A few people have asked me how I have made some N scale vehicles from larger scale masters. Below is a 1948 Chrysler in HO scale, with an N scale copy. I found out about a product called Hydroshrink and how to use it on the 1/87th scale message board: The 87thScale.info messageboard - The Workbench Basically, Hydroshrink is mixed 1:4 with distilled water and quickly poured into a mold. The product sets very quickly, about 15 seconds pot life is all you have to work with. Then, over about a one week period, the water will evaporate out of the part, leaving the cured Hydroshrink part at about 66% of its original size. Presently, the only known US retail source of Hydroshrink is from The Compleat Sculptor, an art supply dealer in New York. It presently sells for $36.99 a quart: The Compleat Sculptor - HydroShrink I carefully peeled the label off of the container to see if I could figure out the source. This container was actually re-labelled from another product called Hydrospan, a similar concept for increasing the size of a casting. Industrial Polymers Corporation doesn't seem to have any direct retail sales, if indeed they are the manufacturer.
The first thing to do is fix any problems with the larger scale master, and put it in a mold box. For this first mold I like to paint my masters with silver enamel paint, as it makes them pop right out of the cured rubber without having to use any mold release. I also try to blend in any bumper guards, and fill in most of the wheel wells with clay. Hydroshrink is not suited for small cross sections as it is pretty soft when cured. Below is a '50 Olds fastback. And here is the RTV mold, this one is from the HO scale 48 Chrysler. Note there is barely any wheel well protruding into the mold.
I learned an awful lot from the 1/87th scale message board on how to work with Hydroshrink. Some of the important things I do: 1) Keep the Hydroshrink and distilled water in the refrigerator until just before you use it. 2) Coating the mold with WD40 and wiping it dry before casting seems to help with small details like side trim. 3) Don't try to mix the Hydroshrink any thinner than 1:4 with water - the resulting casting will be too weak to hold its shape. Anyway, you have to work VERY fast once you start to mix the two together. I like to measure out the water first, and then pour in the Hydroshrink. You MUST mix the two thoroughly, and get rid of any air bubbles, all in about 8 seconds. As you stir them together the mixture will look like milk. You then have about 4-7 seconds to pour the stuff into the mold, so have the mold only inches away from where you are stirring. Do not overfill the mold, as cured Hydroshrink is too soft to be sanded, and too rubbery to be cut. The Hydroshrink part can be removed from the mold in about ten minutes. Below is the HO scale 48 Chrysler, a Hydroshrink casting (Chevrolet) that is about 2 days old, and the completely dried Hydroshrink version of the same 48 Chrysler. Note that the part will curl up like a banana since the water does not evaporate out evenly.
I saw a thread on that somewhere a few weeks ago, and I'm almost tempted to try it. It would sure go a long way towards filling in my need for modern autos.
I think you are limited to shrink things that are primarily solid. Sorry about the picture quality, but here's a Hydroshrink casting that is almost completely dry. You can see there is still some whiteness in the middle of the part. Some people on the 1/87th scale message board say they have been making molds off of the Hydroshrink part before it is completely cured, in order to get the right scale. To me this is asking for trouble, since as you can see the part dries from the outside in, meaning the center will still be swollen a little compared to the ends. In this case (below) the Hydroshrink part is still too large for N scale. I have been using the wheelbase as my guide of scale correctness. To get the rest of the reduction, we need to first make a resin casting of the Hydroshrink part, which I will refer to as mid-scale, since it is somewhere between HO and N scale. To make the mid-scale resin casting, I glue the Hydroshrink part into the mold box with white glue. This time I have to use mold release on the Hydroshrink part. The resin mid-scale copy is shown already painted silver.
Look closely at the roofline of the Hydroshrink part in the previous post and you will see a few bumps. This is due to incomplete mixing of the Hydroshrink and water (remember you have about 8 seconds), so the heavier parts sink to the bottom of the mold when you pour it. One good thing about making the resin mid-scale replica is you can easily sand these things off. I can't think of any way to re-work the Hydroshrink part itself - it is too soft to sand and too rubbery to cut. Plus, you still have a chance to fix any other shape issues with the resin mid-scale part. After getting the 48 Chrysler down completely to N scale, I realized that the front of the car was twisted, and the plane of the front of the car was out of square with the sides of the car. I made two saw cuts in the mid scale resin part - one about 90 % through so that I could give it the Steve Yzerman treatment and get the front end square again, and another cut just behind the headlights so that I could get the twist out. This probably would have been easier on the HO scale original but it wasn't as noticeable on the car I started with.
