Page 344 - Injection Molding Advanced Troubleshooting Guide

340 35 Part Sticking on Cover

35.3.2.1„ Mold: Damage
Any damage to the parting line or the cavity of a mold can lead to sticking prob-
lems and any damage that results in an undercut in the mold will restrict the part
from releasing away from the mold.
Examine the cavity for any dings, scratches, or gouges that need to be repaired.
Also feel along the parting line for burrs that are creating an undercut. If damage
to the mold is discovered it must be repaired to resolve the sticking problem.
Avoiding mold damage is the key reason why steel tools should not be used inside
the cavity. The risk of damaging the mold surface with the steel is to great so part
removal should normally use brass tools. Keep in mind that even soft brass tools
can damage a high polish or textured surface. Everyone must be careful when
using any tool to remove parts from a mold.

35.3.2.2„ Mold: Lack of Draft
Molded plastic parts need draft to allow the part to be removed from the wall. Draft
is the term used to describe the angle of any walls, cores, or bosses that are in the
line of draw for the mold. Because draft angles open up as the part detail ap-
proaches the nominal wall stock the part will quickly be separated from contact
with the mold. If the detail was designed at true zero draft the plastic would re-
main in contact with the mold for the total length of the detail. Without draft the
only thing that allows the part to be freed from the mold surface is shrink.
Depending on the material different draft angles will be required. Stickier materi-
als need additional draft and locations where the part is shrinking onto the steel of
the mold require more draft than shrink-away areas.
Caution with draft must be applied because the angle will impact the thickness of
the detail. A deep rib will release best with extra draft but the wall stock of the rib
may become too thin to effectively fill. Draft analysis should be conducted on all
part designs to verify the correct draft angle is being applied, and evaluate areas
where draft may cause wall stock problems.

35.3.2.3„ Mold: Surface Finish
As texture depth increases, the amount of draft required to remove the part also
increases. Because texture is a series of undercuts, the plastic is stuck and must
have sufficient draft to fall away from the texture. Lack of draft on texture will lead
to either a sticking part or scuffing of the texture.

If a mold is to be textured the depth of the texture must be known to allow accom-
modation of draft angles. Texture companies can help establish the required draft
for a given texture style. A general rule of thumb is to add 1° of draft for every
0.001 in of texture depth. If the appropriate draft is not added to the mold design