inch running at a speed of 4,000 feet per minute (fig. 

10); and two, a circular saw with 4-1/2 teeth per inch turning at 1,770 

revolutions per minute (fig. 11). Both methods were used depending upon the 

availability of the saws. 

5Development of this paper honeycomb core material was made in cooperation 

with the National Advisory Committee for Aeronautics. 

6Appendix III Note 19. 

7Appendix III Note 2. 

Report No. 1574 -4 
Paper honeycomb cores of 1/8- to 3/4-inch thickness (along the cells) 

were required for various test panels. When cut to a thickness of 1/8 inch, 

the outer cells of the paper honeycomb with a density of 6.5 pounds per 

cubic foot were often broken, but this breakage did not develop when higher 

density material (8 pounds per cubic foot or greater) was used (figs. 12 and 

13). None of the paper honeycomb of higher density (8 to 12 pounds per

cubic foot) could be sawed successfully to a thickness less than 1/8 inch, 

but all the paper honeycomb with a density of 6 to 12 pounds per cubic foot 

was sawed satisfactorily to a thickness of 3/16 inch (figs. 14 and 15). 

Later it was found that by reducing the resin content of the outside corrugations, the 6.5-pound material could be cut to 1/8-inch thickness, As the 

blocks were sawed, a constant check was made on the variation of the thickness, which was held to a tolerance of ±0.005 inch. In order to keep the 

variation to a minimum, it was found necessary to lay up the blocks of core 

material on a flat surface. At one time a slight curvature developed along 

the 12-inch dimension of the blocks and produced a slight taper on each 

piece as it was cut that exceeded the allowable tolerance. This was 

remedied by using a heavy piece of plywood, about 1 inch thick, in the core 

jig as a laying-up platform for the cores, 

After the pieces were cut and checked for thickness, they were glued 

together with a phenolic-resin (adhesive N)8, on the high-frequency edgegluing machine. The corrugations of one piece were nested into the corrugations of the other so that a strong glue bond would be obtained and thus 

make the finished core easier to handle. Where cores were required that 

were longer than 38 inches, which was the approximate length of the trimmed 

block, another piece from the same block was butted end to end and glued in 

the same manner by meshing the corrugations. After assembling, the cores 

were sanded with fine sandpaper on a wood block to remove glue squeeze-out 

and other minor surface imperfections. Figure 16 shows a typical piece of 

paper honeycomb core. No attempts were made to make matched cores, since 

the density variation in any one block was often as great as, the density 

variation between different blocks. 

Glass-cloth Honeycomb9 

The glass-cloth honeycomb was received from the manufacturer in 

blocks 3-1/2 by 17 by 18 inches with a cell size of 3/16 inch in the 17-inch 

direction. The density varied from 7 to 10 pounds per cubic foot. In 

making glass-cloth honeycomb about 60 percent resin by weight. was used. The 

resin content was determined by heating small samples to 1,000° F. for about 

one-haif hour, or to constant weight, in an electric furnace. 

From several cutting trials the best method found to cut glass-cloth 

honeycomb into thin sections with the equipment available was to use a 14­

or 24-tooth metal-cutting band saw mounted on the wheels so that the teeth 

8

Appendix III Note 14. 

9 

Appendix III Note 25. 

Report No. 1574 -5-
traveled backwards at a speed of 3,500 feet per minute (fig. 17). Other 

saws with larger teeth or teeth running in the forward, or normal, direction 

caused considerable breakage. The normal thickness used was one-half inch 

and was not difficult to saw, but considerable difficulty developed with 

thicknesses under three-eighths inch because the honeycomb material fell 

apart (fig. 18). All sections were measured and held within the tolerances 

of ±0.005 inch of the desired thickness. The fuzzy character of the surface 

produced by the saw is shown in figure 19. 

Originally the glass-cloth honeycomb was sawed with a band saw by use 

of a solid wood fence, which produced a fins suspension of resin dust in the 

atmosphere and resulted in skin irritations and other effects on the operator. This difficulty was overcome by drilling holes in the wood fence near 

the saw and applying a suction on the far side to draw off the dust (fig. 17). 

The cut sections were glued together with a phenolic-resin (adhesive 

N), in the high-frequency edge-gluing machine by nesting the corrugations of 

the adjoining pieces. Cores made of glass-cloth honeycomb required more 

delicate handling than paper honeycomb cores, probably because of the 

shorter pieces used and the nature of the material. The cores were sanded 

to remove glue squeeze-out and obtain a uniform surface. 

Gotton-cloth Honeycomb10 

The cotton-cloth honeycomb material was received from the manufacturer 

in blocks about 8 by 9 by 125 inches with the 7/16-inch hexagon cells running 

in the 9-inch direction. Since most of the material was to be made into 

panels 1/2 by 36 by 36 inches, the blocks were cut into 38-inch lengths. 

These individual blocks were measured and weighed and found to have a density 

of about 3.65 pounds per cubic foot. Due to the lack of availability of the 

4-ounce cotton duck, another cotton cloth of similar quality was used by the 

manufacturer in some of the blocks, which reduced the density to approximately 3.25 pounds per cubic foot. The cotton-cloth honeycomb was cut into 

thin sections of 1/2-inch thickness with a 4-tooth band saw running at 3,500 

feet per minute (fig. 20). Because of the thickness of the blocks and the 

flexibility of this type of structure a great deal of material was wasted in 

cutting, with the greater percentage of rejections occurring toward the end 

of the block. A tolerance of ±0.005 inch was permitted in the thickness of 

the cut sections. The nature of the surface formed by the band saw Is shown 

in figure 21. 

Due to the hexagonal shape of the cells and large area for contact, 

the sections of cotton-cloth honeycomb were glued together node to node with 

a phenolic-resin (adhesive N), using the high-frequency edge-gluing machine. 

The cores were sanded to remove excess squeeze-out of the adhesive N and 

other nonuniformi ties. 

10Appendix III Note 26. 

Report No. 1574 -6-
Preparation of Face Material 

Plywood 

All of the plywood faces were 0.070-inch, three-ply birch manufactured in accordance with Specification AN-NN-P-511b. The glue used was a 

high-temperature-setting, phenolic dry film glue (adhesive U).11 No special 

treatment was given the plywood other than to condition it for approximately 

1 week at 65 percent relative humidity at 80° F. 

Aluminum 

The aluminum used 

Thickness 

Inch 

0.005 

.012 

.020 

.032 

was described as 

Type 

24SH 

24ST Alclad 

24ST Alclad 

24ST Alclad 

follows: 

Sheet Size 

20 by 42 inches 

2 by 10 feet 

3 by 12 feet 

3 by 12 feet 

The identification marks were first removed by washing with acetone. 

This was followed by cleaning and etching in a sulfuric acid-sodium 

dichromate bath (10 parts by weight of concentrated sulfuric acid, 1 part of 

sodium dichromate, and 30 parts of water) for 20 minutes at 145° F. or 8 to 

10 minutes at 160° F. The sheets were rinsed and if a smooth unbroken water 

film was not obtained, the sheets were returned to the etching solution 

until an unbroken water film was obtained. 

Glass-cloth Faces 

Heat treated glass cloth 12 in combination with various laminating 

resins was used for all glass cloth faces until July 1946. Since that date 

glass cloth that was heat cleaned and treated with a chrome complex compound13 was used. Characteristics of the cloth as received were as follows: 

11 Appendix III Note 20. 

12Appendix III Note 23. 

