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May 22, 2000

3 Min Read
Purchasing Basics:  Front-end components

Many of the questions that need to be answered about screws and barrels that appear in our April 2000 article apply to the front-end components of the injection unit as well. The same information about the machine and the process are an important part of the specs for all the front-end components—valves, end caps, and nozzles.

Common problems
for nonreturn valves

Understanding what component problems relate to which processing conditions can help determine what injection unit component would be a suitable replacement candidate.



Premature valve wear

Inappropriate valve selection

Sinks, non fills

Valve not shutting off


Valve leakage or decompression

Melt overheating

Valve too restrictive


Valve too restrictive

Screw turning on injection forward

Valve not shutting off

Black specks

Degraded polymer

Undersized ring OD

Oversized barrel

Air bubbles

Valve may not be shutting off

Excessive gassing

Improper polymer flow

Brown or yellow milky streaks

Excessive shear

Thread damage

Improper seal at screw

How Fast?
While the design of the nonreturn valve, for example, has a lot to do with the kind of material you are running, and the conditions under which that material runs, probably the most important item is the selection of the proper material for that valve—one that will withstand the environment the valve will face. And the factor most indicative of that environment is screw rpm. So the first question you may be asked relates to just how high your screw rpm is. The higher the rpm, and the backpressure, the more the metal in the valve will overheat, and will be pushed beyond its limit. As one supplier is fond of saying, speed kills nonreturn valves.

The two most common types of nonreturn valves, or check valves, are the sliding ring valve and the ball check valve. The type of materials you most commonly run may determine which type is best for your application. Do you change color frequently? The side discharge ball check may be the first choice. Is your typical material highly heat sensitive? The sliding ring valve may be more suitable. How critical is precise shot-size control? Is your molding machine vented? Do you run high-wear materials?

Rules of thumb

  • Doubling the standard clearance between screw and barrel causes a reduction in the melting rate of 25 percent.

    Nozzle tip size should always match sprue diameter.The smaller the barrel diameter, the greater maximum rpm of the screw can be tolerated by the non-return valve. For example, with a 60-mm barrel a typical four-piece nonreturn valve can withstand a screw rpm of about 150; when the barrel size is 120 mm, the acceptable rpm level drops to 75.

All of these factors will influence the recommendation of the component supplier or the OEM. Once the valve design is chosen, the materials of construction should be selected to meet the criteria for high-wear, abrasive, or corrosive environments.

With our thanks . . .
. . . to the following companies that supply front-end components for their contributions to this article: Glycon, Herzog, Spirex, Westland, and Zeiger Industries.

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