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PoP dryer final prototype
18 replies to this topic
Posted 24 April 2009 - 10:18 PM
The pics show the final prototype of the recycling Plaster of Paris dryer. The construction is self explanatory, but should anyone need more detailed information, I will make it available.
The air is drawn through two mains operated cooling fans. These are very quiet and low power consumption. The air is forced underneath the shelf and up past 3 x 100W bulbs. The bulbs serve as a source of heat. The heated air passes over the mold and back through the fans.
The mold(s) sit on wooden pins, to allow air to pass all around, with at least a 1 air gap. Excess space around the mold is filled with the wooden blocks. The box will dry molds up to 10 x 14, above the shelf and below the shelf, so several molds cold be dried at the same time if required.
As the air recycles, it retains its heat and over a few hours will rise to 35°C (95°F) or more, above room temperature, in my case, giving an internal temperature of 140°F.
The air will become saturated with water and could cause condensation problems. So it was necessary to introduce venting to keep the humidity at a reasonable level. The venting is a compromise, too much and the temperature drops, reducing evaporation, too little and the humidity rises, reducing evaporation.
The venting is through four 19mm dia holes, 2 above the fans and two below. The two holes on the exit side of the fan serve as outlets, due to high pressure. The two on the inlet side serve as air inlets, due to low pressure.
In anticipation of the humid conditions, I sealed the inside of the box. This was not necessary, as the venting strategy and the airflow prevented any condensation occurring. In fact, the fiberglass resin that I used for sealing, gave off a smell with the heat from the bulbs, but after using the dryer a few times, this reduced. Next time, bare wood.
I have run the dryer continuously for 12 hours and no scorching of the untreated lid was observed. However, I intend to cover the entire internal surface of the box with aluminum foil, this should protect the wood and actually increase the temperature some.
The drying time for the small test piece, 4.5 x 3 x 0.5 was 1 hour 50 minutes, to remove 30% of the original weight. A totally dry mold will have a weight reduction of 35%. Once 30% is attained, the evaporation rate reduces and can be considered dry for molding.
The small test piece was to accommodate the weighing limits of my gram scales. The equivalent estimated time for drying a 7 x 5 x 1.25 mold would be four hours. A 10 x 12 x 1.25 mold would take 4.5 hours.
Note the evaporation is constant for the first 30% of water loss and is relative to the surface area of the mold and NOT the volume or weight. Double the weight of the mold does not mean double the time, at least in the recycling air oven.
Obviously, I need to buy some kitchen scales and confirm the estimated drying times for other mold sizes.
I believe these times make the recycling air dryer convenient enough for those in a rush, with no chance of over drying or over heating the mold.
Posted 28 April 2009 - 12:14 AM
Dave, Why the two fans? Could you use one larger or a higher speed? Or is there a reason you chose 2. I am asking because I have everything sitting here in my house to make one of these if it can be done with one fan. I can move 100 CFM with this one fan or use a rheostat to turn it down if needed. Thats a whole lot of air moving through a little box.
Posted 28 April 2009 - 03:57 AM
This type of fan was cheap, readily available and easy to work with (square). It does not have to be two fans. I tried one, but the drying time was not good enough, just a bit too long, plus I had the space. But your 100CFM fan should suck the pips out of a melon and do just fine.
The heater does not have to be bulbs. Again, it was what was readily available. The vent holes are 19mm dia, because that is the size bit I had in my cave. Try a hole, if you get condensation, make it bigger. If may not even need a hole, so try without first.
Use kitchen scales to weigh the mold before drying. You are looking for a 30% weight loss. Example, if you demold weighs 700gm, the dry weight will be 490gm, eventually going as low as 455gm (35%).
Don't bother sealing the wood. As I stated, not necessary.
A thermometer would be nice, just to see what is happening.
Posted 28 April 2009 - 01:50 PM
So you can pour into a Plaster of Paris in twelve hours?
Posted 28 April 2009 - 02:50 PM
Well, the Plaster of Paris mold still has to be sealed. If using the thinned epoxy method of sealing, I would say yes.
The dryer may help with the curing of Elmers sealing, but that has not been tested yet. The temperature may have to be reduced, but this is simply a matter of removing a bulb. I will start a test piece sealed with Elmers and see if it blisters.
Posted 30 April 2009 - 06:24 AM
Professor, what? No coconuts and palm fronds?!? Ginger and Maryann aren't too happy with this design.
Posted 01 May 2009 - 01:05 AM
I did the elmers drying test in the Plaster of Paris dryer. It seemed to work just fine.
I used 50:50 mix, soaked the first coat for a minute or so, drained, then into the dryer. I monitored the weight as the coat dried and gave it 3 hours, until the slight tacky feel all but disappeared. The tacky feel could have been exagerated due to the heat.
