Re-Designing an Injection Molded Part for 3D Printing

Most of the plastic parts you are familiar with have been made by a process called injection molding, whereby plastic is injected into a cavity in a mold and a plastic part is extracted after cooling. The process of injection molding has many limitations of it's own and designs must be made to conform so as to be moldable, strong, and not crack or have defects. We could write an entire paper on these constraints, but suffice it to say they are very much different and share little in common with the constraints and requirements of designing for 3D printing.

Often 3D scanners are suggested for copying parts for 3D printing. Scan - Print - Simple - Right? Well, in actual fact, this is mostly a myth for the following reasons:

1. Scanning doesn't accurately reproduce dimensions within required tolerances.

2. Scanning will often miss interior features like screw holes, threads, cavities etc.

3. Scanning presents you with an organic file for an object (ie. pixels) - uneditable with standard parametric editing programs, so you can't modify it to add things like threads, or holes that the scanner missed.

4. Scanning a part that was injection molded and then applying all those design limitations to 3D printing - which has it's own, incompatible, design limitations - will produce an inferior part that is likely not even 3D printable or will be extremely weak.

To create properly designed 3D printable parts they must be laboriously redesigned feature by feature with constraints and changes appropriate for 3D printing.

So, copying or re-designing a previously injection molded part requires re-thinking the design, both to remove strength limitations which may have been imparted simply because of the injection molding constraints and to take full advantage of the special features which can be designed into a 3D printed part. In this way, the new part will often be much, much stronger than the original injection molded part and can be designed and oriented in such a way as to make for a very much improved part.

In this article we'll look at a boat part which i3DGear sells and was re-designed from an injection molded version to be stronger. This part is no longer available from the manufacturer and has a weakness in it that was inevitable because of injection molding constraints, but was easily designed in a better way for 3D printing.

Look at the bracket at right. The teeth on the bracket are designed to hold a control unit for an Autohelm 4000 marine autopilot system. The control unit slides down into the teeth, mating with them, providing a removable mount for the device. In order to make the part moldable in an injection mold, the two rectangular slots are required behind the teeth, however, they create a weak point and the bracket often snaps exactly in this location.

If you learn about injection molding, you will understand why those slots need to be there - they are for the other side of the mold to create the back side of the teeth. There is no way to mold this part without these slots.

In our 3D printed version, we have no need for the slots. They can be removed as shown at left and you can clearly see that the part will be much stronger being supported the entire length of the teeth.

So, when copying injection molded parts, it's very important to understand what each of the features that you see in the original were for. Replicating weaknesses into 3D printed parts simply makes no sense.

Thinking about the printing orientation of this part is important also, as we discussed in a previous article. The 'obvious' way to print this bracket is laying down flat on the print bed as on the right. But, remember how this bracket is to be used. Those teeth are going to have a lot of force trying to pry them off the bracket and in this orientation, the teeth will quite easily tear off the surface along the layer lines - remember the concept of 'anisotropy' in 3D printing?

So, a much better orientation for this part would be to print it vertically. Then the extruded noodles of plastic will wrap through the teeth concentrically up the part and make the teeth almost impossible to tear off - it's much harder to tear a noodle than to tear a layer. To print in this orientation, we could add support structure under the teeth (right side), or better yet, as we learned in the last Blog post, we can 'build out' at 45 degrees and support the bottom of the tooth naturally (left side), which actually ends up adding a great deal of additional strength to the teeth! A double benefit!

So, there we have it. A properly designed replica of an injection molded part that has removed weaknesses in the original and fully taken advantage of the strengths of 3D printing to make a stronger part that will last. At i3DGear, we don't just replicate parts - we improve them!