Help with huge wax cast for physics project

[Tim E]

Hi there! First off, I must apologise - I am new to this forum, so I apologise if I infringe any ettiquette!

I'm looking for some help with a final year physics project at university, and I think the assembled group of candlemaking experts on here could just provide the answer....

So, our project involves firing microwaves through big wax prisms, but first off, we have to make these prisms. They are semi cyclindrical in shape (think a big rod chopped in half lengthways), with the flat face approximately 12 inches x 12 inches. We're trying to make them at the moment by using a large bucket, placing a plastic sheet down the middle, and sealing it in with bathrom silicone sealant. We've checked the seal by filling half of the bucket up with water, but whenever we pour the wax, the seal breaks, despite it being very thick and, as I say, working with water.

Secondly, when the wax cools, obviously a skin forms and, as the wax contracts, we get a large dip in the top. is there any way of preventing this happening? How do you normally do this when you're making candles? Our latest attempt seems to have a huge hole in the middle of it. We've tried pouring the wax in layers, but these seem to break and we also have found large air bubbles between the layers (despite not having any bubbles when we poured it!).

Lastly, does anyone have any suggestions for making mould release easier? Preferably without adding anything to the wax, if we can help it, we're trying to keep the wax as pure as possible for the experiment. Vurently we've tried a PTFE spray, but it's really hard to get it to release, nonetheless.

All help hugely appreciated, this makes up a significant part of my (and my lab-partner's) degree, and we can't get on and do the actual physics until we've made the prisms! I do have some experience with candle making, ust not one that involves 8 litres of wax!

Tim

[SatinDucky]

I'm NO scientist, just tossing out ideas and guesses here

So, our project involves firing microwaves through big wax prisms, but first off, we have to make these prisms. They are semi cyclindrical in shape (think a big rod chopped in half lengthways), with the flat face approximately 12 inches x 12 inches. We're trying to make them at the moment by using a large bucket, placing a plastic sheet down the middle, and sealing it in with bathrom silicone sealant. We've checked the seal by filling half of the bucket up with water, but whenever we pour the wax, the seal breaks, despite it being very thick and, as I say, working with water.

First thing that comes to mind is making a cylinder of wax then slicing it down the middle with a hot knife or cutting a sheet of metal to fit in the bucket as a divider and fill it completely, separating the halves after it's unmolded. Or, if you can find something to fit and seal tightly as a lid, cut a hole in one side of the lid, lay the mold on it's side to cool after you get the wax in.

Secondly, when the wax cools, obviously a skin forms and, as the wax contracts, we get a large dip in the top. is there any way of preventing this happening? How do you normally do this when you're making candles? Our latest attempt seems to have a huge hole in the middle of it. We've tried pouring the wax in layers, but these seem to break and we also have found large air bubbles between the layers (despite not having any bubbles when we poured it!).

Yup, that's what happens. You can minimize the shrinking some by slowing the cooling process, but not by much. Most likely you'll have to do a repour to fill it in, like we do with candles.

Any wax that shrinks alot is very prone to air pockets. If the top hardens before the wax is done shrinking, it will still shrink inside where it's more pliable, leaving the holes. In candles, we poke "relief" holes in the skin on top and keep it open while it cools, then fill it in with the repour.

Now, I don't know how the filled holes would affect prism qualities though....

I don't know what's practical for you (and I'm NO scientist), but here's my thinking... For prism qualities, a solid pour would be best. The best scenario I see would be to use a tight lid and cool it on it's side, BUT making it 3/4 full. If you insulate it and cool it slow, maybe the dip in the middle would be above the 1/2 way mark. When you take it out, trim of the highest sides. Flip it over and melt it (on low heat) on an electric griddle or something similar to flatten it down to the 1/2 cylinder size. Doing it this way would make air pockets less likely as well since it would be more shallow.

As for mold release... while don't use it often, when I do, I spray the mold with Pam type cooking spray. If the mold surface is very flat, clean and smooth, unmolding shouldn't be too much problem.

This sounds like an odd experiment, but sure sounds interesting...LOL.

Maybe Top will pop in with some helpful suggestions!

Good Luck!

[Vicky_CO]

So, our project involves firing microwaves through big wax prisms, but first off, we have to make these prisms. They are semi cyclindrical in shape (think a big rod chopped in half lengthways), with the flat face approximately 12 inches x 12 inches. We're trying to make them at the moment by using a large bucket, placing a plastic sheet down the middle, and sealing it in with bathrom silicone sealant. We've checked the seal by filling half of the bucket up with water, but whenever we pour the wax, the seal breaks, despite it being very thick and, as I say, working with water.

