Plastic part design can be a complex process under any circumstances, and when you are designing parts with text, numbers, and symbols, those challenges can be greater still. In design you rarely know what little trick or idea is going to make the difficult easy or the seemingly impossible possible; that's why we offer new design tips every month. That said, every once in a while we run across one that seems so important that it's worth revisiting. Here is a "Design Tip Classic" that originally ran in 2008...
There are lots of reasons to add text to a part. It could be an assembly instruction, a part number, a legally-advisable warning, or simply a logo (see Figure 1). Whatever the reason, text characters tend to be the smallest features of a part and, as such, deserve the designer's careful attention.
Figure 1: Off/On switch instructions on a part.
The first thing to keep in mind is that it works much better if text on a plastic part is raised above, rather than recessed into the part (which means it will be milled into the mould). Raised letters on a part are easier to read, and recessed text in a mould allows for polishing, whereas raised letters in a mould make it difficult to achieve a good finish.
The second issue is consistency of wall size in your lettering. Avoid serif fonts, the ones with the little squiggles at the ends of uprights. The serifs are typically narrower than the primary lines of the letter itself, making them too small to mill. Instead, use a sans-serif (non-serif) font like Century Gothic Bold (the default font in SolidWorks). Other common sans-serif fonts are Arial and Verdana. In general, remember that while most 3D CAD programs allow you to use standard Windows fonts, you should resist the temptation to get cute without a good reason.
The third issue is the size of the letters themselves (see Figure 2). Text doesn't need to stand very tall above the surface of a part - 0.508mm is plenty - but even so, the rules for thin ribs apply. You don't need to measure the thickness of every line of each letter; just stick to font sizes of 20 points or more and use the Bold version of the font, and odds are excellent it can be milled (see Figure 3). In some cases, we can mill smaller fonts. If you need to do so, submit the part with the smaller text for a ProtoQuote® interactive quote and the quote will show any required changes or advisories. You can also contact a Proto Labs Customer Service Engineer at +44 (0) 1952 683047 to discuss your project.
Finally, if text is located at the top of a tall feature - a tall rib, for example - the text may have to be larger.
Figure 2: The "o" is too small to mill.
Figure 3: The "L" is a larger font size and allows room for the tool to mill.
In summary, for best results when incorporating text:
- Design your parts with raised text.
- Use a bold sans-serif font.
- Size text to 20 point type or larger.
- Stay away from the tops of tall features.
If you are wondering whether you've designed your text properly, simply upload your 3D CAD model for a free ProtoQuote®. If there are any problems you'll know by the next day.
Watch this short YouTube video on designing text on plastic parts.