About pipe extrusionAn important digit when talking about extruders is the ratio between length and diameter, which varies approximately between 22 and 40. Improvements in the design of extruders have led to a higher efficiency, which means the same output can be reached with smaller (and cheaper) equipment. This is reflected in the L/D ratio, which was limited to approximately 15 in the 1980s. Nowadays a L/D = 30 or more is no exception. For instance a diameter 90 mm twin screw extruder has an output of 600-700 kg/hr, (1300-1500 lbs/hr) 10 years ago this was only feasible on a diameter 125-130 mm extruder. Outputs up to 1800 kg/hr (4000 lbs/hr) are realised now on a diameter 135-140 mm extruder, in earlier days it required a 160 mm extruder or bigger, if feasible anyway for production of a high quality melt.
For the extrusion of plastic, we distinguish two types of extruders: single screw and double screw (counter-rotating).
Single screw extruders
Single screw extruders are mechanically rather simple: a drive with sufficient room for the single thrust bearing, however relatively powerful because PO material needs high specific energy to melt. The barrel is simple without venting for the standard PE and PP resins as the air can escape backwards via the granules, in general at higher backpressures there is less chance on porosity of the pipe wall; humidity in granules is often removed by drying the resin at 80-110 °C. One hour dwell time is sufficient to remove the surface humidity, if is it deeper in the granules up to 4 hrs can be needed. Hotter granules can raise the output as melting goes quicker, on the other hand if the granules melt already in the grooved feed zone the limit is reached.
Barrel intake cooling is mostly done by fluid and the rest of the barrel by air. Screw cooling is absent. The temperature buildup needs to ensure the melt is heated enough to reach a good structure, but does not overheat. Unmolten particles due to insufficient melting capacity of the screw can result in output fluctuations, overheating leads to degradation of the material. In film production unmolten particles are unacceptable. In case of pipes some will say they are not strictly forbidden, as long as the granules are of high quality polymer and a have a good bond/weld with the melt.
Double screw extruders
Compared to single screw systems, double screw extruders offer important advantages for processing pvc: low shear and more control over process parameters such as temperature profile.
A simple mechanical design without external screw cooling and air cooling for the barrel also enhances reliability, low maintenance costs and low energy costs: enforced cooling means a direct waste of energy. Over the past decades, double screw design leaped forward to realise higher rates without sacrificing melt quality in fusion or waviness.
In order to process pvc powder with additives, a twin screw system is the best choice. Pvc is sensitive to overheating, so the residence time in the barrel is crucial. This can best be controlled by using counter-rotating screws. A balanced screw design takes care not to over-gell the dry blend in the venting with the risk that air is already encapsulated by melt and cannot be extracted anymore. On the other hand, a too powdery dry blend in the venting section can lead to powder sucked into the vacuum air stream. The air stream will be limited if there is venting backwards; the powder intake should be the first atmospheric venting to minimise air -that is an insulator- in the first screw sections.
Modern screw designs deliver a good melt at a low back pressure. Screw and barrel wear is reduced by using hardened materials, adapted to the type of material used. A screw/barrel that is worn out can produce pipe that has black carbonised particles in the melt, such a pipe will not pass the pressure test. A worn out screw at the tip will raise the melt temperature and waviness, in the end the DMC test will fail. In general lowering barrel settings and increase of screw filling (if possible) can help in these cases, as well as lowering output. Also formulation changes towards slightly higher lubrication levels can help for a certain period to extend use. However for pressure pipe such a change requires new long term pressure tests.
In all twin screw designs, venting is very important. Insufficient venting can lead to over-gell in the venting zone (with the associated risk of having voids in the pipe wall) or even loss of vacuum (leading to reject of the pipe). Venting is also related to wear, a balanced screw design makes local pressures as low as possible. An example is the pressure build up upstream of the powder lock. In general, wear is reduced at lower pressures also at the screw tip: lower bending forces but also lower spread forces due to calendaring of the melt in between the two screws. To lower the screw tip pressure, die head design is key.
INTRODUCTION DIE HEADS