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Posted: 1/11/2006 12:49:59 PM EDT
I need to get several molds made. Unfortunately, they are bloody expensive. However, at least one of them is fairly simple and I believe I could do the machining myself.

Are there any good sites or books discussing mold design considerations? In particular, how many, umm, hose thingies need to be connected, and where, and how to set it up so the plastic doesn't freeze in the, umm, connecting hose thingies, and air vent holes and how to keep them from filling up, and, er, stuff like that?

Basically, I can cut the cavity without any problem, it's all the other stuff -- actual plastic delivery into the mold -- that I am worried about.

Also, any issues regarding using rubberized material vs. hard plastics?
Link Posted: 1/11/2006 12:58:52 PM EDT

Originally Posted By 71-Hour_Achmed:
I need to get several molds made. Unfortunately, they are bloody expensive. However, at least one of them is fairly simple and I believe I could do the machining myself.

Are there any good sites or books discussing mold design considerations? In particular, how many, umm, hose thingies need to be connected, and where, and how to set it up so the plastic doesn't freeze in the, umm, connecting hose thingies, and air vent holes and how to keep them from filling up, and, er, stuff like that?

Basically, I can cut the cavity without any problem, it's all the other stuff -- actual plastic delivery into the mold -- that I am worried about.

Also, any issues regarding using rubberized material vs. hard plastics?



Umm.. you sound like you don't belong anywhere near an injection molding machine. It's not a garage project, you're dealing with high pressures, high temperatures, and mold clamping forces of a few tons to thousands of tons.

Not sure what you are trying to do but it sounds like this would be better suited for you:

http://www.cloneawilly.com (NSFW)
Link Posted: 1/11/2006 1:04:13 PM EDT
Plastic Part Design by Rober A. Malloy would be a good book to read. It gives an insight of Thermoplastics and Thermosetting plastics. It discusses gate placements and melt flows and all that good stuff. Duke is right though. You should probably leave it up to a dedicated mold shop. Good book to have around though! hippie.gif
Link Posted: 1/11/2006 2:35:50 PM EDT

Originally Posted By SR71:
Plastic Part Design by Rober A. Malloy would be a good book to read. It gives an insight of Thermoplastics and Thermosetting plastics. It discusses gate placements and melt flows and all that good stuff. Duke is right though. You should probably leave it up to a dedicated mold shop. Good book to have around though!



Thanks for the votes of confidence, guys. I'll get the book *and* the clone-a-willy kit. That way, if the book doesn't do it for me, I can always go f*ck myself.

Really, thanks. Good points.
Link Posted: 1/11/2006 2:42:11 PM EDT
I have a friend in the plastics molding business. For the dies he has made, they are $50,000-$100,000 a set.

The steel has to be very high quality, and the heat treating has to be perfect, otherwise the 40,000 psi will crack it.

Plastic, when molten, is not like pouring plaster, molten metal, etc. It is very, very viscous, thicker than tar on hot pavement. It will just barely move, and takes very high pressure to do so. The dies are opened about a minute after injection, and the part dropped out.

At the pressures involved, you need some knowhow on the die making process.
Link Posted: 1/11/2006 2:45:56 PM EDT
Are you sure it needs to be die casted. Sand casting is much cheaper if your design works with it.
For casting molds go to Asia. That is where my company does thier die casting. Mold start at $15,000.
Link Posted: 1/11/2006 3:17:10 PM EDT

Originally Posted By 71-Hour_Achmed:

Originally Posted By SR71:
Plastic Part Design by Rober A. Malloy would be a good book to read. It gives an insight of Thermoplastics and Thermosetting plastics. It discusses gate placements and melt flows and all that good stuff. Duke is right though. You should probably leave it up to a dedicated mold shop. Good book to have around though!



Thanks for the votes of confidence, guys. I'll get the book *and* the clone-a-willy kit. That way, if the book doesn't do it for me, I can always go f*ck myself.

Really, thanks. Good points.



Well you got a great sense of humor at least. I'd recommend you use these guys, it'll probably be the best route:

www.emachineshop.com/

As mentioned, it's a pretty complex process. I'm not sure what machine you are using in your garage but your typical injection molding machine takes a very trained person to operate much less design. You got your mold heating/cooling temperature, resin temp, pressures, hydraulics, etc.. to balance.

