Gloss Differences
Matte/glossy areas arising from the mold surface being replicated differently. Pressure peaks, wall thickness transitions and temperature homogeneity are the main suspects.
Gloss differences are when different areas of the part reflect light differently; even if the color is the same, the surface looks matte in some places and glossy in others. The source is how well the mold surface is replicated — gloss falls in areas where the melt cannot seat fully against the surface.
How to Recognize It
- A polished surface is not glossy enough or there are local differences; overly glossy areas on a textured surface.
- Typical locations: ejector/core areas, around holes, the weld line, part corners, opposing ribs, changes in wall thickness.
- Clouding on a textured surface is caused by venting.
Physical Causes
- Gloss = the smoothness of the surface reflecting light at a narrow angle. If the plastic replicates the mold surface differently in different areas, a difference arises.
- Different cooling/shrinkage conditions, pressure peaks in the mold, areas stretched by deformation.
- Deposits on the mold surface (plate-out) spoil the replication.
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- If the gloss is insufficient: increase the mold wall temperature and the injection speed.
- For differences at ejectors/cores: optimize the switchover point, REDUCE the holding pressure and time (avoid pressure peaks).
- For differences opposite wall thickness changes: holding optimization + adapt the injection profile to the geometry.
- For clouding: reduce the clamping force (with a 20% safety margin), lower the injection speed.
- For melt homogeneity: reduce the remaining cushion, increase the back pressure, increase the nozzle temperature.
Mold, Design and Material Solutions
- Check the mold surface for deposits/wear; apply a uniform surface polish.
- Optimize the venting; round off corners; provide continuous wall thickness transitions.
- Check the effect of coatings on the contact temperature.
- Dynamic temperature control for a uniform appearance on textured surfaces.
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Color Streaks | With a color streak the shade itself changes; it is a dispersion problem. With a gloss difference the color is the same and the surface texture is different. |
| Dull Ring Near the Gate | A dull ring is only around the gate. A gloss difference spreads over wide areas of the part. |
Frequently asked questions
Why is the mold temperature decisive?
On a hot mold surface the melt stays fluid for longer and replicates the surface texture fully. In cold areas it solidifies early, replication remains incomplete and that area looks matte. Temperature differences on the mold surface turn directly into gloss differences.
How much effect does the condition of the mold surface have?
A great deal. If there are deposits (plate-out) on the mold surface, replication is spoiled; the subject is covered on the plate-out page. Also, if different areas of the mold have different surface treatments, the gloss will be different anyway.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice