The Polytek (many rubber and plastic compounds) website advises one and all: http://www.polytek.com/
'Thus, silicones' non-stick quality makes short run resin casting easy, but for high production of many parts per mold, release agents are often used to prolong mold life. If this is the case, some Poly Urethane rubber molds may perform nearly as well at half the price. If your mold costs are high, it may pay for you to review the possibilities of a lower cost rubber with a Polytek representative. Silicone rubber molds are almost never used for cement and plaster casting and are only needed at foundries with the stickiest of foundry waxes.'
Christopher's religion is only his own.
Dan- Do not believe everything you read put out by materials manufacturers . They are skewed by self interest. Urethanes are more profitable for many manufacturers than silicones- its an easy sell to tell someone that they are getting such a deal at 'half the cost' of silicones- they do not mention the higher margin they make on the sale, nor that they have zero experience in actually using their own products in high volume business applications.
What you quote is almost entirely incorrect.
My only religion is the scientific method- I do not care which material is best, I simply test them to determine which is actually the best in practice. In doing consultaion for large casting operations I have had a unique access to both facilites and funding to conduct very comprehensive tests of these issues. I have tested dozens of urethanes products of varying durometer against dozens of silicones of varying durometer and I can promise you that silicones almost always outperform urethanes by far.
Especially with gypsum and concrete mixtures and especially in high volume casting.
To find out which is better I make lots of identical molds in both urethane and silicone and subject them to statistical analysis in a production environment. At one company, we went from replacing 160 - 190 worn urethane molds per day, to replacing only 15 - 20 worn silicone molds per day for a continuous production output of over 3,000 castings per day. This is an obvious improvement in mold life- and the castings were better, too.
Please be aware that we had designed the molds to have no seams or minimal seams- many designs having to be demolded like a glove. We were casting products with very fine sharp edged textures and significant undercuts, and the molds were designed to be operated by minimum wage workers. Given these forces, urethane molds universally wore faster and tore more often than silicone molds.
That being said- it is possible to get very good life from a urethane mold. If the master is relatively smooth surfaced and the mold requires very little flexion or tension in demolding, often requiring mulitple parts to the mold, urethanes can last a very long time in production.
The only truth in your quote is that most gypsum casters use urethanes. In 1978 most businesses in the U.S. used clerks, carbon paper and desk calculators to run their books- The prevalence of ignorence is not indicative of a better method, mechanism, nor material. Most casting companies have self educated, semi-amatuer moldmakers who can not even calculate the true cost of their operations. It is a common mistake to imagine that a lower priced material means you are saving money. This is not always true.
I have recommended urethanes to many companies whose particular applications made urethanes more cost effective than silicone- but this is rare. (Small wax casters and the like)
There is not only manhours, overhead, number of parts, casting quality, and material cost to consider. A good moldmaker must factor in all costs such as possible future litigation from former workers with allergic bronchitis and higher fire insurance costs because urethanes are flammable and make cyanide gas over 350 degrees F. Especially important is the ability to design the mold around the desired product rather than the product around the limitations of the mold material.
I generally recommend silicone even to artists working in solitude because it is relatively non-toxic, reliable, versatile, and stores well. Silicones are the only product with which you can use a single material to pour, brush or spray on molds with complete controlability of thickness and cure time and the ability to cast virtually any medium into them without separation agents. No urethane can do this. This way an artist can become master of one versatile material instead of remaining a novice of many materials.
So you see, Dan, that I am more of an athiest who has spent 20 years seeking to find out what really works at the best cost over the long haul. Verifiably. I am not selling anything, Dan. I offer my opinion only to give others access to this real experience with no margin in it for me other than to be helpful. Take it or leave it, but please, if you must use urethanes, wear your respirator.
Christopher P.S. many companies have developed new tin cure silicone products that do not outgas ethanol at the temperatures of molten wax and that will last 10 to 14 years without signnificant degradation. G.I. 1040 is one of them but there are many others. This elliminates the single area in which urethanes were sometimes a better material than silicones.
Angus MacCauley the master moldmaker at Polytek is very experienced, he gets to play with every type of resin and equipment there is. BUT have you ever noticed all pictures show him using a normal cartridge respirator, even the video where he is spraying urethane rubber in a large ventilated area, he still uses a cartridge. He seems like a young 'family guy', does he know something that we don't?
