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TOPIC: Extrusion screws

Extrusion screws 4 years 11 months ago #1867

I have a problem with the actual piggy back screw. I got the cable with some “snakes” with master batch color, which means that the master batch is not mixing very well with the PVC. I want a screw to mix or whip with high compression. What type of screw you can suggest

Also, I want a list which has photos or drawing of different kind of screws and what each one can do.

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Re: Extrusion screws 4 years 11 months ago #1868

You have asked about extruder screws before and your query was answered.

1. See Forums / Electrical/ extrusion screws information. I have moved this thread from Page 2 to Page 1 for your convenience.

2. There is a drawing of The Standard Plastic Extruder Screw on the Stewart-Hay Associates web site at along with the typical characteristics of that screw. The variables and the screw alloy are of course a function of the plastic designed to be extruded. Similarly, barrier and mixing screws are all offshoots of this screw.

3. As far as your statement "I want a screw to mix or whip with high compression." goes, you have to prevent burning of the PVC on the screw by high shear. That's why a PVC screw by design has a deeper metering section than that used for polyethylene extrusion! Thus your concept is not exactly correct for PVC.

4. Presumably you are putting a skin of color on a base of natural PVC using at least a 20:1 L:D small (Say a 25 mm or 1 inch diameter.) piggyback extruder with good temperature control.

5. Are you using a properly sized gravimetric unit to mix the color masterbatch so you at least have some color mixing precision?

6. Is the piggyback extruder hopper quite small, the gravimetric start/stop sensor low down in the hopper so that a minimum amount of pre-mixed PVC compound is placed there and does the mixer drop the pre-mixed compound directly into the piggyback hopper?

7. Do not use a pneumatic loader to lift pre-mixed PVC compound. Likewise there should be no vibrating devices on the piggyback hopper to help flow the pre-mixed PVC compound into that extruder. In each of these instances, you do not want to begin mechanically segregating the color masterbatch from the base PVC compound as the two different pellets may have different densities.

8. Please advise what screen pack you are using in your piggyback extruder. Is it something like 14/40/100/40/14 or 14/40/60/40/14?

9. You should also look at the thread Forums / General / XLPE curing fail reasons because a couple of the process items discussed for the striping slave extruder there are also appropriate to your piggyback extrusion problem

Kindest regards,

Peter Stewart-Hay
Stewart-Hay Associates
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