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Emilian Popa Injection Molding Design Guide Manual 2012

Resin Transfer molding

Introduction

Resin Transfer Molding (RTM) is a closed mold process in which matched made and female molds, preplacedwith fiber preform, are clamped to form composite components. Resin mix is transferred into the cavity

through injection ports at a relatively low pressure. Injection pressure is normally less than 690 kPa (or 100psi). The displaced air is allowed to escape through vents to avoid dry spots. Cure cycle is dependent on part

thickness , type of resin system and the temperature of the mold and resin system. The parts cures in the mold,

normally, heated by controller, and is ready for its removal from the mold when sufficient green strength is

attained. Processes that are based on similar principles include Structural Reaction Injection Molding (SRIM)and different versions of vacuum assisted RTM (Figs 20.1 and 20.2).

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RTM offers the promise of producing low cost composite parts with complex structures and large near net

shapes. Relatively fast cycle times with good surface definition and appearance are easily achievable. The

ability to consolidate parts allows the saving of considerable parts allows the saving of considerable amount oftime over conventional lay-up processes. Since RTM is not limited by the size of the autoclave or by pressure,

new tooling approaches can be utilized to fabricate large, complicated structures. However, the development o

the RTM process has not fulfilled its full potential. For example , the RTM process is yet to be automate in

operations such as performing, reinforcement loading, demolding and trimming. Therefore, RTM can beconsidered an intermediate volume molding process (Krolewski 1990).

Several unresolved issues in RTM encountered by composite engineers are in the areas of process automation,

performing, tooling, mold flow analysis can resin chemistry. During the last decade, rapid advances in RTM

technology development have demonstrated the potential of the RTM process for producing advanced

composite parts. The advantages and associated disadvantages of the RTM process are summarized. As thedevelopment of this process is rapid, some of the disadvantages may be overcome by the advances made in thi

technology.

Advantages are:

- Class A surface: surface definition is superior to lay-up. In addition, using matched tools for the mold,one can improve the finish of all the surfaces.

- Close tolerance: Parts can be made with better reproducibility than with layup.

- Design tailorability: Reinforcement and combination of reinforcements can be used to meet specific

properties- Fast cycles: Production cycles are much faster that with layup

- Filler: Filler systems can be used to reduce cost, improve fire/smoke performance, surface appearance

and crack resistance.- Gel coat: One or both mold surfaces can be gel-coated to improve surface performance.

- Good mechanical properties: Mechanical properties of molded parts are comparable to other composit

fabrication processes.- Good moldability: Large and complex shapes can be made efficiently and inexpensively. In addition,

many mold materials can be used

- Inserts: Ribs, bosses, cores, inserts and special reinforcement can be added easily.

- Labor saving: The skill level of operator is less critical.- Low tooling cost: Clamping pressure is low compared to other closed mold operations.

- Low volatile emission: Volatile emissions are low because RTM is a closed mold process. The worker

is not exposed to chemical vapors as with the lay-up process.

Disadvantages are:

- Mold design: The mold design is critical and requires good tools or great skill. Improper gating orventing may result in defects.

- Mold filling: Control of flow pattern or resin uniformity is difficult. Radii and edges tend to be resin-rich.

- Properties are equivalent to those with matched-die molding (assuming proper fiber wetout etc) but are

not generally as good as with vacuum bagging, filament winding or pultrusim

- Reinforcement movement during resin injection is sometimes a problem.

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In the following sections, the resin transfer molding process is discussed in terms of the unit operations

involved, to familiarize readers with the basic steps of the RTM process. The discussion covers details such as

materials of construction, mold design, performing, curing and demolding. Processing issues are mentioned ineach individual unit operation. Relevant variations of RTM such as vacuum assisted resin transfer molding and

flexible molding tools are summarized.

Process physics is described with emphasis placed on the principles that govern the RTM process; these areapplied in the use of computer simulations. Through the design tools such as simulation codes for mold filling

analysis, engineers are able to predict or diagnose the problems in gating and venting in the design stage. Theusefulness of such design tools is discussed in detail, giving the relevant advantages and disadvantages.

RTM Process

The RTM process can be viewed as seven unit operations. The general practice and processes issues are

described for each unit operation.

Fiber reinforcement

Selection of the proper reinforcement type should take into consideration loading condition, part geometry(size, thickness), mechanical properties and surface finish. The quantity of parts demanded also determines the

selection. The reinforcement normally carries 90% of the load in a composite and provides over 90% of the

stiffess. The reinforcement in a composite can be designed to match the strength requirements of the part. The

following , characteristics should be considered when selecting fiber reinforcements:

Volume fraction : ratio of the volume of a given mass of reinforcement to the volume of the same component

after molding:

Wash resistance: ability of a reinforcement to withstand movement due to fluid motion or salvation of the

reinforcement binder by the resin.

Wettability : ability of a reinforcement to reach a condition wherein all voids in the reinforcement are filled

with resin;

Sizing : most fibers are coated with size for better wettability and bonding but the size may influence the cure

kinetics during the manufacturing.

