Casting or epoxy rein

[customsrus]

Which is better to cast small parts. Casting resin or epoxy resin. 

[Ace-Garageguy]

It depends on the size of the parts, primarily, but there are other variables to consider.

In general:

"Casting resin" is usually formulated to reduce runaway exothermic reactions that can easily occur if parts the size of an engine block (in 1/24) are cast in a warm environment.

Runaway exotherms happen when the heat generated by the resin and hardener doing their chemical thing go into a "feedback loop", with more heat speeding up the cure, making more heat speeding the cure, making more heat... etc.

The effect is anything from severe bubbling throughout the casting, to the whole mess bursting into flame.

I've had it happen.

Casting resin is also thinner, less viscous, too...which helps to fill voids and air pockets, and release air bubbles introduced during mixing.

Though you "can" cast parts with epoxies designed to be used as adhesives, you're better off using "casting resins" for casting.

OVERVIEW OF "EXOTHERM":

https://www.systemthree.com/blogs/epoxy-files/epoxy-exothermic-reaction

All that said, most molds for model car parts today are made from silicone, and the parts cast in them are usually made of some variety of urethane.

Most of the manufacturers of the materials you'll need have GOOD videos of how to choose and use their products, and they're much more complete and useful than any advice you'll get here.

Edited December 3, 2023 by Ace-Garageguy

[ColonelKrypton]

@Ace-Garageguy 's advise is very good.

If it were me I would use a casting resin for model parts as they don't need the strength of epoxies.

I like Smooth On products https://www.smooth-on.com/applications/  lots of good information on their web site

Also, Alumilite has some good products as well.  https://www.alumilite.com/  They have a line called Amazing that is aimed at crafters and hobbyists.  Amazing Mold Maker and Amazing Casting Resin are two and have worked well for my simple needs.

If price is an issue and you are only making a few small simple parts the common epoxy and even polyester resin will work in a pinch.

cheers, Graham

Edited December 3, 2023 by ColonelKrypton
tinger frouble

[customsrus]

11 minutes ago, Ace-Garageguy said:

It depends on the size of the parts, primarily, but there are other variables to consider.

In general:

"Casting resin" is usually formulated to reduce runaway exothermic reactions that can easily occur if parts the size of an engine block (in 1/24) are cast in a warm environment.

Runaway exotherms happen when the heat generated by the resin and hardener doing their chemical thing go into a "feedback loop", with more heat speeding up the cure making more heat speeding the cure making more heat etc.

The effect is anything from severe bubbling throughout the casting, to the whole mess bursting into flame.

I've had it happen.

Casting resin is also thinner, less viscous, too...which helps to fill voids and air pockets, and release air bubbles introduced during mixing.

Though you "can" cast parts with epoxies designed to be used as adhesives, you're better off using "casting resins" for casting.

OVERVIEW OF "EXOTHERM":

https://www.systemthree.com/blogs/epoxy-files/epoxy-exothermic-reaction

All that said, most molds for model car parts today are made from silicone, and the parts cast in them are usually made of some variety of polyurethane.

I’ve used alumilite in the past and some good results. I’m just wanting to do small parts such as wheels, motor parts, etc. 

[Ace-Garageguy]

2 minutes ago, customsrus said:

I’ve used alumilite in the past and some good results. I’m just wanting to do small parts such as wheels, motor parts, etc. 

There are lots of Alumilite products. https://www.alumilite.com/products/resins/

[peteski]

Most resins used for casting are usually polyurethane-based.  But both 2-part Urethane and Epoxy generate heat when they are curing (the exothermic reaction). Also, since (2-part) polyester resin was mentioned, that also cures by an exothermic reaction.

AFAIK, all 2-part chemically-cured resins are exothermic (they generate heat while curing).

Edited December 3, 2023 by peteski

[Ace-Garageguy]

9 minutes ago, peteski said:

AFAIK, all 2-part chemically-cured resins are exothermic (they generate heat while curing).

The problem, as I mentioned above, is RUNAWAY exothermic reactions when materials are used by the untrained or inexperienced.

 

[Ace-Garageguy]

 

[peteski]

6 minutes ago, Ace-Garageguy said:

The problem, as I mentioned above, is RUNAWAY exothermic reactions when materials are used by the untrained or inexperienced.

Sure, I understand.  But for the relatively small items we cast (even as large as a full 1:25 scale car body), I don't believe that runaway reaction is even something that could happen.  I know that with all the small parts I cast, I never had the molds get more than lukewarm using urethane, and even less so using epoxy.  You are looking at this through the eyes of someone who has done casting on a very large scale.

I actually have more of this issue with CA glue and accelerator (which also sets through an exothermic reaction).  I have had it get hot and even bubble up.

