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The issues with scratch building


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1 hour ago, TarheelRick said:

Ace, will these industrial suppliers sell the smaller quantities needed by us as individual hobbyists?  I know there are many of these companies that do not want to  bypass their retail markets and many require a business license for tax purposes.  What has been your experiences with them?

Most will.

Smooth-On, for instance, will happily sell "trial size" units of about 2 pounds of material (resin and catalyst), for around $30.

The industrial prototyping (using plastics) I've done over the years has been primarily of small medical devices, components for toys and sporting goods, and hand tools...so my purchases have usually been small quantities.

I've never been required to provide any documentation other than a shipping address.

The rules for online sellers collecting sales tax HAVE changed however (if you buy much on eBay you will have seen many sellers now collect it), but as far as I'm concerned, it's a non-issue. Sales tax on $30 isn't much.

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18 hours ago, peteski said:

Couple of observations:

1. Who says that scale model tires have to be soft?  They can be cast form regular hard urethane resin, then painted to  look like rubber.  This is often the way tires in military models are done.  With the right color and sheen, they often look more like rubber than the standard model car sift vinyl  tires.

2. If you really want then soft, Smooth-On has a range of flexible (soft) urethane resins.  Or even cast them using one  of the harder RTV mold-making materials tinted with black dye.

I am using RTV in small quantities, but it makes the tire too soft. I want them stiffer. Using resins is out of my budget... buying a sherline broke the bank... :(

17 hours ago, Ace-Garageguy said:

I would also caution against trying to reinvent the wheel where chemistry is involved.

Several manufacturers make softer casting resins in a variety of Shore hardnesses, specifically for making things like handle grips, movie special-effects masks and prosthetics, and various kinds of modeling.

Polytek has been my go-to supplier for this kind of stuff for decades, and they also offer a range of dyes that are compatible with their products.

Again, not trying to make them soft, trying to make them harder. I am looking at the chemistry actually. I am using silicone caulking to make my molds and using RTV to make  the tires. But the RTV is too soft. Buying large quantities when I haven't tested a method is pointless and will run me into the ground. Instead, I want to test out several methods before actually investing more money. Hell to get 1 gallon of 2 part resin in canada will cost me close to 200-300$ shipped. Not worth it in my opinion if I can find cheaper alternatives that work. But yes, I am aware of the more expensive tried and tested methods. Unfortunately, I have to go the DIY/scratch building route on the cheap.

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1 hour ago, Ace-Garageguy said:

Most will.

Smooth-On, for instance, will happily sell "trial size" units of about 2 pounds of material (resin and catalyst), for around $30.

The industrial prototyping (using plastics) I've done over the years has been primarily of small medical devices, components for toys and sporting goods, and hand tools...so my purchases have usually been small quantities.

I've never been required to provide any documentation other than a shipping address.

The rules for online sellers collecting sales tax HAVE changed however (if you buy much on eBay you will have seen many sellers now collect it), but as far as I'm concerned, it's a non-issue. Sales tax on $30 isn't much.

Found the "trial-size" of casting resin. Yah, already used this stuff. Expensive for the size. I have to improve my castings first. 

Edited by IbuildScaleModels
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Mixing CA with RTV rubber will not do what you expect.  Use the proper materials Luke!  Like Ace and I mentioned, there are urethane (and even RTV) resins which different hardnesses available.

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 @IbuildScaleModels, out of interest, how soft is “too soft” for a rubber tire?  Presumably any RTV mould material will sustain the weight of about any kit and is tough enough to make a tire that you can stretch into a rim.  What’s the problem that you are running into with them?

Mainly asking because I was just contemplating doing the same thing myself - mix some resin dye with RTV and see if I can make some usable tires!

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On 6/11/2020 at 10:43 PM, CabDriver said:

 @IbuildScaleModels, out of interest, how soft is “too soft” for a rubber tire?  Presumably any RTV mould material will sustain the weight of about any kit and is tough enough to make a tire that you can stretch into a rim.  What’s the problem that you are running into with them?

Mainly asking because I was just contemplating doing the same thing myself - mix some resin dye with RTV and see if I can make some usable tires!

i used kleenflo rtv gasket rubber. It feels too flimsy/spongy versus harder rubber. I know it is all in the preparation. maybe need to degas it or let it cure longer. Don't know... still doing tests.

 

On 6/11/2020 at 2:58 PM, peteski said:

Mixing CA with RTV rubber will not do what you expect.  Use the proper materials Luke!  Like Ace and I mentioned, there are urethane (and even RTV) resins which different hardnesses available.

Pete, duly noted. Again, still testing things out. Also, what is available to me on a limited budget isn't a lot. The 2 part silicone, and urethanes are more expensive in Canada (from what I have found)  unless i buy in large quantities.

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I''ve had satisfactory results making tires with Dow Corning Silastic.  Silastic H is white, and is pretty viscous, but it takes 24 hours to set, so you do have lots of time to get it into the mould.  As far as tinting goes, I think you're probably better off using a dry pigment.  You can buy jars of powdered carbon black from art supply stores, which is the stuff they use in full sized tires.

Here's a tire I made for an armoured car project, with a little carbon black added.

RRACtires.jpg

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2 hours ago, Richard Bartrop said:

I''ve had satisfactory results making tires with Dow Corning Silastic.  Silastic H is white, and is pretty viscous, but it takes 24 hours to set, so you do have lots of time to get it into the mould.  As far as tinting goes, I think you're probably better off using a dry pigment.  You can buy jars of powdered carbon black from art supply stores, which is the stuff they use in full sized tires.

Here's a tire I made for an armoured car project, with a little carbon black added.

RRACtires.jpg

ohhh please share a tutorial on your mold making process!

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58 minutes ago, IbuildScaleModels said:

ohhh please share a tutorial on your mold making process!

The technique is pretty much the same as for casting resin.  You can cast RTV in a RTV mould if you make sure everything is coated in mould release.  Since I wanted a hollow tire,  I made a three part mould instead of the usual two parter.  The stretchiness f the master, and the desired part made this a little easier than it usually be.  The viscosity of Silastic H precludes trying to pour it into the mould, so what I did was fill the mould halves with RTV, then stick them together, and trim any flash once it's set.

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Sorry, I didn't take any photos when I did this,  but I thought I was pretty clear that the stuff was too viscous to pour like resin.  However, if you slowly coax it into the individual mould halves, it will fill the various nooks and crannies.

You only need a three part mould if you are trying to make hollow tires.

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22 hours ago, Richard Bartrop said:

Sorry, I didn't take any photos when I did this,  but I thought I was pretty clear that the stuff was too viscous to pour like resin.  However, if you slowly coax it into the individual mould halves, it will fill the various nooks and crannies.

You only need a three part mould if you are trying to make hollow tires.

Do you still have the molds kicking around? I am looking around for 3 part mold tutorials. I'm just not grasping how to do it for the tires. I would prefer the pour line / tree joint to be on the inside of the tire to leave the tire im casting as much in place as possible. 

 

Maybe im just not understanding how to do it properly and could use your or someone else's assistance in doing so. Hell point me in the direction of a good tutorial on how to do it it even and I'm happy. 

 

Thanks. 

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Gerald Wingrove's book The Complete Car Modelled No1 has a tyre moulding process explained in detail within. The book was written back in the 80's but the techniques have not changed. However, newer moulding materials have been developed since then.

Edited by Bugatti Fan
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On 6/16/2020 at 2:00 AM, Bugatti Fan said:

Gerald Wingrove's book The Complete Car Modelled No1 has a tyre moulding process explained in detail within. The book was written back in the 80's but the techniques have not changed. Howevef, newer moulding materials have been developed since then.

Awesome! thanks Noel... going hunting for the book now

 

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Justin,   Wingrove's book has gone to reprint a few times. I think that the latest publisher was Haynes Publishers in the UK. Not sure if still in print or not, but Amazon or eBay may be a good bet if not. HTH.

He has also written a Complete Car Modeller 2 book that mainly describes his Weinberger Bugatti Royale build.       Regards  Noel

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17 hours ago, Bugatti Fan said:

Justin,   Wingrove's book has gone to reprint a few times. I think that the latest publisher was Haynes Publishers in the UK. Not sure if still in print or not, but Amazon or eBay may be a good bet if not. HTH.

He has also written a Complete Car Modeller 2 book that mainly describes his Weinberger Bugatti Royale build.       Regards  Noel

Thanks Noel. Unfortunately I can't find it in pdf format and will have to order it unless someone can photograph just those pages for me. MY 2 part mold works... sorta... but its the hole that is driving me nuts and I understand that is where the 3 part mold comes in. just driving me batty trying to figure this out. and my new mold test works great... good ol corn starch with silicone to act as a catalyst and hardener. might try it out with the rtv by mixing in mineral spirits to thin it but also mix in a bit of starch to help it become a bit stiffer. Forgot to put a vent port in my mold so that makes it a fail. Good old home supplies to help in the build process! 

