Читать книгу Plastics Process Analysis, Instrumentation, and Control - Группа авторов - Страница 23

Simulations and experiments were conducted with gas temperatures of 200°C to 400°C to investigate the impact of external gas-assisted mold temperature control on the quality of the weld line of molding products (29).

The general process consisted of six steps. In Step 1, as soon as the molding cycle was completed and the product was ejected, the hot gas source was transported to the heating area at which the cavity temperature was still low. In step 2, the hot gas sprayed directly into the heating area. Due to heat convection, the thermal energy was transferred to the cavity area, heating that area to the target temperature, after which the heating process stopped. At this time, the cavity remained at a high temperature. In Step 3, the gas drier was removed from the molding area and the two half molds were closed before a new molding cycle began by melt filling into the cavity in Step 4. In this step, the hot cavity allowed the melt to easily flow into the thin-walled area. As soon as the entire cavity filled, the cooling process started with the heat transfer from the hot melt to the mold plate in Step 5. The process stopped when the part temperature reached the ejection temperature. In Step 6, the two half molds opened to eject the part, and the next molding cycle occurred.

In the heating step, the heating rate was 19.6 ^◦Cs^–1 from 30°C to 128.5°C in the first 5 s in a 400°C gas environment (29).

When applied to heating the weld line area of an injection mold, the external gas-assisted mold temperature control improved the appearance of the weld line when the cavity temperature was preheated to 150°C.

For the tensile strength test, a melt flow simulation comparing the packing pressure of different mesh thicknesses revealed that external gas-assisted mold temperature control helped maintain a high pressure in the weld line area in different packing periods.

This was verified by an experiment where the external gas-assisted mold temperature control was applied with 400°C gas to change the mesh area temperature. The result indicated that an increase in the weld line area temperature from 60°C to 180°C improves the tensile strength of all mesh thicknesses, which was more pronounced with thinner parts, especially at 0.4 mm. The simulations revealed that high temperature is concentrated in the weld line area of the cavity surface, thus reducing the energy wasted during heating (29).