Compression molding of composite parts on hot mold surfaces with a short cycle time

A molding system utilizes a cold mold with a large thermal mass and contoured thin mold inserts with a low thermal mass. A composite sheet blank of glass fibers in a thermoplastic matrix resin is placed between the inserts, heated and then transported to the cold mold. The composite sheet is pressed in the cold mold between the mold inserts to flow form and then cool the part. The mold inserts keep the surfaces of the composite sheet hot during the forming process, thereby enabling the matrix resin to flow and form smooth surfaces.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to copending application entitled "Multilayer 
Composite Mold Structure for Molding on Hot Surfaces", Ser. No. 
07/175,078, now abandoned, and "Compression Molding Using Insulating 
Films", Ser. No. 07/176,116 now abandoned, both assigned to the same 
assignee as the present invention. 
BACKGROUND OF THE INVENTION 
The present invention is related to a method for compression molding 
plastics. 
Compression molding of glass reinforced thermoplastic sheets is a promising 
method for producing relatively thin, wide and strong parts such as car 
hoods, doors and panels. One important prerequisite for the use of glass 
reinforced composite products in automobile applications is a Class A 
surface. While there is no universally accepted specification, the Class A 
surface is a glossy, smooth and polished surface which should be as smooth 
as that of current automobile exterior parts made from sheet metal. 
Current molding processes of glass reinforced thermoplastic composite 
sheets begins with the heating composite blanks in an oven, typically in 
infrared or hot air convection ovens. The material is heated above its 
melting point or if an amorphous material at least substantially above its 
glass transition temperature. The hot blanks are then pressed between cool 
mold surfaces (surfaces lower than the melting point or the glass 
transition temperatures), which are typically 175.degree.-250.degree. F. A 
molding pressure of one half ton/sq. in. to two tons/sq. in. is applied to 
the mold during a typical cycle time of 45-60 seconds. 
When the composite blanks are heated, they expand (loft) due to the recoil 
forces within the fibers. The surface of the expanded blanks then cools 
during its transfer to the mold, resulting in "frozen" resins on the 
surface. Compression of this blank in the cool mold produces surfaces 
which are not completely filled with resins, although some hot molten 
material moves from the inner core to the surface. This creates unfilled 
areas in the form of exposed fibers and surface porosity or voids. Since 
the resin at the cold surface is frozen and does not flow, rough 
boundaries between charged and newly formed areas are also produced. These 
exposed fibers, porous areas and blank boundaries are the major 
manifestations of surface roughness, although other physical processes, 
such as differential thermal shrinkage between fibers and resins, can also 
result in surface roughness and/or waviness. 
Recently it has been found that smooth surfaces can be obtained from neat 
resin in blow molding by using hot surface molding. The resin is supplied 
hot to the mold as a parison in blow molding or injected into the mold in 
injection molding. These techniques, which are based on temperature 
cycling of mold surfaces, increase the cycle time of the process. The 
increased cycle time is the major disadvantage of these techniques. In 
addition, composite sheets with continuous mat fibers cannot easily be 
injected or supplied as a parison. Heating of composite sheets causes the 
fibers to loft and extend outside the polymer resin matrix. Attempts to 
obtain smooth surfaces with composite sheets have involved trying to 
change the structure of the composite sheets so that the outside layers on 
the composite sheets have neat resin with barrier layers sometimes being 
provided to prevent the fibers situated in the middle layers from coming 
to the surface. These sheets could then be molded using conventional 
compression molding techniques. 
It is an object of the present invention to provide a method of compression 
molding reinforced thermoplastic composite sheets which result in finished 
products with smooth surfaces, a minimum of exposed fiber, porosity, and 
blank boundaries. 
It is a further object of the present invention to provide a method of 
compression molding reinforced thermoplastic parts which results in short 
cycle times and therefore increased throughput for each press. 
It is still a further object of the present invention to provide a method 
of compression molding reinforced thermoplastic composite sheets which 
reduces the required molding pressure and therefore reduces press size, 
which is particularly significant in large part fabrication. 
It is another object of the present invention to provide a method of 
compression molding reinforced thermoplastic which results in reduced 
thermal decomposition of the thermoplastic resins. 
It is yet another object of the present invention to provide a method of 
compression molding reinforced thermoplastic sheets which preserve the 
original sheet structure after molding. 
