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Old 12th February 2012, 06:48 PM   #271
Squeak is offline Squeak  Denmark
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Quote:
Originally Posted by John L View Post
The only problem here is that the diameter is just 129mm, which comes out to a hair over 5". That wouldn't ever begin to cover the project. It would be something like staring at a full sized person with a tiny head on top. I need at least 13.5"/343mm.

The shape is great, but it would have to be blown up considerably.

I did a search on Google, for possibly another larger product, but no luck. I typed in several possibilities, but again no luck. Unless someone can come up with one that is at least one foot in diameter, I'll have to go with what I already have.
Depending on your crossover frequency you should be fine with a simple cone/dome (which can be can be easily obtained) or even nothing for the BOFU. Also, most of the HF from it will come from the centre and the whizzer, so a smaller cone should be good.
The most important thing is to get good/gentle spread with the tweeter.
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Old 12th February 2012, 06:56 PM   #272
mightym is offline mightym  United States
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Seems to me like your on the right track.
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Old 12th February 2012, 08:27 PM   #273
John L is offline John L  United States
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I have just finished watching some nice Youtube videos on creating two piece molds, and it has me thinking about what to do with the base part that I posted earlier. If the base portion is one piece, I may have a good bit of difficulty removing the casting, even with a silicone mold between the plaster 'mother mold'.

Here are some videos that answer a lot of question, but also create more things to consider that are unique to my situation.

Mold Making: 2 piece mold

Mastering Mold Making Part -1

Two-Part and Complex Mold Making

I think I can modify my two mold bases and be able to come up with a two piece mold. If I remove the wooden corner pieces in the pictures below, I will be able to apply several coatings of silicone rubber, which will make it easy to remove from the poured epoxy lens. Then I can replace the corner pieces which have been cut back some more for allowing plaster around the silicone.

And I will want to keep the wooden frame around the entire project, because I will be using wood fillers inside, so as to keep the amount of plaster at a safe minimum. Keeping the plywood frame will keep the shape of the mold more uniform, and not with a lot or corners and edges.

And I'll be using a lot of wood filler inside the frame. Even plaster is much more expensive than wood scraps. Because I make custom covered things for interior designers, such as padded headboards, cornices, and window treatments requiring wooden railings, I have so much scrap material lying around I have to throw the stuff away from time to time. So I have a lot of scrap, which costs me nothing but my time.

Click the image to open in full size.

Click the image to open in full size.

I will also want to be able to break the entire mold frame apart in order to remove the finished lens. And because I want to keep the outer plywood frame as part of the mold, I will need to take the base for each lens part, and rip them down the middle on my table saw. Then I can take plywood/OSB strips and put the two halves of each one back together. I'll use screws instead of staples/nails because I will need to remove them in order to break the mold in half after the epoxy has cured properly.

Once I have a wall barrier across the middle of the clay sample, and sealed to prevent plaster from running from one half of the mold to the other, I can pour one half of the mold.

When that first half dries, I will remove the barrier, and clean it up. Then I can make recesses in the newly poured plaster, which will act as locators/positioners for obtaining perfect matches for the two mold halves. Once that is done, I can pour the second half.

That should give me a two piece mold of the outside of the diffraction lens. I'll be able to turn it upside down, remove the bottom panel, and pour the inside from that end. When I am through pouring and curing the epoxy, I can then take the two halves of the mold apart and pry the epoxy out of the silicone mold. Then I'll be ready to do another two piece lense.

I think this is going to be the safest and best approach to getting the outside lens correct. I'm still not all that certain what I am going to use as a complete filler for the insides of the lens. I would prefer not to make it hollow, but be light enough not to be top heavy when it is put on top of the cabinets.

I'll still need to work that out. And perhaps someone can help me decide what to use, and perhaps what kind of filler/aggregate to lighten the lens, at the same time not having to use so much epoxy. Polyester resin is about half the cost of epoxy resin, but I could still wind up using a very large amount if I am not careful. And with that part being inside the lense, it doesn't have to be pretty, just light weight and strong.
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Old 29th February 2012, 03:44 PM   #274
John L is offline John L  United States
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I've successfully finished the first step in the mold making process. Yesterday, I took the silicone cast kit(1 quart) and covered the two pieces of the lens. This morning I trimmed off excess silicone from around the pieces and carefully peeled back the silicone, hoping I wouldn't have any problems.

Success! Here are some pictures:

Click the image to open in full size.

I sealed both parts with a clear gloss sealer, and used the entire spray can, by applying as many coats as possible. I applied at least ten coats when the previous one hardened. I was hopeful that the finish would make the pieces strong enough not to start coming off with the silicone.

Click the image to open in full size.