13Appendix III Note 24. 

Report No. 1574 -7-
Roll size - 38-3/4 inches wide 

Thickness - 0.003 inch 

Weight - 2.09 ounces per square yard (without selvage) 

Type of weave - plain 

Minimum breaking strength - Warp - 100 pounds per inch 

Fill - 70 pounds per inch. 

Since all glass-cloth faces were cross laminated, the cloth was first 

cut to size. Cuts were made following a pulled thread to assure edges 

parallel to the threads. 

In early work, the laminating resin was applied to the cloth by hand 

as shown in figures 22 A and B. A film of the resin was produced on a large 

stainless steel plate by slowly squeegeeing the resin to a uniform thickness 

while the plate was slightly warmed by radiant heat. The cloth sheets were 

then laid on the resin film and gently worked down with a dry squeegee. 

After a short wicking period the sheet was removed and weighed for resin 

pick-up. The average resin content of the impregnated sheets was 43 to 45 

percent of the total weight of cloth and resin. The sheets were rolled on 

cellophane-covered steel pipes and stored at 40° F. until ready for use. 

This storage period of 1 day to several weeks permitted the resin to wick 

uniformly through the entire roll. 

The foregoing method was satisfactory for making a few panels, but 

required too much time when a greater number were needed. Therefore, a 

mechanical spreader was used for high-viscosity laminating resins (A and C), 

a6 shown in figure 23. Here the cloth passed around an idler roller in 

contact with a driven roller, was continuously coated by means of a doctor 

blade with a film of the laminating resin, and was then rolled up on a 

paper or copper tube covered with cellophane. The resin content varied 

between 45 and 75 percent depending upon the temperature and thickness of 

the resin fila on the roller. The roll of impregnated cloth was wrapped 

in cellophane and stored at room temperature overnight or at 40° F. for a 

longer period to permit the resin to wick uniformly through the entire roll. 

The cloth was then cut into the desired size and weighed to determine the 

resin content. Usually the resin content was over the desired 45 percent 

and therefore dry sheets were cut and placed between the wet or impregnated 

sheets to produce the desired resin content in the assembled material. The 

number of dry sheets used depended upon tho resin content of the wet sheets. 

After laminating the wet and dry sheets, they were rolled tightly on a pipe, 

and stored at room temperature overnight to allow the resin to penetrate 

the dry shoots, after which they were ready for laying up into a panel. 

For low-viscosity laminating resins (B and D), the procedure was 

simpler and more easily controlled. Here the mechanical spreader was used 

only as a means of rewinding the glass cloth (fig. 24), and the resin was 

poured on as the cloth was rolled up. The resin content varied between 45 

and 50 percent of the total wet uncured weight. The roll of impregnated 

cloth was immediately wrapped in cellophane to prevent the loss of volatile 

Report No. 1574 -8-
components of the resins and then stored in a room at 40° F. until used. 

Because the spread of the low-viscosity resin could be controlled accurately, 

it was not necessary to interleave the impregnated glass cloth with dry 

sheet s . 

Panel assembly Techniques 

Plywood to Balsa, 

Panels with end-grain balsa cores and plywood faces were glued with 

a high temperature-setting melamine resin, (adhesive R).14 The adhesive was 

spread on the plywood by brush to a weight of 24 grams of wet glue per 

square foot. After an open-assembly period of 1 to 7 days the panels were 

assembled and bag-molded on a flat aluminum mold or pressed in a hot press. 

The curing cycle was 15 minutes at 50 pounds per square inch pressure and 

300° F. Panels were removed hot and no trouble was experienced with 

blistering due to internal pressure. 

Aluminum to Balsa 

Method 1.--The work plan originally specified a primary and secondary 

bonding process, therefore all aluminum faces that were later to be glued 

were sprayed with a high-temperature-setting, modified thermoplastic resin, 

(adhesive H).15 Four coats were applied with approximately 30 minutes of 

air drying after each coat. The final coat was air dried overnight and then 

cured for 30 to 45 minutes at 325° F. A light sanding with a fine emery 

cloth prepared the surface of the cured film for the secondary gluing. 

These primed aluminum faces were bonded to the balsa cores with a roomtemperature-setting, resorcinol resin (adhesive P)16 for the durability and 

the first few strength-test panels. A spread of 35 grams of adhesive P per 

square foot was brushed on the treated aluminum faces and the panels were 

pressed at room temperature at 50 pounds per square inch pressure for a 

minimum of 4 hours, either in a bag or in a hydraulic press, 

The use of a wet glue under metal faces obviously trapped considerable liquid, which eventually was absorbed by the cores. In thin panels 

(1/8 to 1/4 inch thick) this trapped liquid raised the moisture content of 

the core to objectionable values for strength-test specimens. For example, 

if the solvent in adhesive P were assumed to be all water (it is actually 

part alcohol) the moisture content of the balsa core in a 1/8-inch aluminumbalsa sandwich would be raised from 6 to about 50 percent in gluing. The 

moisture could not escape except through the edges, therefore the moisture 

content remained excessively high for a period of months. 

14Appendix III Note 17. 

15Appendix III Note 8. 

16Appendix III Note 15 

Report No. 1574 -9-
Method 2.--To eliminate this condition, preliminary tests were made 

on possible "dry-gluing" techniques for bonding aluminum faces, that had 

been coated with adhesive H, to end-grain balsa. A high-temperature-setting, 

thermoplastic vinyl resin (adhesive Q),17 was found to give acceptable bonds 

and, therefore, was used on some of the strength-test panels. After thinning 

with alcohol, it was applied by brushing to the primed aluminum faces until 

a dry spread of 10 grams per square foot was obtained. This required 8 to 

10 coats with a few minutes of air drying after each coat. The final coat 

was allowed to dry at least 24 hours to eliminate most of the solvent. On 

cores thicker than one-eighth inch, one-half of this quantity was applied 

to the core and the other half to the face. Adhesive Q swells wood to such 

an extent that applying it to 1/8-inch cores produced damaging wrinkles. 

The cores and faces were then assembled and the panels were cured for 30 

minutes at 275° F. and 75 pounds per square inch pressure in a bag, or 100 

pounds per square inch in a press, and cooled under pressure. Due to the 

limited flow of the adhesive Q and the inaccuracies of the balsa core, a 

cushion of soft synthetic rubber about one-eighth inch thick or its 

equivalent was required on both sides of the panel when it was pressed 

between platens. 

Thin aluminum-balsa panels of suitable flatness were difficult to 

produce. The hydraulic press used had individual temperature controls for 

each platen, and a temperature differential of as little as 3° F. between 

the two platens, in either the heating or cooling cycle, produced curved 

panels. Thin bag-molded panels molded on a flat mold were always slightly 

curved due to the nonuniformity of heating between the mold and the bag side. 

Panels with 0.005-inch aluminum faces required a special treatment to 

avoid thermal-expansion wrinkles in the faces. The rubber cushion cauls 

were first preheated before inserting the panel to be pressed. The short

period during which the panels were between these hot cauls while the press 

was closing expanded the aluminum faces before pressure was applied and 

eliminated the face wrinkles that were produced if no preheating was used. 

Shortly after initiating this bonding procedure the manufacture of 

this glue was discontinued and a similar glue, (adhesive Q-1)18, having a 

solvent claimed by the manufacturer to produce less wood swelling, was substituted. At the same time a new shipment of adhesive H, which appeared to 

have slightly different color characteristics after curing, was received. 

