As a processor it is our priority to understand areas that are causing you pain. In most injection molding machines, we find that barrel walls, back sides of flights, check rings, and flow channels give us the most problems. This difficulty could come on startup, during a run, or while changing products. When it happens, we find most people resort to running material until it the issues disappear. This results in extended down-time and too much material being scrapped. The great thing about injection machines is your ability to manipulate the machines to clean in situations where you feel helpless.
Here are 4 parameters to optimize for difficult changeovers:
1.) Increase Back Pressure
This parameter is your friend for problematic contamination on the backsides of flights. Increasing back pressure will help clean other areas as well. Understand that the percentage of back pressure is often equal to how full the flights are. You may be able to achieve full flights by other parameters, but the closer you are to full flights, the more likely you will clean the hard to reach backside.
2.) Increase Screw Speed
This parameter is most important for cleaning the barrel walls. The rotating screw will push the material to the outer edge from its rotating velocity. If you have any amount of layering or buildup this is key to scouring the walls and removing the buildup. This cleaning process is best completed with a mechanical purge or what I like to call shear activated. These products are design to clean--not add another layer like production, regrind, or virgin resins often do.
3.) Reduce Shot Size
This parameter is often overlooked during changeovers. Many processors believe ‘full shots’ create the most cleaning. This is simply not true, and I am happy to explain why! Carbon, material, and colorant all love to hang around inside the check ring and nozzle. The more you can move the check ring, the more you are able to ‘jolt’ or break said contamination free. If you change the parameter to 10% or less and do multiple short shots you will not only use less material, you will complete 10 times the movement on the check ring.
4.) Adjust Injection Speed
This parameter is best manipulated for cleaning tooling and adding cleaning power to getting a clean check ring. It contributes to the shear placed on both the check ring and manifolds. Operators or engineers that have difficulty getting these areas clean with their next product, virgin, or regrind resin should highly consider using a mechanical purge compound (shear activated). You will be able to inject it just as fast, but it will be equipped with additives to clean these areas that other products simply flow through.
Ready to reduce your production downtime to protect your profits? Learn more about how purging compounds and process efficiency work in tandem.