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Latex, Epoxy,Dyes,Separators, and Commentary
Aug 21,06.
The following compiled comments are responses to my queries about making Silicone RTV casts and molds from latex molds and alligator skulls, and pouring resin casts from Silicone RTV rubber molds (Grant Hurlburt.) This is simply a compilation. I make no claims of technical knowledge or authority and anyone using this information does so at their own risk.
Grant Hurlburt
A. SILICONE RTVs
SILICONE RTV Suppliers.
1. Silicones Inc.SILASTIC(R) 3496 BASE & SILASTIC 96 CURING AGENT.
a. 211 Woodbine Street | P.O. Box 363 | High Point, North Carolina 27261
Tel: 336-886-5018 • Fax: 336-886-7122 • E-mail: info@silicones-inc.com
b. Silicones Inc.. Pennsylvania Distributor. “PTS” polymer Tooling Systems In
c.
http://www.polymertooling.com/index.htm Email:
Ph. 610-363-5440 • 800-327-8787 fax: 610-524-1004
303 Commerce Drive, Exton, PA 19341.
2. Douglas and Sturgess (artstuf.com).
3. Dow-Corning RTV Silastic silicone rubber.
http://www.dowcorning.com/content/moldmaking/
Artistic Mold-making Document: http://ww.dowcorning.com/content/publishedlit/10-1249C-01B.pdf?DCWS=Moldmaking&DCWSS=Artistic.
4.. Polytek. Development Corp. 55 Hilton St. Easton, PA 18042. Ph. (610) 559-8620. Fax (610) 559-8626. Easton is 20 m. E of Allentown on 33 S.
Polytek supplies a very informative 66 page manual
1) Polytek. Material Polytek. 800-858-5990. Joe (Tech Support). Polytek makes have a liquid thixotropic silicon rubber which you can add liquid to, so that it can be used either for solid molds or for higher viscosity multiple-layer mold-making.
Epoxy Resins are tough on molds due to corrosive nature of the materials in epoxy resins.
POLYTEK MATERIALS AND INFORMATION.
a) Tin-based silicone rubbers e.g..7025. tin-silicone. have a shorter shelf life. 10-1 mix ratio. Degrades over time. Add Tin-Thixx to make it more viscous for making multilayer molds.
Usage: (by GH) Use to make molds from latex casts, then make epoxy resin casts.
7025 Price: 1 pint $21; 1 gallon for $100.00. for first use and free shipping.. Probably provide about six casts before they are destroyed. About $135 for subsequent orders. About 1% shrinkage because alcohol is given off. Less shrinkage than latex .
Tin-Sil. COMPATIBILITY. Clay. Klean-clay is good for both tin- or platinum. Doesnt need separator. Sulphur-based clays need a separator ,espec. for platinum.
“Great for casting polyester resin”.
Tin-Sil. INCOMPATIBILITY:Cannot use with Platinum silicone. Fine for latex. Some oil-based clays contain sulphur which inhibits cure.
Tin-based TOXICITY.
Silicone Separators: Put TinSil 70-25 directly on latex. Doesn’t need a separator.
Recommended.
a) Krylon Crystal-clear (cracks if latex is flexed).
b) Petroleum jelly or vaseline (but degrades latex). Thin with mineral spirits 10-15% vaseline & 85% mineral spirits i.e. paint thinner (not turpentine) or lacquer. Apply with a plant misting bottle. Remove with soap and water.
c) Lacquer can also be used but is harder to get off. .Many oil-based clays have Sulphur and need a separator, espec. for platinum (or maybe it’s tin). Klean-Klay has no sulphur.
Not recommended: Nothing with alcohol (includes Vinac?)Not shellac, bec. alcohol-based.
b) Platinum-based silicone rubbers e.g Plat-Sil. 71-20 (Platinum-based) is an excellent product lasts 10 yrs or more. 1 to 1 ratio to mix so easy to use . More easily inhibited by a variety of materials. Used to cast Sue, according to Joe.
