When filling the mold completely during 1st
stage fill, many processes respond well to pressure transfer when compared to
position transfer. In either case, you will see even better results with a 1st
stage short shot (90-95%) using position transfer.
Question: I recently moved to another company and all the setups are using Injection Pressure as trigger to change from fill phase (Velocity controlled) to Holding ( Pressure controlled). Note that there since there is no pressure sensor in the mold but the machine injection pressure is used. A Engineer here tells me that this was done because while using a screw position switching point they had too much variation of the cushion leading to short shot and flash. What are the pros and cons of a pressure switching point versus screw position switch point?
Regarding Hydraulic Transfer: Hydraulic pressure transfer is used the fills
mold completely during first stage. Once the mold fills, the pressure begins to
increase rapidly and the pressure transfer method is used to identify this
pressure spike and then transfer. When filling the mold completely during 1st
stage fill, many processes respond well to pressure transfer when compared to
position transfer. In either case, you will see even better results with a 1st
stage short shot (90-95%) using position transfer.
mold completely during first stage. Once the mold fills, the pressure begins to
increase rapidly and the pressure transfer method is used to identify this
pressure spike and then transfer. When filling the mold completely during 1st
stage fill, many processes respond well to pressure transfer when compared to
position transfer. In either case, you will see even better results with a 1st
stage short shot (90-95%) using position transfer.
My Recommendations: The process should run with position transfer to
best compensate for the viscosity changes. The part should be 90 to 95% short
shot based on part weight. If this is a multi-cavity mold, 100% full is the
weight of all parts when the first cavity becomes full (this ensures all mold
cavities are short shot). The packing pressure should then be used to compete
mold filling and pack out the parts without flash. You will likely use a
significantly higher packing pressure, but will gain more process consistency
as a result.
best compensate for the viscosity changes. The part should be 90 to 95% short
shot based on part weight. If this is a multi-cavity mold, 100% full is the
weight of all parts when the first cavity becomes full (this ensures all mold
cavities are short shot). The packing pressure should then be used to compete
mold filling and pack out the parts without flash. You will likely use a
significantly higher packing pressure, but will gain more process consistency
as a result.
-Andy
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