Panels pressed with these new materials blistered badly in curing, and it 

was necessary to raise the pressure to 100 pounds per square inch throughout 

the heating and cooling cycle in bag molding. One hundred and fifty pounds 

per square inch pressure was tried on the flat panels, but an exceptionally 

wavy surface indicating compression of the softer growth rings resulted. To 

correct this, a thicker rubber cushion was used at a pressure of 100 pounds 

per square inch. 

17Appendix III Note 16. 

18Appendix III Note 16. 

Report No. 1574 -10-
Because of the slight thermoplasticity of adhesive Q-1 it was possible 

to repress panels having blisters if the blisters were pricked to release the 

entrapped gas that appeared to develop when the air-dried adhesive Q-1 was 

heated. 

So much difficulty with blistered faces was encountered with the adhesive H plus adhesive Q-1 gluing technique that improved bonding procedures 

were sought. 

Method 3.--One of these involved the use of a high-temperature setting, 

modified thermoplastic resin (adhesive F)19 directly on the etched aluminum 

surface. A dry spread of 12 grams per square foot was applied by brushing 

the cement, thinned with a mixture of ethyl acetate and ethyl alcohol, on the 

aluminum. About four coats were required with a half-hour drying period 

after each. The final coat was air dried about 24 hours before pressing. 

Panels were pressed between preheated rubber cushion cauls, or bag molded, at 

2750 F. and a pressure of 100 or 75 pounds per square inch pressure respectively for 40 minutes and removed hot. No blisters resulted but, due to the 

light spread or to the nonflowing characteristics of adhesive F, the cushions 

were necessary on the aluminum-balsa, panels made in a press (fig. 25). This 

produced slightly wavy surfaces conforming to the irregularities of the core. 

The strength of the bond6 was probably sufficient for buckling- and shear-test 

panels, although from stripping tests it did not appear to be so great as that 

of the joints bonded with adhesive H. 

Method 4.--The fourth method was similar to the third, with a hightemperature-setting liquid resin and thermoplastic powder (adhesive G)20-used 

in place of the adhesive F. The thermosetting liquid resin was brushed on the 

etched aluminum to a wet spread of about 20 grams per square foot. While 

the liquid was still wet, the thermoplastic powder was sifted on the surface, 

and, after drying, the excess was removed. These faces were pressed to the 

balsa cores between stainless-steel cauls in a press at 275° F. and a pressure 

of 100 pounds per square inch for 30 minutes and removed while hot. No 

blisters were encountered on the first few panels and the bonds seemed to be 

exceptionally good (fig. 26). Later, after some blistering was encountered, 

three sheets Of 0.020 inch paper on each side were used as cushions for 

panels cured in a press, Some panels were also bag molded in an autoclave 

at 275° F. and a pressure of 75 pounds per square inch for 1 hour. The panels 

were laid on a 1/4-inch aluminum mold, and a 1/8-inch rubber pad was laid 

over the top face. 

Method 5.--An additional bonding method was used that incorporated 

M) 21 a 

high-temperature-setting, modified thermosetting priming resin (adhesive 

applied in six spray coats directly to the etched aluminum, air dried overnight, and cured in an oven at 325° F. for 30 minutes. After sanding, the 

faces were bonded to the balsa core with a high-temperature-setting phenolic 

resin (adhesive N)22, in a press or bag at 230° F. and a pressure of 75 

pounds per square inch for 1 hour, Some panels were made by applying a light 

spray coat of adhesive S23 to the balsa to reduce to the rapid absorption of 

adhesive N during curing. 

19Appendix III Note 6. 21Appendix III Note 13. 

20Appendix III Note 7. 22Appendix III Note 14. 

23Appendix III Note 18. 

Report No. 1574 -11-
Glass Cloth to Balsa and Glass 

Cloth to Cellulose Acetate 

Panels with glass cloth faces and balsa cores were assembled and 

cured in one operation with no additional resin between the faces and the 

core. This is commonly known as "wet laminating.'' 

The normal procedure was to cover the flat caul or curved metal form 

with a parting film of cellophane. The choice of cellophane proved important as some types of film bonded to the resin, such as the self-sealing 

and moisture resistant types. The impregnated glass cloth for one face was 

then laid, one sheet at a time and cross laminated, on a flat caul or 

curved mold. This procedure was repeated for the other face on the matching 

flat caul or, (for a curved mold) on a piece of cellophane taped to a flat 

surface. The balsa core was then laid on one of the faces, and covered with 

the Lay-up for the opposite face. This procedure was preferred to the 

method of laying the glass cloth directly on the core. 

Due to slight waviness of the balsa core surfaces, as described previously, fluid pressure or its equivalent was necessary to assure intimate 

contact between the glass cloth and the core. Large flat panels 1/8-inch 

thick required for some of the test panels were difficult to bag mold to 

acceptable flatness due to nonuniform heat penetration. These panels could, 

however, bo made in a press if suitably cushioned cauls, to simulate fluid 

pressure, were used on each side. Rubber was tried for these cauls but was 

unsuitable for two reasons: first, the rubber available that would withstand the high temperature was too hard; and second, materials contained in 

or formed when this rubber was heated were likely to inhibit the cure of the 

resin, even when covered with cellophane. 

After experimenting with other cushioning materials for pressing, 

such as cloth and blotting paper, a soft-textured wool blanket was found to 

produce the desired results. Four. and one-quarter-pound wool bed blankets 

72 by 84 inches were cut to the desired caul size and laid on 5/16-inch 

poplar plywood. The blanket surface was covered with heavy kraft paper and 

finally with cellophane. These composite cauls provided the equivalent of

fluid pressure and at the same time slowed the heating of the panel to 

approximately that obtained by vacuum bag molding in hot air. For bag 

molding, a 0,020-inch aluminum caul covered with cellophane on one side was 

used. 

The following five assembly methods were used to fabricate the glass 

cloth-to-balsa panels and the first four for the glass cloth-to-cellulose 

acetate panels: 

Method 1.--Both flat and curved panels were fabricated with a highviscosity laminating resin (A)24 and cured in a bag or press at 225° F. and 

a pressure of 13 pounds per square inch for 1-1/2 hours. During this curing 

24Appendix III Note 1. 

Report No. 1574 -12-
cycle resin A apparently became fairly liquid before it hardened. In this 

liquid state it was absorbed somewhat by the end-grain balsa in quantities 

inversely proportional to the density of the balsa. The variations in 

absorptions probably accounted for the somewhat spotty appearance of panels 

with faces of only three plies of cloth (fig. 27). A check of average resin 

loss from the faces showed that three-ply glass cloth faces originally containing 43 percent resin dropped to 36 percent upon stripping from the 

balsa cores after pressing. 

Panels of these combinations were made with core thicknesses of oneeighth, three-sixteenths, one-fourth, seven-sixteenths, one-half, threefourths, and 1 inch, and faces of 2, 3, 4, 6, 8, and 10 plies of glass cloth. 

Acceptable panels could be made by the normal process in all combinations 

except the curved panels with ten-ply faces. In these thick faces, blisters 

between the plies, and between the faces and the core persisted in spite of 

the omission of the aluminum caul and pinpricking of the assembly at 3-inch 

intervals in an attempt to bleed the air. A replacement set of panels molded 

by the same technique but at 40 pounds per square inch pressure had no 

blisters and was acceptable. It is doubtful, in the light of this experience, 

if vacuum pressure is sufficient to insure blister-free sandwich panels with 

thick glass-cloth faces. 

