Injection Molding Troubleshooting Tips

Tip #7: Record Groove Effects

Record grooves are very fine lines that show up on the surface of molded parts and look very similar to the grooves found on a musical record. They are also sometimes called finger prints. They appear as concentric rings coming from the gate, especially if the gate is undersized. If the melt is injected into a cold mold a solidified layer forms behind the flow front due to the high cooling rate. If the cooling is very high the viscous flow front area impedes the direct frontal flow of the melt. So, the hot melt front does not get pushed toward the mold wall but instead gets elongated in the middle. At some point the flow front will touch the mold wall but the cooled down peripheral areas of the flow front have no contact with the wall and this forms the actual groove.

Looking at the possible causes of record grooves we can separate them into material, machine / equipment and processing.

From a material standpoint, one should ensure that the material (for example, nylon) is not over-dried which could reduce the flow properties of the material. Alternately, it may be possible to look at switching to a higher melt flow grade of the material which could help to reduce the record groove effect.

From an equipment standpoint, one should ensure that the runners and gates are adequately sized, and if not, they should be increased. It may also be necessary to add additional gates to reduce the overall flow length that the material needs to be ‘pushed’.

From the processing side typical remedies would be to increase the melt temperature, increase the mold temperature, increase the injection speed or increase the allowable injection pressure.

Using a systematic approach, all potential causes of record grooves should be examined and investigated in order to determine the root cause and take the necessary correction actions to eliminate the defect.

Entec Polymers Technical Service Team

Grooves Duo