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Scientific Injection Molding – Packing Pressure and Gate Seal
This is the third post relating to Paulson injection molding courses and the application of Scientific Molding
Packing pressure and gate seal– If you’ve followed our sequence of machine control settings, you’ve learned about fill rate control and the velocity to pressure timer (VPT).
Now we’ll look at packing pressure and gate seal determination. You’ve learned that fill rate control should be active until the mold is about 95% full. That way, most of mold filling is under the control of the fill rate control system. The next step is to pack the mold using a second stage injection pressure to increase the number molecules in the cavity. This is necessary because when the plastic cools from its melt temperature to room temperature, the polymer molecules move closer together causing the molded parts to start shrinking. To compensate for this shrinkage, mold cavity pressures will be typically average from a few thousand PSI to 10,000 psi, depending on the part design and the dimensional requirements.
Dimensional control is fairly straight forward. In injection molding, dimensional control depends primarily on the maximum plastic pressure in the cavity. To increase dimensions you will increase cavity pressure, to decreased dimensions you will reduce it. If you have a pressure transducer in the mold you can measure cavity pressure directly. Most injection molds don’t have a pressure transducer, but another good measure of cavity pressure is part weight. If a part’s weight remains constant, the cavity pressure is under control.
Making sure the mold cavity gates have frozen before you end pack pressure is another necessary step in controlling part dimensions. The process of determining when the gates freeze is quite simple. The packing timer is set long enough to make sure the gates are frozen when pack pressure is released. Most gates will freeze within 5 seconds. So starting with a pack timer setting of 0.5 seconds and increase it in 0.5 second increments. Then plot plastic part weight versus pack time on a chart. When the plastic part weight stabilizes you have found the proper gate seal time. Cavity pressure is not forcing plastic back into the runner. Once gate seal time is determined, add a bit more time (about 20%) to make sure the gate stays sealed in case there is an increase in the mold temperature, melt temperature or the packing pressure. Any of those conditions could cause gate leakage.
Part Weight is the single most important data point to track as an indicator of a good part.
Please ask your questions and leave you comments below.
This Post Has 3 Comments
Ok so the 20% extra on the gf time. I can see the logic behind the melt temp and mould temps and good point that every mold tech should understand. The pack pressure though is not a function of time and I cannot see any logical linear relationship between these two variables. If I have a holding time of 5 seconds and 10,000 psi and then increase the holding pressure on the next shot to 15,000 psi still with 5 seconds how has that 20% helped. The increase in pressure has not increased the average temperature at the gate vessel. Surely if the gf time had been calculated at say 5 seconds with a 20% buffer then if the holding pressure is changed up or down it just means the average weight of our part will be either lighter or heavier. The gf time is constant and the holding pressure does not effect the gate seal were the mould and barrel temps do.
The extra 20% of added pack time is to insure the gates stay frozen if there is a change in the melt or mold temperature. The statement, “or pack pressure” is not correct. A change in the pack pressure will affect part dimensions because the gate would not yet be frozen when the the change in pack pressure occurred. So the cavity pressure would be affected. Congratulations on your accurate analysis We will correct this error.
How do I determine the correct injection time?