Method 2.--After the glass cloth was treated with resin B,25 a low- viscosity laminating resin (see "Preparation of Face Materials"), the glass 

cloth was rolled tightly in cellophane to prevent the loss of the styrene 

and the drying of the resin. Because of this evaporation and drying effect, 

the treated glass cloth had to be cut to size, weighed for resin content, 

and laid on the caul or mold within 2 or 3 hours. If the spread sheets were 

exposed to the air for a longer period, the glass cloth became so dry that 

the wrinkles (produced in laying up the sheets) were difficult to remove due 

to the lack of slippage between the sheets. Glass cloth treated with resin 

B can be kept for several day6 if wrapped tightly in cellophane and stored 

at temperatures below 40° F. Another method that was used to reduce the exposure time of the individual sheets to the air was to weigh the mold and 

caul, lay the sheets on the mold or caul as soon as they were cut, and then 

weigh the mold or caul with the assembled face for determining the resin 

content. 

The panels were bag molded in a steam-heated oven or pressed between 

cellophane-covered blanket cauls at 250° F. and a pressure of 13 pounds per 

square inch for 1-1/2 hours. 

Method 3.--The details for assembling panels using resin C,26 a highviscosity laminating resin, were similar to those used for resin A. The 

panels were cured at 240° F. and a pressure of 13 pounds per square inch for 

1-1/2 hours . 

25Appendix III Note 2. 

26Appendix III Note 3. 

Report No. 1574 -13-
Method 4.--The details for assembling panels using resin D,27 a 

low-viscosity laminating resin, were similar to those used for resin B, 

except that the curing temperature used was 220° F. 

Method 5.--Panels with resin E,28 a low-viscosity laminating resin, 

were made only with balsa cores. The glass cloth was impregnated by the 

hand method, but could also be done by the method used for the other lowviscosity resins. These panels were Bag molded in a steam-heated oven or 

pressed between cellophane-covered blanket cauls at 275° F. and a pressure 

of 13 pounds per square inch for 4 hours, 

Plywood to Cellulose Acetate and 

Plywood to Cellular Hard Rubber 

Panels of plywood bonded to cellulose acetate and of plywood bonded 

to cellular hard rubber were made by brushing a room-temperature-setting, 

resorcinol resin (adhesive P) on the plywood faces only and pressing the 

panels in a vacuum bag or press at a pressure of 13 pounds per square inch 

at room temperature for a minimum of 4 hours. 

Aluminum to Cellulose Acetate 

The following five methods were used for durability and strengthtest panels made of aluminum bonded to cellulose acetate. 

Method 1.--The primary-secondary gluing method of using adhesives H 

and P, as discussed under aluminum-to-balsa method 1, was used for this combination, except that the pressure was reduced to 13 pounds per square inch. 

Method 2.--Gluing method 2 used for aluminum-to-balsa panels was 

applied to this combination, except a pressure of 40 pounds per square inch 

was used in molding the panels. With this adhesive, it was necessary to 

cool the panels under pressure. 

Method 3. --A high-temperature-setting, modif ied thermoplastic resin 

(adhesive F) thinned with a mixture of ethyl alcohol and ethyl acetate was 

brushed on the etched aluminum to a dry spread of 14 grams per square foot. 

About four coats were required with a half-hour drying period after each 

coat and 24 hours after the final coat. Panels were pressed with three 

sheets of heavy kraft paper on each face or bag molded at 275° F. and a 

pressure of 40 pounds per square inch for 40 minutes and removed hot. 

Method 4.--The etched aluminum was sprayed with 6 coats of a hightemperature-setting, modified thermosetting priming resin (adhesive M) 

allowing 1/2-hour drying between each coat and a 24-hour drying period after 

the final coat. The primed aluminum faces were cured in an oven at 325° F. 

27Appendix III Note 4. 

28Appendix III Note 5. 

Report No. 1574 -14-
for 30 minutes, cooled and sanded with fine sandpaper to remove irregularities of the surface. The faces were spread with a room-temperaturesetting, resorcinol resin (adhesive P) to a weight of 35 grams per square 

foot. The panels were pressed or bag molded at 13 pounds per square inch 

for a minimum of 4 hours. 

Method 5.--The same method used for aluminum to balsa (method 5) 

was applied to this combination, except the molding pressure was reduced 

to 40 pounds per square inch. The spread of the secondary phenolic resin 

(adhesive N) used on the aluminum faces was 10 grams per square foot. 

Aluminum to Cellular Hard Rubber 

Panels of aluminum to cellular hard rubber were made by the following two gluing methods: 

Method 1.--Panelswere made by the same procedure as gluing method 

1 under aluminum to balsa, except that a reduced pressure of 13 pounds per 

square inch was used. 

Method 2.--Panels were made by the same procedure as gluing method 4 

under aluminum to cellulose acetate. The entrapped moisture encountered 

with the secondary adhesive used for both these methods caused no apparent 

difficulty. 

Class Cloth to Cellular Hard Rubber 

Since rubber inhibited the cure of all the laminating resins used in this study when the two were in intimate contact during the curing cycle, 

the normal wet-laminating process as used on balsa and cellulose-acetate 

cores could not be employed. If, however, a suspension (about 15 percent 

concentration) of the catalyst (benzoyl peroxide) in water was sprayed or 

brushed on the rubber core and allowed to dry, the normal laminating 

process could be used with fair results, although difficulties were encountered with these panels. One such example is shown in figure 28. In 

the first trials of this method, an acetone solution of benzoyl peroxide 

was applied to the faces, but tension tests perpendicular to the face on 

panels made by this method gave values below those expected for rubber. It 

was thought that the acetone might be weakening the surface layer of the 

rubber. When water was substituted for the acetone the tension values were 

raised. The catalyst is not soluble in water and must be applied as a 

suspension prepared by adding a small amount of ethyl alcohol (5.0 percent) 

to the water before adding the benzoyl peroxide. The material must be 

stirred or agitated during application to keep it in suspension. 

Cores before and after application of the catalyst are shown in 

figure 29. Panels were made by wet laminating glass cloth faces impregnated with resin A on the treated cores by the same method as that used on 

the glass cloth-to-balsa combination. 

Report No. 1574 -15-
Plywood to Paper Honeycomb; Plywood to Glass-cloth 

Honeycomb; Plywood to Cotton-cloth Honeycomb 

Panels with plywood faces and paper honeycomb or glass-cloth honeycomb cores were glued with a high-temperature-setting phenolic resin 

(adhesive N). A wet spread of 20 grams per square foot was applied to the 

faces and 5 grams per square foot to the core. The adhesive was spread on 

the faces and core with a roller, and then allowed to dry from 1 to 24 

hours before assembling. The panels were assembled on an aluminum mold 

with a pine frame around the panel to prevent crushing of the edges, and 

bag molded in an autoclave at 2300 F. and a pressure of 30 pounds per 

square inch for 1 hour. 

The same techniques used to fabricate the plywood to paper honeycomb 

combination were used for plywood to cotton-cloth honeycomb, except the 

molding pressure was reduced to 13 pounds per square inch. 

