Rigid PVC now acounts for almost 70% of all PVC processed --both worldwide and in the U.S. Although a fair amount is injection molded (pipe fittings, computer housings, etc.), by far the largest amount is extruded---and mainly from powder blend compounds.

ATTRIBUTES OF SINGLE & TWIN SCREW EXTRUDERS

Single Screw Extrusion---Successful single screw extrusion of rigid PVC powder blend is dependent upon developing shear, at high RPM (25-80) and high compression (2.8-3.5:1) in order to generate sufficient "work" to insure adequate fusion, a uniform melt, and full achievement of physical properties.

At these high RPM's, the material's radial velocity in the screw flight differs substantially when comparing material next to the screw root with material next to the barrel. This contributes to shear and frictional heat buildup, as well as non uniform melt viscosity of material entering the die.

A typical single screw extruder capable of handling rigid PVC powder blends has an L/D ratio of at least 24:1 (most now are in the 32:1 range), is vented for removal of volatiles, and has a two stage screw with a decompression zone at the vent. The screw is also cored for oil temperature control.

In a standard screw, material transport depends on the helix angle of the screw flights as well as flight depth. As the helix angle decreases, material transport increases and frictional shear (slippage) decreases. The screw flight lead typically is equal to screw diameter.

It would seem, then. that efficient material transport leading to high output rates and low shear can be readily achieved by reducing the helix angle. However the need to achieve a uniform, homogeneous melt requires certain constraints on screw geometry. Rigid PVC melt starts on the leading edge of the screw flight. Since an uneven melt temperature exists throughout the flight, the coolest (last to fuse) compound is in the center. Therefore, the helix angle must be large enough to provide a certain amount of slippage (shear) of the material to hasten melt uniformity in the flight, but not too large to hinder material transport (output). Higher RPM's are used to help develop the shear which overcomes uneven melt temperatures in the screw flights. Some of the newer single screw extruders are considerably longer (30:1 to 32:1), which permits the use of somewhat less rigorous processing conditions due to longer residence time in the barrel to achieve a more uniform melt to the die. Even so, with every turn of the screw, there is a slight melt pulsation (surge) of the material exiting the die.

The screw design of many single screw machines now includes mixing pins, secondary flow channels, or other features to aid in developing a homogeneous melt.

Twin Screw Extrusion  ---- The rapid growth of rigid vinyl markets in Europe and Japan began several years earlier than in the U.S, so it is not surprising to find many rigid PVC processing and formulating developments originating overseas. One such development, the twin screw extruder, has been in widespread use both in Europe and Japan for many years (mid 60's). Within the last 35 years (early 70's) the twin screw extruder has achieved a significant position in the U.S. rigid PVC market---to the point where the vast majority of extruded rigid PVC is done via twin screw machines. Features of multi screw extrusion that dovetail so well with the processing requirements of rigid PVC have been long recognized by the pipe extrusion industry in this country. Most are now aware that these same features apply equally to rigid vinyl siding, profile, and sheet extrusion processing requirements.

There are a variety of twin screw designs on the market which are characterized mainly by screw rotation and screw geometry. Co-rotating screws (LMP Colombo, W-F, etc.) are available, and used mainly for compound --to pellet--producion, but counter-rotating screws (Cincinnati Milacron, Krauss-Maffei,  American Maplan, Cincinnati-Austria) are the most prevalent for  pipe,profile or sheet extrusion. These extruders can be of either parallel screw or conical screw design, and characteristically have very deep cut flights for high outputs at low RPM's Most counter-rotating screw extruders move material in a diverging direction as seen at the top of the screws, around the flights, converging at the bottom of the screws. Thus, this type of twin screw extruder is in effect a positive displacement gear pump which conveys material at fairly low RPM with low compression and very low friction (shear). This results in a more constant, even melt flow, and lower melt temperatures entering the die. Lower melt temperaures minimize sizing problems of complicated profile extrusions, and the constant melt flow reduces the overweight factor by about half vs. single screw extruders. 

This difference in over weight, as it applies to pipe extrusion as an example, can be significant in terms of a year's production. For example, a typical single screw extruder produces pipe at almost 10% overweight (to maintain the minimum specified wall thickness), while a twin screw extruder produces the same size pipe at 5% overweight. That 5% difference in overweight (material given away--pipe sold by length that weighs more!) is enough to buy another extruder in one or two years, as shown:

              Based on a 6000hr. production year, output of both single and twin screw at 700lbs./hr = 4,200,000 PVC pipe per machine.

              5% overweight difference = 210,000 lbs. of pipe given away. If the cost of the pipe was $0.50/lb (a conservative estimate), material given away = $105,000 per machine per year! Imagine a plant with 5-10 extrusion lines --one single screw, one twin screw!!

Twin screw extruders are sensitive to bulk density variations in PVC powder compounds. Due to the positive displacement attribute, higher bulk density material entering the feed zone at a given RPM translates directly to higher output rates. For this reason, a starve feeder is used to adjust and control the amount of powder entering the feed zone. Screw flights are usually kept full when striving for maximum outpu rates. Running with full screws also helps to ease the screw-barrel wear factor, especially at the "10 o'clock-2 o'clock" wear areas of the barrel.

Additional topics on Rigid PVC Extrusion to follow.