Mold Temperature Control System

What is the Mold Temperature Control System?

The injection mold is first of all a production tool that can repeatedly and in large quantities produce plastic parts with the same structure and the same dimensional accuracy. Secondly, it is also a heat exchanger.

During the injection molding process, the melt is injected into the mold cavity, the temperature is generally between 200~300°C. The melt is formed, cooled, and solidified in the mold cavity to form a plastic part. When the plastic part is taken out of the mold, the temperature is generally 60~80°C, and the heat released by the melt is transferred Die was given.

To ensure normal production, the mold must transfer this part of the heat in time, so that the mold temperature control within a reasonable range.

The heat of the melt is continuously transferred out in the mold, or the mold is heated to the normal injection temperature of the mold, and the part of the structure where the mold temperature is controlled within a reasonable range is called the mold temperature control system. Straight-through cooling water pipes and diaphragm water wells are mold temperature control systems.

What are the Mold Temperature Control Systems and Heating Occasions?

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The occasions where injection molds need to be heated mainly include: For plastics with high viscosity and poor fluidity, such as PC, hard PVC, PPO, PSF, etc. Increasing mold temperature control can better improve their fluidity, and the mold temperature should be controlled at 80-12℃.

For these molds, if the surface dissipates heat quickly, the heat of the melt alone is not enough to maintain the high temperature of the mold, so the mold needs to be equipped with a heating system to heat the mold before or during injection, to ensure the normal production of the mold.

Some molds need to be heated and cooled. The first situation is that in cold regions or large molds, the mold must be preheated before production. When the mold temperature control system reaches the requirements of the plastic molding process, the heating system can be turned off. If after a period of injection, the temperature of the mold is higher than the requirements of the plastic molding process, then turn on the cooling system of the mold to make the temperature of the mold injection molding at the required mold temperature control.

The second case is that the plastic part is large and the wall thickness is uneven, then the part with the larger wall thickness should be cooled, and the part with the thinner wall should be heated to improve the fluidity of the melt.

For small molds with a width equal to or less than 200mm, and when the precision of the molded plastic parts is not too high, it is not necessary to install a heating device or a cooling device, and the mold is cooled naturally

Factors that must be considered in Mold Temperature Control System Design

• Wall thickness, projected area, and structural shape of molded plastic parts

• Production batch of plastic parts

• Properties of molded plastics

• The size and structure of the mold, the way of mosaic molding parts

• The form of the gate and the layout of the runner.

Determination of the cooling time of injection mold

In the molding cycle, the cooling time of a plastic part refers to the time from when the melt fills the cavity to when the mold is opened and the plastic part can be pushed out. “Can be pushed out” means that the melt has been fully solidified, has a certain strength and rigidity, and will not cause defects such as top white or deformation when pushed out. There are three criteria for adequate curing:

1. The temperature of the central part of the maximum wall thickness of the plastic part has cooled to below the thermal deformation temperature of the plastic.

2. The average temperature in the cross-section of the plastic part has reached the specified ejection temperature of the plastic part.

3. For crystalline plastics, the temperature at the center of the maximum wall thickness reaches the solid melting point, or the degree of crystallinity reaches a certain percentage. The cooling time of the plastic part is related to the size and shape of the plastic part, as well as the type of plastic used in the plastic part and the mold material, but it mainly depends on the design of the mold cooling system.

Cooldown times calculated using theoretical formulas are unreliable and meaningless. In actual work, it is often the adjustment engineer who determines a rough cooling time based on the size of the plastic part, structural characteristics, mold size, and experience during the mold test, and then adjusts it according to the molding quality of the product to approach a reasonable value.