How do I make an injection mold?

[gijim]

I have a small 8" x 8" x 3" fiberglass/wood/bondo piece that I want to turn into plastic. It has to be done with an injection mold due to the various angles involved, and it's mostly hollow. Walk me through what I would need to go from prototype to product...

-I have a proto I can take measurements from.
-I assume I need some 3d modeling software.

 

[MikeHalloran]

- If you have a physical prototype, why would you need to build a software model of it? That assumption troubles me. The simplest way to get where you want to go is to engage someone who's done it before to help you test your assumptions.

Failing that...

- That's not a small injection molded part; you will need a lot of money, for the mold, and for machine time.

- Injection molding gets really expensive for hollow parts.

- For some kinds of plastic, and some shapes, hollow parts are more easily made, in cheaper molds, by rotational molding or blow molding.

- You haven't given us a clue about the application environment the part will see, e.g. service temperature, electrical stress, required tolerances on geometry, anticipated loads, etc., so we can't make a recommendation or a guess about which 'plastic' you need.


There are basically half an infinity of 'plastics', and picking one, or one family, needs to be your first step, after the wallet biopsy.








Mike Halloran
Pembroke Pines, FL, USA

 

[gijim]

The prototype is 'flawed', in that it's asymmetric (as best my hand could sand and eye could see ;-) ). I figured if I had a 3d model, I could send it to a CNC machinist, and have them carve out the outer mold portion. Then it would be perfectly symmetric.

It is an automotive spoiler stantion. I already figured ABS would be good for it, for mechanical and temperurature requirements, and good paint adhesion.

I don't mind spending my time learning what I need to do, vs spending money. CNC machine time is cheap compared to people time. ;-)

 

[patprimmer]

Take the model to a toolmaker and get a ball park quote. Just the price of the steel for the mould set might shock you.

You will probably abandon the project at that stage.

If you are still conscious and enthusiastic come back here with some real information and we will give you real advice.

There is an old story about this guy that had a secret product. He sought advice on what material to best make it from, but could not disclose it's design nor use. He also did not know the market size, but claimed he would need many millions of them.

The recommendation was to use this secret material. We cannot disclose it's properties nor price, but it will cost a truckload.

The quality of an answer can at best only reflect the quality of the question.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[gijim]

I have. The estimate was in the 4 figure range. The steel would probably be a couple hundred, I would imagine. I would think that's a bargain, but I would like to do as much as I can to reduce that cost, even if that means learning foreign things, etc. Ideally I would like to get it made for $1000.

I have access to CAD/CAM software at school. What software would be necessary to create a model that a toolmaker can use to create the mold? In other words, what is universally accepted, and what should I focus my time on learning?

 

[patprimmer]

You have an unbelievably cheap toolmaker. I do mean unbelievable.

Why not ask him what file types he can work with.

He should be able to plot one half to create a model, then flop it over to make the other half symmetrical. If he can't do that, he should know someone who can. Plotting a digital image from a model is very cheap these days, with the equipment being a built in option on machine centres.





Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MikeHalloran]

If the part is in the shape of an airfoil like tube, then it will need a deep cavity, and a pretty big piece of mold steel. You probably can't do that for $10,000.

ABS gives you the option of molding it in two halves and solvent bonding them together. That mold will have a much shallower cavity, which is cheaper to sink and to polish, and will require a smaller block of die steel.






Mike Halloran
Pembroke Pines, FL, USA

 

[spider007]

-first of all are you making just one part or are you running a huge production

If you just need 5-10 parts then you could probably get away with using aluminum, with out any complicated ejection mechanisams.

-Software can differ from company to company...I suggest you find out if the software at your school can export files as "iges" or "step" file format. Those formats are standard and can be imported into almost any CAD software out there, so you can use the software you know best

-If you are doing the cad model your self, one of the things to keep in mind is draft. You can't really have any vertical walls, 1 deg draft can be good enough....

-next thing you need is to develop a parting line (along with the designs) I suggest you leave this for the mold shop. But if you insist on doing it your self, I'll help you out...i like a chalange.

 

[gijim]

I'm looking to do a couple hundred parts or more. It depends on the cost of getting the parts made.

I'd rather not have any ejection system, and pay the penalty in increased production time per unit.

I guess the software at school is Solidworks, which I have used before, albeit briefly.

Good info on the draft angle, I likely would have forgot to include that.

 

[spider007]

another coupole of things to keep in mind;

-thick wall parts cool slower and shrink more then thiner parts...keep the material thickness nice and even.

-if you have any ribs, they should be no more then 70% of the part thicness (including the rad on top of the rib) otherwise you will get sink marks on the opposite side...

 

[bright81]

I think you can make a rapid prototype mold first. if that is okay, then you consider make a tool of production.

Bright
Sr.Project Manager&GM
HT-MOLDS

http://www.ht-molds.com

 

[Justin14]

Have you ever thought of sending the part file out and have it 3D printed. We have a 3D printer here at our shop and we create all our proposals out to see the fit and function before we produce a mold for the product. The parts are made out of ABS and we paint them before we send them out to our customer.