Most standard reinforcement materials for composites can be used, but fiberglass, carbon and aramid are the

most common in RTM. One requirement is that the reinforcement should hold its shape during the injection

phase. Therefore, the reinforcements are generally stitched, woven or bonded together. Reinforcement build-

ups in certain areas can easily be included. For example, woven roving and fabric can be combined withcontinuous strand mat and chopped strand mat in applications where higher strengths are required.

Hybrid systems composed of high performance reinforcement such as carbon fiber and aramid fiber and also b

incorporated in RTM laminates. Surfacing materials called veils can be used in the performs to hide the imprin

of fibers, for improved surface finish. Another application of surfacing veil is to achieve a resin-rich skin to

improve corrosion resistance.

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Emilian Popa Injection Molding Design Guide Manual 2012Stitched fabrics (fig 20.3(a)) reduce stresses inherent in the woven roving design and lead to highercompressive strengths in the composite. However, other constructions such as 8-HS style of weave (eight-

harness satin weave) in Fig 20.3(b) have been used because of improved wetting characteristics and

compressive strength compared to bidirectional woven fabrics.

Continuous strand mat is multi-stranded laid in swirled configuration. The mats normally have 4-6 wt % of

thermoplastic binder added. They are thermoformable and can therefore be used for highly complex shapes orwhen the anticipated volume of production makes them economical.

Different sizings can be obtained on many reinforcements. Sizings can be tailored to the type of resin system.

Sizings are available that are compatible with epoxy, vinyl esters or polyesters. The strength variation with typ

of sizing can be as much as 20% so this factor needs to be considered in the choice of reinforcement.

Preform

For a flat parts , the perform can be as simple as a stack of reinforcements that fit in the mold cavity. As

performs become more versatile, various means of producing performs are available. Currently cut-and-sew iscommonly used to assemble performs of various shapes for aerospace applications. Other near net shape

techniques include braiding, spray-up and thermoforming (Fig 20.4)

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If prefabricated performs are not used, then some means must be found to hold the layers of reinforcement

together as they are built up on the tool surface. For example, unidirectional reinforcement is subjected to

washing (washing is unplanned reinforcement movement due to resin movement) if proper precautions are not

taken to prevent it. To improve conformance of fibers, a tacky resin (e.g. epoxy) , dissolved in suitable solvent(e.g. acetone), can be used as a spot glue to hold the reinforcement layers together. The tacky resin will be

washed out during the resin injection cycle and will not interfere with the cure of the part. Sometimes veil can

be used to hold the layers and prevent washing.

Advantages are:

- Fast loading : Preforms allow fast loading of the mold.

- Precise fiber placement: Preform placement can be made precisely without misalignment. This allows

high quality, close tolerance composites for advanced applications molded by the RTM process.

- Net shape performs: If thermoformable reinforcements are used, the stamped performs have excellentdimensional stability.

- No additional tool: For low production volume of the composite, the tool for performing can be the

same as the tool for molding.

The only disadvantage with use of performs is that there is an additional unit operation.

With the obvious advantages, use of performs is advisable when volume of production allows their economica

use. When designing fiber performs, following issues should be considered:

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Emilian Popa Injection Molding Design Guide Manual 2012Corners : The fiber in the bent corner of a preform tends to move to the inside of a radius. This can causechanneling of flow that leads to poor mold filling patterns;

Drapability : This characterizes the ability of a fabric or reinforcement mat to conform to contours of the tool;

Edge definition : The edges of the composite will be resin rich if the preform is not cut to fit closely to the edg

of the cavity or inserts;

Fiber distribution: Uniformity of fiber content in performs, without excessive thinning, wrinkles or folds, is

important;

Permeability : A measure of resin distribution into the cavity. This quantity is also affected by fiber volume

fraction.

Prefabricated performs can be further bonded together, with or without a core, to achieve part consolidation.

For structural composites, this eliminates the need for fasters and adhesives to assemble discrete parts. New

thermiformable reinforcement mats can be used for highly complex shapes or when the anticipated volume ofproduction makes them economical.

Design of performs should go hand in hand with part design. For example, preform corners are sensitive toradii of the shape. Figure 20.5 shows the thickness reduction of preform over different radii. The preform

thickness does not change appreciably compared to those around the corner when the radius is made larger tha

a critical value.

However, if the radius is less than this critical value, dramatic movement of the fibers to the inside of the radiuoccurs. As a result, channeling becomes dominant in the mold filling stage and includes irregular flow patterns

The edge of a preform is another source of the race tracking of resin. In order to avoid the channeling effect, th

preform edge should be cut to fit the edge of the mold cavity. The task of obtaining a good edge definition is

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Emilian Popa Injection Molding Design Guide Manual 2012normally difficult because of the bulkiness of the layers and inter-layer movement (sliding, rotating) during thmold closing when prefabricated performs are not used.

Preforming of fabrics over tool geometry other than simple flat type will induce shear deformation in the fiberreinforcements. For a biaxial reinforcement, shearing of the weave (Fig 20.6) is necessary to conform to the

contours of the tool. This drapability problem, therefore, has a two fold significance in RTM. Because of the

fiber rearrangement, the nonuniformity of fiber distribution should be accounted for in the design of the

composite. Fiber volume fraction and orientation are no longer that of the unreformed reinforcement. Further,such performs exhibit different characteristics to resin flow. Designers should account for this change in

determining the location of vent ports relative to an injection port.

