The quality performance of plastic parts is influenced by the performance of plastic raw materials, mold structure and size, but also related to the development and control of molding process parameters, which is often said to be the three elements of molding: temperature, pressure and time.
1. Molding temperature
Molding temperature refers to the temperature of the mold required for pressing. At this temperature, the plastic melt flows in the mold cavity, fills the cavity and cures. The mold temperature is not equal to the temperature of the plastic melt in the cavity. For thermosets, the maximum temperature of the melt is higher than the mold temperature as a result of the exothermic reaction of the plastic cross-linking. In the case of thermoplastic molding, the temperature of the plastic melt in the cavity is the upper limit of the mold temperature. If the molding temperature is too high, the curing time will be short, but it will cause difficulties in filling the mold, making the surface of the plastic parts dull and lusterless, and even swelling, deformation and cracking will occur. Molding temperature is too low, the curing time is slow, the molding time is long. Therefore, the mold temperature should be determined by considering all factors, it is also the key to ensure the molding quality of plastic parts.
2. Molding pressure
Molding pressure refers to the unit pressure of the press on the projected area of the plastic part during compression molding. Its role is to force the plastic melt to flow and fill the mold cavity, to avoid bubbles, loose structure and other defects inside the plastic part due to low molecular volatiles, to ensure that the plastic part has a fixed shape and size, to prevent deformation, and to improve the inherent quality.
Thermosetting plastics compression molding, need to maintain a certain temperature and pressure for a certain period of time, in order to fully cross-link curing, become a good plastic parts, this time is called compression time. Compression time is related to the type of plastic (resin type, volatile content, etc.), the shape of the plastic part, the process conditions of compression molding (temperature, pressure) and the operation steps (whether exhaust, pre-pressure, preheating). Compression molding temperature increases, the plastic curing speed is faster, the required compression time is reduced: compression pressure increases will also reduce the compression time, but not as obvious as the increase in temperature. In addition, the compression time will increase with the increase in wall thickness of the plastic part. The length of the compression time has a significant impact on the performance of the plastic part. Compression time is too short, the plastic hardening is not enough (undercooked), the appearance of poor quality plastic parts, mechanical properties decline, easy to deformation. Properly increase the compression time, can reduce the shrinkage of plastic parts, improve its heat resistance and other physical and chemical properties. But the compression time is too long, not only will reduce productivity, but also make the plastic shrinkage too much, the internal stress increases, the plastic part is easy to rupture. General phenolic plastic compression time for l ~ 2min, silicone plastic for 2 ~ 7min.