December 2009 Design Tip

Keeping Files in Line

It wasn't long ago that most injection moulds were cut manually by machinists working directly from drawings. Then CAD and CNC (computer numerical control) came along and enabled milling working from digital input. Today, Protomold has added another layer of automation to mould design and CNC toolpath generation. However you get from model to mould, the accuracy of the original design determines the quality of the finished product.

At Protomold, all parts begin as 3D CAD files, but we've learned from experience that some files work better than others (and some don't work at all). Of course, we're always willing to work with customers to adapt files for production, but following a few simple guidelines will speed up the process and help you get exactly what you want without unnecessary delays.

The file formats we can use include:
� SolidWorks Native (.sldprt)
� Pro/ENGINEER Native (.prt)
� IGES (.igs): Initial Graphic Exchange Standard
� STEP (.stp): Standard for the Exchange of Product model data
� ACIS (.sat): Andy, Charles, Ian's System (no kidding!)
� Parasolid (.x_b or .x_t)
� AutoDesk (.ipt and .dwg, 3D only)

One common file format that we cannot use is STL. This format is designed for stereolithography and, though many CAD packages offer it as an output option, it does not contain data that is precise enough for rapid injection moulding. So if you want to submit a design for rapid injection moulding, don't save it as STL. Similarly, 2D files, wireframe models or .dxf (Drawing Interchange Files) do not contain all the information needed for the rapid injection moulding process.

We realise that a part design often requires rework, but if you must edit a design, it is better to undo whatever needs changing than to patch it. For example, if you create a hole that you later decide you don't need, plugging the hole is not the same as deleting the feature and recreating it. Patching can create internal surfaces, which can be confusing as there is no way to be certain if the internal surfaces are errors, parts of an assembly, or a garbled model.

You can, however, "join" separate parts to create a single part (see last month's Design Tip for more detail on this topic). If you design a single part by assembling separate pieces, you must join them within the software; otherwise, your design will include internal faces and, when you submit it, you will receive a message reading:

Assembly needs to be joined.

Conversely, if you are designing two or more parts and submit an "assembly file," that is, a file showing parts that are intended to be separate, you will receive a message reading:

Assembly needs to be separated.

When you export your design, set export tolerances as high as possible, 0.01mm is good; 0.001mm is even better. This ensures maximum accuracy in your final part. In closing, keep in mind that modeling software is very complex. For a variety of reasons, saved files may occasionally appear incomplete. This can often be resolved by resaving the file in a different format. If this appears to be a problem with a file you submit, you will receive a message reading:

There were some missing and/or incomplete faces in the model that we received. Often, saving the model in a different file format will eliminate these issues.

Some of our customers send their designs in two different formats just to be safe. But whatever system you use and whatever you send, we will do everything we can to get you the parts you need in a timely manner.