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Topic Summary
Posted by: fred_55 Posted on: Apr 27th, 2013, 3:51am
I needed some castings but couldn't find a helpful foundry so I tried casting them in plastic. (they are buffer stocks, but they take no load on the model.)
This is the kit. RTV2 casting silicone, sticks to itself but not to anything else.
Posted by: fred_55 Posted on: Apr 27th, 2013, 4:07am
The setup. (I'm single so I can use the kitchen bench)
The purple pack makes the mold, cast in a cup. The mold is quite flexible and will tolerate undercuts. It will withstand 300 degrees so can also take low melt alloy.
The picture doesn't show the mould mixing, but does show 1, the mould (grey tube), 2, the pattern (grey wood) and 3, the finished item (white).  In the back are the cups with coloured spoons, for mixing the polyester.
Posted by: fred_55 Posted on: Apr 27th, 2013, 4:14am
OK  I am starting again.
 
Having made the pattern, next is to work out how to fill it. I decided to use the mounting face which is easily filed flat. the pic shows the pattern suspended in a pattern box (cup) with the mount face upwards.
The RTV mould material is mixed as instructions (use a lollypop stick and take care not to stir in air bubbles) and gently poured away from the pattern and allowed to fill up around to the top face. There is minimal shrinkage and with a set time of several hours, some topping up is possible.
To get the right quantity, first hold the mould while filling the cup with water up to the spot, then empty the water into another transparent cup and mark the depth. Mix the RTV to this mark. Make sure everything is dry before mixing and pouring.
Posted by: fred_55 Posted on: Apr 27th, 2013, 4:18am
Leave for 24 hrs, then you can fill it with polyester. Like fibreglass it is a two part 50-50 mix that goes off quickly.
The orange pack is the polyester resin 50/50 pack. Note the colour coded mixing spoons. A quick stir and pour, then it sets in 100 seconds.
The pattern is grey and the plastic copy is white. The undercut was more than I thought, so I split part of the mold with a scalpel to release the product (simply duct tape it together for the next one)
 
To save guesswork fill the mould with water. This time I used a spoon and counted spoonfulls. Divide this number in half and thats your quantity of each part.
 
I measured part 1 ( with a green spoon) into the mixing cup, then added the same quantity of part 2 with a blue spoon, then stirred with stick. The soiled spoons will keep for several hours if they dont touch each other and are kept in their own cup.
Posted by: moose_the_caboose Posted on: Apr 29th, 2013, 6:00pm
wow!  nice work, pete.
 
now...can you make another casting and show us from start the finish?!  
would be nice to see how you started and what you had to do to replicate the part you used for the pattern.   if you can do it in your kitchen, then maybe we should be able to do it in our shops, garages...er' an maybe the backporch.
 
keep it coming, kindda gets the juices flowing again!
 
moose
Posted by: fred_55 Posted on: Apr 30th, 2013, 6:59am
OK I skipped a bit. This is the mould in a cup recreated for the pic. Using the original mould box keeps the shape, in this case the mould was split to release the finshed item, then returned for the next one.
The polyester has a pot life of 100 seconds, so you have to have mould ready to go. Slow pouring down the side of the mould will minimise air bubbles, but if you set your mould in such a way that bubbles will be on the non-view side, then you get a better result.  
Posted by: fred_55 Posted on: Jan 11th, 2014, 1:22am
You can demould in 60 minutes, so it's a quick production line. I did 8 of these in one evening
Meanwhile, this stuff has reasonable strength, but is best for "decorative" parts, especially underbody detail (generators, battery boxes, air cylinders,etc).  
Secondly, it is not neccessary to split the pattern, as for sand casting. Provided you can support the mold, say in a box or cup, you simply slice the mold partly or fully, through the middle and stretch it to release the part.
It's also great for repetition jobs or sharing with others. The mold  can even be mailed.
Note that there's no shrinkage to calculate, and the finished article can be machined with normal tools ( albeit a bit careful with the vice!!)
Pete
Posted by: fred_55 Posted on: Jan 11th, 2014, 1:27am
Next job is quite a challenge, a 2" scale Westinghouse K valve and tank. I'll describe the mould process a bit more thoroughly, meanwhile this is your version of my valve.
Posted by: fred_55 Posted on: Jan 12th, 2014, 3:09am
And this is my version, turned in wood.  I had the choice of making it in two pieces, or laying it horizontally and filling on top of the air tank where it is out of sight.
In the end I decided that one piece was possible, and air bubbles would escape better if I stood it on end with the awkward tiny details near the top where the bubbles would release more readily
Posted by: fred_55 Posted on: Jan 12th, 2014, 3:13am
Here it is with detail bits added.  The bottom end (sitting in duct tape roll) has less detail.  The moulding box has been cobbled together to give around 1/2" cover (min 1/4" in small areas) and then the joints sealed with bathroom sealer.
Posted by: fred_55 Posted on: Jan 12th, 2014, 3:24am
Then I changed my mind and went for broke by adding the filter and a couple of pipe ends.  This meant that the box was too shallow so I extended it.  The box is best nailed together but leaving the nails just proud, so it is easier to grasp them when the box is to be pulled apart later.
The master has now had a coat of gloss to make it less likely to grab the RTV.
Finally it is suspended in the mould box. I used two batten screws in the ends as I want to later put a 1/4" bar through the mould to strengthen the finished article.
Posted by: Dan Watson Posted on: Jan 12th, 2014, 2:04pm
Very interesting post, Fred. I have been thinking about making some similar castings using RTV molds with low-temperature bismuth alloy. These alloys have melting temperatures between 120F to 300F (depending on the alloy) and the ones towards the lower end shouldn't harm the plastic mold. This one is a bismuth/tin/lead alloy:
 
