Date
1 - 13 of 13
RTV Rubber Mold/Resin Casting shrinkage (was Re: 3D Printed ATSF Tank Cars)
|
Gene <bierglaeser@...>
Tom,
You mentioned shrinkage in the RTV rubber mold and resin casting processes. Others have mentioned shrinkage as well. It must be significant if it is mentioned so often. Is there a given size or volume above which shrinkage matters? Does the rubber mold shrink? Do resin castings shrink a predictable amount or are we talking trial and error? Is shrinkage expressed as a per cent? Do various resin concoctions shrink at different rates or all the same? Gene Green |
|
|
|
Gene <bierglaeser@...>
I've been thinking about my questions below. What I should have asked is, "Who can tell me about shrinkage and what should I know?"
toggle quoted message
Show quoted text
--- In STMFC@..., "Gene" <bierglaeser@...> wrote:
|
|
|
|
Tom Vanwormer
Gene,
toggle quoted message
Show quoted text
I know one producer of resin cast cars who made his first first master 3% oversize only to have the caster tell him after the first run was cast that there was no shrinkage in the process. If you are going to produce resin kits, I would recommend you talk directly with your caster to get those answers. Tom VanWormer Monument CO Gene wrote:
|
|
|
|
Tom Madden
Gene, it depends on the rubber/resin combination. They're all different, and it will be on the materials' spec sheets. I use a low-shrink resin that's more expensive than what many use, and it and the rubber I use (Silicone Inc.'s GI-1000) have a combined shrinkage of 0.004" per inch. That's less than half a percent. A fast resin that cures hot will have much greater shrinkage. It may reach 200 degrees F when it "kicks" and cures, then shrink as it cools down to room temperature. (Coefficient of thermal expansion and all that.) The temperature(s) at which you make and use a mold can have a greater effect than mere resin & rubber shrinkage. Cured silicone rubber has a larger coefficient of expansion than cured urethane resin. If you make a mold in the winter when your room temp is 68 and use it during the summer when it's 80, the castings may well be bigger than the master.
toggle quoted message
Show quoted text
Nothing is as simple as it looks.... Tom Madden --- In STMFC@..., "Gene" <bierglaeser@...> wrote:
|
|
|
|
Gene – I agree with Tom; it’s trial and error. For my primary production master I used a very hard rubber to reduce sink. However, the mold would shrink between the first and second casting and die at 4 or 5. So I would use the second and third castings to match for the production master pair of sides, etc. For production molds I found one that had very little shrink. Since I used heat to cure the castings in the molds to reduce turnaround time, and a controlled environment, there was little variation between castings until the molds were nearing there end-life when stretching of the molds from repeated casting removal became a factor. As I’ve said before, I cannot recommend particular products because they’re trade secrets. – Al
From: pullmanboss Sent: Wednesday, July 24, 2013 12:54 AM To: STMFC@... Subject: [STMFC] RTV Rubber Mold/Resin Casting shrinkage (was Re: 3D Printed ATSF Tank Cars) Gene, it depends on the rubber/resin combination. They're all different, and it will be on the materials' spec sheets. I use a low-shrink resin that's more expensive than what many use, and it and the rubber I use (Silicone Inc.'s GI-1000) have a combined shrinkage of 0.004" per inch. That's less than half a percent. A fast resin that cures hot will have much greater shrinkage. It may reach 200 degrees F when it "kicks" and cures, then shrink as it cools down to room temperature. (Coefficient of thermal expansion and all that.) The temperature(s) at which you make and use a mold can have a greater effect than mere resin & rubber shrinkage. Cured silicone rubber has a larger coefficient of expansion than cured urethane resin. If you make a mold in the winter when your room temp is 68 and use it during the summer when it's 80, the castings may well be bigger than the master. Nothing is as simple as it looks.... Tom Madden --- In mailto:STMFC%40yahoogroups.com, "Gene" <bierglaeser@...> wrote:
[Non-text portions of this message have been removed] |
|
|
|
soolinehistory <destorzek@...>
--- In STMFC@..., "Gene" <bierglaeser@...> wrote:
Gene, It's been a very long time since I've had to deal with this, and you've gotten some good answers already, but I'll throw out some thoughts... All the "industrial grade" mold and casting materials will list their average shrinkage in their tech literature, but this is usually not the case for hobby grade materials. As I recall, shrinkage for each material was <1%, but this can add up when using multiple generations of cast sub-masters. The typical hobbyist makes a mold from a pattern, compares a part cast in the mold with the original master, can't see any real difference, and says, "the process has no shrink." They don't log the dimensions over the life of the mold, so never see the variance that is possible. As I recall, most of the mold making materials expressed their shrinkage as as a value at a specific time interval after the mold was made. That is not to say that the material stops shrinking, far from it. I learned early on not to try to save rubber molds; make them, then run them to death over the normal production cycle, and store the parts. Certain solvents also cause certain RTV rubbers to swell, which causes the mold cavity to grow in size. This was definitely a problem when using the polyester resins that were used to produce early resin kits. I ran some test parts early on, cycling the mold as fast as the parts solidified, and found the third part was one scale foot longer than the first. Given the part was 26 scale feet long, that's almost 4% GROWTH. The polyurethane resins everyone adopted shortly after are not as bad, but I've always been suspicious of Alumilite, since it has volatile fractions that tend to boil off under vacuum. Given all the variables, it's not so much that the process has no shrink as that the shrinkage is indeterminate. If you can't quantify it, you can't allow for it, so people build masters full size and live with the result. Dennis Storzek |
|
|
|
Gene <bierglaeser@...>
--- In STMFC@..., Tom VanWormer <robsmom@...> wrote:
<snip> If you are going to produce resin kits, I would recommend you talk directly with your caster to get those answers. <snip>No plan to produce kits. Just thinking of maybe making some parts, etc. for myself. Gene Green |
|
|
|
Gene <bierglaeser@...>
Thanks, Tom. Very interesting. I've seen "consumer" sized packaging of the rubber and resin for modelers but I don't recall specifications. Maybe inside the package, huh? May I assume that the specific rubber (Silicone Inc.'s GI-1000) mentioned must be purchased in "industrial" quantities?
