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16.3 Blush Troubleshooting 145
16.3.2 Blush Troubleshooting Tooling Issues
The main mold cause of blush relates to gate dimensions:
Gate geometry
Hot runner tip temperature
Cold slug well
Gate location
16.3.2.1 Mold: Gate Geometry
Often the design of a gate will lead to blush problems. If a gate has a long land
length it may increase the blush on the part. Normally, gate land length should
be held to a maximum of 0.030 in and a minimum of 0.005 in distance from the
cavity.
Any setback of the gate from the class “A” mold surface may lead to blush. If the
gate is not flush to the mold surface there will be an area of back fill that will leave
a disturbance to the plastic entering the part. If possible reduce this setback to the
minimum acceptable offset from the class “A” surface as this will normally help
with the blush appearance.
Try widening the gate, because a wider gate will allow more material flow and may
minimize blush at the gate. Gate thickness may also play into the blush; however,
do not assume that a thicker gate will eliminate blush. Gate thickness can go either
way when it comes to gate blush, and there are times that thinner gates will
improve blush. Figure 16.2 shows typical gate blush.
Figure 16.2 Gate blush
