We know 3D printing has some advantages over larger volumes of manufacturing, such as plastic injection molding. You can completely encapsulate objects and not worry about undercuts, part lines or tolerances.
What if you want to use your 3D print to cast a mold to make many more from? It is possible to produce small runs of plastic products reasonably cheaply with silicone molds – this is common practice for prosthetics and props in the film industry, where dozens or hundreds of objects may be necessary.
I’m going to show you a simple plastic part line example – that you can apply to your 3D prints – if you ever need to do small scale plastic molding.
You can see 0.20 millimeter dimension. Thats the tolerance I’m giving between part lines to ensure the two parts will fit, even if there is slight variations from the molding and cooling process (common, even in silicone rubber injection molds). Even iPods have visible partline tolerances. Although companies like Apple are very clever with visual techniques to either hide them or make them a feature.
I gave the outer rim a height of 2.5mm. (A fairly arbitrary number as I’m not sure what this product is going to be used for, nor what machine is going to manufacture it.) This should help keep the parts together.
Draft Angles are implemented in the design to help remove a part from a mold once it is cooled. a few degrees off 0º should be fine for low volume silicone tool molds. Silicone rubber is an extremely flexible tooling material, unlike hard tooling in steel or aluminium.
Both parts together, you’ll note the inner side’s part line is higher (1.2mm) than the other, producing a gap in between. This is a common visual trait in product part lines that hides any wavy imperfections from molding. If they were to fit perfectly together it can make any minor tolerance variations appear rather glaring.
All that is needed now is to add a screw hole and boss for fixing the parts together. You can learn a lot by taking apart products and studying how they fit together. This is actually common practice in some design consultancies who document interesting fixings for future reference.
David is an industrial designer from Wellington, New Zealand. He contributes weekly 3D print articles for Ponoko. You can follow him on Twitter @dizymac