an introduction to common hand-layup methods …an introduction to common hand-layup methods with...

AN INTRODUCTION TO COMMON HAND-LAYUP

METHODS WITH COMPOSITE MATERIALS

by

Justin Cooley

A senior thesis submitted to the faculty of

Brigham Young University - Idaho

in partial fulfillment of the requirements for the degree of

Bachelor of Science

Department of Physics

Brigham Young University - Idaho

December 2018

BRIGHAM YOUNG UNIVERSITY - IDAHO

DEPARTMENT APPROVAL

of a senior thesis submitted by

Justin Cooley

This thesis has been reviewed by the research committee, senior thesis coor-dinator, and department chair and has been found to be satisfactory.

Date Kyle Kinghorn, Advisor

Date David Oliphant, Senior Thesis Coordinator

Date David Johnson, Committee Member

Date Todd Lines, Chair

ABSTRACT

AN INTRODUCTION TO COMMON HAND-LAYUP

METHODS WITH COMPOSITE MATERIALS

Justin Cooley

Department of Physics

Bachelor of Science

Composite materials are made by combining multiple materials to achieve ex-

tremely high specific weight and stiffness qualities while keeping the weight

of the part low. Composites consist of a matrix and a reinforcement. Two

of the most common types of matrices include polyesters and epoxies. Two

very common reinforcements are carbon fiber and fiberglass. Carbon fiber and

fiberglass have very similar strengths, but carbon fiber is lighter, stiffer, and

more expensive. Composite parts are unique in that they can be designed to

withstand loads from specific directions. Manufacturing composite parts is an

extensive process and several methods are discussed in brief detail. Some pro-

cesses include infusing a reinforcement with the matrix, curing a reinforcement

that has the matrix infused already inside it, or performing an open-mold, wet

layup technique. Finishing procedures include drilling holes, trimming parts

down, and quality checks.

ACKNOWLEDGMENTS

A sincere thank you to my advisor, Kyle Kinghorn, and my committee

member, David Johnson, for giving invaluable advice and corrections.

To ACT-Aerospace for allowing me to intern and learn more about com-

posites manufacturing.

To my professor, David Oliphant, for mentoring me and encouraging me.

Thank you.

Contents

Table of Contents xi

List of Figures xiii

1 Introduction 11.1 Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Typical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Material properties of composites 52.1 Properties of Composites . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.4 Most Common Composite Materials . . . . . . . . . . . . . . . . . . . 10

3 Composite Manufacturing 113.1 Tooling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.1.1 Important Terms . . . . . . . . . . . . . . . . . . . . . . . . . 113.1.2 Composite Tools . . . . . . . . . . . . . . . . . . . . . . . . . 143.1.3 Metal Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.2 Layup Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.2.1 Preparing Plies . . . . . . . . . . . . . . . . . . . . . . . . . . 173.2.2 Wet-Layup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.2.3 Resin infusion . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.2.4 Pre-Preg Layup . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.3 Heating/Pressurizing . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.4 Finishing Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.4.1 Drill and Trim . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.4.2 Paint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.4.3 Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Bibliography 25

xi

List of Figures

2.1 Comparison of various material properties between steel, aluminum,and composite materials. [3] . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Different fiber orientations, unidirectional (a), randomly oriented (b),bi-directional weave (c), and a multi-directional weave (d) [8]. . . . . 9

3.1 Mock-up example of a male tool (bottom) and a composite part restingon top. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2 Mock-up example of a female tool (bottom) and a composite part rest-ing inside. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3 Example of draft angles for a female tool. 90◦ is no angle, > 90◦ ispositive draft angle, and < 90◦ is negative draft angle. Angles are largeto aid visualization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.4 Example of a two-piece tool. Note the upper right hand corner of theimage, it would be impossible to remove the part without splitting thetool in two [9]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.5 Example of how plies can be cut in the same shape, but with differentfiber orientations. [10]. . . . . . . . . . . . . . . . . . . . . . . . . . . 18

xiii

Chapter 1

Introduction

On any scale, from microscopic to planetary, the most important factor that con-

tributes to building and creating things is material. Typically, there have been three

main material classifications; ceramics, metals, and polymers. Most everyday items

are built using one or more of these three types of materials. A new type of ma-

terial has been classified within the last 100 years; composites. This material is a

combination of multiple materials that bond together to form superior qualities.

