A hot mold surface keeps the plastic surface liquid for […]
A hot mold surface keeps the plastic surface liquid for a long time, enough to build up pressure in the cavity. If the cavity fills up and the pressure in the cavity presses the soft plastic against the metal before the frozen skin hardens, the temperature of the cavity surface is high. On the other hand, if the plastic entering the cavity under low pressure is suspended, no matter how short the time is, then its slight contact with the metal will cause stains, sometimes called gate stains.
For every type of plastic and plastic parts, there is a limit for the surface temperature of the mold. If this limit is exceeded, one or more adverse effects may occur (for example, the components may overflow with burrs). Higher mold temperature means lower flow resistance. On many injection molding machines, this naturally means faster flow through the gate and cavity, because the injection flow control valve used does not correct this change, and faster filling will cause a higher efficiency in the runner and cavity pressure.
It may cause flash burrs. Since the hotter model does not freeze the plastic that enters the flash side area before the high pressure is formed, the melt can flash burrs around the ejector rod and overflow into the gap of the dividing line. This shows the need for good injection rate control, and some modern flow control programmers can indeed do this.
Generally, the increase in mold temperature will reduce the condensation layer of the plastic in the cavity, making the molten material easier to flow in the cavity, thereby obtaining greater part weight and better surface quality. At the same time, the increase in mold temperature will increase the tensile strength of the parts. Many molds, especially engineering thermoplastics, operate at relatively high temperatures. If the mold is not insulated, the heat lost to the air and injection molding machine can easily be as much as the injection cylinder. Therefore, insulate the mold and the machine board, and if possible, insulate the surface of the mold. If you consider using a hot runner mold, try to reduce the heat exchange between the hot runner part and the cooled injection part. This method can reduce energy loss and warm-up time.