Thermal Expansion of Molds

In the case of plastic injection mold, since it is necessary to maintain the temperature of the mold within a range of 20 to 200°C, and since there will be a difference between this temperature and room temperature, there will be thermal expansion of the parts of the mold (in some cases, there may be thermal contraction).
Each part of a mold has its own shape, and since a number of such parts are assembled in a complex manner, there will be sliding or friction movement.
Even the state of thermal expansion is extremely complex and it is very difficult to predict it by theory.
The practical method of using know-how accumulated by experience is very common regarding the problem of thermal expansion.
The method of supplying oil is modified, the contact surface area is reduced, and the amount of deformation is absorbed by providing an escape.
In addition, a technique is used of balancing the thermal bias by using additionally the temperature controlling function.
A cooling water flow path is provided near the heater or the manifold, or an air thermal insulation layer is provided, or a thermal insulation plate is provided.
Positioning rails are placed in the case of sliding parts.
Ball bearings are used in the case of parts lubricating which is difficult, or even a non-lubricating type alloy is used.
In order to balance the heat accumulation in the mold base, the excel thickness of unwanted parts are removed.
Regarding hot runners, it is particularly important to calculate and study thoroughly the thermal balance between heat supply and loss. It is necessary to give proper considerations about the balance between the quantity of heat on the fixed side and the quantity of heat on the movable side of the chip parts, manifold parts, etc.
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