A brief history and overview of composites will be introduced, followed by more

technical details of some common composite materials. Different forms of matrices

and reinforcements will be discussed. The largest chapter will provide a more detailed

overview of how composites are manufactured.

The purpose of this document is to provide a brief walk-through on what com-

posite materials are and how they are made. This does not attempt to cover every

manufacturing method, nor does it include every detail behind the chemistry of com-

posite bonding. This is intended to be a guide to enable the reader to gain a basic

understanding of composites and how they are manufactured.

1

2 Chapter 1 Introduction

1.1 Composites

Composites are a special kind of material in that they consist of multiple materials

that are combined to make a new material. The new material utilizes the properties of

its constituents and makes them more useful than either would be on their own. This

ability to combine materials and their properties opens up many doors to application

possibilities.

Generally, a composite material has two main parts- a matrix and a reinforcement.

A very common example of this is rebar-reinforced concrete. Concrete by itself has

very high strength when in compression, but does poorly in tension. Rebar does

extremely well in tension, and can be added into the concrete to give the structure

this quality. In this situation, rebar would be called a reinforcement, or a material

that adds certain strength qualities to another material that surrounds it (concrete),

called the matrix.

With the development of new composite materials come many different manu-

facturing processes for making them. Engineers must design a product, determine

what material will be used, perform a stress analysis, and then lay out a fabrication

procedure. New methods in manufacturing help engineers to achieve tighter toler-

ances with composite parts, and new studies in material science continually provide

engineers with more advanced composite materials to work with.

1.2 History

Composites have been in use throughout human history even before they were called

composites. Brick, made of straw and mud, has been used to build houses for cen-

turies. Even living things are made, in one way or another, of composites. ”Cell

wall might be considered as a composite material, made of fibre (cellulose), a matrix

1.3 Typical Applications 3

(lignin, hemicellulose) and fillers (water, simple organics, tannins)” [1]. The basic

principle behind composites is that there is a matrix and a reinforcement that are

combined to make a new material.

More recent improvements in composites include the combination of resins with

carbon fiber and fiberglass materials. Just in the last 100 years fiberglass was first

used to reinforce resin matrices. Today, there are thousands of different materials used

for composites, and their applications are what propel technological advancement.

Fabrication processes have improved over recent years as well. One common

method is open, wet layup, where the resin is applied with simple hand tools. Two

other very common process types include resin infusion and pre-impregnated ply

layup. Any composite factory will have one or more of these processes efficiently

developed to produce high-quality composite parts.

1.3 Typical Applications

As discussed before, one of the most ancient applications of composite materials is

for civil engineering. Just in the construction of a house, there are composites in

the foundation, in the walls, and on the roof. In this case, composites not only help

structurally, but they provide weather resistance and insulation as well.

Sports and recreation provide a large market for composite science. Boats are

commonly made of reinforced plastics, which allows them to float easily as well as

remain light enough to travel at high speeds on the water. Smaller items, such as golf

clubs, bicycles, and guns can implement composites to improve performance.

Aerospace applications possibly drive the greatest incentive for composite research.

Weight and strength is a huge issue when it comes to making things fly, and composites

excel in both areas. ”Today, it costs $10, 000 to put a pound of payload in Earth

4 Chapter 1 Introduction

orbit” [2]. Since composite materials have such a high strength and stiffness to weight

ratio, they are frequently the best option for material choice in aerospace applications.

