Some thoughts on resin warping & shrinkage


Tom Madden <tgmadden@...>
 

Can't offer much in the way of solutions to this very interesting
set of problems, but if you'd care to consider some possibly useful
insights, read on….. (Most of you know a lot of this already, but
it's good to get it all down in one place.)

As Pierre says, styrene and cured urethane resin are two different
materials. They have different physical properties, and they respond
to heat in entirely different ways.

Styrene is a thermoplastic material; it can be re-melted and formed
into new shapes many times over. The heat necessary to melt styrene
is applied externally, and styrene solidifies when it cools. Any
shrinkage is due entirely to its coefficient of thermal expansion.
If the injection mold is held at a fixed temperature and the cycle
time is locked in, all parts will match exactly. Subsequent heating
and cooling of a styrene part will not change its physical
properties or dimensions. (As long as you don't melt it!)

Urethane resin is a thermosetting material; two liquid components
(base and catalyst) are mixed together and contained in a mold until
the casting cures. The heat needed to cure the casting is generated
internally, and comes from the chemical reaction between base and
catalyst. The casting solidifies at an elevated temperature when the
reaction "trips" and massive cross-linking occurs. THE TEMPERATURE
AT WHICH THIS TAKES PLACE DEPENDS ON THE AMOUNT OF MATERIAL USED,
AND THE GEOMETRY OF THE PART.

Castings that cure at higher temperatures exhibit greater shrinkage
than castings that cure at lower temperatures. Makes sense, because
they have further to go to get back to room temperature, and
coefficient of thermal expansion is a harsh mistress!

But, you say, that should only be true with different materials.
Surely a given material will always react the same way, release the
same amount of heat energy, and the castings will all cure at the
same temperature, n'est pas??

Mais non, mon ami! Yes, the energy released per unit of material is
always the same, but if the casting is very thin, a lot of that heat
will be dissipated into the mold and be unavailable to cure the
casting. Thin castings cure at lower temperatures and as a result
exhibit lower shrinkage. But they are not fully cured – i.e. not
fully cross-linked. (That's why a casting can be rock-hard, while
the flash surrounding it is still sticky.)

Which brings us to the second source of urethane casting shrinkage:
post-curing of incompletely cured castings, which I suspect is the
cause of our problems. The specs for fully-cured urethane are
derived from thick (from our point of view) slab castings, either
1/8" or 1/4" thick and large enough so that the mix reaches its
maximum attainable temperature when the reaction trips. The
percentage of heat dissipated into the mold is relatively small, so
the resulting samples are fully cured and fully cross-linked. A
freight car side around 0.040" thick has few geometrical
similarities with such a slab, even if it contains the same amount
of resin.

The problem comes because the urethane really wants to be fully
cured. Heating castings to flatten them will help, but if no
additional heat is supplied, the castings will continue to cure at
room temperature, even if it takes many (sometimes many, many)
months. This additional cure is really additional cross-linking, and
as this proceeds the molecules in the castings are pulled closer
together. And the castings continue to shrink. It shouldn't take too
much imagination to visualize an assembled resin freight car body:
slab roof, which is a pretty thick casting to begin with and is most
likely fully cured; two ends which are relatively small compared to
the roof; and two sides, still needing to shrink and will do so with
very noticeable results if they are only restrained at the edges.

One more factor needs to be considered, then we'll wrap up this
exposition. And that is Heat Deflection Temperature, or HDT. This is
the spec'd temperature at which cured urethane deforms, and it is
typically quoted in multiple ways. One might be: "135F (24 hrs RT
cure); 150F (24 hrs RT cure followed by 24 hrs @ 180F)". In this
case the shrinkage would be quoted as "0.001" per inch (RT cure
only); 0.003" per inch (post-cured)". Parts have to be supported
during post-cure so they don't deform.

