IMM 7/97

I am looking for information about how to purge the hot runner systems of molds running PVC during the shutdown process. Can anyone help?

Place a spare set of bushings over the leader pins in the mold. Zero the mold at very low tonnage-I use 15 tons on our 275-ton Van Dorn (hydraulic clamp). The pressure switch to start injection will need to be adjusted. Cycle the machine with an appropriate purging material in semi-auto, purging the resin into the open space between the mold halves. Return machine parameters to original settings, making sure to rezero the clamp.
-E. Fisher, Steelcase-Attwood Subsidiary, Lowell, MI, (616) 897-2202.

Because of the very small processing temperature range of PVC, it can be one of the most difficult materials to run, let alone purge. Generally speaking, hot runner systems are not recommended for injection molding of PVC-it is highly corrosive. Your molds should be made from stainless steel. My first question is what procedure, if any, does the manufacturer of the hot tip nozzles recommend. Purging compounds are probably the best method of cleaning up degradation left behind, which is typically an easy task. But, your concern is the hot runner, a different beast altogether. The Chisso Corp. has developed a specific grade of Z Clean purging compound engineered for PVC. Most importantly, Z Clean is engineered for hot runner systems with very small hot tip nozzles and gates.
-M. A. Billings, LarSan Chemical, Akron, OH, (800) 837-5510.

I used the following purge formulation (with K-67PVC) for twin screw extruder cleanout very successfully. Using a lower K-value PVC for molding, I am sure the same formulation would purge your hot runners nicely. Formulation: PVC (K-55-58)-100 parts; CaCO3, 1-2 µm-100 parts; Ca Stearate-4.0 parts; 165 paraffin wax-3.5 parts; Tin Mercaptoester liquid stabilizer-10 parts.

I ran this as a powder blend in the extruder, but if you can have it pelletized, it may work better in a screw injection molding machine. The neat thing about this purge was the fact that it could be left in the machine overnight, cooled, reheated the next morning, and pushed out with production compound. There is absolutely no sticking tendency with this blend, due primarily to the lubricant content.If you try this in molding, you'll be on the cutting edge of PVC technology.
-S. Thacker, PVC Technical Services, Cincinnati, OH, (513)561-0661.

When shutting down a hot-runner-equipped molding system that's running PVC, it's important to purge all of the displaceable PVC out of the system. Let's consider what happens if you don't do this. Suppose late on Friday night the operator runs a few pounds of natural HDPE or a stiff styrene through the system, moves the big red switch to off and goes home. For the next three hours or so, as the machine slowly comes down to room temperature, the small amount of residual vinyl left in the system is acquiring heat history. 

At midnight on Sunday, the third shift comes in and moves the big red switch to on. Two cups of coffee and a quick cribbage game later, the heats have lined out and they're ready to go. Unfortunately, that leftover PVC has been quietly degrading in a warm, dark place. So parts start coming out with black specks in them. It takes half a shift of messing around before you get consistent good parts. Just another typical Monday morning. 

Incidentally, this scenario isn't confined to PVC. It happens with any resin, but the consequences are most obvious because PVC is sensitive. Using a stiff material such as low-melt-index HDPE or a high-heat GP styrene, or even a conventional purging compound that relies on physical displacement to purge before shutdown will get the great bulk of the vinyl out of the system. But it will inevitably leave behind a small amount in dead spots, crevices, and restricted flow areas-especially in a hot runner manifold. And any PVC processor is painfully aware that a small amount of PVC will, when properly cooked, yield a very large quantity of carbon particles.

We believe the only really effective approach is to employ a high-performance chemical purging compound. When employed in accordance with the manufacturers' instructions, this class of products will remove virtually all of the PVC from the system and provide for trouble-free restarts. Note, however, that the system needs to be free of degraded material to begin with, and that you have to be committed to using the purging material on every shutdown-planned or unplanned. It only takes one unprotected thermal cycle to start making black specks.

For this type of compound, the shutdown scenario would not change very much from that described above, except that the operator would run the system empty after flushing with HDPE, then fill it with purging compound and hold it for a heat-soak period (with occasional cycling to maintain active material in the mold) then run it empty again before cutting the heats. Start-up would go as before, except there would be no black specks. We're talking here about the most simple case-purging process material at operating temperature. For more complex situations- such as a dark-to-light color change or dealing with a system in trouble after degradation has already shown up, the procedures are more complex and time consuming
-F. Van Haste, Novachem, Bridgeport, CT, (800) 762-3984.

My first response is "very carefully." Our purging compounds have been specially formulated to absorb HCI and chlorine gas when purging chlorinated polymers like PVC and PVDC, and we have had the opportunity to work with major hot runner manufacturers who have given their input to perfect hot runner purging. For tailored PVC purging instructions when shutting down, contact us.
-B. Giaquinto, Technical Dept., RapidPurge, Trumbull, CT, (800) 243-4203 or (203) 372-5677.

As a major producer of molding grades of rigid vinyl, we have evaluated many different types of purging compounds. While many do a satisfactory job of cleaning the barrel, only a few can be used in a hot manifold system. While the additive type compounds (to be mixed with the vinyl) work, we prefer purging hot runner systems with a nonvinyl compound.

Dyna-Purge Pulsar manufactured by Shuman Plastics is used at normal processing temperatures and does an excellent job purging both the barrel and hot runner manifold. After the air shots, the purging compound is introduced and purged through the system.
-C. Toensing, The Geon Co., Avon Lake, OH (800) 438-3466.