> ![](media/image1.png){width="1.4847222222222223in"
> height="1.1458333333333333in"}

**CENTER FOR**

COMPOSITES MANUFACTURING

**FABRICATION GUIDE**

**JUNE 2003**

**FTA REPORT NUMBER FTA-AL-26-7001-2003.2**

<table style="width:100%;">
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</colgroup>
<tbody>
<tr class="odd">
<td colspan="12"></td>
</tr>
<tr class="even">
<td colspan="8">REPORT DOCUMENTATION PAGE</td>
<td colspan="4"><p>Form Approved</p>
<p>OMB No. 0704-0188</p></td>
</tr>
<tr class="odd">
<td colspan="12">Public reporting burden for this collection of
information is estimated to average 1 hour per response, including the
time for reviewing instructions, searching existing data sources,
gathering and maintaining the data needed, and completing and reviewing
this collection of information. Send comments regarding this burden
estimate or any other aspect of this collection of information,
including suggestions for reducing this burden to Department of Defense,
Washington Headquarters Services, Directorate for Information Operations
and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204,
Arlington, VA 22202-4302. Respondents should be aware that
notwithstanding any other provision of law, no person shall be subject
to any penalty for failing to comply with a collection of information if
it does not display a currently valid OMB control number. <strong>PLEASE
DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.</strong></td>
</tr>
<tr class="even">
<td colspan="2"><p><strong>1. REPORT DATE</strong>
<em>(DD-MM-YYYY)</em></p>
<p>06-06-2003</p></td>
<td colspan="6"><p><strong>2. REPORT TYPE</strong></p>
<p>Final</p></td>
<td colspan="4"><p><strong>3. DATES COVERED</strong> <em>(From -
To)</em></p>
<p>From 4-26-02 to 7-31-03</p></td>
</tr>
<tr class="odd">
<td colspan="8"><p><strong>4. TITLE AND SUBTITLE</strong></p>
<p>Center for Composites Manufacturing</p></td>
<td colspan="4"><strong>5a. CONTRACT NUMBER</strong></td>
</tr>
<tr class="even">
<td colspan="8">Fabrication Guide</td>
<td colspan="4"><p><strong>5b. GRANT NUMBER</strong></p>
<p>FTA-AL-26-7001-01</p></td>
</tr>
<tr class="odd">
<td colspan="8"></td>
<td colspan="4"><strong>5c. PROGRAM ELEMENT NUMBER</strong></td>
</tr>
<tr class="even">
<td colspan="8"><p><strong>6. AUTHOR(S)</strong></p>
<p>Klaus F. Gleich and Thomas E. Jackson</p></td>
<td colspan="4"><p><strong>5d. PROJECT NUMBER</strong></p>
<p>A634</p></td>
</tr>
<tr class="odd">
<td colspan="8"></td>
<td colspan="4"><strong>5e. TASK NUMBER</strong></td>
</tr>
<tr class="even">
<td colspan="8"></td>
<td colspan="4"><strong>5f. WORK UNIT NUMBER</strong></td>
</tr>
<tr class="odd">
<td colspan="8"><p><strong>7. PERFORMING ORGANIZATION NAME(S) AND
ADDRESS(ES)</strong></p>
<p><strong>AND ADDRESS(ES)</strong></p></td>
<td colspan="4"><p><strong>8. PERFORMING ORGANIZATION
REPORT</strong></p>
<p><strong>NUMBER</strong></p></td>
</tr>
<tr class="even">
