In my last seminar I was again asked another common question. The same question has been discussed a few times in the past on my forum.
Question: If one uses the Decoupled Molding technique (service mark of RJG Inc), should you fill the mold 95-98% full by volume or by weight?
Here is my take on this. To begin with let me give you my theory on filling the part less than full. As the plastic melt cools down it shrinks. As it shrinks the volume reduces. Molten plastic is injected into a cold mold where the melt begins to cool down as soon as it touches the mold. Therefore we need to get the plastic inside the cavity as soon as possible. This is the injection phase. In the injection phase, we fill the mold cavity with molten plastic. But if we stop there, the plastic will shrink and the part that is ejected out of the mold will have sink. To compensate for the shrinkage and eliminate the sink, the second stage called the pack stage is applied. During this stage, the plastic must enter the cavity at the rate of the shrinkage that is occurring. The third stage is the hold stage that is applied such that more plastic does not get into the cavity nor does the plastic that is under tremendous pressure get out of the cavity. It is applied till the time the gate freezes off. So in summary, we fill the cavity, compensate for the shrink and hold till gate freeze.
When we say ‘fill the cavity’ – in theory we need to fill it up a 100% with molten plastic … and that is where the problem arises. Plastic melt is highly compressible like a ball of rubber bands. So we do not know if the volume of plastic that was injected into the mold was equal to a 100% of the volume of the cavity or was it more than a 100%. It could have very well been more that a 100% since the melt could have been compressed. Compressed melt is not desirable since it can lead to issues such as stress and flash in the parts. For that reason, the mold must be filled just a little less than a 100% and then the pack and hold should be turned on. That is where the number of ‘less that 100%’ comes from. An added benefit of this is also to slow down the injection speed before hitting the point of transfer into pack and hold to achieve consistency of the point of transfer. It is like slowing down at the traffic light before coming to the light and not slamming on the brakes at the zebra crossing.
So does it then matter if we are going to transfer by weight or volume? To begin with, Weight = Volume X density. Since the melt density is going to be constant, 95% by weight = 95% by volume! I think the question is answered. It does not matter. This can get confusing in some situations:
- On thick parts you may fill the skin first and then the inside will get packed out. So there is not visual sign of the less that 100% in injection.
- On thick parts once the skin is fully filled, the vents are closed and there may still be air inside the cavity. So you may need to inject slowly to vent out the air and then pack and hold. In this case a ‘visual short’ may be the way to go. This may appear to be 98% short by volume but may be only 90% by weight.
- On thin parts, since there is not much shrinkage a 98% by volume will be very close to 98% by weight.
Comments on the % number: So should it be 95% or 98% or why can it not be 90%? The answer is that there is no definite number that must be followed. Here are some general guidelines:
- The thinner the part, try to use closer to 100%. In some cases, where there are very thin sections such as filter mold I was working on, I had to get 100% of the material in there in the injection phase with no pack and hold pressure but with some hold time to seal off the gate.<