What are characteristics of injection molding of thermoset plastic products?

Time:2023-05-15 09:05:51 / Popularity: / Source:

Molding and processing of thermosetting plastics began in early 1960s. Because of their high heat resistance, deformation resistance and good electrical properties, they have been rapidly developed and widely used.
Commonly used equipment for molding thermoset plastic products is plunger injection molding machine and screw injection molding machine. Thermosetting plastics are added into barrel. Heated barrel and rotating screw make raw material melt and plasticize into a viscous fluid state. At this time, a physical reaction occurs in raw material, then it is pushed by screw (or plunger), molten material is injected into molding mold at a higher injection pressure and a certain injection speed, heated and raised, under high pressure and high temperature conditions, it interacts with curing agent added at the same time to cause a crosslinking reaction. This chemical reaction releases low molecular substances such as water and ammonia. After product is hardened, it can be taken out of forming mold.
injection molding of thermoset plastic products 
Process characteristics of thermosetting plastic injection molded products are as follows
① Plasticizing temperature of thermosetting plastics in barrel should be strictly controlled. Generally, thermostatically controlled hot water circulation is used to heat barrel; barrel temperature is controlled at about 90℃. When temperature is low, plasticization of raw material is not good, melt fluidity is poor; when temperature is too high, melt fluidity will be reduced or even hardened. During injection, heat is generated due to friction between melt and nozzle, temperature of material at this time should be between 100 and 130℃. Temperature of molten material injected into mold is controlled at 160~170℃.
② When thermosetting melt is cross-linked under action of high pressure and high temperature in forming mold, low-molecular substances are precipitated, clamping mechanism is required to meet requirements of exhaust action. That is to say, clamping mechanism must not only have clamping force to ensure high-pressure injection of molten material, but also have action requirements to open mold in time and discharge gas due to chemical process reaction of raw materials.
③ Residence time of thermosetting plastics in barrel cannot be too long, and temperature cannot have large fluctuations to prevent melt from solidifying in advance. Structural requirements of screw: compression ratio is 0.8-1.2, length-to-diameter ratio L/D is 12-16, thread groove depth is deeper than thread groove of screw used for thermoplastic plasticization, screw head is 60° tapered, usually with a backstop ring.
④ For thermosetting plastic molding, injection pressure of melt is higher than that of thermoplastics. This is because viscosity of thermoset plastics is relatively large, this viscosity will increase sharply with time under temperature, so in order to ensure complete filling of melt, high-pressure fast filling must be used. This high-pressure injection and high-pressure holding pressure are also beneficial to product structure, size stability and a certain degree of compactness. However, this kind of high-pressure injection will cause product to produce relatively large internal stress, defects such as cracks, pores and flash will easily appear. Note that the injection pressure should be selected to satisfy complete filling of viscous melt, pressure should not be too high.
⑤ Raw materials for thermosetting plastic products are mostly made of resin and fillers with some other additives. For this multi-component raw material, it should be noted that melt has better fluidity and a wider plasticizing temperature range, and can be plasticized at a lower temperature. In addition, thermal stability of raw materials is also required to be good. When residence time is long in barrel, curing reaction cannot occur; after entering molding die, it must be quickly cured to shorten product molding cycle.

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