Silicone mold casting of replacement parts with plastic resins

I've recently broken a plastic joystick on my Menzi Muck excavator. The parts were quoted at $173+ for each half of a plastic handle, not including any electronics or shipping from Switzerland. I also need some impossible to find switch covers that are "melting" from age or chemical reactions - quoted $85 - not sure if that's for one or all six.


After recovering from a near cardiac moment from that quote, I started looking at 3D printers with carbon fiber media as a possible solution. Not really a cost effective approach for the few parts that I need to prototype or build and lots of computer time with CAD drawings. I would have to dredge up what I can remember from CAD and do a lot of relearning. Not the best option for me at close to $5k initial investment.

I then looked into custom casting with silicone molds and plastic resins. I could get into that for about the price of the joystick halves. For about $500, I could get enough tooling and supplies to get well underway and learn/perfect another skill. I enjoyed sand casting metals back in high school so I already had some basic knowledge of the process. Youtube is full of silicone mold videos so I watched enough to learn before I decided to make the plunge.

I already had a vacuum pump and compressor so for under $90, I purchased a pressure paint pot with spray gun that I needed anyways. I now have a pressure painter for big jobs, a pressure pot for resin casting and with the addition of a few fittings, a vacuum chamber for degassing silicones and resins. Xacto knife set at HF under $20, hot glue gun under $20, gallon silicone kit + 2 different hard plastics + 1 hard rubber plastic + modeling clay + mold release made up the balance of the budget and got me under way.

Starting small: These little button covers were getting soft whether from age or chemicals, I don't know, but they were melting/ripping and starting to fail. They were so bad that I had to make new patterns on the lathe. I could not find any plastic round stock so I cut the plastic patterns from the handle of an old screwdriver, made the center core from an old pencil, adjusted the depth of the center core to proper depth and then epoxied into place for the final patterns. I used an old cat food can as the mold container and poured up the mold in 2 halves with 2 buttons in the mold. I used the firm rubber texture resin to cast the buttons, cut the flashing off after curing and I have perfect replacement parts. I've now made 4 sets of castings from that mold and it works great. I have all new buttons when I get to put things back together.



Getting bigger: The joystick halves are much more complex. I had to glue the broken parts back together to make a good mold pattern. I also added modeling clay to the areas that I wanted to build up stronger with more material. I filled in the major gap with some aluminum ducting tape to provide a partition line. I cut out a tight fitting cardboard pattern to set my molding line and glued that with the hot glue. Built a mold box and then poured the first half of the mold. I did not degas this first silicone pour with the vacuum chamber and there is a noticeable difference from the later pour that was degassed for 5 minutes prior to pouring - the pressure pot is worth the difference in quality.

I still have to pour up the first joystick half and finish the mold of the second half. I will continue to edit and add to this post about results.

Making buttons out of firm rubber resin. About 3 cc fills the cavity to full.

You can see the extra modeling clay that I added to strengthen the next casting in the left side of this pic and above the middle trigger switch. Here, I was test fitting electronics with the addition of the modeling clay so that I did not build something that would not fit properly. Original button in dirty red and 2 new mold patterns that I built.


Pressure pot / vacuum chamber


2 halves of joystick poured and ready to split.

Joystick mold after separation - note extra modeling clay already removed from pattern when pic was taken.

Yup. I’m impressed.

I was able to cast my first handle half yesterday AM. I'm always learning more as I go along. I did not remember to put in a good sprue on the mold and with the amount of flash leakage, I had a slight void near the base of the handle. That is small enough that I could fill it with more plastic or even JB Weld. Otherwise, I am very happy with the results. The added clay to the original built up the weak spot on that handle. The plastic feels extremely strong with just a little bit of give on a hard squeeze. I'm well satisfied even if there is a small void in it.

Prior to casting the handle, yesterday, I started on the left half of the joystick mold. I found some acorn nuts and axle covers to properly build my mold keys with - I forgot those on the first half and had to cut them in. The back side of the pattern is glued to the cardboard cutout with hot melt glue. I built the form inside of the other box for ease of holding the side walls while waiting on the glue to set. Seemed to work faster that way. You can see all of the added modeling clay inside the original handle to provide a stronger casting than the original handle. Some of the clay is also filling in missing pieces from the original handle where part of it had cracked off.

I have also learned to keep the mold space a bit smaller so I don't waste too much silicone. The hot glue gun is really good for this cardboard mold building but waiting on the glue to set sucks. The cardboard works well to follow the contours with ease yet provide a stiff wall structure. Another thing I learned is that rubbing alcohol will release the grip of hot melt glue sticks. I was skeptical that it would work but it works like a charm after 2 minutes or so. I can peel up over 4" at a time with this method when I need to separate the cardboard from the pattern.

