Thermosetting resins compression molding

An important step in the manufacture of any plastic product is the fabrication or the shaping of the article. Most polymers used as plastics when manufactured are prepared in pellet form as they are expelled from the reactor. These are small pieces of material a couple of millimeters in size. This resin can then be heated and shaped by one of several methods. Thermoset materials are usually compression molded, cast, or laminated. Thermoplastic resins can be injection molded, extruded, or blow molded most commonly, with vacuum forming and calendering also used but to a lesser extent. 

The PAI resin is utilized for small, simple parts. Because of its stiff melt flow, the resin is available in compression moldable powder. Grades include neat, filled, and glass-reinforced resins it is also available in solution for use as an enamel. Because of high prices ( 33—44/kg) the market is limited, estimated at 150—300 tons for the molding resin. Resins competing with PAI include PEI, polyethersulfones, and thermoset resins. 

The resin matrix can be either thermosetting or thermoplastic. Thermosetting resins such as epoxy, polyimide, polyester, and phenolic are used in applications where physical properties are important. Polyester and epoxy composites make up the bulk of the thermoset composite market. Of these two, polyesters dominate by far. Reinforced with glass fiber, these are known as fiberglass-reinforced plastics (FRPs). FRPs are molded by layup and spray-up methods or by compression molding either a preform or sheet molding compound (SMC). 

By analogy with compression molding, thermosetting resins plus fibrous reinforcement can be pressed between matching mold halves, with heat and moderate pressure, and cured directly into finished products. Typical cure cycle is about a minute. Half of all reinforced thermoset products are made in this way. Major applications are in the automotive, electrical/electronics, appliance, and business machine industries. 

Injection mold machines are rated by their ability to mold polystyrene in a single shot [3], Mold pressures can range from 55 to 275 MPa and cycles as low as 15 s. It is possible to injection mold a thermoset resin, although it is very difficult. Compression molding or transfer molding is used in these cases ... 

Compression molding is an older method of polymer processing that is still currently used for parts such as polyester thermoset engine covers or polyamide transmission seals. The resin will be placed in half of the mold (generally the female half shown in Figure 5.4). The male half of the press will compress the resin to a pressure of about 15 MPa. The resin will be heated simultaneously in both halves of the mold, which will cause it to begin to cross-link. 

With thermosets, compression molding is commonly applied. The starting material is a molding powder comprising the resin, curing agent, and filler. 

Solution In compression molding of thermoset, once the resin is cured the molded part can be ejected while still hot and allowed to cool outside the mold. Qn the other hand, compression molding of thermoplastics involves heating followed by cooling to solidify the molded part. Therefore, in compression molding of thermoplastics the relatively longer heat-and-chill cycle times involved are imeconomical for large volume production. 

Molding under pressure is called compression molding. In this process, powders or mold materials are usually preheated, and then simultaneously pressed and hardened (Figure 36-1). Generally, only thermosets containing a great deal of filler are used as mold materials, i.e., phenolic, urea, melamine, and unsaturated polyester resins. Inlays such as mats and fabrics are also frequently used. 

---