In practice, to modify the permeability of performs, various flow including media or mechanisms have been

suggested. Application of such high porosity to the preform or inclusion of a runner system in mold design can

alter the mold filling pattern.

Resing system and injection

The resin used in the RTM process forms the matrix in the composite after solidification. The solid structure i

a result from polymerization. To select the resin system, one must take into account of the rheological changeand resin cure kinetics.

The formulation of the resin system depends on many factors. For example, the resin system can be combinedwith promoters, fillers, internal mold releases, pigments, etc. Typical fillers such as clay or calcium carbonate,

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Emilian Popa Injection Molding Design Guide Manual 2012may reduce cost. The optimum viscosity for RTM should be less than 500 cP s. Mixing is normally required toform a suspension.

Properties requirements (mechanical, chemical, fire retardancy, etc) can also affect resin selection; the resinmix can be formulated to meet specific needs. Attributes to look for in resin systems are:

- consistent reactivity

- ability to wet out the reinforcement- rapid cure after gel

The ester-type resin mix is combined with an appropriate catalyst, such as emulsifies BPO, MEKP, cumene

hydroperoxide, at the mixing head and transferred into the RTM mold. Low profile additives have been

developed especially for polyester resins to improve surface appearance. In addition, epoxies, urethanes, vinyl

esters, nylon and other hybrid resins are available for RTM. The newer resins may require modifications to thepumping/injection unit to meter and condition the resin mix prior to injection. These new systems offer a rang

of cost and performance options for the RTM process.

Influencing parameters are viscosity, pot life, tensile modulus, glass transition temperature, tensile elongation

and moisture absorbance. In considering a new resin system, the choice of the proper resin system for RTMmust satisfy the following system criteria. Failure to meet these criteria usually means that the resin system isimpractical for RTM.

Low viscosity. High viscosities can cause mold pressures that are too high in both the mold and the injection

unit. Raising the temperature of the resin system is effective to lower its viscosity, but pot life may be adverseaffected.

Sufficient pot life. This is the time it takes the resin systems viscosity to reach a level that no longer becomfortably handled by the equipment.

Tg point. The glass transition temperature should be as high as possible. As a rule of thumb, the glass transitiotemperature should be at least 30C (50F) and preferably 55C (100F) higher than the service temperature.

Toughness: Toughness in a resin system is exhibited by its tensile elongation. If sufficient damage tolerance is

required, the elongation should be at least 3%.

Youngs modulus: This modulus must be over some threshold value or the composite compression strength

will be less than the optimum value. A high tensile modulus is required to adequately support the fiberreinforcement and prevent premature buckling.

The effect of the resin system on hot-humid performance is important in the composite part. The modulus of a

typical resin remains essentially constant until the temperature is close to the ultimate Tg when it falls off tozero.

Under wet conditions, the strength of the resin usually falls off at the same rate as the modulus because of the

effect of absorbed moisture. Absorbed moisture plasticizes the resin matrix and lowers the strength of thecomposite in non-fiber dominated directions. The amount of moisture absorbed by the resin matrix should be

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Emilian Popa Injection Molding Design Guide Manual 2012small, normally less than 2%. The limits the amount of mechanical performance degradation at elevatedtemperatures.

One final topic to consider is the injection of the resin system (schematics shown in Figs 20.1 and 20.2). Itemsto control in the resin mix to assure a consistent, smooth running process include:

- resin mix temperature

- ratio of catalyst or curative to resin- resin mix viscosity

- amount of air entrained in the resin mix. Presence of air in the mix can lengthen the gel time/induceporosity in the composite and/or affect the mix viscosity.

Most successful production resin transfer molding operations are now based on the use of resin/catalyst mixin

machinery using positive displacement piston-type pumping equipment for accurate control of the resin tocatalyst ratio. Back pressure at the mix head may change when a mixed resin is injected into a cavity filled wit

the fiber reinforcement. Static mixers greatly simplify the process and are easily cleaned at the end of the

injection cycle. A static mixer sends the proportioned resin and catalyst through flexible hoses to an injectionhead employing a motionless mixer to thoroughly blend the materials together immediately prior to injection

step.

Mold

RTM mold design and construction is the most critical factor in successful resin transfer molding. The mold

must be constructed so that resin reaches all areas. RTM molds require special considerations compared toother composite tooling. Figure 20.7 shows two possible configurations in RTM processing. The mold be

designed to account for the following factors:

Mold materials: The material of construction dictates life cycle of mold, temperature control and press

requirement.

Cavity design: The RTM mod should consolidate as many assembly steps as possible. A good design should

take advantage of this ability.

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Emilian Popa Injection Molding Design Guide Manual 2012Gate and vent: The critical part of the mold design should allow complete wetout with minimal resin wastage.

Mold sealing: A perimeter gasket is necessary to keep void content low. Tight sealing is important when

vacuum is used.

Heating/cooling: A typical RTM cycle consists of a wide range of temperature for initiating the chemical

reaction, curing and final demolding. Hence, proper heating/ cooling channels need to be designed.