There are a number of implications to all this. First, you know none
of our hobby resin kit providers are post-curing their castings in
any meaningful way. This is good, because it leaves room for us to
heat and flatten castings when necessary. In fact, it might be
useful to heat large thin castings for a couple hours at 150F or so
as a matter of course, to make post-assembly differential shrinkage
less of an issue. You'd do this before sizing the parts, because
post-curing drives castings to a higher state of cure and introduces
additional shrinkage. Secondly, you cannot repeatedly heat and
reheat castings to modify their shapes and expect them to respond
the same way every time. It's that "higher state of cure" thing
again. Third, Tim's suggestion of bonding true structural members to
the backside of thin castings is a good one. Properly done, the
castings may still want to shrink, but they won't be able to – at
least, not so much as to deform. Fourth, the above applies more to
flat cast parts than to one piece bodies. Yes, all of that can
affect one piece bodies to some extent, but more likely causes of
deformed one piece bodies are removing castings from molds too soon
(which can stretch parts irrevocably), or inadequate support for the
castings when they are shipped or stored. Keeping the HDT in mind,
you'd like the castings in a resin kit to be packed so as to survive
being left in the back of a UPS truck sitting on an asphalt parking
lot in El Centro, CA, over the 4th of July weekend. Think that's
ridiculous? You haven't been in my attic recently. I have all sorts
of styrene kits surviving nicely up there, but all my resin kits are
on shelves in my workshop.

Personally, I prefer thicker sides so deformation isn't an issue.
But urethane resin is expensive, and minimizing the amount used is
one way of controlling costs – so long as manufacturer and consumer
are aware of the problems.

Thanks for your attention!

Tom Madden


Walter M. Clark
 

--- In STMFC@yahoogroups.com, "Tom Madden" <tgmadden@...> wrote:

Can't offer much in the way of solutions to this very interesting
set of problems, but if you'd care to consider some possibly useful
insights, read on….. (Most of you know a lot of this already, but
it's good to get it all down in one place.)<SNIP>.

Thanks for your attention!

Tom Madden
So, Tom and the rest who have commented on this topic, it seems there
are two additional things I need to do to the resin kits before I
build them.

First, at least for the larger pieces such as roof, ends, side, doors
(maybe) and floors/underframes, heat to about 150 F and hold at that
temperature for a couple of hours before matching/adjusting major
dimensions at the start of assembly (which is a problem because I
don't have an oven that will hold that low a temperature. Would it
work to get the oven up to that temperature, established with an oven
thermometer, turn off the gas and put the kit pieces on a sheet of
glass slid into the oven on the bottom rack, then watch the
temperature and when it drops too low turn on the oven for a little
while to bring the temperature back up? Or would that take a chance
on igniting the resin castings? I don't need a resin smoke bomb in
the oven; my wife's patience only stretches so far).

Second, glue braces inside the sides to remind the sides they are
supposed to stay flat. How about if I use a heavier brass H or
channel along the lower portion of the side and a lighter brass angle
near the top? Would that both keep the sides flat and add weight,
concentrated near the bottom so the car doesn't get top heavy? Of
course I need to be sure the final assembly including trucks, couplers
etc. isn't too heavy, but that is easy to handle.

Of course open top cars wouldn't receive the inner bracing. Would
that really matter too much since hoppers and gondolas don't get too
far from the erecting floor before becoming bent, battered and abused
in service (just don't let it get so bad that it becomes a caricature
and not a representation).

So, what am I missing/where did I go wrong/what suggestions for
improvement can you give me?

Walter M. Clark
Time stopped in November 1941
Riverside, California


Tom Madden <tgmadden@...>
 

So, Tom and the rest who have commented on this topic, it seems
there are two additional things I need to do to the resin kits
before I build them.
Is there really that much of a problem? I've not heard rumblings of
shrinkage-induced warping before this exchange. I've never
experienced it in the Westerfield and Sunshine kits I've assembled,
nor in the castings I've done for various manufacturers and
historical societies. I will admit to never having built a model
featuring what I'd consider "very thin" large parts, except for the
NP stock cars I did for Northern Specific models - and I use an
industrial resin which has to be pressure-cured and has better
properties than the resins most hobby manufacturers use. I've seen
many "dished" car side castings, but that's a molding issue that
results in parts of non-uniform thickness. That's different from
shrinkage-induced warping of an assembled car, and my previous post
was a "teach a man to fish" attempt to help list members better
understand the casting process so they can identify, understand and
resolve problems when they occur.