<td colspan="4"><p>Southern Research Institute</p>
<p>757 Tom Martin Dr.</p>
<p>Birmingham, Al 35211</p></td>
<td colspan="4"></td>
<td colspan="4"></td>
</tr>
<tr class="odd">
<td colspan="8"><strong>9. SPONSORING / MONITORING AGENCY NAME(S) AND
ADDRESS(ES)</strong></td>
<td colspan="4"><strong>10. SPONSOR/MONITOR’S ACRONYM(S)</strong></td>
</tr>
<tr class="even">
<td colspan="7">Quon Y. Kwan, Office of Technology (TRI-20)</td>
<td colspan="2"></td>
<td colspan="3">FTA</td>
</tr>
<tr class="odd">
<td colspan="7">Federal Transit Administration (FTA)</td>
<td colspan="2"></td>
<td colspan="3"></td>
</tr>
<tr class="even">
<td colspan="7">400 Seventh St., SW, Room 9401</td>
<td colspan="2"></td>
<td colspan="3"><strong>11. SPONSOR/MONITOR’S REPORT</strong></td>
</tr>
<tr class="odd">
<td colspan="7">Washington, DC 20590-0001</td>
<td colspan="2"></td>
<td colspan="3"><strong>NUMBER(S)</strong></td>
</tr>
<tr class="even">
<td colspan="7"></td>
<td colspan="2"></td>
<td colspan="3">FTA-AL-26-7001-2003.2</td>
</tr>
<tr class="odd">
<td colspan="12"><p><strong>12. DISTRIBUTION / AVAILABILITY
STATEMENT</strong></p>
<p>Unlimited</p></td>
</tr>
<tr class="even">
<td colspan="12"><strong>13. SUPPLEMENTARY NOTES</strong></td>
</tr>
<tr class="odd">
<td colspan="12"><strong>14. ABSTRACT</strong> The objective of this
report is to describe thermoplastic composite materials and processes
and to demonstrate fabrication methods for molding these materials into
transit bus flooring components or other large components for use in
buses and other mass transit applications. The primary goal of this work
is to demonstrate that these technologies can provide lower cost,
lighter weight, and improved performance structures for mass transit
applications. This fabrication guide was written to outline the basic
requirements involved in design and fabrication of large long-fiber
thermoplastic composite parts by Vacuum Assisted Resin Transfer Molding
(VARTM)or by Thermoforming woven prepreg.</td>
</tr>
<tr class="even">
<td colspan="12"><p><strong>15. SUBJECT TERMS</strong></p>
<p>Long fiber woven, thermoplastic composite, VARTM, thermoform,
manufacture cost, weight reduction</p></td>
</tr>
<tr class="odd">
<td colspan="5"><strong>16. SECURITY CLASSIFICATION OF:</strong></td>
<td><p><strong>17. LIMITATION</strong></p>
<p><strong>OF ABSTRACT</strong></p></td>
<td colspan="4"><strong>18. NUMBER OF PAGES</strong></td>
<td colspan="2"><strong>19a. NAME OF RESPONSIBLE PERSON</strong></td>
</tr>
<tr class="even">
<td><strong>a. REPORT</strong></td>
<td colspan="2"><strong>b. ABSTRACT</strong></td>
<td colspan="2"><strong>c. THIS PAGE</strong></td>
<td></td>
<td colspan="4"></td>
<td colspan="2"><strong>19b. TELEPHONE NUMBER</strong> <em>(include area
code)</em></td>
</tr>
<tr class="odd">
<td colspan="11"></td>
<td><p><strong>Standard Form 298 (Rev. 8-98)</strong></p>
<p><strong>Prescribed by ANSI Std. Z39.18</strong></p></td>
</tr>
</tbody>
</table>