This is because they can withstand the same stresses while keeping the weight low,

thus reducing costs of flying.

Chapter 2

Material properties of composites

2.1 Properties of Composites

There are many different kinds of stresses that materials experience during their in-

tended use. Some materials exhibit large strength in compression, others in tension.

Some materials are brittle, and others ductile. By knowing what stresses an applica-

tion will need to handle, deciding what material to use becomes easier. As shown in

Figure 2.1, when compared to steel and aluminum, composites are lighter, have lower

thermal expansion, higher specific stiffness and strength, and much higher fatigue

resistance.

In the case of composites, they have an extremely high resistance to tension and

torsion. In many cases, carbon fiber has a better resistance to these forces than steel.

Weight also plays an important role in choosing a material. Composites tend to have

a lower weight to volume ratio than metals do, which makes them a popular choice

in aerospace applications.

In brief, composites are strong against tension and torsion, but weaker in com-

pression. They are lighter than metals, but also very brittle in comparison to some.

5

6 Chapter 2 Material properties of composites

Figure 2.1 Comparison of various material properties between steel, alu-minum, and composite materials. [3]

Composites can be designed to withstand forces in certain directions. This quality

makes them very versatile, and therefore if they can be made into a certain shape,

they are often times superior to other materials in that application.

2.2 Matrix

The matrix of the composite is what gives the part its shape. Before curing, it usually

starts as a viscous fluid. This fluid, called resin, consists of a long chain of molecules

called polymers. Generally, a polymer with larger molecular weight will be stiffer,

harder, stronger, and more resistant to abrasion. ”A resin is a polymeric material

that has not yet been molded into its final form. A plastic is the material after it has

its final shape” [3]. A finished composite part therefore consists of a reinforcement

2.2 Matrix 7

fiber and a plastic matrix.

The matrix has several purposes; it gives shape to the part, it distributes load into

the reinforcement fibers, and it protects the reinforcement from environmental wear.

It is able to give shape because it starts as a viscous fluid and thus makes a malleable

material when combined with the reinforcement before curing. When undergoing

stress, the matrix relies heavily on the reinforcement to take the load, otherwise it

would crack. The matrix also waterproofs the fibers, making them more suitable for

outdoor use. It also acts as an adhesive to keep the fibers bonded together.

As stated before, resins in composites are generally made of materials called poly-

mers. Polymers can be categorized in two parts, thermoset and thermoplastic poly-

mers. Both of these polymers will harden upon curing, but a thermoset will not soften

again after curing, and a thermoplastic will become soft again when heat is applied.

The factor that defines whether a polymer will be a thermoset or a thermoplastic

is if the polymer molecules are capable of cross-linking, which means the polymers

are bonded in several different locations along their chain. Thermoplastics do not

crosslink, while thermosets do.

Thermoplastics can be heated and reheated to change their shape. The heating

causes them to soften, making them malleable. They can also be recycled. ”The

most useful members of the thermoplastic group are acrylics, cellulosics, polyamide,

polystyrene, polyethylene, fluoroplastics, polyvynils, polycarbonate, and polysulfone.”

[4] These materials are cheap, and come in many different forms. Due to their na-

ture to soften under higher temperatures, they are not generally used in the kind of

composite applications that are discussed here as much as thermosets.

In contrast to thermoplastics, thermosets can only be melted once to allow them to

flow into a shape. Once they harden, heating them up to their previous melting point

will damage them, rather than melt them. ”Members of the thermosetting group in-


clude aminos, casein, epoxies, phenolics, polyester, silicones, and polyurethanes.” [4].

Thermosets are commonly used as matrices in composites because they do not tend

to soften when exposed to higher temperatures. The two most common thermosets

used in composites are polyesters and epoxies.