Next, I prepared the silicone mix and applied three coats. As soon as the first coat became tacky, a little over an hour, I went back and brushed on another coat. I did this three times before running out of silicone.

And I could have used more if I had it. The instructions called for applying at least four coats, and a finished shell of 3/8" thickness. When I started peeling back the rubber, I noticed that it was not as thick as the optimum.

But it should still be thick enough. I thought that using two pints of mixer, for one quart, would be enough. But I probably should have bought two quarts, and made the molds thicker. I'm hoping this will not be a problem. But I am not going to be making dozens of pourings, so it should hold up. That's one lesson learned about making molds.

Click the image to open in full size.

In the second picture you can see where I trimmed off the excess silicone, leaving a flap beyond the lens. Then I carefully peeled back the silicone, and sliced the mold from the edge to the center point.

Note that I made a mark on the lens. That is to ensure that I get the silicone back in the exact place where it was poured. No matter how much one tries to make parts perfectly even, it is impossible. If I didn't put it back perfectly I could have a problem when I actually pour the plaster in the next step.

Now that I have this part successfully completed, I am ready to move on to the pouring of the plaster cast. If you look closely at the frame above, you can see where I cut the mold in half on my table saw. Then I took strips of plywood and screwed the two pieces back together. This will allow me to make the plaster mold into two pieces. When the resin is poured and cured, I can break the mold apart and then peal back the silicone, exposing the finished lens. At least that is what I hope anyway.

Last edited by John L; 29th February 2012 at 03:53 PM.
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Old 2nd March 2012, 08:28 PM   #275
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This is inspiring work John! Wish I lived closer so I could lend a hand.
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Old 3rd March 2012, 10:14 PM   #276
John L is offline John L  United States
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Originally Posted by wiredbecker View Post
This is inspiring work John! Wish I lived closer so I could lend a hand.
Good Lord, I wish you were here too Chris. I just had a little set-back on the mold. I don't know how big yet, but my first plaster pouring was a disaster.

I can't show you any pictures of me jumping through my 'backside', because I was in a hurry to put out the fire. But I did take three shots of the bigger lens that I was going to do last.

Here's what I mean:

Click the image to open in full size.

Click the image to open in full size.

Now imagine the frame as the smaller top section that has a slight round hump. I had replaced the silicone shell over the positive piece of clay, and then started building up a partition in order to create two halves. That way I would be able to break the mold in half, remove the poured resin mold, surrounded by the silicone shell, which I could quickly peel off and place back within the mold assembly.

I carefully scribed a 1/4" piece of plywood to match the shape of both Positives, splitting them down the middle. I did what I saw in one video and covered the partition with aluminum foil so as not to have the plaster stick to the partition. And then I used Pam spray all over the partition, and silicone mold cover.

I actually though I had everything covered, and had worried about the plaster running out the wooden frame, but that didn't happen. I just overlooked one tiny, but significant issue.

After I thought I was set up, I moved the frame holding the upper portion to the basement floor, so as not to make a mess. I would have done it outside, but it is raining here in the Raleigh area. Then I carefully followed the directions on mixing the plaster with water, by measuring the water and then equal parts of plaster, like the dental supply company said to do.

I knew it was going to be thinner than I thought it could be, but I followed the directions, and mixed the batch until everything was evenly mixed. Then I started carefully pouring, very slowly, in one spot and allowing the plaster to fill up on its own. Everything was going nicely.

After I had poured in about two inches along the outside edge, I noticed that there was some plaster on one corner of the other side of the wall. What? How could that be? After all, I had carefully sealed the edges and partition with modeling clay, and it shouldn't be running out.

Well, I pulled off two paper towel sheets and dabbed up the runny plaster, but then noticed that the plaster was seeping in from underneath the silicone mold cover. Oh no!, the plaster was running under the positive and silicone and coming out on the other side of the frame. Horrors!

I immediately started going through paper towels and it just got worse. Pretty soon I had a pond of wet plaster on the bottom of the frame on the other side!

I had been so concerned about everything except making sure the clay positive and the silicone shell were sealed. It was so obvious and I had completely overlooked it.

So I made a command decision and aborted the pouring and broke down the setup. I quickly removed the partition, and rushed the entire frame upstairs to the kitchen sink, and spray rinsed all the plaster starting to set up. Then I removed the silicone and the clay positive and cleaned them off.

But in the process I had to get the clay piece wet again, which meant I would have to let it dry for several days before I could start again. I could kick myself for this.

But there is another problem. The clay positive has now developed a hairline crack, running from the corner where the plaster had run under it, and the crack has traveled up about 1 1/2" and then made a right turn for another 2-3 inches. This has me very worried, because it just might break off and ruin everything.