Aluminum to Paper Honeycomb 

Five adhesives were suggested and approved for gluing aluminum faces 

to paper honeycomb cores. The methods of preparing panels with these 

adhesives were as follows: 

Method 1.--The procedure used for preparing the aluminum faces and 

spreading the adhesive G, a thermosetting liquid resin and a thermoplastic 

powder, was the same as used in the aluminum-to-balsa combination, except 

that a wet spread of 35 grams per square foot of the resin instead of 20 

grams per square foot was used. This additional fila thickness appeared 

necessary to obtain better adherence to the core. In addition, the core 

was coated only with the thermosetting liquid of adhesive G by a rubber 

roller to a spread of 10 grams per square foot on each side. After drying 

for 24 hours, these panels were molded on a 1/4-inch aluminum mold, with a 

wood frame around the edges of the panel, and the top face of the panel 

was covered with a 1/8-inch rubber pad. The rubber pad was used to reduce 

the conduction of heat to the top face and make it comparable to the heat 

reduction caused by the mold. The panels were cured in an autoclave at 

275° F. and a pressure of 30 pounds per square inch for 1 hour. 

Method 2.--The method of applying adhesive F, a thermoplastic rosin, 

to the aluminum faces was the same as used for the aluminum-to-balsa combination. The core was given two roller coats of adhesive F, with a halfhour period between coats to allow the solvent to escape, which gave a 

spread of approximately 2 grama per square foot, The assembly for bag 

molding consisted of an aluminum mold, a wood frame around the edges of the 

panel, and 1/8-inch rubber pad over the top face. The panels were cured in 

an autoclave at 275° F. and a pressure of 30 pounds per square inch for 1 

hour. 

Report No. 1574 -16-
Method 3.--Adhesive L, 29 a thermosetting resin and synthetic rubber, 

was applied directly to the etched aluminum by brush to an air dry spread 

of 15 grams per square foot. The adhesive L, as received, was thinned with 

acetone from 30 to 50 percent by volume. About four coats were required to 

obtain the required glue film, allowing at Least one-half hour between 

coats and 24 hours' drying after the last coat. Adhesive L was applied to 

the paper honeycomb core with a hard rubber roller until a dry spread of 

5 grams per square foot was reached. These flat panels were bag molded in 

an autoclave at 2600 F. and a pressure of 30 pounds per square inch for 

1 hour. Like the previous panels, a wood frame was used around the edges 

to prevent crushing and a 1/8-inch rubber pad was laid over the top face to 

prevent a curvature in the panel after molding. 

Method 4.--Adhesive J,30 a thermoplastic and thermosetting resin 

also may be applied directly to aluminum when properly prepared, as etching 

in a hot sulphuric acid and sodium dichromate bath. For brush application, 

this adhesive was thinned with a special thinner to 30 percent by volume. 

About four brush coats were required to build up a dry film thickness of 

0.006 to 0.010 inch, or about 20 grams per square foot on the aluminum faces. 

One hour or longer of drying was allowed between each coat, and 24 hours 

after the final coat. A dry spread of 10 grams per square foot was applied 

to the core with a rubber roller. The panels were assembled for bag molding 

and cured in an autoclave at 275° F. and a pressure of 30 pounds per square 

inch for 1-3/4 hours. The precure and cure that were recommended for 

adhesive J were carried out in one operation under closed assembly. The 

panels were removed hot and no blisters were apparent. 

Method 5.--The primary adhesive M, which was a primary hightemperature-setting, modified, thermosetting resin, was sprayed on the 

etched aluminum sheets in six coats to an approximate 0.003-inch film thickness. After drying overnight the sprayed sheets were cured in a press at 

320° F. for 30 minutes. Prior to applying the secondary adhesive N, which 

was a secondary high-temperature-setting, phenolic resin, the cured primary 

adhesive M was sanded lightly and wiped with a clean cloth and methyl 

alcohol. A wet spread of 20 grams per square foot of the secondary adhesive 

N, was applied with a rubber roller and dried for 1 to 24 hours before 

assembling. The paper honeycomb core was coated with a wet spread of 5 

grams per square foot on each side by a roller and dried for the same period. 

The panels were then assembled and bag molded or pressed at 230° F. at a 

pressure of 30 pounds per square inch for 1 hour. For bag molding, the 

panel was laid on an aluminum mold with a wood frame around the edges, and a 

1/8-inch rubber pad was placed over the top face. For pressing, three 

sheets of heavy paper were used as cauls to conform to the irregularities 

due to the cutting and fabricating of the paper honeycomb cores. The panels 

were removed while hot. 

29Appendix III Note 12. 

30 Appendix III Note 10. 

Report No. 1574 -17-
-- 

Aluminum to Glass-cloth Honeycomb 

The five adhesives that were discussed under aluminum to paper honeycomb combinations were also used for aluminum to glass-cloth honeycomb. 

Since the glass-cloth honeycomb has essentially the same structure and 

sufficient compressive strength to withstand the fabrication pressures at 

elevated temperature used on paper honeycomb, the same spreads and conditions used for the various adhesives under aluminum to paper honeycomb were 

used for glass-cloth honeycomb. 

Aluminum to Cotton-cloth Honeycomb 

The same five adhesives used for aluminum to paper honeycomb panels 

were used to make aluminum to cotton-cloth honeycomb panels. The conditions 

of fabricating the panels were also the sane, except that the pressures were 

reduced to 20 pounds per square inch for pressing operations and about 15 

pounds per square inch for bag molding in an autoclave or steam-heated oven. 

The pressures and equipment used for curing the cotton-cloth honeycomb 

panels are summarized as follows: 

Method 1.--Whenadhesive G was used the panels were cured in a press 

at a pressure of 20 pounds per square inch. 

Method 2.--When adhesive F was used the panels were cured at a pressure of 13 pounds per square inch in a steam-heated oven. 

Method 3.--Whenadhesive L was used the panels were cured at a pressure of 18 pounds per square inch in an autoclave. 

Method 4.--When adhesive J was used the panels were cured in a press 

at a pressure of 20 pounds per square inch. 

Method 5.--When adhesives M and N were used the panels were cured at 

a pressure of 15 pounds per square inch in an autoclave. 

Glass Cloth to Paper Honeycomb; Glass Cloth to Glasscloth Honeycomb; and Glass Cloth to Cotton-cloth Honeycomb 

Five contact-pressure laminating resins were included in the working 

plan for impregnating glass cloth as faces for panels with the various 

honeycomb cores. The manufacturer could not supply the low-viscosity 

laminating resin (E) within a reasonable time and therefore it was not used 

in combination with the honeycomb cores. The methods of preparing the 

panels with the four laminating resins are discussed as follows: 

Method 1.--The impregnating of the glass cloth for the faces with 

the high-viscosity laminating resin is presented under "Preparation of Face 

Materials Glass cloth faces," and the procedure used to fabricate the 

panels followed the same steps as used for the glass cloth-to-balsa combination. At first no resin was put on the core material, but from strength 

Report No. 1574 -18-
tests, (table 1) it was found that such an application greatly improved the 

bond. A spread of 10 grams per square foot was therefore applied to each 

side of the paper honeycomb and glass-cloth honeycomb cores and 5 grams per 

square foot to each aide of the cotton-cloth honeycomb cores. The 

impregnated cloth was used for several weeks after it was treated with 

resin A. Panels were assembled on an aluminum mold, covered with cellophane 

and bag molded in a steam-heated oven at 225° F. and vacuum pressure for 1 

hour. Other panels were made in a press at 225° F. and a pressure of 15 

pounds per square inch for 1 hour with blanket cauls. 

