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  • Insulation method for injection molds

Insulation method for injection molds

Mold temperature is the most important variable in injection molding ---- No matter what kind of plastic is being injected, it must be ensured that a basic wetting of the mold surface is formed. A hot mold surface allows the plastic surface to remain liquid long enough to build pressure within the cavity. If the cavity is filled and the cavity pressure can press the soft plastic against the metal before the frozen skin hardens, then the cavity surface replication is high.

On the other hand, if plastic entering the cavity at low pressure is suspended, no matter how briefly, then its slight contact with the metal can cause stains, sometimes called gate stains.

For each type of plastic and plastic part, there exists a mold surface temperature limit beyond which one or more undesirable effects can occur (e.g., the component can spill burrs). A higher mold temperature means less resistance to flow.

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 for this change, and faster filling causes higher effective pressure in the gate and cavity.

This can cause overflow burrs. Since the hotter model does not freeze the plastic that enters the overflow edge area before high pressures are developed, the melt can overflow around the ejector bar and spill into the parting line gap. This indicates the need for good injection rate control, and some modern flow control programmers can indeed do this.

Typically, an increase in mold temperature will reduce the amount of plastic that has a condensation layer in the cavity, allowing the molten material to flow more easily in the cavity, resulting in greater part weight and better surface quality. Also, the increase in mold temperature will result in increased part tension strength.

Mold insulation methods

Many molds, especially engineering thermoplastics, operate at relatively high temperatures, such as 80 degrees Celsius or 176 degrees Fahrenheit. If the mold is not insulated, as much heat can easily be lost to the air and the injection molding machine as is lost to the injection cylinder.

To insulate the mold a skeleton board and, if possible, insulate the surface of the mold from heat. If considering a hot runner mold, try to reduce the heat exchange between the hot runner section and the cooled injection molded part. This approach will reduce energy loss and preheating time.
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