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July 28, 1999

6 Min Read
Cooling the barrel:Reduced temperature helps prevent leaking

Editor's Note: Paul Allen is the president of Logic Corp. and a veteran of injection molding for more than 40 years. More than 10 years ago he hatched the idea of cooling the end of the barrel as a way to reduce material degradation, prevent leakage, and reduce check ring and barrel wear. He has since updated the concept and presents the idea here.

As molders know, to properly inject plastic into the mold, the tip of the screw must be sealed against the inside of the barrel to prevent molten material from slipping back during injection. This is done with the check ring, which allows melt to extrude in front of it, but shuts itself off in the opposite direction to build pressure during injection. Between the OD of the check ring and the ID of the barrel a clearance of .004 to .006 inch is typical on a 2-inch diameter barrel. But with metal-to-metal wear this dimension increases. When it does, melt can slip back through the clearance between the check ring and barrel wall. Available shot capacity is lost behind the check ring, and some material is reworked by the screw in subsequent shots.

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Figure 1. By cooling the front zone of the barrel, a region of viscous resin builds up along the wall. This acts as a seal to prevent material from leaking past the check ring. Material in the center of the barrel remains at temperature as it's refreshed each cycle.

Cooling is Key
Obviously, the best way to prevent such leaking is to seal the clearance between the check ring and the barrel. Logic Corp. has developed a system that does just that, cooling the front of the barrel to reduce the temperature and increase the viscosity of the resin between the barrel and the check ring (Figure 1). This higher-viscosity region acts as a seal and prevents material from slipping backward past the check ring. Because plastication occurs in the middle zone of the barrel, and because fresh melt is forced to the front zone in each cycle, the melt at the center of the barrel remains at the proper temperature for injection into the mold.

This system consists of cooling rings installed around the front of the barrel with steel or copper tubing wrapped around the barrel. Air moving through the tubing passes out of several small holes drilled into the side of the tubing that faces the barrel (Figure 2). Care must be taken to make sure the holes do not blow directly on the pyrometer that controls the front heat zone; this could produce false low readings and activate the heaters. Airflow should also be adjusted to prevent excessive cooling (i.e., the heaters stay on or come on too frequently). With a little more investment, a heat controller can be purchased to turn on the air only when it's needed.

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Figure 2. Cooling coils, consisting of copper or steel tubing, can be installed between heater bands. Air passes through holes in the tubing to cool the barrel.

The temperature at which viscosity increases such that a seal can be developed is an inexact science, dependent largely on material type, barrel size, shot size, and the cycle time. Trial and error may be required to help determine the best temperature for the machine and material.

An Example
The problem with even a little leakage-aside from pressure loss-is that material that gets reworked by the screw can degrade. Such was the case with a polycarbonate part molded with a small patch of highly degraded material near the gate. The hot tip was blamed and removed, but the problem persisted.

The part, which had a highly polished surface, had been molded for 18 years without this problem. It wasn't until the customer asked for a light texture on the polished surface that the tint appeared. Parts in stock that were produced with the polished surface were pulled and found to have the same blemish visible in the right light. It didn't become obvious until the texture was applied.

The answer: Some material was being trapped between the check ring and the barrel wall. When injection forward stopped that material was not reworked into the good material, but flowed into the injection stream and was later mixed by the sprue, runner, and gate. This degraded polycarbonate appeared in the part in the form of the discolored spot. Cooling the barrel and increasing the viscosity of the polycarbonate along the barrel wall would have eliminated the leakage and discoloration.

There are other benefits of cooling the front of the barrel. By reducing leakage the screw maintains a consistent pressure on pack and hold, the screw does not bottom out, and part surface texture is enhanced. This is particularly evident in clear parts, especially engineering resins such as polycarbonate. Heat-sensitive resins also show appearance improvement.

Increased Capacity
If cooling the barrel reduces material slippage, more material is left in the shot, effectively increasing shot capacity. But even in a robust system with a new check ring and minimal slippage, cooling the barrel can increase shot size.

A mold that produced 14-oz parts was run in a machine with a maximum shot capacity of 14 oz. Not only was the shot size too small to pack out the part, but many parts were shorted. A cooling system was installed, enabling the part to be adequately packed out with a cushion to spare.

Why did this occur? On the original system, before barrel cooling was introduced, even though capacity was maximized it was difficult to build all 14 oz required because the check ring floated freely in the barrel and was not immediately and fully seated against the screw. When the screw did move forward, the check ring tended to move with it.

By cooling the barrel, the higher-viscosity material that develops holds the check ring in a relatively centered and straight position in the barrel. When the screw moves forward on injection the check ring is held by the friction of the material. The screw then effectively moves through the check ring before it seals itself against the seat. The consistency and stability of the check ring's orientation produces repeatable processing conditions, shortens the shutoff distance, and eliminates variations in shot size that caused the shorts in the earlier test.

This system has been successfully applied many times in molding programs. Molders are encouraged to test this system and report their findings. This process is not licensed.

Contact Information
Logic Corp.
Sandy Hook, CT
Paul Allen
Phone: (800) 325-6442
Fax: (203) 426-5966
Web: www.logicseal.com

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