Polyester and vinylester resins are inexpensive and relatively easy to use in many

applications, even for non-professionals. ”Polyester resins ... are the workhorse of the

composites industry and represent approximately 75% of the total resins used.” [5]

They begin as a viscous liquid, and do not cure quickly on their own. ”Polyester

resin is cured by the addition of a catalyst.” [6]. After the addition of a catalyst, the

resin can be room-cured (meaning no heating/pressurizing is required). This process

is called polymerization, which links the various polymers together. A downside

to using polyesters is that they tend to be weaker than epoxies. ”Unfortunately,

polyester tends to have some of the lowest strength of the common composite resins.

Due, in part, to the high rate of shrinkage it experiences during curing – about a 7%

volume reduction from its liquid form – stress is built up within the cured resin, taking

away some of its overall useful strength” [7]. Vinylesters are similar to polyesters,

and tend to have better strengths in comparison, but at a higher monetary cost.

Epoxies are more expensive than resins (they can be over $150 per gallon), but

offer higher strength. ”Epoxy resins are used in applications where superior strength,

durability, and chemical resistance are needed” [7]. They also tend to shrink much

less. Epoxies are often used in pre-impregnated fibers, where the epoxy is infused

into the fibers without curing. Epoxies have high strength-to-weight ratios, and are

very commonly used with carbon fiber. Aside from the price, other negative qualities

of epoxies include poor UV resistance and sometimes the vapors from the uncured

resin can cause allergic reactions.

2.3 Reinforcement 9

Figure 2.2 Different fiber orientations, unidirectional (a), randomly oriented(b), bi-directional weave (c), and a multi-directional weave (d) [8].

2.3 Reinforcement

The reinforcement is the fiber material that is encased by the matrix. It takes the

loads that are applied to the matrix, and adds the stiffness and strength. In an ideal

matrix-to-reinforcement bond, the amount of strain experienced from any given load

will be equal in the matrix and the reinforcement.

There are several factors in the reinforcement that add to the composite’s mechan-

ical properties. These include the orientation of the fibers, the material of the fibers,

the bonding of fibers to the matrix, and the amount of fibers used. Since the fibers

can take the most stress in tension, their orientation can be designed in such a way

that they are aligned with the planned stresses. Figure 2.2 shows several different

fiber orientations.

Other properties determined by the reinforcement include the CTE (Coefficient

of Thermal Expansion), the conductivity, and the thermal transport [3]. These prop-

erties are best achieved through strong chemical bonds between the matrix and the


reinforcement. The science behind the chemical bonds will not be discussed in detail

here.

2.4 Most Common Composite Materials

Fiberglass, as the name implies, is a ”finely drawn molten glass” [3]. It was originally

used as insulation, but later engineers found that it could be combined with plastics

to make them stronger [3]. When made into a cloth or a mat and combined with a

matrix, is is able to withstand large loads.

Fiberglass finds frequent use in construction, aquatic, and aerospace applications.

As mentioned earlier, one of its original uses was for insulation because of its poor

thermal conductivity. It is used in speed boats frequently because of its water resis-

tance, lightweight, durability, and cost. In aerospace, if the application does not need

to endure large loads but must be lightweight, fiberglass is a common choice.

Carbon fiber is another highly used reinforcement. While it is about as strong

as fiberglass, it is much lighter and stiffer. ”Today, carbon/graphite fiber has among

the highest specific strength and highest specific modulus of any material” [3]. For

this reason, it is used frequently in aerospace applications. While its mechanical

properties exceed those of many other materials, it can be very expensive.

Chapter 3

Composite Manufacturing

3.1 Tooling Methods

3.1.1 Important Terms

There is some important nomenclature that must be addressed before delving into

much detail. When creating a composite part, a mold must be made for the part to be

created against. Molds are commonly called ”tools” in the manufacturing industry.

