Ideal Mould Group Blog
Plastic Mould Defects-Dull spots near the sprue:
Dull spots near the sprue are mainly caused by:small gates
Due to high injection speeds, small gate cross-sections and by-passes behind the gate, extremely strong orientations of the molecule chains are formed during injection. There is not enough relaxation time directly behind the gate, the peripheral layers of the melt are frozen while still strongly oriented.
Such peripheral layers can only be stressed to a minimum degree and crack under the impact of the high shear stresses. The hot melt inside flows to the mold wall and forms very small notches. The dull appearance is caused by the wide spread reflection in this area.
Melt flows into the cracked peripheral layer (formation of micro notches)
Can the injection speed be reduced? If yes, then
(1) reduce injection speed or use injection profile (slow-fast)
Can the gate be modified? If yes, then
(1) round off transition from gate to cavity
(2) increase gate diameter
(3) move gate (The gate should be placed so that the melt does not have to flow around sharp edges)
(1) increase melt temperature (The effect of those parameters is small)
(2) vary mold wall temperature (-) (The effect of those parameters is small)