Manufacturing composite products is quite different from traditional production processes. With composites, materials are built up in layers to create and shape the end product. In traditional production, material is removed from the basic wood, metal or other stock material to form the final shape. The difference between these methods is so great that composite products are rarely manufactured using a wood or metal design. Rather, it is important to design specifically for composite materials and to consider which type of process – Open Mold or Closed Mold – is being used.
Composite designers can choose from a variety of fiber reinforcements and resins to develop the part laminate. The part’s design, or fiber architecture, details the ply schedule, or arrangement of individual plies (fabric layers), to be used. Each ply imparts specific mechanical and physical characteristics to the final product.
In order to design a satisfactory product, it is important to know about the properties of each raw material involved. Basically, the reinforcement material provides mechanical properties such as stiffness, tension and impact strength, while the resin system (matrix) provides physical properties including resistance to fire, weather, ultraviolet light and corrosive chemicals.
The other factor is cost. An over-designed part costing more to produce cannot compete with products already established in the marketplace. A well-designed part, using the right materials and process to meet the application requirements, is usually commercially competitive, especially when installation and maintenance are factored into the total cost.
Three factors must be considered when choosing reinforcements: fiber (most commonly fiberglass, but also aramid and carbon); form (roving strands, mat and fabrics); and orientation (fiber direction in the part). Fibers can run parallel (uni/longitudinal, 0º), circumferential (bi-axial, 90º) or helical (biased, ±33º to 45º) along the length of the part, and/or with random continuous strands. Strands can also be varied, producing a virtually isotropic laminate with equal strength in all directions. Fiber volume (glass to resin ratio) must also be considered. Resin is heavier than glass; so having higher fiber content will result in a stronger, but lighter weight, part.
Resin (polyester, vinyl ester, epoxy) and form (wet lay-up or prepreg, a reinforcement saturated with resin) must be carefully chosen to ensure a successful design. Formulators can modify resin with chemicals and fillers to help meet product performance requirements. Resin viscosity, usually expressed in centipoise (cps) units, is important in achieving optimum flow rates for specific manufacturing processes.
Laminate design, size and complexity, as well as cost, volume, production speed and market conditions, determine whether the part will be built through Open or Closed Mold processes. To produce a strong and durable laminate by any process, the resin must thoroughly saturate the reinforcements, and the wet laminate must be compacted to remove excess resin and entrapped air.
Gel Time Test
Gel Time Tests, which assure that the resin or gel coat being used meet specifications prior to production, are recommended with each new batch number. Gel Time Tests can also verify if older materials in the shop are still useable.
The following materials are needed to perform a Gel Time Test:
- Current Data Sheets for each material being tested
- 100+ grams of each test material
- Catalyst, usually MEKP (Methel Ethel Ketone Peroxide)
CAUTION! EYE PROTECTION AND GLOVES MUST BE USED WHEN HANDLING MEKP!
- An accurate scale that weighs in grams
- An accurate thermometer (up to 400° F)
- A stopwatch or accurate clock with a secondhand
- A polyethylene pipette or glass eye dropper for dispensing MEKP
- Small (4 oz) disposable cups
- Stirring sticks
- Acetone and rags for clean up
Put the cup on the scale and tare the scale (i.e., zero out the scale so that the cup’s weight is not included in the test.) Pour 100 grams of resin or gel coat into the cup, and check its temperature with the thermometer. The material needs to be 77° F for the test to be accurate. If necessary, hold the cup in a warm hand or set it in a small pan with an inch of warm or cool water until the proper temperature is reached.
Put the cup back on the scale. Using the pipette, add the proper percentage of catalyst listed on the Material Data Sheet. If not listed, the general rule of thumb is 1% for resins and 2% for gel coats. One percent of one hundred grams is one gram; two percent is two grams.
Start the stopwatch while mixing catalyst with the test material. NOTE: Wooden stirring sticks must be pre-wet or inserted into the cup away from the catalyst. Otherwise, some catalyst will be absorbed into the stick and catalyst ratio will be inaccurate.
Insert, remove and check the stir stick every few minutes to see if the material has changed from a liquid to a gelled state (similar to jello or gelatin). As soon as the material gels, record the time and insert the thermometer.
Continue checking the temperature every three to five minutes until the material is at its highest temperature. Then record how much time it took from mixing in the catalyst to reaching its highest temperature (peak exotherm).
Take the cup outside, away from any flammable materials, and pour cool water into it. The cup can be placed in the trash when its contents return to room temperature.