CENTER FOR

> COMPOSITES MANUFACTURING
>
> FABRICATION GUIDE
>
> JUNE 2003
>
> Prepared by
>
> Klaus F. Gleich and Thomas E. Jackson
>
> Southern Research Institute
>
> 757 Tom Martin Drive
>
> Birmingham, AL 35211
>
> Prepared for
>
> Federal Transit Administration
>
> U.S. Department of Transportation
>
> Washington, DC 20590
>
> [http://fta.dot.gov](http://fta.dot.gov/)
>
> Available from
>
> National Technical Information Service (NTIS)
>
> 5285 Port Royal Road
>
> Springfield, Virginia 22161
>
> 703-605-6000

6900. fax

> Email \[orders@ntis.fedworld.gov\]
>
> Report Number
>
> FTA-AL-26-7001-2003.2

**Table of Contents**

**Page**

**Table of Contents 2**

**List of Figures 3**

**Foreward/Notice 4**

**Acknowledgement 5**

**Executive Summary 6**

1 Introduction 7

2 Objectives 8

3 General Description of Thermoplastic VARTM 9

> 3.1 Material Guidelines 9
>
> 3.2 VARTM Processing Guidelines 9
>
> 3.2.1 FASTRAC VARTM Process 10

4 General Guidelines for Woven Thermoplastic Broadgoods 18

> 4.1 Processing Guidelines for Thermoforming Woven Broadgoods 20
>
> 4.2 Guidelines for Double Belt Press 22
>
> 4.3 Guidelines for Discontinuous Woven Material Handling
>
> and Processing 23
>
> 4.3.1 Fabric Cutting 23
>
> 4.3.2 Vacuum Bag Thermoforming 23

5 Design Guidelines for Woven and VARTM Thermoplastic Composites 26

**Glossary of Abbreviations and Acronyms 27**

**Metric Chart 28**

**List of Figures**

**Figure Page**

3.2-1 Low Cost VARTM Processing 10

3.2.1-1 Illustration of FASTRAC Process for Rapid Processing 11

3.2.1-2 High-Temperature Setup for Nylon VARTM Processing 12

3.2.1-3 Heated Tooling Schematic for Flat Plate 13

3.2.1-4 Various Stages of Processing of FASTRAC Processing 14

3.2.1-5 Wet Out Characteristics of Nylon to E-glass Fiber 15

3.2.1-6 Carbon Fiber Preform Bagged under Vacuum and Prepared for
Infusion with Caprolactam 16

3.2.1-7 Resin Infusion of Caprolactam in Progress 17

4-1 Roll of Woven Polypropylene/E-glass Fabric 18

4-2 Close-up View of Woven Polypropylene/E-glass Fabric 19

4-3 Consolidated Sheet of Woven Polypropylene/E-glass 19

4-4 Close-up of Consolidated Woven Polypropylene/E-glass 20

4.1-1 Picture of Loom Weaving Thermoplastic Prepeg 21

4.1-2 Take-up Accumulator for Woven Prepeg Fabric 21

4.1-3 Schematic of Double Belt Press 22

4.3.2-1 Aluminum Mold Half Showing Inserted Cartridge Heaters 24

4.3.2-2 Mold Prepared for Vacuum Bag Thermoforming 24

4.3.2-3 Part Bonding with Low Temperature Melt Adhesive 25

4.3.2-4 Placement of Secondary Part to be Adhesively Bonded 25

**Foreward**

This fabrication guide describes steps required to apply the technology
of long fiber and continuous woven thermoplastic composites in transit
bus applications for the U. S. Department of Transportation and the
Federal Transit Administration. The goals of improved safety, reduced
weight, and lower cost are very important to the transportation
industry. This report describes the design guidelines and fabrication
methods related to the thermoforming and resin transfer molding of
long-fiber thermoplastic composite transit bus components.

**Notice**

This document is disseminated under the sponsorship of the U.S.
Department of Transportation in the interest of information exchange.
The United States Government assumes no liability for its contents or
use thereof.

The United States Government does not endorse manufacturers or products.
Trade names appear in the document only because they are essential to
the content of the report.

**Acknowledgment**

The authors wish to express their appreciation to the Federal Transit
Administration for the support of this work. We thank the C.A. Lawton
Company for the use of their plastication equipment and North American
Bus Industries for their guidance in component selection and design
guidelines. We acknowledge the following for their contributions to the
technical effort of this project: George Husman, Tim Hartness, Uday
Vaidya, Greg Janowski, Krish Chawla, Joel Dyksterhouse, Patric Moriarty,
Mike Dyksterhouse, Andy Grabany, Chad Ulven, Shane Bartus, and Brian
Pillay. We acknowledge and thank Lisa White for her assistance with the
preparation of reports.

**Executive Summary**

The Fabrication Guide for Woven Prepreg Thermoforming and Vacuum
Assisted Resin Transfer Molding (VARTM) outlines part design, processes
requirements, material selection, tooling, and equipment specifications
related to the manufacture of large parts for the transportation
industry using these methods.

This guide describes the process by which woven thermoplastic prepreg is
produced and process as well as outlining the VARTM process for similar
parts. There is discussion of the expected mechanical properties and
design constraints. Descriptions of the critical equipment and tooling
are given with an emphasis on equipment capabilities and specific
performance characteristics.

**1. Introduction**

The Federal Transit Administration (FTA) has encouraged innovative
technologies to be implemented in mass transit applications,
particularly when improvements in performance, cost, weight reduction,
or safety may be realized. The results of efforts made by Southern
Research Institute under FTA Project AL-26-7001-01 were directed to the
fabrication of woven prepreg thermoplastic composite bus flooring
subcomponents with demonstration of several contributing technologies.
Primary considerations for this application were weight savings, to
reduce fuel consumption, and cost savings to encourage usage in
commercial applications. The choice of bus flooring as a thermoplastic
composite application goal was the result of consideration of overall
weight reduction calculations for several large-area bus components. The
primary selection issue was based on the total weight of flooring
installed on a standard transit bus as well as the potential increase in
functional lifetime over conventional flooring structures.