[Ace-Garageguy]

1 hour ago, peteski said:

Sure, I understand.  But for the relatively small items we cast (even as large as a full 1:25 scale car body), I don't believe that runaway reaction is even something that could happen. I know that with all the small parts I cast, I never had the molds get more than lukewarm using urethane, and even less so using epoxy.  You are looking at this through the eyes of someone who has done casting on a very large scale.

Well, you're wrong.

Car bodies are thin sections and as such usually bleed enough heat into the adjacent mold to prevent a runaway.

A 1/24 scale engine block is a relatively thick section and CAN runaway exotherm...and so can the container the material is mixed in if the user isn't careful...which is why material designed to be runaway-exotherm-resistant is preferable.

I've had as little as a 14 gram mix of epoxy get hot, smoke, and sputter violently.

FOURTEEN GRAMS is hardly "looking...through the eyes of someone who has done casting on a very large scale". 

 

 

Edited December 4, 2023 by Ace-Garageguy

[Oldriginal86]

I’ve used West Systems in the marine world for years. I’ve also used it to mold small parts for modeling. Just last weekend my son and I repaired his golf club with epoxy, one pump from each container as recommended. The left over mix was left outside on the deck so as not to heat up in the trash. The cup was melted when I checked it later. The amount left over was about the size of a 1/24 engine. Whatever is used be careful and use common sense and show pics of the final product.

[robdebie]

11 hours ago, customsrus said:

Which is better to cast small parts. Casting resin or epoxy resin. 

I've used both, and I was always impressed how much harder, stiffer and stronger epoxy is. But epoxy 'eats' the molds much quicker, and casting time is generally much longer. Therefore it remains the less practical option. I used epoxy mostly for parts that required the extra strength (like the bracket below) or transparancy. For the latter I bought a special epoxy, with a pot life of 10 hours, and a cure time of 4 days. Talk about impractical! But the parts came out very nice. The second photo shows the original kit part plus the epoxy casting.

Rob

[Rick L]

Generally, the longer the pot life the less heat will be produced. If you’re working with small parts with thin walls such as small scale model car parts you shouldn’t have any issues with the castings. The mixing cup on the other hand may be an issue if it’s a fast curing plastic or epoxy such as the reaction Bill experienced. If you’re unsure of the reaction you may get just cast the part and leave the mold and mixing cup outside.

[Ace-Garageguy]

3 hours ago, Rick L said:

Generally, the longer the pot life the less heat will be produced. If you’re working with small parts with thin walls such as small scale model car parts you shouldn’t have any issues with the castings. The mixing cup on the other hand may be an issue if it’s a fast curing plastic or epoxy such as the reaction Bill experienced. If you’re unsure of the reaction you may get just cast the part and leave the mold and mixing cup outside.

This is generally the case.

In the particular case I mentioned, the epoxy was MGS 285 with the slow 287 hardener. It's a high-performance material certified for use on several aircraft (most German sailplanes and the Cirrus SR20-SR22 for example). MGS 285 has a nominal pot-life of 45 minutes to 4 hours depending on hardener. It reaches full room-temp cure in 24 hours, and full strength after an additional 24 hour elevated-temperature post-cure cycle.

In critical aviation use, it HAS to be mixed on a gram-scale to a ratio of 100:40 by weight (100:50 by volume) accurate to within about 2 percent...or it doesn't reach its full strength (and for aircraft use, we often have to have "test coupons" from EVERY mix validated by an FAA-approved lab, and the test results included in the paperwork before the aircraft can be signed-off and returned to service).

I also use it for modeling and a wide variety of real-car applications because of its exceptional strength, and always mix it on an accurate scale. It allows me to make almost-scale-thickness fiberglass model-car panels that are many times stronger than styrene, and are stable up to 176 deg F if post-cured properly.

10 grams of 285 plus 4 grams of hardener is about as small a quantity you can mix with reasonable accuracy for non-aviation work, so after I mixed that amount and used less than half (on a warm, humid day), I forgot about what was still in the cup and walked away. I heard "sizzling" coming from the bench area, and turned around to see a plume of smoke rising from the sputtering mass of resin. Luckily it wasn't sitting on anything flammable.

The actual lamination I'd used the stuff on was just fine, because it was spread over a large enough area so any heat from curing dissipated quickly.

Lesson one is that thicker masses, even small masses, may runaway-exotherm. It's easy enough to avoid if leftover material is simply poured into a flat pan with enough surface area so heat isn't concentrated in a confined volume like a mixing cup.

Metal paint pans work well for smaller amounts, and metal under-car drip-pans work well if you have more waste to deal with.