I then gave three more thin coats, giving each an hour in the dryer.
BUT, after drying, I noticed that I could mark the surface with my thumb nail. so I gave it a few more hours, no change. I then left the mold in the room for a couple of days, still no change.
Either force drying Elmers is a bad thing OR the thumb test is normal OR my batch of elmers has deteriorated, it is a couple of years old and was a bit thick and may have needed extra thinning.
So, the result was inconclusive. Any comments?
Posted 01 May 2009 - 01:53 AM
Just a guess but I am leaning towards force drying elmers is a bad thing. I was kind of thinking this is how it might turn out when you said you were going to try it. If it was me I would attempt with a new batch of elmers just to find out but I think the same will happen. I am trying to figure out why I was thinking it wouldnt work when you posted it and for whatever reason, (probably because its almost 1:00am here) I cant remember what I was thinking. But who am I to question the genious of the Great and Noble Vodkaman. I think the Plaster of Paris dryer is a great accomplishment. I can wait for glue to dry. LOL
Posted 01 May 2009 - 04:54 AM
Nothing noble, just searching.
I will pick up some fresh elmers on my next shopping safari. But just 'cos you pay for a new bottle, in Indonesia it does not guarantee fresh. I always have to open it and check the consistency.
Posted 02 May 2009 - 07:50 PM
I haven't made a mould since I was in cub scouts, back when there was no "under God" in the pledge of allegiance, so I'm just thinking out loud here.
When I read this thread, I dropped down and reread the other thread about the dryer, and the Plaster of Paris mix.
Years ago, in another life, we used milk when we mixed our fixall to smooth floor joints, because it made fixall stronger and helped it stick to the sub-floor surface.
I wonder if mixing Plaster of Paris with milk might result in a pre sealed, stronger mix?
Posted 03 May 2009 - 05:05 AM
Mark, I tried a google search for the milk idea, but came up with nothing. I did read, that milk was used to make paint.
I did another test, air drying the Plaster of Paris sealer. The results were the same, easily marked with my thumb nail. So my conclusion was that speed drying made no appreciable difference.
Then thinking about your post, I did an elmers/water mix. I replaced 20% of the water with elmers. The new test piece took longer to dry, just over 3 hours to get to 30%. I continued drying for another 2 hours and got to 32.5%. I then tested it with my nail, the surface was much harder and very difficult to mark.
From this I can only conclude that the problem was the plaster and not the elmers. With the elmers in the mix, the surface strength is far superior.
Posted 03 May 2009 - 07:50 AM
Glad the glue worked. Seems like a logical thing.
Just watch the amount you add, because glue can cause shrinkage when it sets.
I ran out of tapping mud on a drywall job once, so I mixed carpenter's glue into the topping mud I had on hand, and it worked. But it was murder to sand, because it was hard as nail. Tapping mud has some kind of glue in it, to give it strength and adhesion to the drywall tape and board, but it also shrinks more than the topping mud, which is softer and easier to sand, and which shrinks less.
Before they started coloring them differently in the factory, we used to add latex paint, a small amount, to our topping mud, so we could tell what had been coated and what hadn't. We used to argue about what color to use (my personal favorite was blue), until we hit on yellow, because it colored the mud well in small amounts, and didn't bleed through when you primed the walls for painting. If you added too much, the mud shrank.
I even mixed fixall with carpenter's glue, when I didn't have any milk, but it didn't work as well as milk.
Posted 04 May 2009 - 10:26 AM
Very nice Inspector Gadget. Looks great.
If you make it out of Spanish Cedar you dont need to seal it and it can second as a humidor. I have several filled with sticks but might have to convert one now:wink:.
Posted 04 May 2009 - 03:21 PM
A mini kiln is a good idea, I have used it to dry off some unsealed prototypes. Actually, I am finding new uses for the Plaster of Paris dryer every day.
One of my favourites is to switch off the bulbs and put my dinner plate inside. Cools the food down in a minute.
Speed drying socks, when you run out. I am not admitting to trying that one, yet.
Posted 04 May 2009 - 04:06 PM
That's why hair dryers were invented.
Posted 29 August 2011 - 03:38 PM
Very informative post Vodakman! I was wondering if the percentages can be applied to water putty as well?
Posted 30 August 2011 - 12:51 AM
Easy enough to find out:
Weigh the mold once cured and write down.
weigh the mold at regular intervals.
When the weight no longer reduces, the mold is dry. Divide the final weight by the start weight and x 100.
The numbers are useful, because any mold that you make in the future, you can check the drying progress and even predict how much longer is required. As long as you record the start weight. Surprisingly, molds dry at a constant rate.