If you’re experienced in injection molding, you probably can identify warpage in your sleep. Your average consumer certainly notices when a component that should be smooth and flat is misshapen. But pinpointing the cause? That can be more complicated. And it’s important because plastic injection molding offers opportunities to reduce or avoid unintentional bending. But rather than waste hours guessing at the most effective combinations of material, design, orientation, or temperature, you can manage warpage with one tool: injection molding simulation.
First, let’s walk through some sources of warpage and the most effective ways to stop it. We’ll also look at how the right technology—such as plastic injection molding simulation software like Autodesk Moldflow— can help you validate solutions before the manufacturer cuts mold steel.
Plastic materials can shrink during and after manufacturing for a variety of reasons. Those reasons go down to the molecular level, influenced by factors like the type of polymer and whether any filler or fiber reinforcement is present. Those can play a role in what happens when plastics melt and cool.
- Amorphous Materials: This category includes materials such as ABS, polystyrene, and polycarbonate. With a random molecular orientation in their natural state, the forces holding molecules together weaken and they move away from each other as the materials melt. The shear experienced during the injection phase also causes those molecules to uncoil and align with the direction of flow. When the flow stops, the molecules return to a state of random orientation; intermolecular forces bring them closer together until cooling temperatures freeze them in place. This leads to uniform shrinkage, but the relaxation effect causes more shrinkage in the direction of flow.
- Semi-crystalline Materials: These materials have regions of highly ordered, tightly bundled molecular structures. Melting causes those structures to loosen with the molecules aligning with the direction of flow. But the molecules don’t relax when the materials cool; they stayed oriented in the direction of flow and as they recrystallize, they shrink. The shrinkage tends to be much greater in the direction perpendicular to flow.
- Fiber-reinforced Materials: Manufacturers often combine fibers with polymer materials – and that addition can counteract the shrinkage effects described above. Fibers don’t expand or contract in response to temperature changes.
It’s not the shrinkage itself that causes warpage, but rather, the variations in shrinkage that cause it. A part that shrinks into a smaller but perfect version of itself is going to look the same. It just won’t be the right size. But when different areas shrink at different rates, the internal stresses can disturb the structural integrity – guaranteeing warpage when the part is ejected from the mold.
You’ll see four types of variations:
- Regional. Simply put, one region of the part shrinks more than another – usually the difference being between the thicker (gate area) and thinner (EOF) areas.
- Through the Thickness. The shrinkage differs between top and bottom. Sometimes this causes the part to bow.
- Directional. Molecular or fiber alignment can cause differences both parallel to and perpendicular to the material’s orientation.
- In-plane vs. Thickness. Polymers can shrink more in the thickness direction than in the plane of the surface, thanks to mold restraint. This difference in shrinkage can cause warpage, particularly in the corners.
So, what causes these variations? There are several drivers. High cooling rates give crystalline structures less time to form, which can decrease total volumetric shrinkage. Orientation due to filling and mold restraint can also cause uneven shrinkage. Finally, temperature differences in the thickness of a part, or the areas closer to the gate, can cause shrinkage variance.
To make this even more confusing, some effects can cancel each other out or worsen each other, which can further obscure the culprit.
The Benefits of Injection Molding Simulation
Hopefully, you’re not too daunted at this point. Given the above factors, yes, managing warpage can be tricky. But injection molding simulation software can make it so much easier.
Consider simulation tools like Autodesk Moldflow, which helps you tackle warpage earlier in the product design cycle. By running analyses based on part materials, design, and processing conditions, you’ll spot future warpage before it happens. Just one simulation can present multiple processing options and how each affects warpage. You’ll end up with a good idea of how much shrinkage and warpage is likely – and even compare your results to other simulations.
Also helpful: iterating designs through manual or automated workflows. You can tweak designs and processing conditions to identify the ideal combination. For instance, Moldflow offers automated optimization analyses that spell out exactly how the design, material, and process changes influence part warpage. Moldflow simulations can also fast-track your ability to evaluate solutions, such as picking a new material or mold cooling, before creating parts.
Hopefully, you now feel more confident about the multiple causes of plastic warpage. While it can definitely be confusing, you don’t have to figure this out on your own. Injection molding simulation does the heavy lifting for you – so you can triumph over confusion and find the best and most cost-effective solution for your design.