Making a distributor cap

[Peter Twissell]

As the title suggests, I need to manufacture a distributor cap for my freelance 7 cylinder radial engine.
When I drew the design for the distributor a few years ago, the plan was to mould it in phenolic resin, selected for its dielectric constant and resistance to water absorption. At that time, it was possible to purchase resin kits under the name Novolac.
I have now arrived at the stage when I am about to start making the mould and I can't find any source for phenolic resin.
I have not been able to find anything useful with web searches.
Has anyone else moulded distributor caps or other HT ignition components and if so, with what materials and what results?

Thanks in advance,
Pete.

 

[stevehuckss396]

Most including myself machine distributor parts. I have used delvin and have made many from a sheet of lexan that was given to me. Unless you are trying to replicate something for an exact look, it might be less of a headache to machine one from solid plastic. I cant remember the last time I saw someone pour one. I hope to see you do one. That would be interesting as heck to see one out of the mold.

 

[Peter Twissell]

Thanks Steve. My back-up plan is to machine the part from solid, as you suggest. Even so, I would like to go through the process of making the mould and pouring the part, if only for the experience.
I have designed the cap with a traditional hemispherical shape. I can easily machine the mould for that shape, but a machined cap would need to be a compromise for manufacturing.
Some other parts of the engine, most notably the supercharger housing on the front, have been machined using a contrived copy attachment on the mill, giving the appearance of a 4 axis CNC machined part. Another option for the cap would be to mould a part in any resin, then use it as a pattern and 'copy mill' the finished part.

 

[Peter Twissell]

IMG_20191211_175828

• Peter Twissell
• Dec 18, 2019
• 3

I have now made the mould for the distributor cap, which is to be cast in high density polyuerethane resin.

 

[awake]

Impressive mold - I look forward to seeing the results!

At the risk of sending this off-topic ... has anyone experimented with 3d printing a distributor cap? It would involve some supports, of course (unless, maybe, printed at 45°? Have to think about that). Not sure what the dielectric strength of PETG or PLA or ABS is ...

 

[Peter Twissell]

I had considered it, but I concluded that the porosity of 3D prints would be too much of an issue with regard to water absorbtion. It's no good having a high dielectric constant if the part is a wet sponge!

 

[awake]

Yes, good point. OTOH, I have seen successful prints of water bottles that do not leak, so ... maybe??

 

[Peter Twissell]

Not leaking is one thing, zero porosity is another.
Have you ever seen a canvas bucket? They used to be a popular camping item and they don't leak!

 

[awake]

Aha! I had not thought through that difference. Definitely something to investigate.

At the risk of beating the now rapidly dying horse ... would it work to print the distributor, but then coat it with epoxy to seal it? (Thinking about the way that one can make a boat using plywood that then is sealed with epoxy ... and fiberglass, but the latter is for strength and toughness, not for sealing.)

 

[Peter Twissell]

Yes, that is a possibility. However, I'm a stubborn old bugger and I'm going to cast mine! I have some out of date epoxy resin, so I'll try with that before committing to the PU resin.

 

[awake]

Oh, I definitely think you should cast yours - you've already made a beautiful mold! I was thinking more along the lines of someone like me, who might need a distributor but doesn't have a mold ... and certainly not the skills to make one with the detail that yours has!

 

[petertha]

Hi Peter. I've had a bit of 'dabbling' experience with 2-part urethane's. They come in a variety of viscosity's, cure time, hardness numbers... But they are pretty straightforward to use & there are lots of online examples to examine as to suitability. Some general tips
- I cant speak to the dielectric properties if that's an issue, you would have to check
- don't substitute or short cut or the releasing agent or bad things can happen - improper cure or worse, stuck to your mold. test on scrap material first
- air bubbles are removed by simple vacuum chambers. Mix the resin, insert into chamber, vacate (allow for expanding froth head), ready to go. People are commonly casting clear resins where this basically makes or breaks the look of the part, so its pretty established work flow. Some guys build their own from sealed pots & a spare pump but you can get turnkey assemblies now from Asia

This is one vendor, but there are many.
https://www.smooth-on.com/category/urethane-resin/

ps (now that you have put all that work into a female mold). I have done some work using flexible (urethane or silicone rubber) molds made from male or positive shapes. Sometimes, depending on the part geometry & acceptable tolerances, this an easier way to go. Some very intricate parts even with improper draft can be reproduced (using the same hard urethane casting resin). You can see examples of this with hobby figurines & such. Your distributor is a shell, so may or may not lend itself to this.