Depending on the geometry and size of the part, this tool will be made either male

or female. A composite part is typically laid up around a male tool (like wrapping a

blanket around it), while parts are generally laid up inside a female tool (like putting

pie crust into a plate). Figures 3.1 and 3.2 show the difference between a male and

female tool. Draft angle in the tool refers to the angle the walls of the tool make

with the horizontal. This is important because if the tool has no draft angle, then the

part will be difficult to remove, and if it is angled the wrong direction, the part may

be impossible to remove. Figure 3.3 shows the difference between negative, positive,

and no draft angle. From the figure, it can be seen that it would be very difficult to

remove a part with a negative draft angle. The various compositions of tools will be

11

12 Chapter 3 Composite Manufacturing

Figure 3.1 Mock-up example of a male tool (bottom) and a composite partresting on top.

explained later in more detail.

When purchasing fiber by the roll, there are several common forms. The ones men-

tioned here will include woven fabrics, or ”cloth”, chopped, or ”mat”, pre-impregnated,

or ”pre-preg”, and filaments, known as ”strands” or ”yarns”. Woven fiber comes in

many types of weave. An entire chapter could be written on the different forms of

weaves and their properties. For simplicity sake, this form of composite material will

be defined as a cloth with patterned, interwoven fibers that work together to hold

their general form. Chopped fiber has randomly oriented fibers that vary in length.

This is a more economical option when compared to woven fabric, but lacks in com-

pared strength. ”Chopped strand mat is not as rugged as a woven fabric. The fibers

are bound loosely and so when handled, some of the fibers may fall from the mat” [3].

A ”pre-preg” is a cloth that has resin infused into it already, but not cured. This

saves time in the layup process, but generally is more expensive.

3.1 Tooling Methods 13

Figure 3.2 Mock-up example of a female tool (bottom) and a compositepart resting inside.

Figure 3.3 Example of draft angles for a female tool. 90◦ is no angle, > 90◦

is positive draft angle, and < 90◦ is negative draft angle. Angles are large toaid visualization.


An autoclave is a pressurized oven that is used to enhance the curing process,

and can be used for precise heat-treatment recipes. Some operations only require an

oven, which does the curing without controlling pressure. A release agent is a chemical

applied to a tool that allows for a cured part to be separated easily from the tool.

”Laying up” refers to the process of making a composite part. EOP is an acronym

for ”edge of part” and is used as a datum reference in documents. The tool surface

of a part is the surface that will be in direct contact with the tool. This is important

because the tool surface usually is the smoothest surface of the composite part, and if

aesthetics play an important role in the part’s purpose, this surface must be defined

before creating a tool. CTE is an acronym for ”coefficient of thermal expansion”

which dictates how much a material expands when exposed to higher temperatures.

A ”ply” refers to an individual layer of dry cloth or mat that is laid down into a tool.

3.1.2 Composite Tools

Composite tools, as the name implies, are molds that are made of a composite mate-

rial, and tend to be less expensive than metal tools. However, they also wear easier

than metal tools and are not able to withstand as high of temperatures. For these

reasons, composite tools are used more frequently in parts that do not require high-

temperature cures or are not intended to make as many parts. Often times, composite

materials are used to make tools for parts that are very large. Another factor to take

into consideration when creating a tool is the amount of thermal expansion the tool

goes through during the curing process, which in the case of composite tools, is very

close to the expansion of the part in comparison to a metal tool.

There are several methods for making a composite tool. If an existing part is

on-hand, it can be used as its own tool to make a copy. A release agent is applied to

the clean part, and then fiber is laid up on the part and cured. Generally, the fiber is

3.1 Tooling Methods 15

laid up a certain distance past the EOP, and in some cases flanges are incorporated

to allow for side supports which prevent the newly formed tool from warping. The

part is then removed from the tool, and this same process can be repeated on the

tool in order to make more parts.

If a CAD model of the tool is available, and it requires tighter tolerances, then

machinable foam may be used. This type is very similar to a metal tool in that it is

machined, but machinable foam wears much faster than metal, and in many cases is

only good for a few parts. To improve this design, a tool can be made by machining

the foam and then laying over it with composites in order to produce the tolerance

necessary and extend its lifespan.

