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FAQ

Question:
Why is my extruder shutting down on startup?

Answer:
Most likely your motor load amps were high just before it shutdown, and this was caused by insufficient preheating of the equipment. You need to allow enough soak time to melt any residual material in the barrel and downstream tooling. Just because certain measured points indicate adequate temperatures, it doesn't mean that the whole screen changer, or die, or resin left in the machine are up to temperature.

If you do not properly preheat your line, when you hit "Start" you can break your screw, or pop out a rupture disc.

Contact your OEM for additional information.

Question:
We would like to get more output from our main extruder. Can we just increase our speed from 175 to 225 rpm?

Answer:
This one depends on several factors, including the hp of your extruder, the gear reducer style and rating, the amount of torque required to process the material you are extruding, and of course whether or not you have adequate cooling capacity downstream. Your OEM will be able to help you determine the answer and solution to increasing your output for your particular system.

Question:
Can I add coextrusion to my sheet line?

Answer:
This depends on what you now have and what you would like to achieve.

Assuming that you now have a sheet line it is quite easy to add a coextruder and a coextrusion feed block to make two color sheet or glossy HIPS sheet with a thin layer of crystal PS. You must have the space as well as the added cooling capacity on your take off for the addition.

More complex systems are not readily retrofittable because of controls and capacity limitations.

In some cases the addition of a film lamination system will serve you better with less cost and easier operation.

Question:
What other materials can I run on my line?

Answer:
This will depend on the polymer you run now, and what you want to add. First best thing is to ask your machine supplier. Conversions and interchangeability are often possible with minor changes or additions.

Question:
Why are there black spots, or carbon flakes in my material?

Answer:
Black spots typically indicate contaminated or degraded material. Possible causes could include:

Contaminated material
Running all or part of the line too hot, or too slowly, thereby allowing the material to degrade.
Not having adequately purged old material from the machine.
In some cases, equipment may need to be disassembled for thorough cleaning.

Question:
How can I determine if I have screw or barrel wear?

Answer:
While there is very little wear with most unfilled materials, it is important to keep an eye on this for performance reasons. Cylinders will usually wear only a small fraction of screw wear because there is much more surface area and the cylinder is usually also harder.

You should always keep a log of your running conditions. From your log, compare your throughput and melt temperatures for each product. If you see that throughput has been decreasing and melt temperatures have been increasing, this may indicate wear.

Misalignment is another possible cause of wear. If components are not properly aligned, the cylinder and screw can bend and cause rapid wear.

Question:
How can I measure my screw and cylinder?

Answer:
You will first need to extract and clean your screw. Using precision key stock, span 2 or 3 flights, and take measurements at several points with a micrometer. Deduct the key stock thickness from your dimensions, and compare to your original screw dimensions.

Cylinder measurement beyond the easily accessible front end requires special tools and is best left to your machine builder or other specialist with the necessary equipment.

Question:
How do I balance a 2 stage screw on a single screw vented extruder?

Answer (see illustration below):
Vented extruders are equipped with specially designed two stage screws, with a decompression section which allows volatiles or entrapped air to be extracted through a hole in the cylinder as part of the process.

The screw design is essentially two stages. The first one plasticizes the material and drops it into the feed section of the second stage, which is the vent section. This is followed by re-compression and a second metering zone, which pumps out the material through the die.

The metering zone of the first stage is always only about half the depth of that of the second stage to provide the decompression.

The two stages must be carefully balanced for precise uniform output or the machine will surge more or less because the second metering zone is starved and only partially full. It must be balanced by bringing the head pressure up to fill the second stage just below the pressure where material floods the vent section.

This can be done with a throttling valve at the extruder discharge, or by adjusting the gear pump suction pressure where available. Such adjustment is essential with all vented extruders for precision and must be made at the operating pressure.

Venting may be just atmospheric or with the aid of a more or less intense vacuum system in extreme cases to extract the volatiles from the polymer. With fluid polymers like PET the vacuum must be applied carefully in stages to permit the expanding foam to settle at the beginning of the vent section before it reaches the vent opening.

Welex's Technical Service Staff can answer this and many other questions you may have about your Welex Sheet Extrusion System.

FEED ZONE TO THE FIRST METERING SECTION:
The material is conveyed forward from a water cooled feed section through the rear barrel temperature zone. At the beginning of the second barrel temperature zone is the transition area of the screw. This is the beginning of the first metering section. The material should be melted at this point by way of barrel temperature and screw compression and shear.

FIRST METERING SECTION:
The first metering section determines the output of the screw. If one knows the depth of the flights and screw diameter in this area, a throughput calculation can be made.

VENT AREA:
The material next goes through a decompression or vent area. Here the vacuum draws off any moisture or volatiles in the material.

TRANSITION AND SECOND METERING SECTION:
The second metering section is slightly deeper than the first metering section. This may cause surging. To counter-act this, an adjustable melt valve is fit after the cylinder flange or screen changer to create backpressure.

The rule of thumb for adjusting the valve is to close the valve in stages until material bleeds from the vent. The valve is then opened until the vent stops bleeding. This back-pressure ensures that the second metering section remains filled with material, preventing surging.