The collapsing of pipe ends is one of the most common problems in the extrusion of pipes and occurs especially when high production speeds are used on the production lines. The reasons for the geometric shape deformation are internal stresses. The reduction of the problem is often possible with simple means.
To avoid pipe end incidence, it is important to change the cooling situation of the product in such a way that as little residual stresses as possible are generated in the product. In the first part of this article, you heared what the causes for the residual stresses can be. In this article we would like to show you how you can reduce the residual stress formation in pipe extrusion. The causal relationships and the results can, in principle, also be transferred to other product geometries (plates, hollow rods, solid rods, flat rods, profiles, etc.).
The collapsing is analyzied on a pipe with the dimensions: 250 x 22.4mm (SDR11) made of polyethylene (PE Borealis). The production mass throughput is 600 kg/h, resulting in a production speed of 0.53 m/min. The cooling section consists of 4 spray cooling tanks, each 9m long (2 x vacuum) and a further spray cooling tank with a length of 6m. All spray cooling tanks are supplied with cooling water at a temperature level of 12°C. In addition, the two vacuum spray cooling tanks and the third and fourth spray cooling tanks are connected so that no air gap remains between these tanks (this is often done to reduce the replacement of seals or water loss, but avoiding the air gap often shows disadvantages in terms of residual stress formation, as we will see below).
In the simulation result shown above (generated with chillWARE V3.9.3) you can see the design of the cooling section (lower part of the image) and the cooling situation of the product (upper part of the image). The plastic pipe enters the cooling section at a temperature of 210°C. Due to the intensive contact with the cooling water, the outside wall temperature of the pipe (line shown in green) decreases very strongly in temperature. The inner wall temperature of the pipe remains at a high temperature level for a very long time. The blue lines represent different radial layers of the pipe and their temperatures. From a cooling section position of approx. 23m, the exothermic crystallization process of the material becomes visible, the material hardly cools down on the inner wall and remains at a high temperature level (plateau area) for a long time. This behavior is typical for semi-crystalline materials. At the end of the cooling section the product reaches a temperature of 35.3°C (outer wall) or 43.1°C (inner wall) – it is thus completely cooled.
The following diagram shows the residual stresses resulting from this process parameterization. The abscissa shows the radius of the tube, the ordinate shows the residual stresses. Positive residual stresses represent tensile stresses, negative residual stresses represent compressive stresses. The maximum values here are 1,616 N/cm² for the tensile stresses and -4,210 N/cm² for the compressive stresses. The tube produced in this way thus exhibits high residual stresses in typical characteristics (-> tensile stresses inside / compressive stresses outside – see part 1 of the article). Such an internal stress distribution leads to the formation of various quality deficits, such as the formation of pipe end incidence.
In the following, different possibilities for process optimization using computer simulation will be calculated and evaluated with regard to their effectiveness. For this purpose, various optimization steps are carried out.
Why the use of air distances between cooling tanks can make sense
In the first optimization step, short inline annealing zones (air gaps) between the first 4 spray cooling tanks are realized. These short air distances in which no active cooling takes place can cause the effect that the outer wall of the pipe can heat up again above the VICAT softening temperature, so that all residual stresses built up to this point in time are suddenly released again.
The following diagrams show the cooling situation for this process parameterization, in which the air gaps between the two vacuum spray cooling tanks and between the two other spray cooling tanks have been reintroduced. This simple approach makes it possible for the surface of the product to become that warm again (due to the inherent heat of the pipe) that the internal stresses are significantly reduced.
The corresponding residual stress diagram shows that the residual stresses have already been significantly reduced by this simple measure. The value of the tensile stresses decreases by approx. 8%, but the compressive stresses are reduced by more than 33%. The attached graphic shows a direct comparison between the reference process (shown in blue) and the variation with added air gaps, with otherwise completely identical process control.
It should also be noted that this measure extended the cooling distance by 2m (2 x 1m air distance), but the product temperature at the end of the cooling distance could also be lowered. While the product was previously 43°C or 35°C warm, the surfaces are now 33°C or 29°C warm.
In the next optimization step, two further solution approaches will be examined. In this case, the cooling water temperature in the second vacuum spray cooling tank was raised from 12°C to 18°C and at the same time the air distance behind this cooling section segment was extended by 50cm. Although the general cooling situation has not changed significantly compared to the initial process, the effect of the cooling process on the formation of residual stress is very intensive.
Residual stress reductions of more than 50% possible?
The following diagram shows the comparison of the residual stresses between the reference process and the optimized process. Overall, the simple measures shown in the design of the cooling section and in the process parameterization have now led to a reduction in compressive stresses of more than 24% and a reduction in tensile stresses of more than 55%.
The implementation into reality shows that considerable process optimizations are possible due to the presented possibilities. Computer simulations are a fast and cost-effective method to gain insight into the process and to identify the perfect process parameter for any product without production of waste material.
Another very interesting example shows the situation when the inner diameter of such a tube subject to residual stress is turned off, as shown in the following figure. In this case, those areas of the pipe that have the highest tensile stresses are removed. The result is that a new stress state (compressive AND tensile stresses) is created in the pipe, which is then in equilibrium again. To achieve the state of equilibrium, however, a deformation takes place at the same time, which increases the overall diameter of the pipe.
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