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Chopper Gun Applications

Applications include making of custom parts in low to medium volume quantities. Bathtubs, swimming pools, boat hulls, storage tanks, duct and air handling equipment, and furniture components are some of the commercial uses of this process.

The basic reinforcement material for this process is glass-fibre rovings, which are chopped to a length of 10 to 40 mm and then applied on the mould. For improved mechanical properties, a combination of fabric and chopped fibre layers is used. The most common material type is E-glass, but carbon and Kevlar rovings can also be used. Continuous strand mat, fabric, and various types of core materials are embedded by hand whenever required. The weight fraction of reinforcement in this process is typically 20 to 40% of the total weight of the part. The most common resin system used for the spray-up process is general purpose or DCDP polyester; isophthalic polyesters and vinylesters are also sometimes used. Fast-reacting resins with a pot life of 30 to 40 minutes are typical. The resin often contains a significant amount of filler. The most common fillers are calcium carbonates and aluminium trihydrates. In filled resin systems, fillers replace some of the reinforcements; 5 to 25% filler is used by weight.

Steel, wood, GRP, and other materials are used as mould materials for prototyping purposes. The mould can be a male or female mould. To make shower bathtubs, a male mould is used. In the boating industry, a single-sided female mould made from FRP (fibre-reinforced plastic) is used to make yacht hulls. The outer shell of the mould is stiffened by a wood frame. The mould is made by taking the reversal of a male pattern. Several different hull sizes can be made using the same mould. The length of the mould is shortened or lengthened using inserts and mould secondaries such as windows, air vents, and propeller shaft tunnels.

Processing Requirements

The processing steps are very similar to those in hand lay-up. In this process, the release agent is first applied to the mould and then a layer of gelcoat is applied.

The gelcoat is left for two hours, until it hardens. Once the gelcoat hardens, a spray gun is used to deposit the fibre resin mixture onto the surface of the mould. The spray gun chops the incoming continuous rovings (one or more rovings) to a predetermined length and impels it through the resin/catalyst mixture. Resin/catalyst mixing can take place inside the gun (gun mixing) or just in front of the gun. Gun mixing provides thorough mixing of resin and catalyst inside the gun and is preferred to minimize the health hazard concerns of the operator. In the other type, the catalyst is sprayed through two side nozzles into the resin envelope. Airless spray guns are becoming popular because they provide more controlled spray patterns and reduced emission of volatiles. In an airless system, hydraulic pressure is used to dispense the resin through special nozzles that break up the resin stream into small droplets which then become saturated with the reinforcements. In an air-atomized spray gun system, pressurized air is used to dispense the resin.

Once the material is sprayed on the mould, brushes or rollers are used to remove entrapped air as well as to ensure good fiber wetting. Fabric layers or continuous strand mats are added into the laminate, depending on performance requirements. The curing of the resin is done at room temperature. The curing of resin can take two to four hours, depending on the resin formulation. After curing, the part is removed from the mould and tested for finishing and structural requirements.

Manufacturing Process
The mould is waxed and polished for easy removal.
The gelcoat (a smooth, hard polyester resin coating) is applied to the mould surface and is given time to cure before re-applying.
The barrier coat is applied to avoid fibre print and a rough surface through the gel coat.
The barrier coat is oven cured, then brought to room temperature.
After curing, Calcium Carbonate and Aluminium Trihydrate fillers are added using a high shear mixing unit.
A wax-like additive is added into the resin to reduce styrene discharge by 20% during lamination.
A fibreglass chopper is mounted on the spray gun.
The mixture of catalyst, resin and fibreglass is then sprayed evenly in a fan-like pattern to assure even coverage.
A roller is used for compaction after each layer has been applied, this removes trapped air.
Where desirable, wood, foam, or honeycomb cores are embedded into the laminate to create a sandwich structure. Corner and radius coverage is also checked.
The part is then cured in an oven and left to cool to room temperature.
The mould is removed and is ready to be waxed and polished for the next manufacture cycle.
Finishing is done by trimming edges of excess fibreglass and drilling holes that are needed.
After this the part is assessed by quality control personal; it is weighed, structurally checked and surface finish is inspected prior to packing and shipping.

It is a very economical process for making small to large parts.
It utilizes low-cost tooling as well as low-cost material systems.
It is suitable for small- to medium-volume parts

Vacuum Assisted Resin Transfer Molding (VARTM) or Vacuum Injected Molding (VIM) is a closed mold, out of autoclave (OOA) composite manufacturing process. VARTM is a variation of Resin Transfer Molding (RTM) with its distinguishing characteristic being the top portion of a mold tools use of a vacuum to assist in resin flow. The process involves the use of a vacuum to facilitate resin flow into a fiber layup contained within a mold tool covered by a vacuum bag. After the impregnation occurs the composite part is allowed to cure at room temperature with an optional post cure sometimes carried out.

Typically, this process uses a low viscosity (100 to 1000 cP) polyester or vinyl ester resin along with fiberglass fibers to create a composite. Normally the process is capable of producing composites with a fiber volume fraction between 40-50%. The resin to fiber ratio is important for determining the overall strength and performance of the final part, with mechanical strength being most influenced by the type of fiber reinforcement. The type of resin used will primarily determine the corrosion resistance, heat distortion temperature, and surface finish. Resins used in this process must have low viscosities due to the limited pressure differential provided by the vacuum pump. High performance fibers, such as carbon fiber, can also be used. However, their usage is less common and is mainly for the fabrication of high end parts.

This process offers the benefit of not requiring an expensive autoclave while also being capable of producing large, complex aerospace-grade parts. Products produced using this method vary widely in their application with parts being used in transportation, wind energy, marine, infrastructure, and aerospace applications. The process’s ability to create large and complex parts has allowed it to effectively reduce manufacturing costs when utilized to produce parts that are traditionally constructed of numerous small components. For instance, LOCKHEED Martin Space Systems (LMSS) experienced a manufacturing cost saving of up to 75% when it began to produce the quarter section of the equipment bay for the Trident II D5 missile using VARTM.

Hand laid up Fiberglass.  Hand laying up fiberglass is a process close to spray up. However instead of using spray up processing equipment the fabricator uses highly skilled hand techniques. Once the gel coat is either sprayed or brushed the laminate will  be cut to size. Most production will use  glass, caron, graphite, or a fabric of his choice. The fabric will then be wet out using Iso tooling, polyester, vinylester, or epoxy resins. Typically squeegees or rollers will be used to work the now wet fiber into the cavity of the mold.

The hand lay up process is a lot slower than the spray up process but tends to be better controlled. The laminator can easily control thickness, cure time, and resin to glass fiber ratio making a higher quality part. Typically in high FRP production shops the hand lay up process is reserved for mold making in which the molds need to be high quality to last the lifetime of production.