Compare the gel time and time to peak exotherm against the data sheet to see if the material is within specifications.
Open Mold Processes
Open Mold Processes include spray-up and open contact molding (hand lay-up) in one-sided molds. These low cost processes are commonly used for making boat hulls and decks, RV components, truck cabs and fenders, spas, tubs, showers, and other fiberglass composite products.
In a spray-up application, the mold is waxed, sprayed with gel coat, and then cured in a heated oven at 120º F.
After the gel coat cures, the mold is sprayed with a mixture of catalyzed resin (polyester or vinyl ester, 500-1000 cps viscosity) and chopped fiberglass roving (E-glass cut with a chopper gun). Using low-styrene and suppressed-styrene resins, fillers and high-volume/low-pressure spray guns or pressure-fed roller applicators help reduce the emission of volatile organic compounds (VOCs).
The spray-up is rolled out so the laminate can be compacted. Wood, foam or other core material may then be added. A secondary spray-up layer imbeds the core between the laminates (sandwich construction). The part is then cured, cooled and removed from the reusable mold.
Fiberglass (typically E-glass) continuous strand mat and/or other fabrics such as woven roving is manually placed in the mold. Each ply is sprayed with catalyzed resin (1000-1500 cps). Brushes and rollers are used to work the resin into the fiber, wetting out and compacting the laminate.
Hand lay-up and spray-up methods are often used together to reduce labor. For example, fabric might first be placed in an area exposed to high stress. A spray gun then applies chopped glass, completing the part. Balsa or foam cores may be inserted between the laminate layers in either process. Typical glass fiber volume ranges from 15-35%, with spray-up at the lower end and hand lay-up at the higher end.
Fiber content can be increased up to 50% by curing the part in a vacuum bag at 2-14 psi vacuum pressure and a cure temperature below 350º F. It can be increased up to 70% by using vacuum-assisted resin transfer molding or infusion molding. The applied vacuum compacts the preform while helping the resin penetrate and wet-out the fiber.
Spray-on surface materials, are available to finish parts made through Open or Closed Mold processes. This spray-on surfacing material bonds to fiberglass and other materials. Available in granite-look color blends, solid, accent and custom colors, it provides an attractive finish that is more durable then premium solid surface materials, but as economical as plastic laminate.
Closed Mold Processes
Light Resin Transfer Molding (Light RTM), Closed Cavity Bag Molding (CCBM) and Vacuum Infusion Process (VIP) are versions of technology used to produce precision parts in a variety of industries, especially for applications requiring closer tolerances. With Closed Molding it is possible to produce better parts, in less time, with less waste and greatly reduced emissions.
By moving to Closed Mold you will:
- Find new solutions to meet MACT standards
- Make better parts more consistently
- Obtain more consistent parts trimming
- Reduce labor costs
- Reduce overall costs by making parts faster with fewer molds
- Use improved processes resulting in less operator control
- Reduce protecting clothing and equipment
- Reduce post work
With Closed Mold you will have a cleaner, more compliant and more productive manufacturing facility. And with a more pleasant workplace, you will also have happier employees.
At Composites One we offer a complete line of products and mold construction materials. Whether just getting started with the Closed Mold Processes or already enjoying the benefits of this technology, we’ve got all the products you need, when you need them!
Education & Training
As a process technology Light RTM, CCBM and VIP offer enormous potential for quality and productivity improvements over the open molding or similar composite molding processes. However, the use of these technologies will remain below its optimum potential to serve the industry unless you receive an appropriate training on how to use them.
Composites One offers educational opportunities, on-site training plus all the assistance you need to get started! We also organize demo days – daylong demonstrations that will give you a unique opportunity to learn more about these processes.
Several acronyms describe the different methods to deliver and distribute the resin. A common goal of all systems is to get the resin to its final destination in the faster and easiest way possible. A general distinction is that:
- Any system that uses higher-than-atmospheric pressure to drive resin into the mold cavity is RTM (Resin Transfer Molding).
- Any process that uses a lower-than-atmospheric pressure to drive resin is VIP (Vacuum Infusion Molding).
Think of it this way:
- If a pressure pump provides the motive force to the resin, the process is RTM.
- If a vacuum pump provides the motive force in moving resin into the mold cavity, the process is VIP.
For those using CCBM (Closed Cavity Bag Molding), on the other hand, either a pressure pump or a vacuum pump can be applied to move the resin into the mold cavity, resulting in greater flexibility.
