Plastics Technology: February 2012 Archives

Unfortunately, here is no standard for machine response from fill to pack. I have seen some machines stop the screw immediately when 2nd stage is turned off, yet allow extensive overshoot once it is turned on. Other machines have different settings for how over or under-damped you want the machine to behave. You should determine the behavior of your actual machines and then make a procedure specific for those machines so everyone can setup a process the same way.

-Andy

My employees were asking me to explain the following statement 'Back pressure causes additional material to flow over the flights of the screw'. I wanted to hear your explanation before discussing it with them.

During screw recovery, the screw is pumping material forward down the flights of the screw. If unhindered, the material will flow to the front of the screw, thus pushing the screw backward to build the next shot. Back pressure provides resistance to the backward movement of the screw, causing some of the material pumped forward to either 1) flow back within the screw channels or 2) flow back over the flights of the screw between the flights and the barrel.

There will always be some degree of backflow within the channels and over the flights, but increased back pressure will significant increase this effect.

-Andy

note: when discussing the troubleshooting process, they have investigated most aspects of the process, but it turns out they never considered trying differential mold temperatures.

Although I try to avoid using differential coolant temperatures when possible, it would be a good option to investigate for this mold/process combination. Since PP has a high degree of crystallinity, decreasing the temperature of one side will cause a tendency to bow towards the opposite side due to differential shrinkage. In this case, the coolant temperature differential between one side and the other might only be 5 or 10 degrees, but it might be enough to counteract the bowing.

Before using differential heats, it would be beneficial to measure both the coolant temperature entering and exiting the mold as well as the temperature of the mold steel. In many cases, you may be using differential coolant temperature, but the result may be similar mold surface temperatures.

-Andy

Once you have determined the tonnage that should be used on a tool, I recommend you put a plaque on the side of the mold stating the tonnage to be used. If the recommended tonnage is stamped right on each tool, it is hard for a technician or setup person to miss it.

If you are having a problem with a specific issue, such as tonnage, always look for a simplistic way to draw attention to it.

-Andy

It is common to believe that advanced training will automatically cause your employees to function at a higher level. Unfortunately, advanced training does little to change behavior if your employees lack a good understanding of the fundamentals. A great example of this is the decoupling of 1st stage filling from 2nd stage packing.

Let's presume you have a technician who has been filling the mold completely during 1st stage fill for the past 15 years. Now you send this  person to a class focused on the finer points of optimizing filling and packing based on the foundation of a short shot during first stage fill. This employee would learn very little which is directly applicable to his job since he still believes filing the mold completely during 1st stage fill is the best way to process.

-Andy

Companies who use individual water lines rather than a manifold should color-code everything using blue lines for incoming water, red lines for outgoing water, and extra colors such as yellow for line which exit and re-enter the mold.

Once you have this established, place colored plaques on each connection of the mold. This will ensure that your employees cannot accidentally connect an in-line to an out connector.

-Andy