One of many valuable online resources for learning how to design plastic parts is the use of Protomold's automated website. Protomold not only make parts quickly, their automated system informs you of problem areas in your design and explains why.
Starting from ground zero: Before jumping into CAD and designing a part, I highly recommend taking a quick look at the library of white papers on the fundamentals of molding. It will provide all the basics needed in designing your first plastic injection molded part. Understanding these 12 basic molding principals will help you go a long way in your design (there are 13 on their site, but the “Pricing Guide” doesn’t really count; still a good reference though).
Prototyping: You don’t have to incorporate everything in your first iterations, keeping the basic principals in the back of your head while designing will aid in cutting down the time between prototype to final part. Iterating with 3D printed parts is great, but it can give a false sense of manufacturability. Understanding the limitations of molding will force you to design the right way the first time.
Step by Step Design: It starts off with simple geometry for the main body, then add on features from there. Always remember to watch your wall thickness and avoid undercuts; these are probably the two most commonly overlooked principals. After the part is fully featured it is time to add the drafts, rounds and chamfers. These types of features should always be at the bottom of your feature tree for good modeling practices.
Get it Reviewed for Free: Using the automated online quote system is quick and easy. All you need to provide is a little personal information, a part file, and material. This will supply you with a basic quote in which you can change other options like surface finishes, material and lead-time; it allows you to make on the fly changes and will update the price instantly. The built in molding advisor points out issues like draft, wall thickness, sink, and undercuts that are highlighted for easy reference. Once you have updated your model you can upload new revisions within the same quote and quickly see if your changes made the difference. Remember this an automated system that is based on rigid rules, as a beginner, it would behoove you to stay within those bounds. Now, you will be able to iterate faster and get to a manufacturable design with less iteration.
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