Casting new ball joint and tie-rod end boots for 411/412

[raygreenwood]

These are the 2nd half mold enclosures getting ready for mold release and then top half mold resin.
I would say this is about stage 5 of about 12-15 stages to make the molds over about 4 days.

When the molds are done, I have to install inlet ports and vents via drilling with micro-drills.

Then I will cast a set and see how they look and fit. They will be high temp RTV about 30% thicker than stock.

Ray

[dcat917]

Great work, I did a horn button for my Studebaker using this approach.
_________________
1981 Puma GTB http://www.thesamba.com/vw/forum/viewtopic.php?p=7794566#7794566
1971 Karmann Ghia http://www.thesamba.com/vw/forum/viewtopic.php?t=591290&highlight=
Past Projects:
1951 Studebaker Business Coupe https://goo.gl/photos/eHPuNo89m9PXZwBM8
1953 Studebaker 2-Door Sedan https://goo.gl/photos/pe38HEShzUrVSwcV8

[raygreenwood]

Thank you!
Some updates and general information. Both success and failure.

First the success: One of the ball joint and both tie-rod end molds were successfully de-molded from the original example part.

The failure: One of the ball joint boots (this was expected) failed to de-mold and was destroyed.

The difficulty (to be explained below):

I have done quite a lot of casting and molding over the years including some injection molding for a living. Many people on these forums use the overly simplistic term "re-pop" to describe remolding a part from an original....as if it were actually that simple (oddly...a small percentage of the time it actually is!)

There is an endless variety of casting resins, liquids, powders, metals, rubbers, elastomers etc.
Each one has attributes, works with a range of materials for originals (sometimes a narrow range), .....each mold making material has a specific operating temperature range, moisture absorption, shrinkage or expansion rate....exothermic reaction rate (it gets hot or cold)...curing time, demolding time....enclosure material requirement....release agent requirement...working viscosity...mixing requirement...mixing ratio.....etc. etc. etc.

Sometimes on a rigid part that is fairly two-dimensional...its really easy. Pick a resin that fits the detail...if any....coat the part with release agent (if needed)....mix some resin....and pour a puddle around the part. Then separate after curing...clean and relube the mold and pour in what you want to cast with.

Most are not that simple.

In the last picture you can see one risk of this part. I am making an outer shell and an inner form or"buck". The bulbous lower part of the ball joint boot means that unless the rubber of the original ball joint boot is flexible enough...I may not be able to get the inner form out.

Also the upper collar or cuff will be fully enclosed on three sides by the outer mold...so the release agent must work and the boot must be flexible enough to pull out.

What type of mold:
Also a big part of the planning is what type of mold you are going to make....and what type of resin/rubber/metal are you going to cast. The viscosity and required pressure of what casting material you are going to make the finished part from makes some of your decisions for you.
From those decisions....whether the mold will need to have a vacuum pulled on it or be heated...makes more of the decision for you.

After that ...on a part like this for instance.....it will dictate whether you use a two piece mold, three piece mold or four piece mold.

Two piece...and inner and outer is the best. Smoothest nearly factory result with no flashing seams on either the inner or outer surface. But all of the details have to be spot on...casting liquid viscosity (this mold will require both pressure and vacuum to operate), core sizing, mold casting temps, the original parts etc. Support structure for the original parts while casting either first outer or first inner. Easiest to make with the most to worry about and fail.

Three piece:....Two pieces outer and one inner (or vice-versa). Very hard to make and cast the outer mold section, requires modeling bulkheads for casting and using a very fast curing...low shrinkage outer casting resin (or metal)...with a high reaction temperature. Lots of work to make...but very low risk when actually casting the final part. Will have to have the final parts deflashed and deburred.

A four piece mold has a two part inner and outer. Lots and lots of work...but very successful molding of finished part with complicated molding resins.


Inner form cast:
I knew the above listed risks going into this....so I cast the inner form from a mid grade epoxy. Nice detail but slow cure rate...meaning high shrinkage rate.
It is in this calculated way that I kept exact proportions and got the inner form to be smaller to make the boot thickness 25-30% thicker than stock.

In reality the boots...both old and new were juuust not quite flexible enough so the inner forms had to be destroyed to remove them. More work but no problem.

Outer form cast:
This is only the third time I have ever used this particular resin on a mold. I will not use it for a rubber part again.
It has precise detail level....I'm still trying to get a handle on its exact chemistry (it has properties of both polyurethane and epoxy...its a Polyol....so I suspect its closer to a Urethane).
Generally with high detail two part catalyzed resins....the more detail and less shrinkage they have...the faster they must cure....so they outgas less and shrink less. They have to cure fast to freeze their detail and volume.

In doing this they get VERY hot.

That is what damaged the older ball joint boot. This is the problem with casting from old parts.

You cant just take an old rubber part...and simply Re-pop/remold it because:

1. Over time that part has experienced shrinkage and distortion. So many times a primer must be used over the outside to carefully build up the dimensions by a thousandth or more....and sometimes only in certain critical areas. Better be good with masking and fine paint brushes.

2. No matter how clean you get it...its always dirtier than you need it to be...and.....just like old tires that get cracks in the side wall....the rubber part gets micro cracks.

That is what happened to the destroyed ball joint boot in the middle picture.
The very high temp of the outer molding resin.....about 170F expanded the micro cracks all over the surface...and inserted resin into them. As the resin cured to a solid....in less than 3 minutes ....and cooled....the surface of thee boot close over the resin trapping the boot. It defeated the release agent.

The black you see in the mold is literally cracked rubber lifted from the outside of the boot. The boot came out rough as pavement with the top collar ripped off.

I now have to re set outer forms on the surviving boots, and recast inner cores and then center those cores on the outer molds and attache them to the molding board. Then drill vents and inlet holes.

To not have core removal trouble again....I will probably cast the cores from indium alloy with a melt temp of around 135F or maybe 150F.
In this way after I vacuum pump the silicone into the molds and it cures....i simply heat it with a heat gun to about 150F and the inner cores will melt right out.
Sounds simple but that means a permanent mold to make new cores each time from recasting the indium alloy.

And...because the indium Alloys are so precise....in order to get the thicker wall section in the right areas in the final part...i will have to spray the indium with machinist dye...and sand it to size....which is also tedious and dangerous as Indium casting alloys have a high proportion of lead in them (mask, gloves and vacuum).

I have used nearly pure indium alloy for casting before......but the cost is prohibitve. The version I am using is already running a little over $100 a pound.

Pure indium metal...for reference....is running about $1000 a Kilo

Just thought you might want to know what goes into low volume, tooling free casting/molding of parts like this. Ray

[Lars S]

Impressive work Ray! We learn a lot from your excercises, hope you will succed!


/Lars S
_________________
Porsche 914 -72, Bahia Red daily driver
VW411 2-d -70, White, sold
VW412 4-d, -73, Gold Metallic, daily driver
Suzuki T500, -69, Candy Gold, sold
Suzuki K50, -77, Black, daily driver
BMW R69S -69, White, sold
Husqvarna 118cc, -47, Black, Sold

[raygreenwood]

[Lahti411]

What if i made a 3d-model for you Ray, and you could print the mold in, for exmaple, Shapeways? It would be really easy to make a two piece outer mold with this method and include all the alignment studs etc. It'll take something like 15 min for me to model that simple piece and you would have an accurate mold in couple of days.
_________________
My cars: 1972 VW 411 Variant; 1973 VW 412 4d sedan

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Thank you. Someday this little bit of knowledge, just knowing it is possible will come in handy.

[raygreenwood]