Also don't forget to account for shrinkage (for your IM part, not the clone-a-willy).
Link Posted: 1/11/2006 3:25:57 PM EDT
I used to work in an injection molding factory, you need more than just a mold, the mold goes in a special machine that has a vat for the plastic stuff, all kinds of hydralic crap, it's not something you just throw together.
Link Posted: 1/11/2006 3:36:50 PM EDT
[Last Edit: 1/11/2006 3:41:26 PM EDT by the_great_snag]
Sounds like you need an injection molding press for starters. I can't imagine trying to injection mold plastic without one. Shouldn't cost more than a few thousand to get a decent used one assuming you have a large concrete floor and sufficient electrical hookups. OH and a ton of training on how to operate it and maintain it.

The molds themselves are pretty complex too. You need to think not only about shrinkage, but parting lines, draft angles, ejector rods, coolant channels, and a half a million other details. And that's assuming you aren't talking about adding slides block and angle pins or hydraulically actuated inserts, etc...

There are other ways to skin a cat though. What are you making and how many parts are you planning to make?

You can pretty easily make molds for epoxy resin and it can give good results, especially if you don't mind doing a little trimming an tweaking of the finished parts. There are also other compounds and techniques out there for home or hobby molding that are quite good and much much easier, especially for small runs and prototypes.

Model railroaders use this technique quite a bit for making various scenery elements, etc. I wish I had some links for you but I'm drawing a blank. I would suggest looking up some model building websites and look for information on this.

EDIT: rubbery materials generally have much higher shrinkage rates and tend to flash through much smaller vents, and can even flash on the parting line. Most plastic parts require no deburring except maybe breaking off the gates and runners. Rubber parts generally need a lot of trimming and deflashing.

And.. if you're talking about true rubber molding it's even worse, because you bring temperatures and curing times to the equation, and there's no regrinding your scrap. True plastic molding basically just needs sufficient temperature to plasticize your material, and pressure to fill your cavities. Then it's a simple matter of letting the cooling system on the press cool it down to a solid state and letting the ejector system spit out the part. MUCH faster cycle times than rubber parts.
Link Posted: 1/11/2006 4:25:15 PM EDT
How big are the parts that your trying to mold? I know a guy that has a 75 ton newberry for sale.
Link Posted: 1/11/2006 4:45:59 PM EDT
You're in the center of the Universe of injection molding; it can't get any cheaper, or easier to find a mold designer and production shop.
Link Posted: 1/11/2006 4:58:09 PM EDT
Some plastics can be machined, such as Delrin. This is a strong, self lubricating plastic that is used as bearings, gears (R/C Cars, R/C helicopters, many other things), and other products. It can be precisely machined.
Link Posted: 1/11/2006 5:00:28 PM EDT
I am a toolmaker, and the book we used during our classes on molds is called MOLDMAKING &DIE CAST DIES FOR METALWORKING TRAINEES by John Kluz. I am sure you could find this book through MSC or other shop supply companies or maybe Amazon.

The hose thingies you speak of are for circulating water or sometimes hot oil around the mold to aide in cooling which in turn increases cycle time, ie production. Also the venting you asked about is very important. This lets the gasses created by the hot plastic escape. If these gasses are not realeased it can cause many problems such as burn marks or partial fills. These vents, depending on the type of plastic can be from .0002 - .0007 deep and around 1/8 wide (at parting line). Also you can vent through pins in deep sections in the cavity.

As far as rubberized vs hard, it will really depend on the design of the part you want to produce. Each has its own qualities.

Someone on here mentioned very high quality steel. This would be these case if you were building a long running high production tool. If you are looking for small runs I would suggest going with a grade of steel called P20. It is a good quality steel that is already hardened, but can be sawed, milled, or turned pretty easily. I have built a good amount ot molds from this steel.
Link Posted: 1/11/2006 5:20:45 PM EDT
I'm a tool and die maker with a side business doing composites work. If you only need a few hundred parts, you could make a composite (i.e. glue and fiberglass) mold off of a male plug. This would be a lot easier to do than making an injection mold, and the plastics are a powder/liquid mix that flows like water with no heat needed-all you have to do is get it into the mold without any bubbles. A friend of mine is doing this for a company that wanted a few hundred trial parts before going to China to get a mold made. He did 5 molds off of his original plug (plug is still good) and is making parts as we speak. I have no experience with this plastic, but the molds and plug are just like I do. Here's some of my work:

F150 Lightening Carbon Fiber intake plenum and 1/2A Racing airplane molds:

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