Sphere Mold - Part 3

Registration marks on the mold.

Last time I posted I blogged about experimenting with making a mold of a sphere from flannel and glue.  My first experiment was whether I could make a rigid mold using those materials.  And yes, the mold held its shape once the mold was cut open and the ball removed.  The mold release allowed the ball to pull away from the mold, leaving me with a hollow sphere.  So far, so good.  Step one succeeded. 

Mold wrapped in layers of tape.

Foam flowed out of the ball  mold.

My next question was whether once the mold was separated into half spheres whether the mold could be put back together again.  With that in mind, I made and numbered registration marks at the four points of the mold's equator before cutting the ball open.  I had this question in mind because a previous experiment with a papier mache' mold had shown me that the lip of the mold was too thin.  It was hard to keep the hemispheres in place because the lip was so narrow.  The new mold cast with three layers of flannel and glue (see the two previous blog posts for more information on the construction method).  When I cut open the mold, I could see that the thickness of the mold was three-sixteenths of an inch thick.  It was enough lip to place one hemisphere on top of the other and have it stay there...more or less.  In future molds, I think I will put down at least five, maybe six layers of flannel.  I think that it would be easier to put the mold together if it had a slightly thicker lip.

Before I put the mold together, I had to think about using a mold release.  My hope was that I could reuse the mold to make a number of balls.  In any case, I wanted the ball to come out easily even if I never made another ball.  I coated both halves of the mold with a generous layer of petroleum jelly.  Was petroleum jelly going to work?  It did release the flannel mold from the original ball.  How a different casting material would react with that mold release I did not know. Again, an experiment to see what I could see.

Foam removed from the mold, mostly in one piece.

Foam leaked slightly from under the tape.

My plan from here was to put the mold back together and fill it with an expanding spray foam insulation.   Before putting the mold together, I cut a small hole in the top of one hemisphere so I would have a place to insert the spray foam nozzle.  Then I started putting the mold back together.  I guess this is one case that poor planning lead to something akin to slapstick comedy.  When I was making this mold, I was only playing around with some scrap materials in the studio.  I was working without a full plan, although I had an idea for putting the mold back together.  I know that professional quality molds have bolts that bolt the pieces of the mold together.  I was not planning to go that far ahead with this mold.  I had really only trying to see if a mold could be made.  My thought was that we had a lot of clamps in the downstairs workshop that I would use to clamp the hemispheres together.   I knew that the clamps opened a couple of feet wide.  What I did not think about was that the depth of the clamp was only about three inches.  My ball was roughly eight and a half inches.  Once I actually pulled out a clamp and examined it I realized that there was no way that the clamp was going to clamp onto the ball.  Then I had to come up with a Plan B.

Lower hemisphere.

Upper hemisphere.

Plan B was tape.  Lots of tape as it turned out.  As it turned out, the glue saturated flannel was practically impervious to both masking tape and duct tape.  I was trying to tape around the equator.  It was a rather trying experience.  I would line up my registration marks and get the ball set just right.  I'd get tape about three quarters of the way around and the first pieces of tape I had put on would start peeling off.  I would go back and try and press the tape down.  Too much pressure would cause the top hemisphere to fall off.   I would put the hemisphere back on and the tape would fall off.  It was probably hilarious if you were watching it, but not so amusing if you are trying to put something together.  Anyway, I finally managed to get the ball together by wrapping my body around it like a contortionist to hold the ball in place and then wrapping a long piece of  tape completely around  the ball so that the tape could stick to itself. 

I started with masking tape but I could see that it was barely sticking to the ball.  I repeated the process with duct tape.  The duct tape did not stick to the mold well either.  After numerous wraps of tape I finally felt I had the mold situated so that it would stay together when the expanding foam was added.  It looked like a big mess.
 
I had some strong concerns that the ball might not stand up to the pressure of the expanding foam. I put on my oldest paint clothes and went outside.  I also wore rubber gloves and eye protection.  Who knows what can happen when something is put under pressure.  I was just hoping that if the worst case scenario happened, the foam would not get stuck in my hair.

The can of spray foam did not enlighten me as to how much area the can could be expected to cover. All it said was that it could fill a crack up to one inch wide.  However it did not state whether it would fill a one inch wide crack for a certain number of feet.  There was no way with the information available to calculate how much volume of foam would be created.  Without that information I just had to guess whether it was going to be enough to fill the sphere or whether the sphere would be forced apart by pressure from too much foam.  I decided that I would start filling the sphere and quit adding foam once I saw that it was getting close to the hole I had cut for the spray nozzle.  My hope was that as the foam expanded it would fill the rest of the cavity and any excess foam would flow out of the hole for the nozzle. 

I added the foam and it did fill the cavity and then start to overflow out the hole.  There were a couple of places on the equator where the tape did not seal well and a little bit of foam about the size of a grape emerged.  The rest of the foam flowed out the intended hole.  I was wiping the excess foam away from the hole for about twenty minutes.  I did not want the foam to harden before it was finished expanding.  When the foam settled down to blurping more slowly and looked like mud blurping at a hot spring I went inside and left the ball to its own devices overnight hoping that it would dry up and seal off the hole. 

The next morning I went outside to see what had happened overnight.  The foam had continued to flow from the ball and formed a mound flowing down the side of the ball.  I took the ball inside to work on it at the downstairs workshop table so could use the shop vac if the foam made a mess when I cut it off the ball.  As it turned out, I did not need to use the shop vac.  The whole block of foam pulled off in one piece.  Apparently it was not very amenable to sticking to the mold either.

I used a utility knife to cut through the foam around the seal of the mold.  The mold was starting to move easily.  I was getting excited.  Then I opened it up and was in for a big disappointment.    Apparently the flow of the foam acted as a siphon.  There was barely any foam left in the mold.  Fortunately just before I opened the mold, I decided I should put down plastic for "just in case".  What foam that was left in the mold was still sticky wet.  Although it was supposed to be set up in twelve hours, it was still quite damp at nearly twenty-four hours.  I guess with hindsight I am lucky that the foam had flowed out of the mold.  That would have been a real mess to clean up if I had opened the mold and it all just fell out onto the worktable.  Note to self:  always unmold your experiments outside.  Anyway, the lower hemisphere of the mold held some foam.  The upper sphere was coated with a thin layer of foam.  I left them on the workbench to finish drying.  Just to see if the mold release was going to work. 

So that is the end of the experiments with this mold.  You win some, you lose some.  Conclusions:  the flannel and glue worked to make a rigid mild; future molds will need to have brackets of some sort for bolts and nuts to hold the mold together; spray foam may not work as a casting medium because it needs air to help the foam set. 

So there you have it.  An experiment gone awry.  But it did give me some insight into mold making and casting, so that is worth something.  Check back on Sunday for a blog post on a different subject.