Mold Materials

The low pressure requirements of RTM allow the use of more type of mold materials than can be used in othe

composites manufacturing. The choice between metal molds and polymeric composite molds is chiefly one of

volume and processing temperatures. High volume and high temperatures dictate metal molds.

Steel, the most suitable mold material, provides superior face life. Aluminum is good for construction of

prototype molds since the metal is easy to machine, is lightweight and has a reasonably high heat transfer rate,

but also galls easily. Cast aluminum and spray-metal tooling volume applications. Cast copper alloys are beingconsidered for use in RTM molds due to the potential for increased throughput via heat management and bette

durability.

Composites, for example reinforced polyester and epoxies, are most frequently used for making RTM molds.

They can be expected to last for approximately 2000 parts (Isorca, 1992). Higher production volumes may

justify the use of higher cost spray-metal or metal tools.

In some cases, the mold must be backed up in order to maintain its shape. Conventionally, the backup can be

done cost-effectively with core material or steel frames to add rigidity to the cross section and to support

composite mold faces. The closure of the mold is achieved by mating of the mold surfaces against a perimetergasket. Therefore, guide pins are usually employed to align the mold halves both laterally and vertically to kee

resin from leaking.

Advancement in adapting composite tooling to the needs of RTM is underway. For example, lengthening the

life of the composite tool face is desirable and effective to maintain quality while keeping costs low. The

factors that cause deterioration of the mold face are temperature fatigue and attack by solvents or mold release

agents. An electrolytically or vapor deposited nickel shell is a new technique that will extent face life.

Mold Cavity Design

One of the most important design rules for RTM parts is to reduce the number of assembly steps. Therefore, th

mold designer should incorporate this rule in the design of the mold cavity. Instead of joining several

substructures or onto a major structure after molding, it is structurally more effective and efficient to

incorporate them into the part before fabrication. This can be easily achieved by joining substructure performswhen practical.

In production the number of molds or cavities required is determined by needed throughout. This should take

into account the cycle time. For small parts, the designer can incorporate several cavities in a mold.

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Emilian Popa Injection Molding Design Guide Manual 2012High surface quality with excellent dimensional control can be achieved by electroplating the mold face withnickel. The appearance surface of a part is usually placed on the bottom of the mold. Pinholes are more likely

to collect on the top surface. Mold preparation is similar to that used for hand lay-up. A new tool must to be

treated with several coats of release agent.

New mold materials provide flexibility in mold design for RTM. For example, to demold a part with vertical

sides, it is common to allow several percent draft in the vertical dimension. Flexible silicone rubber has been

used for RTM molds in the form of a bladder mold half which is capable of being inflated or deflateddepending on the process requirement. During mold filling, the flexible mold wall is pressed against the rigig

wall by inflating the bladder with a pre-determined pressure. During the injection cycle, the mold can deform tenhance resin flow. Upon completion of mold filling, the flexible tool can be further inflated to consolidate the

composite component. Part removal in this case is easy since the flexible half can be deflated. This technique

allows fabrication of complicated parts that are not ordinarily possible to demold.

Injection port and vent design

The injection port allows the resin to be transferred into the mold (Fig 20.1) and its design may be critical. Thelocation of inlet ports must allow the resin to reach all areas without bypassing part of the reinforcement. Air

vents help control internal pressure, bleed out air and provide a visual indication of mold filling.

Race tracking, or channeling, in the mold is usually the reason why the resin bypasses areas of the

reinforcement. Since the resin will not flow backwards, this tends to create dry patches. The engineering way t

ensure complete initial wet out is to gate the mold correctly in the design. This may be difficult even for an

experienced mold designer. Use of computer simulations as a design tool has become popular in conventionalinjection molding. Without an engineering design tool, gates and vents can be pout in the mold after molding

some trial parts, but many trial runs may be prohibitive in some applications. In the next section, new

engineering tools adapted for RTM mold filling will be discussed to overcome the problem.

Mold designers have found that RTM molds must be vented to allow the air within the mold to be pushed out

by the resin. Gate at the lowest point and vent at the highest point is generally a good design practice.Experienced designers may use symmetry to design the inlet ports and outlet vents to remove entrapped air.

Venting ports must be placed to draw the resin through sections of the part that are difficult to wet out. They

are best placed at dead ends where the resin would not flow by itself.

After the resin has finished bleeding, both injection and venting ports must be sealed off. This allows pressure

to build up in the mold, and forces the resin to further wet out other sections of the part. This packing stage

allows the part to gel under pressure, decreasing void content in the finished part.

Sealing the Mold

The perimeter gasket seals the edges of the mold to prevent loss of resin and injection pressure. In addition, it an absolute necessity when vacuum is used. Sealing the mold to achieve cavity pressure of 690 kPa (100 psi) o

higher is necessary if the void content of the part is to kept low. The only practical way to accomplish this is touse O-rings. Machining the face of the mold to close tolerances is prohibitively expensive. It is also usually

impossible to maintain the mold absolutely flat to achieve a metal-tight seal.