http://www.rotometals.com/product-p/LowMeltingPoint212ALLOY.htm?gclid=COfsirup-bsCFSHNOgod0EoAzw
Posted by: fred_55 Posted on: Jan 12th, 2014, 5:49pm
Amazing what is available on the www these days!
When I was casting bits for "O" scale I used pewter scrap from printing press type. It was cheap, ran about 300 degrees and was hard as it had a higher tin content.
Here's a link to someone with better writing skills than me.  I notice he refers to polyurathane and not polyester (fibreglass) as suitable resin.
http://johnsblogworld.blogspot.com.au/2010/11/building-custom-usb-flash-drive-part-1.html
Posted by: fred_55 Posted on: Jan 14th, 2014, 1:35am
Filled the mould with water to the desired depth then dumped it in  the tupperware measuring pot on the left.
It measured 1200 ml which is the exact ammount in the rtv kit. That made measuring easier as I mixed it in the kit container, but it only just covered the pattern, so I put in some scrap bits of wood where the rtv was thickest and by scraping the barrel, just made it.  
This morning I checked it and there is a soft spot, possibly from scraping the pot and getting mixture with less hardener.
I'll leave it a day or three as it could still cure.
 
The experts say that a vacuum chamber will reduce bubbles dramatically, but this lump is way too big for my home made vacuum setup. Instead, that bit of coiled wire is a bubble catcher, waving it around and under the pattern very gently tends to pursaude the bubbles to let go and rise.
Pete
Posted by: moose_the_caboose Posted on: Jan 14th, 2014, 4:50pm
on Jan 14th, 2014, 1:35am, fred_55 wrote:       (Click here for original message)
Filled the mould with water to the desired depth then dumped it in  the tupperware measuring pot on the left.
It measured 1200 ml which is the exact ammount in the rtv kit. That made measuring easier as I mixed it in the kit container, but it only just covered the pattern, so I put in some scrap bits of wood where the rtv was thickest and by scraping the barrel, just made it.  
This morning I checked it and there is a soft spot, possibly from scraping the pot and getting mixture with less hardener.
I'll leave it a day or three as it could still cure.
Pete

 
well? inquiring minds want to know!  how did they turn out?  and btw...gonna do some in 1/6 scale?  i need 5 of them.
 
keep the pics coming!
 
moose
Posted by: tomc Posted on: Jan 14th, 2014, 9:45pm
Holy Crap Moose!  5!  where ya goin to keep all the equipment?
 
