Over the past week, I have received a few follow-up questions regarding the 20% rule for barrel capacity.
In a previous blog entitled The 80-20 Rule For Available Shot Size I discussed the importance of processing parts where the shot size falls between 20 and 80 percent of the overall shot size capacity.
The questions all revolve around ways to get around the issues of processing shot sizes below 20% of the shot size. A typical questions is ‘Is possible to have a process control with this type of problem?’ ‘Are we going to be able to have shot to shot consistency?’ ‘I suddenly encounter shorts or flash without any warning… could this be the cause?’ ‘We are considering better process controls… will this help?’ ‘We have to purge the barrel often… might this be the cause?’
I have addresses these individually… but would also like to address this in general terms.
Basically… when you process at 20% barrel capacity, you typically have 5-10 cycles of material in your barrel. If you use 10% capacity, you have will generally have 10-20 cycles of material in your barrel. Likewise, 5% capacity jumps it up to 20-40 cycles. This means that the material will cook, breakdown, and degrade while sitting in the barrel.
Additionally, each time the screw moves forward to inject… you cause additional mixing amd material migration within and over the screw flights. This will cause an increase in the residence time distribution. This means that if you put a pellet of colorant in your barrel, the time form when you start seeing the color, to the time you stop seeing the color increases.
With the increases in both residence time and residence time distribution… you significantly increase the risks associated with degradation, viscosity variations, mechanical property loss, as well as all the related part defects that occur.
A good way to see your property losses would be to mold some parts with virgin material record the peak pressure during first stage as well as perform some mechanical tests on the parts. After this, regrind the parts, mold the parts again and compare the loss in peak pressure as well as the loss in mechanical properties. Most materials will exhibit 5-10% property loss when processed properly.
There are also other factors which affect the stability of the process… but those are the two biggest when you are processing with an over-sized molding machine.