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Purchasing Basics: Screws and barrels 19666

May 23, 2001

6 Min Read
Purchasing Basics: Screws and barrels

There is almost no part of a molding machine that is more important than the engine that melts and injects the plastic. The screw, the barrel, and the mechanical front end of that combination can have a significant impact on the quality of the melt, the quality of the molded part, and the economics of operation. How you address the basic issues of the screw and barrel, treated on these pages, and front-end components—tips, check rings, nozzles, valves—that are discussed in another Purchasing Basics article, can be a drag on your operation or can make it successful. 

The Hardware 
Since many screws and barrels are purchased as replacement parts, the first place your supplier will want to start is understanding what you presently have installed in your machine. Whose machine is it? What model, serial number, and year manufactured? What is its clamp force and rated shot size in GP polystyrene? What about stroke, maximum screw rpm, and maximum injection pressure? 

Understanding wear

There are three basic types of wear that occur on screws and barrels. Understanding which type is occurring on your equipment will help you apply the right corrective measures, either for an existing barrel or screw or when ordering new or replacement components.

What is the screw diameter and L/D? What type of screw design is it—metering, mixing, general purpose, barrier? Single-stage or two-stage? What condition is the screw in? The barrel? Is the problem abrasive wear, adhesive wear, or corrosive wear (see box at right)? 

Maybe you are looking at an upsizing or downsizing project—replacing the screw and barrel to achieve better processing conditions. Do you need to upsize because you can now mount a mold for a large part in a smaller press? Do you need to downsize to avoid excessive residence time for the material in a small part that needs to run in a larger press? Your supplier will want to know the details of that particular part—material, shot volume (including the sprue and runner), and cycle time. 

Process Conditions 
Your supplier will also want to know the exact nature of any problems, and what the conditions are for that problem. Do you have an existing screw recovery time problem? What melt temperatures are you running? How does that compare to the material supplier's recommended setting? How does the barrel temperature setpoint differ from the actual in front, center, and rear zones? 

Are you looking to improve your existing cycle time? Are you experiencing melt decompression? What is the normal screw rpm? What about backpressure? What is the screw motor hydraulic pressure? 

The Material 
What are the materials you want to run in this machine? It's important to know what additives or reinforcements are in the resin, particularly flame retardants, glass fibers, or minerals. Outgassing of volatiles is a major cause of corrosive wear on screws and barrels. Is the material difficult to melt? What is the percentage of scrap that is being reused? Can you calculate the actual residence time for the material? 

Mixing Issues 
How important is the mixing function to your operation? While we don't have room to cover the subject here thoroughly, screw designs generally favor either distributive mixing or dispersive mixing. 

Distributive mixing is basically the process of dividing one component into another. Dispersive mixing is the process of reducing the size of the smaller component to improve its distribution within the major component. Generally, distributive mixing is preferred, because dispersive mixing involves high shear and a resulting temperature increase. 

Understanding how screw design affects mixing is basic to this choice. If you are using strictly precolored material, and are not mixing scrap back in, melting the plastic is your primary goal, not mixing it. But if you are metering color or additive concentrates at the machine, or have a high percentage of scrap reuse, the mixing function of the screw will be essential to good part quality. 

Additive or color concentrates that have a large particle size require more mixing than those with small particle sizes. High concentration levels within the color concentrate mean that fewer pellets are required, and thus mixing is more difficult. When additives are fed near the throat, it usually means that there is less dispersion of the additive when it enters the barrel. 

Finally, the specific resin type makes a difference as well. Crystalline resins are more difficult to mix than amorphous ones, because they melt with less ease. Inorganic additives such as talc or calcium carbonate can agglomerate under pressure, requiring substantial shear to be broken down a second time. 

With our thanks . . .
. . . to the following companies that supply or repair screws and barrels for their contributions to this article: Concor Tool & Machine, Inductametals, New Castle Industries, Spirex, and Westland. 

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