Light RTM is a vacuum-assisted RTM (Resin Transfer Molding) low-pressure resin injection system. The peripheral vacuum is used as a clamping mechanism, while positive pump pressure, in conjunction with a controlled cavity vacuum, produces consistent parts.
The RTM concept is to inject the mixed resin and catalyst into a closed mold containing a fiber pack. Once the resin has cured sufficiently, the mold can be opened and the part removed.
The vacuum pump supplies two vacuum levels – a full vacuum for mold closure and a partial vacuum to assist resin flow.
The cycle time for a Light RTM part can range from 45 minutes to several hours. It is faster than hang lay-up or spray-up open molding. However, temperature controlled tooling can decrease cycle times substantially.
Advantages of Light RTM
Light RTM results in significantly lower styrene emission (the only VOC’s generated are from gel coat application), more attractive reproduction on parts (they will have good cosmetics on both sides), better part consistency, larger production runs, less need for operator control and reduced labor costs.
Light RTM vs. Standard RTM
Light RTM offers lower cost mold production with lower volume production runs per part, while standard RTM, which requires a high volume of parts, results in higher tooling costs. Light RTM mold costs are basically half the price of equivalent standard RTM. The process provides molders an attractive introductory route into closed mold production.
The advantages of closed molding for either Standard RTM or Light RTM are that they offer working environments that are far more comfortable and environmentally friendly than in open molding. Even though it is true that Light RTM will not meet the production rates that are enjoyed in traditional RTM, it will provide more cost effective production.
The simplicity of the system means that it is possible to start production without major investments. The equipment list to get started is rather short and includes: a vacuum pump and an injection machine. Basic injection equipment can be customized and automated which will reduce operator labor cost.
As with any composite closed mold production technique, Light RTM demands high quality, accurate composite molds in order to provide good mold life and consistent production of good parts. This is a key element to successful molding and is a common oversight for those who venture into Light RTM on their own.
It has often been implied that building Light RTM molds does not require the expertise or accuracy that is demanded for the construction of Standard RTM molds. However, the temperature resistance and the accuracy are equally important criteria for building successful Light RTM molds. Building successful tooling requires specific Light RTM tool making experience.
Light RTM involves a mold and a counter mold. Molds must be designed and engineered to accept vacuum channels and resist pump pressure within the mold cavity. Injection ports and vents must be correctly located. The counter mold is semi-rigid and its lightweight makes it easy to handle. The gel coat and the resins used should have an HDT of at lest 250° F (121° C). The cavity should be made with a high temperature calibrated wax.
In Light RTM, part prototyping is important for:
- Insuring the correct thickness of the cavity between the mold and the counter mold.
- Considering overlaps of the reinforcements when constructing the counter mold. This should be the first step for determining part thickness prior to construction of counter molds.
Plant Layout & Application Sequence
Light RTM typical plant layout is in a circular configuration and includes the following steps:
Closed Cavity Bag Molding (CCBM) is a patented vacuum infusion system. It utilizes a durable, reusable silicone vacuum bag. The reusable, repairable bag has an expected life cycle of 1,000 parts.
Vacuum bagging has been reserved for high tech industries such as aerospace manufacturing and specialty products. However, Closed Cavity Bag Molding is the faster process to implement today. It reduces material waste and labor costs while improving part quality.
CCBM works as follows:
- Adding an 8-inch wide flange for vacuum clamping modifies a new or existing open contact mold.
- A form fitting reusable silicone bag is made to reproduce the inside contours of the part.
- The mold is gel coated and then dry reinforcement is placed into the mold.
- The CCBM bag is fitted over the reinforcement and sealed to the mold by a peripheral vacuum clamping assembly.
- Vacuum is applied to the vacuum clamping assembly to seal the bag to the mold and evacuate air and volatiles from reinforcement load in the mold.
- A specific volume of resin is catalyzed and injected through special ports in the bag into the reinforcement load.
- The resin fills the evacuated space between the reinforcement fibers. Resin cannot escape the mold cavity because of special vents that only allow air to pass through.
- After gelation, the bag is removed and the part demolded.
- There is no clean up of the bag or mold required and the cycle can begin again immediately.
Advantages of CCBM
As Light RTM, CCBM results in significantly lower styrene emissions, increased production of parts, increased production rates and reduced labor costs.