Also Polytek does not sell or encourage polyester or epoxy tooling resins (no/low shrink, high heat resistance). For any type of production this is the way to go. I have been able to convert over 90% of my pieces into rigid molds, some up to four pieces. I would have had four pieces even if there was rubber inside. Look at the mother mold and see if you can eliminate the rubber altogether. If you can, you end up with a super tough, lightweight, inexpensive mold that can also be sanded and repaired easily. New fibreglass materials for resin transfer molding make casts (and extra molds) as easy and as fast as making waffles, the hinged mold clamps on the composite and resin all in one process. New mold releases and spray gel coats make the job even easier. All this is not rocket science or high budget. The smell of fresh fibreglass can be eliminated by soaking the cast in warm, soapy water and then rinsing well. Large items, like statues are made in pieces and then resined together, sanded out. Once you get the hang of it, it seems to take the same time as finishing plaster or concrete, but does not require a helping hand because of low weight. I once worked in a shop where one guy could make a twenty foot high corinthian column. There is now a huge variety of colors and finishes. I have been playing with a crystal clear 'marine'gel coat, which can be lightly tinted with transparent dyes, you back up with a bright white gelcoat and surface veil, and then buff the finished piece with cloth wheel and eventually wax. Makes some great fountains.
Gary There are many excellent moldmakers who still can not compute casting cost accurately. I know several. They can make a fantastic mold, but ask them to shave 40 cents off of each of 50,000 castings and they are at a loss.
Of course- for the average sculptor, such considerations are nearly meaningless- I still prefer silicone for the versatilty alone, if not the safety.
Oh, and Gary, about polyester- a well known moldmaker and patination expert hereabouts was recently hospitalized for accumulated MEK exposure. He came within a few minutes of dying with multiple organ failure before the doctors finally realized it was the MEK- They advised that MEK is absorbed not only in fumes but even more efficiently through the skin- And the toxic effects are cumulative over time, my friend had about 15 years of near daily exposure. He now preaches like a Baptist about being sure to keep the resin and MEK completely off the skin and to wear a special respirator rated for Ketones.
I only just heard about this, and thought to pass it on- wear your gloves, friend, and a respirator.
Gentlemen, You are starting to convince me. I did not bring this up to flog a dead horse (kinky?!?) but because I figured Polytek did make more on sils than 'thanes. I possibly will die from acetone. When I started my business, I knew nothing of urethane rubber, just the casting plastic. My first molds were of natural latex and silicone. Latex took forever and is not cheap either. The silicone gave me problems: tearing, degrading, even warping. See my Washington Ave. project: 60 concrete headers from a silicone mold, FRP backup. Had to stop and mend the mold pretty often. And was I ever careful! No shovel-holes.
Then I started pouring urethanes but this involved incredibly intricate casting-cases. There had to be a better way, and it was the paint-on urethanes. So I've stuck with those until today. Are you saying that sils are tougher and stronger now? Back then there was no paint-on sil, but I'd laminate cheesecloth with a PermaFlex sil. You could add cabosil but that decreased tear strength. Andrew Werby has been corresponding directly on this question, don't know why not keep it all public. Tell me more about paint-on sils and about creating pouring-cavity molds.
Here's the part the rest of you missed:
Date: Sunday, July 30, 2000 12:32 PM Subject: Re: Silicone according to
[Used the way I do, the stuff goes a long way. With a little cheesecloth reinforcement, it will work fine at 1/8', although I reinforce the seams and edges with thickened material or caulk.(I wait for the hardware store to clear it out at $2.50 a tube). I paint it on, so I don't need to mess around with vacuum- it goes on so thin I get every nuance of texture in fine depressions, where I'd get a field of bubbles in urethane. And it's good to the last drop, unlike about 3 gallons of Smooth-on that's going to cost me money to get rid of at the toxic waste dump because I opened the cans and used some- silly me. Making all the molds I do, I'm very conscious of economy, but at least for my purposes, the advantages of silicone are worth the extra price.]
Dan Spector wrote
Dan, the article that Chris has at Andrew's site leaves little, if anything, out.
