INJECTION MOLDING: Purging: A to Z

Changing color or resin in an injection molding machine is a significant hassle confronted by all molders. Changeovers take time (both machine and operator) and cost money in the form of wasted material. This is time and resin you cannot sell to your customer.

The challenge for molders is to minimize these losses. I do not have a magic procedure to pass on that will allow you to make changeovers in 5 min. Besides, one procedure will not work for all resins. But in this column I will make some suggestions that I have used to get the job done as quickly as possible.

First, let’s make sure the goal is clear: Get all the flow paths of the resin clean so the next resin or color is not contaminated. The flow path of the resin starts with the manufacturer’s package—bag, box, truck, or railcar. Once arrived at the molding shop, the resin has to be unloaded, then sent to the machine. Its path may consist of conveying lines, silo, more conveying lines, dryer, machine hopper, feed throat, screw/barrel, nozzle body, nozzle tip, and perhaps a hot runner to the final destination—the part. That’s a lot of flow paths, so where should you start?

1. Resin supplier and carrier: Nobody is perfect, and the resin supplier and carrier (trucking company, railroad, etc.) can contaminate the resin. So before unloading or accepting delivery, pull samples and inspect. (How to sample bags, gaylords, hopper trucks and rail cars is a subject for another day.) It is important that you do not contaminate your equipment upon delivery. Broken bags or gaylords should be suspect.  Also, make sure that the unloading line is clean with no breaks or holes,  and that contamination from another resin, dirt or oil does occur.  

2. Plant storage: Whether silo or warehouse, it needs to be checked periodically, not only for contamination sources from dust to bird droppings, but for cleanliness and appropriate space and equipment to store bags or gaylords without contaminating the resin.

3. Transport to dryer or machine: Usually not a major source for contamination problems, but anything can happen in a molding shop. Elbows wear, lines do not mate properly, and so forth, so fines, angel hair, stringers, and contamination are all possible.

4. Machine hopper and feed throat: The hopper is supposed to be easy enough to clean but is this actually the case? Check the hopper for proper angle and cleanliness—no fines, etc. The feed throat is not as easy to get at and rarely gets the attention it deserves. Take time to inspect it and remove any buildup of oils, waxes, resin, rust (yes rust, there are times when you can actually see condensation) or other contamination. 

5. Screw and barrel: This is the last and biggest hurdle to overcome when changing color or resin. Before I begin the stepwise process for purging, it is important to state that a lot of this would not be necessary if molders paid more attention to screw design. As fellow Plastics Technology columnist and extrusion expert Jim Frankland has stated: “A general-purpose screw is a no-purpose screw.” Now to optimize the time- and money-wasting procedure that negatively impacts your profit margin:

• a. Make sure you have a free-flow nozzle tip. General-purpose nozzles have a dead spot in the nose and take about 50 shots to purge clean. I recommend throwing them out. If you’d rather not, take off the general-purpose tip and clean it.

• b. Check that the nozzle body is as short as possible for this mold and there is no obstruction in the flow channel.

• c. Purge the barrel of the existing material. I don’t change shot size or temperatures until the barrel is empty. With the screw fully forward, I rely on screw rotation (not high-speed), until there is no melt stream coming out of the nozzle.

• d. With the hopper and barrel empty, pull the screw back without rotation and check all the feed flights you can see. That is, pull the screw back as far as possible while looking down the feed throat (with proper eye protection). Do this in increments if working alone. If there is any melted or solidified material sticking to flights in the feed zone, you probably have a leaking check valve. There should be no melted material on these flights. Clean any melted or solidified material off with a plastic rod, tube, or (second best) a wooden dowel. Please don’t use metal. The screw needs to be smooth and scratch-free to work properly. Plastic should stick to the barrel and slip off the screw if things are working properly.

• e. Add the purging compound of your choice, rotating the screw until it backs up to about 25% of the barrel capacity. If there is too much backpressure to allow the screw to back up, lower the backpressure or use manual decompression to do this in increments.

• f. Let it sit for a few minutes.

• g. Purge the purging compound.

• h. If introducing a new material, adjust temperatures as required.

• i. When at the new temperatures, feed another 25% of the barrel with purging compound.

• j. Purge the barrel until empty.

• k. Start feeding the new material—use 100% regrind if available—to 25-50% of the barrel capacity.

• l. Purge until empty.

• m. Build shot size for the new material.

• n. Adjust backpressure to 750-1000 psi (50-70 bar) plastic pressure, providing this is appropriate for the resin you are working with.

• o. Start shooting plastic into the mold with first-stage injection only (hold pressure very low…near zero). Again, use 100% regrind if possible.

• p. Adjust screw rpm to provide a reasonable recovery time.

• q. If all is OK, add hold pressure for the next shot.

• r. As you are making parts with 100% regrind or prime material, vary backpressure from very low to high, up and down, as the screw turns on one shot.

• s. On the next shot, adjust injection speed to as fast as you are comfortable with, make a shot and vary screw rpm from low to high.

• t. Repeat the two previous steps two or three times, starving the screw periodically to allow the screw to rotate without building a shot. Vary injection speed, screw rpm, and backpressure. Do not keep the hopper full; hand-feed if necessary.

• u. Drain the hopper if you were working with 100% regrind, clean if necessary.

• v. Add production-quality resin—100% virgin or appropriate ratio of virgin and regrind.

• w. Set backpressure to your requirements.

• x. Adjust screw rpm to take all but 2 sec of cooling or mold-closed time on normal cycles. For fast cycles, adjust this timer to ensure the cycle is determined by the timers, not screw-recovery variances. Remember, you cannot set screw-recovery time. It will vary during production.

• y. If you have a hot runner that is contaminated or releasing black specks, try a purging compound specifically made for hot runners and raise the tip temperatures about 50° F (25° C), providing the resin can handle this higher temperature.

• z. Say your prayers and check the parts. Ideally, our nefarious friend “Murphy” fell asleep during your purging process and you have a clean changeover. If parts show problems such as black specks, contamination, etc., you will have to make a decision to keep purging or pull the screw. No easy answer here.

Bottom line: Look at the time and resin costs for color and material changeovers. Would you be better off if you found a better way? Even if it is not your money, how about the time and hassle you put in? Talk to your manager, ask the embarrassing question: Is the above factored into the “just-in-time inventory” strategy of your plant?

EDITOR’S NOTE: The author thanks Joe Cascarano, technical service manager at Colors For Plastics, Elk Grove Village, Ill., for his contributions to this column.

ABOUT THE AUTHOR

John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Mich., a provider of training and consulting services to injection molders, including LIMS, and other specialties. E-mail john@scientificmolding.com or visit scientificmolding.com.