• When evaluating the gloss of plain molding surfaces, an analogous fine processing or polishing quality of the mold contours must be assumed. For structured visible surfaces, special attention must be paid to the fine structures.
    • The occurrence of weld lines can be clarified by geometric considerations (swelling flow) or by computer-aided filling pattern simulation. Possible quality reduction (visibility; strength reduction) has to be decided according to requirements and experience.
    • Higher degrees of crystallization in semi-crystalline polymers result, among other things, in greater stiffness and heat resistance, improved media resistance, lower post-shrinkage with increased processing shrinkage, and significantly improved abrasion resistance.
    • Abrupt cooling can cause "internal tensile stresses" in the molded part, which in the case of brittle plastics (e.g. PS) cause "hairline cracks" or, when exposed to specific media, stress cracks. Analogous effects are possible when overmolding non-preheated metal or ceramic inserts.
    • Molded parts for optically active applications (mostly made of acrylate polymers) require residual stress-free slow cooling.


Corresponding qualities of the mold contour surfaces can be derived from design and functional requirements for the molded part surface quality (smoothness, gloss) and its manufacturability (demolding, post-treatment, film formation). Their gradation is given below with the arithmetic mid-roughness value Ra according to DIN EN ISO 4287:


    • Technically rough (without fine machining): Ra ≥ 3 µm.
    • Smooth (with fine finishing. e.g. grinding): Ra = 0.8 to 1.6 µm
    • Glossy (coarse polish): Ra = 0.2 to 0.4 µm
    • High gloss (fine polish): Ra = 0.05 to 0.10 µm
    • Reflective (optical fine polish): Ra = 0.012 to 0.025 µm

When deciding on the surface quality, it must be taken into account that with increasing polishing quality, the costs for manufacturing and maintaining the molds increase significantly, while the imaging precision on the molding surface decreases. The type of plastic and processing conditions can also have a significant impact on imaging reproducibility. In particular, the special optical grade (mirror-like surface) should therefore only be provided for transparent plastics for special requirements (e.g. lenses, scales, CD).

Molded parts for optical applications require very low residual stresses (density constancy) and long-term holding pressure effects to avoid sink marks in relatively thick-walled parts. Both conditions are realized, among other things, by high mold contour temperatures. For this purpose, a special orientation for high contour temperatures is specified in the software, which is only slightly below the glass temperatures of transparent thermoplastics.

The "plate" or "shell" is to be selected as the geometry element for lenses and the like, and average wall thicknesses are to be estimated from the courses of the radii of curvature.