SUMMARY OF THE INVENTION 
In one aspect of the present invention a method of compression molding 
reinforced thermoplastic composite sheets to obtain smooth surface 
finishes and short cycle times is provided. The method comprises the steps 
of heating the composite sheet above the temperature which the matrix 
material in the composite sheet becomes molten and heating a mold insert 
having the desired contour of the molded composite sheet. The heated 
composite sheet is placed on the heated mold insert and transported on the 
mold insert to a cooled mold. The mold insert and composite sheet are 
placed in the mold with the mass of the insert being much less than the 
bottom half of the mold upon which it is placed. The mold is then closed, 
the composite sheet and insert having been heated sufficiently and the 
insert having sufficient mass to allow the thermoplastic resin to flow and 
fill the insert. The molded composite sheet is allowed to cool inside the 
mold. The mold is then opened with the molded composite sheet as well as 
the mold insert removed.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawing and and particularly FIG. 1 thereof. A 
composite sheet 11 comprising thermoplastic resin, including reinforcing 
layers, is shown being preheated to approximately the glass transition 
temperature of the resin in an oven 13. Oven 13 can comprise a convection 
oven as shown or an infrared oven. Preheating allows the sheet 11 to 
soften to conform to the contours of mold inserts 15 and 17 when it is 
placed between the two thin mold inserts. The composite sheet can 
comprise, for example, approximately 30-40% glass fiber mat and 70-60% 
polymer resin. The glass fiber mats can be fabricated from continuous 
strands sized with a sizing compatible with a matrix resin being used. 
Depending on the application, a variety of polymer matrices such as 
polycarbonate, polyesters, polypropylene, polyamide polyimides, 
polyphenylene oxide, polystyrene and blends of the above can be used. The 
mold inserts 15 and 17 with the sheet 11 in between are then further 
heated above the temperature at which the matrix material in the composite 
sheet becomes molten, preferably by conduction heating between plates 21 
and 23 containing heating elements 25. The temperature of the sheet will 
exceed the glass transition temperature if an amorphous resin is being 
used or exceed the melting point if a crystalline resin is being used. The 
mold inserts 15 and 17 can be fabricated from a material which can 
withstand the compression forces at elevated temperatures at which the 
composite material melts without distorting and has a smooth surface for 
shaping the composite materials. Examples of such materials are metals, 
ceramics, high temperature plastics, plastic composites. Conduction 
heating of the mold inserts is preferred over convection or infrared 
heating because of the faster heat transfer. The inserts with the sheet in 
between are transported then to a press 22 and between the cold upper and 
lower mold halves of a cold press 27 and compressed. The mold halves can 
comprise tool steel or to reduce cost can comprise a softer less expensive 
material such as aluminum or plastic composites. Since the mold inserts 
are used in direct contact with the composite, the mold halves are not 
subjected to wear by the material to be molded. The mold halves with which 
the hot inserts come in contact are maintained at approximately 
100.degree. F. by liquid cooling through passageways (not shown) in the 
mold halves, not shown. Initially, the resins in contact with the hot mold 
inserts remain molten and fill the insert mold surface under pressure. The 
composite part and mold inserts cool in the cooled mold halves. The 
composite product is released from the mold inserts and the mold inserts 
reused. If only one smooth surface is required, one mold insert may be 
sufficient. The optimum thickness of the mold insert is determined by the 
minimum mass required to provide hot surfaces, the cooling time desired 
and the mechanical strength needed to withstand the compression molding 
process. 
Another embodiment of the present invention is shown in FIG. 2. A composite 
sheet blank 31 and mold inserts 33 and 35 are heated separately. The blank 
may be heated in a infrared or convection oven above the temperature which 
the matrix material in the sheet becomes molten, which is above the 
melting point of the resin if a crystalline material is used or above the 
glass transition temperature if an amorphous material is used. A tunnel 
convection oven 37 is shown providing a steady supply of heated blanks. 
Heating the blanks allows the glass fibers to loft. The blank is then 
placed between preheated inserts which have been heated between plates 41 
and 43 containing heating elements 45. The inserts and blank are 
transported to a cooled press 47 and compressed. The mold inserts 33 and 
35 are shown with a varying wall thickness profile which simplifies the 
design of the molding system. It has been found that a simple shape can be 
used at the interface between the cold mold halves 48 and 49 and the mold 
inserts based on tests of mold inserts with different wall thicknesses. 
Varying thickness mold inserts resulted in good resin flow and filling of 
resins at the surface of the part molded. The composite was free of 
exposed fibers, porosity and blank boundaries. The surface of the molded 
part was a reproduction of that of the mold insert. 