O-ring design is well established. The slot has to be cut so that the O-ring can deform when the mold is closedand maintain a seal. Either square or round O-ring grooves can be used. The type of O-ring material used

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Emilian Popa Injection Molding Design Guide Manual 2012depends on the maximum temperature the O-ring will experience during the fabrication cycle and the type ofsolved used to clean the mold. Nitrile rubber material can be used satisfactory up to 120C (250F). Over 120

silicone rubber can be used to temperature approaching 177C (350F). If help is needed in sealing around inl

or outlet tubes, tacky sealant can be used. This type of sealant is useful for making an O-ring where grooves dnot exist.

Heating and Cooling Design

The mold should have good temperature control. The RTM mold should be able to heat and cool the part

during the fabrication cycle. Most resin systems cure faster at elevated temperatures. During demolding,lowering the temperature is sometimes helpful in removing the part. Even molds that are intended for room

temperature-cured resins should be well insulated so that environmental conditions do not change the gel time

and viscosity of the resin. Some molds are heated or designed to go into ovens to achieve faster cures at higher

temperatures.

Normally, the mold is heated and cooled using either hot water or oil. The mold is constructed to allow the

heating/cooling fluid to flow through channels. (Fig 20.8) in its interior. The fluid is heated and cooled byconventional means, such as a gas-fired heated and heat exchanger.

For larger molds, the heating and cooling times will be longer if the heat transfer area does not increase in

proportion to the weight. At some point, the production cycle time becomes limited by the rate at which heat

can be added or removed, and becomes independent of the curing characteristics of the resin system.

Under development is low thermal inertia technology that allows the tool face to be heated by electrical wires

buried in the face. The construction of the mold face is such that the heat flows into the mold face and notoutward toward the mold support structure. This is accomplished by use of a foam core that insulates the bulk

of the mold from the tool face. This novel technology, if successful, will allow a more instantaneous transfer o

heat where it will do the most good at the mold face.

Mold Filling

Resin injection is to pump the base resin system to a mixing head through either a single or two pot system.

Impingement mixing of the components occurs in the mixing head. The catalyzed mix is then pumped through

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Emilian Popa Injection Molding Design Guide Manual 2012a static mixer which completes the mixing of the two components. The injection nozzle is attached to theinjection port on the mold and the resin system is injected into the mold to pack the mold to a predetermined

pressure. When the mold is filled, the pumping system is shut off and immediately flushed, and the part is

allowed to cure.

Successful configurations demonstrated in the industry show a common factor: that is, the flow of resin is

symmetrical about the vent ports, in a manner such that the volume of air left in the reinforcement decreases.

This compression effect helps sweep the remaining air out of the part.

When the flow path is arranged in such a way that the resin flows into a configuration with increasing volumethere is a tendency to bypass part of the reinforcement. This situation can happen when core materials is used.

For example, when there is reinforcement on either side of a core, it is possible that slight misalignment in the

core thickness will cause dry spots in the part. To overcome this problem, the resin must be introduced on

either side of the core simultaneously. Holes may be drilled through the core to allow the resin system to flowto the other side. When this is done, the core floats on the wet reinforcement and equalizes itself.

When the injection pressure is too high or reinforcement tends to move in the mold, the following remediesmust be considered.

Multiple gates: partition the mold along the flow path such that travel distance for resin is reduced.

Runner system: allows the delivery of resin to various parts of the reinforcement quickly without using high

injection pressure.

Flexible mold wall : allows the deformation of the bladder wall to facilitate mold filling.

There are several techniques to modify the flow patterns. Application of high porosity media on the preform oinclusion of a runner system in mold design can alter the mold filling patter. This is helpful in reducing

injection pressure or displacing air.

All resin movement must be accomplished within the time allowed before the onset of gelation. Additionally,

the resin injection process should be done at low pressure so that the mold will maintain its shape without

requiring massive backing.

Vacuum may be used to facilitate filling the mold and simultaneously assist in removing air from the laminate

This requires good mold sealing and the use of a vacuum pump. Vacuum up to 740-760 mm Hg (29-30 in Hg)

has been reported in assisting RTM mold filling (Mosher 1995). Note that the tooling must be large enough toaccommodate the perimeter gasket, air vents, injection ports and guide pins.

Curing

To convert a resin system into useful products it must be cured or cross-linked by chemical reaction into a thre

dimensional network. The reaction usually involves either a step growth polymerization, a chain growth

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Emilian Popa Injection Molding Design Guide Manual 2012polymerization, or a combination of both. The accompanying rheological change in process is shown in Fig20.9.

The curing step constitutes a major portion of a typical RTM cycle. During curing, rheological propertychanges of the resin system and heat transfer between the mold wall and the resin dictate the cure cycle.

Simultaneously, modulus and strength begin to build up at a rate depending on the type of resin and catalyst

used and the chemical kinetics of the resin system. Curing can continue after the part is demolded.

Cure cycle is dependent on part thickness, the ratio of catalyst or curing agent to resin and the temperature of

the mold and the resin system. In some cases, the part is removed from the mold immediately after gel occurs.

The part must develop sufficient green strength for handling prior to its removal from the mold. Green strengthis the strength a composite exhibits after the resin gels, but prior to complete cure. Gel time is the interval of

time between introduction of catalyst or curing agent to a thermosetting resin and the formation of a gel.

