Ultracal Vs Hydrocal Dwell Time

Chris Wescott
Chris Wescott ✭
in How To Make A Latex Rubber Mask Part 1
Hi. Is there a difference in latex dwell times between ultra-cal vs hydro-cal molds?

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  • Matt Winston
    Matt Winston Admin
    Hi Chris,

    Due to its more porous properties, latex dwell times are shorter in Hydrocal molds than in Ultra Cal 30 molds.

    However, Ultra Cal 30 molds last longer than Hydrocal molds and do a better job of retaining surface detail after multiple castings.

    See below for some additional information on this topic from our friends at The Monster Makers. Original article here.
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    Ultra Cal 30 vs. White Hydrocal

    Latex mask making uses a slip casting process very similar to the one used for ceramics. For those new to this medium, the basic concept is that when liquid latex is poured into the mold, an even layer of gelled rubber begins to build on the mold interior. The porous nature of the mold has a capillary effect on the rubber which first causes it to build up in the mold. The thickness of the rubber build-up is defendant on how long the liquid rubber is left in the mold. Then, once the excess latex is poured out of the mold, this same capillary effect of the mold combined with air exposure will cause the latex to dry for removal.

    Ultra Cal 30 – Ultra Cal is a high strength, very low expanding stone or gypsum product that works similarly to plasters. Similar in the sense that once mixed with water, it will begin to react and after about 30 minutes, it will harden like plaster. That is where the similarity ends though. Unlike plasters, ultra cal will set to a very hard stone. Special effects artists have traditionally liked to use ultra cal 30 especially for foam latex molds because it’s high tooling accuracy. The benefit for using it in slip cast molds for masks is that it has a long service life. Ultra Cal will hold pattern detail much longer than white hydrocal.

    Another benefit of that Ultra Cal 30 for mask molds is that it can be built up in layers without forming stress cracks.. This is again due to its strength and low expansion abilities. For this reason Ultra Cal 30 is the perfect material to use for making one piece, full head production molds for masks. The downside of Ultra Cal 30 for mask molds is that molds take at least twice as long to produce. It also takes quite a bit of practice to learn how to produce UC30 molds that will absorb the deposited latex evenly. Ultra Cal 30 molds that are not carefully made will have surface spots known as resistance areas where latex will not build up at all.

    White Hydrocal

    Many mask makers like to use white hydrocal for the majority WH is the primary choice for mass production. The main of their molds and advantage is that overall mold and mask production is significantly faster than UC30. Once WH is mixed with water, molds will set within 10-15 minutes on average depending on how thick it is mixed as opposed to 25 to 35 minutes or more for Ultra Cal 30 molds. The big downside with WH is that it has to be used in mass. In other words you mix up what you need in one shot and build it up over your sculpture until it completely sets. You cannot build up the mold in multiple layers and batches of gypsum like you can with UC30. WH molds made like this end up with stress fractures throughout the mold. These will show up as significant defects right from the get go when you go to cast your masks.

    The second major drawback in molding with WH is that is has a very short service life. This is especially true if your mold has lots of detail. Each time you pull a latex piece from your mold, a small amount of surface detail is removed with the mask, and tiny air holes will begin to open up. When defects occur on the mold those are also defects that will show up on your mask. With each cast the defects gets progressively worse.

    The same erosion effect happens with UC30, but it happens at a much slower rate. Given these problems, you may be wondering why anyone would choose WH for a mask mold. There are two main reasons: First, if only a few casts of the mask are required. And second, as silly as it sounds, some mask designs actually lend themselves to an eroding mold. So what kinds of designs are perfect for WH? How about Zombies, Corpses, and anything where defects will not detract from the mask. They might even help! In fact, most large mask companies will use WH and will simply make a large number of molds and use them for production until the quality of castings is no longer acceptable. When the mold reaches that point, the manufacturer will use a stored, thick latex copy of the mask (known as a production model), to make as many new molds as they need. Half masks are especially suited for WH since they only require a one piece mold and they can be easily replaced.

    To sum it up, if you only need to make a few copies of your mask or if your design lends itself to mold erosion, than White Hydrocal is a good choice. If however, you have a highly detailed mask that you plan on making a couple dozen copies or more, than the clear choice is Ultra Cal 30. The extra time and effort you put into the mold will be worth it in the long run.
  • Chris Wescott
    Chris Wescott ✭
    Thanks but that wasn't my question. I know the difference between HC and UC. I'm asking the difference in latex dwell time between the 2. 
  • Chris Wescott
    Chris Wescott ✭
    Thanks. That does help. Is it absolutely necessary to bake a UC mold? Can you just let it dry awhile before pouring latex? 

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