Solid Thinking: Go With the Flow

14 Feb, 2005 By: Greg Jankowski Cadalyst

SolidWorks' MoldflowXpress analyzes plastic part designs so you can address moldability issues early on

When designing an injection-molded part, there often are questions concerning moldability -- the ability to create a repeatable, per-specifications part. MoldflowXpress is included as an add-in to SolidWorks. It allows a designer to perform a quick moldability analysis of the part early in the design process. Like COSMOSXpress (an add-in for finite-element analysis), MoldflowXpress is a simplified version of a complete package (in this case, Moldflow), and is geared toward the design engineer who wants to perform a quick what-if analysis to determine whether changes should be made to design for moldability and have better questions (i.e., gate location) for the mold designer.

Many factors contribute to designing and creating a good plastic part. These fall into four main areas:

  • Part design. Many design characteristics make up good plastic part design. This article will only touch on a few of these items. MoldflowXpress can be used to help validate the design and determine potential changes. Some guidelines also are included in the Help file.
  • Material selection. The material used matters because the same part molded from different plastics can differ in final dimensions. Materials have different shrink rates so they melt and cool differently. Therefore, it follows that the next two items are also important to making good parts.
  • Molding. The temperature, fill and cooling cycles will affect the final part and production rate.
  • Tooling. The mold designer will define and design a number of things (i.e., number of cavities in the tool, parting lines, injector location and ejector pin locations) that are required to make the tooling used to manufacture the part. It's always a good practice to understand these items and discuss them with the mold designer. In particular, tool related items that may have a visual implact on the design.

In this example (figure 1), we have an injection-molded part with three potential gate locations. We will use MoldflowXpress to determine whether the part design, material selection, molding and tooling considerations are acceptable.

Figure 1. Example part with three gate location options.

Creating an Analysis
To create an analysis, start MoldflowXpress from the Tool pull-down menu. MoldflowXpress is a wizard-based application. Define the information required for each tab from left to right. The Next and Back buttons can be used to go from tab to tab or just select the next tab at the top.

A couple of additional notes are that some of the tabs require the previous information to already be defined. So if the Results tab is selected before the material is selected or even the analysis is run, the results will not be available. The other note is that once defined, the information about the analysis will be saved within the SolidWorks document so the analysis can be rerun without adding all the information again.

Step 1: Define the injection location. Here we see the location of the injector (figure 2). This can be a predefined point on the part. Select the location on the part and click Add. Select Next to continue. To remove the current location, select the coordinate and click Delete. A new injector location can be added.

Figure 2. Injection Location dialog box.

Step 2: Define the material. In the Materials tab (figure 3), select a material that is closest to the desired material. Select Next to continue.

Figure 3. Materials tab selection dialog box.

Step 3. Define the molding condition. In the Conditions tab (figure 4), you'll typically use the default conditions. Select Next to continue.

Figure 4. Conditions tab dialog box.

Step 4: Run the analysis. Under the Analyze tab (figure 5), select Run to start the analysis. After the analysis is complete, select Next to continue.

Figure 5. Analyze tab dialog box.

Step 5. Review the results. In the Results tab (figure 6), review the animation by selecting one of the animation controls -- for example, run, loop, time slider or flap. Review any advice that is listed under the Advice button. Select Next or Close to finish.

Figure 6. Results tab dialog box.

Analyzing the Results
Now that the analysis is done, what's next? The Results tab has playback and Advice functions (figure 7).

Figure 7. Visual results from the analysis.

The following should be reviewed after an analysis:

  • Did the part fill well (animation) and were any issues listed within the Advice area?
  • Are there any areas that restricted the flow of material? This could be due to different wall sizes, lack of radii on corners and so forth. Also remember that putting large radii on all corners may help plastic flow, but also can cause other issues, such as sink, because the large radii created a thick section.
  • Review the part for plastics design practices: equal wall sections, radii based on the wall section and so forth.
  • Should other material types be analyzed?
  • Does the gate location work for both molding and design? If not, analyze other options and locations. Have a feeling for potential gate locations before taking your design to the tooling designer. This will make for better, more informed dialog with the tool designer.

The three locations were run and the location shown in figure 3 filled most quickly. The second option filled 0.1 seconds slower but otherwise achieved similar results. The third location would have been my initial best guess for a gate location, but it turned out the part did not fill well using this location. The assumption would be that we should choose either the first or second location for the gate. Deciding the gate location is driven not only by reasons of moldability but also cosmetic appearance. The gate can leave a small mark on the final part, so the gate should be on a surface or location that will not be easily visible. This is why having more than one option for potential locations is important early in the design process.

The next step may be to send the results to the toolmaker. On the Results tab, the Export button will export the results to an eDrawing that includes the fill animation. This eDrawing can be a good basis for the conversations with the tool designer.

MoldflowXpress is a limited version of Moldflow, so naturally it has limitations you should keep in mind. The two most significant limitations are:

  • Limited materials library. The materials listed in MoldflowXpress are a good cross-section of material types, but they do not represent all SolidWorks materials settings. Again, keep in mind that this tool is a simple one intended for limited use by the designer.
  • Limited molding analysis. You can analyze only one part and injection location at a time. Including the runner and examining multiple parts is not part of MoldflowXpress. These are tool design questions that will be decided later in the process.

Your tool designer is really the expert at detailed, in-depth moldability analysis. As with COSMOSXpress, the intent of MoldflowXpress is to provide the design engineer with a tool that can be used to verify design attributes -- in this case, wall size, fillets, and so forth -- and to be sure material selection does not cause any unforeseen potential issues. Using this tool, you can discover these items up front and address issues, if any, before involving the tool designer.

When designing a molded part, the more you can do up front to ensure the part is designed with the end process in mind, the better the end product. While a mechanical designer is not a mold or tool designer, the simple analysis you can perform using SolidWorks MoldflowXpress can provide valuable information you can use as a basis for discussion with your toolmaker.

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