Typical gel times range from several minutes to about an hour depending on the factors mentioned above.

The glass transition temperature Tg for an RTM resin system depends in thermal history. For a given

temperature, the Tg increases dramatically with time until it levels off. As the curing temperature is raised , th

Tg reaches a steady-state value at a faster rate. The steady-state value for Tg is a function of the curingtemperature, and usually approaches the curing temperature. However, the limit is bounded by the degradation

temperature of the resin system.

Demolding and post processing

The minimum the curing step must accomplish is to develop sufficient green strength so that the part can be

removed from the mold. While cost is an important factor, it is not the only criteria in choosing a method to

remove a part from an RTM mold. For example, part weight and complexity, and throughput are important

considerations. In many ways, the choice of ejection methods parallels the choice of clamping methods.

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Emilian Popa Injection Molding Design Guide Manual 2012A few precautions are required to facilitate demolding. Before opening the mold halves, it is necessary torelease the part from one mold surface. The force required is approximately that to overcome the adhesive forc

between the mold and the composite. Typically, tears of surface skin or flash, both resin rich, can be found

around the corners or edges of the part.

If the adhesion to the mold face is too strong, even exceeding the strength of the composite, it can be reduced

by spraying release agents, normally fatty ester soaps or waxes, on the mold surface.

After the two mold halves separate, the part can be removed from the cavity. Part removal methods range from

the use of plastic/ wooden wedges and rubber mallets to the use of knock out pins. A mold designed for lowthroughput with hand operated clamps producing a relatively simple, lightweight part would most likely be

removed using a wedge and mallet. Sophisticated hydraulic ejection systems can be used for high volume,

complex or heavy parts. To be pushed out, the part needs enough green strength to survive considerable

bending stresses.

The most common test for sufficient bending strength is to fold over a corner of the part immediately after

demolding. If the corner survives the bending without cracks or a crease, the part is accepted. Otherwise,measures to improve its green strength include any of the following steps:

- allow the part more time to cure in the mold;- increase the mold temperature

- modify or change the resin system , e.g. increase the catalyst level.

There is often excess resin at the edges of the part and in the vents. Considerable trimming, part of the postprocessing, is common when reinforcement is clamped in the parting line. Trimming is required for almost all

items made by the RTM process. Accurate preform placement and precise alignment can reduce the labor in

this step.

Postcure, one of the post processing operations, is used for various reasons. A molding cycle including postcur

can increase production throughput. While post curing in an oven, the temperature is not restricted to thatallowed for the mold materials. Therefore a higher conversion of reactive groups can be achieved. It can also

prevent the reaction exotherms of a resin system from damaging a composite tool. It is important to hold the

part shape during the process of postcuring and cooling to prevent distortion or warpage.

Process physics and use of simulations as a design tool

The processing defects addressed in the previous section are often caused by lack of a systematic treatment inRTM part design and process planning. Among the unresolved issues in RTM encountered by composite

engineers, those related to the physical processing have developed rapidly during the last decade. The

advancement in RTM technology demonstrates the potential of RTM becoming a primary process for

producing many composite parts.

In this section, the issues in reinforcement performing, alternative tooling, mold flow analysis, and cure kinetic

are revisited. The focus is on the use of models to describe and enhance the understanding of the physical

phenomena. The models are built on the experimental evidence and observations, the goal being to reduce the

scope of experiments in the engineering applications. Reducing engineering experimentation is achieved bycombining three elements: mathematical models, numerical methods and computer software, into a simulation

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Emilian Popa Injection Molding Design Guide Manual 2012One example of such software is LIMS (Liquid Injection Molding Simulation) which has been developedspecifically for mold filling of complex structures in RTM and can be used also as a design tool for

manufacturing of complex structural parts as shown in Fig 20.10.

The topics will be presented in the order found in the unit operations of RTM. Draping of reinforcement plays

the role of distributing fibers in a way that depends on the tool geometry. Simulation of reinforcement draping

allows an engineer to estimate the fiber content distribution. This distribution can change the volume fraction a

well as the orientation in the molded part and therefore is of extreme importance.

Tooling and mold construction are critical factors in successful RTM. By considering several alternative

configurations, both the injection pressure and the filling time can be reduced. These alternative designs are

valuable as the injection pressure tends to rise rapidly when inhomogeneous fiber distributions are present as a

result of performing.

Physics governing RTM processing and numerical simulation

Darcys law for flow through porous media is conventionally used to describe the resin flow in the fiber

reinforcement. The generalized form expresses the superficial velocity of resin flow in terms of a factor, which

is permeability divided by fluid viscosity, multiplied by pressure gradient. This expression together with themass conservation in the mold are solved together using various numerical methods. A typical example of this

method combined finite element method with control volume method. The solution is moved forward in time

after the pressure field is obtained during the filling process.

The pressure solution obtained from the mold flow analysis can be used to position the gate and vent. This

lends a design engineer infinite options when facing the task of mold design. The design rules are no longer

restricted to the rule of symmetry used by experienced designers to position the inlet and outlet ports. Instead, composites engineer would be able to optimize the overall design based on criteria such as minimizing the

injection pressure.

