Web-exclusive expanded content: Basic Elements: Purging with compound or resin: How do you decide?

Purging with a commercial compound rather than with resin is on the upswing. Should you be using it if you aren’t already? Finding out for certain takes organization and honesty. Yes, honesty. And the benefits are higher productivity and cost savings.

Talking with a number of commercial purging compound (CPC) suppliers as IMM recently did, you expect to hear different approaches regarding the how-to of machine and mold purging. But you might be surprised how unanimous these suppliers are about “why to.”

Increased productivity, they all say, gained through increased uptime, is the main reason CPCs are replacing resin when purging. The uptime increases are not trivial, and to further sweeten the deal, there is money to be saved on the resin used for purging, as well as that consumed just getting a good part.

You may be thinking, OK, that’s all good, but purging compounds are expensive; or maybe that the scrap resin you are using is virtually free. When molders began trying CPC as it appeared about 20 years ago, cost was a reason many decided against it—often $7/lb or more. In addition, a number of them were difficult to use and some even had safety issues. That was then, this is now, and purging compounds cost as little as $2/lb and more commonly around $3/lb. But take note: Cost-per-pound is a fairly minor element in calculating the cost/benefit of CPC. Any meaningful analysis has to focus on the cost-per-purge and add on the increased productivity. There are molders using relatively expensive CPCs and saving money.

Many molders ask the CPC supplier for an upfront guarantee of a certain level of savings and/or productivity. There are too many variables for that. Asaclean, for example, says the norm is 40% to 70% savings, but it depends on the molder’s work, machines, and more. The best way to find out how it will work for you is to make your own thorough comparison. By thorough, we mean well organized and tightly controlled, but above all, honest.

Where the Wild Costs Hide

Since you are still reading this, it is likely you make frequent mold, color, or material changes, and that you know only too well how much time and money they take. Sorry to say this, but it’s probably worse than you think. It is true that skillful job scheduling—dark colors follow light, change molds but run the same material/color—is a good practice, but what do you do when a customer calls needing JIT parts tomorrow? Or if your job mix simply leaves you little choice?

Even if you have those long runs other molders envy, regular purging keeps machines and molds clear of internal buildup and running better. Machines and molds have places where material can hide and cook. CPC supplier Novachem is very clear about the need for getting to the source of longer-term contamination, and the long term can be a weekend shutdown with material left in the barrel. It may take 3 or 4 hours at startup to get good parts without black spots, and once again we are talking about lost production time.

For most molders, however, the biggest motivator for using CPC rather than resin is high change frequency. The more changes you make—especially dark to light colors or different materials—the more uptime you lose. This is straightforward enough, but since many factors enter into the process—machine size, hot runners, material types, and more—there is no single yes/no question to determine if you should be using purging compound.

Frank Van Haste of Novachem says, for example, that if changes take 45 minutes or more, a compound should help your productivity. Neutrex notes that if straight resin or regrind is going through a machine for 30 minutes or more and still not coming out clean, you are losing money. Asaclean says that, while downtime is the leading factor, combining it with lower scrap and fewer rejects easily justifies CPC. Tim Cutler of Dyna-Purge thinks “getting back to making good parts ASAP is how a molder makes money.” By good, he means parts without specks, gels, or streaks, either at startup or later if the machine is restarted. The common denominator, then, is regaining lost production time, and other savings are added in.Suppliers say that if a machine’s time is booked, but it is down as a result of a changeover, you owe it to yourself to learn how CPC can cut the downtime cost-effectively. And that brings us to the thorough—and thoroughly honest—comparison between CPC and resin compounding.

If you wonder whether you even have the time to make such a comparison, take heart. Many molders have already done the studies and the suppliers are happy to give you references (see “A sample cost analysis,” right). However, if you need to see the results with your jobs on your machines, the suppliers have the methodology all worked out. They are convinced that if you make a truthful comparison—emphasis on if—you will see the value clearly and join the purging faithful.

Nothing is Free, Resin is Something

Here are cost factors that Dyna-Purge lists for CPC:

The idea is to include every CPC cost, which also must be done with resin purging costs for an honest comparison—but it may not be so easy to do. CPC is an easily identified line item bought by the pound. The cost of virgin resin used for purging often is not broken out of the overall resin cost for the job. As well, some molders do not cost out scrap resin at all. The Buddha said nothing is free, and he would not have excluded resin. Material cost should be identified clearly on both sides. Then there is the how-to of the comparison.

Kathleen Jarvis of Neutrex, maker of Purgex compounds, says companies committed to lean manufacturing or Six Sigma, and thereby used to performing scientific analysis under tightly controlled conditions, have fewer problems, such as accounting for the cost of purging resin. Equally or more important, they make sure the procedures for using CPC are followed strictly: Too much means waste, not enough adds time, and using the right amount but not allowing it the right time to work inside the machine (usually called soaking) can foul the results. Jarvis says those who do an impartial analysis invariably turn CPC into SOP (standard operating procedure) after seeing the numbers.

Overcoming Bad Habits

Another thing suppliers agree on is the difficulty of changing habits in a molding shop used to purging with resin, yet this is critically important when comparing resin and CPC purging. Operators used to switching resins and running the machine until it clears (break time?) will be tempted to continue doing just that.

Management has to go hands-on with this one. Do the training, set up a procedure, be sure the procedure is followed, scrupulously log the results, and then analyze the comparisons. Managers, take note: CPC makers already have the procedures, know the pitfalls, and for obvious reasons, are eager to support you.

Although we cannot use the name of the company, IMM has the following from a reliable source: Top management at an auto supplier recently needed to cut costs to meet its OEM customers’ requests—a familiar story. Word came to the molding department manager that XX% (double X is your only clue) had to be cut from the budget. This shop was already efficient and personnel cuts were looming.

At that same time, a well-structured comparison between purging compound and resin purging, which had been SOP, was in process. Since this plant makes a lot of mold, color, and material changes, it had a big multiplier. Using CPC, the molder found enough savings from increased uptime and reduced resin consumption to meet the XX% goal—with all hands still aboard.

Not too long ago, the downtime issue was important only to certain types of molders, but as competition gets tougher, more productivity can create a competitive advantage and boost profitability for any molder. The basic story here is this: Commercial purging compounds reduce the nonproductive time from when you stop making one product until you start making good parts of another. And compared to purging with in-house resin, CPC is very likely faster, better, and cheaper. You can use the resin you save to mold more good parts.