Page 338 - Injection Molding Advanced Troubleshooting Guide
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334 34 Jetting
stream back onto itself, air can be trapped leading to splay on the surface in addi-
tion to the jetting defect.
34.3.2 Jetting Troubleshooting Mold Issues
Most jetting problems have a root cause in mold design. Important mold-related
items are:
Gate location
Gate size
Gate land length
34.3.2.1 Mold: Gate Location
One of the key items of importance concerning jetting is the location of the gate. A
key consideration of gate location is not to locate the gate in the middle of the part
wall stock. If the gate is located in the middle of the wall stock the plastic entering
the cavity will not be able to adhere to the walls of the cavity and may jet as a
result. Locate the gates in line with either cavity or core wall steel.
It is desirable to locate a gate so that the melt entering the cavity will impinge on
something such as a mold detail or wall section. The term impinging means that
the melt stream hits something as it enters the cavity. If there is nothing for the
melt to impinge on when entering the cavity one trick is to add an ejector pin, core
pin, or other standing steel that protrudes into the part wall stock. This detail will
give the melt something to impinge on and breaks up the jetting. This detail can
usually be a minimum of 50% of the nominal wall stock for the depth into the wall
stock. Be careful with this, though, because the change in wall stock may create a
problem with read-through.
34.3.2.2 Mold: Gate Size
Gate thickness can lead to jetting. Frequently, a thin gate leads to jetting; however,
there are cases where a thin and wide gate can resolve the problem, so it is import-
ant to keep an open mind. Figure 34.4 shows an example of a very thin gate with a
long land length.
If jetting is a problem and the gate is excessively thin try to open the gate depth
gradually to determine the impact on the jetting. As the gate is thickened the gate
seal time increases, which adds cycle time and as a direct result adds cost to the
molded part. The case study below shows that troubleshooters must keep an open
mind to gate dimensions; do not assume that thicker gates will always improve
jetting.