The surfaces of composite parts distributed fibers throughout the sheet 
produced by hot surface molding of composite sheets. The molded parts were 
analyzed using a mechanical profilometer (Feinpruef Model M4P). Table 1 
shows the comparison of the average roughness and the peak-to-valley 
height of composite parts produced by hot surface molding and cold surface 
molding. Samples made by hot surface molding with fine fibers and coarse 
fibers had an average roughness of 8.4 and 9.4 microinches and a peak to 
valley height of 65 and 78 microinches. Compared to values of 30 and 47.5 
microinches respectively, for average roughness and 400 and 450 
microinches for peak-valley height respectively, on parts made by the cold 
surface molding. The significant reduction in surface roughness is due to 
elimination of imperfections such as exposed fibers, porosity and voids at 
the surfaces. 
The shape of the contact area between the mold inserts and the cold mold 
halves is important because of the thermal shrinkage that occurs to the 
mold inserts when cooling. Since the shape of the portion of the mold 
insert contacting the mold halves does not have to conform exactly to the 
mold insert contours, a simple geometry which provides good contact 
between the cold mold halves and surface element can be used. 
TABLE 1 
______________________________________ 
Comparison of Surface Roughness 
Average Peak 
Average Roughness 
Valley Height 
(micro. in.) 
(micro in.) 
______________________________________ 
Hot Surface Molding 
fine fiber composite 
8.4 65 
coarse fiber composite 
9.4 78 
Cold Surface Molding 
fine fiber composite 
30 400 
coarse fiber composite 
47.5 450 
______________________________________ 
A one dimensional transient heat transfer mathematical analysis was made on 
a multilayer system comprising five layers; 
mold/insert/composite/insert/mold. The heat transfer between the insert 
and the composite was assumed to take place by conduction. The temperature 
profile of a 1/8" composite sheet situated between two 1/8" metal mold 
inserts in contact with a cooled mold is shown in FIG. 3. Dimensionless 
temperature was used in the calculations with dimensionless temperature 
defined by the ratio ((T-T.sub.c)/(T.sub.i -T.sub.c), where T, T.sub.i and 
T.sub.c stand for temperature as a function of time and location, initial 
temperature, and boundary temperature, respectively. A constant boundary 
condition was used where the mold inserts were assumed to be heated to 
550.degree. F. (T.sub.i) and cooling fluid in the cooled mold, with which 
the mold inserts where in contact, provided a constant temperature of 
100.degree. F. (T.sub.c) at a distance of 1/4 inch from the mold insert 
surface. 
Thermal conductivities of composite sheets were measured for use in the 
calculations and the thermal conductivities of composites made with 
continuous fiber mats were much higher than both the neat resin and short 
fiber filled composites. The composites were assumed to have a 40% fiber 
mat content. 
If the resin used as a matrix in the composite material is polybutylene 
terephthalate available from a General Electric Co. as Valox.RTM. 
thermoplastic with a melting point of 450.degree. F., its dimensionless 
temperature is 0.8. Referring to the FIG. 3 the melting temperature is 
maintained at the mold insert surface for 0.6 seconds. The temperature of 
the resin further away from the metal surface is maintained above the 
melting point longer than 0.6 seconds. If a high speed press fills the 
mold surface with resin during this time period, the surface will be free 
of voids. Using a maximum temperature of 250.degree. F. as the desired 
final sheet temperature during cooling before release from the mold, 
cooling can be accomplished within 20 seconds. If the resin in the 
composite material is bisphenol A based polycarbonate an amorphous 
material with a glass transition temperature of 284.degree. F., available 
from the General Electric Company as Lexan.RTM. 141 thermoplastic, the 
glass transition temperature is equivalent to 0.4 on the dimensionless 
temperature scale. A high speed press could fill the mold surface with 
resin during the period of time that the metal surface is maintained above 
the glass transition temperature and the surface will be free of voids. If 
a maximum temperature of 250.degree. is again used as the desired final 
temperature during cooling before release from the mold, cooling can then 
be accomplished within 20 seconds. This indicates that the use of a hot 
mold insert fabricated from 1/8 inch metal sheets makes it possible to 
mold composite sheets free of voids. 
A comparison of the calculated temperature decay of the surface of a 
composite sheet including a crystalline polymer in both a cold mold press 
with hot inserts and a cool surface press without inserts is shown in FIG. 
4. The temperature used in the calculations are 550.degree. F. for the 
preheat temperature of the composite sheet and mold inserts, 225.degree. 
F. for the mold surface temperature in the cool surface molding, and 
100.degree. F. for the mold temperature of the cold mold into which the 
mold inserts are placed. Initially, the temperature of the metal surface 
in contact with the composite sheet during hot surface molding with hot 
mold inserts is much higher than during cool surface molding allowing 
increased flow time of the polymer resin achieving a smooth surface 
without voids. 