3.1.3 Metal Tools

For parts that are in high-volume production and require autoclave curing, metal tools

are generally the best option. They can withstand high temperatures and pressures,

wear very little over time, and are able to hold very tight tolerances. Metals do,

however, have a higher CTF than composites, and therefore must be scaled down

when modeling their tools.

Steel and aluminum are typical choices for metal tools. They are easily accessible,

easily machined, and their properties are well known. To make a metal tool, raw

material is first cut into a general block size. For example, if the material comes in

6 foot lengths, and the part is only 15 inches long, then the metal will be cut closer

to that size. Once the rough cuts have been made, it is either placed into a CNC

machine, which automatically makes all of the cuts by reading information from a

computer file, or it is given to a machinist who makes the measurements and cuts by

reading information from drawings made by engineers. Some layup processes involve

laser-assisted positioning and orienting of the plies into the tool. For these tools,


Figure 3.4 Example of a two-piece tool. Note the upper right hand cornerof the image, it would be impossible to remove the part without splitting thetool in two [9].

holes must be drilled to allow for datum markers that tell the laser where to point.

Some parts have no draft angle, meaning that the walls of the part are 90◦ to the

horizontal. In some cases, this can make the part impossible to remove. To solve this

issue, a metal tool can be made into two halves that are fastened together with screws.

The two halves are made with rough cuts, and their adjacent faces are machined to

rest flush with one another. They are then machined to hold against each other, and

when fastened into their final positions, they are then treated as one tool and the rest

of the tool is machined. By doing this, the part can be laid up to meet tolerances

within the tool, and when it is time for removal, the screws are simply loosened, and

the part comes out easily. Figure 3.5 shows why a two-piece tool might be necessary.

3.2 Layup Methods 17

3.2 Layup Methods

3.2.1 Preparing Plies

Due to the effect that fiber orientation has on the resistance to stresses and strains of

the finished part, it is important to know before layup how each ply will be oriented.

”The engineering lay-up requirements are set from the engineering stress and stiffness

calculations.” [3]. Woven cloths have patterns with a distinct direction, and this

direction is what layup technicians use to determine how to lay down the cloth.

There are four different directions that the plies can be oriented; 90◦, ±45◦, and 0◦.

Layup notation tells engineers and technicians which direction to lay down the plies.

This is written using square brackets, with forward slashes separating the different

plies as shown

[90/0/ + 45/− 45/0/90] (3.1)

It is important that this information is communicated before the fabric is even cut

because it is actually cut in these directions. Reference figure that shows how these

can be cut in different orientations.

The simplest way to cut plies is with scissors or a razor knife. For higher pro-

duction, a cutting table is very efficient. In order to correctly cut the plies, the same

shape is cut from the cloth, only the orientation of the shape changes to match the

ply layup. The plies are then labeled and sent to the layup technicians.

3.2.2 Wet-Layup

Wet layup is a basic process that involves laying down the reinforcement and applying

the resin all by hand. The basic preparation for this process is the same as others.

The tool surface is prepared with release agents to allow the part to separate easily


Figure 3.5 Example of how plies can be cut in the same shape, but withdifferent fiber orientations. [10].

after cure. All plies should be cut before this step so that all of them can be laid down

in the same step. All of the layup procedures in this section follow this preparation

procedure.

Next, the first ply is set down on the tool surface, followed by a layer of resin.

There are a variety of tools available for applying the resin. A hard, plastic roller

(much like a paint roller) can be used, and is very effective when applying resin to

chopped mat. A hard plastic spatula is a good tool for applying the resin to woven

cloth. The rest of the plies follow this pattern of ply followed by resin.

For some parts, this is enough to finish the process. The curing can be done by

simply leaving the part out to dry. A common issue with leaving the part to cure this

way is that the plies can have excess resin or air bubbles between them. This can be

resolved by using vacuum pressure to compact the plies.