7120 Price . 2 pint size. $31.00. 2 gallon. $185 for first time trial includes UPS ground freight. Repeat usage. ($235.00/gallon for subsequent uses).
Can it be extended?
Plat-Sil. COMPATIBILITY: Klean-Klay (Polytek supplies this) but not sulphur-based clays.
Plat-Sil INCOMPATIBILITY: Sulphur in sulphur-based clays. Cannot use with tin-based silicone. Not good with latex even with layers of separator. Something in latex retards curing
Plat-Sil Separators: Krylon Crystal-clear for tin or platinum.
TOXICITY. Quite benign.
RECOMMENDATION 1.. Get a longhandled paint stirrer stick with flat sides to scrape sides of tubs, and/or use a smaller amount..
RECOMMENDED METHOD FOR PLATINUM SILICONE.(This follows the Polytek Manual, pp.40-41).
PlatSil. 71-20.This uses equal amounts of A and B by weight. B is much pastier..
Choose a tub which will be two-thirds full after A and B are used.. Measure out the desired amount of B into the tub, with the (tared) tub on the scale.. Then spoon or pour an amount of A that is equal in weight to the amount of B, so that the weight on the scale is doubled.* Mix thoroughly, scraping along the sides and bottom. The combined mixture is thick and pasty, and required a sturdy stirrer.
Deaerate under vacuum at 28-29” mercury, to remove air bubbles. Final cure properties at 7 days, but may be used with car after 24-48 hours.
COMMENTS. *Part A does not pour well, so that if you weigh it in a separate container and pour it, you won’t get 100% of the material way. Part B doesn’t pour at all, being very pasty, and is even harder to move from one container to another.
GH. So far I find Plat-Sil 71-20 more viscous than latex. It has been easier to make endocasts with latex than Plat-Sil 71-20 but I may have made some errors mixing it.
CLEAN-UP. Use ethanol for cleanup. Loose amounts of Parts A and B are still sticky after two days
c) Polyurethane Rubbers e.g. 7430 do much better with epoxy than silicones. Don’t shrink. Need very good and well-spread release agents. Bonds very tightly to porous material and nearly impossible to remove after bonding to porous material.
TOXICITY Need respirators or a fume hood.
COMPILED SILICONE RTV COMMENTS.
1. Try Silicones inc. instead of Dow, they have a very large selection of silicones, (note: the platinum-based is is more expensive but has no shrinkage, although the tin-based is fine for most applications, the shrinkage isn't even measurable until to get over a foot in length), and they deliver to the middle of nowhere, Wyoming so you'll be fine. the GI 1100, 1110, and 1000 silicones are the ones I use, and you may want to consider foam core-ing you hollow casts I've found that it makes the larger ones a lot stronger.. Syn air offers several types of plastics if your looking for variety. I'm not familiar with the current Dow viscosity grades of silicone
RTV (there are several different formulations available), but the choice of viscosity (or thixotropy) usually depends upon what you are molding, how you are molding it, and how much detail you want. If you are pouring solid molds rather than painting/laying up glove molds, the lower viscosity would likely be best. In your case, without vacuum, entrained air bubbles might be easier to eliminate with low viscosity RTV. To prevent bubbles from spoiling the mold surface, you can use a stream of air to blow small amounts of RTV around the surface of the object before pouring the rest (if you are making poured molds.) Be sure to check the RTV specs for materials that cause cure inhibition. Some RTVs will not cure in contact with certain clays, resins, etc. I don't
know if latex is a known inhibitor for any silicone RTVs, but you should make sure before molding .
2. In my experience, silicone rubbers are the best for molding this sort of thing; in perusing Silicone, Inc.'s material, they've got so many choices that I'm not sure what would be best in terms of strength (multiple casts will probably be pulled) and flexibility (given the fragility of the fossils, I think more flexible would be best).