**2. Objectives**

# 

# The intent of this report is to provide general guidance in the design, and fabrication of a large area component for use in a commercial transit bus using Vacuum Assisted Resin Transfer Molding (VARTM) or continuous woven broad goods processes. The major manufacturing issues addressed involve part material selection, processing and tooling.

**3. General Guidelines for Thermoplastic VARTM**

The use of VARTM methods with thermoplastic resin as the finished matrix
of the composite is quite different from the usual VARTM process that
utilizes thermoset resins in conjunction with a given fiber
reinforcement. In the case of thermoset resins, there are a large number
of viable candidates that possess the low viscosity and relatively low
cost characteristics that are needed for commercial transportation
applications. In the case of thermoplastic resins, two candidates were
identified as feasible resins, Nylon 6 and cyclic polybutylene
terephthalate (c-PBT). Only the Nylon 6 was economically attractive
enough to pursue. The use of a Nylon 6 precursor, ε-caprolactam, was
chosen because of its low viscosity and relatively low cost. Other
thermoplastic resin precursors may become available in the future that
are both inexpensive and amenable to VARTM processing methods. The Nylon
6 system used in our studies consisted of materials commercially
available through the Brüggemann Chemical U.S, Inc. (Newtown Square,
PA). The Brüggemann C10 catalyst and C20 activator were the raw
materials used along with the generic ε-caprolactam in our trials . The
manufacturer's guide specified using 1.2 to 3 % catalyst and 1.0 to 2.5%
activator in an inert atmosphere.

**3.1 Material Guidelines**

The successful application of thermoplastic VARTM processing depends
upon relatively low viscosity of the prepolymer and the compatibility of
the resulting polymer resin with the fiber and the fiber sizing
formulation. The prepolymer must have a viscosity less than 1000 cp so
in order that it can effectively permeate the fiber perform in a
reasonable period of time using vacuum forces alone to draw the
prepolymer through the entire part. The pot life of the prepolymer must
likewise be sufficient to insure that the prepolymer will make its
transit through the part before significant polymerization occurs that
could leave dry spots or poorly wet-out sections in the perform.

The issue fiber sizing compatibility with the resin cannot be ignored
since the wetting of the fiber by the resin or prepolymer depends
critically upon the compatibility of surface energy for fiber wetting
and chemical compatibility in order that excellent coupling occurs. The
realization of the fiber reinforcement mechanical potential depends
critically upon the compatibility of the prepolymer and polymer with the
fiber. Additional problems can also occur if the fiber sizing interferes
with the rate of polymerization by either accelerating or inhibiting it.
Acceleration of the rate of polymerization can prevent filling of the
part due to increased viscosity of the matrix and inhibition of
polymerization may increase the cycle time to unacceptably long periods
or quench it altogether.

**3.2 VARTM Processing Guidelines**

In the particular case of polymerization of ε-caprolactam, moisture
content of the prepolymer must be less than 200 ppm to avoid quenching
the reaction. All resin lines and containers must be dry as well.

Recent developments in composites processing methods have focused on
cost-effectiveness and ability to produce integrated structures using
liquid molding techniques. Vacuum assisted resin transfer molding
(VARTM) has emerged as a leading cost-effective process that utilizes
innovative developments in one-sided tooling and vacuum-bag technology.
VARTM is cost-effective for small or medium sized production runs
because it eliminates many of the tooling costs associated with long
high-temperature and pressure cycles incurred in an autoclave process
environment. This process is a very attractive alternative to spray-up
or impregnation methods, and is far less expensive than conventional
manufacturing large-scale production methods such as compression or
injection molding. The VARTM process is reduced to a simple single-sided
tooling that requires only consumables in place of expensive molds.
During the VARTM process, resin is infused into dry fabric preform
placed in single-sided tooling that covered with an inexpensive vacuum
bag film as illustrated in Figure 3.2-1.

