|Oct 12, 2011, 12:18 AM|
Making a clear cast piece of art from a pylon racer using silicone molds and urethane
Welcome to my project in turning a small pylon racer airframe into a clear, cast piece of artwork! This tutorial and demonstration may present new materials and concepts to those use to using rigid composites (fiberglass and epoxy) to make our typical flying parts.
The materials themselves are not too far different than the typical epoxies and other two-part material systems we are use to. They have certain ratios to measure and mix just like you would a laminating epoxy. But there are some new concepts and methods in processing that may take some getting familiar with. I will go over the equipment used for my demo and how you might approach this type of a project in case you don't have access to it.
Since I am already building an airframe of the Micro Stinger (designed and produced by my good buddy Bret "Pdawg" Becker) I decided it was small enough to try this concept of molding and casting a solid, static model for display. I received permission from Bret to take a mold of this design on the premis that it would be for personal use and not for profit. I am already in the business of selling these materials and am not looking to produce cast parts for sale but rather show other people how it can be done for their own projects. Enough of the semantics, let's get on to the good stuff...!
The mold material I am going to use for this is a standard in the industry for casting two-part polyurethanes; Silicone. This is not the silicone found at local hardware stores for sealing windows and tubs; it is a two-part castable siicone that has a given working time and cure time. The advantages to using silicone in a casting application would be that the mold seals like a gasket around the edges and the poured liquid silicone doesn't stick to clean, smooth surfaces like plastic, fiberglass, and metal when cured. Plus the silicone when cured is a non-stick surface; so things cast against it don't stick except for other silicones (there is a limit to this non-stick quality we won't delve too deep in). In this case, I am using BJB Enterprises TC-5040 which is a clear/translucent, 40 Shore A Durometer, platinum-based catalyst silicone and and also TC-5045 which is also a platinum-based silicone but is an opaque pink color (we'll go over why we're using two types later). In casting silicones there are two primary types; platinum-based (aka addition cured) and tin-based (aka condensation cured). The tin-based silicones are noted to be user friendly and can be useful with many castable polyurethanes but there are some materials that can't be cast into a tin-based mold; namely the clears I am going to use. Tin-based silicones as a general rule are also not as durable long term and their library life is shorter than a comparible platinum-based. Platinum based silicones typically have good mold service life, have a wider variety of available hardnesses (Durometer), are stable over long periods of time, can handle higher heat than a comparible tin-based, and you can pour just about anything into them. Their downside is that they can be sensitive about the surface you plan to cast a mold against. Some examples would be polyester based products (the trapped styrene causes issues), natural rubbers, and some artist-type clays containing sulfer. They are also picky about certain types of paint because like polyester, they contain certain chemicals the platinum-based silicones won't cure well against. Catalyzed paints like urethane and epoxies are typically safe. For many years a quick fix to seal and prevent issues with unfriendly surfaces for platinum silicones was to use a can of Krylon Crystal Clear; a sprayable acrylic coating. Due to some formulation changes in the product there are some reporting to me that it isn't doing what it use to do. Lucky for me I have some older version of Crystal Clear to use on this project but I will have to do some testing in the future to see what the real story is.
The airframe serving as my master(s) is being finished just like I would prior to applying a paint scheme of my choice. But I'm stopping short of doing multiple visible colors (the Micro Stinger needs it!) to simply apply several good covering coats of the Crystal Clear. Once dry, I wet sand from 1000 grit to 2000 grit and then polish to a high gloss. The beauty of this process is that when you go to cast a silicone mold off of your master, the silicone will pick up this high gloss and duplicate it in your future parts. This is actually easier than if you were doing a composite mold since you don't have to worry about mold releases potentially dulling your finish. Since you can't come back and polish the silicone mold, and it's a pain to sand & polish each part out of the mold, you are better off putting some extra work into your masters.
The first part I finished to take a mold of was the V-tail. I used the clear TC-5040 so I could cast over tail as a solid block and then come back and manually cut through the silicone creating a parting line; they call this a Jeweler's Cut. Using some bamboo skewers lightly CA'd to the tips to suspend the part in the mold box, the skewers will also create my fill port and vent on the opposite side when removed from the cured mold. The mold box you can see is super simple; some smooth carboard from a discarded disposable-glove box. I bent, folded, cut, and hot glued the carboard on all overlaps and along the base to seal it from leaks. I then suspended the part just below the top level of my mold box using more sticks and hot glue. I pre-measured enough TC-5040 to fill the mold box to the top. Silicones are typically much higher in viscosity (thicker and stickier) when compared to many other two-part resin systems so mixing them thoroughly is super critical. The mix ratio is 100 parts A to 10 parts B by weight. I mix for several minutes in a quart-size cup and do what we call a double-cup mix where the material is then poured into a new cup and mixed some more. The idea is to leave any unmixed residue sticking to the sides and bottom of the cup behind and lower the risk of it ending up in your mold. The next step is where we cross the leap from amateur to pro... The mixed silicone is placed into a vacuum chamber and an extremely high level of vacuum is achieved to pull the trapped air bubbles out. If you mixed correctly, you will have a lot of air stirred into the silicone and unfortunately those bubbles will not magically rise to the top and pop on their own. The vacuum chamber is not a Shop-Vac hooked into a coffee can and even some vacuum bagging pumps used in composites are too small for the job. We need to be pulling at least 26-28" of mercury (or almost 1 atmosphere) to effectively pull bubbles out. Now if you have no access to a vacuum chamber it may be necessary to use a much lower viscosity silicone and use some industry tricks to help eliminate air (we'll discuss later). You have to allow a lot of open vertical space in the mixing container because the silicone will begin to foam and rise. A good video of the whole process here:
Once the silicone is de-aired after several minutes, the silicone is carefully poured into the mold box in a thin stream away from the part and the rising level of silicone is allowed to flow naturally over the part displacing any air as it goes. Once you filled to the top you may see a few accidental bubbles. Don't fret as these will likely pop on their own considering the working time is quite long. You can see in the pics this is exactly what occured.
The next day the silicone is cured and, using a new razor blade, I carefully cut through to the trailing edge of the tail starting from one end to the other connecting the holes made by the bamboo skewers. I then carefully extract the V-tail master using some compressed air to pop it loose and help force it out.
Up next, casting a clear part.... stay tuned.
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|Oct 14, 2011, 12:57 AM|
Troy, platinum silicone does not require any special de-airing... but I found it extremely sensitive to household objects (tape, pigment) but if sopmeone wants to give this a try, and the deairing is intimidating you, look for platinum two part silicone (e.g. TAP)
|Oct 14, 2011, 11:50 AM|
Yes and No. A Platinum-cured silicone is a two-part material available in a huge variety of hardness'/stiffness; from skin-like up to the stiffness of a car tire. The silicone you refer to is really soft and would need a lot of support structure to make a larger mold. I'm going to be showing another option where you can use a stiffer Platinum silicone with good dimensional stability that will have a better chance at making molds with no vacuum de-airing. I'm going through the options to show how it's done professionally and also how a DIYer in the garage may approach this. Going to YouTube, some DIYers build their own vacuum chambers if they need to achieve certain results. The wing mold is up next...
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