I will try to help here. Bathroom sealant will not work because of the temperatures involved. I would get sealant that will work for heating and cooling ducts. Go to the auto parts store and get one there. You need one that can take high temps over a longer period of time.

Now I have a question What temerture are you pouring your wax at? I have this strange feeling you are pouring really hot.

Do you know what kind of wax you are using? Where did you get it? What is the melt point?

[van_yulay]

Wow, this sounds like fun. I would love to be in this project!. Ok, well since I make molds, I can help with some ideas. I would first of all try not to make a divide, instead make a whole piece and use a hot knive to cut it in half. Or you can use a plastic bucket cut in half and glue gun hard plastic to the edge. A teflon pans release real easy.(good will) I know when I was little I used cardboard once and wax paper to make a mold, LOL..The ideas are endless with me. You can use candle realse that you can buy at micheals, or if you have some pam spray will work. The wax you use could be cheap wax also, that is why you are getting a lot of skrinkage and pot holes. ( or you are pouring to cold) Pour a little hotter and let it cool slower( not in a cold room). Repour when it is cooling to top it off. Hope this helps ! Monica

[BruceCarvesWax]

Pour a full bucket of wax, repouring as needed to keep it level on top and cut it in half with a band saw.

Bruce

[Tim E]

To Satin Ducky:

Many thanks for your help, I'm not wonderfully adept at using quotes, so I'll try to explain without. Firstly, we did consider filling the whole bucket, and then cutting it in half, interestingly enough we do have a hot-wire cutter which we have used on this wax before, but it didn't work, as the wax seals itself up behind the wire, so you get the wire right through it, but it doesn't cut it! Also, we've found it hard enough getting the prism out of the bucket with only half of it full - at least having an empty half means we have space to get our hands in and try and get it out! I'm worred if we fill the whole thing, we'll never get it out, particulalry as there would be no room to deform the bucket. Also, the bucket will take about 16-17 litres of wax (sorry for metric units, I'm from the UK!), and we simply dont have facilities to melt that much wax in one go! So, I think that's out.

Secondly - relief holes; when you poke these holes in the skin, do you then pour more liquid wax in through these holes into the liquid wax below, or do you just poke the holes and leave it? Or do you let it set fully with the dip in it, and then just pour some more on top to fill it in? You're absolutely right - we think a solid pour is the way to go, we're not really sure what will happen if it's in lots of layers!

I really do like the idea of pouring it with the lid on and the bucket on its side (assuming we can find a lid for it!). I'm slightly worried we'll get even bigger problems with a dip in it, as there will less open area on the surface, but when it contracts the volume change will be the same, so the dip will be deeper... I'm just findng it difficult to work out what will hapen when we try that! Nonetheless, could well be worth a try. We also have a large hotplate we can use to make the surface flat, so that shouldn't be a problem.

Many thanks for the advice on the cooking oil though, that sounds like exactly what we need.

Many thanks!

Tim

[Tim E]

Vicky,

Many thanks, I think you're right about the bathroom sealant and the temperatures - it would probably be worth finding heat-resistant stuff.

As for the temperature, we're using a temperature controlled hotplate in a physics lab, so we know we're melting at around 75 degrees Centigrade.

As for the wax, we bought it from a company online, it's on this webpage,

http://www.4candles.co.uk/wax/paraffin_wax.html

candle shack 280P. Due to the nature of the experiment, we need to get reasonably pure paraffin wax, so we were careful about our source.

Thanks,

Tim

[snow]

I have no advice for you because I am still pretty new to candles, but this sounds so interesting! Please let us know how it all turns out and what method worked. Some of these people on here have such great advice and seem so knowledgable about wax and how it all works. GOOD LUCK (keep us posted)

[SatinDucky]

Pour a full bucket of wax, repouring as needed to keep it level on top and cut it in half with a band saw.

Bruce

What a great idea! I never would've thought of a band saw

Tim - In candles we strives for a few repours as possible, hence the relief holes and waiting until completely cool (done shrinking) for the repour.

For your purpose of limiting any layering, Bruces suggestion of adding more hot wax AS it shrinks and keeping it full/level is a better idea. The hot wax will keep the top soft and limit the air pockets as well. Also the hot wax will bind better with the warm wax and there shouldn't be any layering effect.

[Tim E]

Wow, this sounds like fun. I would love to be in this project!. Ok, well since I make molds, I can help with some ideas. I would first of all try not to make a divide, instead make a whole piece and use a hot knive to cut it in half. Or you can use a plastic bucket cut in half and glue gun hard plastic to the edge. A teflon pans release real easy.(good will) I know when I was little I used cardboard once and wax paper to make a mold, LOL..The ideas are endless with me. You can use candle realse that you can buy at micheals, or if you have some pam spray will work. The wax you use could be cheap wax also, that is why you are getting a lot of skrinkage and pot holes. ( or you are pouring to cold) Pour a little hotter and let it cool slower( not in a cold room). Repour when it is cooling to top it off. Hope this helps ! Monica

Monica,

Yes, it's not bad as far as projects go, unfortunatley the actual physics is less fun than the wax pouring! And the fact that the project is really important to our degrees, so all the problems and delays are really frustrating! Which is why all help is much appreciated!

