Is JB Weld Any Good?

[CNC Molds N Stuff]

JB Weld melts at a lower temp than lead.  Why do people use it? 

Lots of people use JB Weld, because the amount of time its above melting is measured in fractions of a second.  It will burn up/out eventually, but that may take a long time for smaller castings.  Even bigger castings can run more than once or twice.  Silicone burns out when casting (or spin casting) lead too, but people still make and use silicone molds.  Same with Bondo.  Many a saltwater casting or flutter jig has been cast in a Bondo mold. 

Sure, an aluminum mold properly machined (or die cast) for the result you want is generally best for hand pouring or pouring from a pot, but even that is a compromise.  Die steel will outlast an aluminum mold, but its more expensive to machine and cools much slower / holds heat longer.  Aluminum allows much faster cycle times, and is cheaper to make than steel. 

Silicone, Bondo, JB-weld have a much shorter life, and doesn't transfer heat as well as aluminum, but its generally easier and therefor cheaper for some people to make. 

IT IS ALL ALWAYS A TRADE OFF.  People looking for that one best, most bestest, perfectest, solution are doing themselves a gross disservice and as a result not always passing on the best information for everybody that might be talking to. 

Will it fail?  Yes! Absolutely it will fail.  Every solution will fail eventually.  Its just a choice based on a variety of factors not the least of which might be time to market of cost (of which time is a factor) per failure. 

I'll post an unrelated reply that illustrates my point after this post.

[CNC Molds N Stuff]

I was a licensed contractor for 23 years. Before that I was a technician for other companies doing the same or similar type work. Sometimes directly as an employee. Sometimes on a per job basis. Starting out I couldn't afford to spend 2-300 dollars on a Milwaukee drill. I could afford 29.95 for a 1/2 inch Black & Decker from K-Mart. If I had the attitude of only the best or nothing I would never have done any of that work and never made any money.

I bought the Black & Decker from K-Mart and did dozens if not hundreds of jobs with it, but after getting paid for only a few jobs I then had the money to buy the more expensive drills. I've bought a bunch of them over the years, and many of them did fail too, but if I hadn't taken that half step knowing it would fail I would never have been able to afford that better drill.

My first circular saw was also a cheap Black & Decker from K-Mart. It was the cheapest way for me to cut steel. LOL. Seriously. I made thousands of cuts with it using an abrasive blade before I got a torch or a chop saw or a plasma cutter. Now I own all those tools, but the Black & Decker circular saw paid for them. Sure now I have several circular saws including one designed for cutting steel with a carbide steel cutting blade and a Skil Worm Drive (the best IMO) for breaking down lumber, but that Black & Decker from K-Mart paid for them.

I'm not advocating cheap tools or stop gap measures over better tools and methods. I'm saying you have to make the best choice for you at the moment.

Now I make aluminum molds on CNC milling machines. I still can't afford a half million dollar machine or a multi million dollar building, but my little toy Taig desktop machine paid for the machines I have now.