Resin casting is a plastic casting process where liquid synthetic resin is filled in mold and allowed to harden. Traditionally this process is chiefly used for small- scale production such as industrial prototypes and dentistry products. It can also be used by amateur hobbyists and manufacturers to create collectible toys, scale models, models of objects, figurines, and small-scale jewellery production. Casting resins are relatively fast-curing and very easy to use.
The development of various types of composites has increased the demand for resin casting. Lightweight composites are widely used in marine, automotive, aerospace sectors, etc.
The simplest process for resin casting is gravity casting. In this process, the resin is poured into the mold and pulled down into all the parts by gravity. When the resin is mixed, air bubbles may be created in the liquid, this can be removed in a vacuum chamber. The casting can also be performed in a vacuum chamber especially when using open molds, to extract the bubbles. It can also be performed in a pressure pot to reduce the size of the air bubbles to the point where they are not visible. Finally, pressure and/or centrifugal force can be used to push the liquid resin into the fine details of the mold.
Today casting resins are being used to create water clear plastics and water-based acrylic casting materials with mechanical properties matching those of engineering plastics due to the advancements in cast plastic chemistry. Vacuum degassing and even pressure casting can be used to create bubble free castings as well.
Types of Casting Resins for Composite Manufacturing
There are several types of casting resins available in the market:
- Polyurethane casting resins – are used along with a silicone rubber mold to produce precise plastic castings of an original part or a rapid prototype. These resins have excellent thermal stability, ultra-low viscosity, high heat resistance, and can be easily pigmented to attain a wide range of colours. They are capable of reproducing very unique surface details. It is comparatively inexpensive, and can be cost effectively used even for larger castings. These are also very machinable and very easy to de-mold.
- Water clear polyurethane casting resins – These are high performance, ultra-clear casting resin that can be used in clear casting, rapid prototyping, and object embedding/encapsulation. They are polishable to a high gloss and are UV stable.
- Water clear polyester casting resins – These are suitable for object embedding, clear sculpture casting, jewellery making and set design.
- Aluminium-filled epoxy casting resins – These are designed for high temperature tooling applications such as vacuum forming tools. They are known for their very hard-wearing property.
Composite Casting Resin Materials
- Acrylic – There are several types of acrylic resins. For example, the methyl methacrylate type of synthetic resin is used to manufacture acrylic glass such as Plexiglass, which is more of a plastic polymer rather than glass. These resins are ideal for embedding objects.
- Epoxy - Epoxy resins have a lower viscosity than polyurethane resins. These are polyether resins containing more than one epoxy group. They are capable of being converted into the thermoset form.
- Unsaturated polyester - Unsaturated polyester resins are manufactured by condensation reaction between acids like phthalic anhydride, maleic anhydride, isophthalic acid, and glycols (propylene glycol, di-ethylene glycol, mono-ethylene glycol). These are general purpose resins that are used for reinforced plastic applications.
Applications in Composite Materials
The following are the major application areas of composite casting resins:
- Artificial limbs and other related applications
- UV stabilised specially modified resin for translucent sheets
- Encapsulation potting for chokes and transformer for electrical insulation applications
- Pultrusion applications
- Vacuum forming and other repetitive pressing/molding applications
- Vacuum forming male tools
- Platen press tools
- Garage kits
- Applications in which extreme clarity and absence of color are required
- Decorative and artistic applications.
Sources and Further Reading