You can get rubber delivered almost anywhere, usually from the manufacturers or their distributors website. I generally use Silicones Inc, GI 1000 and others viscosities, so I'm not really familiar with Silastic, although most silicones are generally similar.
The consistency you want depends on how you are going to make the molds. If you plan on building a "box" and pouring a solid mold you can use a thinner mix. If you plan on painting on layers of rubber to make a thin walled mold, you'll probably want a thicker rubber. Some of this depends on the size of the endocast (ie poured for small, painted for large) - looking at your previous work I would think a solid poured mold would be fine.
I think you should invest in at least a cup sized vacuum chamber - it's sad to spend the time and material to make molds and have them be full of air bubbles, usually they show up in just the places where the information is that you want to see. I'm assuming you have access to a fume hood? All of these materials are toxic (including epoxy resins) so you'll want to work safely.
3. Peter is working now with another type of silicone rubber, two part, that works very well. I think you can get it in 1 kg buckets, and five or 10 kg buckets. It’s cheaper than the Dow RTV silicones and works just as well. It comes from the States, so you should be able to get it easily enough.
You might consider using a company called Douglas and Sturgess (artstuf.com). They sell silicon molding compound with an additive you can mix in called “viscosil” that will thicken the compound to brushable viscosity. If you don’t add the viscosil it acts like normal silicon molding compound. You do have to order the concentrated catalyst to go with this though – I just got in a large order last week – I can look up the product numbers if you’re interested.
4. Silicones – two types, platinum cure and tin cure. Platinum cure is most expensive, highest detail, very easy to inhibit the cure. Tin cure cheaper and less touchy. The main thing to worry about is sulfur in the clay you use to make mold flashing, so you have to order sulfur free clay for making mold flashing. Permoplast from American Art Clay is good – the tan color specifically is designed for molding. Kleen Klay is also designed to be molding compatible, but they did this by substituting a sulfur-free oil in the clay base. I find it to be very sticky, and the slightest amount of kneading heats it up enough that it smears all over the fossil, your hands, the counter top, etc.
Another thing to keep in mind is that generally a good separator for silicones is petroleum jelly (bad for latex, though).. Even the spray separators for silicon are just basically vaseline in spray form, but latex will swell in the presence of petroleum products. You might need some sort of wax-based separator for the latex so it won’t swell, and that won’t inhibit the cure of the silicon. I’ve never tried what you’re doing, but maybe a spray shellac? I would try that on a test piece first.
Douglass and Sturgess are very helpful – I just ordered from them last week., material got here in Alabama from California in about 4 days. The best thing about them is they will sell in smaller volumes, so you can buy a pint of silicon, or a quart of casting material and not have to deal with larger volumes going bad in storage before you use it up.
5. Following from http://www.dowcorning.com/content/publishedlit/10-631.pdf?DCWS=Moldmaking&DCWSS=
“De-airing is recommended for all silicone moldmaking rubbers when not using automatic dispensing equipment. The small air bubbles that result from handmixing become trapped in the mixture and, if not removed by de-airing, can interfere with exact surface reproduction. One exception is Dow Corning 3110 RTV Silicone Rubber. Due to its low viscosity, this material can be used in many applications without de-airing. Because the mixture of base and catalyst or base and curing agent will expand during de-airing, it is important to use a container that is between three to five times the volume of the material itself. The higher viscosity silicone moldmaking rubbers will expand the most. The mixture can be quickly and easily de-aired in a vacuum chamber. This important step usually takes just a few minutes. Entrapped air may be removed by applying a vacuum of 26 to 29 inches of mercury. Dow Corning HS series RTV silicone rubbers should not be de-aired for more than 5 minutes. Any more de-airing will change physical properties. In all cases, the material will expand and then recede to its original level. For approximate time to de-airing, consult the individual product data sheets.”
