Pipes are produced by horizontal extruding molten polymer through an annular opening in to several sizing, cooling devices that stabilize the final dimensions.
Finished products made from plastic take many different forms, but they all start in the same place and of the same raw materials. In plastic extrusion, resin beads are melted down, filtered for uniform consistency, pushed through a die cut for the final product shape, and then cooled.
Plastic extrusion is used to produce a wide range of products on the market, from building materials to consumer products to industrial parts. Pipes, window frames, electrical covers, fence, edging, and weather stripping are just a few of the common items made by plastic extrusion, along with thousands of custom profiles.
The process itself is highly technical. However, it can be summed up in just five steps.
Step 1: Raw plastic materials like granules, pellets, or powder are loaded into a hopper and then fed into a long heated cylindrical chamber called an extruder. It is moved through this chamber by a revolving screw. Some extruders can have either one or two revolving screws.
Step 2: The raw materials flow from the hopper down through the feed throat and onto a large spinning screw operating within a horizontal barrel.
Step 3: Not all materials have the same properties, so the raw material is processed by a heating temperature specific to the type of raw material being passed through. At the end of the heated chamber, the molten plastic is forced through a small opening called a die to form the shape of the finished product.
Step 4: As the plastic is extruded from the die, it is fed onto a conveyor belt for cooling. Cooling can happen by air or water. The process is similar to the injection moulding process except that the melted plastic is forced through a die instead of into a mould.
Step 5: The die for an individual plastic profile is designed so that the plastic flows smoothly and evenly from the cylindrical profile of the extruder into the final profile shape. Consistency in this flow is critical to achieving an end product with integrity.
And pipe extrusion is one of most common extrusion process will be recommended in this article
As other kind of plastic extrusion, pipes are produced by horizontal extruding molten polymer through an annual opening into several sizing, cooling devices that stabilize the final dimensions.
For the pipe production the materials may have to be compounded. For example, many pipe producers use PVC. The resin is heat sensitive and under the normal processing conditions, it has high viscosity, thus it can’t be processed-Its needs compounding. However appropriate grades of other polymers, such as PE or ABS, can be directly use as supplied by the manufacturer, without compounding.
PVC compounding is process in which additives are mixed with the base resin to get a homogeneous mixture. The additives are added to either improve process ability or enhance product performance. The following ingredients are added to PVC for pipe production:
The pipe extrusion line comprises an extruder pipe die, sizing unit, cooling through, puller (haul off) & cutter (saw)
The role of an extrusion die is to form the molten material delivered by the screw in to a required cross section. The extrusion die can be classified as straight: through cross head and offset dies, base on their attachment to the extruder.
The extruded emerging from the pipe die usually shows variation of diameter, caused by variable of the rhelogical melt property. Thus the pipe has to be calibrated or sized while hot.
During the sizing also the pipe gets cooled.
Cooling is done by spraying water over the surface of the pipe.
This length of the cooling through PE is longer than for PVC.
Pulling machines maintain uniform pipe size
Pullers are either the caterpillar type of the gripping wheel type.
The puller speed is controlled by electrical electronic or magnetic speed vartiators.
They provide automatic measurement and control of pipe size
In one system the capacitance of the pipe is measure and is related to the thickness
The sensing head moves around the pipe, showing when the pipe is out of round and when it is generally too heavy or too light.
All these systems can be made of feedback to control the puller or the extruder drive.
Circular say cutters with blade special designed for cutter plastics are used to cut the plastic in to specified length.
Automatic cutting device are also used. They travel with the pipe to cut than after cutting go back to pick up and the next pipe length.
Larger size are sawed to length
Smaller diameter PVC pipe are coiled on a large center wood reel.
In simple methods pipe is coiled by a band rank.