Method 2.--The impregnated glass cloth faces made with the lowviscosity laminating resin B were prepared in the same manner as for glass 

cloth-to-balsa panels made with resin B. Since increased strength was 

obtained by applying resin to the cares, the paper honeycomb and glasscloth honeycomb cores were coated with 10 grams per square foot of resin B 

with a roller, and the cotton honeycomb cores with 5 grams per square foot. 

For bag molding, the panel was assembled on a cellophane-covered aluminum 

mold, and a cellophane-covered, 0.020-inch, aluminum caul was used over the 

top face. These panels were bag molded in a steam-heated oven at 250° F. 

and vacuum pressure for 1 hour. When pressed, the panels were cured at 

250° F. and a pressure of 15 pounds per square inch for 1 hour with blanketcushioned cauls as described previously. 

Method 3.--The methods used for resin C, a high-viscosity laminating 

resin, were similar to those used for resin A of glass cloth-balsa, except 

that the curing temperature was 240° F. instead of 225° F. The three honeycomb core materials were also coated with resin C with a roller as discussed 

previously. 

Method 4.--The techniques used to fabricate glass cloth panels with 

resin D, a low-viscosity laminating resin, were the same as those used for 

resin B, except that the curing temperature was 220° F. 

Curved Sandwich Panels 

A few tests were made to determine the limits of curvature of various 

core materials and few sandwich constructions. These bending tests and 

fabrication trials were limited to single curvature. 

Panels with only slight single curvature could readily be molded by 

merely draping the core and faces over a convex mold or laying them in a 

concave mold and later inserting the whale assembly in a bag after temporarily taping the pieces to the mold. An assembly of this kind on the 

convex side of a steel mold is shown in a bag in figure 30. 

As the curvature became more severe, it became more difficult to 

bend the core to shape and to draw it down firmly to the mold surface. The 

anticlastic (saddle-shaped) curvature, which the core materials tend to 

assume upon bending, caused some difficulty even at moderate curvatures. 

This phenomenon was particularly noticeable with honeycomb cores. The 

limitations imposed on curvatures of core materials by their tendency to 

Report No. 1574 -19-
assume an antielastic curvature has not been fully investigated in this 

study, but panels having 1/4-inch thick paper honeycomb cores have been 

molded to a 30-inch radius with no difficulty. 

Often the severity of curvature to which a panel can be molded by 

the "draping method," or one-step molding process, is limited by breakage 

of the core material. An attempt to evaluate this limitation was made by 

determining the approximate breaking radius of 1-inch wide strips of four 

core materials in a variety of thicknesses from 1/8 to 1/2 inch. The 

results of these tests are presented in figure 31. The four lower lines 

represent the average approximate breaking radii and therefore cannot be 

used directly in estimating the minimum working radii. If a factor of 

safety against breakage of about 2 is applied to the radii, however, it is 

believed that reasonably safe working radii will be obtained. This factor 

has been investigated, in an exploratory manner, by bending larger sheets 

of core material between sheets of thin aluminum, and the upper line in 

figure 31 represents the results, which can be used in determining an 

approximate safe working radius for all four of the core materials tested 

in the form of strips. 

If it is desired to make curved sandwich panels having radii smaller 

than the safe working radii presented in figure 31, other special means of 

forming the core must be employed. Some core materials, such as cellular 

cellulose acetate, cellular hard rubber, and paper honeycomb, lend thenselves fairly well to post-forming. Another method, which perhaps is less 

cumbersome is to glue or laminate one face to the core in the first operation, bend the assembly to approximately the proper shape, the faced side 

being the convex side, and glue the inner face to the core in a second 

molding operation to produce a sandwich panel of the desired shape. Figure 32 shows the bending operation being done in a conventional hand-operated 

sheet metal rolling machine. The panel being bent is end-grain balsa glued 

to an aluminum face. 

The limits on minimum radius for this bending operation were again 

investigated on strips, and it was found that 1/2-inch thick strips of any 

of the four core materials with either an aluminum or a glass-cloth face on 

one side could be bent to a radius of lese than 2 inches without excessive 

damage. 

After bending, the molding operation can be completed on either a 

male or female mold by use of fluid pressure. Both methods were used 

experimentally in this study to explore in a preliminary way the advantages 

of each. Several steps in the use of a male wood mold are shown in figure 

33. The parts to be molded are first assembled on the mold and temporarily 

held in place (in this case by screws) (fig. 33A). Several layers of heavy 

canvas are wrapped around the mold (and appropriate projections from the 

mold) to provide adequate pressure at the point of severest curvature (fig. 

33B). The assembly is then inserted in a bag and a vacuum drawn before 

molding in an autoclave (fig. 33C). After the part is molded and cooled 

its curvature is not exactly the same as that of the mold (fig. 33D). This 

difference in curvature between mold and product is e fundamental 

Report No. 1574 -20-
disadvantage of the use of a mold having considerable body, and is caused 

by nonuniform temperature conditions between inner and outer faces of the 

product at the time the glue sets. 

After this curved piece was trimmed, as shown in figure 34, it was 

used as a female mold in molding a similar piece. Four steps in this molding operation are illustrated in figure 35. The parts, after being bent as 

before, were placed in the mold (fig. 35A). It was known to be exceedingly 

difficult to force the parts into a concave mold of this type. Therefore a 

device utilizing a fire hose backed up by a support. which in turn was 

restrained by a canvas around the mold, was used to force the parts into the 

region of severest curvature before a vacuum was drawn on the bag. This 

device is shown in figure 35B. After the vacuum was drawn the assembly 

appeared as shown in figure 35C. It was then molded by conventional methods 

in an autoclave. The untrimmed finished part is shown in figure 35D, and 

again the curvature of the part did not conform to tho curvature of the mold. 

The nonconformance was less than before due to the relatively thin mold. 

In spite of the precautions taken to provide adequate pressure at the 

point of greatest curvature the molded piece had a poor bond, and actual 

gaps occurred between the inner face and the core at this point, again 

emphasizing the difficulty of molding parts of U shape. 

Report No. 1574 -21-
Balsa Lumber Specifications 

Specification used through June 1945: 

Lumber, Balsa, S23, weight 6 to 9 pounds per cubic 

foot, kiln dried, random widths of 3 inches and up, 

and random lengths with a minimum length of 6 feet. 

Thickness shall be 1-1/2 inches or 2-1/2 inches with 

not less than 50 percent of the material of 2-1/2­

inch thickness and not more than 50 percent of the 

material of 1-1/2-inch thickness with an allowable 

tolerance not more than 1/8-inch plus or minus. 

Specification used after July 1945: 

Balsa Lumber, rough, weight 5 to 8 pounds per cubic 

foot, kiln dried, 3-inch thickness, random widths, 

random lengths with a minim of material under 6 

feet in length, to be graded in accordance with tho 

Foregin Economic Administration Grading Rules of 

March 1, 1944 for Grade AA. 

Report No. 1574 -22-
APPENDIX II 

Preliminary Experimental Work 

Before adopting the manufacturing and fabricating techniques used 

on the test panels, many other methods were tried and discarded. Some of 

these, although not considered satisfactory, are worthy of discussion as 

a matter of record and as a guide in future work on sandwich manufacture. 

Band Sawing End-grain Balsa 

The band saw set-up shown in figure 8, which produced good results 

on cellulose acetate, was tried on balsa. The end-grain surfaces were 

fuzzy and were not suitable for gluing or wet laminating. If band sawing 

were followed by sanding on an accurate drum sander, the surface would 

probably be acceptable. 

