Page 326 - Injection Molding Advanced Troubleshooting Guide

33.3 Gloss Variation Troubleshooting 321

ƒ Fill velocity
ƒ Fill only weight
ƒ Mold temperature
ƒ Melt temperature

33.3.1.1„ Molding Process: Second-Stage Pressure
The most important process setting for gloss on molded parts is the pressure ap-
plied to the plastic in the cavity. If not enough pressure is applied to the plastic the
molded part will not replicate the finish of the mold well. This applies for polished
surfaces, textures, and sand-blasted finishes. Think about the surface of the molded
part compared to the mold surface: is the plastic being packed into all of the micro-
scopic surface detail of the surface of the mold?
Examining mold surfaces under magnification will show a series of polish scratches
on polished surfaces and a series of peaks and valleys on a textured part. To have
adequate replication of these microscopic surface details the plastic has to be pres-
surized at the appropriate levels. If the pressure is not high enough, the surface
will not reflect the details of the mold surface.
Variation in pressure across a mold will result in a potential variation in gloss
levels across the part. Low-pressure areas will not replicate the surface as well,
and will typically have a different gloss appearance when compared to better-pres-
surized areas. This can often manifest as a distinct difference between areas near
and farthest from the gate. Also watch for areas that fill quickly and freeze before
being packed out.
One of the challenges faced in injection molding is trying to minimize the pressure
drop across the mold cavity. Some of the keys to minimizing pressure drop include
filling fast to avoid viscosity variations during first-stage injection. Proper gate
placement is also critical to ensuring a minimized pressure drop. To determine
®
gate quantity and location a flow analysis software such as Moldflow is a valuable
first step before steel is cut.
When troubleshooting for gloss problems check that the second-stage pressure is
set at the correct value (do not forget to account for intensification ratio). Also
check to see if the correct nozzle and nozzle tip are being used because a mixing
nozzle will result in a large pressure drop between the machine and the end of the
cavity.
Cavity pressure transducers will provide very useful data on the actual pressure
within the mold. If a transducer is located near the gate and at the end of fill loca-
tion, accurate data on what is occurring in the cavity will be easy to obtain.