Meaning
The nominal distance between the opposing surfaces of a moulded plastic component is an important factor in determining its mechanical strength, cooling time, and ease of processing. This design dimension, known as wall thickness, directly influences the flow behavior of the polymer melt within the mould cavity. It must be kept as uniform as possible to prevent differential shrinkage and warping.
Thicker walls increase part strength but also extend the required cooling time and overall cycle time.
Processing Influence
A uniform thickness ensures that the polymer melt flows evenly through the cavity during the injection phase. If the flow encounters a sudden change in thickness, the pressure drops and the flow slows down. This flow variation can cause structural weakness and cosmetic defects like hesitation marks.
Tooling designers use flow simulation software to optimize gate placement for a given part thickness.
Cosmetic Defect
Variations in thickness across a part lead to uneven cooling and thermal stresses. Thicker regions cool more slowly than adjacent thin regions, causing localized shrinkage. This difference in shrinkage results in sink marks and internal voids.
To avoid these defects, designers use transition zones to blend different thicknesses gradually.
Optimization Principle
Minimizing thickness reduces material consumption and cycle times. The minimum thickness is limited by the polymer flow length. Sufficient thickness is required to withstand operating loads.
Balancing these requirements is key to part design.
