When plastics are injected into a mold, they are subjected to a new set of conditions that affect how they shrink. Specifically, shear and extensional forces act on the polymer during the filling and packing phases. Plastic molecules tend to align themselves in the direction the polymer is flowing. This alignment, or orientation, determines linear shrinkage. Orientation can vary in direction and magnitude, meaning that many polymers shrink more parallel to flow and less perpendicular to flow.
Figure 4: Orientation effects on a center gated part show the difference between high parallel shrinkage that causes a saddle left and high perpendicular shrinkage that causes a bowl right. Orientation effects can align molecules in ways that are difficult to predict. In some cases, extensional forces can take over at the center of the part to align molecules perpendicular to flow.
Skin laminates have no distinct orientation pattern. Outer laminates inside the frozen layer exhibit high shear rates and are oriented in the direction of flow. Transition laminates have medium shear rates, but no distinct orientation. And inner laminates have lower shear rates and tend to be oriented perpendicular or transverse to flow.
The thicker the part, the more influence extensional flow tends to have. Gate type and location can also contribute to this effect. Figure 5: Compare purely shear-induced orientation effects with those caused by shear and extensional flow. When the part exhibits both extensional orientation effects in the middle laminates and radial orientation effects in the outer laminates, whichever type of shrinkage is higher will determine the direction of warpage.
This also correlates to whether the material is filled see Figure 5. Another orientation effect to be aware of is transient flow, or underflow. This refers to a flow front that changes direction during filling, typically due to a filling imbalance. The change in direction creates variations in shrinkage that create residual internal stress in the part. The greater the difference in flow between the parallel and perpendicular directions, the higher the internal stresses will be and the more warpage the part will exhibit.
The difference between isotropic and anisotropic shrinkage can change the total effect of shrinkage on the shape of the part. Managing shrinkage is a complicated task, given the number of factors involved and how each one can affect the others.
Simulation software can make this work easier by allowing engineers to address the problem earlier in the product design cycle.
Using simulation tools such as Autodesk Moldflow allows you to set up and run analyses to visualize how much shrinkage to expect, given the current part material, design, and expected processing conditions. Results can be scaled for easier interpretation. Then engineers can change the processing conditions or part design and run the simulation again to see how much shrinkage is reduced.
Simulation tools also make it faster and easier to consider a wider range of potential solutions, such as changing the material or the size of the mold, all of which is more convenient than dealing with shrinkage after it has already occurred.
However, understanding how and why plastic shrinks gives engineers an edge when trying to control its effect on a part and develop an appropriate solution that conforms with your budget and schedule. Free Trial. She has a degree in Plastics Engineering Technology from Pennsylvania State University, and has experience as a facility and project engineer at a custom injection molder. Kristen joined Autodesk in as a technical support specialist, focusing on working with Moldflow customers.
Hence the longer time that a polymer remains between T g and T m the greater will be the amount of crystallisation.
In many cases a warm mould which reduces the cooling rate will increase the amount of shrinkage. The remaining shrinkage may take up to 3 months to complete. Raising the temperature speeds the process and in boiling water full shrinkage takes place within an hour.
For PA 66 a similar situation exists as for PP, i. T g is only slightly less than room temperature and after-shrinkage can occur for up to two years after manufacture. In such cases it is normal to anneal the moulded article for a short time at the maximum crystallisation temperature to force the polymer to equilibrium.
Because polymers have a very low thermal conductivity, compared with metals, cooling from the melt proceeds unevenly, the surface cooling more rapidly than the interior. This leads to variations in the structure and crystallinity through the section thickness and can result in the formation of voids or holes due excessive internal shrinkage. As a general rule thick sections take longer to cool and thus develop more shrinkage for all types of polymers. Mould cooling should be connected such that water is fed in first at the gate area and led to outer edge as it warms up.
This will counteract the temperature gradient in the melt. It is important to note that distortion of a moulding may not be solely due to shrinkage of either amorphous or crystalline polymers but may be due to high levels of moulded-in strain. High pressure and low temperature moulding can lead to low initial shrinkage but high levels of moulded in strain.
Components can be tested for this by immersion in boiling water for 10 minutes. If they are undistorted after this then they are unlikely to warp later. The most critical of these are:. As noted above, shrinkage in amorphous polymers is mainly due to thermal expansion and shrinkage may be minimised by the following:.
It is important to note that most of these factors will also reduce the total cycle time and give optimum production rates. Most of the factors affecting amorphous polymers also affect crystalline polymers and the same remedies apply. In the case of crystalline polymers a basic decision needs to be made. This is 'Do you force crystallisation and increase shrinkage at the moulding stage or not? For materials which do not continue to crystallise at room temperature crystallisation at the moulding stage should be minimised as it will not occur later.
For materials such as PP, which continue to crystallise at room temperature shrinkage at the moulding stage should be maximised to reduce the amount of post-shrinkage that occurs and to allow assessment and acceptance of mouldings as soon as possible after moulding. The above setting guide assumes that wall thicknesses remain relatively constant.
If wall thicknesses vary greatly through the moulding then the lower cooling rates in the centre of thick sections can lead to varying degrees of crystallinity and hence shrinkage variations from point to point. This uneven shrinkage also sets up stresses which can lead to distortion. See the accompanying illustration for some examples of unbalanced orientation.
Processors sometimes do not recognize the effects of unbalanced orientation and how to combat it. In the case of the sheet, the middle had higher flow stress or orientation through the die because the die was pinched down more in the center than the edges.
With uniform cooling across the sheet, the result is more residual orientation in the center than the edges, causing the sheet to shrink more in the middle. This results in both edges wrinkling up to match the length of the middle portion.
A similar situation exists in the profile. The thinner cross-section sees more flow-induced stress in the die and drawdown because of the narrower opening, resulting in more orientation in that section. Assuming uniform quench cooling, the thinner cross-section shrinks more than the less oriented circular section. This causes the part to distort as it seeks its relaxed state. As a result, they tend to make die adjustments with little success.
Relaxation cannot occur below the glass-transition temperature Tg , but many of the most useful semi-crystalline polymers have glass-transition temperatures well below room temperature see table so they continue to relax shrink indefinitely until they reach their equilibrium structure. The biggest problem is that this distortion might not show up for several days, weeks, or even months, if the part was quenched quickly and is kept cool after processing.
Changing the part geometry is usually not an option to resolve the unbalanced orientation, so the operating conditions must be changed.
In most cases, simply reducing the cooling rate and reducing the drawdown will alleviate much of the unbalance. For more difficult situations, zoned cooling, additional heating in the highly oriented sections, interruptions in the cooling phase to allow thermal homogenization, and, finally, secondary annealing steps may be required.
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