The Designer's Edge: Addressing Mold Issues In the Press

Randy shares various mold issues that can be addressed in the press versus pulling the tool.


Facebook Share Icon LinkedIn Share Icon Twitter Share Icon Share by EMail icon Print Icon


A dovetail burr with a positive stop ground on it is a safer method for grinding undercuts versus a round burr.

There are many benefits to correcting a mold problem by making a physical change to the mold rather than processing around the problem. When processing around problems you can make the process window smaller and put yourself at risk for other part quality issues. Here, I am going to share a few mold issues that can be addressed in the press versus pulling the tool.

Let’s begin with part sticking in cavities. There are numerous root causes, but in most cases the part stays with the ejector/moving half of the mold unless the stationary/non moving half has reverse ejection. If the part is sticking to the wrong mold half, you need to investigate a few things before processing around the problem or playing with water temperatures.
First, check the cavity surface finish for burrs, cutter marks, etc. For example, a polish can be "too shiny" and cause sticking. Simply using 320 paper draw polish can address this problem. If the cavity is polished properly for the plastic being used, it will most likey not be the root cause, so you need to look at the opposite half and determine what can be done to help the part stick to it.
Typically, undercuts are added. Although a good option, they are sometimes not allowed as per part criteria. When adding undercuts with a grinder, I prefer a dovetail bit that has been ground with a positive stop to prevent going too deep, which can cause flaking, sink marks or read-through on the part.
One trick is to add a very fine stipple/texture to the side walls to retain the part. I use a Dremel engraver with a fine carbide tip to add the fine texture to the side walls. This is a $40 tool with various tip configurations that can also be used for texture repair. However, this is only recommended if you are experienced with texturing.  
If the part is sticking to the proper mold half, but is sticking too hard causing pin push, hanging up on lifters or slides or other defects, look again at the surface finish for the items mentioned earlier. If the surface finish is considered good, consider things that cannot be done in the press with the mold design. For example,insufficient ejection, lifter travel, etc. 
Later this month I will continue with more repairs in the press.