Injection molding: the causes of deformation

Shown is a screenshot from Autodesk Moldflow showing a warped injection molded plastic part.

Anyone, including the average consumer, can tell when a part that should be naturally flat is bent or twisted right after it is manufactured. If this warped part was made by plastic injection molding, this could have provided an opportunity to reduce or even prevent the inadvertent bending.

The first step is to understand why the part is warping. This is a complicated task, but it is essential to alleviate the problem. Learning the concepts behind plastic warpage can lead to an approach to reduction being developed. And with the help of technology such as simulation software for plastic injection molding Autodesk Moldflow– Engineers can validate these solutions before the manufacturer cuts shape steel.

Use Autodesk Moldflow Adviser early in the design process to ensure the success of the injection molding.

The shrinkage of the material during and after manufacture plays an important role in the warping of injection molded plastic parts. Before we get into part warpage, it’s important to understand how and why plastics shrink. To do this, we have to examine exactly at the molecular level what happens when plastics melt and cool. For the most part, the melting and cooling properties depend on the type of polymer and whether filler or fiber reinforcement is present.

1. Amorphous materials

One type of polymer is referred to as amorphousThese include materials such as ABS, polystyrene and polycarbonate. They have a random and intricate molecular orientation in their natural state, much like a bowl of spaghetti. When these materials melt, the forces between the molecules weaken and they move away from each other. In addition, the shear during the injection phase (which is similar to friction) causes individual molecules to unwind and align with the direction of flow. When the flow stops, the molecules relax and return to a state of random orientation. The intermolecular forces pull them closer together until the temperature drops enough to freeze them in place. These forces result in even shrinkage, but the relaxation effect causes greater shrinkage in the direction of flow.

Amorphous polymers and how they react to temperature, causing shrinkage

2. Semi-crystalline materials

In contrast to amorphous materials partially crystalline Materials have regions of highly ordered, tightly bundled molecular structures. When they melt, the crystalline structures loosen and the molecules align with the direction of flow, similar to amorphous polymers. But when the materials cool, they don’t relax. Instead, they maintain their orientation in the direction of flow and the molecules begin to recrystallize, resulting in significantly higher rates of shrinkage. In this case, however, the effect is much greater in the direction of flow.

How semi-crystalline polymers react to temperature and how they shrink

3. Fiber reinforced materials

Fibers are often combined into a polymer material to add strength and other properties. When fibers are introduced into the plastic, they can counteract shrinkage effects due to the molecular orientation described above. Fibers do not expand or contract when the temperature changes, so fiber-filled materials typically experience reduced shrinkage in the direction of their orientation.

Polypropylene, middle and end of the drawbar

This shows shrinkage of the polypropylene tensile bar without Fiber optic at two gate locations: end of part (left) and middle of part (right). (Autodesk Moldflow)

Glass filled

This shows shrinkage of the polypropylene tensile bar With Fiber optic at two gate locations: end of part (left) and middle of part (right). (Autodesk Moldflow)

The underlying cause of warpage is deceptively simple: fluctuations in shrinkage. Simply put, if a part shrinks evenly evenly in all directions, it will shrink but keep the correct shape. However, if one element of the part is shrinking at a different rate than any other element, the difference creates internal stresses. If these stresses exceed the structural integrity of the part, the part will warp when it is ejected from the mold.

Shrinkage variation of the injection molded part

Shrinkage variation of an injection molded part, as simulated in Autodesk Moldflow.

In general, there are four types of shrinkage:

1. Regional

This is the case when the shrinkage rates on the part vary between the regions closest to the gate and the regions closest to the end of the fill (EOF). Typically this variance is between the thicker (gate area) and thinner (EOF) areas of the part. One region is shrinking much more than another.

2. Through the thickness

When looking at the cross-section of the part, this difference in shrinkage occurs when the shrinkage on the top is different from that on the bottom. This deviation can cause the part to bend because one side can shrink more and is therefore smaller in relation to the other side.

3rd direction

Due to the molecular or fiber orientation, differences in shrinkage can occur both parallel and perpendicular to the orientation or direction of flow of the material. As mentioned before, an amorphous material tends to shrink more in the direction of flow. For crystalline materials, the shrinkage is usually higher perpendicular to the flow.

