Intro: Quick Repair for Chipped Plastic
Repairing the plastic handle of my son's paintball gun with pourable urethane.
Step 1: Prep
The plastic handle of my son's Spyder paintball gun had chipped around a captive nut and broken off leaving a small crater so that the nut had fallen out and the CO2 cartidge fitting could no longer be bolted up securely.
You can see in this first picture that I took a standard #8-32 SHCS, started it into a nut, and set it into the now fully exposed slot where the old nut had once sat. I did no prep work to the broken area other than wrapping electrical tape around the end of the handle to hold the urethane in and form the vertical side. I assumed that the rough surface of the fracture would contribute to more area and a better grip between the two mating surfaces that would be formed.
I obviously could have done a better job getting the electrical tape to lay straight, but I wasn't too concerned as the folded over portion was in a non-critcal area and I knew from experience that this urethane is easy to work with after it sets up. As a matter of fact, I couldn't have cared less about the final appearance of this project - it was to be a function-only fix.
I possibly could have used JB Weld, but I feel that this pourable urethane epoxy was a much better choice in this particular case for a number of reasons. Most notable is that it will flow down into every nook and crevice, which in this case means that it will automatically form a perfectly fitted "foot" that hooks under the unbroken portion of the handle, which will ultimately strengthen the repair's hold. I would be afraid that JB Weld (or any equivalent putty-type epoxy) would only form a nice mold of the cavity, but then just fall out as a plug releif - and this very thing has happened to me on more than one occasion.
In JB Weld's defense, I once used it to seal a small leak from a heat transfer fluid resevoir. The 15 gallon resevoir had gotten a bottom corner worn through by 30 years of being rubbed up against by hydraulic hoses, no less. Without shutting the machine down I stuck a JB Weld ball on the leak, being sure to get it wedged in the hole as well as on both the vertical and upside down surfaces. These surfaces were also at about 250 degrees F, mind you. This was in 1998 and as of mid-2006 that band-aid job was still holding the Ucon heat transfer fluid in the barrel coolant resevoir.
Step 2: Mix & Pour
While the Spyder's handle was held in the vice I mixed the urethane parts A & B into a 1:1 ratio as per the instructions. This particular epoxy sets up pretty quick and only has a 2 or 3 minute pot life (OK, I know that I mentioned the instructions just now... but I haven't looked at them since I bought the material almost a year ago...) This means you want to mix it up and get it poured fast.
I used a small plastic case that I found laying around the garage and had wiped out with a shop towel to mix the urethane in. I used two capfuls of each part. Any reliable source will advise you to use clean, dry equipment and to measure your materials carefully.
You can see in this image that I poured the urethane into the cavity until it was roughly to the top of the cracked out area. Again, this particular urethane has a very thin viscosity which helps it pour very quickly and lets bubbles come to the surface and pop, rather than being trapped or needing help popping as is the case in thicker pouring resins.
Step 3: Wait
I always enjoy watching this stuff set up! When it is first poured it's a clear amber fluid. As it begins to harden, it loses a little clarity and then little wisps and fingers start to coil outward from one spot. You can actually see the hardened area grow, like a time lapse video of a pond freezing over
Step 4: Finish
After the resin had set up, but before it was fully cured, it was still a little soft and pliable. At this point I gently unscrewed the machine screw a little, then screwed it back and forth a couple of times. I did this to prevent a permanent bond between it and the urethane.
I also used a razor blade to trim the excess material from the top and sides. If I was worried about the finish I could have sanded it out - or even bought some black pigmented epoxy and waited for it to ship for that matter.
Step 5: Voila
After waiting a couple of hours for the resin to further cure, I bolted the CO2 adapter fitting to the handle to make sure everything lined up. It was off just a tad so took a little finagelling (sp?), but she got on there. We put the handle grips back on and gave it another 16 hours of unstressing rest to fully cure before Nick took the Spyder back out into the field where he reports that it performed like a champ!
It actually took me longer to find 2 #8 by 3/4" screws and nuts than to mix, pour, wait, and "finish" the resin. The total time from placing the screw into the handle to bolting the fitting back on the handle was about 15 minutes and I estimate that the cost of the rigid urethane resin used was about 30 US cents - less than the cost of mailing a letter.
The resin I used was RP-40 from Dascar Plastics with whom I am not affiliated in any way, just very happy with this product. It is has a shore-D hardness rating of 75 and ran me about $25 including shipping for 2 quarts.