6. The silicone we use is HS-II from Dow Corning. They make a thixotropic additive that will thicken the product to a brushable or spatulable (is that a word?) consistency. Avoiding sulphur-based clays is very important. We've had good success using this silicone with the urethane casting resins. The molds can handle some heat, but
with repeated castings they will tend to become more likely to rip and tear. Keeping in mind that the greater the volume is, the hotter the temperature will get can help you to decide what methods and materials are best for castings. If it is a huge volume, a layed-up hollow cast (subsequently filled with urethane foam) is a better choice. It also
seems that molds made without the thixotropic are stronger, perhaps because they cure more slowly. We often put layers of cheese cloth into our silicone molds to help with tear resistance. I hope this helps you out. As pointed out earlier on this thread, there are many correct ways to mold a cat! Good luck.
3. Dow-Corning RTV Silastic silicone rubber.
http://www.dowcorning.com/content/moldmaking/ Artistic Mold-making Document: http://ww.dowcorning.com/content/publishedlit/10-1249C-01B.pdf?DCWS=Moldmaking&DCWSS=Artistic.
4.. Polytek. Development Corp. 55 Hilton St. Easton, PA 18042. Ph. (610) 559-8620. Fax (610) 559-8626. Easton is 20 m. E of Allentown on 33 S.
Polytek supplies a very informative 66 page manual
1) Polytek. Material Polytek. 800-858-5990. Joe (Tech Support). Polytek makes have a liquid thixotropic silicon rubber which you can add liquid to, so that it can be used either for solid molds or for higher viscosity multiple-layer mold-making. Epoxy Resins are tough on molds due to corrosive nature of the materials in epoxy resins.
POLYTEK MATERIALS AND INFORMATION.
a) Tin-based silicone rubbers e.g..7025. tin-silicone. have a shorter shelf life. 10-1 mix ratio. Degrades over time. Add Tin-Thixx to make it more viscous for making multilayer molds. Usage: (by GH) Use to make molds from latex casts, then make epoxy resin casts. 7025 Price: 1 pint $21; 1 gallon for $100.00. for first use and free shipping.. Probably provide about six casts before they are destroyed. About $135 for subsequent orders. About 1% shrinkage because alcohol is given off. Less shrinkage than latex . Tin-Sil. COMPATIBILITY. Clay. Klean-clay is good for both tin- or platinum. Doesnt need separator. Sulphur-based clays need a separator ,espec. for platinum. “Great for casting polyester resin”. Tin-Sil. INCOMPATIBILITY:Cannot use with Platinum silicone. Fine for latex. Some oil-based clays contain sulphur which inhibits cure. Tin-based TOXICITY.
Silicone Separators: Put TinSil 70-25 directly on latex. Doesn’t need a separator. Recommended.
a) Krylon Crystal-clear (cracks if latex is flexed).
b) Petroleum jelly or vaseline (but degrades latex). Thin with mineral spirits 10-15% vaseline & 85% mineral spirits i.e. paint thinner (not turpentine) or lacquer. Apply with a plant misting bottle. Remove with soap and water.
c) Lacquer can also be used but is harder to get off. .Many oil-based clays have Sulphur and need a separator, espec. for platinum (or maybe it’s tin). Klean-Klay has no sulphur. Not recommended: Nothing with alcohol (includes Vinac?)Not shellac, bec. alcohol-based.
b) Platinum-based silicone rubbers e.g Plat-Sil. 71-20 (Platinum-based) is an excellent product lasts 10 yrs or more. 1 to 1 ratio to mix so easy to use . More easily inhibited by a variety of materials. Used to cast Sue, according to Joe. 7120 Price . 2 pint size. $31.00. 2 gallon. $185 for first time trial includes UPS ground freight. Repeat usage. ($235.00/gallon for subsequent uses). Can it be extended? Plat-Sil. COMPATIBILITY: Klean-Klay (Polytek supplies this) but not sulphur-based clays. Plat-Sil INCOMPATIBILITY: Sulphur in sulphur-based clays. Cannot use with tin-based silicone. Not good with latex even with layers of separator. Something in latex retards curing Plat-Sil Separators: Krylon Crystal-clear for tin or platinum.
