MoldRite 25: Silicon is not just for CPUs


So, I find myself in need of some little one inch or so thick fan ducts for a recessed or “ducted fan” type treatment on a case fan. Simple enough, I just use the old technique of gutting a similar sized fan and modding the housing as a duct.

Problem is, this takes stuff; an old fan of the right size, a good deal of filler work to fill in gaps used by the original fan, and time (which I never have). Oh, did I mention I need six of these? The solution is to make one and mold and cast a half dozen or so. Unlike Joe Turtle though, latex compound and plaster are not going to do the trick. This requires maximum molding amperage in the form of room temperature vulcanizing (RTV) silicon and later some hard urethane poly for casting (um…ok, rubber and plastic if you really must know). So, I picked up a couple pint kits of MoldRite 25 from Dick Blick’s to see what kind of messes I could create in Manta’s workshop this week.

MoldRite 25 is a two part RTV silicon rubber system that will allow you to make studio quality molds with “forensic grade” detail. Of course, my part doesn’t really require CSI levels of detail, but this type of RTV is also known for making durable molds that require little or no release agent and can be used 20+ times (in fact, with release agent they can be used longer). Best of all, it’s fume free, so I can mix it in the kitchen using the kitchen scale while certain other parties are at work (NOTE: covert operations are run at your own risk – we will deny any knowledge of you if you get caught). And, a scale you will need. There are two ways to mix most casting and molding compounds. One is by volume (generally preferred) and one is by weight. MoldRite requires mixing two compounds in a 10:1 ratio by weight. And that requires a scale. Unless you are mixing epoxy or other material in tiny batches, you can use a digital scale that will measure increments of 1 gram (0.03 oz) and can measure up to 1kg (read this as digital kitchen scale).

Once you have obtained said scale, the process is simple. Weigh out the parts, mix them together, and as soon as you have them well mixed (and I mean work fast), pour the material into your mold container. MoldRite 25 will harden to usable in about 24 hours, but if you added too much catalyst, it will become unworkable pretty fast (within minutes)! So don’t dally. Besides, covert operations should always be done quickly!

After about 24 hours, gently “massage” your positive (model, subject, core – what have you), out of the mold. Take your time doing this. MoldRite is pretty durable, but you don’t want to rip the mold you just made.

The results? I have to say impressive. The material reminds me of materials I’ve used in a lab (right down to the blue color). It’s sturdy and durable, which means many casting and very low distortion in the final product (a must for a functional part like I’m going to cast). In fact, I have gotten dozens of castings from a single silicon mold in the past. That’s far more than you will get from latex typically. There were no fumes or noticeable heat when working with it and the kit comes with just about everything you need to work with the material (including CSI grade latex gloves). And consider, this took about 30 minutes from start to finish where a latex mold can take a week to complete.

There are two drawbacks, however. First, the stuff is barely pourable. It’s the consistency of cold honey. This can be a pain to deal with when you go to make the mold and you feel you are in a hurry. Just be patient. Second, you pay for quality (and quantity). MoldRite comes in small kits. I had to use two to make a mold of my fan duct. Each kit will run you around $35 USD (ouch). It’s also not as easy to find as, say, plaster or latex mold compound. You can get it at some art supply stores or you may have to order it online.

Ah, and some tips if you decide to use this or any pourable mold material:

  • Make sure your subject parts are very clean.
  • Use double stick tape to secure part(s) to whatever you are pouring into (or it may float).
  • Don’t let anything distract you once you start.
  • Wear the protective gear! (Mainly eyewear and gloves).
  • Any containers you use will be disposable. Even if you didn’t want them to be.

Next week I’ll cast this part in a urethane poly I've never tried so you can see the real fun part - the final product (or pile of plastic goo - which ever appears first).

‘Till then, Happy Modding!

Our bit of bling is how-to from and ModNation. DIY Printed Circuits anyone ?

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