Wax Molding Technique and Temperature Control

I’ve been doing more testing with making molds out of wax that will be used to make parts out of carbon fiber, and it’s a bit more difficult than I originally thought. The wax that I’m using is carnauba wax, which is extremely brittle and hard. It also has a very low heat capacity, so it hardens almost immediately once it isn’t being fed more heat. This has been causing problems with molding, because it means that the wax must be poured into the mold and then reheated to completely liquid while in the mold. The test mold I’m using isn’t sealing properly, so I haven’t been able to get a good wax mold out of it yet. If the mold isn’t reheated after pouring in the wax, then the wax has patterns of cold and warm sections and cracks very easily. I think that if I heat the wax to its melting temperature while in the mold and it doesn’t leak out, then I will get a very high quality molding that can be used to make carbon fiber parts.

One of the biggest problems that I’ve been dealing with is that I’m using a low quality toaster oven to heat my wax and mold. I have a temperature sensor in the oven when I’m using it to monitor the temperature, and I’ve found that it usually holds the temperature within 100°F of where you set it. This is obviously terrible and means that I have to adjust it by hand to get it to the correct temperature.

The solution to this is to make a temperature control unit. There are many plans for how to make a water heater for sous vide cooking online that can hold the temperature within 0.1°C. I studied those plans and have bought the parts needed to make the same system, but for my toaster oven. The system will switch the power to the oven on and off using a PID control algorithm to keep the temperature stable. I opted to get high quality parts so I could also use this system to control a heated water bath for sous vide cooking.

These are the plans I am loosely following: http://seattlefoodgeek.com/2010/02/diy-sous-vide-heating-immersion-circulator-for-about-75/

This system will let me set the oven to 190°F (or whatever temperature I find works best), walk away, and know that it will be correctly controlled. I can also cool it down at exactly the pace I want, to prevent the wax from cracking. Luckily this system isn’t too terribly expensive, and has uses far and wide outside of my trumpet project. I think making a custom temperature control system is much better than simply buying a higher quality toaster oven, because that would be possibly more expensive and it would still have really terrible temperature control, since commercial systems all look to be drop dead simple. I opened up the controller side of my toaster oven and found ridiculously simple electronics. It’s no wonder it can’t hold a stable temperature! The temperature sensor is completely analog and is insulated from the heat of the oven by a metal wall. It is a terrible temperature control system that needs to be supplemented.

I did get part of my test mold to have a pretty good wax mold, so I’m going to make a test carbon fiber piece using it to confirm that this method will work, although melting out the wax will be bothersome until I get this temperature control system setup. I’ll post pictures of everything mentioned here once I get it working reliably.

4 thoughts on “Wax Molding Technique and Temperature Control

  1. Hi! I’m trying to make a kick scooter with CF and find your posts exceedingly helpful.
    If I tipped my hat every single time I learn something it would fatigue crack in no time!

    • Thanks! I haven’t been working on this project in quite some time. I’ve been on a team for the last year and a half where we build a race car from scratch. We do a massive amount of carbon fiber work on our car. Check it out! http://cppfsae.com
      If you have any questions about CF work, let me know.

  2. You must be an expert in carbon fibre moulding now after experience with FSAE. We used fibre glass panels in our car.
    I wanted to ask you something. I have been making something of my own with carbon fibre, actually a bike frame, and I was wondering if compression moulding (The one with inflating bladder) is a good enough method to get a good finish on carbon fibre. I want to use it over vacuum bagging but I also want a pristine finish that I don’t have to paint over. What are your thoughts?
    Hope you see this!

    • Somewhat of an expert I suppose 😛
      You will get a good surface finish if your mold has a good surface finish and you have enough pressure. I know that a lot of bike companies actually use the bladder method for their CF frames. In fact, you should be able to get a better surface finish for tubes with the bladder method, because the mold can be absolutely perfect and shiny. With vacuum bagging, there are always wrinkles and seams.
      The hard part about the bladder method is getting a bladder that works well. The companies that use that method usually make custom bladders for whatever shape they are making. That way the bladder stretches the same amount everywhere and applies the same amount of pressure on every area of the CF part. For a symmetrical tube, you will probably be fine with some sort of stretchy tube. But you will need to do lots of experimenting. Definitely one of the most difficult CF methods.

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