Where to start in mold design?

hey there, I'm looking for information (books, sites, etc) that give some guidelines on injection mold design, I have been getting some of my customers asking if I can fabricate parts in plastic in quantity and I wind up referring them to another company to design the molds, I feel like this is lost money since I have a 4 axis haas vf2 and would invest in a sinker edm if I thought I could keep it in use. I am fluent in solidworks, I just need to know how to build molds to interface with the machines, basically looking for how they are set up, standards for different gates, recommended temperatures, troubleshooting, etc. I know it's a rabbit hole but I'd like to get my foot into it.

IIRC Penn State University, Behrend Campus (here in Erie) offers a degree
in plastic molding design.

They probably have some classes you could take.

smp4616 said:

I know it's a rabbit hole but I'd like to get my foot into it.

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It's a sink hole! If you've never built a mold before, you have a lot to learn and will need a lot more equipment than a machining center and EDM. Most of what you'll need to know isn't in a book somewhere. The good designers can look at a part or drawing and know how to build the mold. Where to put the parting line. Where to put the gate(s). What type of gates. Then, you get into mold finishes which are in the hundreds. It's a specialty that some excell in and others flounder. If there is any quantity to the run than pricing can be down into the $0.0001 range. Then you start talking hot runner and lots of cavitities. It is a very competitive business to be in. Most of the shops are competing with Chinese shops now.
JR

Plus^^^^^ most shops that build molds have presses to try out the mold and do short production runs. So not only do you need to be able to design and build a mold, you have to be able to run it.

Tom

The only thing I could add is learn to set up and troubleshoot the molds in presses for a few years first. If you only know how to make the molds you will have no idea how much the set up guys are dicking you around when they are doing your test shots, then telling you your mold has problems. Plasitc injection molding is complicated, and pretty much everything is interdependent.

JRIowa said:

It's a sink hole!
JR

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On top of a volcano...
Where to start? Everything JR said is right, and Tdegenhart knows an awful lot about molds. Machining is the easy part. You'll almost certainly need a surface grinder too, and experience with bench/hand-work and fettling.
It would be far easier to sub the design out to someone who knows what the are doing, and then see if you think you can make it.
I've been in the industry all my working life, made a few molds, designed a hell of a lot more more molds than I've made - and I'll still sub out the design on some really complex parts.

i worked at a mold shop for a year, i have a grinder, and the tools to grind and measure radii, angles, etc. I have everything for the grinder that we used at the mold shop, so I'm familiar with the tolerances required and can hit them, I'm more looking for standards for mold base sizes, design principles for runners, gates, where to place ejectors, how injection machines are typically set up and operated, I could go back to work at a mold shop but frankly I enjoy making decent money and not listening to dad rock on satellite radio all day... again, not looking to start taking in all kinds of quotes for complicated plastic parts, just looking to learn how to build molds for the mostly simple things my customers are asking for, which are almost all 2 part molds with maybe a slide or two but mostly very simple. I think I had enough hands on training to build the things, I just need really to know where to start to learn how to design them... I'd also not be opposed to buying a small injection machine if I felt confident enough with a few successfully designed and built molds completed.

DME's catalog is a good place for common mold frame design, but not all molding machines have the knockouts in the same locations so you may want to confirm with the molder. The knockout location issue may have changed some in the last 25 years, it's something that bit my in the ass when I started making molds at home. I learned the trade by doing, I did everything involved from part design to running the press in production with our 200 ton press. My employer was a mold maker and the part design was driven by need, it was all structural sailboarding components. Common design rules for mold making were something I learned after I learned the trade, thankfully, as they are almost all complete BS. Common wisdom on draft, amount of ejection needed, wall thickness variation in a part, family molds of vastly differing size parts, etc have nothing to do with reality, in my experience. 90% of getting a mold to work well are gate design and good cooling, with gate design being most of that. Really, learning by doing is critical to making molds that run well. The best information I ever got was through the material suppliers, Dow and Dupont have some really good design manuals. Sorry but I don't know of any books. If you do find some books read them through completely, but do not blindly believe anything they say, there really are no hard and fast rules. As I said earlier it is very complicated, everything is interdependent, and can change quite a bit depending on what resin you are using, without even getting into compensating for shrinkage.

Maybe talk to some of the big resin suppliers in your area to see what info they can offer.

I've been building molds fr 40 years, and have done a fair amount of design. Simple parts aren't all that difficult to design for, if you know the basics. Many parts just aren't that critical, and sometimes a customer need parts by the hundreds or thousands, not millions. Even when I had my own shop, though, I often subbed out mold design. It helps to be in a good relationship with a local molder that can sample the tool. Most small molders that I worked with also knew a fair amount about mold design, and we would usually meet to discuss the design before committing it to steel.

