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Distributor Making Adventures

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petertha:
The finished plug sits on a 3DP base plate, centered on a 3mm pin. A 3DP mold dam fits over the plate. The inside dam corners are chamfered to save mold material which doesn’t really contribute to anything. I made one corner a slightly different size to act as a key register so the mold is uniquely orientated if removed. A releasing agent was sprayed on the assembly. I used Ease-Release-200 for everything.
https://sculpturesupply.com/products/ease-release-200?_pos=1&_psq=ease&_ss=e&_v=1.0

The silicone was mixed & then de-gassed in a vacuum pot. The mixture foams & rises as entrained air bubbles expand & escape. Then it collapses on itself leaving the mix mostly bubble free which is the objective. Degassing only takes about a minute but helps make a void free mold surface if all goes well.
The silicone is slowly poured into the cavity from one corner, allowing it to flow out. Once cured (overnight) it becomes the female mold for casting the distributor part from a chosen resin. The mold is semi flexible, about the durometer of a soft eraser for reference. It can be re-used multiple times. The mold should stay resident in the 3DP dam when casting the resin. That maintains dimensional stability as much as possible, but still allows the part to be easily demolded.

The shrinkage % of silicone is amazingly small but it’s all relative to the application. In my case even if the cap diameter was out 0.005-0.010” it’s not really the end of the world because the critical dimensions are machined from the solid cast urethane part anyway. YouTube is filled with interesting videos of very complicated parts cast from silicone molds, many with severe undercuts. These would be very difficult if not impossible with ‘hard’ molds. 

petertha:
Silicone
I tried 2 different brands of silicone. Links attached so you can read the specs. Smooth-On Moldstar 40-T which is a milky clear color.
https://sculpturesupply.com/products/mold-star-40t?_pos=5&_fid=dbb982474&_ss=c

A product from China available on Amazon called BBDINO 30A which comes colored blue.
https://www.amazon.ca/dp/B08CVSD6MZ?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_3&th=1
The BBDINO was less expensive (to me). Both worked well enough for this purpose.

Both are 1:1 mix by volume. 40 & 30 refer to the corresponding durometer ratings. I wanted it to be high enough that the mold would not distort much, but flexible enough to de-mold the finished part. It seems like durometers above 40 in silicone are less common, at least in small volumes. I also wanted to use the same stuff to make other kinds of molds as well as ignition related parts like spark plug boots & seals which might favor a lower durometer. Anyway, both products seemed to work for the cap, but I kind of favor the 40. BBDINO claims that no degassing is required but I don’t quite agree with that, or maybe its part specific. It foamed madly as well in the vacuum chamber. BTW silicone can be colored with dedicated pigments.

Soft durometer urethanes are also available & can generally be used in the same manner as silicones. I intend to experiment one day soon. But urethanes have a much shorter shelf life, even when using a dedicated gas blanket extender. But they also come in harder durometers which may be an advantage in certain applications.

petertha:
Urethane
The distributor cap was made by casting urethane resin into the silicone mold. I used Smooth-Cast 300 which becomes a hard plastic ~70 D Shore when cured. It is an easy 1:1 mix by volume but you have to be pretty exact. I don’t think I ever figured out dielectric properties beyond anecdotal reference it should be similar to other plastics. The spec sheet says 50 deg-C heat deflection temp which should be OK.

https://sculpturesupply.com/products/smooth-cast-300
https://www.sculpturesupply.info/pdfs/technicalbulletin/smoothcast300series.pdf

This particular resin has a pretty short pot life, 3-4 minutes, which means only about a minute of mixing time & you need to be pouring. Fortunately, the cap volume is small so not a big issue. I’m going to try 305 next which has a slightly longer pot life. This resin mix is only 80 cp viscosity which is quite thin & a desirable property because it flows nicely into cavities & better allows entrained air bubbles to escape on their own. Voids are still possible depending on the mold shape, or if you pour too quick. Urethane could be degassed in a vacuum pot but you are really working against the short pot life clock, it’s probably impractical. What I see more often is after pouring, the mold is transferred into a pressure chamber which squishes micro bubbles to where they are not noticeable.

Demolding is very easy. You just squeeze around with your fingers & out it comes. The urethane cures white but can be colored opaque with their specific line of pigments, which is how I did the red & black ones. I’ve read that black plastics may contain carbon in the pigment & might adversely conduct electricity. It was hard to be certain but the black pigmented cap seemed to spark the same as others so maybe different black colorant or concentration?

I also tried rattle can painting some caps. It seems to have bonded & cured so maybe another option. You absolutely have to scrub any release agent residue with soapy water & paint prep or the paint will fisheye.

I also cast one cap from epoxy resin with some milled glass fibers added as a test & it turned out OK, so I’m confident that the same ‘soft mold’ technique could be used. Epoxy is typically a higher viscosity, especially if its more paste like with additives, which means higher probability of entrained air & void potential. OTOH, epoxy usually has longer pot life, so maybe could be put in the vacuum pot during cure just like regular composites work. A benefit of this kind of mold is making multiple parts if desired.

So why cast a completely solid distributor cap body versus something more like a finished cap with internal cavity? Because it much easier & the resin waste is negligible. A solid allowed me to machine different ceiling heights (counterbore depths) corresponding to different rotor sizes & the skirt lip feature. I think it would be finicky to pour into a finished dimension shell mold with only 2mm wall thickness, the mold would be more complicated.

I did some tests on cured urethane samples by drilling holes & bonding metal like brass, mimicking terminals in the towers. I was satisfied that ordinary CA & epoxy seems to hold it quite well as long as it’s a sanded or machined surface. When I tried this same test on some Delrin, I really struggled to make it stick which seems consistent to what I’ve read. Apparently, there are specialized glues & primers but expensive. I think for Delrin the parts need to have some kind of knurling texture, be more interference fit or somehow mechanically keyed.

petertha:
Machining
The urethane part cures hard after about 10 minutes & can be demolded, but I usually gave it a couple hours to stabilize before machining. To machine I held the round part of body in a chuck or collet, then skimmed the base off flat to dimension in the lathe. Urethane machines exceptionally well.

One could either machine the internal cavity first & glue in the terminals second. I initially did this so that the terminals extend ~1mm proud of the cap ceiling. To position I first put a few layers of tape over the rotor mimicking the desired air gap & pushed the terminals down until they contacted the tape. Alternatively, one can glue the terminals into tower holes beforehand so that machining the cavity trims both the urethane & terminal ends simultaneously in one go. This would make the ends flush with the ceiling or making the corner birds beak profile or whatever it’s called

petertha:
Terminals
I used 2mm brass banana plug connectors sourced from RC model hardware. The fit is reasonably tight. They have pretty low resistance, nominally rated for 20-30 amps continuous. 16 AWG spark plug wires were soldered to the male plug end which has a little solder pot feature. The matching tubes become the terminal contact once glued into the cap. The tubes are 2.5mm OD x 2mm ID x ~15mm length. A bit longer than I would have preferred, but they worked out OK. You just need to be careful trimming what will become the terminal ends inside the cap without cutting through & exposing the tube portion, not a lot of wiggle room here. I tried a few drill jig styles to hold & orient the cap. 3DP to the rescue again. The 2 mounting lugs were drilled M1.6 clearance using the same jig.

On another cap I used brass M3 set screw glued into tapped holes. They were landed proud of the terminal top to accommodate a nut, so spark plug wires come in laterally & attach to the set screw via a M3 ring terminal. The cap terminal towers could be trimmed shorter in this mode if desired. One could also make your own terminals & center tap a hole. Many possibilities on the same basic design.

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