Molded Part Detail Drawings 2

[griffithbuilt]

Where can I get a primer on detailing injection molded parts with variable draft angles?

Is there an asme standard on detailing molded parts and draft angles?

I've recently joined a new engineering firm that has a lot of injection molded parts and needs some clarifying of the drawing standards when it comes to detailing these parts and I would like to know of any tips or tricks in detailing, notating and simplifying the detail process of these potentially complex parts.

Thanks in advance. I'll gladly expand this topic and ask more pointed follow ups as the discussion moves along.


Jason

 

[MikeHalloran]

Showing, or even indicating, the draft angle on a part implies that you know the best shape of the parting surface.

In general, you don't.

Make a preliminary drawing of the part with no draft. Include the dimensions and features that you actually need for function, mating and interchangeability. Be sure to indicate areas where you need best properties and best finish.

Go over that drawing in detail with the molder on your team, and let him tell you how much draft you will need and in what direction, how and where the part will be gated, and where the ejector pin marks will be. The results of that meeting may surprise you; molders are clever.

Different molders are clever in different ways. That's why I said 'the molder on your team', not 'any molder'. You need to customize the design in accord with the preferences and customs of the guy who'll most likely be making the actual part, in order to maximize the chances of actually getting the part you want. Molders and molds do not interchange, at least not for free.




Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Also, required minimum draft angle depends on a few variables, being polymer type, specific type, section thickness, mould surface temperature , quality of finish and direction of polish, precision required, colour or other additives that might vary, cooling time, effective injection pressure at various points and the list goes on.

Like Mike says, talk with your moulder and maybe raw materials supplier.

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.

 

[insideman]

I have found that every dimension affected by draft should be dimensioned by X.xxx+-.oox +xDEG/side. Or equivalent. This takes more time and space. But the discipline that makes you think about each dimension helps prevent errors. Toolmakers love it!

 

[griffithbuilt]

I'm not so much looking for design tips on what draft angle to use as much as I am looking for information on how to detail the parts and draft angles that are already designed (in tandem with the moldmaker) at the drawing level.

I'm guessing based upon the responses I'm getting that there is no set ASME or similar drafting standard that determines the way the dimensions specific to draft angle should be put onto a drawing and thus, the molders prerference becomes paramount to a global standard.

Seems to me like an excellent way to get what I want from my team member...but what happens when that team member has left or another molder is brought in to work on new products, now i've got two standards to follow (or three or ten or fourteen depending on number of molders in the shop).

I certainly agree with the + X°/Side addition but it seems to me that there may be a standard that has a lot of additional tricks for efficient ways to detail injection molded parts at the drawing level.

Thanks,


Jason

 

[MikeHalloran]

Molders don't interchange as well as MBAs think they do.
Moving a mold among molders usually involves a near total rebuild on the receiving end. If the mold has only one or two cavities, it's probably cheaper and quicker to just build a new one.


I've had some confusion with dimensions like x.xxx +n deg draft, because of disagreements about which way the draft is intended to go, i.e. which way is '+', and because of trig errors in converting the angle to an incremental dimension.

I've had fewer confusion problems with drawings that put a dimension and a tolerance on both ends of a drafted surface, eliminating the need for anyone to do any trig, and with section views that greatly exaggerate the draft angle and specify the exact location at which measurements are to be made.



Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

Mike,

As a moulder - a star for your comments!!

Perhaps I should not say this, but it is reality...

As a moulder - tools pre-made and sent to us are worth $$$! We can say things like "the coolings rubbish", "well, this is what it makes!" etc, etc. and consequently the customer pays whatever it costs. (Remember: "The one-eyed man is King in the land of the blind!)

Alternatively:

Customers who sit down with us and our toolmakers get the optimum designed part, the optimum material choice (noting that a survey in the UK about 10 years ago found that 90% of moulding were "overdesigned"), get a good price - we have even got work up against the Chinese!

Cheers

Harry

 

[MikeHalloran]

Thanks, Pud.

I hate to say this, but every mold that's ever been moved for me (I have _never_ done it voluntarily) was held for ransom just as you describe.

The ransom was paid, the mold eventually made good parts again, and ... not one of them damn idiot managers learned a damn thing.





Mike Halloran
Pembroke Pines, FL, USA