Planing End-grain Balsa 

The surface produced on end-grain balsa by a high-speed, 30-inch 

cabinet planer is shown in figure 36. Although the knives were in very 

good condition, the fibers were not cut off square and clean and the surface was unsuitable for gluing. 

Pressing End.-gain Balsa 

In an attempt to correct the wavy surface produced on balsa by its 

nonuniform density, sample cores were pressed between smooth surfaces at 

progressively higher pressures and temperatures to determine the crushing 

pressure at various temperatures. A pressure of 300 pounds per square 

inch at 300° F. for 1 minute produced flat cores with smooth surfaces by 

crushing the high (harder) rings and thicker slabs down to the thickness of 

the thinnest spots. This process was not used because of the possible 

physical changes in the slightly crushed balsa. 

Spreading High-viscosity Laminating Resin (A) on Glass Cloth 

The viscosity of laminating resin A is too high to make direct hand 

spreading practical for resin contents below 50 percent. An indirect 

method, such as shown in figure 22A and B must be used. For large-scale 

production, a mechanical coater should be used. One such type, a knife 

coater, is shown in figure 37. 

Report No. 1574 -23-
-- 

Some serious defects were encountered when the resin content of the 

faces was too high. Figure 38 shows the results of high resin content in 

the faces combined with high pressure and temperature. The resin while in 

the liquid, heated state flowed into the slightly open edge-glued joints 

and into checks in the end-grain balsa core and actually compressed the 

balsa sideways or at right angles to the motion of the press. This panel 

had, approximately 55 percent resin content in the faces and was pressed in 

a press at 250° F. and 75 pounds per square inch pressure. 

A single sheet of glass cloth with a resin content of 43 percent molded between cellophane sheets at 225° F. and vacuum pressure is shown 

greatly magnified in figure 39. Each square in the weave contains a small 

air bubble. Figure 40 shows a glass cloth face made of three crosslaminated sheets with 43 percent resin content and molded under the same 

conditions. The irregular pattern was caused by the register of the weave. 

'If the squares registered exactly, large air bubbles were formed; if they 

meshed, practically no bubbles resulted, Thicker faces presented the same 

irregular pattern. It was particularly evident on the finished panel in 

reflected light. 

Strength Determinations to Evaluate Fabrication Techniques 

In order to evaluate the fabrication techniques and the glues to be 

used, preliminary panels, 1/4 by 12 by 12 inches, were made and tested by 

three methods tension, shear, and strip. 

The Forest Products Laboratory tension specimens are made by bonding 

a 1-square-inch aluminum cube grip to each side of the sandwich. These 

specimens and testing apparatus are shown in figure 41. In conducting the 

test, a testing machine cross-head speed of 0.05 inch per minute was used. 

The shear-test specimen used was similar to that employed as the 

standard for plywood shear tests, 1 by 3-1/4 inches, with a test area of 

1 square inch. The ends of the specimens were gripped by modified jaws 

inserted in the standard machine for testing plywood in shear, Side plates 

were added to the standard type jaws so that support could be applied

laterally to keep the specimen from twisting and bending under test. The 

load was applied at the rate of approximately 600 pounds per minute. Two 

shear-test specimens and the gripping jaws with the special side plate 

attachments are shown in figure 42. 

The strip test was used to measure the energy required to strip the 

face from an area 1 by 3 inches from the center portion of a specimen 3 

inches square. The aluminum or glass cloth face extended approximately 1 

inch beyond the area to be stripped. Two saw cuts, 1 inch apart, were made 

through the metal face and about two-thirds through the core parallel to 

the sides, making a test area of 1 inch by 3 inches. The specimens were 

tested in the Forest Products Laboratory toughness testing machine with 

special gripping attachments. The force was applied at the 1-inch edge at 

an angle of 60° to the face. The initial angle of the pendulum was 45°, 

Report No. 1574 -24-
-- 

and from the angular position attained by the pendulum upon its release, 

a toughness value was obtained in inch-pounds. A tested specimen in the 

toughness testing machine is shown in figure 43, which also shows the 

method of attachment to the drum. 

These tests were of a preliminary nature and were used only to indicate the best gluing and fabrication procedure to be used. Table 1 

summarizes some of the results of the tests on panels with honeycomb cores 

and glass cloth, plywood, and aluminum faces. 

One of the series of panels was made by using glass cloth faces 

(impregnated with five various laminating resins) and paper honeycomb and 

glass cloth honeycomb cores. The resin-impregnated glass cloth faces were 

"wet laminated" to the paper honeycomb and glass cloth honeycomb cores. 

The panels were then cut into tension, shear, and strip test specimens and 

tested as described previously, Additional panels, using the same combinations of face, cores, and resins, were made by the same conditions, except 

that extra resin was applied to the core. Ten grams per square foot, of 

the resin used to impregnate the face, was roller spread on the core before 

the panels were assembled and cured. The addition of resin to the core 

generally increased the strength of these panels when subjected to tension, 

shear, and strip tests. The results of these tests are summarized in 

table 1. Because the additional application of resin to the core increased 

the bond strength, it was used as standard procedure thereafter with all 

honeycomb core combinations. 

Compression of Core Materials at Elevated Temperatures 

Although the compressive strengths at room temperature of some of the 

core materials were known, the strengths that these materials would develop 

at molding and pressing temperatures (200° to 300° F.) were open to question. 

Compression tests were conducted in a 14-inch steam-heated press, the 

pressure being controlled by a hand operated hydraulic pump. The temperature at the platen was controlled within ±2° F. of the desired reading. An 

asbestos fence was built around the platens to reduce the cooling effect on 

the outside edges of the test specimen. 

The specimens were placed between aluminum cauls and put in the 

press, and the platens were brought together until they just touched the 

cauls. By keeping the specimens in this position for 15 minutes, the cores 

were brought up to the temperature of the platens before pressure was 

applied. Pressure was applied in increments of 10 pounds per square inch 

until a compression of 0.050 inch or more was observed, as shown by a dial 

indicator attached to the side of the platens. After the first load of 10 

pounds per square inch was applied, the dial indicator was set on zero, and 

that pressure and each consecutive increase of 10 pounds per square inch 

was maintained for a period of 15 minutes. Readings of core compression 

were taken immediately after the pressure was applied and after the pressure 

was maintained for 15 minutes. 

Report No. 1574 - 25-
The results of these tests on the six core materials used for sandwich constructions are summarized in table 2. In general, the compressive 

strength of all the core materials was affected by temperature, decreasing 

as the temperature increased. 

From the data obtained, cellular hard rubber was found to be the 

weakest material. This was also verified by the panels that were made with 

rubber as a core, since molding and pressing pressures could not exceed 15 

pounds per square inch without considerable crushing. Balsa, on the other 

hand, was the strongest both at room and elevated temperatures, and the 

strength values far exceeded the capacity of the press (400 pounds per 

square inch). 

Since the tests of compression strengths were determined by pressing, 

the values are more applicable to pressing than bag molding. In pressing, 

the load is not absorbed evenly over the entire area of the specimen, but is 

absorbed to a greater extent on the stronger areas and a lesser extent on 

the weaker areas, Therefore, the value obtained in pressing is an average 

of the entire area. In bag molding. where a fluid pressure is obtained, 

the load is distributed evenly over the entire area, and failure may occur 

in one area before it does in other areas. This was demonstrated by a panel 

made with aluminum faces and cotton honeycomb core, in which parts of the 

core were weaker as shown by a difference in color. When bag molded at 

275° F. and a pressure of 30 pounds per square inch, crushing occurred only 

in the light-colored or weaker areas. 