Tom C.
Posted by: fred_55 Posted on: Jan 15th, 2014, 2:06am
Hi Moose. Bulk RTV costs $10 litre, this kit cost $90 for 1.2 litres. Two days out in the sun has seen it get much stiffer,  I think I'll wait a bit longer.
This item is at 1:6 or 2" scale, so would suit you.  I'll keep an eye on costs in case its worth making them for you . Either that or I can mail you the mould and you make your own and then return it.
Tom.   Holy Crap Moose!  5!  where ya goin to keep all the equipment?.  He can put two of his cabooses on one of your flatbeds .  Have you found any bedposts yet?
Pete
Posted by: Dan Watson Posted on: Jan 15th, 2014, 2:13pm
As far as a vacuum chamber, how about putting the mold into the clean tub of a shop vac, putting a plug into the inlet of the vac, and let it run for five minutes?
Posted by: moose_the_caboose Posted on: Jan 17th, 2014, 10:11pm
on Jan 15th, 2014, 2:06am, fred_55 wrote:       (Click here for original message)
Hi Moose. Bulk RTV costs $10 litre, this kit cost $90 for 1.2 litres. Two days out in the sun has seen it get much stiffer,  I think I'll wait a bit longer.
This item is at 1:6 or 2" scale, so would suit you.  I'll keep an eye on costs in case its worth making them for you . Either that or I can mail you the mould and you make your own and then return it.
Tom.   Holy Crap Moose!  5!  where ya goin to keep all the equipment?.  He can put two of his cabooses on one of your flatbeds .  Have you found any bedposts yet?
Pete

 
 
hi pete!
 
after having a day or two to think about it, all i really need is 1 (one).  for a modest fee, i can have it scanned in as a 3-d shape and then printed in one of several materials.  hi-res printing is coming down in price and there is now a handful of choices for printing materials.  once scanned, the object can be scaled to any desired size.  if this interests you, pm me and we'll work out the details to your satisfaction.  you've done the work, you'll own the file.  you've saved me a lot of work!   keep it coming, this is a wonderful project.
 
to tom c: i'll have to build an elevator.
 
to brian t:  all stations alert...i'll be on the high seas through 1/23, she's in your hands...take care of her.
 
moose
Posted by: fred_55 Posted on: Jan 18th, 2014, 3:36am
Vacuum in a shop vac....might just work.
Moose, whatever works... it must be a lot cheaper to do 3D in U.S.  I have a fail in the mold, which is frustrating. The air tank end is good so I might do a test cast. It could take a little while to save up for more RTV material to try again.  
Pete
Posted by: Dan Watson Posted on: Jan 18th, 2014, 7:33am
Sorry to hear about the mold failure. But I'm not following how you separate the two halves of the mold.  Is it cut apart after the pour?  
 
If so, why not pour half of it at a time?  First fill it half full to the midpoint of the pattern and let it harden,  Then put some kind of parting medium on the first half (could be cooking oil spray or wax paper) and then pour the top half. Would be much easier to separate the mold.
 
If not, then I'm not following your method.
 
As far as the cost of 3D printing, it appears to be way cheaper here than it is elsewhere.  What's really strange is that Shapeways (which I believe has their plant in the U.K.) charges a lot more for the same part there than they do here. I heard of one guy in England ordered a part from them, had it delivered to a U.S. address, then had it shipped it back to the U.K. for less total cost than having it shipped there in the first place (?).
Posted by: fred_55 Posted on: Jan 19th, 2014, 6:51am
Part of the benefit of RTV is its stretch, so when cut partway (after molding) it holds registration real tight and leaves no parting mark.
However I have obviously found the limit and exceeded it. This mold for this item will have to be done in 2 halves.
I also discovered that incompletely cured RTV reacts with the plastic, look what happened to the test pour
Very sparing petroleum jelly stops the bond, but keeping the joint sharp is difficult.  Registration pegs will be  required.  A split top and bottom is probably the way to go, and would minimise air bubbles in the filter area.
 
3D printing is crazy expensive here. I enquired about a headlight, one quote was over $400 and had very  poor definition. Perhaps Moose could get a ballpark, then I might get him to make them!  
Pete
Posted by: Dan Watson Posted on: Jan 19th, 2014, 12:24pm
The other trick in holding down on the cost of RP parts is to make them hollow.  Generally they charge by the number of cc's of material used, not by the size of the item, so if you make it hollow it costs much less.  It's also easier to mold if the pattern is made in halves.  Then it's easy to fill the cavity with something cheaper like plaster of paris, and mount each half on either side of a matchplate.
Posted by: Steen_Rudberg Posted on: Jan 21st, 2014, 6:31am
Hi Fred
 
Maybe you should split it up in smaller parts, and join them later.
 
