The Path to Increased Productivity

In my opinion not enough people have paid attention to one of the most critical parts of the molding process - the non-return valve. How much do you know about yours? Along the way I will ask some questions and maybe you'll need to rethink some things.

CHECK RING

"When the screw rotates, does the check ring rotate as well?

Throughout most of my career this was the valve of choice, but it has idiosyncrasies. I believe there is one machine manufacturer that builds non-return valves where the check ring rotates with the screw. While this may decrease some variability it may also lead to problems such as premature wear of the barrel or shear degradation of the plastics at the barrel wall.

At what speed does your screw rotate?

Spending enough time will show you that as the non-return valve wears, material can be abused to a greater extent. Usually the flow channels that the material will pass through can get narrower and in some cases almost close itself off.

How often do you inspect (or replace) your non-return valves?

One may not know that many check ring designs have different angles on the front and rear seat.

Do you always check yours to make sure it's not installed backwards?

My experience shows that starting with slow injection rates might give one a hint that a check ring assembly is leaking. Another sign is to watch for the screw turning when injecting, or see if the screw bottoms out.

BALL CHECK

The second of the two most popular types has been shown to be the more repetitive of the battle with the check ring assembly.

Why don't you use them?

The enclosed ball tends to wear as well but the failure mode is not as noticeable on the machine output. If the ball wears unevenly it may not seat properly and could cause production of defective product.

NEWER DESIGNS

Over the last several years there are newer designs that have proven to be even more effective at minimizing variation in the process. Products I have tried included one from US Valves and one from Spirex. The designs are similar in that there is a poppet style sliding component that travels within the valve - instead of a ball. My limited experiments have shown that proper use can virtually eliminate intermittent underfills (if the process is properly set up).

Have you ever tried one?

SO, WHERE DOES THIS LEAVE US?

My experience has been that the technicians prefer the newer designs; however, the machinists who maintain the equipment prefer the ball check. The battle may be between how repetitive the process should be versus how often one has to disassemble the injection unit to check or replace the non-return valve. It seems that most technical papers I've read comparing performance have been written by the non-return valve builders, so one would have to decide how credible the sales pitch is. You should decide who wins. You might just try a new design. Your problem may go away.

Jerry Golmanavich