![](media/image3.wmf){width="5.999305555555556in"
height="1.1555555555555554in"}

**Figure 3.2-1 Low Cost VARTM Processing**

A distribution mesh aids in distributing the resin evenly over the fiber
preform. Large structural parts with inserts or multiple layers can be
produced rapidly. In multi-layered constructions, the VARTM process can
be effectively utilized to produce parts by co-injecting resin to cause
simultaneous part wet-out. Other advantages of VARTM are low process
volatile emissions, higher fiber-to-resin ratios and good repeatability.
While most studies have considered VARTM for thermoset type systems,
similar processing of thermoplastics has been limited to their
inherently high viscosity. For VARTM to work successfully, the resin
should lie in the 100-500 cps viscosity range.

# **3.2.1 FASTRAC VARTM Process** 

We have investigated the feasibility of using the FASTRAC VARTM approach
for thermoplastic systems for production of transit bus floor
structures. A cross section of the process is shown in Figures
3.2.1-1and -2. The concept uses a double vacuum bag. The top bag covers
a channeled tool which is placed over the bottom bag that holds the
plies under vacuum. Applying a vacuum between the two bagging materials
causes the bag at the top surface of the plies to be pulled towards the
channeling of the tool. This causes channels wide enough for the resin
to race through. Once the resin reaches eighty percent of the distance
of infusion, the channeled tool and the top bagging is removed. The
vacuum now pulls the bagging tight over the plies helping to wet out the
plies. This process provides significant time efficiency and ability to
produce/process very large flat shapes. Figure 3.2.1-3 illustrates the
kind of tooling heating needed to effectively polymerize the resin.

![](media/image4.wmf){width="5.384722222222222in"
height="7.125694444444444in"}

**Figure 3.2.1-1 Illustration of FASTRAC Process for Rapid Processing**

![](media/image5.wmf){width="5.668055555555555in" height="8.4in"}

**Figure 3.2.1-2 High-Temperature Setup for Nylon VARTM Processing**

![](media/image6.wmf){width="5.647916666666666in" height="5.63125in"}

**Figure 3.2.1-3 Heated Tooling Schematic for Flat Plate**

The FASTRAC processing system has been successfully implemented using
higher viscosity thermoset resins with very short gel times. We have
successfully used the process in castable thermoplastic systems such as
reactive Nylon 6 resin.

Thermoplastic resins polymerize at temperatures of between 100° C to 200
° C. Nylon 6 polymerizes at 120° C to 150° C and cyclic polybutylene
terephthalate (c-PBT) at 180° C to 200° C. Therefore, there is a need to
provide a controlled high temperature processing setup for VARTM.
Furthermore, the sensitivity to moisture during the polymerization step
necessitates a controlled inert processing environment for Nylon 6.

An example resin wet-out of Nylon 6 on E-glass fabric was demonstrated
and illustrated in Figure 3.2.1-4 and -5 .

![](media/image7.png){width="4.010416666666667in"
height="6.708333333333333in"}

**Figure 3.2.1-4 Various Stages of Processing of FASTRAC Processing**

\(a\) Bagged Preform, showing infusion channels

\(b\) Flow of resin through the channels

![](media/image8.wmf){width="4.122916666666667in"
height="3.0930555555555554in"}

**Figure 3.2.1-5 Wet Out Characteristics of Nylon to E-Glass Fiber**

The Nylon 6 system described here consists of caprolactam monomer using
Brüggolen C10 catalyst and Brüggolen C20 activator (Brüggemann Chemical
U.S., Inc., Newtown Square, PA). The manufacturer's guide specified
using 1.2 to 3.0 % catalyst and 1.0 to 2.5% activator in an inert
atmosphere.

The following process was employed using carbon fiber preforms.

-   Teflon tubing was slit and used for resin distribution over the
    width of the preform.

-   Four layers of 210 x 210 mm de-sized woven carbon fabric were used
    in the lay-up.

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-   The woven carbon preform was vacuum bagged on a heated processing
    table. High-temperature bagging materials and sealant tape were used
    to seal the preform.

-   The caprolactam (200g) plus activator (3g) was heated at 75 degrees
    C in an oil bath until it reached a fully liquid state. The solution
    was continuously agitated using a mechanical stirrer.

-   When the entire solution was in a liquid form, the catalyst (6g) was
    added to the solution.

