Cavity Surface Texture Effects on Thermoplastic Heat Transfer and Shrinkage
Cavity surface texture increases interfacial thermal contact resistance, delaying resin skin freeze and increasing semi-crystalline thermoplastic shrinkage.
Skin Layer Thickness
Meaning
Rapid solidification of the polymer melt as it contacts the cold mold walls forms a highly oriented, frozen boundary layer on the surface of the molded part. This skin layer thickness is determined by the melt temperature, mold temperature, and fill speed during the initial injection phase. The dimension of this oriented layer directly influences the cosmetic quality and the flexural modulus of the finalized part.
Flow Influence
High injection speeds and hot mold temperatures reduce the rate of solidification and lead to a thinner skin layer. Conversely, when the polymer freezes too quickly due to slow injection, the skin layer thickness increases, which can restrict the flow path for the remaining melt core. Moulders must control these processing variables to maintain a consistent flow cross-section and prevent premature freezing.
Mechanical Strength
The highly oriented molecules in the outer layer of the part provide increased stiffness under bending loads. Adjusting the process to yield a larger skin layer thickness can enhance the flexural modulus, but it may also increase the internal stresses within the molded component. Processors must manage this structural trade-off to ensure both strength and dimensional stability.
Surface Finish
The quality of the outer layer affects how well the polymer duplicates the texture of the mold surface. A very rapid freeze that increases the skin layer thickness can prevent the melt from replicating fine details, leading to surface blemishes or uneven gloss. Higher mold temperatures keep the skin layer pliable longer, allowing for better replica quality.