As per the other casting tutorial, the first thing you have to do is get yourself some marine epoxy resin, like for example this stuff, the important thing is that it has low viscosity so it will flow well for casting purposes, and that it sets as a solid resin.
1) cut out a rubber disc slightly smaller (around 0.5mm or 0.02 inches) diameter as the tube in which your piston will travel. Drill/punch out (using a sharpened tube) a hole in the middle through which you can insert a bolt with a washer. If necessary, apply JB weld or araldite to ensure that the seal between the bolt and washer are airtight.
2) lubricate the sides of the tube in which you're going to cast with a very thin layer, anything from automotive grease to KY jelly will do as long as its not a substance that will disolve in the epoxy. Insert the rubber disc and make sure its face is at 90 degrees to the tube. It's important that the space between the disc and the tube is sealed, if the fit is tight enough you can smear grease to keep it sealed, if not a thin application of JB weld to fill the gap will do.
IT'S IMPORTANT THAT YOU HAVE EVERYTHING ALIGNED AND SEALED BEFORE YOU EVEN THINK ABOUT MIXING THE EPOXY - it will be too late to make adjustments afterwards, this method of construction is VERY unforgiving.
3) Once you're satisfied the seal is airtight, mix the marine epoxy and pour it. The length of the whole piston should be at least as much as its diameter. Note that epoxy is very heat sensitive, store it at room temperature before mixing - if not, use a baine marie to heat it up before mixing - (some valid pointers on cold temperature bonding). I find that syringes are ideal to mix it in the correct ratio, and always calculate roughly how much you need so you don't mix too much and have to throw it away - if you take care of it, a pack will last you through many different projects. Clean up any spills immediately or you'll regret it, this stuff sets hard!
Allow 24 hours (or more in cold weather) for complete curing, then tap out the piston with a wooden dowel or similar. Lightly sand the sides of the piston until it's a loose enough fit in your chamber tube to allow you to push it easily with one finger. Do this little by little as its very easy to remove too much material. The piston should now be ready for use.
To make a lighter weight hollow piston, follow the second diagram - cut a cylinder of foam approximately half the diameter of the piston and attach it to the bolt before pouring the epoxy.
Any queries, feel free to ask
Edit: some useful advice on making a built-up piston, including how to centre the tubing (which also applies if one needs to centre the barrel in a coaxial chamber)
A useful troubleshooting guide:
US Composites wrote:1. My epoxy is taking forever to dry. What went wrong?
The reasons listed here cover 99.9% of possible scenarios. One or more of them should ring true to your situation.
* The ratio for resin to hardener was not correct. This is most commonly a function of poor or imprecise measuring. Eyeballing the epoxy for your ratio is not good enough. A graduated cup should always be used for measuring.
* Improper usage of the pumps. If you did not prime the pumps properly it can throw off the ratio. An indication of an improperly primed pump would be when small amounts of air are expelled when dispensing. It may also be a result of losing count of the number of pump strokes.
* The resin and hardener ratio was reversed. Odd as this may sound, we've heard about this happening many times. If your hardener is a 3:1 ratio, this mean 3 parts of resin to 1 part of hardener.
* The batch was too small. Often when customers are testing epoxy for the first time they will only mix a few teaspoons of epoxy. We encourage testing but when done in such small batches a slight deviation in ratio is greatly magnified. We therefore recommend that your test batches be at least a few ounces, be accurately measured and be thoroughly mixed.
* The resin was applied too thin. Very thin applications take longer to cure especially when applied without fiberglass. In these cases patience is required. If the epoxy still does not cure refer to the other scenarios listed in this section.
* Something was mixed into the batch. We have heard of people mixing strange things into their epoxy, with water being the strangest. Always use uncontaminated containers and clean stir sticks when mixing the product.
* It just wasn't mixed thoroughly. This is more likely to occur if your epoxy is cold. Ideal mixing temperature is about 75 to 80 degrees. If you can't meet these temperatures just be certain your are getting a complete mix.
* Low air temperature increases dry time. Temperature plays the biggest factor in the speed of drying. Even when you are using our fastest hardener low temperature always increases dry time. If the resin can be kept between 75 and 80 degrees, i.e. indoors, before mixing, a lower working temperature is not a problem. But again patience is required.
* The hardener may need to be shaken. If your hardener has not been used for a month or more, we recommend shaking it at least 45 seconds to agitate any settling that may have occurred. This is not necessary for the resin.
* Epoxies cure slower than polyesters. If you are used to working with polyester resins, epoxy cure times will seem to be dramatically longer. They are! And unlike polyesters, there is nothing you can do to speed up the process. Epoxy ratios must be accurate and consistent.
2. I mixed up a quart of epoxy and before I could get it out of the cup it started smoking and cured immediately. What happened?
The amount of time you have before your resin begins to kick in the container is referred to as Pot Life. {link to pot life Q&A ). When determining the size of each batch three factors should be considered. First, the larger the mixture the shorter the pot life. Second, the method of application can play a significant role. Ideally, after the epoxy is mixed you can pour it out of the container and then begin to apply it. Once out of the pot you will have a greater amount of time, referred to as working time, before the epoxy begins to cure. If this method is not possible, smaller, multiple batches should be used. Third, air temperature can effect pot life. A 10 degree difference from one day to the next will affect your pot life. Reduce your batch size to accommodate the higher temperatures.
3. I applied epoxy outside under a covered awning but it rained last night and now the surface is cloudy and sticky. What do I do?
Caution must be taken when using epoxy in cool, humid conditions. Despite working under cover, if it rains while the epoxy is still wet, moisture contamination can occur and the epoxy may never cure properly. If this happens it will have to be removed by mechanical means, either scraping and/or sanding away the contaminated epoxy.
4. I made a part and it is too flexible. Why?
Epoxies are flexible by nature. If you are accustomed to working with polyester resins you will definitely notice that epoxy is more flexible. Cured epoxy will become stiffer after a few days. If a fully cured part is still too flexible it is most likely that additional layers of reinforcements are necessary. Generally, two layers are more than twice as strong as one, etc. Only you can determine your strength and performance requirements and this can only be done by experimentation and testing. Finally, the stiffness of a part can be a function of the system(hardener) chosen. Our 2:1 hardener is our most flexible post-cure system. Our 4:1 hardener is our most rigid system which is still flexible enough to not be brittle. Our most popular system, the 3:1 hardener combines the properties of the 2:1 and the 4:1 systems.
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Whilst Im on embarrassing topics, and off topic, how do you get a picture near your name, because everyone has them and Im curious!
that's simply the rest of the pipe...
