The compression or transfer molding process
There is a basic difference between thermoplastics and thermosetting resins which should be understood before the intricacies of the various molding methods are considered. Thermoplastic molding materials are usually supplied as polymers which soften when heated and harden when cooled. Thermosetting molding materials are supplied as partially polymerized materials which soften sufficiently to mold on initial heating, but which harden and cure (completely polymerize) when heating is continued. They cannot be softened by subsequent heating.
As a rule, thermosetting materials are molded by compression or transfer techniques, though in some cases thermoplastics may be formed by these techniques, vinyl phonograph records. Thermoplastic materials are usually molded by injection molding or by extrusion.
In compression molding, the granular or preformed molding material is loaded into a heated mold cavity. The mold is closed under relatively low pressure until the two mold halves exert pressure on the material. This application of heat and pressure softens the material and as higher pressure is applied, the material is forced into all parts of the mold. The mold remains closed and under pressure until the part is set or cured, after which the part is removed.
Four basic types of molds are used in compression molding. Though usually the selection of the type of mold should be left up to the molder,a general understanding of the types may be helpful to the designer or engineer.
- Fully positive molds are used where maximum density and superior electrical and physical properties are required. The full molding pressure is exerted directly on the material throughout the entire molding cycle. The mold plunger telescopes into the cavity, preventing escape of excess material. The mold is stopped by the compressed plastics material, determining the height of the piece and the thickness of the bottom. This technique is generally not recommended for multi-cavity molds.
- Landed positive molds are so constructed that the travel of the plunger is stopped at a predetermined point by metal lands. They are sometimes used to control the height and thickness of parts, as well as to compensate for variations in weight of charge. This is considered poor practice when maximum density is desired in the molded part.
- Semipositive molds contain such features as flash ridges, telescoping parts and external lands. As the mold closes, excess plastics material escapes between the flash ridges, equalizing differences in weight of charges. The subsequent telescoping action traps and compresses the material with direct pressure, helping to assure proper density of the part. When the mold is fully closed, some of the molding pressure is borne by the lands.
- Flash or overflow molds, less expensive than the other types, are used for molding flat or shallow parts, especially in multiple cavities. They are designed so that the mold halves register exactly. The plunger travel is regulated by areas of reduced size called flash ridges or cut-offs.
Developments in recent years have been toward more completely automating the compression molding cycle, and thus increasing production rates. Improvements have been brought about in mold design, in handling operations and in auxiliary press equipment, especially dielectric preheating units. Tooling： for compression molding is less expensive than that for transfer molding and is suitable for production of large parts.
In transfer molding, the granular or preformed material is loaded into a heated chamber connected to the mold cavity or cavities by sprues and runners. The beat and pressure applied to the outer chamber force the material through the sprue and runners into the heated, closed mold cavity or cavities. Heat is continued in the closed mold until the material has set or cured, after which the mold is opened and the part or parts removed.
Since the mold is closed initially, the amount of flash which must be removed is less than in compression molding. Also, more delicate mold pins ,more and inserts, and thinner sections, can be used. Transfer molding also provides material handling advantages in that only one preform, rather than several, is loaded for each mold cycle.