Compression molding process parameters The quality and […]
Compression molding process parameters
The quality and performance of plastic parts is not only affected by the properties of plastic raw materials and the size of the mold structure, but also related to the formulation and control of molding process parameters. These are the three elements of molding that are often said in production: temperature, pressure and time.
1. Molding temperature
The molding temperature refers to the mold temperature required for pressing. At this temperature, the plastic melt flows in the mold cavity, fills the cavity and solidifies and forms. The mold temperature is not equal to the temperature of the plastic melt in the cavity. For thermosetting plastics, the maximum temperature of the plastic melt is higher than the mold temperature due to the exothermic result of the plastic cross-linking reaction. When thermoplastics are compression molded, 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 difficulty in filling the mold, making the surface of the plastic part dull, dull, and even swelling, deforming and cracking. If the molding temperature is too low, the curing time will be slow and the molding time will be long. Therefore, the determination of mold temperature should comprehensively consider various factors, and it is also the key to ensuring the quality of plastic parts.
2. Forming pressure
The molding pressure refers to the unit pressure of the press on the projected area of the plastic part during compression molding. Its function is to force the plastic melt to flow and fill the mold cavity to avoid defects such as bubbles and loose structure inside the plastic part due to low molecular volatiles, ensure that the plastic part has a fixed shape and size, prevent deformation, and improve the internal quality.
When thermosetting plastics are compressed and molded, they need to be kept at a certain temperature and pressure for a certain period of time to be fully cross-linked and solidified to become a good plastic part. This period of time is called the compression time. The compression time is related to the type of plastic (type of resin, volatile content, etc.), shape of the plastic part, process conditions (temperature, pressure) of compression molding, and operation steps (whether exhaust, pre-pressing, pre-heating), etc. When the compression molding temperature increases, the plastic solidifies faster, and the required compression time is reduced: an increase in compression pressure will also reduce the compression time, but it is not as obvious as the increase in temperature. In addition, the compression time will increase as the wall thickness of the plastic part increases. The length of compression time has a great influence on the performance of plastic parts. The compression time is too short, the plastic is not hardened enough (under-cooked), the appearance quality of the plastic parts is poor, the mechanical properties are reduced, and it is easy to deform. Appropriately increasing the compression time can reduce the shrinkage of plastic parts and improve their heat resistance and other physical and chemical properties. However, if the compression time is too long, it will not only reduce the productivity, but also cause the plastic parts to shrink too much, increase the internal stress, and easily break the plastic parts. Generally, the compression time of phenolic plastic is 1~2min, and that of silicone plastic is 2~7min.