To get the remaining scale reduction, the next step is to make another RTV mold using the mid-scale resin part as the master. This time the RTV rubber is mixed with Naphtha. The Naphtha will evaporate out of the cured RTV over about 5 days, shrinking the entire mold. The amount of Naphtha used will determine the amount of shrinkage. I think you can get up to at least 20% shrinkage using this method. Here again is our 48 Chrysler mid-scale part in front of the shrunk down RTV mold. For this process I have to paint the mid-scale part with a lacquer based silver as my mold release, otherwise the Naphtha will attack an enamel paint. I try to make these molds as small as possible but this one is a little too small - the sides can distort a little too easily. Why not just make another plain RTV mold and pour a reduced ratio Hydroshrink part?? You can, but the scale reduction you get with Hydroshrink is highly dependant on the water/hydroshrink ratio. When you are mixing 50 cc total, a tiny measurement error will cause a big difference in the reduction. Plus, you get to wait a week to see if you mixed it right or not. With Naphtha, the amount of total reduction you can get is less, but a tiny error in the mixing ratio will not be as noticeable. Naptha/RTV molds have their own learning curve. The largest problem I found was that adding Naphtha will slow down the curing time of the rubber dramatically. As the rubber is curing, the Naphtha will slowly sink to the bottom, and the mold will not shrink uniformly. To get around this, we need the RTV to cure quickly. I couldn't locate any real fast curing RTV, but instead I noticed that Smooth-on sells catalysts that can be added separately. Pictured is their catalyst for tin-cure RTV : The catalyst needs to be added to about 0.5% of the weight of the RTV rubber used. This can be tricky as most scales cannot weigh such a small amount. For example, I am mixing about 1 to 1.5 ounces of RTV for two molds, so 0.5% is less than 0.01 ounces which is all my scale will read. I've had one batch turn solid before I could pour it out of the mixing container, and a few others have had to be re-done because of air pockets forming as I poured them in to the mold. Lately I have been adding 3 drops of catalyst and praying. The steps for mixing a Smooth-on 10:1 RTV with some oddball percentage of Naphtha are as follows: 1) Zero out the scale with the desired mixing cup 2) Pour the desired volume of the RTV part A into the cup. This can be slightly less than the total desired mold volume depending on how much Naphtha you are going to use, but make the volume exactly at one of the reference marks on the mixing cup. 3) Weigh the amount of the RTV part A you have in the cup and write it down somewhere. 4) In a second mixing cup, pour in the exact same volume of Naphtha. Now, if you want some oddball percentage of Naptha, you can simply weigh the Naphtha and remove or add Naphtha to get the desired weight of Naphtha. For example, if you want an 89% volume mix ratio of Naphtha and you have 100 cc of RTV, there are no mixing cups with a mark at 89 CC. But if you know what the Naphtha weighs, it's simple to get the desired volume. I don't recommend trying to simply add the Naphtha right on top of the RTV, as as soon as you pour it in the total volume will not be the sum of the RTV and Naphtha. 5) Once you have the desired weight of Naphtha you can pour it in to the RTV part A. At first it will look like these two are never going to mix together, but just keep gently stirring and after a few minutes you will have a uniform soupy mixture. 6) Now, put the RTV part A / Naphtha soup back on the scale. Look up the weight of the RTV part A that you wrote down and add 10% of that weight of the RTV part B. Mix these together thoroughly - this shouldn't take very long since the mixture is very thin. 7) Make sure your molds are ready, add a few drops of the catalyst. Quickly re-stir the whole batch again as fast as you can, and pour it into the mold(s) I do this all in my spray booth with the blower running - remember Naphtha is flammable.
