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Hi all, I've got a few questions regarding the 'new' product 'XenCast Flexible Polyurethane Rubber' advertised on your website. Basically I'm making a couple of moulds with the intention of casting 'Vac-Cast Epoxy Resin' into them, what I need to end up with are two perfectly reproduced tooling plugs for vacuum forming my final parts into ABS plastic. The following image will hopefully explain in greater detail my procedure. I've recently used a product from 'Smooth-On' called 'OOMOO 30', but had a disastrous result. I must admit, as good as it seems to be I just hate working with it. Here's what my original mould box looks like with my plug secured in the base, I then poured the silicon rubber into the mould box ......... it was an awful lot of work, trying to mix what was a consistency of partly molten lava, and the impossibility of pouring made the job needlessly difficult, the result was a failed pour and a very badly formed mould ! >>  Pouring the mix in a thin stream in a low corner was just impossible, I almost had to resort to 'trowelling' it in ! >>  When I demoulded the following day, it wasn't too bad, but strictly speaking an appalling result. Not something I would want to cast resin into that's for sure ......... I put it down to a good learning experience >>  Because the silicon rubber wasn't 'fluid' enough, full coverage of my plug wasn't achieved, leaving large voids and air pockets >>  So basically, my question is, would that 'XenCast flexible Polyurethane rubber' be an alternative to OOMOO 30 ? I've been looking at the 'XenCast PX30', it seems to have the same characteristics as the OOMOO 30 ( both Shore A of 30 ), how will it react to Vac-Cast Epoxy Resin being poured into it ? |
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Group: Forum Members
Posts: 101,
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Hi guys ....... ( and girls if there are any on this forum ) ! it seems to me that this is more of a male dominated world ......... so ladies, introduce yourselves ! 
This is going to be a lengthy post, because I'm simply going to cut n paste from my original forum group. So bare with me, my intention for doing so is to see if any of you clever guys can put me straight. Even though it worked, I got great results ......... but where can I make life easier for myself, and is it possible to get better results in a quicker time.
Anyway, onto my quad headlight project ........... In the last update I left you all with this. Two perfectly formed silicone moulds for casting aluminium resin tooling plugs for vacuum forming. The thing is, you couldn't simply poor ( cast ) resin in those as you would end up with a solid block of resin. Because of the intricate shape of the tooling plug itself, there must be vent holes coming from underneath the plug >>  There aren't too many ways of accomplishing this task, making a hollow tooling plug. One way the industry do it is to use sheets of a special hard wax, an A4 size sheet of wax cost £19 ! and the thickest they do is only 5mm. That's no way thick enough for the walls of a vacuum tooling plug, as mine have to be at the very least 20mm thick as they have to be able to withstand at least 20 pounds of pressure per square inch or the plugs would simply crack under pressure, so I would need dozens of sheets of wax. Making that particular method ridiculously over priced. Spending over £200 in wax on a mould that already costs in the region of £400 is not a cost effective method of producing hollow moulds. There is another method however, known as 'blocking-out'. Some people simply use blocks of wood ........ but I've had problems in the past using solid materials against one another in mould construction. Another method I believe would work would be the use of plasticine, but I haven't got enough of it in stock, so I came up with this .......... >>  I began by cutting out some templates indicating the area to be 'blocked-out', leaving me with about a 20mm gap as a side wall >>  I then transferred all those dimensions to a sheet of 5mm PVC foam, and cut them to shape >>  And then assembled the 'skeleton' with the aid of a glue gun ........ I now have my basic shape >>  You then have to block-out the gaps using a polyurethane foam, to create a solid structure >>  The reason why it's a good idea to use PVC foam for the ribs and PU foam for the blocks is, when you move onto the sanding stage, the sanding is so much easier as the PU foam is very soft and the PVC ribs are a little firmer, it's easy to see how far you have to sand ....... once you hit the 'green stuff', stop sanding ! >>  And you should end up with this ........ the inside of what will be your cast resin mould ....... minus 2cm all the way round >>  Then you apply a skim of body filler to harden the whole thing, and sand that down. but it doesn't end there ........ you now have your 'block-out' mould, but you have to be able to release it from your resin cast afterwards >>  I then turned my attention to working out how much resin I would need for casting. So I filled the silicone mould with dry rice to find out the volume >>  As it's so important to remove the 'block-out' mould from my resin cast afterwards, without damaging the resin tooling plug, I decided to use a flexible material as a 'glove' as sorts. People 'blocking-out' using wooden blocks in their resin casts, would have to chip out the wood with a chisel as resin sticks to wood like sh!t to a blanket, I chose a different method. I simply used the remainder of that OOMMOO 30 silicone rubber I had from the last fiasco, and just painted a few millimetres of it onto my foam plugs. I also found two tupperware containers about the same size as my 'block-out' mould and deducted them from my silicone mould volume >>  Now that I know the actual amount of resin I'll be needing to fill my mould, I still need to workout what that is as weight as the rice thing is it's volume ( the amount of space the resin will take up inside the mould ). You can't workout how much catalyst you need just by volume, you need it's weight. It so happens that the volume of the rice was equal to 2000ml on my measuring container. As I'll be doing the casting in two pours, I put an equal amount of dry rice ( 1000ml ) in each mixing container, then weighed that. So we get the weight of the rice which is 947 grams, but lets not forget the density of the resin ....... this particular resin because it has aluminium in it weighs almost twice as much as the rice ........ that is to say 1.80 grams per cubic centimetres 947 g X 1.80 ( g/cc ) = 1704.60 grams of resin And as the resin/catalyst mix is a 100:6 mix ratio ( for every 100 grams of resin you need 6 grams of catalyst )So the sum is 1704 x 0.06 -------- 102.24 grams of hardener required for mixture -------- Now it's just a case of prepping everything, you must give the resin a though mix before use, as the aluminium would of sunk to the bottom >>  As I'm doing this in two pours, I prepared both the resin and the catalyst, you don't want to be mucking about with weighing scales midway through casting resin >>  The two pour technique is to minimise on air entrapment, so I used Archimedes' principle on 'fluid Dynamics', where any object submerged in a body of water displaces it's own volume of said fluid. Same if you were to fill your bath tub up to an inch from the top, then stepped into your bath it would overflow. So I poured the first container into the silicone mould >>  I then positioned the 'block-out' mould very carefully into the resin being careful not to trap air underneath. An air pocket trapped under the mould would most certainly destroy my completed resin cast >>  I then poured the rest of the resin from the second container to fill the silicone mould, after securing the block-out mould in place. You'll also notice on this close-up that there's a millimetre gap between the silicone mould and the wooden side of the mould box ........ that's not good ! ........ that's due to shrinkage, most materials will shrink after they have had time to cure. The drawback with this is, the mould could end up a millimetre too small !  Now I just got to wait for it to cure .......... curing time is 12 hours >>  A couple of hours later I went back into the workshop to check on the progress, and saw this ! ........ that's not good As careful as I was, there had been an air pocket trapped under the block-out mould :eek: it caused the resin under the mould to escape thus causing this bubble burst ! ......... nothing I can do about it now, I just have to pray and hope ! The interesting thing is, notice the tape has come unstuck ....... that's down to the amount of heat that the curing process produces. The 'exothermic' reaction caused even the wood to heat up to a point where is was as hot as a cup of coffee, and even dried the adhesive on the tape ! >>  Moment of truth .......... I stayed up like a little kid at Christmas eagerly awaiting Father Christmas with my presents :confused: So at 4 AM it was time to demould, the foam block-out plugs ( or 'Bucks' Gaz ) are simply a temporary measure and will not be used agian, so I can simply break them apart. >>  Well, that worked out a treat ! after releasing the tension, the rest of the 'buck' just peeled away. The cavity isn't pretty to look at, but who cares ......... I've got a hollow mould. And it doesn't look like the air pocket did any damage to the inside of the resin cast. Actually, I think it was completely expelled, leaving that bump of cured resin on the sidewall >>  I struggled for almost 2 hours trying to demould this cast as the resin was stuck to the silicone mould ......... then I hit upon an idea, so I squirted some soapy water into the silicone mould. This is where I cut my fingers to shreds, trying to force my entire hand in between the cast and the silicone mould to break the 'seal', those edges are like razor blades ! It's amazing just how strong and flexible this RTV silicone is, it's tear strength is amazing. At one point I had both my hands up to my wrists inside that mould and it still didn't split ! And this is the result .......... an absolutely flawless resin cast ! T: >>  I am well happy with that not a single blemish on the resins surface, nice solid 20mm walls, should be strong enough to produce many vacuum formed parts. I've just got to tidy up the base a little, sand off that resin bump and smooth out the edges which are really like a knife edge >>  Was all this really worth the effort ? ........ you bet ya ! From a PU foam plug covered in Pattern-Coat resin to a silicone mould to a foam buck to a cast aluminium resin tooling plug ........ a lengthy process and an expensive one >>  I've learned a great deal from this entire process and had a lot of fun doing it, and now I'm pretty sure I can create just about any shape, that will lead to producing just about anything .......... I'll only be restricted by the size of my vacuum forming machine. Just look at the surface finish of that ........ flawless :db >>  And just one last shot of my first attempt at making a tooling plug. This thing is so heavy that it actually feels just like a solid block of aluminium >>  Next up will be to drill some vent holes in it, in order to allow the vacuum to form the shape required for the part. I'll have to work out where the holes will be positioned, as it's not just a case of drilling anywhere. And I also have to make another one ! Yes folks ....... the drawbacks of designing and fabricating headlights, every process is done in two's. I didn't have enough Vac-cast Epoxy resin left to start the other one, but it's already been ordered and it's already on it's way. I know that most if not all of you lot are well past this amateurish stage, but there's always a novice newbie out there ......... hopefully it will also help others out. Hope you enjoyed the update peeps ........ plenty more to come
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