Mixed Color

[rodney garrison]

Hi, I am new to the forum and fairly new to injection molding as well. I have been tasked with reducing scrap from injection machines at my company. My question is about how to handle color changes. Currently we have two IMM. One machine, we do a running color change and rarely have more than four pieces of scrap for mixed color. On the second machine, we purge about 70 lbs. of material and still have around five pieces of scrap from mixed color. The second machine is much older than the first and is hydraulic vs. electric. We do not use any purging compound and the company prefers not to use. Also, on the machine we periodically have colors start blending in towards the middle and end of the run. Any ideas or suggestions? Thanks!

[Shearstressin me out]

Rodney,

A couple of follow up questions.

Do both machines have the same shot size?

Do the IMM have the same nozzle tips/Screws inside the barrel?

[iautry1973]

Purging compounds are helpful. You could pull the screw and clean it, but that is normally a lot of work compared to just purging. This may seem basic, but do you start witht he lighter colors and then move to the darker ones? just makes it a little easier.

[rodney garrison]

Thanks for the reply. Both machine have several molds ran on them. The shot size changes with each mold. My guess is that they do not have the same nozzle and screw. One is an older Toshiba 2500, and the other is a UBE 3300.

Is it possible that the screw and/or barrel have wear that would allow material to build up and drop down into the next color?

[rodney garrison]

I have done investigation with the color mix and feel that the color changes are ok. Is it possible that the screw and/or barrel may have wear and allow material to build up and gradually drop down? If I run straight through without purging, I may have 20 shots that have to be scrapped before the previous color goes away. Whereas, on the other machine, I have maximum of four shots of scrap before good parts come out. The problem machine is a Toshiba 2500 and the good machine is a UBE 3300.

Thanks!

[Suhas]

Rodney,
Yes, it could be the barrel and the screw are worn. Also check to see if the cushion is consistent. That can some times tell you if there is a wear on the checkring.
Suhas

[Shearstressin me out]

The flight radii on your screw could also be too small causing an Eddie. Material will sit at the bottom of the flight and degrade. It can then break off throughout a production run causing foreign material in your parts.

We have seen this before when running PC with a general purpose screw.

[JGentry]

I agree, if purging compound isn't a expense the company wants to add then your only other option is to pull the screw to clean it that way and eat the down time and labor for that task. We do a lot of material changes from ABS and PC ABS to materials such as ASA, TPO and AES. Raising the back pressure all the way up as well as your RPMs will help the purge over process when switching material.

[rodney garrison]

Rodney,
Yes, it could be the barrel and the screw are worn. Also check to see if the cushion is consistent. That can some times tell you if there is a wear on the checkring.
Suhas

Can you explain what you mean when by "cushion?

Thanks!

[rodney garrison]

Can you explain "Eddie"?

[jnewmanco1]

A properly chosen and properly used purging compound would most likely be cheaper and more effective than what it sounds like you are doing. Why does your company not want to use a purge compound?
James

[jmullan]

Cushion is the lowest point the screw reaches in the injection process. Monitoring this position is a good indicator of how stable your process is. Ex. if it stays the same its stable, if it fluctuates its unstable. If it fluctuates its most likely the screw tip or check ring is worn.

If thats the case, you will need to replace them.

Secondly, tell the company that the investment in purging compound will balance itself out between purge scrap, part scrap and machine downtime. For the most effective purging no matter the compound you are using, max out back pressure and raise rpm's, this should help push the color out faster.

But i seriously recommend purge compound. Asaclean in particular. it is highly effective.