Accurate cooling time calculation is needed for cost estimation of injection moulded parts. Many cooling time formulae for injection moulding contain errors, though they may have been used and accepted for decades. The purpose of this paper is to review the available formulae and to clarify correctness from a theoretical standpoint. Ejection criteria may be based either on mid-plane or average temperature. Deviation from the assumptions of the theory for both thin and thick parts is also discussed.
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