 
Posted by: BobbyT Posted on: Jan 21st, 2014, 7:28pm
Pete,  
So sorry about the mold issue. I really like the possibilities this has for our hobby.
 I had read an article once about using RTV molds. In the article they only filled the casting box half full, and placed 4 marbles, one in each corner into the still wet RTV. Once the first half hardened they removed the marbles, coated the first half with a release agent and then poured the remainder of the mold. The marbles provided a nice clean registration pin. Just a suggestion.  
 
Posted by: fred_55 Posted on: Jan 21st, 2014, 10:40pm
The marbles idea has merit. Making smaller parts makes sense, but because there's no shrinkage, it only makes more work....we'll see.
Hollow parts is definately worthwhile. Balsa wood or polystyrene inserts  are OK I've been told,  perhaps here I could skewer them, like a kebab!
Meanwhile I have some axle box ends to copy that shouldn't cause too many dramas.
I'm liking the idea of RP a lot more. I'm not sure about scanning the pattern, why not just draw it from the original specs. Maybe someone who is digi-savvy could jump in.
Pete
Posted by: fred_55 Posted on: Feb 2nd, 2014, 12:17am
Went back to the shop and told my sad story. "Too much hardener" she said. Anyway she gave me a discount on the new kit.
Meanwhile I ordered a little kitchen scale from ebay for $7.99,    0-5000 grammes, just right.
So here's the new setup.  I need 650 grammes, so set up scales with empty container, and zero it..........
Posted by: fred_55 Posted on: Feb 2nd, 2014, 12:19am
Pour in 600 grammes of goo  (scuse the "modern" measures, you'll just have to convert it to U.S. yourself  )
Note how is outgassing when left still. With a pot life of 45mins it can be left stand for a while
Posted by: fred_55 Posted on: Feb 2nd, 2014, 12:23am
And add the 10% hardener.   That total is a bit more than I need, but it needs slightly more than the water used for calculations.
.
Posted by: fred_55 Posted on: Feb 2nd, 2014, 12:30am
And I can report that the half mold has hardened and the second layer is going off nicely.
While I was messing about I tried another SMALL item, a dummy axlebox.  It has a taper, and the tin bowl also has a taper, so it should release easily.  It did and the test piece is nearly right,(needs a bit more trimmig of the joints).
On the plus side the tapers mean the mould doesn't have to be split, but I won't use tin bowl again as it needs to be flexed to release the RTV.
Pete
Posted by: Steam290 Posted on: Feb 2nd, 2014, 9:57am
on Jan 19th, 2014, 6:51am, fred_55 wrote:       (Click here for original message)

 
3D printing is crazy expensive here. I enquired about a headlight, one quote was over $400 and had very  poor definition. Perhaps Moose could get a ballpark, then I might get him to make them!  
Pete

 
I've been going down the 3D printing path. My casting masters for the Burlington # 7 truck have used this technology. And some parts on the truck are plastic and metal 3D printed. I have used Ponoko and Shapeways. Sometimes their prices align, other times there is a large divergence. I have found the most cost effective approach is have your longest dimension on the X axis, next Y, and shortest dimension Z. Cost is substantially different depending on part orientation. And then overall volume matters.
 
Not trying to hijack the thread. But here is an example. The side bearings on my do not need to be metal. They hold a block or roller. I put the 3D drawing together during a lunch hour. Cost for the plastic or aluminide (plastic with aluminum filler) was $4.11 and $4.76 respectively, plus shipping. they are about 0.5" by 0.4 x 0.5". Picture of each below. The pedestals, in rough numbers, 6" x 4" x 1" in plastic are low $50. My brake hanger brackets (red part upper right in second photo) in bronze impregnated stainless were $8+. I see the quote now is $9.48. But it is a little part that would cost about that by the time I went through the lost wax silicon bronze casting process. And that would have a sprue on it that I would have to cut/machine off. It's a small part, hard to hold. I might have to get a casting quote on it, just to compare.  
 
Back to the thread at hand. Here in Atlanta, GA USA, there is a place that carries all kinds of casting materials. http://www.theengineerguy.com/ I have used some of their stuff to do reproduction locomotive trust plates for a full size locomotive.
Posted by: Steam290 Posted on: Feb 2nd, 2014, 9:59am
Here's the truck jpg with the brake hanger.
 
