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.
Ra
Meaning
Surface quality quantification for industrial components relies on the mathematical averaging of height variations across a reference plane. Ra is the most common parameter used to quantify the surface roughness of moulded plastic parts and the tool steel they are produced from. This value provides a general measure of the texture but does not describe the shape or frequency of the peaks and valleys.
It is used to specify the finish of a part, from high-gloss mirrors to heavy EDM textures.
Mould Texture
Transfer of the tool surface to the plastic part is never perfectly 100 percent efficient. Factors like melt temperature, injection pressure and the presence of fillers influence how well the resin replicates the steel finish. A glass-filled resin will typically result in a higher ra value than a virgin amorphous resin on the same tool.
Friction Control
Surface roughness directly affects the haptic quality and the sliding friction of a component. Parts that must move against each other require a low ra to prevent wear and noise. Conversely, a higher roughness may be specified for parts that need to be painted or glued to increase the available surface area for bonding.
Measurement Method
Stylus profilometers or optical scanners are used to capture the profile data.