toggle quoted message
Show quoted text
In my situation I can "dial in" the desired room temperature and humidity. That came about merely because I didn't want wood bench work to expand and contract with changes in humidity, not that such changes are extreme in this area anyway. Right now it appears that in my situation trial and error will likely get me to my goal sooner than anything else. I had no idea resin could get so hot in the curing process. One could get burned. Gene Green --- In STMFC@..., "pullmanboss" <pullmanboss@...> wrote:
|
|
|
|
Gene <bierglaeser@...>
Dennis,
Thanks for your input. Both you and Al W. mentioned changes in the size of the rubber mold over the course of its use. THAT is really a surprise. Your last sentence - "If you can't quantify it, you can't allow for it, so people build masters full size and live with the result." - may well sum up the situation in which I would find myself. In this case and that of 3-D printing, why can't things be simple? VBG Gene Green |
|
|
|
Jack Mullen
Gene,
toggle quoted message
Show quoted text
I'm coming from the same direction you are - thinking of producing parts primarily for my own use. My take-home on this is that it would be useful to pick some reference dimensions on the master and measure the same on each casting produced, keeping a table of the data. It would aid one's learning curve, if nothing else. Jack Mullen --- In STMFC@..., "Gene" <bierglaeser@...> wrote:
|
|
|
|
Rio Grande Ltd <rgmodels@...>
I have been casting over 35 years and never have turned down a inquiry as to how to do something or what products I use. The more competition the better.
toggle quoted message
Show quoted text
Eric Bracher Rio Grande Models As I’ve said before, I cannot recommend particular products because they’re trade secrets. – Al -----Original Message-----
From: westerfieldalfred <westerfieldalfred@...> To: STMFC <STMFC@...> Sent: Wed, Jul 24, 2013 5:28 am Subject: Re: [STMFC] RTV Rubber Mold/Resin Casting shrinkage (was Re: 3D Printed ATSF Tank Cars) Gene – I agree with Tom; it’s trial and error. For my primary production master I used a very hard rubber to reduce sink. However, the mold would shrink between the first and second casting and die at 4 or 5. So I would use the second and third castings to match for the production master pair of sides, etc. For production molds I found one that had very little shrink. Since I used heat to cure the castings in the molds to reduce turnaround time, and a controlled environment, there was little variation between castings until the molds were nearing there end-life when stretching of the molds from repeated casting removal became a factor. As I’ve said before, I cannot recommend particular products because they’re trade secrets. – Al From: pullmanboss Sent: Wednesday, July 24, 2013 12:54 AM To: STMFC@... Subject: [STMFC] RTV Rubber Mold/Resin Casting shrinkage (was Re: 3D Printed ATSF Tank Cars) Gene, it depends on the rubber/resin combination. They're all different, and it will be on the materials' spec sheets. I use a low-shrink resin that's more expensive than what many use, and it and the rubber I use (Silicone Inc.'s GI-1000) have a combined shrinkage of 0.004" per inch. That's less than half a percent. A fast resin that cures hot will have much greater shrinkage. It may reach 200 degrees F when it "kicks" and cures, then shrink as it cools down to room temperature. (Coefficient of thermal expansion and all that.) The temperature(s) at which you make and use a mold can have a greater effect than mere resin & rubber shrinkage. Cured silicone rubber has a larger coefficient of expansion than cured urethane resin. If you make a mold in the winter when your room temp is 68 and use it during the summer when it's 80, the castings may well be bigger than the master. Nothing is as simple as it looks.... Tom Madden --- In mailto:STMFC%40yahoogroups.com, "Gene" <bierglaeser@...> wrote: [Non-text portions of this message have been removed] [Non-text portions of this message have been removed] |
|
|
|
soolinehistory <destorzek@...>
--- In STMFC@..., Rio Grande Ltd <rgmodels@...> wrote:
Since Al just recently sold his line, he likely is signed into an agreement not to disclose processes, so cut the guy some slack. Back when resin casting was just beginning, Al Westerfield was one of the guys who published a LOT of articles about resin casting, and is likely responsible for many others entering the field with products. Dennis Storzek |
|
|
|
Tom Madden
It's like a reverse catch-22. If your resin casting skills haven't developed past the hobby level, knowing what professional-grade materials are used by commercial casters won't help you at all. Any resin and rubber will serve for practicing the craft. You learn and improve as you go along. Once you do know how to make molds and cast efficiently with, say, 95% casting yield and minimal waste, you'll realize you don't have to know what materials the other guys use. You'll have figured out what works for you, and you'll go with that.
toggle quoted message
Show quoted text
I use industrial materials. A 5-gallon bucket of silicone rubber costs over $400, and 8 gallons of my principal casting resin (4 gallons each of Parts A and B) costs $750 delivered. Those are daunting figures if you're just looking to cast a couple of boxcar doors. Go with what you can find at Micro Mark or a local craft supply store. As the old saying goes, "walk first, then run." Tom Madden --- In STMFC@..., "soolinehistory" <destorzek@...> wrote:
|
|
|