High-Pressure Water: A New Concept for Screw Cleaning

Ever wonder how many shots are injection molded each day? An enormous number, no doubt, yet not all shots make acceptable parts. Shot size, melt uniformity, color mixing, splay, contamination, material changes, and black specks are only a few of the causes for rejects.

These part problems cost companies huge sums of time and money. While there are a number of possible causes for these problems, they all share a critical component of a molding machine—the screw. In many ways, it’s the forgotten piece of the puzzle when it comes to troubleshooting. Consider some of these demanding functions the reciprocating screw has to do on every cycle:

 1.  Compress the granules for the transition zone,
 2.  Drive air and volatiles out the feed throat,
 3.  Feed resin to the transition zone,
 4.  Uniformly melt various plastics without discoloring 
or degradation,
 5.  Accommodate resin and color changes with the minimum amount of time and wasted resin,
 6.  Build a consistent shot size, 
 7.  Develop consistent backpressure.

Each is difficult to accomplish consistently. For this column, I’ll explain why a clean, well-maintained screw will perform these functions more efficiently and provide less hassle. Then I’ll address a topic everybody hates: How to mechanically clean a reciprocating screw.

Many molders use one or more of the commercially available purging compounds for screw cleaning. That’s sensible: Compared with pulling a screw for cleaning, purging a screw is easier, faster, takes less time and is less expensive. That said, there comes a time when screw needs to be pulled and cleaned. It’s unavoidable.

Pulling out and cleaning a screw is a tough job that drains profits because the machine is not producing parts. It is difficult, hot, dirty, laborious, and time-consuming. Not many enjoy the day when the order comes to pull a screw for cleaning. Typically, these are the steps of disassembly (the order may be different depending on manufacturer or type of machine):

1.  The barrel is heated to the last resin’s melting temperature;
2.  The barrel is emptied of resin; a purging compound may be used to start the cleaning process;
3.  The feed block is pulled off;
4.  The nozzle plus heater band(s), thermocouples, and end cap are disassembled;
5.  The drive mechanism is uncoupled or detached;
6.  The screw is pushed/pulled out with everything from gloved hands to hoists to fork lifts and a push bar.

Each one of these steps can be a nightmare in itself. The end-cap screws, bolts, wires, etc. can be encrusted with molten or degraded plastic. Bolts can be rusted, seized, or broken. Removing the nozzle tip and check ring is often a significant physical experience, especially if someone forgot to use anti-seize or used the wrong type. Each of these steps can be frustrating and time consuming.

The nightmare can get worse in the process of cleaning any of these components. The screw can get damaged, for instance, or while cleaning it you may find damaged flights or other problems that may result in costly repairs and delays. And let’s not forget about the pressure to get the machine back into production. Everyone is on the same page on this topic: Put off pulling a screw for cleaning until there is no other alternative.

But a clean, well-maintained screw can improve the bottom line via better throughput, shorter screw-recovery time, less contamination, and faster resin and color changes. So, how do we get there with the least cost, effort, and downtime? 

First, hedge some bets, ask some questions. How has the screw been performing? If you suspect the check ring is leaking, order a replacement. Truth be told, too many molders have to have the faulty part right in their hands to be sure  what is needed. Are there any known bolts, thermocouples, wires, heater bands, etc. that need to be replaced? Order them before the teardown begins. Make sure you have the proper tools to do the teardown. With the expected replacement parts and tools on hand, start the teardown.

As the screw is being pulled out hot, try to get as much of the molten polymer off the flights as possible. I like to run a high-molecular-weight polyethylene through the screw just before pulling it. As the screw comes out—if you have some helpers—use an air knife and brass pliers to peel the PE off. Often the PE is tacky and will pull some of the residual resin and degraded material off the flights.

Here are some typical cleaning methods:

1. HAND WORK: Once the screw is out and safely mounted on supports, patiently use tools like copper mesh, brass putty knives, and wire brushes to begin cleaning it. I emphasize patiently because impatient maintenance personnel sometimes go at it with screw drivers, electric- or air-driven wire wheels, and other sharp devices—and before you know it you’ll have a screw full of scratches.

Getting rid of the caked-on carbon and degraded plastic takes aggressive methods. But the resulting scratches only worsen the situation. Why? In the melting process plastic needs to “slip” on the screw and stick to the barrel wall. Scratches are similar to undercuts; they hold up the plastic and accelerate degradation or formation of black specks. Cleaning takes significant time, and, with pressure building to get the machine on line, it’s no surprise that folks often bypass the final step of buffing and polishing the screw with an appropriate buffing/cleaning compound such as Semichrome.

2.  BURN-OFF: Another method is to burn off the residual resin and degraded material. This works and is easier than hand cleaning, but the temperatures the screw is exposed to in this burn-off process can sometimes cause it to warp, scorch, and/or change its hardness.

3. DRY-ICE BLASTING: A newer technique, which works nicely, is to use CO₂ pellets to “sand blast” the baked-on carbon or degraded resin. This requires new equipment with the right kind of CO₂ pellet and pellet generator. It is also a manual method.

4.  HIGH-PRESSURE WATER: Here’s another newer approach. The process is clean, can be automated, and will do no damage to the screw (though some coatings may be compromised). We’ve all seen high-pressure washers used to clean decks, etc., but there are super versions of these things that cost more money and can produce a steam of filtered water at 20,000 to 50,000 psi. Some are used to cut and trim plastics and are expensive, but we do not need the sophisticated cutting capabilities. The point is that they cut plastic and will not cut the metal of the screw, provided there is no dirt in the water—hence the need for filtering.

To start, buy, borrow, or rent an appropriate waterjet cutting system. Mount it in a waterproof box with safeties that require the box to be closed before it can be powered on. Ensure there are adequate shields and baffles around the box and on the bottom to keep the water from cutting through. Provide supports inside and outside the box to hold the screw but allow it to rotate and move forward and backward.

Don’t forget a drain for the water! Ensure that no one can stick his hand in the waterjet stream to test how powerful it is. The box front panel should be hinged to allow screws to be placed inside. When the panel is closed, that engages the safety switches to allow the waterjet to be turned on. Provide a mechanism that both turns and pushes the screw slowly through the box. When ready, turn on the waterjet. As the screw moves through the box, the carbon and degraded plastic will be cut and blown off the screw. As the screw comes out the other side, provide an electric- or air-powered buffing wheel with an appropriate metal polish. 

This procedure will yield a clean and polished screw with minimum manpower, time, and effort. It isn’t free, but it’s so easy that it might become a yearly routine for all your presses.