Compression molding cycle time

The preform is compression molded to full density. A variety of molding cycles (time, temperature, and pressure profiles) have been used to manufacture the UHMWPE/ HA materials depending on the size and geometry of the sample and mold [81-84]. 

Typical mold cycle times range from less than a minute for injection molding to up to 10 min for thick parts made by compression molding. Molding temperatures are typically between 175°C and 205°C. Most formulations and end-use will require a post-cure cycle ranging from 1 to 24 h, at temperatures between 200°C and 240°C. 

Either crystalline and amorphous types (Chapter 1) can be used with flow molding because the plastic is melted prior to forming. The forming temperature is usually lower than for injection molding or extrusion. Plastics need not be trimmed, as the composite is compression-molded to completely fill the mold cavity (Chapter 14). Very important, the flow molding process permits more complex parts to be formed than solid-state forming. The process cycle time is usually about 1 min. 

The mold can be modified to meet certain different shapes. The molding can be made either by withdrawing cores or by special press motions that partially open the mold halves (such as the compression molds used in coining to provide 2-D action 3-D mold actions are also used). The degree of foam density, wall thickness, and surface finish depends on the foam mixture (constituents and amounts). The machine controls the time cycle and the mold action required. 

Through a compression and decompression action the density of the mold contents is increased, and the more intimate contact with the mold walls improves the heat reduction. The surface of the molded part is substantially improved and voids as well as sink marks disappear. The improved dissipation of the heat reduces the cycle time up to 15% depending on the polymer. 

When compared with compression molding, transfer molding provides better product consistency, shorter cycle times, and better bonding of rubber to metal [55]. However, considerable amount of material is lost as scrap in the transfer pad, sprues, and flash. The basic three-plate multiple cavity mold is more complex and expensive than a comparable compression mold but is suited better for intricate parts or securing inserts [56]. 

Compression molding involves limited flow and, consequently, less strain than injection molding. Consequently, shear sensitivity typical of polymer blends is less critical. On the other hand, the cycles are usually longer and the long soak times at high temperatures can affect blends morphology through coalescence. 

In the case of thermosets, numerous articles are made by compression molding, such as bulb fittings, switches, wall plugs, crockery, and many other technical and household articles. The process is very economical because no expensive machines and molds are required and the cycle times are short (the finished article can be taken out of the mold without cooling down). 

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. 

Phenol-formaldehyde molding compositions are traditionally processed on compression- and transfer-molding machines with a very small amount being extruded. However, the injection-molding process as modified for thermosetting plastics is being increasingly used and it is today the most common method used to process phenolic compounds. The shorter cycle times and low waste factor available with... 

The CDs are manufactured either by injection compression molding or injection molding. Injection compression molding minimizes molded-in stress but requires longer cycle times than injection molding and produces flash. For both processes very clean room conditions, up to 1,000 times cleaner than medical molding, are essential. 

Compared to both compression and transfer CM, injection molding requires shorter vulcanization and cycle times as well as better automation. On the other hand, mold costs are so high that only large-series production is economical. 

Compression molding is a relatively simple and inexpensive procedure for the manufacture of moderate piece counts. For the production of bipolar plates compression molds with surface structured inserts were designed and manufactured. For the molding, the granulate can directly be molded in a heated mold under high pressure. Another way is the direct extmsion of a profile (semifinished material) into the structured mold, forming the plates from this plasticized polymer. In this variant the tool temperature is kept constant. With a multicavity-mold the cycle time can accordingly be shortened, and the output quantity of plates per cycle increased (Fig. 12). 

An LFT-D technique is applied in a compression mold with glass mat reinforcement to produce the supporting structure for the dashboard in large series, i.e., with cycle times of approx. 30 s. This is in keeping with the state of the art. 

In case of the metallocene-catalyzed random copolymer of ethylene and alpha-olefin (POP and POE), incorporating more comonomer along the polymer backbone reduces density and crystallinity and hence, increases flexibility and softness. However, as the density is decreased, the melting point, crystallization peak temperature and heat resistance decrease and cycle times in injection molding increase. These deficiencies have limited the use of POEs in applications where heat resistance, high temperature compression set, and faster cycle times are desired. 

---