Lesson two is try to not mix more material than you can use within the pot-life window, and pay attention.

EDIT: Lesson three is that higher ambient temperatures and higher relative humidity will have a very noticible effect on shortening pot-life...so PAY ATTENTION

 

Edited December 6, 2023 by Ace-Garageguy

[Rick L]

6 hours ago, Ace-Garageguy said:

This is generally the case.

In the particular case I mentioned, the epoxy was MGS 285 with the slow 287 hardener. It's a high-performance material certified for use on several aircraft (most German sailplanes and the Cirrus SR20-SR22 for example). MGS 285 has a nominal pot-life of 45 minutes to 4 hours depending on hardener. It reaches full room-temp cure in 24 hours, and full strength after an additional 24 hour elevated-temperature post-cure cycle.

In critical aviation use, it HAS to be mixed on a gram-scale to a ratio of 100:40 by weight (100:50 by volume) accurate to within about 2 percent...or it doesn't reach its full strength (and for aircraft use, we often have to have "test coupons" from EVERY mix validated by an FAA-approved lab, and the test results included in the paperwork before the aircraft can be signed-off and returned to service).

I also use it for modeling and a wide variety of real-car applications because of its exceptional strength, and always mix it on an accurate scale. It allows me to make almost-scale-thickness fiberglass model-car panels that are many times stronger than styrene, and are stable up to 176 deg F if post-cured properly.

10 grams of 285 plus 4 grams of hardener is about as small a quantity you can mix with reasonable accuracy for non-aviation work, so after I mixed that amount and used less than half (on a warm, humid day), I forgot about what was still in the cup and walked away. I heard "sizzling" coming from the bench area, and turned around to see a plume of smoke rising from the sputtering mass of resin. Luckily it wasn't sitting on anything flammable.

The actual lamination I'd used the stuff on was just fine, because it was spread over a large enough area so any heat from curing dissipated quickly.

Lesson one is that thicker masses, even small masses, may runaway-exotherm. It's easy enough to avoid if leftover material is simply poured into a flat pan with enough surface area so heat isn't concentrated in a confined volume like a mixing cup.

Metal paint pans work well for smaller amounts, and metal under-car drip-pans work well if you have more waste to deal with.

Lesson two is try to not mix more material than you can use within the pot-life window, and pay attention.

EDIT: Lesson three is that higher ambient temperatures and higher relative humidity will have a very noticible effect on shortening pot-life...so PAY ATTENTION

 

Also note that some casting materials, particularly polyurethanes, need to be cast at higher temps in order for their properties to work. Or they become brittle when fully cured. This mold is set in an oven preheated to 150 degrees before injection cast with 2 part urethane mix.

[customsrus]

Thanks everyone for your comments. 

[Ace-Garageguy]

On 12/6/2023 at 5:36 PM, Rick L said:

Also note that some casting materials, particularly polyurethanes, need to be cast at higher temps in order for their properties to work. Or they become brittle when fully cured. This mold is set in an oven preheated to 150 degrees before injection cast with 2 part urethane mix.

Fascinating. I wasn't aware of that. All the urethane-based products I've ever worked with are used at room temp. This is definitely something I'll follow up on.

What are those...housings?...for, anyway?

[ColonelKrypton]

On 12/6/2023 at 11:12 AM, Ace-Garageguy said:

mixed on a gram-scale

Mixing by weight is far more accurate and produces more consistent results than mixing by volume.

I use an inexpensive jewel scale ( check Amazon ) which measures to a resolution of 0.1g. There are those that will measure to 0.01g or finer.  Whether these are truly accurate to those levels is not so important as is consistent and repeatable.

I can consistently mix 5g ( 2.5 + 2.5 grams ) of casting resin this way. I mix silicone mold rubber this way and even mix fast cure 5/15/30 minute epoxy this way. I used to mix by volume but will never go back once I realized the value of mixing by weight.

cheers, Graham

 

 

[Rick L]

16 hours ago, Ace-Garageguy said:

Fascinating. I wasn't aware of that. All the urethane-based products I've ever worked with are used at room temp. This is definitely something I'll follow up on.

What are those...housings?...for, anyway?

I spent 40 years building mockups, engineering prototypes and mold masters for short run casting. Mostly for the medical diagnostic field. That casting is one of many enclosures we produced for a long time client.

[Bugatti Fan]

This thread has been a fascinating read.

What with other threads on 3D parts programming and printing, as well as various others on adhesives, paints and properties of different materials one could almost gain enough information from this forum to take a degree in materials technology!   LOL

[Oldriginal86]

The knowledge base of the members here span a wide spectrum in their profession and yet we all share the same interest in building models.