Referring now to FIG. 5 a comparison of the calculated temperature decay of 
the centerline of a composite sheet including a crystalline polymer in 
both a cold mold press with hot inserts and a cool surface press without 
inserts is shown. The temperatures use in the calculations are the same as 
used in connection with FIG. 4, 550.degree. F. for the preheat temperature 
of the composite sheet and mold inserts, 225.degree. for the mold surface 
temperature in the cool surface molding and 100.degree. F. for the mold 
temperature of the cold mold into which the mold inserts are placed. 
Initially, the temperature of the centerline of the composite material 
between the hot inserts and in the cool mold are the same. As cooling 
continues, the temperature in the middle of the composite in the hot 
surface molding process cools more rapidly than in the cool surface mold, 
showing that it is possible to have a shorter cycle time with the hot mold 
insert than with conventional cool surface molding. 
The use of a thicker insert, 1/4 inch for example, does not result in much 
of an increase in cooling time (17-20 seconds in this case) when a mold 
insert fabricated of high thermoconductivity material such as metal is 
used. Increasing the composite thickness to 1/4 inch however, increases 
the cooling time to almost a minute, while the time the composite surface 
temperature remains higher than its melting temperature stays the same. 
The temperature selected for the cooled mold can be in the range of 
50.degree. to 300.degree. F. The lower the temperature the quicker the 
cycle time. However, for smooth surfaces the resin must be molten during 
press closing limiting the temperature selected. The cool mold 
temperatures in the upper part of the range are used with resins having 
high glass transition temperatures while the temperatures in the lower 
portion of the range are used with resins having low glass transition 
temperatures. 
Referring now to FIG. 6, the reduced pressure requirement in compression 
molding for hot surface mold inserts in a cold mold and a conventional 
cool mold is shown. For all the ratios of area of a part after compression 
molding to area of a part before compression molding, it is seen that the 
compression molding can be accomplished at reduced pressure when the hot 
surface insert mold technique is used. The reduced pressure requirement 
results from the higher composite sheet temperature in the early molding 
stages and resulting slow initial cooling at the composite surface. 
Referring now to FIGS. 7A-G another embodiment of the present invention is 
shown. A single mold insert 51 is used as a carrier to transport a hot 
composite sheet 53 to the press 55. The composite sheet is heated above 
the temperature at which the resin used as a matrix material becomes 
molten in an infrared or convection oven. A convection tunnel oven 57 is 
shown in the Figure. The single mold insert 51 is preferably heated by 
conduction heating between plates 59 and 61 which are heated by electrical 
resistance heating 63, for example. The heated sheet is then situated on 
the heated mold insert and transported to the press where it is placed on 
the fixed bed 65 of the press 55. The press 55 is shown with a two part 
movable head 67a and 67b. The press fixed bed and upper mold half core are 
cooled by cooling passages 69 carrying cooling liquid. As the press 
closes, as shown in FIG. 7C, the first part of the movable head 67a, 
located on either side of the upper mold half 67b, contacts the mold 
insert 51, clamping it in place against the fixed bed of the press. The 
second part of the movable head 67b containing the upper mold half presses 
the sheet 53 as seen in FIG. 7D. When the press opens as shown in FIG. 7E, 
the shaped composite part 71 is lifted with the upper mold half, ejector 
pins 73 shown in FIG. 7F in the upper mold half release the shaped 
composite part 71 which is shown in FIG. 7G with its smooth defect free 
surface facing down. The mold insert is removed and used again. 
The use of the mold insert to transport the heated sheet blank from the 
oven to the press is another important feature of this invention. Since 
the heated sheet blank is very flexible, a carrier which supports it, is 
required, particularly when transporting a large blank. The insert can be 
used as a carrier for this purpose. 
Mold inserts in contact with the composite blank reduce surface cooling of 
the blank during its transfer from the oven to mold. While composites can 
be heated to a higher initial temperature to compensate for cooling which 
occurs when mold inserts are not used, thermal decomposition of the resin 
in the composite sheet will increase due to the longer heating times, 
higher temperatures, and increased exposure to air. The use of mold 
inserts allows plastics with a narrow operating range of temperature to be 
more easily accommodated. 
The foregoing has described a method of compression molding reinforced 
thermoplastic composite sheets which result in finished products with 
smooth surfaces and a minimum of exposed fiber, porosity and blank 
boundaries. Parts can be formed using the present invention that require 
short cycle times and reduced molding pressures. 
While the invention has been particularly shown and described with 
reference to several preferred embodiments thereof, it will be understood 
by those skilled in the art that various changes in form and detail may be 
made without departing from the spirit and scope of the invention.