Classification of plastic molds②

In the last issue, I explained the classification by gating system, and today I will introduce the classification by molding.

Then the molding classification can be divided into six categories.

(1) Injection molding

The plastic is first added to the heating barrel of the injection machine, the plastic is heated and melted, and driven by the screw or plunger of the injection machine, it enters the mold cavity through the nozzle and the mold gating system. Due to physical and chemical effects, it is hardened and shaped into injection molded products. Injection molding consists of a cycle consisting of injection, pressure holding (cooling) and plastic part demoulding, so injection molding has periodic characteristics. The molding cycle of thermoplastic injection molding is short, the production efficiency is high, and the wear of the molten material on the mold is small. Plastic parts with complex shapes, clear surface patterns and marks, and high dimensional accuracy can be formed in large quantities; but for plastic parts with large wall thickness changes, it is difficult to avoid molding defects. The anisotropy of plastic parts is also one of the quality problems, and all possible measures should be taken to minimize it.

(2) Compression forming

Commonly known as compression molding, it is one of the earliest methods of forming plastic parts. Compression molding is to add plastic directly into the open mold cavity with a certain temperature, then close the mold, and the plastic melts into a fluid state under the action of heat and pressure. Due to physical and chemical effects, the plastic is hardened into a plastic part with a certain shape and size that remains unchanged at room temperature. Compression molding is mainly used to form thermosetting plastics, such as phenolic molding powder, urea-formaldehyde and melamine formaldehyde molding powder, glass fiber reinforced phenolic plastics, epoxy resin, DAP resin, silicone resin, polyimide and other molding compounds. It can also form and process unsaturated polyester dough (DMC), sheet molding compound (SMC), prefabricated bulk molding compound (BMC), etc. In general, according to the matching structure of the upper and lower molds of the compression film, the compression molds are divided into three types: overflow type, non-overflow type and semi-overflow type.

(3) Extrusion molding

It is a molding method that makes plastic in a viscous flow state pass through a die with a specific cross-sectional shape at high temperature and a certain pressure, and then shape it into a continuous profile of the desired cross-sectional shape at a lower temperature. The production process of extrusion molding is preparation of molding materials, extrusion molding, cooling and setting, traction and cutting, post-processing of extruded products (tempering or heat treatment). During the extrusion molding process, pay attention to adjust the temperature of each heating section of the extruder barrel and the die of the die head, the number of screw revolutions, the traction speed and other process parameters in order to obtain qualified extrusion profiles. Special attention should be paid to adjusting the rate at which the polymer melt is extruded from the die of the die. Because when the extrusion rate of the molten material is low, the extrudate has a smooth surface and a uniform cross-sectional shape. However, when the extrusion rate of the molten material reaches a certain limit, the surface of the extrudate will become rough and lose luster, and phenomena such as shark skin, orange peel, and shape distortion will appear. When the extrusion rate is further increased, the surface of the extrudate is distorted, and even fragmented and broken into melt fragments or cylinders. Therefore, the control of extrusion rate is very important.

This issue will stop here for everyone, and the next issue will continue to explain the classification of plastic molds by molding.We welcome more people to pay attention to our company, ZHUHAI BAOLI FOAM SPRAY PUMP CO.,LTD.