Page 377 - Injection Molding Advanced Troubleshooting Guide

374 39 Scuffs and Scratches

Texture on side walls must be accounted for during part design because adequate
draft must be used for the chosen texture. A lack of draft will lead to a situation
where the texture is “scrubbed” from the mold or reduced to accommodate part
ejection without scuffing. Bear in mind that for every 0.001 in of texture depth the
draft angle may need to increase by as much as 1°.
It is important to understand that parts that are molded with very low gloss levels
(under 2.5) will tend to show marring very easily. A slight difference in gloss level
can provide a part that provides a much more stable surface appearance.
Another detail to watch for is parting lines that have been rolled over creating an
undercut. These undercuts in the steel will cause a drag or scuff when the part is
ejected from the mold. A ruby stone can quickly clean up a rolled parting line. To
detect a rolled parting line drag a fingernail across the suspect location; if there is
a rolled edge your fingernail will catch on the edge.

39.3.2.2„ Mold: Lack of Ejection
A mold must have enough ejector area to successfully remove the part from the
mold. If there is not enough ejection area in a mold the part may be trapped on the
mold. Lack of ejection area can lead to a variety of damage including pin push.
Whenever ejector pins are designed in a mold the best situation is pushing the part
off the mold rather than pulling the part off the mold. In other words, the ejector
pins, sleeves, and lifters should be pushing on the deepest areas of the part when-
ever possible. If the ejectors are pulling the part off the mold the chances of a rib
or boss sticking is elevated.
Another area that can create damage is if a part sticks on a lifter or slide and tries
to travel with the component during ejection. To avoid this problem ensure that
components have an adequate draw polish for ease of movement. Another trick
that can help with this problem is to add “donuts” to the ejector pins near the lift-
ers. These “donuts” are rings that trap an ejector pin on the part; this will help
hold the part steady so it does not travel with lifters.

Guided ejection is another aspect that is important relative to potential ejection
damage. If parts are not ejected with the ejector plate moving forward in a square
orientation there may be a chance for part damage to occur from sticking or scuff-
ing. Guided ejection will help to keep the ejector plate square as it travels forward.

39.3.2.3„ Mold: Parting Line Damage
If the parting line of the mold becomes rolled or has a burr it can cause a scratch
or scuff on the molded part. When the scratch is in a consistent location examine
the mold for damage that could be leading to the defect. Damaged parting lines
need to be repaired to eliminate the defect.