Report No. 1574 -26-
Table 1.--Results of tension, shear, and strip tests on panels having three 

honeycomb core materials 

Report No. 1574 -27-
Table 2.--Compressive strength of six core materials as determined in 

a heated press on specimens 1/2 inch thick 

Report No. 1574 -26-
APPENDIX III 

Description of Resins and Adhesives 

Note 1. Resin A. A high-temperature-setting, high-viscosity, contactpressure, laminating resin of the polyester type. 

Note 2. Resin B. A high-temperature-setting, low-viscosity, laminating resin of the styrene monomer, polyester type. 

Note 3. Resin C. A high-temperature-setting, high-viscosity, contactpressure, laminating resin of the polyester type. 

Note 4. Resin D. A high-temperature-setting, low-viscosity, contactpressure, laminating resin of the polyester type. 

Note 5. Resin E. A high-temperature, low-viscosity, laminating varnish 

of the low-pressure type. 

Note 6. Adhesive F. A high-temperature-setting, modified thermoplastic 

metal-to-wood glue. 

Note 7. Adhesive G. A high-temperature-setting, two-component resin 

with a thermosetting liquid and thermoplastic powder. 

Note 8. Adhesive H. A high-temperature-setting, thermoplastic resin 

with thermosetting resin and pigment. 

Note 9. Adhesive I. High-temperature-setting mixture of thermoplastic 

resin and synthetic rubber. 

Note 10. Adhesive J. A high-temperature-setting 

and thermosetting resins. 

mixture of thermoplastic 

mixture of thermoplastic 

mixture of a thermosetting 

mixture of thermosetting 

Note 11. Adhesive H. 

resin and 

Note 12. Adhesive L. 

resin and 

Note 13. Adhesive M. 

resin and 

Note 14. Adhesive N. 

A high-temperature-setting 

synthetic rubber. 

A high-temperature-setting 

synthetic rubber. 

A high-temperature-setting 

synthetic rubber. 

A high-temperature-setting, acid-catalyzed, 

phenolic resin. 

Note 15. Adhesive P. A room temperature-setting resorcinol resin. 

Report No. 1574 -24-
Note 16. Adhesives Q, and Q-1. A high-temperature-setting thermoplastic 

vinyl-resin adhesive. 

Note 17. Adhesive R. A high-temperature-setting, melamine-resin 

adhesive. 

Note 18, Adhesive S. A synthetic resin sizing. 

Note 19. Adhesive T. A high-temperature-setting phenolic resin adhesive. 

Note 20. Adhesive U. A high-temperature-setting, phenolic, dry-film 

glue. 

Note 21. Cellular hard rubber. A core material, black in color, 8 pounds 

per cubic foot density (including skin), 

Note 22. Cellular cellulose acetate. An extruded, unoriented, multicellular form of cellulose acetate containing a small percentage 

of glass fiber as a filler, 7-8 pounds per cubic foot density 

(including skin). 

Note 23, Heat-treated glass cloth. A glass cloth that has been heated to 

a temperature sufficient to char the lubricant retained from the 

weaving process. 

Note 24. Glass cloth. Heat-cleaned cloth chemically treated with a chrome 

complex bath after the lubricant has been completely removed by 

exposure to a high temperature. 

Note 25. Glass-cloth honeycomb. Honeycomb core material with 3/16-inch 

hexagonal cells, made by impregnating glass cloth with resin to 

give a density of 7 to LO pounds per cubic foot. 

Note 26. Cotton-cloth honeycomb. A 4-ounce cotton sheeting impregnated 

with a phenolic-type resin to a density of approximately 3.75 

pounds per cubic foot. 3/8-inch to 7/16-inch hexagonal-cell 

size. 

Report No. 1574 -30-

Figure 2 .--End-grain balsa slab showing variation in width of growth 

rings. 

Z M 74052 F 
Figure 3 .--Photomicrograph of sawed end-grain balsa slab. Lower half 

was low-density material and produced a valley in the surface, while 

upper half was higher-density growth ring, which produced a slightly 

raised surface when sawn. Z M 74053 F 


Figure 6.--End-grain balsa core of uniform density in the 

range of 6 to 9 pounds per cubic foot. Z M 74056 F 



Figure 10.--Band saw set-up used for cutting paper 

honeycomb. 

Figure 11.--Circular saw set-up used for cutting 

paper honeycomb. 

Z M 74060 F 
Figure 12.--Paper honeycomb showing the effect of density 

on cutting with a band saw. 

Z M 74001 F 
Figure 13 .--Paperhoneycomb showing the effect of density 

on cutting with a circular saw. 

Z M 74062 F 

Figure 15.--Paper honeycomb showing effect of sawing to a 

thickness of 3/16 inch. Z M 74064 F 

Figure 17.--Band saw set-up used for cutting glass 

cloth honeycomb. 

Figure 18.--Glass cloth honeycomb showing the effect of sawing thin sections on a band saw. 

Z M 74066 F 



Figure 22.--Procedure used in applying high-viscosity laminating resin A to glass cloth by hand spreading. A Applying film of resin to stainless-steel sheet. B . Working 

glass cloth into resin film to impregnate the cloth. 

Z M 74070 F 
Figure 23.--High viscosity, laminating resin being spread on glass 

Z M 74071 F cloth with a mechanical spreader. 
Figure 24.--Low viscosity laminating resin being spread on Z M 74072 F glass cloth. 



Figure 28.--Sandwich panel with glass-cloth faces and cellular 

hard rubber core showing blisters and crushing of the core 

during curing due to high catalyst content on the core. 

Z M 74076 F 


Figure 31.--Approximate breaking radii and safe working radii 

of balsa, cellular cellulose acetate, cellular hard rubber, 

and paper honeycomb cores. 

Z M 72999 F 

Figure 33.--Molding an aluminum-to-balsa panel on a male mold. A.-Preformed aluminum-balsa assembly held temporily in place on a male 

mold preparatory to molding to the inner face. B.-Canvas wrapping 

applied to assembly to provide additional pressure at the point of 

greatest curvature. C.-Vacuum being drawn on bag previous to the 

molding operation. D.-Difference in shape between the mold and the 

Z M 74080 F sandwich panel molded on its outer surface. 

Figure 35.--Molding a sandwich panel on a female mold. A.-Preformed 

faces and core laid in female mold preparatory to gluing inner face 

in place. B.-Vacuum being drawn on bag after pressure is applied to 

inner curvature by means of a fire hose jig. C.-Vacuum completely 

drawn and fire hose jig removed. D.-Difference in shape between mold 

and product. 

Z M 74082 F 
Figure 36.--End-grain balsa core after planing in a freshly 

sharpened cabinet planer operating at 3,600 revolutions 

per minute. 

Z M 74083 F 


Figure 39.--Photomicrograph showing openings between weave of one 

sheet of glass cloth with 43 percent resin content after curing 

between cellophane sheets. 

Z M 74086 F 
Figure 40.--Photomicrograph of three-ply glass cloth face with 

43 percent resin content, after curing between sheets of 

cellophane, showing variation in air-bubble size depending 

upon register of plies. 

Z M 74087 F 
Figure 41.--Forest Products Laboratory type tension 

specimen for sandwich materials assembled in testing 

apparatus. 

Z M 74088 F 
Figure 42.--Shear specimens and gripping jaws with 

special side-plate attachments. 

Z M 74089 F