Preforming

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Emilian Popa Injection Molding Design Guide Manual 2012For directional mats, woven or stitched, draping an arbitrary tool surface depends on two deformation modes:shear deformation and inter-yarn slip (Potter , 1979). A mat of this nature is treated as a net that consists of

many cells. Therefore, draping over a surface of double curvature requires the net to map on the surface by

changing the internal angles in each cell. The four sides of a cell are made up of fiber tows. These tows, underthe performing condition, are inextensible. At high deformation regions in a reinforcement, slippage may be

necessary to drape the tool surface. The length of the cell segment can be changed as a result of slippage to

accommodate this effect.

A dome shaped part will serve as an example of this draping simulations. First, a square bidirectional mat is

draped. The workpiece is initially configured so that warp and weft tows are perpendicular to each other. Thendraping starts at an arbitrary point on the tool. The initial constraints used in this case study are prescribed

along the central tows in both the warp and weft directions. The length of the cell segment is assumed to be

constant.

In the draped configuration shown in Fig 20.11, the degree of deformation varies from cell to cell. The minor

angles in the preform range from 90 to a minimum of 35. The shear also results in fiber volume fraction

increase up to 70% for the dome. This information can assist a designer in material selection, setup ofprocessing conditions and part design: a process engineer can use this information to find out where to make

necessary cuts in order to accommodate for induced deformation. As a rule of thumb, formability of preformmat relies on absorption of such deformation by the reinforcement material. A good material can withstandhigh deformation without wrinkle formation.

Alternative tooling

One benefit of this process is that it can consolidate several complex three dimensional parts into one molded

piece. The key to accomplish this is tool design. From the design point of view, a flexible mold wall is very

desirable to mold certain parts with difficult or impossible to demold geometry. While a hard tool makesclamping and demolding difficult, the flexible mold provides a convenient alternative for mold design of thesetypes of parts. Figure 20.10 shows an example of possible features which may be molded using this concept. I

this part, one can easily see the small draft angle and the stiffeners which can make demolding difficult.

Moreover, the beads and the flanged opening in the bulk-head of this frame are features that are impossible tomold using conventional rigid molds.

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Emilian Popa Injection Molding Design Guide Manual 2012To avoid unconstrained wall movement, the bladder pressure is higher than the injection pressure. The motionof the flexible wall or the preform is driven by the pressure difference. Therefore, the equation of motion is a

function of the bladder as well as the reinforcement material. On the preform part, the compressibility of the

reinforcement in the thickness direction plays a major role. On the bladder part, factors such as inertia,damping, and rigidity of the elastomeric material can also be included when they are significant.

From lateral compression tests, the load-deflection curve of the fiber reinforcement material behaves like a

nonlinear spring. The elastic constant of the preform depends on its state. Preform permeability is a function ofiber architecture and porosity. Since the porosity of the preform changes with the thickness, the permeability

can be expressed in terms of the cavity thickness.

Figure 20.12 was obtained from the numerical simulation of two cases, one with rigid walls and the other with

a flexible mold wall. In the case with rigid mold walls, the pressure drops linearly with respect to the flow

distance.

This is caused by the constant permeability of the perform inside the mold. The pressure curve for the case wit

a flexible mold wall reflects the fact that the fluid flow in the filled region exhibits a smaller pressure drop. Threduction is beneficial to the molded parts as it causes less fiber washout and preform deformation due to the

resin.

Figure 20.13 shows the results of computed gap thickness of the 1-D mold with a flexible mold wall. Thestraight line shows the thickness in a rigid tool. From this distribution, one can see that the gap height is a

function of pressure. Near the injection gate, the resin pressure balances the applied pressure from the bladderand increases the gap thickness to its maximum in the 1-D mold. As a result, the resistance to the incoming

flow has reduced significantly as shown in the previous figure.

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Emilian Popa Injection Molding Design Guide Manual 2012Through the numerical study, the potential of the flexible tool design has been demonstrated. It has theadvantage of reducing the pressure drop and overall filling time which is impossible to attain simultaneously i

conventional tooling.

Gating, Venting, and Void Capture

In this section, computer simulations for RTM mold filling are discussed to overcome the gating and venting

problem. Mold filling simulation is an effective way of positioning injection and vent ports. Gating and ventinare critical in the mold design because they determine whether complete wet out is achievable under normal

operating conditions.

A gate designed at the lowest part and vent at the highest point is generally a good practice to allow the air

within the mold to be pushed out by the resin. Experienced designers may use symmetry to design the inlet

ports and outlet vents. However, the picture is often complicated by the geometry or the presence of inserts.

The engineering way to ensure complete wet out initially is to gate the mold correctly in the design.

Figure 20.14(a) shows a square plate with two cutouts in the part. The injection port is first positioned at thecenter of the lowest part. The flow fronts corresponding to the gate design are indicated by the curved contour

Contours in this figure indicate different time steps. For example, the contours closer to the gate represents are

that is filled first and the contours closed to the vent the last filled region. As a result of colliding flow fronts inthe middle and top portion of the part, the figure demonstrates the capturing of dry patches or macro-voids.

