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Posted 26 December 2007 - 09:59 PM
Everyone agrees that using the household oven is not a good idea. Commercial plaster operators use drying ovens. There is no reason why the small production/hobbyist cannot do the same. So I have introduced this thread so that we can brainstorm the problem and arrive at a workable solution.
I see the solution as a box, deep enough to mount a household bulb with sufficient air space around it. At the bulb end, a 2 inch dia hole. This is the air inlet. At the other end, a small 12v cooling fan (battery operated), as used for cooling computers. These fans move about 4 – 5 cubic feet per minute.
The plaster moulds would be mounted on wire racks, probably two deep, to occupy the space, thus increasing the airflow around them.
The box would be made of thin ply or chip board. It should be sealed because of the moisture and lined with aluminum foil, especially around the bulb. Long enough for your production needs. The required air gap around each cast is ½ inch. This figure will ensure a reasonable velocity of airflow. If the gap increases, the airflow slows. Redundant space could be packed out with wood or dummy boxes or even old castings. A removable lid for access.
The required temperature is 120°F, this should be easily achievable. I would suggest testing with a 40 watt bulb and measure the temperature. Adjust the wattage until a suitable temperature is obtained. The air inlet is positioned close to the bulb, this will help to prevent it from overheating in a confined space.
You could really get carried away with this design, with humidity sensors and temperature control, but that would be unnecessary complication. The best way to assess if the plaster is done is by weight. Weigh a new cast and weigh when dry. The percentage loss in weight will act a rough guide for future casts.
This box is not going to dry your casts in 30 minutes, but it will be faster than air drying. I would guess over night. The materials are cheap, everyone has an old computer lying around with a cooling fan inside. For someone who regularly produces plaster moulds, this box could be worth building, I hope someone tries it out.
Posted 27 December 2007 - 11:23 AM
Good idea! Should be easy to build and produce good results.
Posted 03 January 2008 - 07:46 PM
I throw my Plaster of Paris molds in a 12 in. square cardboard box then set the top of my food dehydrator on it. Run in for 8 hours then pour.
Posted 04 January 2008 - 10:45 AM
Guys, How bout using a thermostat controlled heater that you would use in a incubator. I remember as a kid we had a box that we would put duck eggs in to keep them warm untill they hatched.
Posted 04 January 2008 - 11:07 AM
It would work, but would take a lot longer without the airflow. The required temperature for ideal drying is 120°F . This temperature is little more than a hot room. The introduction of moving air keep the humidity at the Plaster of Paris surface low and promote evaporation. All comercial ovens are based around an airflow.
Posted 11 January 2008 - 11:42 PM
This sounds a whole lot like the food dryer I built using plans from our local county extension agent. It used aluminum screens built by a local screen repair shop. They kept sets on hand just to fit the dryer.
Posted 16 January 2008 - 04:57 AM
Hey Vodka, out in the field on construction sites when we couldnt get a microwave on the job we constructed a hot box, which was a wooded box made of plywood with a hinged lid on top, and took some 1" insultion foam board and glued it to the inside and on the outside of the foam applied aluminum foil so that the box would be lined to refelect heat, and installed a 100watt light bulb into a screw shell lamp holder, just the cheap plastic style thats in your attic, and plugged it into the nearest socket. We would put our food to be warmed up in around 9.00am and by 11.30am its ready to eat,
Posted 16 January 2008 - 05:46 AM
Ross, good one. I've used a similar design when I used to do wine making. Heated by a 15w bulb and a thermostat to control the required tempreature.
I did not mention insulation, as the air will be in and out. But I think a lining of ALUMINIUM foil would not hurt any.
Posted 16 January 2008 - 02:46 PM
Couldn't you retrofit a large ice chest? It's already insulated, and the plastic ones are cheap.
Posted 10 February 2008 - 07:16 PM
I use my food dehydrator. I just put it in there overnight and it works great!
