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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.

-Andy

What is the best method for measuring mold temperature? Should I trust the thermolator measurement?

When measuring mold temperature, it is actually best to measure these two factors… coolant temperature into the controller and coolant temperature out of the controller. If these are documented when the process is molding acceptable parts, then you can better replicate the conditions in the future.

These measurements should be taken using a surface probe in contact with the in and out connectors of the temperature controller.

The return temperature provides a good indication of the mold temperature. The difference between these two numbers provides a good indication of cooling effectiveness. Higher differences may indicate a poor coolant flow, while a low difference may indicate a too much coolant… or an improperly connected coolant line.

Some molders use the thermolator measurements with good success… but these measurements should be calibrated and verified often with a pyrometer with a surface contact probe.

Mold surface temperature measurements can be helpful, but do not provide relevant data about the overall effectiveness of the mold temperature controller.

-Andy

The most common defects we encounter are flash and short shots. I would think that these two defects would be complete opposites, so why do we always see both?

I agree that these two defects sound mutually exclusive… the part has too much material with flash, and too little material with short shots. Surprisingly, when these two defects are seen together… it often indicates a poorly established fill, and I’ll explain why.

When establishing a molding process… If the part is completely full during first stage, then the process cannot compensate for variability. A drop in material viscosity will cause too much material to enter the mold during fill, causing flash. A rise in material viscosity will cause too little material to enter the mold cavity, causing shorts. 

When the process is established with a short, then 2nd stage pack is used to fill out the part. any typical variation in viscosity during fill will then be compensated by pack.

The most solid and reliable processes are short during first stage fill… and then packed out during second stage.

-Andy

Such bushings are a great option for the longevity of your tooling. These demonstrate significantly less wear when compared to regular brass or steel bushings.

The only drawback to this option would be with your tooling maintenance department. If they have not used such a system, they should communicate with the manufacturer to ensure they understand the operation and maintenance of such bushing systems.

I always recommend spending a little extra money for the better leader pin bushings. The price might be a little higher, but the long term value far outweighs the initial cost.

-Andy