TOXICITY. Quite benign.
RECOMMENDATION 1.. Get a longhandled paint stirrer stick with flat sides to scrape sides of tubs, and/or use a smaller amount..
RECOMMENDED METHOD FOR PLATINUM SILICONE.(This follows the Polytek Manual, pp.40-41). PlatSil. 71-20.This uses equal amounts of A and B by weight. B is much pastier.. Choose a tub which will be two-thirds full after A and B are used.. Measure out the desired amount of B into the tub, with the (tared) tub on the scale.. Then spoon or pour an amount of A that is equal in weight to the amount of B, so that the weight on the scale is doubled.* Mix thoroughly, scraping along the sides and bottom. The combined mixture is thick and pasty, and required a sturdy stirrer. Deaerate under vacuum at 28-29” mercury, to remove air bubbles. Final cure properties at 7 days, but may be used with car after 24-48 hours.
COMMENTS. *Part A does not pour well, so that if you weigh it in a separate container and pour it, you won’t get 100% of the material way. Part B doesn’t pour at all, being very pasty, and is even harder to move from one container to another.
GH. So far I find Plat-Sil 71-20 more viscous than latex. It has been easier to make endocasts with latex than Plat-Sil 71-20 but I may have made some errors mixing it.
CLEAN-UP. Use ethanol for cleanup. Loose amounts of Parts A and B are still sticky after two days
c) Polyurethane Rubbers e.g. 7430 do much better with epoxy than silicones. Don’t shrink. Need very good and well-spread release agents. Bonds very tightly to porous material and nearly impossible to remove after bonding to porous material. TOXICITY Need respirators or a fume hood.
COMPILED SILICONE RTV COMMENTS. 1. Try Silicones inc. instead of Dow, they have a very large selection of silicones, (note: the platinum-based is is more expensive but has no shrinkage, although the tin-based is fine for most applications, the shrinkage isn't even measurable until to get over a foot in length), and they deliver to the middle of nowhere, Wyoming so you'll be fine. the GI 1100, 1110, and 1000 silicones are the ones I use, and you may want to consider foam core-ing you hollow casts I've found that it makes the larger ones a lot stronger.. Syn air offers several types of plastics if your looking for variety. I'm not familiar with the current Dow viscosity grades of silicone RTV (there are several different formulations available), but the choice of viscosity (or thixotropy) usually depends upon what you are molding, how you are molding it, and how much detail you want. If you are pouring solid molds rather than painting/laying up glove molds, the lower viscosity would likely be best. In your case, without vacuum, entrained air bubbles might be easier to eliminate with low viscosity RTV. To prevent bubbles from spoiling the mold surface, you can use a stream of air to blow small amounts of RTV around the surface of the object before pouring the rest (if you are making poured molds.) Be sure to check the RTV specs for materials that cause cure inhibition. Some RTVs will not cure in contact with certain clays, resins, etc. I don't know if latex is a known inhibitor for any silicone RTVs, but you should make sure before molding .
2. In my experience, silicone rubbers are the best for molding this sort of thing; in perusing Silicone, Inc.'s material, they've got so many choices that I'm not sure what would be best in terms of strength (multiple casts will probably be pulled) and flexibility (given the fragility of the fossils, I think more flexible would be best). You can get rubber delivered almost anywhere, usually from the manufacturers or their distributors website. I generally use Silicones Inc, GI 1000 and others viscosities, so I'm not really familiar with Silastic, although most silicones are generally similar. The consistency you want depends on how you are going to make the molds. If you plan on building a "box" and pouring a solid mold you can use a thinner mix. If you plan on painting on layers of rubber to make a thin walled mold, you'll probably want a thicker rubber. Some of this depends on the size of the endocast (ie poured for small, painted for large) - looking at your previous work I would think a solid poured mold would be fine. I think you should invest in at least a cup sized vacuum chamber - it's sad to spend the time and material to make molds and have them be full of air bubbles, usually they show up in just the places where the information is that you want to see. I'm assuming you have access to a fume hood? All of these materials are toxic (including epoxy resins) so you'll want to work safely.
