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Simulating deformation of inserts or core pins

| Editor: Steffen Donath

Simulative comparison of the deformation of core pins made of tungsten carbide (left) and 420 stainless steel (right) at the end of the filling of the cavity — the material with the lower modulus of elasticity deforms three times more under constant process conditions.
Simulative comparison of the deformation of core pins made of tungsten carbide (left) and 420 stainless steel (right) at the end of the filling of the cavity — the material with the lower modulus of elasticity deforms three times more under constant process conditions. (Source: Sigmasoft)

Virtual moulding can help make decisions in mould design. Comparing different mould materials in the simulation can help prevent deformation during the filling.

There are many questions that injection mould manufacturers confront daily regarding the variety of mould materials which are available in the market. What type of steel should be used for which part of the mould? Whether it is the thermal conductivity of the steel used for the cavity insert, the diameter of a suitable ejector pin or even the mechanical stability of the core pins.

In Sigmasoft every mould material, e.g. steel, isolation, etc., is taken into account with its thermal and mechanical properties. As an example, one can simulate the heating-up phase or the development of hotspots during several injection cycles having the thermal conductivity and specific heat capacity of all the mould materials. Besides the prediction of shrinkage and warpage of plastic parts, Sigmasoft also simulates the deformation of inserts and core pins during filling. These calculations are based on possible imbalances in the filling of the cavity and of course the mechanical properties of the steel.

Sigma Plastic Services, Inc. (IL), the American subsidiary of Sigma Engineering GmbH, simulated an interesting project in co-operation with two companies, Cavaform (FL) and Crafts Technology (IL). In the project the deformation of core pins made of different materials during filling of the cavity was evaluated. The investigation was done for an injection mould with 16 cavities, which is used to produce centrifuge tubes. In eight of these cavities, core pins made of tungsten carbide are integrated to form the inside of the tubes. 420 stainless steel is used for the other eight core pins. Simulative analysis of the filling phase shows an imbalance, which comes to existence after about 85 % of filling has passed and becomes more obvious near the end of filling (Figure 2). This imbalance is caused by the asymmetrical geometry of the screw in the cap area of the tube and leads to a force, which tends to deform the core pins during the filling of the cavity. Due to the lower module of elasticity of 420 stainless steel, pins made of this steel are deformed about three times more compared to the pins made of tungsten carbide (Figure 1).

The described case is just an example of the many details of an injection mould that can go wrong during the mould design phase. Sigmasoft Virtual Moulding provides even the most experienced mould manufacturers with a detailed insight of the injection moulding process. With a very low effort and based only on the thermophysical and mechanical phenomena, even before ordering the mould units, they can examine and evaluate the effect of changes in the mould. Thus, simulation provides a sound basis for decisions and supports the mould design from the beginning.

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