I’m often contacted by companies that are having issues with a production tool that has a manifold/hot runner system. Color hangs up in the manifold and causes extreme amounts of scrap and excessively long changeovers. Regardless of whether the issue originates at the drop, nozzle body, or somewhere in the manifold, all of their problems are very similar.
Most times they are not using a commercial purging compound. In the cases where they are using a CPC, they are either using it improperly or just using the wrong purge material. With today’s JIT production no one has the room for excess downtime or scrap.
Just a few quick things to consider:
Nozzle tip orifice on the molding machine – Is it the same diameter as the orifice on the sprue bushing?
Manifold and Nozzle body and nozzle tip heaters -- are they working properly?
Are the heater controllers set to temperature (auto mode) and not set on percentage (manual mode)?
Are you using all of the drops in the hot runner?
Looking at where the defect is in the molded part will help you isolate where the problem originates.
Color streaks all through the part or farthest from the drop are generally coming from the screw and barrel, check ring, or nozzle of the molding machine.
This is sometimes hard to identify when the process is set up to use only some of the drops that the manifold was built with. The processor will automatically assume that it’s the manifold and not the molding machine injection unit.
Let me explain.
The processor will set up a process using only part of the drops built into the hot runner system. This is usually done trying to eliminate or hide weld lines. This does produce a visually acceptable part. This plan does backfire more times than not. They may have a better looking part, but they will fight color changes for the entire life of the program. This can cause a huge drain on the already small margin they may have on the product. Excessive machine downtime, excessive scrap at change over, excessive labor hours performing changes.
The manifold has 7 drops and the processor has set up the process to use only 6 drops. The drop not being used just had the heats turned off on that leg of the manifold. Eventually, after running for several hours, material will seal over the intersection where that leg intersects and will not color streak. It is best to turn on the heats and clean the entire manifold with purge material. Once the manifold appears clean, turn off the heats to that leg of the manifold. At this point you can now successfully make the color change in the manifold.
In many cases the processor will have the gate welded up at the drop that is not used. They will keep the manifold heats on to the leg not used. Understand that they will still need to open mold purge to clean that leg with purge material prior to the color change as in the above example.
These are just a few scenarios, but hopefully you now have a better understanding of possible causes and easy solutions to your problems.
If you are having issues cleaning your hot runner/manifolds, I would be glad to discuss your issues and provide solutions. Please contact me.
Learn about the importance of implementing proper purging procedures by downloading this free industry guide.