-   Dry nitrogen was used to continuously purge the system to prevent
    any moisture from getting into the flask.

-   The carbon lay-up was heated to 120 degrees C and also purged using
    dry nitrogen.

-   The resin was then infused into the preform.

-   After the resin filled the preform, the temperature of the heated
    table was raised to 150 degrees C to complete the polymerization

The best conditions for processing were achieved by using 3 % catalyst
and 1.5 % activator. The two tanks were heated to 90 °C, then mixed
dispensed into the fabric and heated to 150 °C. Under these conditions,
the polymerization process was achieved in approximately 3 minutes.

Figures 3.2.1-6 and 3.2.1-7 il1ustrate the infusion process.

![](media/image9.jpeg){width="3.3744094488188976in"
height="2.530708661417323in"}![](media/image10.jpeg){width="3.3744094488188976in"
height="2.530708661417323in"}

**Figure 3.2.1-6 Carbon Fiber Preform Bagged under Vacuum and Prepared
for Infusion with Caprolactam**

![](media/image11.jpeg){width="5.372916666666667in"
height="4.029861111111111in"}

**Figure 3.2.1-7 Resin Infusion of Caprolactam in Progress**

The primary caveats for the Nylon 6 polymerization is the exclusion of
moisture and rapid infusion of resin throughout the preform to insure
uniform time and temperature history.

**4. General Guidelines for Woven Thermoplastic Broadgoods**

The use of woven thermoplastic composite broadgoods is an efficient
method for laying up a substantial area and thickness of composite for
processing in a thermoforming environment. An important requirement for
the effective thermoforming with vacuum bag pressure is that the prepreg
tape used to weave the fabric must be evenly and completely wet-out by
the resin. The degree wet-out of the fiber by the resin is dependent on
the compatibility of the resin and the fiber sizing as well an even
application of the resin to the fiber under appropriate processing
conditions to avoid dry fibers. The prepreg tape can be woven into
broadgoods with various weaving patterns appropriate to the application.
The choice of fiber type, resin, and fiber content may be adjusted to
fit the mechanical and cost constraints of the final part desired.
Generally, in the case of polypropylene/glass tape the percentage of
fiber by weight can be set from 30 percent to 60 percent without great
difficulty.

Polypropylene/E-glass tape was produced in a 12.7 mm width and then
woven in to a fabric as shown in Figure 4-1. This material is about 1
meter wide. A closeup view is shown in Figure 4-2.

![](media/image12.jpeg){width="4.711111111111111in"
height="3.546527777777778in"}

Figure 4-1 Roll of Woven Polypropylene/E-glass Fabric

![](media/image13.jpeg){width="4.675in" height="3.50625in"}

Figure 4-2 Close-up View of Woven Polypropylene/E-glass Fabric

The woven fabric was consolidated in a heated double belt press formed
the raw woven fabric into a dense composite. A sheet of that material is
shown in Figure 4-3.

![](media/image14.jpeg){width="4.746527777777778in"
height="3.5597222222222222in"}

Figure 4-3 Consolidated Sheet of Woven Polypropylene/E-glass

A close up of the same material is shown in Figure 4-4. The consolidated
forms can be used as-is as structural panels, subsequently formed in
standard thermoforming operations, or joined to other components by
fusion or adhesive bonding.

![](media/image15.jpeg){width="4.728472222222222in"
height="3.546527777777778in"}

Figure 4-4 Close-up of Consolidated Woven Polypropylene/E-glass

The woven material can be consolidated by vacuum bagging and or bagged
and autoclaved if additional pressure is need to form the part. A
prototype bus floor subcomponent was fabricated from this woven form of
thermoplastic composite material using vacuum bagging methods with a
heated tool.

**4.1 Processing Guidelines For Thermoforming Woven Broadgoods**

The particular materials chosen for the woven broad goods were
polypropylene (BP/Amoco 9965) and E-glass (Owens Corning 225 4588). The
tapes were made using the DRIFT process and then woven into broad goods.
Figures 4.1-1 and 4.1-2 show the weaving process that was performed by
American Iwer, Greenville, SC.