First, at least for the larger pieces such as roof, ends, side,
doors (maybe) and floors/underframes, heat to about 150 F and hold
at that temperature for a couple of hours before
matching/adjusting major dimensions at the start of assembly
(which is a problem because I don't have an oven that will hold
that low a temperature. Would it work to get the oven up to that
temperature, established with an oven thermometer, turn off the
gas and put the kit pieces on a sheet of glass slid into the oven
on the bottom rack, then watch the temperature and when it drops
too low turn on the oven for a little while to bring the
temperature back up? Or would that take a chance on igniting the
resin castings? I don't need a resin smoke bomb in
the oven; my wife's patience only stretches so far).
There's nothing magic about 150 degrees. I routinely run HDT tests
over 200F. The dogbone samples I work with may go completely limp
and collapse, but they don't melt, don't shrivel and deform, and
don't char at that temperature. If potential shrinkage is a concern,
there'd be nothing wrong with pre-heating your oven to 180, popping
your parts on a flat glass plate and putting that in the oven, then
turning it off. In an off-list email Richard B. said the kit that
gave him problems smelled like raw resin when he opened it. That
certainly sounds like incompletely cured castings, which would just
as certainly benefit from additional heat curing. But as a general
process for all castings? Not unless there's a compelling reason to
do so - like a noticeable odor, or castings that feel too soft, or
you've had bad experiences with a particular manufacturer's castings.

Second, glue braces inside the sides to remind the sides they are
supposed to stay flat. How about if I use a heavier brass H or
channel along the lower portion of the side and a lighter brass
angle near the top? Would that both keep the sides flat and add
weight, concentrated near the bottom so the car doesn't get top
heavy?
Now you're talking! I haven't tried it, but K&S makes lengths of
square brass tubing that would be perfect for that. Light, very
strong, with nice flat bonding surfaces.

So, what am I missing/where did I go wrong/what suggestions for
improvement can you give me?
Walter, I do my best to explain processes so you can draw your own
conclusions on how to proceed. The members of this list,
individually and collectively, are amazingly talented and you & they
deserve better than "do this, this and this" cookbook solutions.
Those get you past problems without letting you understand what
caused them. I'll be interested in hearing if others feel this is a
widespread problem, or just a few bad apples. In the meantime,
additional internal bracing sounds like a good idea.

Tom Madden


Paul Lyons
 

Tom,



I am over a hundred completed resin kits from all manufacturers and I have never had the problem. I also, always internally brace my resin models.

Paul Lyons
Laguna Niguel, CA

-----Original Message-----
From: Tom Madden <tgmadden@worldnet.att.net>
To: STMFC@yahoogroups.com
Sent: Mon, 23 Jul 2007 11:44 pm
Subject: [STMFC] Re: Some thoughts on resin warping & shrinkage







So, Tom and the rest who have commented on this topic, it seems
there are two additional things I need to do to the resin kits
before I build them.
Is there really that much of a problem? I've not heard rumblings of
shrinkage-induced warping before this exchange. I've never
experienced it in the Westerfield and Sunshine kits I've assembled,
nor in the castings I've done for various manufacturers and
historical societies. I will admit to never having built a model
featuring what I'd consider "very thin" large parts, except for the
NP stock cars I did for Northern Specific models - and I use an
industrial resin which has to be pressure-cured and has better
properties than the resins most hobby manufacturers use. I've seen
many "dished" car side castings, but that's a molding issue that
results in parts of non-uniform thickness. That's different from
shrinkage-induced warping of an assembled car, and my previous post
was a "teach a man to fish" attempt to help list members better
understand the casting process so they can identify, understand and
resolve problems when they occur.