I did add the sprue to the other mold half that I finished up and poured this AM. You can see the clay ball at the end of the handle which will be the pouring sprue. The acorn nuts leave a nice, clean key system for a positive lock down of the mold halves. A good saturation of mold release spray and then I cast the second half of this silicone mold this AM. It will be ready to use with plastic resin tomorrow AM.

Very cool i always wanted to cast stuff and last year i cast a aluminum slug to turn down. I want to do some more.

So I finished up the mold for the left half and got it poured. I demolded a little early and the plastic was not set rigid, yet. I also had the mold set it a little too tight between the boards and that caused some deformation. Repoured with rubber bands set to hold the mold together, lightly. Resulted in more flash but a better pour. Mixed the resin for about 3 minutes, vacuumed down for 4 minutes then poured, vibrated and squeezed the mold, poured, more vibration , poured to top off and set into pressure chamber by the 10 minute pour time mark at 60 psi for at least 20 hours cure time.

This resulted in a much more rigid casting and the thin areas of the previous casting were no longer of a concern. Lessons well learned. It's well worth a couple of minutes to vacuum that resin prior to the pour to remove as many bubbles as possible prior to setting it under pressure. Keep light pressure on the mold to hold it together without distorting it. Extra flash is easier to remove than working another pour.

All of the studs that hold the switches are perfect. The switches drop in with ease but likely have less than 0.010"-0.015" clearance. All of the button holes are perfect and I like my buttons better than the originals - more stick out and more required pressure to work the switches so less unintentional activation of functions.

Test fitting of the finished halves fit perfectly without any force required to install and absolutely no slop. I'm impressed. I can see a lot of potential for custom parts if/when needed.

Unfortunate that the prices are so high that that becomes necessary. Although at my own rate per hour I would have probably just bit the bullet and bought the thing.

One other issue that I had with the controller was a missing black button. I've searched the web for these and had no luck finding any. Why not cast up one and several extras?

I popped off one of the buttons and, in order to make molding it in clay easier, I drilled a small hole into the top of it then placed the drill backwards into the hole to act as a handle. I sprayed mold release onto the button to keep it from sticking into the clay.



I worked up a small ball of modeling clay with my hands to warm it up and them made a flat piece to work with. I gently pushed the "pattern" button into the clay to full depth of pour and then carefully removed it being sure not to deform the mold hole. I filled up most of my pattern clay with holes, let the clay cool to stiffen and then sprayed on mold release into the clay hole patterns.

The entire piece fit into a small Gatorade scoop. I mixed resin in another smaller plastic scoop, vacuumed that down, poured into the mold, vibrated, topped off the mold and set it into the pressure tank until tomorrow AM. I only left a thin layer of overpour so my flash should be minimal. I should have some white replacement buttons by morning and this repair job will be complete for the left side controller. I'll have to do some more pours to make up the right side halves but I now have much better confidence in my methods to produce better results. All the buttons are made up, I just need to pour and finish the halves to get this project completed 100%.


Another interesting note is that the vacuum tank brings out the heat in the resin. I think that degassing actually speeds up the chemical reaction. You can see the difference in temp from my welding table to the resin just out of the vacuum chamber via the FLIR camera on my phone.

Birken Vogt said:

Unfortunate that the prices are so high that that becomes necessary. Although at my own rate per hour I would have probably just bit the bullet and bought the thing.

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I would have saved time and therefor money just paying for the parts outright. No doubt in my mind about that and no argument otherwise. However, I am now equipped to build more custom items and be more self sufficient saving critical downtime in the future. I've also added more knowledge to my skills bank.

It cost me about the same to get fully set up with lots of extra resin as it would have to purchase the parts outright. I'm satisfied with the path that I chose because time was not critical for repairs, this time.

Thank you for the photos and your working through the problems. That is very impressive.

John C. said:

Thank you for the photos and your working through the problems. That is very impressive.

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I hope this solution can be of help to others. If nothing else, for me, it is a great repository for notes on how I've solved problems in the past.

I finished up the tiny buttons this AM. After removing the mold from the pressure pot, I removed the casting from the clay and then cut off the flashing. I did lose one button to breakage of the tiny pin that hold the button in place. The resin still seems a little soft on the pins as of now. Another 12 hours and it should be completely cured without question.

I did not bother to sand these down for a perfect profile. They fit perfectly and work as they should. If I have too much breakage on my cast replacement units, I think that I would try the rubber feel urethane resin for the next batch. All in all, this has been a success and I've established some new shop skills for future needs.

Quite impressive results

Nice work! You should go into the aftermarket parts business..