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.

Vacuum bagging updates

I recently mentioned that I was working on vacuum bagging as a method of molding parts. I’ve done some work with this and I think it is viable, but there are some significant challenges I have to overcome first.

The first challenge is very basic, and that is that my vacuum chamber isn’t able to get to a low enough vacuum. It only gets down to 26.5 in-Hg, which is not enough to remove all the air bubbles in the epoxy. I’m not sure where the problem in my system is. It might be that the vacuum pump itself simply cannot pull a low enough vacuum because it is a cheap harbor freight pump. Or it might be that there is a leak in the chamber. Both of these situations are very likely and it might be that there is a tiny leak and the pump can keep up with it all the way up to 26.5 in-Hg. I’m going to have to work on narrowing down the problem, but so far I’m having a hard time figuring out the problem. I tried replacing the oil in the pump, but that didn’t help much at all.

The next challenge is getting a ziplock bag that is actually airtight. So far I’ve tried a few different bags and brands and all of them, even the one designed specifically for vacuum bagging food, don’t stay sealed at high vacuum. When I seal them they seem to work very well, but then after about a day they have air in them. A few hours after sealing, they’ve lost all negative pressure, making them useless for compression molding. Maybe I just need to find a different brand or type that works perfectly, but it doesn’t seem like any ziplock bags are going to work for this. I was really excited about this method of vacuum bagging because the sealing method was working really well, but if these bags can’t hold a vacuum then they’re useless for this.