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Prediction of shrinkage and warpage in injection moulded components using computational analysis
Author(s)
Riddles, Mornay
Date Issued
2003
Type
Thesis
Publisher
Peninsula Technikon
Abstract
Injection moulding is a process by which molten polymer is forced into an empty
cavity of the desired shape. At its melting point, polymers undergo a volumetric
expansion when heated, and volumetric contraction when cooled. This volumetric
contraction is called shrinkage. Once the mould cavity is filled, more pressure is
applied and additional polymer is packed into the cavity and held to compensate for
the anticipated shrinkage as the polymer solidifies. The cooling takes place via the
cooling channels where the polymer is cooled until a specific ejection criterion is met.
Heat from the polymer is lost to the surrounding mould, a part of this heat reaches the
cooling channel surfaces, which in turn exchange heat with the circulating cooling
fluid.
Due to the complexity of injection moulded parts and the cooling channel layout, it is
difficult to achieve balanced cooling of parts. Asymmetric mould temperature
distribution causes contractions of• the polymer as it cools from its melting
temperature to room temperature. This results in residual stresses, which causes the
part to warp after ejection.
Given the understanding of the mathematical model describing the heat transfer
process during the cooling stage, the objectives of this study were three fold. Firstly,
an alternative numerical model for the heat transfer process was developed. The
proposed model was used to investigate the cooling stress build-up during the
injection moulding process.
cavity of the desired shape. At its melting point, polymers undergo a volumetric
expansion when heated, and volumetric contraction when cooled. This volumetric
contraction is called shrinkage. Once the mould cavity is filled, more pressure is
applied and additional polymer is packed into the cavity and held to compensate for
the anticipated shrinkage as the polymer solidifies. The cooling takes place via the
cooling channels where the polymer is cooled until a specific ejection criterion is met.
Heat from the polymer is lost to the surrounding mould, a part of this heat reaches the
cooling channel surfaces, which in turn exchange heat with the circulating cooling
fluid.
Due to the complexity of injection moulded parts and the cooling channel layout, it is
difficult to achieve balanced cooling of parts. Asymmetric mould temperature
distribution causes contractions of• the polymer as it cools from its melting
temperature to room temperature. This results in residual stresses, which causes the
part to warp after ejection.
Given the understanding of the mathematical model describing the heat transfer
process during the cooling stage, the objectives of this study were three fold. Firstly,
an alternative numerical model for the heat transfer process was developed. The
proposed model was used to investigate the cooling stress build-up during the
injection moulding process.
Additional information
Thesis (MTech (Mechanical Engineering))--Peninsula Technikon, 2003
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