Other advantages are:
- Reusable, repairable bags
- Existing molds easily converted
- No resin waste
- High glass to resin ration
- Conversion time
- No disposable components
- Excellent part-to-part consistency
- Weight reduction
- Suitable for all part sizes: The limiting factor for part size is simply the ability to handle the bag.
- High complexity parts: Negative draft and very intricate shapes are easily made using the CCBM process. Multi pieces split molds can also be accommodated. Insert, bosses and cores are integrally molded at the same time.
- Offers a competitive edge
- Offers a better-looking product
- Offers higher profitability and lower manufacturing costs
- Improves the working environment by reducing VOC emissions
- Reduces consumable shop supplies
- Help comply with EPA regulations
Simple modification of existing molds is all that is required to convert to Closed Cavity Bag Molding. Usually this can be done in a very short time with minimal costs.
The CCBM process is not pressure or volume sensitive. It provides a high level of performance using a gravity feed, pressure pot delivery system or a resin injection machine. Usually existing resin equipment can be modified to give acceptable performance.
The edges of the mold are horizontal and 8-inches wide. It is therefore easy to transform an existing mold into an infusion mold; it is merely necessary to add horizontal flanges. NOTE: the complexity of the mold surface can have a critical effect on the final cosmetics of the parts made from it.
Plant Layout & Application Sequence
As for Light RTM, CCBM typical plant layout is in a circular configuration and includes:
Vacuum Infusion Process (VIP) is a process used to manufacture fiber-reinforced plastic (FRP) parts in which disposable films are applied over the laminate.
Dry materials are stacked onto a male or female mold surface and a thin plastic vacuum bag or semi rigid counter mold is sealed around the part perimeter. A vacuum pump is used to evacuate the air and apply atmospheric pressure to consolidate the dry materials and create a “vacuum cavity”. Resin is then introduced into the cavity via strategically placed resin feeder lines.
The pressure differential between the cavity and the outside atmospheric pressure pushes the resin through the porous materials until the part is completely saturated. The vacuum is maintained until the part the part cures to ensure consolidation.
The filling time for a VIP part is determined by the following elements:
- Viscosity of the resin: According to Henry D’Arcy equation for aqueous flow through porous media , viscosity is inversely proportional to the resin flow speed, or the more viscous the resin, the more time it will take to saturate the part. For optimal results, viscosity should range between 100 and 300 cps.
||Equation: D'Arcy Porous Flow
||Viscosity – Resin Viscosity
||Permeability of the Medium – Flow Medium, Fiberglass.
||Volume-Average Fluid Velocity – Flow Speed.
||Pressure Gradient or Pressure Differential Between Atmospheric and Inside the Cavity.
- Porosity/permeability of the reinforcements: The more permeable (higher ? value) the materials are, the faster the resin will flow. Flow medias are essential for proper infusion, unless a scored or grooved core is used to transport the resin.
- Applied pressure difference: As for permeability, the higher the pressure differential is, the faster the resin will flow. Full vacuum must be achieved for optimal results.
- Flow distance: It should not be greater than 54 inches.
The temperature of the resin must be 72° F (22° C) or higher for proper flow.
Therefore, to theoretically optimize an infusion process, the resin needs to be very thin (low viscosity), the materials need to be very permeable, and the pressure differential needs to be as high as possible.
The cycle time for a VIP part is generally slower than hand lay-up or spray-up open molding. It depends on size of the part and how fast resin can be pulled into the mold. A long infusion time requires a long gel time resulting in a long cure cycle.
Advantages of VIP
The Vacuum Infusion Process (VIP) is a superior method for construction of composites parts. Parts produced using this method are stronger, lighter, and cheaper to produce. In addition, quality control and quality assurance issues are much easier to deal with than with hand laid parts. Inspections can be easily carried out before the resin is introduced into the part, and with the use of clear gel coat, the part is very easily examined for flaws after it has been infused. Furthermore, the process is very environmentally friendly. Volatile Organic Compounds (VOCs) and Hazardous Air Pollutants (HAPs) are drastically reduced. This also means that the working environment is greatly improved. VIP also allows unlimited setup time because the resin in not catalyzed until all the materials are in place.
Today, VIP is drawing current interest because of its low capital investment and easily manageable learning curve.
Other benefits are:
- The vacuum bag evenly applies pressure, conforming to both simple and complex shapes.
- Application of vacuum provides control of part thickness by compressing the laminate during cure.
- Application of vacuum removes air, excess resin and volatiles resulting in a stronger laminate.
