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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 mate- rial. 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 manufac- turer'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 com- pany, railroad, etc.) can contami- nate 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 Purging: A to Z is important that you do not contaminate your equipment upon delivery. Broken bags or gaylords should be suspect. 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 contam- inating 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 extru- sion expert Jim Frankland has stated: "A general-purpose screw is a no-purpose screw." Now to optimize the time- and money-wasting procedure that nega- tively impacts your profit margin: Use these steps as a guide to minimize the time and money it takes to switch materials and color on your presses. By John Bozzelli Get more insights on Injection Molding at: short.ptonline.com/moldingKH Learn more at PTonline.com KNOW HOW INJECTION MOLDING In this purging procedure the melt temperature is too low; the indvidual swirls should melt together. 30 AUGUST 2016 Plastics Technology PTonline.com K now How INJECTION MOLDING