4. In the plane vs. thickness

Polymers tend to shrink more in the direction of thickness than in the plane of the surface. This effect is caused by shape retention (or lack of shape retention due to the thickness of the part). The difference in shrinkage between the in-plane direction and the thickness direction can cause warping, especially in the corners of the part as these are sometimes thicker than the nominal wall thickness.

While it is clear that different rates of shrinkage can cause distortion, it is also important to understand why these differences occur in the first place. Here are five of the most common reasons:

1. Cooling rates

With every semi-crystalline material, a high cooling rate leads to a shorter time for the formation of the crystalline structures. This effect reduces the overall volume shrinkage. The same effect applies to amorphous materials, but because the overall shrinkage is less, the degree to which high cooling rates reduce the shrinkage is reduced.

Fiber orientation tensor on elements, simulated with Autodesk Moldflow Insight

Autodesk Moldflow screenshot with fiber orientation. Red = fibers are aligned with the flow;
Blue = aligned perpendicular to the flow

2. Orientation by filling

Initially, the orientation of long, filamentary polymer molecules is caused by shear stress during flow. If the polymer is still at a high temperature and the shear stress is removed, the orientation relaxes. (Orientation is only fixed when shear and freezing occur simultaneously.) When this relaxation occurs in amorphous materials, there is generally more shrinkage parallel to the flow. Since the molecules of crystalline materials are oriented in the direction of flow, most of the crystallization occurs perpendicular to the flow, which leads to greater shrinkage in that direction.

Differential shrinkage

Differential shrinkage

3. Mold restriction

While the part is in the mold, it cannot shrink in the plane of its surface – but it can shrink in the direction of its thickness. This has two effects. First, there is more shrinkage in the thickness direction. Second, the polymer accumulates stresses in the plane of its surface. After ejection, these stresses can relax as the part continues to cool, causing deformation. The higher the mold temperature, the slower the cooling rate and the more stresses are released from the part. Mold retention is also material dependent. Materials that resist creep (and relax more slowly) have higher linear shrinkage, while materials that relax faster have less linear shrinkage.

Differential cooling

Differential cooling

4. Temperature differences through the thickness

If the mold temperature is different from the other on one side of the cross-section, the side-to-side shrinkage will not be uniform. Essentially, the plane shrinks more on one side of the part, making it smaller than the other side, creating a bending moment that can cause deformation.

5. Variations in thickness and uneven packaging

With different thicknesses of the part, thick areas take longer to cool, which can result in higher shrinkage. A similar effect occurs in areas that are far from the goal. When a constant packing profile is used, areas closer to the gate will be denser and cooler at a different rate than areas further away from the gate, resulting in shrinkage variance.

Managing faults is a complicated task given the number of factors involved and how each factor affects the other. Simulation software can make this work easier by allowing engineers to address the problem earlier in the product design cycle.

Use Autodesk Moldflow to perform cooling analysis in both gas-assisted injection molding and two-shot overmolding.

Using simulation tools such as Autodesk Moldflowenables engineers to set up and perform analysis to visualize how much shrinkage and warpage to expect given the current part material, design, and expected processing conditions. Using the visualization tools, the results can be scaled and anchored to facilitate interpretation and comparison with other simulations.

Warpage simulation with Autodesk Moldflow

A simulation is displayed in which various processing options and their effects on part warpage are compared. (Autodesk Moldflow)

By iterating designs through manual or automated workflows, engineers can change the processing conditions or part design to identify a combination that makes an acceptable part. Moldflow’s automated optimization analyzes provide direct feedback on the influence of design, material and process changes on part distortion.

Moldflow simulations also make it faster and easier to consider a wider range of possible solutions, such as: B. changing the material or mold cooling, which is more convenient than dealing with warpage after molding a part.

What makes fault diagnosis so difficult is that there is very rarely an answer. In most cases, multiple effects can counteract or exaggerate each other, making it difficult to isolate the contribution of each effect. When you understand how and why plastic chains give engineers an edge in analyzing through simulation to come up with a suitable solution that fits their budget and schedule.

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