Composite Fabrication Methods

July 26, 2016

Various methods of composite manufacture offer various benefits – some methods allow for large amounts of production in a short time while others offer low startup and materials costs. An engineer must make a smart choice when determining which composite fabrication process will yield the best results.

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APP's CryoTek Pipe Shoe is manufactured using the vacuum-assisted resin transfer molding process.

Most methods of composite manufacture involve some type of molding to shape the resin and reinforcement into the desired finished shape. Care must be taken when choosing a composite fabrication method, as not all methods are equally suitable for all applications.

Hand Layup

The most basic method for fabricating thermoset composites is the hand layup method. In this process, dry fabric layers (or “plies”) are laid by hand to form a laminate stack and then resin is applied once the stack is completed. In a variation of this method, known as wet layup, the plies are coated with resin before it is laid, and then the stack is “debulked” or compacted afterwards.

Open Contact

Open contact molding in one-sided molds is similar to the hand layup method, but is quicker because the catalyzed resin and reinforcing fibers are sprayed into the mold before the laminate is compacted by hand using rollers. Core material can then be added and a second laminate skin can be added to form a sandwich structure composite. However, this method, also known as sprayup has begun to fall out of favor because it is difficult to limit workers’ exposure to and emission of the harmful styrene used in the resin.

Resin Transfer Molding (RTM)

Resin Transfer Molding (RTM), on the other hand, allows for faster production rates and makes use of a two part, matched, closed mold. First, the dry reinforcement fibers are inserted, and then a low viscosity resin and catalyst are pumped into the mold at low pressure. This type of fabrication method offers a myriad of benefits because the dry preforms and resins for this process are quite cheap and can be stored at room temperature, this process eliminates most post fabrication work, and this process is significantly faster than layup and sprayup techniques.

Vacuum-Assisted Resin Transfer Molding (VARTM)

A more recent, and fast-growing, development is Vacuum-Assisted Resin Transfer Molding (VARTM) in which the resin is drawn into the mold solely under the influence of a vacuum, and not pumped in under pressure. This process can efficiently produce large, complex parts in one shot, which is how APP's CryoTek Pipe Shoe is made.

Resin Film Infusion (RFI)

Resin Film Infusion (RFI) involves a dry preform placed in a mold on top of a layer of high-viscosity resin film. Then, under heat, vacuum, and pressure, the resin is liquefied and drawn into the preform, causing the resin to be uniformly distributed. This happens in spite of the high-viscosity of the resin because of its very short flow distance.

Compression and Injection Molding

Common methods used in high volume manufacture are compression molding and injection molding. Compression molding uses expensive, but very durable, metal dies which make this method a good choice when production will require thousands of parts. This process begins with a sandwiched composite sheet material that is assembled on a heated mold which is then closed and clamped. The material’s viscosity drops, and the composite flows to fill the mold cavity. In contrast, injection molding is fast, high volume, and low pressure. The process is similar to compression molding, only instead of beginning with a sandwich composite which is compressed to fill the mold, injection molding begins with resin that is then injected into the mold through ports. This process has the benefit that after cure and ejection, it needs minimal finishing.

Various methods of composite manufacture offer various benefits – some methods allow for large amounts of production in a short time while others offer low startup and materials costs. An engineer must make a smart choice when determining which composite fabrication process will yield the best results.

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