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11 hours ago, IbuildScaleModels said:

Thanks Noel. Unfortunately I can't find it in pdf format and will have to order it unless someone can photograph just those pages for me. MY 2 part mold works... sorta... but its the hole that is driving me nuts and I understand that is where the 3 part mold comes in. just driving me batty trying to figure this out. and my new mold test works great... good ol corn starch with silicone to act as a catalyst and hardener. might try it out with the rtv by mixing in mineral spirits to thin it but also mix in a bit of starch to help it become a bit stiffer. Forgot to put a vent port in my mold so that makes it a fail. Good old home supplies to help in the build process! 

Corn starch in your rubber mix?  You sure use some unconventional ingredients Professor Michaud.  :D

If you use mineral spirits to thin the rubber mix, the volume of mineral spirits you added will eventually have to evaporate, and the part you cast will shrink.

Edited by peteski
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11 hours ago, Ace-Garageguy said:

Who knows? He might discover the secret to table-top cold fusion. 

hahaha... I've heard that exact comment too many times. I have also heard I should work for Microsoft or Nasa. Just not interested...

Also the corn starch with the rubber does harden it. Tested it last night. Unfortunately it makes it too brittle. I will try today mixing the starch with the mineral spirits first to make a thick syrup base and then mix the RTV rubber in it to see if it will work. Unfortunately mineral spirits greatly increases the drying time. If I had a vacuum chamber I would be able to do a more accurate test.

Luckily, the corn starch in the silicone makes the mold super solid and gives it great detail. I think I will use a gloss spray coat on the silicone as a mold release instead of a vaseline (petroleum jelly)/mineral spirit mix like they use in cinema prop molds. the petroleum jelly causes the mold to lose some definition as it is adding an extra layer of interference. Maybe if I airbrushed it on it would work better... Just thinking out loud right now.

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1 hour ago, IbuildScaleModels said:

hahaha... I've heard that exact comment too many times. I have also heard I should work for Microsoft or Nasa. Just not interested...

Also the corn starch with the rubber does harden it. Tested it last night. Unfortunately it makes it too brittle. I will try today mixing the starch with the mineral spirits first to make a thick syrup base and then mix the RTV rubber in it to see if it will work. Unfortunately mineral spirits greatly increases the drying time. If I had a vacuum chamber I would be able to do a more accurate test.

Luckily, the corn starch in the silicone makes the mold super solid and gives it great detail. I think I will use a gloss spray coat on the silicone as a mold release instead of a vaseline (petroleum jelly)/mineral spirit mix like they use in cinema prop molds. the petroleum jelly causes the mold to lose some definition as it is adding an extra layer of interference. Maybe if I airbrushed it on it would work better... Just thinking out loud right now.

Like I said, adding solvent to rubber material which normally hardens by a chemical reaction (not by solvent evaporation) will make the final product shrink.  But if you want a solvent, use Naphtha instead of mineral spirits. It is also a petroleum distillate, but it have much faster evaporation rate.  But it will shrink.  I know people that use it specifically to produce molds smaller than the original (shrink it).

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1 minute ago, peteski said:

Like I said, adding solvent to rubber material which normally hardens by a chemical reaction (not by solvent evaporation) will make the final product shrink.  But if you want a solvent, use Naphtha instead of mineral spirits. It is also a petroleum distillate, but it have much faster evaporation rate.  But it will shrink.  I know people that use it specifically to produce molds smaller than the original (shrink it).

From what I read about naphtha is that it is a mineral spirit with higher purity and faster evaporation rate. Doesn't sell here in Montreal from what I saw. 

You can use mineral spirits/naphtha to either shrink or grow a mold. Also silicone molds shrink over time and you soak them in mineral spirits to get them to grow. 

I'll look into it more Pete, thanks for your help! 

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18 minutes ago, IbuildScaleModels said:

From what I read about naphtha is that it is a mineral spirit with higher purity and faster evaporation rate. Doesn't sell here in Montreal from what I saw. 

You can use mineral spirits/naphtha to either shrink or grow a mold. Also silicone molds shrink over time and you soak them in mineral spirits to get them to grow. 

I'll look into it more Pete, thanks for your help! 

In USA it is available in quarts and gallons in hardware stores. It is labeled VM&P Naphtha (Varnish Makers & Painter's Naphta) - a solvent used by painters.  If unavailable, I'm sure you can find Ronsonol lighter fluid, which is pure Naphtha, but at much higher price.  I use Naphta in my hobbies (mainly as a solvent and degreasing fluid).

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