The vacuum bagging process requires a few more plies of different materials. After

the last ply has been laid, a release film, or peel-ply, is laid over the part to prevent

vacuum bagging materials from bonding to the part. Next, a breathing material is

3.2 Layup Methods 19

laid on top of that which allows for uniform pressure throughout the part during the

vacuum process. A vacuum bag is laid over that, and a few holes are made in the bag

for vacuum ports. These are two-part fixtures that connect on either side of the bag

and adapt to the vacuum hose to apply vacuum pressure to the entire part. Time

under vacuum pressure is determined by engineers. The ending effect is that the plies

lay closer together, making it more compact and stronger.

3.2.3 Resin infusion

Resin infusion, as the name implies, is a type of resin-injection into the dry fibers.

It uses vacuum pressure to move the uncured resin throughout the entire part. The

benefit to this process is that applying the resin to the fibers is much easier, as it

takes all hand layup tools out of the process. Also, the vacuum-pressurizing process

is combined into this step, so it achieves two things in the same step.

The resin infusion process sets up similar to a regular wet layup. The plies are

laid up dry into the mold, and a peel-ply layer is then laid on top of the part. After

that, a channel is made through the part for the resin to flow easier. There are

several methods of doing this. A common method is by using a narrow folded strip

of fiberglass cloth around the perimeter of the part, and sometimes down the middle.

After this, breather is laid over the part to allow for air to escape, and vacuum

pressure is applied through vacuum ports, as described in the vacuum process in the

last section. Some ports in this process are made for resin transfer. Instead of pulling

air out, the negative pressure inside the bagged part pulls resin from a container

throughout the whole part.

After the resin has been infused through the whole part, it is left in this pressurized

state to cure. Some parts require that it be heated in an oven for curing. When

finished, the disposable materials are peeled off, and the part is sent to the finishing


stages.

3.2.4 Pre-Preg Layup

Pre-impregnated fibers have resin already infused into them. Because of this, they

must be stored at a low temperature so that they do not cure while not being used.

Since the resin is already in these fibers, only the plies need to be purchased, and the

layup process is much cleaner.

The tool preparation process is the same for these fibers as it is for the previously

mentioned ones. For laying up, it is cut to size, and a technician uses hand tools to

press it into place against the tool. These plies stay in place well while being laid up

due to the resin making them somewhat sticky. In order to keep the finished part

free from air voids, the part goes through a ”de-bulking” process every few layers.

De-bulking is the vacuum bagging process, but is done for a shorter amount of time,

specified by the engineers.

After all plies have been laid up and de-bulked, the part is bagged and prepared

for cure, either in the oven or in an autoclave. This process requires that the part be

under pressure during the curing process so that air voids do not form. After curing,

it is sent to the finishing stages.

3.3 Heating/Pressurizing

When curing parts, heating and pressurizing can not only speed up the process, but

can also reduce air voids within the part. Heat recipes can be prescribed by engineers

who have tested the material and much like metals, can use these recipes to achieve

certain properties. For ovens, the recipes are only temperature controlled. Autoclaves

can control the temperature and the pressure of the parts.

3.4 Finishing Parts 21

When using an oven or an autoclave, the part is left in its bag under vacuum

pressure. This adds to the pressure created by the autoclave, and also ensures that

the plies will not peel apart during curing to create air voids. Some ovens and most

autoclaves will provide a detailed report during and after the process to show the

temperature and pressure differences.

One benefit to having these reports is that if something goes wrong in the part,

or it fails in some way, the cause can be diagnosed if it was due to something in

the oven/autoclave. If the part did not follow the exact heat treatment recipe, its

temperature/pressure curve can easily be compared with the prescribed curve. Heat

treating is the last stage before sending the part off to drill and trim.

