Screws and barrels for fun and profit

Last month, I described very briefly the core of every injection molding machine: the screw. There are literally volumes of engineering literature about screws, barrels and the behavior of resins.

But for plastics professionals without the steel rim glasses and pocket protector, the key points are:

* For maximum productivity, screws are optimized for a specific resin type and grade;

* They have multiple functions including metering, mixing, compression, devolatilizing, heating, homogenizing and injecting the melt, and;

* They’re expensive to replace — damned expensive — meaning care and common sense can save a lot of money.

How do you care for your screw and barrel? Rule number one is to keep resin in the hopper, and ONLY resin. Tramp metal is a sure way to kill a screw, and with multiple detection and separation devices on the market, it’s not hard to protect yourself. It’s very cheap insurance.

Secondly, look at your resin handling procedures. Can a stray nut or bolt get into the resin supply? Are Gaylords or supersacks sealed until use? A great (and cheap) solution is to cap the hopper whenever it’s exposed. Many machines ship with a neat lid, which is usually lost when the loader goes on. Rack it next to the press, or on your technician’s ‘crash cart.’

Also, make sure if you’re feeding from a press-side Gaylord or tote, that is it covered. Take a look overhead to see if something can fall in. Overhead conveyors are risky, as is mezzanine racking. And I can speak from personal experience — wasn’t my fault, I swear –but due to a questionable housekeeping procedure. Non-returnable Gaylords were used to collect scrap cardboard for recycling; they were crammed full of everything from cigarette butts to pop cans. Inevitably, somebody dumped floor sweepings into some press side polypropylene. Dirt is bad, but the handful of #10 Robertson self-tapping screws got everyone’s attention.

And I can’t stress enough: Don’t reuse any container that once held resin, at least not anywhere near the production floor. A rarely seen faux pas these days is the cold start breakage, where starting the machine with solidified material inside snaps the screw like a toothpick. It’s worth the price of computer control if only to avoid that horror.

Sometimes the stuff you want in the barrel can cause problems, too. A good example is heavily-filled resins. Fillers and additives are terrific ways to cut costs and add desirable properties, but they’re dumb as a post inside the barrel. Abrasive and corrosive, or both, additives and fillers will happily wear your expensive screw and barrel into oblivion if they’re not plated or lined with resistant materials.

A common issue starts with a minor engineering change, where the customer changes the resin compound specification, for example. You know the resin processes differently because operating parameters have changed, but you have to consider how the condition of your barrel will affect your hard-learned settings.

Remember that the majority of heating happens by shear, not by externally applied energy, so the heat history of the resin shifts as wear becomes a factor. You can compensate, but eventually you’ll need a rebuild. Hard-plating of the screw and bimetallic barrels are typical precautions, and if done correctly, they can give your press better-than-new durability. But as usual, the economics can’t be ignored.

I’ve seen situations where it’s more cost-effective to replace the worn press entirely. Shifting the worn unit to a less sensitive (quality-wise) job, and increasing capacity there can recover some of the cost. Unless it’s a completely separate profit centre, the real replacement cost is the machine cost minus the cost that would have been incurred to increase capacity in the less critical job. And newer machines generally have higher throughputs for a given barrel length and diameter, so it’s possible to get growth in the same press footprint.

There is so much more to say about screws and barrels that it’s fortunate that there are so many resources out there to fill in the gaps. Just remember: they’re the heart of injection molding and extrusion machines, so treat them with respect and they’ll take care of your bottom line.