![](media/image16.jpeg){width="6.020138888888889in"
height="3.8631944444444444in"}

**Figure 4.1-1 Picture of Loom Weaving Thermoplastic Prepreg**

![](media/image17.jpeg){width="6.020138888888889in"
height="3.8631944444444444in"}

**Figure 4.1-2 Take-up Accumulator for Woven Prepreg Fabric**

Each layer of fabric when consolidated is about 0.6 mm thick. Flat parts
can be made in various thicknesses and widths using a heated double belt
press. The flat portion

Multiple layers of woven prepreg may be consolidated into a single flat
sheet using a belt press concept like that shown schematically in Figure
4.1-3.

![](media/image18.wmf)

Figure 4.1-3 Schematic of Double Belt Press

**4.2 Guidelines For Double Belt Press**

The major considerations for the selection or use of a double belt press
for consolidation of woven prepreg are cost, size and production rate.
Double belt presses are expensive; therefore, matching the size and
production capacity to the need is very important. Plan for 80 percent
operational time based on a 24 hours per day/7 days per week schedule.

The thickness of the product is determined by the numbers of plies fed
into the press. The size of the press determines the width limits and
the rate of production. Raw material is introduced at room temperature
and then heated under compression until the sheet is consolidated. The
sheet remains under pressure until the cooling zones are passed and the
material can be handled. A cutting station may be located near the
output of the press for cutting the sheet to practical dimensions.
Material can be processed at a fairly high rate. The linear velocity of
a large belt press is nominally 5 meters per minute for a 3 to 4 mm
thick material about 1.25 meters wide. This rate of production could
produce about 2 metric tons per hour of 60 percent glass/polypropylene
sheet material. Power requirements for such a press would be in excess
of 1000 kw, and that is primarily associated with heating and cooling
functions.

**4.3 Guidelines for Discontinuous Woven Material Handling and
Processing**

The handling and processing of discontinuous woven materials has
application for single part fabrication that cannot be achieved with a
continuous constant profile process. The primary areas to be mentioned
here are fabric cutting and vacuum bag thermoforming.

**4.3.1 Fabric Cutting**

Cutting woven prepreg is an operation that should be minimized during
part design with a consideration for tooling constraints as well. The
chief difficulty that one encounters in cutting a pattern out of a woven
material is that of having tapes on the edge of the cutout falling out
during the cutting operation or during handling. For any significant
volume of production, cutting with a water jet or laser technology is
needed to reduce edge damage. Laser cutting has the advantage of not
wetting the material, but heat-sealing the edges that would reduce
handling damage. For low volume applications, mechanical cutters are
acceptable as long as the handling damage is minimized.

**4.3.2 Vacuum Bag Thermoforming**

A processing method that has similarities to the VARTM process is the
vacuum bag thermoforming process. The tooling for vacuum bagging is
single-sided which reduces cost. The tooling can be made from a variety
of materials depending on the volume of parts and the rate of production
needed. In addition, all of the bagging components and tooling need to
gracefully handle the process temperatures encountered with the resin
system utilized. Aluminum tooling has the advantage of machinability and
high thermal conductivity which can reduce both heating and cooling
time. Steel is the most durable tooling, but also the most expensive.
Kirksite is a zinc-based alloy that can be used for prototyping tool
that may be recast and recycled after its use.

Regardless of the choice of tooling material, one must size the heating
and cooling function so that the respective heat and cool cycles are
acceptably rapid. For most applications, direct electrical heating is
the most inexpensive approach to getting the tooling and prepreg up to
processing temperature. Cooling can be effected with water spray into
internal channels of the mold that will turn of steam and be vented away
safely. It is also feasible to use circulating oil as a heating and
cooling medium, which eliminates the wiring associated with the
cartridge heaters. Sizing of the heating capacity of the tooling is
dependent on the mass of the tool, the heat capacity of the tooling
material, and how rapidly the tool is required to heat up. The same
constraints hold for cooling capacity. Figure 4.3.2-1 shows an example
of an aluminum mold with associated cartridge heaters. This particular
mold weighed about 320 kg and could be raised to a temperature of 50˚C
in about 6 minutes. This required about 42 kw of heating.

