May 2005 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, CNC (computer numerical control) came along and automated milling, working from digital input. Today, Protomold has added another layer of automation to the process, but 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 (really… no kidding)
  • Parasolid (.x_b or .x_t)

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 enough data for rapid injection moulding. Because data lost in saving a file in STL format cannot be recovered, the problem of data loss cannot be solved by opening an STL file and resaving it in an acceptable format. 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 realize 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 confuse the automated milling software.

You can, however “join” separate parts to create a single part. 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
It appears that your part is modeled as several components. In order to quote your part we will need the components combined into a single model.

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
It appears that your part is modeled as an assembly. In order to quote, we will need a file for each individual part output in one or more of the following file formats: SolidWorks native (.sldprt), Parasolid (.x_t or .x_b), STEP (.stp), IGES Surface (.igs), or ACIS (.sat).

When you export your design, set export tolerances as high as possible. 1/1000th is good; 1/10,000th 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.