3. Peter is working now with another type of silicone rubber, two part, that works very well. I think you can get it in 1 kg buckets, and five or 10 kg buckets. It’s cheaper than the Dow RTV silicones and works just as well. It comes from the States, so you should be able to get it easily enough. You might consider using a company called Douglas and Sturgess (artstuf.com). They sell silicon molding compound with an additive you can mix in called “viscosil” that will thicken the compound to brushable viscosity. If you don’t add the viscosil it acts like normal silicon molding compound. You do have to order the concentrated catalyst to go with this though – I just got in a large order last week – I can look up the product numbers if you’re interested.
4. Silicones – two types, platinum cure and tin cure. Platinum cure is most expensive, highest detail, very easy to inhibit the cure. Tin cure cheaper and less touchy. The main thing to worry about is sulfur in the clay you use to make mold flashing, so you have to order sulfur free clay for making mold flashing. Permoplast from American Art Clay is good – the tan color specifically is designed for molding. Kleen Klay is also designed to be molding compatible, but they did this by substituting a sulfur-free oil in the clay base. I find it to be very sticky, and the slightest amount of kneading heats it up enough that it smears all over the fossil, your hands, the counter top, etc. Another thing to keep in mind is that generally a good separator for silicones is petroleum jelly (bad for latex, though).. Even the spray separators for silicon are just basically vaseline in spray form, but latex will swell in the presence of petroleum products. You might need some sort of wax-based separator for the latex so it won’t swell, and that won’t inhibit the cure of the silicon. I’ve never tried what you’re doing, but maybe a spray shellac? I would try that on a test piece first. Douglass and Sturgess are very helpful – I just ordered from them last week., material got here in Alabama from California in about 4 days. The best thing about them is they will sell in smaller volumes, so you can buy a pint of silicon, or a quart of casting material and not have to deal with larger volumes going bad in storage before you use it up.
5. Following from http://www.dowcorning.com/content/publishedlit/10-631.pdf?DCWS=Moldmaking&DCWSS=
“De-airing is recommended for all silicone moldmaking rubbers when not using automatic dispensing equipment. The small air bubbles that result from handmixing become trapped in the mixture and, if not removed by de-airing, can interfere with exact surface reproduction. One exception is Dow Corning 3110 RTV Silicone Rubber. Due to its low viscosity, this material can be used in many applications without de-airing. Because the mixture of base and catalyst or base and curing agent will expand during de-airing, it is important to use a container that is between three to five times the volume of the material itself. The higher viscosity silicone moldmaking rubbers will expand the most. The mixture can be quickly and easily de-aired in a vacuum chamber. This important step usually takes just a few minutes. Entrapped air may be removed by applying a vacuum of 26 to 29 inches of mercury. Dow Corning HS series RTV silicone rubbers should not be de-aired for more than 5 minutes. Any more de-airing will change physical properties. In all cases, the material will expand and then recede to its original level. For approximate time to de-airing, consult the individual product data sheets.”
6. The silicone we use is HS-II from Dow Corning. They make a thixotropic additive that will thicken the product to a brushable or spatulable (is that a word?) consistency. Avoiding sulphur-based clays is very important. We've had good success using this silicone with the urethane casting resins. The molds can handle some heat, but with repeated castings they will tend to become more likely to rip and tear. Keeping in mind that the greater the volume is, the hotter the temperature will get can help you to decide what methods and materials are best for castings. If it is a huge volume, a layed-up hollow cast (subsequently filled with urethane foam) is a better choice. It also seems that molds made without the thixotropic are stronger, perhaps because they cure more slowly. We often put layers of cheese cloth into our silicone molds to help with tear resistance. I hope this helps you out. As pointed out earlier on this thread, there are many correct ways to mold a cat! Good luck.