I use those bar clamps and love them. The actual bar on them is metal and the ends are plastic. Irwin sells 3 sizes of them. The smallest generates 150 lbs or pressure, one does 300 lbs and the big daddy can do 600lbs. THe one CF is using is the 150lbs. The original ones I bought were that size. They still work great 2 years later with 5+ days a week shooting. Now when I buy new clamps I get the 300 lbs. THey definitely bite better. A tip on the bar clamps... If they start to slip you can take some steel wool or sand paper and run it along the metal bar to give the mechanism something to grab.

I still revert back to my 2 possible problems... Using too much pressure on the injector or something was in the mold not allowing it to close. Bear uses thinner stock on some molds and with the big 5/8 runner you can get some flex in the mold if you get on the injector a tad. I found this out with my crappie dancer mold. Should take much more pressure than the weight of your arm to fill the mold.