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Multi-component | DFM


designing a multi-shot injection component: l Hard component first – the injection order should always be to mould the first shot hard and second shot soft;


l Consider shrinkage rates – different shrinkage rates for the two component materials can lead to high stress, distortion and/or poor adhesion between the component materials;


l Think about gating – the gate point of the first shot should be covered by the second shot wherever it is possible to do so;


l Consider wall thickness – keep wall thickness as even as possible to reduce risk of re-melting or cosmetic defects;


l Consider flow properties – the minimum wall thicknesss of each shot should be checked against each materials’ flow properties, especially when both materials are overlapping;


l Split the design – the design for each of the different materials should be checked to achieve the most robust tooling and minimal risk of visual defects such as flash;


l Think about venting – venting of the second shot is


more difficult to achieve due to the shut-off between the first shot and the mould steel;


l Take care with material selection – very good chemical bonding can be achieved with appropriate material combinations.


l Manage temperatures – the temperature of the first and second shot materials will have a significant effect on adhesion.


l Analyse the process – mould filling analysis software can predict gate positions, weld line and venting locations, temperature differentials, cooling times, and shrinkage and stress behaviour. Always make full use of these analytical tools.


About the author: André Eichhorn is general manager of Germany-based AST Technology. This is the fourth instalment in a series of articles presenting a step-by- step discussion of the Design for Manufacturing (DFM) process. Previous instalments in this series can be viewed here and here. Part five, which will be published in the next edition of Injection World, will discuss how mould construction considerations can be integrated into the product development process. ❙ www.ast-tech.de


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