Anyway, I'll have to cross my fingers and pray it stays intact. I had sealed the upper surface, but didn't seal the underside, and now I am paying the price for my mistake. So when it dries, and if it is not messed up, I am going to use one or two spray cans of clear sealer/finish, and make certain this doesn't happen again.

Anyway, that's the story so far. I have the wet clay part turned upside down and resting on a padded clay vase, and a fan blowing nearby in hopes of drying things quckly.

If it breaks apart, I'm going to have to take another ceramics class, so I can use the potters wheel again. And this time I'm going to fire it at a high temperature so this won't happen again.

Sigh. this is so frustrating, but I can't let it beat me down again. Click the image to open in full size.
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Old 6th March 2012, 12:59 AM   #277
Carlp is offline Carlp  United States
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John,

Sorry to hear about the nightmare, but thanks for posting the update. Can you remind me why you didn't fire the clay the first time? I thought it was due to shrinkage, but if you're gonna fire next time, I must have been wrong?

Ironically, I was given a kick-wheel several years ago and have been looking for an excuse to use it, but firing is not an easy proposition in my NH area, so I've been slow to use it. Wish I could offer it up for your use.

Cheers,
Carl
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Old 6th March 2012, 01:57 AM   #278
John L is offline John L  United States
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Quote:
Originally Posted by Carlp View Post
John,

Sorry to hear about the nightmare, but thanks for posting the update. Can you remind me why you didn't fire the clay the first time? I thought it was due to shrinkage, but if you're gonna fire next time, I must have been wrong?

Ironically, I was given a kick-wheel several years ago and have been looking for an excuse to use it, but firing is not an easy proposition in my NH area, so I've been slow to use it. Wish I could offer it up for your use.

Cheers,
Carl
I just got disgusted with the process. So I set everything on the walk-in closet shelf and left it there.

I know, I have no excuse for my dereliction.
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Old 7th March 2012, 02:01 AM   #279
John L is offline John L  United States
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I managed to pour one half of the upper part of the lens assembly this evening. I set things up by using soft modeling clay I had picked up at Michaels last week. I took small portions, and rolled it in my hands until I made thinner, longer, round lines of clay, and then set them down on the frame, under where the blank would be sitting.

Once I got it all laid out I set the blank on the clay and gently applied pressure all around until the clay spread out and formed a water barrier. Then I added more clay to the edges and did the same thing with the silicone layer of the mold. Unfortunately, the blank cracked in three or four more places, so after I finish with this, the blank is going to have to be tossed out.

Also, I decided to try out the 25 pound bag of plaster I picked up at Lowes, because I wanted to see if it had different characteristics from the more expensive dental plaster I had ordered earlier. I made up a smaller amount to use as the initial pour, so as to seal the seams and set up a little before pouring more and completing the pour.

The instructions state that it will set up in fifteen minutes. So I went upstairs to pour more water into a couple of containers. When I came back, about five minutes later, WOW, it was already setting up.

Holy Cow, I was totally surprised, because the dental plaster was nothing like that. So I quickly made another bigger batch and poured it into the mold half, but I still needed a little bit more in order to make it flush with the frame walls. By the time I got that little bit mixed, the entire mold was already firm, so I had to ladle the remainder into the recesses and wait for it to harden a little bit more.

Then I took a 24" metal framing square and used it to level the hardening plaster with that of the top of the wooden frame. Gosh, I had no idea this stuff would set up this fast. Had I used the construction grade plaster to begin with I would never have had my initial problem. Live and learn.

Here is what the thing looks like after I had somewhat leveled the plaster, and before cleaning up the mess. I'll do that tomorrow when I can use a scraper to get up the dry plaster.

Click the image to open in full size.

Once I get back from work, I'll trim off all the dry molding clay, and clean up any loose pieces of plaster from the other side. And this time I will make one complete pour now that I know I will have to really work fast from now on.

I also have another idea just in case the upper part does not come out as well as I hope. I'll talk about that later, after I have the second half poured and have broken open the cast to trim up everything. Then I can better see what I have to work with.

And here's another thought. If this type of plaster from Lowes sets up so quickly, I probably don't need to worry all that much about any more leakage from one side of the mold to the other while pouring. It won't have time to leak before starting to set up.
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Old 7th March 2012, 02:53 AM   #280
Greg B is offline Greg B  United States
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Very cool project. It'd be a good idea to gouge a couple key holes into the side of the mold if your intention is to make a two piece mold. That's the usual trick for alignment.

You probably could have gotten away without using the silicone rubber compound for this, since the shape doesn't have fine detail, but it certainly won't hurt.
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