CBCNSfan said:
However the costs of heating the plastic for the injection process can be costly also not to mention the labour put into designing and making the injection mold.
Had this whole message typed when I apparently tilted my laptop the wrong way and it threw a fit. Oh well...my fingers need some exercise...I've got a lab report due Monday
Ahh...injection molding...now there's a subject I can understand!
Our molding machine is relatively new. And sure it's huge, has 110 tons of clamping force, heats plastic to 400+ degrees and shoots it into a mold at several hundred PSI, but it's actually a bit of a gentle giant. It doesn't take as much electricity as one may think to heat up the plastic...it's not really that much different than an electric stove. Yes it's larger, and therefore requires more energy, but the idea and energy consumption is fairly relative. It's actually a rather quite machine (unless the bored operator starts singing, in which case you are better off turning on a bench grinder, or two
). It's an extremely sensitive machine...which is many times learned the hard way. The other night my dad and I were making a chute of sorts in an attempt to eliminate an extra conveyer and excessive static that was causing the roof walks we were running to cling to
everything, and I closed the mold while it was cold. Since it hadn't expanded as it does when it is heated during production, it moved and extra three or four thousandths, and threw a fit.
The cost of building a mold is something that most people don't understand, and I'm glad you brought that up. Many hours go into design before any metal is ever touched. Researching blue prints, finding photographs, and sometimes even finding and measuring the car itself (made a nice trip to the PA rail museum in Strasburg
and got the whole family out into the 'yard') adds up. Then many more hours are spent in Auto CAD and BobCAD drawing, scaling, programming, etc. Some of the details have to be burnt into the mold via EDM, which means making an electrode, which means writing a CNC program (which is relayed to the CNC via a decade old computer running Windows 3.1
) and often times creating cutters that are find enough to produce the detail. Even before building the mold you have to have a mold base to put it in, and when you get into doing one-piece bodies, you have to have a mold base capable of handling slides, and it can become an extremely complex project. All the while you have to plan out ejector pins so that they can be built into areas in which they won't affect the detail or leave visible marks, and take into account the shrink rate of the plastic, and gate and vent everything and...well...you get the idea.
It's kind of annoying when some "Joe" walks in off the street with some little gadget they want made in plastic and expects to pay a hundred bucks for a full-fledged mold. It's not hard to see why the cost of a mold can easily reach five figures (or much much more if you don't already have a mold base to work with). After the initial investment, each finished part only has a couple of cents worth of plastic in it, but you have to pay for all of the costs it took to get the mold to that point. It only takes about 15-30 seconds to produce a plastic part, which surprises a lot of people, but it takes months to construct the tooling.
I actually made a Power Point presentation about this process for school last year. I built a mold for an HO scale coal load (the Bowser H-43 to be exact). It didn't cost me much since I did the labor and had access to the machinary, but it was also a fairly simple mold (except for achieving the curves in the load...and then programing the CNC to machine them...). They look pretty good once I glue real coal to them. I had hoped to produce a series of these, but I don't have the time to manufacture large numbers. Speaking of time, I think I just spent a half an hour writing this...
...guess I better go do something constructive now