Dale Grice
Posted by: fred_55 Posted on: Feb 4th, 2014, 2:45am
No highjacking, it's all relevant to plastic parts.
I did some comparison from your link. My RTV costs $90 litre,That's a bit over a quart. Engineer guy's platinum cure BC 9060 is $50 a quart. Big difference, especially when you think that the westinghouse mold took over a litre(without considering the epic fail cost)
My polyurathane is $75 a litre ($325 a gallon), engineer guy has Smooth Cast 320 for $40 a Gallon, which makes the excercise worth while on your side of the pond.  
So my finished Westinghouse is $25 for plastic, $90 for mold.
Your link would be about $3 for plastic, $50 for the mold.
Dang..
 
Meanwhile it's fill-em-up and move-em-along
Posted by: fred_55 Posted on: Feb 4th, 2014, 2:51am
And pop-em-out at 30 minute intervals.   Some of the bolt detail could be better, but I figure anybody lying down looking up at a passing wagon might not notice
I gave up on kebab-ing the interior to save material, it just wasn't worth the agro. (although one of them has a mixing stick dropped in it! doh)
Maybe the RP method is better for higher quality stuff.
Pete
Posted by: fred_55 Posted on: Feb 4th, 2014, 6:47pm
Not much on TV tonight
Posted by: BobbyT Posted on: Feb 4th, 2014, 7:47pm
WOW Pete, you keep this up and you will have to build more cars to put these on. From the photos they look like they came out real nice.
Posted by: tomc Posted on: Feb 5th, 2014, 11:03am
Pete,  looks like your trials and tribulations have paid off.  Glad hands next?  Rare Earth Magnets work great to hold and let apart.
 
Tom C.
Posted by: moose_the_caboose Posted on: Feb 7th, 2014, 8:57pm
on Feb 4th, 2014, 7:47pm, BobbyT wrote:       (Click here for original message)
WOW Pete, you keep this up and you will have to build more cars to put these on. From the photos they look like they came out real nice.

 
heck!  keep it up pete...i'll pay the freight on a couple!  i know a couple more guys that would love to have one or two.  just sayin'...they are looking good, when they meet your approval, let us know.
 
moose
Posted by: fred_55 Posted on: Feb 8th, 2014, 3:04am
on Jan 17th, 2014, 10:11pm, moose_the_caboose wrote:       (Click here for original message)

 
 
hi pete!
 
after having a day or two to think about it, all i really need is 1 (one).  for a modest fee, i can have it scanned in as a 3-d shape and then printed in one of several materials.  hi-res printing is coming down in price and there is now a handful of choices for printing materials.  once scanned, the object can be scaled to any desired size.  if this interests you, pm me and we'll work out the details to your satisfaction.  you've done the work, you'll own the file.  you've saved me a lot of work!   keep it coming, this is a wonderful project.
 
 
moose

 
Back to playing trains, eh
 
Result is good enough for me, but not good enough to sell. I don't need any more, and I have the mold, so if you want the pattern, I'll give it to you so you can RP it.  PM me with your address if you interested.
Pete
Posted by: moose_the_caboose Posted on: Feb 8th, 2014, 5:50pm
on Feb 8th, 2014, 3:04am, fred_55 wrote:       (Click here for original message)

 
Back to playing trains, eh
 
Result is good enough for me, but not good enough to sell. I don't need any more, and I have the mold, so if you want the pattern, I'll give it to you so you can RP it.  PM me with your address if you interested.
Pete

 
pete, pm sent!  thanks!
Posted by: fred_55 Posted on: Feb 10th, 2014, 3:24am
It's on the way Moose.  Meawhile I mounted one up on the country coach.
Posted by: tomc Posted on: Feb 10th, 2014, 9:32pm
nice detail.  just enough to say I got brakes
 
Tom C.
Posted by: moose_the_caboose Posted on: Feb 26th, 2014, 5:23pm
on Feb 10th, 2014, 3:24am, fred_55 wrote:       (Click here for original message)
It's on the way Moose.  Meawhile I mounted one up on the country coach.

 
...it's finally here...and it looks great!  thanks pete!  starting to get my ducks in a row so we can get this thing scanned!
 
moose the ecstatic!