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Creating Mold Bases with NX Expressions

By Murat Ugur, April 29, 2013

In this article, I introduce Siemens PLM Systems NX expressions, part families, and the visual parameter editor. I show how to create a custom mold set wizard. I have experience with other CAD systems, but this is my first time with NX.

As with other MCAD packages, expressions are mathematical formulas that can be used to control the sizes of objects and distances between them parametrically. Siemens PLM Systems licenses NX’s Mold Wizard as a separate module. The module has extensive capabilities and can be customized.

About NX

NX is the high-end CAD system from Siemens PLM Systems. (Sometimes, you can still hear people calling it by its original name, UGS, short for Unigraphics.) It uses the Parasolid geometric modeling kernel, D-Cubed sketch and constraint engine, Synchronous Technology for direct modeling and editing, and outputs JT-format files for data exchange. That it uses all this technology is not surprising, because Siemens PLM owns it all.

It is used for CAM, CAM, and CAE by some of the largest manufacturing companies in the world, such as General Motors, United Aircraft, NASA, and HP. These companies use NX for parametric solids and freeform surface modeling, reverse engineering, sheet metal and electrical design, digital mockups, and mold design. It supports PMI [product and manufacturing information], analytics, validation, and NC [numerical control]. Its models can be tested through FEA [finite element method], kinematics, CFD [computational fluid dynamics], and thermal analysis. It is a very complete CAD system, and unlike most other CAD systems, it runs on Linux, Mac OS X, and Windows. The current version is v8.5.

Customizing Mold Design

Over time, CAD vendors add more functions to their software, but sometimes we can’t wait that long. When it comes to mold design, some function may not be available in all countries. This creates a disadvantage. For example, I live in Turkey and so I need to use Turkish standards for design work. This creates a disadvantage. For example, I live in Turkey and so I need to use Turkish standards for design work.

A solution available to everyone who spends a bit of time thinking about it. I created a simple but fast method that I use now: a generic mold design that is driven in NX by expressions. By changing values input to the expression, NX draws different -size molds.

I started by drawing a tree of the workflow. This gives me an outline of the tasks I need the expressions to do (see figure 1).

Figure 1: The workflow for customizing a specific mold design

The sizes of different parts are specified by labels, such as the size of bushings and the distance between columns (see figure 2).

Figure 2: The relationship between labels and parts of the mold

The values of the labels change as the use of the mold is specified. The values are shown below in figure 3, after putting together the expressions from a file I named "ASB."

Figure 3: Typical values of expressions

I wanted to create the mold using these values of the components of the structure created in figure 4. All transactions will be shown in the figure when the assembly is given in figure 5.

Figure 4: Variables used by expressions

Figure 5: Parts tree in NX

I created the structure by selecting the part family in an Excel spreadsheet (see figure 6). This table uses the mold standards in Turkey.

Figure 6: An Excel spreadsheet of parts designed according to Turkish standards

Well, it worked to create individual part files. Variables used in each assembly file created my ASB file with the link to the spreadsheet. I tried different values, and the result of each set was a success. After this, I mated the parts from the asb.prt file and I created links to get values from ASB file in figure 7.

Figure 7: Expressions in NX (at left) and the resulting mold (at right)

Here, for example, is the function I used (as described by Asb M8 M12) adjusts the diameter of the hole when the value is changed:

P17 = if(M==12)(10.2)else(if(M==16)(14)else(if(M==20)(17.5)else(19.5)))
P18 = if(M==4)(3.3)else(if(M==6)(5)else(if(M==8)(6.8)else(if(M==10)(8.5)else(p17))))

Thickness usually varies from the size of sets of mold-centering columns. These are a part of a set of a family members in the mold. I can click with the mouse in the image to pick the visual parameter, and then modify the measurement.

Figure 8 shows how I can manipulate control points. NX supports the slightest variations among functions in the system.

Figure 8: Parameter values and their location (at left) and isometric view of the mold (at right)


Figure 9: Changing the dx parameter

In figure 9, I change the value of dx to 0 for the two bushings pointed out in figure 10.

Figure 10: The result of changing DX to 0


I have described a simple design of a mold to test the capabilities of NX in using expressions to control the size of the mold. By using expressions, I am not stuck in the early design stage by setting a specific size. I can change sizes parametrically by selecting the appropriate set of values. This shortens the design time and allows the manufacturing process to continue more quickly.

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About the Author

Murat Ugar's expertise is in Industrial design, injection and vacuum molds, and thermoforming machines. He is a mechanical engineer with 20 years experience, and uses Siemens PLM NX and Delcam PowerMill in his work. Mr Ugar works and lives in Turkey.

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