As far as the paint on sils, I've only done one, and I was completely satisfied with the material. I brushed two coats of a pourable silly on the face, then backed that up with a material that came out of the can as a peanut butter consistency. Worked out great. Start to finish, with a four piece case, in less than a day.
As for the cavity pour molds, I do a couple of dozen a year. I'm working on an illustrated 26 page series of webpages that will be finished soon. Different technique from CP's article describes.
'Tell me more about paint-on sils and about creating pouring-cavity molds.' is pretty open ended. Do you have any specific questions??
A significant portion of the strength of silicones is due to the elasticity of the material. Cheesecloth acts to prevent the material from stretching, and, under significant stress WILL cut the rubber internally- Disect an old mold that has been sorely stretched and you can see the tears.
I/8 inch thickness is not a good idea for any mold rubber- at that thicknerss rubber has very little dimensional stabitlity and will often 'fall' away from the inside of the casing- particularly when making hollow castings such as foundry waxes. Many moldmakers combat this by studding the surface of the mold with little 'snap' keys- but these are un-necessary in a mold of proper and uniform thickness.
I should add here for Battersby's benefit that many gypsum casters do prefer urethanes for their storage capability- If you are casting a part in very low numbers over a long period of time, urethanes can be more cost effective than silicones. Many urethane formulations remain very stable for long periods of time.
Their principle weakness is that they are easily 'abraded' and easily torn. When you have to pull very hard gypsum castings with fine sharp detail from them, the detail on the demolding castings will quickly 'carve' the rubber away leaving bald spots in the inner surface. Also, if the mold must be significantly yanked or stretched to demold, urethanes are more likely than silicones to fail and tear.
While a few urethanes boast a 300% elongation, this comes at the price of a 40 pound or less tear strength. What I meant was that no urethane offers over 300% elongation WITH a 100 pound plus tear strength. Only silicones and latex achieve this.
I should qualify my analysis of urethanes to state that silicone molds achieve optimum strength 72 hours after they are poured, and when then put into production where they are cast non-stop- many rounds per day, until they fail, they will generally outlast urethane molds by well over a month.
But if you only pull a few castings from the molds- then shelve them for a few months, then pull a few more, then shelve them... In this kind of small operation, I have often recommended urethanes over silicones. At this level of prodction the economies of scale afforded by silicone do not outwiegh the cost of affording the silicone itself.
I still prefer the silicone over the urethanes for the artist because of all of the previously stated advantages regarding versatility and toxicity. I can elaborate by saying that I keep a 5 gallon pail of silicone base in my shop. I have 3 different catalysts for this base that will give different properties to the finished rubber- one even gives it the ability to withstnd 400 degrees F. without swelling. I further have a bottle of thixotropic agent that I can add to the silicone with any of the catalysts to make the mixture thick like peanut butter for brushing on molds. I have an ultrafast catalyst that I can add to control the cure time from 18 hours down to 15 minutes. I have a bottle of diluent that I can add to make the mixture more fluid should I need it so. All of this and it won't burn, don't stink, cures on anything, is highly tolerant of off ratio mixing, and I can pour anything into it except silicone without a release agent. Great stuff- but I always advise anyone to do their own cost vs vbenefit analysis and stick with whatever they are most facile and profitable with.
Chris, The pourable peeyou that I use most is SO 121-30, SmoothOn claims 80 pound tear, 1000% elongation. My favorite brush on right now is the SO BrushOn 40. For this one they claim 60 pound tear, 1000% elongation. Both of these are well above the numbers you mention above.
There are six or seven SO urethanes that claim double and even triple the numbers that you are saying 'Only silicones and latex achieve this.'. I've never done any lab testing on any rubber, only quoting SO.
The values reported by Smooth-On are based on internationally recognized, standardized ASTM tests - of which the parameters are open to the public. Fact is the evolution of materials in providing improved physical properties has increased dramatically over the last decade, due to advancements in the raw chemicals industry.
Whether you use silicones or urethanes for your project, you're fortunate to live in an age where such materials are available in the quality that they are presented - regardless of the manufacturer (even though some products are indeed better than other products). Which ever material you use, you must use those materials as prescribed by the manufacturer. If you deviate from the recommended processing parameters, then you may encounter cure issues.... so do your research.