The Vacuum Provides Excellent Cooling Against The Tube

The vacuum trough method provides excellent cooling with little frictional drag acting against the tube. There are no theoretical limitations to the production rate. The trough has to be longer for higher output rates, which has a bearing on the available space. Large pipes (>100 mm or 4 in outside diameter) are hard to keep submerged under the water without distorting them or marring their surface. On these occasions, the trough is not filled, but water is sprayed and cascaded all around the pipe while it is passing through. The entire inside space of the trough is maintained under vacuum. Smaller pipes and tubing (OD 12.5 mm or 0.5 in) are usually sized without the use of vacuum.

The extended mandrel technique, provides internal cooling and support to the tube. The shrinkage of the plastic as a result of cooling causes a tight contact between the tube and the metal mandrel that can extend up to 30 cm (12 in) beyond the die. Longer mandrels have been difficult to use due to mechanical difficulties. The mandrel is tapered with the larger end closer to the die. The diameter difference between the large and small ends is a function of the polymer type in addition to the variables related to the tube type and size. The end of the mandrel is slightly larger than the final pipe size to allow additional shrinkage. There is a passage for cooling water inside the mandrel. To ensure a constant temperature, the passage must be uniform. Water cascades accomplish additional cooling over the tube. A small split ring is placed around the tube just as it leaves the die to prevent oscillation of the water, which could otherwise cause water marking. Water cascades further cool the tube after it leaves the mandrel. It finally enters a water trough equipped with sizing plates.

Pipe and tubing can also be made by extrusion through a crosshead die, similar to that used for wire and cable coating.