CAD Craft

A resource to CAD Craft specialists, and a showcase of my work — in both material and virtual worlds

Design Impact: Materials & Processes

The role of fabrication process & choice of materials is one of significant impact on any given design. With the advent of 3D printing, a rapid tooling or prototyping process, it is sometimes even more critical since this approach is an additive process — building up material instead of removing it, as in a machining process. It is also significantly different from molding or casting since there is no die or ‘container’ present for support in critical areas. For those coming into CAD from rendering tools like 3D Studio, Maya, etc., or from virtual world environments, the significance — and subtleties of these issues may not be readily apparent.

When designing for a regular machining process, one of the key considerations is the space allowance for the tool to be able to get in toperform its task. This is why various forms of casting or molding are used in certain instances, particularly for metals. With most plastics, this mode of fabrication is primary. When designing for casting or molding operations, some specific design considerations include draft angles, split lines, set backs & the avoidance of ‘captive features’ — a feature on the part which, once molded, cannot be removed from a rigid mold without damaging or destroying the mold. This is the purpose of such things as draft angles. It should be mentioned here that for special situations where such constraints as ‘captive features’ cannot be avoided, there are other molding/casting methods which inherently sacrifice some elements of the die, or mold. This technique is generically referred to as investment casting, and the lost wax method is a very ancient form of this.

In Fig. 1 below, there is an image of a part which would be very difficult — and expensive — to machine out of a solid piece of material. There are certain features which may seem, at first glance to contravene the rule I just mentioned about ‘captive features. The upper lip and the lower ring create a situation where this part could not be effectively cast.  This ring would therefore be added after the rest of the part was molded.

 

Figure 1 - Front section view

Figure 1 - Front section view

 

However, this brings us to the issue of materials, and, specifically, material thickness. The primary thickness for all features except the bosses is .06. For most plastics, this part would be insufficiently stable without the ribs shown, to provide wall stiffness. There are 8 ribs, equally spaced about the diameter, As shown in Fig. 1-4. As mentioned above, there is also a bottom ring which provides to important services: it provides greater lateral stability, augmenting the vertical/crush strength of the ribs while also providing a surface for the mounting bosses, one of which is visible above. The upper lip also provides stbility & strength at that end. 

 

Figure 2 - Oblique section view
Figure 2 – Oblique section view
 

These bosses are simply areas of thickened material, in this case to provide an anchor for a threaded insert, allowing for the securing of this part to a support framework using loose hardware. Obviously, you cannot effectively thread most plastics with any significant reliability, particularly if you are using metal screws. In this case, the screws would be mounted from below, using a locking threaded insert, eliminating the need for washers or nuts. 

Figure 3

Figure 3

 

This approach of providing ribs & stiffeners is a standard design practice facilitating the use of thin materials. This approach is used with metals as well. It should be noted that most rounds and fillets are produced in the molding/casting process, though they may be ‘cleaned up’ afterwards in the final machining process, even with durable plastics

Figure 4

Figure 4

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1 Comment»

  Shapeways | Blog wrote @

[...] his own minatures company, he uses 3D printing and scuplting and has some kewl stuff on his siteSimply fantastic post about designing for particular production techniques and materials by Jamal Ali over at CAD [...]


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