RELEASE AGENTS: (Parting agents or mold release)
Regardless of the type of reinforcement or the resin used, the parts have a tendency to stick to the mold surface. Hence some type of release agent is always used to facilitate removal of the part from the mold. The release agent functions by forming a barrier film between the mold surface and the part. Various materials used as release agents can be classified as:
1. External release agents--applied to the mold surfaces.
2. Internal release agents--combined with the resin.
· EXTERNAL RELEASE AGENTS:
They can be further classified as--1) films, 2) film formers and 3) wipe-ONs.
It can be obtained in roll or sheet form. The film may be applied as a flat sheet cut to size or as thermoformed sheet made to conform to a 3-D contour e.g. Regenerated cellulose (Cellophane), PVA, Polyester, nylon, and Teflon films are usually used as flat sheets and PE, cellulose acetate, PVC and various fluorocarbon films can be easily thermoformed.
The film will satisfactorily prevent mold sticking and will usually stick to the part after curing so that it can be easily stripped off.
Temperature at which the part is to be cured should be well below the softening temperature of the film material.
They can be applied either by brush or spry gun on the mold surface. Most common materials are: cellulose acetate lacquers, PVA and nylon solutions, heat cured fluorocarbon coatings and possibly varnishes and/or automobile type paint fillers which can be sanded to a high gloss finish.
These materials are generally used over porous surfaces like wood or plaster of paris. They are also used where the surface are to be filled and smoothed to remove scratches, pits, or the grain pattern of a wooden form. They are frequently used along with a wax, silicone, or fluorocarbon type wipe ON material.
They are applied with a wiping rag (for certain waxes), a brush (for oily material) or a spray, usually in the form of an aerosol can. In most cases after the initial application the mold is wiped with a rag to smoothen the mold surface. Most common materials are:
WAX: There are number of wax preparations ranging from pure carnauba wax to various soft paste waxes similar to automobile waxes. The pure carnauba wax is generally applied to hot molds.
1) High gloss can be obtained if the material is correctly applied and polished.
2) The mold can be used immediately with no drying or curing period required.
Various Silicone materials are available in solution form and are wiped or brushed on. Some are air dried and others are heat cured. Silicones are also available in the form of aerosol solutions which are either air drying or heat curing.
The aerosol spray cans of fluorocarbon particles dispersed in a volatile solvent carrier have advantage like-- the excellent release properties and high temperature resistance. This allows them to be used with polymers which are curing at the highest temperature. However, the final gloss may not be as high as that obtained with paste wax.
In general, the use of wrong mold release can result in increased time for part removal or may result in damaged surface or parts due to mold sticking. A mold release should be chosen according to the post molding process being carried out.
· INTERNAL RELEASE AGENTS:
The chemical compounds used as internal release agents are added to the resin by the manufacturer to minimize the mold sticking after cure. Metallic soaps like zinc or calcium stearate are used for polyester and phenolic resins. Silicone oils and high melting point waxes are used in epoxies. Organo-phosphates and waxes are used in polyester resin. The internal release agents work by being squeezed or exuded on the surface during cure of the resin.