The developments in Vacuum Infusion Processing during the last decade involve advances in introducing resin into the vacuum bag. The use of well designed resin distribution manifold systems and the introduction of flow media in the vacuum bag has allowed greater control over the transport of resin into the bag. Additionally, the use of a flow media on the laminate surface or of an incorporated inflow media inside the laminate, allows faster and more efficient reinforcement saturation. Also see CCBM method.
The equipment list to get started is short and includes: a vacuum pump, a leak detector, a thermometer, a few resin collectors, clamps, hoses, vacuum gauges and the usual vacuum bagging supplies. However, the major cost for a larger shop is a good fail-safe vacuum system with two pumps, filter, reservoir and permanent vacuum lines throughout the shop.
VIP tooling can be virtually identical to standard open molds. An adequate perimeter flange is required to mount the vacuum bag. Building tooling requires no special expertise other than open mold fabrication. However, in case of pre-existing molds, the biggest cost associated with VIP conversion will be the mold modification and the overhaul of the plant layout to adapt it to the VIP process.
As for the tooling types, typical open molding type tooling is adequate. Polyester tooling gel coat/polyester laminate molds are also widely used. Epoxy tooling is an option. Flat panels may be molded from vacuum tight metal surfaces. Vacuum source is required. Plant vacuum system is needed for serious processing.
Plant Layout & Application Sequence
VIP typical plant layout is in an assembling line configuration and it includes:
There are different ways to learn the technology of VIP. You can spend money on licenses and training, or you can experiment in a corner of the shop. A VIP base course usually lasts one week, but it takes two or four weeks to train the average fiberglass worker to a point where he has production-ready skills. A training budget may vary. Depending on the chosen system, the technology can be open or may require licensing.
Other Closed Mold Processes
Other Closed Mold Processes such as Resin Transfer Molding (RTM), Compression Molding, Injection Molding, Filament Winding and Pultrusion are being used to manufacture composite products in a variety of industries.
Resin Transfer Molding (RTM) is a relatively low-pressure vacuum (100 psi) process that molds near complete shapes in 30-60 minutes. It results in two Class A finished sides, providing the highest quality surface technically achievable.
The process uses fiberglass or electroformed molds capable of producing up to 10,000 parts. Gel coat is first applied to one or both mold halves. Continuous or chopped strand mat and the core (if used) are then placed into the bottom half before the mold is closed.
Resin transfers into the mold through injection pressure, vacuum pressure or both. Cure temperature depends on the type of resin used (polyester, vinyl ester, phenolic or epoxy at 350-450 cps viscosity). Heater blankets can be used to heat the mold up to 395º F. Sensors in the mold detect resin flow position and monitor resin gel and cure data.
Compression molding is a cost-effective process for higher production runs. It uses forged steel dies capable of turning out up to 200,000 finished parts. While more expensive, these longer-lasting dies pay for themselves when production quantities exceed 10,000 parts.
Faster cycle times and lower unit cost are possible when sheet-molding compounds (SMC) are used in the process. Fiberglass thermoset SMC cures in 30-150 seconds, and overall cycle time may be as low as 60 seconds. Major SMC markets worldwide include high-volume electrical components and the transportation industry.
The SMC sheet consists of chopped fiberglass sandwiched between two layers of resin paste. A metering device called a “doctor box” transfers a layer of resin paste onto a moving film carrier. Chopped glass fibers are dropped onto the paste, followed by another layer of resin from a second film carrier. Rollers compact the sheet, saturating the glass with resin, while squeezing out trapped air. From its original molasses-like state (20,000-40,000 cps), resin paste thickens to the consistency of leather (about 25 million cps), its ideal molding viscosity, within three to five days.
Ready for molding, the SMC is cut into sheets and the “charge pattern” (ply schedule) is assembled on a 250-325º F heated mold. The mold is closed, clamped, and applied with pressure at 500-1200 psi. Material viscosity drops, letting the SMC flow, filling the mold cavity. After the resin cures, the mold is opened and the part can be removed manually or by using integral ejector pins.
Different molding pressures are used for different types of material. Each two square inches of area on a part requires one ton of force. The formula for determining press tonnage is: Area (inches) x psi molding pressure ÷ 2000 = Press tonnage required
For example, a 50-inch x 30-inch part (1500 inches) molded at 1000 psi requires a 750-ton press. Molding pressure can be greatly reduced by using a low-pressure molding compound (LPMC) requiring as little as150 psi.
