Page 249 - Injection Molding Advanced Troubleshooting Guide

240 26 Dimensions

„ „26.3„ Dimensions Troubleshooting

Most factors that impact dimensions will be capable of either reducing or increas-
ing a part size depending on which direction the change is made.

26.3.1„ Dimensions Troubleshooting Molding Process Issues

There are many process factors that will impact dimensions on a part including:
ƒ Second-stage pressure
ƒ Second-stage time
ƒ Fill velocity
ƒ Fill only condition
ƒ Melt temperature
ƒ Mold temperature
ƒ Cooling time
ƒ Decompression
ƒ Cushion

26.3.1.1„ Molding Process: Second-Stage Pressure
One of the biggest impacts on dimensions will come from adjusting second-stage
pressure. Normally higher pressure will pack more plastic in the mold leading to a
larger part. Lower second-stage pressure normally leads to a smaller part.
With pressure it is important to realize that once the gate or wall stock freezes
there will be no more pressure transferred through the part. This is an important
aspect of maintaining uniform nominal wall stock. If a part has varying wall stock
the thin areas will tend to freeze, which leads to additional shrink in the thicker
areas, which in turn may limit impact on dimensions. Once the gate or wall freezes
there is no longer a chance to pack the part. If the area of concern is a thin wall
section the wall stock may freeze so fast that there will be very little change in
dimensions.
Although keeping things simple is always the preferred choice there will be cases
that using a profiled hold may be required to impact a challenging part. Sometimes
using a high second-stage pressure for a couple of seconds will help pack out a thin
section. It is important to understand, however, that adding a profiled second-stage
pressure increases the complexity of the process and creates another thing that
needs to be validated when running parts.