Thermal Resistance Basics in Textured Plastic Injection Molds
Textured mold cavity thermal resistance stems from trapped micro-air gaps, reducing interfacial heat transfer coefficients down to 800 W/m²K and extending cooling cycle duration.
Surface Gloss Variation
Meaning
Inconsistency in the reflective quality of a moulded part’s surface indicates a lack of control over the cooling or filling process. Surface gloss variation appears as dull spots or patches of high shine on an otherwise uniform component. This defect is particularly problematic for cosmetic parts in the automotive and appliance industries.
Cavity Finish
Polishing of the tool steel sets the baseline for the appearance of the plastic part. Surface gloss variation occurs when the resin does not stay in contact with the tool surface long enough to replicate the texture. High-polish surfaces require higher mould temperatures and packing pressures to ensure a consistent look across the entire part.
Shear Effect
Flow of the melt through the runners and into the cavity influences the alignment of the polymer molecules at the surface. Surface gloss variation can be caused by changes in injection speed, which alter the shear stress and the temperature of the material. If the melt is too cold when it reaches the end of the fill, it will appear duller than the material near the gate.
Thermal Gradient
Differences in the cooling rate across the tool surface lead to localized changes in how the resin solidifies. Surface gloss variation is often a result of poor cooling channel design that allows some areas of the cavity to remain hotter than others. In semi-crystalline materials, these temperature differences also affect the size of the crystals formed at the surface.
A moulder can use a gloss meter to quantify the difference in reflection and identify the areas that require adjustment. Solving this issue usually involves balancing the cooling flow or increasing the hold time to ensure the part remains pressed against the tool.