Walking through an injection molding simulation workflow.

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Written by: Drew Buchanan, Engineering Manager at TrueInsight, a certified Altair Channel Partner.

I have spent my engineering career at various companies in different industries, and a common thread among all of them was the need to talk to manufacturers overseas whenever I went to manufacture a part. Often, I would find myself working in my office past normal working hours to align time zones with our overseas distributors, so that we could discuss an upcoming part to manufacture. For many of those calls, I would wonder—as I’m sure many others have—if there was an easier way to avoid all these design hiccups and conference calls that go late into the evening. One positive of these calls was that I was able to see a variety of beautiful sunsets on my way home from the office. That said, I yearned for a better solution.

One of many beautiful sunsets after a late conference call. (Image courtesy of Drew Buchanan.)

Fast forward to 2022. There are ongoing global supply chain delays and engineers designing plastic injection molded parts face even more challenges. For example, if an engineer sends a part to a manufacturer to be molded, and the manufacturer realizes there is an issue in the part, delays are significantly longer to resolve the issue versus what they were pre-pandemic. As a result, engineers are looking into ways where they can reduce these types of delays, and one popular approach has been injection molding simulation.

Injection Molding Simulation Tools

Traditionally, molding simulation software has been very costly and has required significant expertise to set up scenarios (i.e., including the full mold and tooling in the system). Traditional molding software tools require advanced requirements to setup scenarios like modeling the complete mold. While all these tools are successful at simulating the injection molding process, they require substantial expertise to run injection molding simulation. The use of these advanced requirements, in addition to the investment in cost and expertise, has traditionally steered engineers away from considering traditional molding simulation tools in their process.

With the high cost of investment in expertise and price to use traditional injection molding simulation tools, advanced simulation vendors are looking at ways to democratize injection molding simulation so design engineers can utilize injection molding simulation easily. Tools like CADMOULD by Simcon and Altair Inspire Mold have embraced democratization of injection molding tools, so design engineers can run injection molding simulation scenarios easily. Let’s run through a workflow in Altair Inspire Mold.

Simplifying Injection Molding Simulation

Altair Inspire Mold is intended for design engineers, as the workflow is set up to be very easy and to help designers figure out problems early in the design process. Let’s look at a workflow for a plastic part that we would send to be molded. The software can import various CAD formats; in our case, we are importing a Parasolid part file to simulate.

Plastic part to be injection molded. (Image courtesy of Altair.)

The next step in the workflow is to designate the part’s material. During this step, users can access a library of polymers to utilize. But if a situation comes up where a user needs to create a custom material, this can be done too. In our case we will utilize a generic ABS material, which is already within the material database.

After specifying the material, we need to set or verify the melt temperature and assign a gate. A gate is where the hot polymer material flows into the mold cavity. We can apply a predefined gate, or we can assign CAD entities as a gate.

In our case, since we only have a single CAD body, we’ll utilize the predefined gate feature and apply a gate on one of the sides of the part. Note that later in the simulation workflow, there is a tool to optimize gate location. This means that if we do not know where to assign a gate, we can evaluate the part and the software will suggest a location for us.

Gated part. (Image courtesy of Altair.)

One of the big challenges of performing injection molding simulation is the need to know the process parameters (e.g., the length of the filling stage). Traditionally, this is where most people have shied away from these kinds of simulations. This is because design engineers typically do not know these process settings since they are correlated to the injection molding machine. In Inspire Mold, there is an automated feature called “Molding Window” which will automatically tell us the process settings to utilize in our scenarios (i.e., melt temperature, mold temperature and fill time).

In our case, we will see that to maintain our designated melt temperature, we can utilize a fill time of up to 1.1s to run our molding simulation. Often, faster is better for most parts to minimize pressure gradients and warp.

Our automated process settings. (Image courtesy of Altair.)

At this point, we can now run the scenario by specifying a fill time of 1s. Once we run the filling simulation, we will better understand the potential issues that may occur during this stage of the process. Note that the workflow includes advanced capabilities to evaluate the packing, cooling and warpage that can affect the molding process. For now, our primary concern is to evaluate filling defects, so we will only run a filling study.

Our mold run settings. (Image courtesy of Altair.)

After running a quick simulation, we can look at various plots to determine if our part will have any significant defects. In our case, we first want to see if our part can be filled easily. We can do this by animating the flow front pressure. The simulation shows us that our part can be filled easily, and as a result we do not need to make any significant changes in our CAD part design (e.g., adjusting thickness or adding ribs).

Plastic part flow pattern. (Image courtesy of Altair.)

Two other things that a lot of designers worry about are Air Traps and Weld Lines, as these are defects that affect the overall part aesthetic and can affect the part strength.

Air Traps occur when the polymer flow front surrounds and traps air in the mold cavity, and they will often cause several surface defects on the part including shiny regions, dull regions and grooves along weld and knit lines, and can even leave carbon scoring that will damage the tool over time from adiabatic compression of the trapped air (i.e., dieseling).

Similarly, weld lines occur when two flow fronts meet and cause a surface defect that looks like a small line. Weld lines and knit lines always start at an air trap.  In both instances, designers do not want these situations to occur.

With simulations, we can visualize both results. In our model, we have some small air traps and knit lines that we will need to resolve via changing the geometry or adding vents to the mold. Note that in a lot of cases mold makers can resolve Air Trap and Weld Lines through venting.

Plots show knit lines on left and air traps on the right. (Image courtesy of Altair.)

The truly valuable thing about simulation is, we can capture these defects and issues before we reach out to our manufacturer. We can clearly understand if the part we designed can be injection molded easily or if we will encounter unnecessary challenges. Tools like Inspire Mold are enabling designers to reduce the product lifecycle and overcome some of the existing supply chain issues through reducing the time for rework via effective and easy simulation.

To learn more, visit altair.com/manufacturing.