Determining Polymer Wall Conductance on Smooth Tool Steel
Polymer wall conductance on smooth tool steel peaks above 3500 W/m²K under packing pressure and drops below 600 W/m²K upon thermal gap detachment.
Polymer Shrinkage Gap
Meaning
Physical separations occur at the interface between the mould surface and the solidifying part as the material reduces in volume. The polymer shrinkage gap creates a layer of air that acts as a thermal insulator between the part and the tool. This gap reduces the efficiency of heat transfer and slows down the cooling process.
Managing the timing and size of this separation is a major part of mould design.
Thermal Contraction
Formation of a polymer shrinkage gap is an inevitable result of the molecular realignment that happens during cooling. Volume decreases as chains pack together. In crystalline materials, this change is more pronounced than in amorphous ones.
The gap typically begins to form at the corners and edges first.
Interface Resistance
Interface resistance arises because the presence of a polymer shrinkage gap increases the thermal resistance at the boundary of the mould. Heat can no longer pass through direct conduction and must instead cross the air space through less efficient means. This causes the internal temperature of the part to remain high even if the mould is cold.
Simulation software must account for this resistance to provide accurate cycle time estimates.
Warpage Result
Non-uniform development of the polymer shrinkage gap across the surface of a part leads to internal stresses. If one side of a part pulls away from the tool while the other remains in contact, the cooling will be lopsided. This imbalance causes the part to bend or twist after it is ejected.
Careful placement of cooling channels can help even out the cooling and minimize the negative effects of the gap.