First, at least for the larger pieces such as roof, ends, side,
doors (maybe) and floors/underframes, heat to about 150 F and hold
at that temperature for a couple of hours before
matching/adjusting major dimensions at the start of assembly
(which is a problem because I don't have an oven that will hold
that low a temperature. Would it work to get the oven up to that
temperature, established with an oven thermometer, turn off the
gas and put the kit pieces on a sheet of glass slid into the oven
on the bottom rack, then watch the temperature and when it drops
too low turn on the oven for a little while to bring the
temperature back up? Or would that take a chance on igniting the
resin castings? I don't need a resin smoke bomb in
the oven; my wife's patience only stretches so far).
There's nothing magic about 150 degrees. I routinely run HDT tests
over 200F. The dogbone samples I work with may go completely limp
and collapse, but they don't melt, don't shrivel and deform, and
don't char at that temperature. If potential shrinkage is a concern,
there'd be nothing wrong with pre-heating your oven to 180, popping
your parts on a flat glass plate and putting that in the oven, then
turning it off. In an off-list email Richard B. said the kit that
gave him problems smelled like raw resin when he opened it. That
certainly sounds like incompletely cured castings, which would just
as certainly benefit from additional heat curing. But as a general
process for all castings? Not unless there's a compelling reason to
do so - like a noticeable odor, or castings that feel too soft, or
you've had bad experiences with a particular manufacturer's castings.

Second, glue braces inside the sides to remind the sides they are
supposed to stay flat. How about if I use a heavier brass H or
channel along the lower portion of the side and a lighter brass
angle near the top? Would that both keep the sides flat and add
weight, concentrated near the bottom so the car doesn't get top
heavy?
Now you're talking! I haven't tried it, but K&S makes lengths of
square brass tubing that would be perfect for that. Light, very
strong, with nice flat bonding surfaces.

So, what am I missing/where did I go wrong/what suggestions for
improvement can you give me?
Walter, I do my best to explain processes so you can draw your own
conclusions on how to proceed. The members of this list,
individually and collectively, are amazingly talented and you & they
deserve better than "do this, this and this" cookbook solutions.
Those get you past problems without letting you understand what
caused them. I'll be interested in hearing if others feel this is a
widespread problem, or just a few bad apples. In the meantime,
additional internal bracing sounds like a good idea.

Tom Madden





________________________________________________________________________
AOL now offers free email to everyone. Find out more about what's free from AOL at AOL.com.


Denny Anspach <danspach@...>
 

Paul Lyons writes-

I am over a hundred completed resin kits from all manufacturers and I have never had the problem. I also, always internally brace my resin models.
Paul leads a charmed life, indeed, especially if he is referring only to one piece resin house cars.

Out of the last five one-piece house car kits that I have purchased, two have been seriously warped, one yet to be tackled, and one to the extent that it had to be internally braced with a whole forest of styrene shapes- the process of which exhausted my supply of small clamps (and it still has a warp- hopefully disguised by the weathering and details).

Routinely internally bracing resin house cars is probably a very good idea.

Denny



--
Denny S. Anspach, MD
Sacramento


Paul Lyons
 

Denny,



I have never had a problem as the one Tom Madden was describing with shrinkage from resin not totally cured. I have had more than my share of distorted castings (for whatever reason, but probably heat) in?both one piece bodies and flat kits. If you would try the hair dryer on your one piece body as?I suggested earlier, your clamps would still be in the toolbox and your styrene supply would not be depleted.



Paul Lyons

Laguna Niguel, CA

-----Original Message-----
From: Denny Anspach <danspach@macnexus.org>
To: STMFC@yahoogroups.com
Sent: Tue, 24 Jul 2007 8:27 pm
Subject: [STMFC] Re: Some thoughts on resin warping & shrinkage







Paul Lyons writes-

I am over a hundred completed resin kits from all manufacturers and
I have never had the problem. I also, always internally brace my
resin models.
Paul leads a charmed life, indeed, especially if he is referring only
to one piece resin house cars.

Out of the last five one-piece house car kits that I have purchased,
two have been seriously warped, one yet to be tackled, and one to
the extent that it had to be internally braced with a whole forest of
styrene shapes- the process of which exhausted my supply of small
clamps (and it still has a warp- hopefully disguised by the
weathering and details).

Routinely internally bracing resin house cars is probably a very good idea.

Denny

--
Denny S. Anspach, MD
Sacramento




________________________________________________________________________
AOL now offers free email to everyone. Find out more about what's free from AOL at AOL.com.