For almost three decades this air control box distributed air through the cabins of various Porsche 911 models. It's a modular design employing 5 base parts which in different configurations make up a left and right hand assembly consisting of 7 components each. After decades of use some of these parts are often broken or missing. Neither this assembly nor any of its components are available from Porsche. Good second hand ones are rare and expensive and typically sold as complete units which is frustrating when only one or two of the components need replacing but can't be sourced individually.

Reproducing this item using additive manufacturing poses some challenges. To start with, none of the geometries are ideal for 3d printing with lot of large thin walls, extreme overhangs and inconveniently placed protrusions. Every groove, curve and and indentation is there for a reason and affects in one way or another the part next to it. This leaves very little scope to redesign or tweak anything. We know that only one or two components will typically be replaced so the restored assembly will almost always consist of a mix of original and printed parts. In other words, despite being adversely shaped for additive manufacturing, the printed parts have to fit each other and the originals equally well while being strong, heat resistant, neat and accurate. And all of this needs to be repeatable.

Step one was to scan all the parts in high definition. Some of them had bits missing such as the example below. Fortunately there's a lot of repetition between the parts so we could fill in the gaps in CAD.

Step two was to design new parts from the scans and from measurements of the original. Fortunately the sizes and shapes of the parts made them fairly easy to scan and all the details were picked up accurately including the locating indents and grooves 

The genius behind this mechanism quickly became evident. The centre part which on face value is a fairly complex shape, turns out to be a mirror image of itself along 2 axes which means only a quarter of it needs to be designed and then mirrored twice to make up the complete part. This also means that there's no wrong way to start the build as the cornerstone part is always oriented correctly to slot into the rest of the assembly, which is also made up of parts that are mirror images of themselves. What a shame that such a clever design has to be hidden deep inside the body of a car.

There are two flaps inside the assembly which are notorious for breaking. The challenge here is to make them as strong as possible while adhering to the original dimensions. The originals were made from a bacallite-like material and, while being extremely rigid and heat resistant, it also means that they were brittle.

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Porsche clearly over-engineered this mechanism for maximum strength and durability so why would it break?

Under normal circumstances these parts would last forever and we see instances where they are as good as new. However over the course of half a century there are some moments in a car's life when circumstances aren't normal. It often has to do with an accident which causes a small fracture which takes years to propagate to the point where the part finally breaks.

With this in mind it's essential that the replacement parts are not only rigid but also durable and impact resistant. This eliminates many of the commonly used thermoplastics as well as almost all the resins currently available.

These renderings show what the complete assembly looks like in exploded view and with each part in its place. The challenge now is to find a way of printing each part so that it uses the minimum support, no post-processing, is durable and dimensionally spot-on. 

A number of delicate features limit the ways in which the some of the parts can be printed. One example is a narrow groove on both sides of the centre piece which needs to be clean and sharp. No support material must be generated in this region and the flat mating surface needs to be totally smooth.

The first photo is of a prototype printed with resin. It does achieve a high quality finish but resins tend to be brittle which limits their use in this instance. Durable resins do exist but they have a low heat deflection temperature (HDT) which makes them unusable anywhere in a vehicle's heating system.

The decision was therefore made to use carbon fibre nylon. It is both rigid and durable with a heat deflection temperature much higher than required for this use case. The second photo is of the carbon fibre nylon part. It took some fine tuning but the result is in every way as good as that of the resin printer.

After countless attempts and many failed prints our efforts finally paid off. Below are some photos of the final product which is now sold to customers worldwide. 

© 2020 by Martin Wiesner