Everything I see at a swap meet, my first thought is "Can I shrink that down??" There are definite limitations, though. You might be able to do an entire box car if you left it solid. The best quote from the 1/87th message board goes something like "The closer the shape is to a loaf of bread, the better your chances are of reproducing it in hydroshrink". Below, once the Naphtha has evaporated, you can cast a normal part. Using Naphtha will also reduce the tear strength of the RTV, so the number of parts you can cast straight from this mold will be limited. Okay, so how much Naphtha should I mix in?? The relationship is certainly not linear, and it probably also depends on the RTV being used. Here are a few from my notes: 13% reduction = 90% Naphtha 17% reduction = 100% Naphtha 9% reduction = 20% Naphtha 20% reduction = 112% Naphtha Some models I've been able to get them correctly scaled in one go. On one model the Hydroshrink part came out perfect in N scale with no bumps in the roof. On other models, I'm on my third attempt at the right amount of Naphtha. I try to get the wheelbase correct within about 0.010 inches if I can, or make sure the vehicle fits in with the rest of the ones on the street! To get the body hollowed out is simple. Take a Dremel tool and remove everything that is not a '48 Chrysler: I also use files and X-acto knives to open up the window openings. You could then make a two part mold to make the rest of the offspring easier.
Oh yea Rasputeen, just remove everything that's not a '48 Chrysler... Maybe I will finally get that 900 class ATSF steam engine nobody will ever produce commercially; grab a steel bar and just remove everything that's not the 900! Why didn't I think of that... Seriously, this reduction technique just blows my little N scale mind.....when I read the first post I had to check the calendar to make sure it wasn't an early April fools' joke!!! I' sitting here just stunned, with the little wheels just barely beginning to spin, thinking about the possibilities.....Nice work, WOW! Otto
Thanks. So it looks like about $50 worth of materials in addition to normal resin and RTV products and your set. Well worth the price. Now all those great die cast HO cars Walmart sells are within reach.
In all seriousness, you NEED to write an article on this for one of the N scale mags. This is truly fantastic stuff.
I thought about writing an article on several topics, but 1) I have too many other things to do, 2) my photography skills aren't the greatest, and 3) this how-to thread probably totals more key strokes than my total message board entries on all forums to date. One item I keep thinking about shrinking is a cast iron front of an HO scale building, but see reason number 1 above. Oh and another thing about Hydroshrink - it is reported to have a pretty short shelf life once opened, so plan your projects ahead of time.
This is outstanding information and thanks for sharing. I would like to encourage you to rethink writing an article. I'm sure there are TrainBoard members who could help with the editing and photography. Andy Tetsu Uma
One other thought in here as well that I'm sure Rasputen will mention is that Naphtha is extremely flammable and not good to breath! Make sure you have lots of ventilation when working with this stuff! I know first hand from woodworking and how many hoops we jump through at work when it gets spilled.. But aside from that, this is really a cool idea. Thanks for sharing!
Found the Industrial Polymers Corp. site. Hydrospan 400 is the shrinking stuff, 100 is the expanding stuff. HydroSpan 400 Shrinking Resin : Industrial Polymers Note the shrink %age figures given - since it's apparently the same as what is labelled Hydroshrink, the figures there are not correct from what I've found in online demos, including Rasputen's. Hmph. But what the table does seem to mean is that you can fiddle with the mix to try to get the closest possible to N, though that may mean more warping. Then again, consider it might be that slightly less water = less shrinkage in the first molded piece = less/no warping.
A few questions: 1. does the catalyst work with all silicone molds? I have experience with the 1-1 part silicone from Micro-Mark? 2. When cutting the RTV with Naptha, is the table Naptha % to tatal RTV rubber or just part A? Again I ask this because a 10-1 mix rubber is going to be much different than a 1-1 rubber. I have to order more rubber and resin at Micro-Mark soon, so I think I may try this as well.
I'm not a huge expert on RTV Silicones but I know there are basically two types - tin cure, and platinum cure. I think the Micro-Mark one-to-one is also a tin cure type, but I'm not certain. I would definitely test a few drops of catalyst in a mix and see how it hardens before I would risk making a mess over my master part! I'll give it a try sometime and report my findings since I already have some. I would use the Naphtha relationship for the total volume of RTV when using 1:1 mixes. In theory you could start off with a Naphtha/RTV mold and go for 20% shrinkage right off of the larger scale master. I should have done this on the 1/64th scale master I started with, as my dried out Hydroshrink part is still too large for me to fine tune with Naphtha. Steamghost, thanks for clarifying the source. I'd like to try that Hydrospan 100 stuff on some other projects but I think that is even more expensive than the 400.
I have not, but the 1/87th scale info message board members have. The principals are the same, but it would take more mold making supplies. I'm shrinking my first 1/64th scale vehicle, a '50 Olds 88 2 door.
