part quality. The most promising area is in mold pressurization. It is well known that applying a small amount of pressure internally to the mold at the correct point in the heating phase accelerates coalescence of the polymer particles during the melting, producing a part with fewer bubbles in less time than at atmospheric pressure. This pressure delays the separation of the part from the mold wall due to shrinkage during the cooling phase, aiding cooling of the part. The main drawback to this is the danger to the operator of explosion of a pressurized part. This has prevented adoption of mold pressurization on a large scale by rotomolding manufacturers. Mold release agents A good mold release agent (MRA) will allow the material to be removed quickly and effectively. Mold releases can reduce cycle times, defects, and browning of finished product. There are a number of mold release types available; they can be categorized as follows: Sacrificial coatings: the coating of MRA has to be applied each time because most of the MRA comes off on the molded part when it releases from the tool. Silicones are typical MRA compounds in this category. Semi-permanent coatings: the coating, if applied correctly, will last for multiple releases before requiring to be reapplied or touched up. This type of coating is most prevalent in today's rotational molding industry. The active chemistry involved in these coatings is typically a polysiloxane. Permanent coatings: most often some form of polytetrafluoroethylene (PTFE) coating, which is applied to the mold. Permanent coatings avoid the need for operator application, but may become damaged by misuse. Materials More than 80% of all the material used is from the polyethylene family: crosslinked polyethylene (PEX), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and regrind. Other compounds are polyvinyl chloride (PVC) plastisols, nylons, and polypropylene. Order of materials most commonly used by industry: Polyethylene Polypropylene Polyvinyl chloride Nylon Polycarbonate These materials are also occasionally used (not in order of most used): Aluminum Acrylonitrile butadiene styrene (ABS) Acetal Acrylic Epoxy Fluorocarbons Ionomer Polybutylene Polyester Polystyrene Polyurethane Silicone Various foods (especially chocolate) Natural materials Recently it has become possible to use natural materials in the molding process. Through the use of real sands and stone chip, sandstone composite can be created which is 80% natural non-processed material. Rotational molding of plaster is used to produce hollow statuettes. Chocolate is rotationally molded to