![](media/image19.jpeg){width="3.911111111111111in"
height="2.186111111111111in"}

**Figure 4.3.2-1 Aluminum Mold Half Showing Inserted Cartridge Heaters**

Figure 4.3.2-2 shows a mold prepared for vacuum bagging of a woven
thermoplastic material. This prototype mold has material hold-down
clamps to keep the prepreg conformed enough to the mold until proper
vacuum bagging can occur. In a production situation, automatic slides
would perform the hold-down function to achieve a shorter cycle time.
The thickness of vacuum bag consolidated parts is controlled by the
number of layers of woven prepreg stacked in the mold. In case of the
sine wave rib structure, eight layers were stacked to obtain a 5mm thick
part. The eight layers were consolidated in two steps because of the
difficulty in manually handling all eight layers at once.

![](media/image20.jpeg){width="3.928472222222222in"
height="2.946527777777778in"}

**Figure 4.3.2-2 Mold Prepared for Vacuum Bag Thermoforming**

Once a part is consolidated, it can be used or bonded to another
component. In production circumstances, vibrational or ultrasonic
bonding is very effective. On a more limited prototype production, hot
melt adhesives can be used effectively as shown in Figure 4.3.2-3. On a
large part the adhesive can be applied in the proper place and quantity
and permitted to cool before the secondary operation of putting the
parts together and heating to bond, Figure 4.3.2-4.

![](media/image21.jpeg){width="4.3375in" height="3.2527777777777778in"}

**Figure 4.3.2-3 Part Bonding with Low Temperature Melt Adhesive**

![](media/image22.jpeg){width="4.230555555555555in"
height="3.1729166666666666in"}

**Figure 4.3.2-4 Placement of Secondary Part to be Adhesively Bonded**

It is usually desirable to minimize extra bonding operations for the
sake of both economics and strength.

**5. Design Guidelines for Woven and VARTM Thermoplastic Composites**

The basic guidelines for woven thermoplastic or VARTM components are
similar to those encountered in other composite structures.

The most important guidelines are the following:

-   Design with the process and tooling in mind

-   Simulate when possible

-   Do not assume isotropic mechanical properties of the materials

-   Use continuous processes such as belt press consolidation or hot
    roller forming when feasible

-   Avoid adhesive bonding by designing part and process to form it in
    one step if possible

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-   Avoid changes in thickness where ever possible for the sake of
    mechanical performance. At least minimize it. Thickness variations
    encourage warpage due to different cooling dynamics. If changes in
    part thickness cannot be avoided, it is best to taper the change
    gradually over a long distance. The length of the transition in
    thickness should be at least 3 times of the change in thickness.
    Avoid changes in thicknesses that are more than 3 mm. Stage
    thickness transitions with prepreg layers stepped to minimize
    thickness steps.

-   Avoid sharp edges and sharp radii in a part design. These can cause
    a high level of stress concentration and is often the reason for
    part failures. Sharp edges and transitions may also increase fiber
    damage.

Ribs located on one side of the part may be seen on the opposite
surface. If this is a problem in the design, avoid ribs in this area or
alternatively reduce the mass of material by reducing the thickness of
ribs thereby minimizing sink marks. Ribs should be drafted with
approximately a 1^o^ angle to assist in part extraction. The thickness
of the rib should not exceed 0.6 times the wall thickness. A radius
between rib and wall reduces stress concentration. True ribs are not
feasible with continuous weave prepreg so that the caveats that are true
for the VARTM process are not strictly applicable with the woven
thermoforming process.

**Glossary of Abbreviations and Acronyms**

Cadpress Software for mold-filling of long fiber thermoplastic
composites, The Madison Group, Inc.

c-PBT Cyclic polybutylene terephthalate

Copolymer Combination of two or more polymers into one molecular chain

DRIFT Trade name for a directly impregnated thermoplastic composite

Dynatup Trade name for an instrumented falling dart impact test

E glass Common grade of fiberglass used in commodity composites

Falling dart test Impact test defined in ASTM standard D-3029

FASTRAC Trade name for a resin transfer molding method

GMT Glass mat thermoplastic composite

High melt flow Polymer with melt flow index greater than 50 g/10 minutes

Homopolymer Polymer consisting of similar subunits in the molecular
structure

Izod test Impact test defined in ASTM standard D-256

Longitudinal Sample orientation in the direction of predominant fiber
orientation

PA6 Nylon 6, polyamide

PP Polypropylene

Random Material with fiber orientation equal in all directions

Size Coating applied to fiber bundles to assist with respect to handling
and surface compatibility

Transverse Sample orientation orthogonal to predominant fiber
orientation

VARTM Vacuum assisted resin transfer molding

#  

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