 

[petertha]

Vacuum chamber general principle (there is much more info on net). I find I use mine for all kinds of composites projects now, they are good to have.
https://www.smooth-on.com/products/vacuum-chamber/

example offshore kits with/without pump depending on what you might have handy.
https://www.ebay.ca/itm/Small-Mini-...054360?hash=item4d93607698:g:Bx4AAOSwyi5czsbb

 

[Peter Twissell]

Awake: Don't overestimate the skill in machining. The distributor cap is 60mm diameter, so it's not an intricate piece of jewellery. The turning was done on a Drummond roundbed lathe which my grandfather bought new in 1926. Spherical forms were generated by calculating the X and Z locii at 10 thou intervals, then finished with emery. Both parts of the mould were then drilled to produce the form of the cap with one central and seven peripheral contact insulators. Nothing fancy at all. Polished with Autosol and a rag (if you can't make it good, make it shiny!)

Petertha: Thanks for the tips. I occasionaly use 2 pack resins, mostly for potting electronics. The epoxy I have to play with is in a divided sachet with no air space. It is mixed by removing the dividing clip and combining the resin and hardener in the sealed sachet, avoiding entrapment of air. Only when it is fully mixed is the sachet opened to dispense the resin, avoiding the need for vacuum de-aereation. I have used other resins in dual syringe type mixers, again avoiding trapping air in the mixing process, and reducing waste as the resin is mixed as it is dispensed directly into the mould.

I intend to use wax release agent. By heating the mould in boiling water, wax runs very cleanly over the surface and 'wets' the polished aluminium, getting right into the smallest details. The mould inserts which support the brass conductors will also serve as ejector pins, allowing the part to be pushed out of the mould with evenly distributed force. There is a short section at what will be the base of the cap with no draft angle. In that area, the part is effectively a Ø60mm ring with a 3mm wall thickness and I am confident that it will not present a problem.

 

[awake]

Doesn't have to be fancy to be well executed - and I have no doubt it took many hours!

 

[sniffipn]

As the title suggests, I need to manufacture a distributor cap for my freelance 7 cylinder radial engine.
When I drew the design for the distributor a few years ago, the plan was to mould it in phenolic resin, selected for its dielectric constant and resistance to water absorption. At that time, it was possible to purchase resin kits under the name Novolac.
I have now arrived at the stage when I am about to start making the mould and I can't find any source for phenolic resin.
I have not been able to find anything useful with web searches.
Has anyone else moulded distributor caps or other HT ignition components and if so, with what materials and what results?

Thanks in advance,
Pete.

is this the material?

https://www.adkwik.co.uk/prefere-re...MIpaSYs7PC5gIVg7HtCh0xjgE4EAQYBCABEgIZ-_D_BwE

 

[Mike Henry]

At the risk of sending this off-topic ... has anyone experimented with 3d printing a distributor cap? It would involve some supports, of course (unless, maybe, printed at 45°? Have to think about that). Not sure what the dielectric strength of PETG or PLA or ABS is ...

I 3D printed a small distributor cap for a friend's IC engine, a little less than 1" OD. It was printed on a Zortrax M200 FDM printer in black ABS which is supposed to have decent dielectric characteristics. The print failed, mostly in the details which were a little too fine for the FDM process to accommodate. I'll try again with a SLA or MSLA printer when I get one of those next year some time. The resolution on those is a lot better than with FDM. In many cases, they look like cast parts.

 

[Rickus]

Know nothing about dielectric properties, but what about using epoxy resin with "milled fiberglass" mixed in? I don't know if you have access to a mold or could make a general mold to pour one, but this might work. A bit of experimenting would be in order to get the ratio right, but I have used this while active duty in the Air Force and the parts came out strong and rather well. Of course that was good enough for government work!!! Oh, be sure to place the resin in a vacuum chamber to degas it. removing the mixed in air makes for much better machining.

 

[Ken I]

I did a tutorial some time back on moulding with polyurethanes - unfortunately the mental midgets at ImageShack and Photobucket etc have ruined that - and my faith in any image sharing site as I am sure they will convert you to a paying customer just as soon as you have enough data invested to extort money from you.

https://www.homemodelenginemachinist.com/threads/injection-vacuum-moulding-in-polyurethane.15298/

This site has learned the lesson and now allows images to be uploaded - but it is sad how much was lost.

Fortunately I also made an MS-Word file as backup - attached as a PDF - the site doesn't accept *.docx

One final tip not in the attachment - use a release agent specified by the resin manufacturer for the product and compatible with the mould material concerned. I learned this the hard way and as a result I have one aluminium mould which is permanently bonded together.

Regards - Ken

 

[petertha]

That's a top notch write-up, Ken. Thanks for sharing your expertise & nice project examples. My experience ends with simple vacuum degassing, careful pour and a prayer to the bubble gods. I can see in larger or more sophisticated parts that will only go so far & pressure enters the picture.

I always wondered how the intermediate vessels & lines were used without the resin locking those up & rendering them useless. I assumed there was a sacrificial bladder inside them or something. You put releasing agent in the metal canister & it cleans out OK? Do you do valve off line so you can get at it when the excess resin is semi flexible, or?..

Do you find the hard casting urethanes are sensitive to shelf life?