Small black areas (spots) inside the material, mostly present in transparent resins.
Small air or gas-filled hollows trapped inside the moulding, cooling voids.
Blush and flow marks are the result of variations in material temperature, caused by the temperature gradient between machine nozzle and mould sprue bushing.
A halo around the direct sprue is the result of cold material in the nozzle tip section.
Eliminating the temperature gradient will minimize the blush and halo effects.
Burn marks are often like brown streaks. They are usually caused by overheating the material due to entrapped air (diesel effect) and this causes the darkening in colour.
Separation of layers in the moulded part that can be peeled off. flaking of surface layers. It results from insufficient layer bonding due to inhomogeneities and high shear stresses.
Excessive shrinkage means part dimensions differing from expected ones; amorphous resins behave differently to semi-crystalline materials.
The appearance of areas in the moulding with a deviating colour.
A film of material attached to the moulding at the mould parting line. " Excess plastic".
A turbulent flow in the resin melt: the melt strand enters the cavity in an uncontrolled way. Due to cooling down, the strand is not fused homogeneously with the melt that follows and it shows as a serpentine line on the part surface.
Too little restriction when filling the cavity, material is injected in empty space.
Pitting is the presence of unmelted particles due to difficult dispersion of additives, wrong mixing or crosslinking during processing.
Resembles the grooves of gramophone records. At slow speed - as material enters cold tool - it loses its flow (below HDT) before actual contact. The melt that follows flows over cooled melt, to repeat the cycle.
At end of cycle, the moulding does not release from the mould but sticks in the cavity (female mould side).
At end of cycle, the moulding does not release from the mould but sticks on the core (male mould side)..
Resulting from incomplete filling of the mould: parts of the moulding are not formed.
Visible defects resulting from insufficient cooling before removal from the mould.
A heavy rib intersecting a thin wall may show up sink marks. Quite difficult to eliminate by varying processing conditions.
Too high holding pressure – useless when gating is too small – creates very high stresses in gate areas.
Splay marks, silver streaks, splash marks are the result of:
- moisture on the pellets which should be removed under recommended drying times and temperatures
- products of degradation due to overheating
- residual non-aqueous volatiles in material
(a) and (c) will produce fine lines emanating from the gate all over the part whereas (b) will show up as coarse lines, lumps in sections of the moulded parts.
Stringing is the appearance of a thin plastic string coming from the sprue.
Vacuum hollows (‘empty bubbles’) in the moulding, due to thermal shrinkage that draws material away from the fluid core of the part.
A dimensional deformation of the moulding resulting from frozen-in stress, or because the part was taken too hot from the mould. Basically it is due to pressure differences between areas.
These lines occur where two plastics flow fronts in the mould meet. The streams of plastic should be hot enough to fuse adequately. Weld lines are not just surface marks, but can be points of weakness: notches, stress raisers.