3.4 Finishing Parts

3.4.1 Drill and Trim

Generally, parts are made a bit larger than what is specified in the actual drawings

of the part. This is because the parts can be sharp at the edges after curing, and it

leaves some space for technicians to trim them down to size. If parts have holes in

them, they will be drilled out in this stage as well.

Safety is a very important factor to consider in this stage. When drilling and

trimming, tiny particles are thrust into the air which can be dangerous to breathe in.

Eye protection, face masks, and gloves should be worn during this stage to prevent

any injuries. Power tools are often used in this step, so care should be taken when

using them as well.

A pencil grinder or other rotary cutting tools are typically used to trim parts to

their intended size. The outer edge is defined with lines, and the part is cut along

the lines. The same process is used to drill out the holes. Sometimes, if the part is


large and there are complex contours, the lines can be traced from a laser projection.

This ensures very precise positioning of holes and cut lines.

3.4.2 Paint

There are multiple purposes for painting parts. An obvious reason is for aesthetic

purposes, but some composite parts can wear faster when exposed to sunlight. Paint

can also protect against other weather-related wear.

To prepare for painting, the part is masked with tape in places where paint should

not go. In some cases, if there are small holes that could easily collect excess paint,

cork is inserted into the holes and trimmed down against the surface of the part.

After the painting process, all of these masking materials are removed.

Paint is usually applied with a sprayer. A primer must first be applied to the part

to ensure the paint adheres well. The amount of layers to be sprayed and the color

of the paint are generally decided by the customer or the engineers. By painting the

part, it becomes better looking, and better prepared for outdoor use.

3.4.3 Quality

When a composite part is completely done being fabricated, it must pass quality

inspections before it can be shipped to the customer. There are many different tests

that the quality technicians must perform to see if it is an acceptable part. If there

are any major defects, the part must either be reworked into acceptable condition,

or it must be scrapped. One downside to using composite parts is that they are not

recyclable like metals are.

A visual inspection of the part is one of the most important operations in a quality

inspection. FOD, or foreign object debris, can be caught in the visual inspection.

3.5 Conclusion 23

Some customers require that a part be scrapped even if just a hair is found infused

into the part. Other, larger FOD is obviously unacceptable and will cause a part to

fail inspection.

Some facilities have more advanced screening equipment. One example of this is

an ultrasonic scanner. This scanner checks for imperfections within the part where

they are not visible. It does this by using sound waves underwater and producing a

computer image of where the imperfections are.

Many customers require that the composite manufacturer prove that they can

actually find imperfections in their parts. To accomplish this, test panels are made

with planned imperfections at measured locations. The quality technician will use

the scanner to find those impurities, and if they are found in the exact locations, then

they are able to show the customer proof of their capabilities.

3.5 Conclusion

In short, matrices give a composite part its shape. Polyester resins are cheap and

durable, while epoxies are stronger and more expensive. The reinforcements, such as

fiberglass or carbon fiber, help to take loads, making the entire part extremely strong

while keeping the weight low. Parts can be designed to resist forces in different

directions by orienting the fibers to take those loads.

The manufacturing process includes receiving, testing, and combining materials

to make composite parts. The materials are combined in a layup process, and then

are cured, where they are fused together. Finishing the part includes drilling holes

and trimming edges, then a quality check and a paint job.

Other materials that could be researched for further discussion might include

aramid fibers, or any of the many other forms of matrices. Generally, manufacturers


of raw composite materials will have catalogues with specifications for each of their

materials. These catalogues can be used to determine which material would be best

for the given application.

Bibliography

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25

26 BIBLIOGRAPHY

[9] Thermwood validates direct additive manufacturing of yacht hull molds.

Davide Sher. https://www.3dprintingmedia.network/thermwood-validates-direct-

additive-manufacturing-of-yacht-hull-molds/

[10] Laminates and Sandwiches https://www.oxyblack.com/index.php/en/composites/laminates-

sandwiches

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