Volumes of production
The process in all these methods is always slow due to the labor involved. However, the nature of the spray-up method makes it faster than the hand lay-up process.
Unit price vs. capital investment
Tooling can be inexpensive but the time taken to form parts makes them expensive to mass-produce as a high-volume process.
Speed
Depends on the type of hand lay-up technique and the size of the molding. Spray lay-up is faster, but larger areas mean the unit speed is not always quicker.
Surface
The reverse of moldings will have the fibrous texture of the reinforcement. Gel coats can be applied to the mold to enhance the components’ surface finish. Other thermoformed skins can be applied in a secondary process for a superior surface. Vacuum-bag and pressure-bag methods allow much greater surface detail.
Types/complexity of shape
All methods are limited to open shapes with fairly thin cross-sections. Only slight undercuts are possible, depending on how far the component can be flexed when removing it from the mold.
Scale
As big as you want. Hand lay-up allows a much thicker wall thickness to be built up than spray lay-up, which reaches its maximum at about 5/8 inch. The scale of components using the bag methods is limited only by the size of the bags.
Tolerances
Due to shrinkage, tolerances are hard to control for all methods.
Relevant materials
Reinforcement materials include advanced fibers such as carbon, aramid, and glass, and natural materials such as jute and cotton. Polyester is the most widely used thermosetting resin; others include epoxy, phenolic resin, and silicone. Thermoplastics are far less cost-effective.
Typical products
General glass-reinforced plastic (GRP) items such as boat hulls, car panels, furniture, bathtubs, shower pans, and cheap seats on the decks of small Greek ferries.
Similar methods
Transfer molding (p.176) can achieve a similar strength. Gas-assisted injection molding (p.201) and reaction injection molding (p.199) can be used to create large parts, but without the strength. Other alternatives include vacuum infusion (VIP) (p.154), filament winding (p.158), and autoclave molding (p.156).
Sustainability issues
All the processes are largely labor assisted and use a fairly low amount of energy. Use of natural fibers minimizes the use of nonrenewable materials. Composites are difficult to recycle at the end of their life. Yet their excellent strength and rigidity ensures a prolonged lifespan.
Further information
www.compositetek.com
www.netcomposites.com
www.compositesone.com
www.composites-by-design.com
www.fiberset.com
Vacuum Infusion Process (vip)
The vacuum infusion process (VIP) is a method of forming composites that achieves density and strength in the end product by sucking the resin and reinforcement fibers together into a dense, solid mass. In essence, it is an advanced form of contact molding (see p.152) and, compared with similar techniques for forming composites, it is a clean and highly effective process through which the two main ingredients can be combined in a single step.
In traditional hand lay-up methods in contact molding, the reinforcing fibers are laid over a mold before the liquid resin is brushed or sprayed into it. In the VIP process, the dry parts of the material are stacked up over a mold. This is then covered with a flexible sheet and a seal is formed between the sheet and the mold. The air is pumped out from inside, forming a vacuum, and the liquid polymer resin is then fed into the fibers. The action of the vacuum means that the resin thoroughly impregnates the dry material, which gives the final component its density and strength.
1 A boat hull being covered in a flexible plastic sheet, ready to be sealed prior to the application of a vacuum.
2 The sheet is inspected to make sure it is completely sealed.
3 The vacuum pumps that suck the air from between the sheet and the hull.
– Economical use of resin due to the efficient ratio of fiber to resin.
– Clean.
– Eliminates air pockets.
– Higher strength-to-weight ratios than contact molding (see p.152).
– Complicated setup.
– High degree of trial and error.
– High failure rate.
Volumes of production
This is a slow production method that relies on the luxury of a fairly long setup time to build the part.
Unit price vs. capital investment
VIP can be used in a small workshop with basic equipment, which can be purchased from various suppliers. However, it requires a lot of trial and error, and you may suffer a high failure rate.
Speed
Slow.
Surface
Gel coats can be applied to provide the parts with a high surface finish.
Types/complexity of shape
A common application for VIP is in the manufacture of boat hulls, which should give you an idea of the level of its complexity and its scale.
Scale
The process is suited to large parts. It is difficult to make anything smaller than around 12 by 12 inches, because the fiber needs to be draped over or inside the mold.
Tolerances
Not the kind of process for high tolerances.
Relevant materials
As in any plastic composite method, typical resins used are polyester, vinyl ester, and epoxy, combined with reinforcements such as fiberglass, aramids, and graphite.
Typical products
Propellers, marine components, and equipment such as a stretcher used in rescue operations, which features an aluminum frame over-molded with a vacuum-infused composite.
Similar methods
Contact molding (p.152), transfer molding (p.176), and autoclave molding (p.156).
Sustainability issues
This process is often carried out on a large scale, and its high error and failure rate results in increased waste, most of which cannot be reused. However, the fibers and resin are laid by hand which helps to balance out the high amount of energy required to heat the resin and power the vacuum. Furthermore, the vacuum ensures that only the minimum amount of resin is introduced to the fibers; any excess is sucked out, which reduces material consumption and increases the strength of the product.
Further information
www.resininfusion.com
www.reichhold.com
www.epoxi.com
Autoclave Molding
Advanced composite materials have applications across a range of industries, from premium branded sports products to engineering components. These materials offer superior strength in a lightweight molding. However, the combination of the two distinctly different ingredients (various fibers and polymer resins) in advanced composites provides manufacturers with a challenge. They must find new ways to bring these raw materials together in a cost-effective manner that is suited to industrial production. The use of heat and pressure is a very common element within manufacturing. In autoclave molding the combination is used to compact the raw materials together to offer the highest level of strength.
Autoclave molding is a modified form of pressure-bag forming (see contact molding p.152)—the composite is formed in what is essentially a pressure cooker. As a result of the applied pressure, it is one of the methods of forming advanced composite components with particularly high density. The process begins with the application of reinforcing fibers and resin onto a mold, which can be achieved through a variety of methods, such as hand or spray lay-up techniques (see contact molding, p.152). A flexible bag is then placed, a little bit like a quilt, over the surface and the whole thing is placed in an autoclave (a sealed chamber), where heat and between 50 and 200 psi of pressure are applied, forcing the bag to squeeze itself into, or around, the mold, compressing the resin and fibers together. This forces out any potential air gaps and allows for a relatively fast curing time, compared with hand or spray lay-up. It is the squeezing together of materials under pressure, with the application of heat, that gives the final component a very high density.
– Increased density, faster cure times, and void-free moldings compared with molding methods that use neither heat nor pressure.
– Potential for molded-in color.
– Suitable only for making hollow parts that have thick, dense walls.