Hi again jhov:
When I approach a project like this, the first thing I ask my customer is:
How many parts do you want to make and how fast?
The second thing I ask is : How long must the tool last between major rebuilds?
These two simple questions define a great deal about the build and set the conditions for the next question which is: What's your budget?
To help you with this here are a few rules of thumb I like to use to give me a reality check:
1) A single cavity mold running on a 30 second cycle working three shifts can produce 60,000 parts per month.
2) An aluminum tool runs a bit faster than a steel tool (better heat conduction) but is quite a bit more fragile and quite a bit less fixable.
3) An aluminum tool is typically pretty much done with 60,000 shots but it can last as little as one shot if the molder fucks it up. By contrast a steel (prehard P-20) tool will often go 300,000 shots and still be usable.
4) An aluminum tool is not really cheaper to build than a steel tool, unless a ton of material has to be milled to turn it from raw blocks into a finished mold.
Aluminum is faster to machine, but a pain in the ass to surface grind (it's non magnetic); it costs more to buy; thermal expansion has to be considered and mitigated in a different way; it can't tolerate shallow draft shutoffs on features like parting surfaces and pass cores etc etc.
5) Hot tip and hot runner molds are to be avoided unless you need them for something you can't get with a simple cold sprue especially for short run projects.
6) A fully hardened mold for high volume production (Class 101) is going to cost around three times what the equivalent pre-hard steel or aluminum mold is going to cost.
Moving on to your part specifically; without seeing a part design and without knowing all the details I see a mold consisting of two big slides with the part cavities cut so the part is molded standing up between the slides, upside down and stripped off the main core that come from the cold side with a floating stripper that carries the slides.
There's about a gazillion reasons why this part orientation is attractive to me; not the least of which is that I can eject the part without having to accept ejector pin marks on it, and I can gate it on the bottom and achieve good plastic flow naturally out to the periphery.
A mold like this would probably be about 10" x 16" x 10" tall, cost around thirty grand Canadian to build out of mold base alloy with nitrided P-20 slides, case hardened locks, a P-20 prehard core, a fully hard stripper insert and a simple DME cold sprue
If you don't know what all of those terms mean, I recommend you look them up or ask here.
I'm sure Dennis will have more to say about what he sees as the best way forward, and it may well differ from mine!
Be aware if you mold these in one piece, the plastic will be awfully thick, which brings its own problems...if you do it in two halves you have more options to core it out and also a simpler mold to build with no slides and no stripper and a simple ejector plate and etc etc. (assuming there are no features I can't see in this one picture).
Cheers
Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
Oh yeah, while I'm thinking of it...you wrote about scrapping unworkable molds...are you aware of just how much you would have to scrap if you inadvertently build a dog?
The mold I sketched out above would be around 200 to 300 pounds and about 200 hours of work...that's a lot to do over if you didn't get it right the first time.
MC