These voids can degrade the properties of the molded composite significantly. Void capturing is important in

the process simulation to avoid formation of such defects. Figure 20.14(b) shows an alternative design thateliminates venting in the middle of the part. As a result of injection in the corner, the vent port has to be

positioned differently. This demonstrates the power and simplicity of the design tool. The situation would be

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Emilian Popa Injection Molding Design Guide Manual 2012much more complicated if mold filling is coupled with phenomena such as preform deformation andchanneling in the corners and along the edges.

At the microscopic level, heterogeneities always exist in the preform media. For example, the fiber tow mayhave a permeability several orders lower than that of the interstices. Therefore, micro-voids form when the

orientation of the fiber tow does not allow the displacement of the air inside the tow. A novel approach in mol

filling analysis is reported by modifying the equation of mass conservation to account for the fluid absorbed by

the fiber tows . Void entrapment inside tows is found to be dependent on the microstructure, the vent pressureand the ratio of the difference in the permeability of the tows and the permeability of the preform.

Sensor Controlled Injection

Sensor controlled injection is multiple injection on an intelligent way without involving a complex control

algorithm. It requires placement of gates along the flow path at a number of locations. The injection gate is als

a sensor capable of detecting the arrival of resin. These gates are then activated or deactivated in the order offirst on, first off, and , therefore, allow the mold to fill in a series of steps.

For example Fig 20.15 is a simple mold which has four injection gates. To help visualize the concept

effectively, a 1-D mold is used. The T-column represents different time stages in the filling process. In this

example, only one gate is allowed to open at a time. As the injection starts at T1, the first gate is open and theremaining three are closed. At the flow front progresses through the mold, it hits the second gate location at

time T2. The injection unit shuts off the resin to gate one and opens the second injection gate. Instead of havinthe resin flow through the whole length in the mold, the length is divided up into a number of intervals.

Therefore, the overall flow resistance decreases as the effort required is for the resin to flow from one gate to

the next closest gate in the flow path.

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Figure 20.16 shows the pressure calculation from the mold filling simulation. The pressure drops linearly in thone dimensional flow. As the flow front progresses from the inlet forward the vent under a constant flow rate

boundary condition at the inlet, the pressure build-up looks like the schematic shown in the lower left figure.For the multiple gate with sensor controlled injection, the pressure at the first gate increases up to a limit when

the flow front hits the next sensor. When the next gate is open, the previous gate is shut off. So the pressurebuild-up is only limited by the length of the interval. Therefore, the maximum pressure seen in the mold is onl

a fraction of the pressure compared to the lower left figure.

Table 20.1 shows the results from two sets of computer simulations. For either case, only one gate is open atany time during the mold filling stage. The first column uses constant flow rate and the second column uses

constant pressure.

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If only one injection gate is used, the pressure under the constant flow rate boundary condition will reach a

maximum. Compared to the sensor controlled injection with four gates, the pressure at the gate is only 48% of

the pressure reached by the single gate injection. In terms of filling time, the two molds are subject to aconstant pressure boundary condition. Results show that the mold filling for the single gate injection takes

almost three times that for the sensor-controlled injection.

An example is shown in Fig 20.17 , which elaborates on how one can utilize a sensor to eliminate a dry spot

during molding. In Fig 20.17(a), where no sensors are implemented and the injection gate is at the location as

shown, a dry spot will appear in the middle of the part. However, an extra gate in the middle as shown in Fig20.17(b) , if triggered at the point the fluid reaches the midframe, can prevent this void, as indicated by Fig

20.17 (c). This feature is incorporated in a numerical simulation such as LIMS and can be systematically

studied for a given geometry to decide the best strategy when in situ sensing capabilities are incorporated in th

fabrication phase .

Mold Filling with Resin Delivery System

Conventionally, an injection port serves as a point source where fluid is pumped. The drawback of a point

source is that the pressure value tends to rise rapidly to an extent that could be detrimental to the preform. Byextending the point source into other forms proves to be effective in reducing the pressure build-up. To

implement this concept, one can use multiple point sources as discussed previously. A line source has beenpopular in vacuum assisted RTM because of its ability to fill the mold using 1atm of pressure.

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Emilian Popa Injection Molding Design Guide Manual 2012A line source may be modified to serve as a runner by allowing more fiberfree space in this deliveru system.This is the channeling effect, now used to advantage in mold filling. Further extending the fluid source may

possibly yield a plane source. The actual implementation of a plane source may include a high-porosity laye

in the stack-up of the reinforcement mats. The layer can possess a permeability several orders higher that thatof the fiber preform. The result of this is a three-dimensional mold flow with fluid propagating, rapidly throug

the spreading plane or surface first followed by percolation of the resin through the thickness of the perform.

For three-dimensional flow, venting the mold may become less intuitive. In practice, vacuum assistance can

provide part of the solution.

Conclusions

Resin transfer molding is a practical process for much of the composite industry. The quality of RTM molded

parts can equal that by conventional autoclave processes and its economic advantages are obvious. Although

the underlying principles of RTM appear at first to be simple, this is often not the case. The challenge for RTMis to bring together the disciplines of performing, mold design and process development with existing fibers

and resins. Thos can be best achieved through an understanding of the physics governing RTM and by current

simulation technology.

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