Well, I mentioned about the hot-knife idea and why we'ver found that a problem. Similarly about the difficulty with getting a full bucket load of wax out, as well as trying to melt 16 litres of wax! We considered cutting the bucket in half, but didnt want to use a glue gun, as the glue melts when it gets hot! A teflon pan would be ideal, however, the size of the thing is really important for the experiment, and we've not been able to find one the right size. What do you think of the wax we're using? I'm no expert, but it seems pretty good quality, and, like I say, we dont want too many additives in it....

Tim

[Tim E]

Unfortunately Bruce, The absolute minimum size of exposed bandsaw blade that we need is over a foot, and we just can't find one to do it with! Also, as I say, we can't melt 16 litres of wax in one go, and I'm worred about getting it out of the bucket. Also, we need to get the flat face really flat, and I'm not sure my band-saw skills are up to it!

However, I do like the sound of topping it up whilst it is hot, I'm glad that's viable, I think we'll deifnitely try that for the next pouring.

Thanks,

Tim

[NattyCat]

It's a shame you're not near Kent - you could have used my wax melter which holds 50lbs of wax and melts in like an hour.

[Donita]

What is the bucket made out of? Plastic or metal? Many candle molds are made so that the top is larger than the bottom....just slightly so the candle will unmold better. I have made many large candles but I do have large melters. If you have four burners on a stove you can set up four large pans with cans or metal pitchers in them and put water in the outer pan so it becomes a double boiler. That is safe if you aren't used to working with wax. I use silicone sealer on molds and it holds up to temperatures over 200 degrees. I like Bruce's idea of cutting the bucket and sealing a plain flat sheet onto the side. The silicone sealer could do that if you don't want to use a hot glue gun. To help release the wax once cooled, you can poke a hole in the bottom of the mold and that helps to release the pressure. Satin Ducky does what I do. I poke relief holes as the candle is cooling.....every once in awhile. Then when it is completely cooled I do a final pour into the holes with hot wax. Good luck. Donita

[Wiscus]

Make a mold out of the bucket...permanently attach a lid to the bucket....cut the bucket in half...place on it's side and support it with rolled towels....this should give you the desired shape and the wax should easily release from it...can make two from one bucket!!...

[BruceCarvesWax]

Ok new idea!

Do what you did the first time since you already had it set up and know how to do it. You said it would hold water with the bathroom sealer so fill up one side with water to keep pressure in that side and then pour the empty side with the wax. With the water on the other side it will keep the wax from going into the water. Wax floats on water so the water will have more pressure on that side than the wax will have since its more weight so that will keep it from leaking! I would use room temp water... or maybe even hot water to keep the wax from setting up too fast.

Bruce

[SatinDucky]

Ok new idea!

Do what you did the first time since you already had it set up and know how to do it. You said it would hold water with the bathroom sealer so fill up one side with water to keep pressure in that side and then pour the empty side with the wax. With the water on the other side it will keep the wax from going into the water. Wax floats on water so the water will have more pressure on that side than the wax will have since its more weight so that will keep it from leaking! I would use room temp water... or maybe even hot water to keep the wax from setting up too fast.

Bruce

Sounds like a great idea if you use a heat resistant sealer

[Donita]

I don't care what they have been saying about you Bruce....but you are truly a genius (smile) .....I would never have thought of that. Donita

[BruceCarvesWax]

I don't care what they have been saying about you Bruce....but you are truly a genius (smile) .....I would never have thought of that. Donita

Well hell if it didnt work and the seal broke he would atleast have one heck of a water candle!

Bruce

[topofmurrayhill]

In the course of going from a liquid to a crystalline solid, paraffin shrinks more than just about any material you could work with. Attempting to fill the voids with melted wax will create discontinuities that you might like to avoid, even if you do it while the paraffin is still warm.

For a more seamless result I would envision reheating the wax and mold in an oven. The outer portions would melt and the sinkhole would be filled while the center remained solid. Any subsequent shrinkage would be more minor and even than the first time around. The process could be repeated if necessary.

Slow cooling of such a big chunk of unmodified paraffin must be producing an object full of snow spots, no? I wonder how that would affect your microwave experiment. Using a small amount of additive could get you a more uniform and cohesive crystal structure.