Resin shrinkage

[Chris V]

I decided to post this topic in the Model Building Q&A-section rather than the aftermarket section of the forum, as it is more of a general question concerning the use of resin parts, namely larger parts like car bodies.

Let me start by saying, that as an experienced modeler I am familiar with working with resin and have both built and modified several resin models. 

However I was recently tasked with sorting the collection from the estate of a fellow modeler, sadly no longer among us. Among his kits were a number of relatively recent resin bodies from a well known manufacturer. When I compared some of the more "conservative" conversions to the suggested donor-kits it became apparent that some bodies were considerably smaller: For instance, one resin body was roughly 2 mm (or approximately 2 scale inches) narrower by the cowl than the hood from the kit it was based on and intended for use with.   Likewise I found a resin body that had been cut in half and only one of the halves painted. Comparing the two halves to each other, it was very clear that the painted half had shrunken to a point where it would no longer line up with the other half. In fact, if it hadn't been for the multiple corresponding irregular cut-marks, it would have been hard to tell that the two halves were even from the same body.

Anyway I've previously heard of some Japanese model kits with factory supplied resin body kits, that became unbuildable after just a few years due to shrinkage. This experience makes me wonder how common shrinkage issues are, and whether they're a more common occurrence with some resin manufacturers than others?  It would be a shame to put countless hours of work into modifying and detailing a model - only to see it distort over time...

Edited April 10 by Chris V

[robdebie]

That's an amazing story - I had never heard of resin shrinking after being cast. I do know that polyester can shrink a bit after curing, in boats etc the 'fiber print through' will often develop over a week of so. But I'm guessing these car bodies are polyurethane as usual, and I've never ever read accounts or warnings of shrinkage after curing.

Maybe your account of the two body halves contains a clue: it sounds like the paint caused the shrinkage. But even that I've never heard of.

It's probably no help to you, but here's what I wrote about shrinkage on my webpage about vacuum casting (https://robdebie.home.xs4all.nl/models/casting.htm😞

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Shrinkage

A few words on the subject of shrinkage. Shrinkage can occur both in the mould and in the casting:

• regarding the mould: one of the reasons for choosing addition cure (platinum catalyst) silicone rubber over condensation cure (tin salt catalyst) silicone rubber is that the former has close to zero shrinkage. The latter shrinks during cure, and (amazingly) continues to shrink during its life. Therefore my choice is simple: I use addition cure silicone rubber, and the mould will not show shrinkage.

• regarding the castings: polyurethane resin itself shrinks very little during the curing reaction. However, there's a big 'but': when the resin heats up during the cure, because the cure is exothermic, it expands, pushing out some resin from the mould cavity. When it cools down after the cure reaction, the natural thermal shrinkage occurs, making the part smaller than the mould. The resin's potlife mostly determines the shrinkage: a fast cure means it gets hot, and a slow cure means it will hardly heat up. Fast curing resins can get so hot that they form steam bubbles in the centers of the castings! I use a 7 minute potlife resin (SmoothCast 305), and I never felt a temperature increase in the small parts that I produce. Therefore the resin hardly shrinks. SmoothOn lists less than 0.1%.

I tested the above theory by measuring my largest casting, the ALE-2 chaff pod. The master is 87.5 mm long, and of the five castings that I measured, four were 87.7 mm, and one 87.9 mm. I blame the length increase on temperature differences of the days I cast the mould, and the days I made the castings. Silicone rubber has a large coefficient of thermal expansion: I found values ranging from 200 to 300E-6/°C. Compare that to polystyrene 70E-6/°C and aluminum 21-23E-6/°C. That means a 10 °C difference makes a 0.25% larger silicone rubber mould, and that equals the size increase from 87.5 to 87.7 mm of my ALE-2 pod.

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Rob

[hedotwo]

Very interesting!

[peteski]

Do those resin-cast items have noticeable odor?  I have not experienced any shrinkage of automotive model kits (made using polyurethane resin with no odor), but I have some other model RR castings which I believe were made from polyester resin, which has a strong mothball-like odor, and those have shrunk over time.

[robdebie]

Building on Peteski's comments, maybe you can scrape the resin parts, to release the original smell.

Rob

[peteski]

4 hours ago, robdebie said:

Building on Peteski's comments, maybe you can scrape the resin parts, to release the original smell.

Rob

In my example the resin stinks without need to scrape it. Some of the items I haven't used (or painted over) still stink after 30+ years!

[Chris V]

The bodies in question are cast in regular polyurethane resin by a well known US caster/vendor. There’s absolutely nothing unusual about the color, feel, texture or smell of the resin. They are fairly simple conversions for commonly available kits, but (now) underscaled.

Some years ago I bought some soft resin tires from another, smaller caster. Those tires shrunk considerably as well, but I attribute that to either the resin compound having exceeded its shelf life or the mixing ratio being off, as they gave off fumes that attacked plastic as well (they left a haze on nearby plastic parts and “melted” the wheels just like old Revell tires)m.