When does extrusion profile warpage occur, and how it can be reduced? In this post, we look at how a faster or more complex profile extrusion can cause warpage.

When extruding plastic profiles, the output is usually controlled by the cooling of the profile and the ability to hold the part in the correct shape while it is being cooled.

Warpage comes as a result of extruded parts that do not cool uniformly. The reasons include running a faster production to get more product out on the same production line and speeding up an extruder to get more output.

Simple shapes like round pipe and tubing are easier to cool down faster. However, the difficulty of cooling increases when the complexity of the profile increases. Hence window profiles and other complex parts are very difficult to cool uniformly.

Plastics shrink as the temperature of the plastic decreases, but they usually shrink a lot more than other materials. Plastics shrink at a different slower rate when they are in the solid (frozen) state than when they are still soft or in the molten state.

The profile extruder needs to control this shrinkage when cooling the hot plastic, coming out of the extruder, all the way down to room temperature.

In an example of a flat sheet where one side cools faster than the other, both sides are shrinking at the same rate when the sheet is soft. Even if one side is cooling faster and shrinking faster, the other side is still pliable enough to come along with the other shrinking side.

However, once one side cools past the crystalline temperature or its glass transition temperature, two things happen. First, that material stiffens and is no longer pliable enough to follow the other side, and the rate of shrinkage goes down significantly. It is as if the stiffened side is no longer shrinking while the other pliable side continues to shrink.

Therefore, as the pliable side continues to shrink it is pulling on the stiffened side and causing a bow in the direction of the side that cooled last. In this example, and in other simple profiles, the part will bow in the direction of the material that cooled last.

Furthermore, in more complex profiles the parts may twist, distort, or warp in different ways, all depending on which sections of the part cooled last.

Finally, plastics are good thermal insulators, meaning that they do not transfer heat very fast. It becomes difficult to pull all of the heat out of the extruded part in the first place, let alone doing it uniformly, which causes the warpage.

Find out more about extrusion profile warpage on Plastics Technology.

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