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Principles of work of molds

The correct material selection for mold parts

Materials used for the manufacture of injection molds must withstand pressures and forces arising during the casting process. To such materials high demands are made, as they should provide:

  • good heat transmission (thermostat control of the mold);
  • high wear resistance (processing of composite polymers);
  • high corrosion resistance (chemical effect of polymers);
  • high dimensional stability (heat resistance when working with polymers at high casting temperatures);
  • good processibility.

These materials include carbon steels. The carbon steels, according to their intended use are divided into instrumental and structural steels. For plugs and matrixes, depending on the complexity of the configuration, apply:

  • tool steel U8A, U10A, etc.;
  • structural alloy steel 40X, 20X, etc.

The hardness of the surface of the shaping elements varies between 45-65 HRC.

Depending on the processing of the polymer material, the surface of the forming materials is treated with nitriding, nickeling, chrome plating, coating of hard alloys.

Correct choice of inclinations for parts, area of material injection

Technological inclinations

When designing molds for casting parts from plastics, technological inclinations must be taken into account. Technological inclinations are made for better extraction of parts from the mold. The inclinations facilitate lighter flow of the polymer melt during filling of the mold cavity. Technological inclination of internal surfaces and holes should be greater than the inclination of the external surface. The direction of the inclination depends on the opening of the mold, as well as on the moving forming parts of the complex molds. The size of the inclination depends on the size of the parts spots.

Area of material injection

Very important elements in the gating system are ingates. The gating point is the place through which the melt enters the mold cavity. The quality of filling the cavity of the mold depends on their number and location. The location and number of ingates should provide:

  • short way of filling of the cavity;
  • simultaneous filling during streaming of two melt flows;
  • avoiding the separation of material flows;
  • when casting a part with variable wall thickness, the inlet should be placed from the thickened part.

It is recommended to place the gating point when casting products with variable thickness walls from the side of the thickened part.

Material preparation

The quality of plastic parts is greatly influenced by the preparation of the polymer material. It is necessary to follow closely the recommended rates of moisture content in water-absorbing polymers. The residual moisture content in the polymer due to improper preparation can lead to various product defects. Before loading the material into the hopper of the injection molding machine, the raw material is dried in a shelf oven, or dry or hot air dryers are used, which significantly reduces the drying time. There are many recommendations on temperature ranges and drying times for thermoplastics, but in each case it is better to follow the supplier's recommendations.

When you receive a new batch of material, certainly read the passport (certificate) from the supplier. All laboratory data are recorded in the passport and presented to the customer.

Drying

There are many grades of polymer material, some of them are capable of absorbing water steam from the air. Polymeric materials that absorb liquid are called water absorbing. Water absorbing polymers must be dried; the residual moisture content must adhere to the recommended standards for a particular polymer. Exceeding the percentage of moisture in the polymer due to improper preparation leads to various defects in the product. Before loading the material into the hopper of the injection molding machine, the raw material is dried in a shelf oven, or dry and hot air dryers are used, which significantly reduces the drying time. There are many recommendations on temperature ranges and drying times for thermoplastics, but in each case it is better to follow the supplier's recommendations. It is also necessary to fulfill the conditions of material storage. The material is stored in dry storage rooms at least 1 m from the heating devices, excluding direct sunlight. Before processing, in cold weather they stand a day in the shop floor.