Posted 17 February 2008 - 11:08 PM
This thread reminds me of my brother inlaw. He heats his lunch behind his radiator on his truck. He made a wire rack on the back of the radiator. Wraps his lunch in tinfoil puts it in always has a hot lunch. Dry your mold on your way to work.
Posted 09 April 2008 - 01:16 PM
Vodkaman, I am very interested in making a small Plaster of Paris dryer. I do have a question regarding the design. If I am correct the idea is to basically remove moisture. So the dryer the air passing over the mold the more moisture that can be removed. I have access to medical grade air drying chemicals (like dessicants) used for drying nitrogen that is put into auto racing tires. I am thinking of folding up an aluminum sheet metal box with an incadesent light fixture. Then using a computer fan to pull air through this pellatized chemical, to remove humidity, into the box. Then vent air out. Any thoughts, or is there little advantage to drying the air?
Posted 09 April 2008 - 06:11 PM
This idea was borne from a discussion on oven drying and its problems. It is based on commercial drying oven design, taken from the internet.
The most important things are to keep the air moving to promote evaporation and don't let the temperature of the air rise above the safe max for the plaster (about 120F, I mentioned it earlier in the thread).
My only concern with the drying chemicals method that you're thinking about, is health and safety. If you can find a solution to these issues, go for it.
You could probably build it so that it uses both methods. Then you can report back with the comparison results.
Posted 09 April 2008 - 09:51 PM
I would hold to the basic ideas of 120 degrees and airflow. The idea is simply to pass the incoming air through a pellatized material that absorbes moisture (no health issues, just don't eat eat), in turn drying the incoming air. The drying material is similar to desicant packs that you find inside a pair of new shoes. The theory being that incoming air with high moisture content cannot remove moisture as effectively as dry air. The theory sounds good, I was just curious if anyone else thought it might actually be effective.
Posted 09 April 2008 - 10:35 PM
Sounds good, your theory is sound.
Is it possible to dry this substance in the oven, thus making it re-usable? We are talking a lot of moisture, I have noticed 30 - 40% weight reduction of the Plaster of Paris mold during the drying process.
Posted 09 April 2008 - 10:41 PM
That is an interesting question, I do not know if you can recharge this material, that is something I will look into. I am going to get something going on this "easy bake oven" and I will post what I come up with. Thanks for you thoughts.
Posted 10 April 2008 - 12:01 AM
Would a cheap toster oven work? That way you don't destroy the boss'es (wife) oven. Would a smoker like the one's in the netcraft catalog work? Could using a chemical to dry cause toxic fumes when you pour? airhog had a good idea simple and works.
Posted 11 April 2008 - 12:10 AM
Just my opinion, for what its worth and thats about nothing, but I spent 18 years in the Heating and AC buisiness. A temp controll is cheap and I would think absolutely necesary for what your trying to do. You can get by without the humidity controls but I would bet anything by the time you are done screwing around with different lightblbs and faan speeds and moving it to different locations based on outdoor weather temps ie. sun beating down on it vs shade(assuming this is located outside which it should be).Throw a 20.00 temp control on there and save yourself the frustration. In fact look in granger and you may even find the small fan you need that will be 110volt so you wont need to use 12v either, but thats really not that big of a deal.
Posted 11 April 2008 - 12:25 AM
After 18 years in a relevant industry, your opinion is worth a lot. I do agree with you, a temp control would be an advantage. I have designed circuits for temp controls for brewing equipment and for my pet tarantula!
I did not include such a control, because this usually scares people off the project as too complicated, also, I doubt that the temperature is that critical for what we do, considering most members (including myself) dry the Plaster of Paris in an oven. The temperature references were included because they were quoted in the industrial design article that I based the idea on.
The object was to come up with a quick, cheap and efficient way to dry Plaster of Paris without using the bosses oven. As I live in a rented apartment from the landlord from hell, I am quite happy to use his oven.