Injection molding is a fast, high-volume, low-pressure (5000-12,000 psi) process capable of producing up to 2000 small parts per hour. Over the past 20 years, it has replaced traditional thermoplastic and metal casting processes previously used for manufacturing electrical, automotive, appliance and other products in a variety of industries.
The process uses thermoset bulk molding compounds (BMC), a low-profile formulation of thermoset resin with 15-20% chopped fiberglass. Very thick when mixed, BMC loses viscosity and liquefies when heated to curing temperature.
A ram or screw-type plunger forces a metered “shot” of BMC through the machine's heated barrel and injects it into a closed, heated mold. Liquefied BMC flows easily along runner channels into the mold’s forming cavity. Heat build-up is carefully controlled to minimize curing time. After cure and ejection, parts need only minimal finishing.
Parts with thick cross-sections can be compression molded or transfer molded. Transfer molding, like injection, is a Closed Mold process in which a measured amount of BMC is placed in a “pot” with runners leading to the mold cavities. A plunger forces the material into the cavities, where the product cures under heat and pressure.
Filament winding is an automated, high volume process used to manufacture products with cylindrical shapes such as pipes, tanks, shafts, tubing and pressure vessels. Machine sophistication varies from basic two-axis mechanical chain-drive operation, to computer-controlled multi-axis and multi-spindle systems.
A winding machine pulls dry fiberglass through a resin bath and around a mandrel. To ensure part performance, all fibers must be wound using the same tension. Resin is typically worked into the fibers by roll coaters or breaker bars in dip tanks.
As the mandrel rotates, the roving delivery system (“feedeye”) reciprocates along the length of the mandrel. Reciprocation speed and rotation are synchronized to hold a pre-set winding angle, usually 7- 90º. Optional filament winding axes of motion enable the feed eye to move into a perpendicular position relative to the mandrel axis, rotate 360º, and perform a yaw pivot. Patterns can get quite intricate, such as with computer numerically controlled machines providing up to 11 axes of motion for single or multiple spindles.
The cycle time for using E-glass reinforced epoxy to produce a compressed natural gas pressure vessel measuring 75 inches long by 15 inches in diameter is one hour for winding and 2.5 hours for curing at 250º F.
The winding action compacts the laminate, eliminating the need for vacuum bagging or other compaction methods.
A continuous, automated Closed Mold process, pultrusion is cost-effective in high volume production runs of constant cross section parts. Pultruded custom profiles and standard shapes (channels, angles, beams, rods, bars, tubing and sheets) have penetrated virtually every market.
The process relies on a reciprocating or caterpillar-type puller/clamping system to pull the fiber and resin continuously through a heated steel die. Fiber roving is wet out in an open resin bath, and pulled under shaped bushings that squeeze out excess resin. The compacted package then enters a heated steel die. After curing, the part is pulled from the die and into a saw at the end of the machine, where it is cut to a preset length. Computer controls are used to synchronize puller and saw motions and speed.
Alternative wet-out systems inject the resin directly into the heated die. This eliminates the need for compaction tooling, significantly reduces emissions from open resin baths and simplifies clean up.
Multiple streams can be pultruded in a single die with several cavities. To form hollow or multiple-cell parts, the material wraps around heated mandrels that extend through the die. If off-axis structural strength is required, mat and/or stitched fabrics may be folded into the material package before it enters the die. This can add substantial loads to the machine frame and pulling system, however. Hydraulically driven systems are capable of pulling up to 100,000 lb. structural profiles measuring 103-inches wide by 15 inches deep.
Heat control is a critical part of the pultrusion. For that reason, controllers are available to monitor and maintain a pre-set temperature in various zones throughout the die and mandrels. For all-roving pultrusion, a radio frequency (RF) generator can be used to accelerate cure by pre-heating the wet material package. RF energy initiates cure before the material enters the die, reducing cure time and increasing running speed. For example, the running speed for all-roving 1/2-inch rod is about 21 inches per minute. With RF pre-heating, running speed can increase to 10 feet per minute. RF is not generally recommended where mat and/or fabrics are pultruded.
Pultrusion typically uses fiberglass and thermoset resins such as polyester, vinyl ester, epoxy and phenolic. It can also be done using thermoplastic composites, which bring strength, toughness, reformability and reparability to the finished part. The process essentially reverses thermoset pultrusion, using heat to soften the thermoplastic, and cooling to harden it.