I have a pretty good mold design book somewhere- I'll post the title if I can find it.

thanks, that was what I needed, I looked at the dme catalog and found the 3d files for the mold base assemblies, that helped a lot, jogged my memory a bit. if you can find that title that would be great, I've still got a lot of questions about how things worked that I built, but never saw operating.

The D-M-E catalog is great for standard mold bases and components - as is the PCS catalog. For more design info for side actions and such, check out Omnimold:

MOLDs by OMNI Mold Systems

Dennis

Just my opinion but if you really want to do this find a mold shop you can work with and become really, really good friends. If you want to design molds then you have to know how they work, it's not rocket sience and it's not magic. If possible watch the molder do some test shots on a new mold, this is where the most information you need right now is. You also want to be there when your molds get their test shots, do favors for this molder, be their buddy. Do this and mold design is not difficult, you may make a few mistakes, no big deal as long as it's only cheaper molds. Under 250,000 ish shots you can make your molds out of 6061 aluminum, above that I used QC-7. Ejector boxes and backup plates were all 6061. One nice thing about aluminum is the molding cycle time is around 40% less than a steel mold, and it's a hell of a lot easier to machine than 4140.

smp4616 said:

I'm looking for information (books, sites, etc) that give some guidelines on injection mold design

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The attached image shows books at the nearest university library to me (in New Zealand). Unfortunately, this forum's software will compress the titles and authors to be unreadable but if you care to suggest an email address, I'll send the full-sized photo. Dubois' "Plastics Mold Engineering Handbook" ISBN: 9780412989513 would be a good start.

Suggestions: Offer your machining skills to a friendly mold-shop, to repair or to modify an existing mold, without being too ambitious at first. You'll later need their help to try out any mold you might make.

Construct a first mold for an item that you wish for yourself or have a personal interest in.

Mold-making and injection moulding are two separate skills. Fix one molding problem, then two new issues arrive, and so it goes on.

-Brian McKenzie

I have some books I use for reference but also farm out the high cavity and or complex molds for designing.It just saves time and a good designer can have the design finished by the time you can figure out where to start. Just recently I was stuck on a part design change and spent several hours working out what I thought would be the best way to go. I sent the file to the outside designer we use for his opinion and within a minute of looking at what I came up with he said how about doing it like this, which was less work and less complex then my idea. My idea would have worked but would have taken more time and expense.
Here are some books for basic injection mold information.

Plastic Injection Molding by Douglas M. Bryce

Injection Mold Design Fundamentals by Glanvill and Denton

Runner and Gating Design Handbook by John P. Beaumont

Also check Hanser Publications for technical references as well as SPE (Society of Plastics Engineers)and SME (Society of Manufacturing Engineers).
Good luck.

JRIowa said:

It's a sink hole! If you've never built a mold before, you have a lot to learn and will need a lot more equipment than a machining center and EDM. Most of what you'll need to know isn't in a book somewhere. The good designers can look at a part or drawing and know how to build the mold. Where to put the parting line. Where to put the gate(s). What type of gates. Then, you get into mold finishes which are in the hundreds. It's a specialty that some excell in and others flounder. If there is any quantity to the run than pricing can be down into the $0.0001 range. Then you start talking hot runner and lots of cavitities. It is a very competitive business to be in. Most of the shops are competing with Chinese shops now.
JR

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I'm sure most mold shops would be glad to share their experiences with you . . .
My instincts tell me that unless you have an established corporation that willing to support you because they have a ready market . . . it is best not to pursue this line . . . as mentioned it is basically a closed market of seasoned veterans that have all the support they need . . . and more.

HuMor said:

I'm sure most mold shops would be glad to share their experiences with you . . .
My instincts tell me that unless you have an established corporation that willing to support you because they have a ready market . . . it is best not to pursue this line . . . as mentioned it is basically a closed market of seasoned veterans that have all the support they need . . . and more.

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How many molds have you designed ?
How many molds have you machined ?

smp4616 said:

hey there, I'm looking for information (books, sites, etc) that give some guidelines on injection mold design, I have been getting some of my customers asking if I can fabricate parts in plastic in quantity and I wind up referring them to another company to design the molds, I feel like this is lost money since I have a 4 axis haas vf2 and would invest in a sinker edm if I thought I could keep it in use.

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Scrounge up Santa Cruz Mike, he's gotta be bored with Nawth Cahlina by now. Send him over to Dongguan for the occasional "purchasing" trips and he'll hang with you forever

If he doesn't have a heart attack ... them three-ways take a lot out of a guy !

"Injection Moulding of Plastic Components" by John Bown is a good general introduction that deals with mould design, gate sizes, etc. Still, mould design and injection moulding is really a separate profession and seldom economical to do it yourself for an occasional part.