Drawing Reqs: Molded Parts 4

[MadMango]

The manufacturing company I work for builds equipment out of 80% sheet metal components, 20% machined components. We are updating some of our product lines, and are starting to incorporate molded parts. My (broad) question centers on what drawing requirements are needed to receive components as specified on the drawing?

For example, do I need to specify the parting line if its non-critical? Shouldn't I leave this up to the vendor?

Do I cover the major draft angle in a general note, and dimensionally specify other draft angles that deviate from the note?

Any opinions and help welcomed.

[green]"Art without engineering is dreaming; Engineering without art is calculating."[/green]

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[MikeHalloran]

Don't specify a parting line unless it's a design feature; it hardly ever really is, and a clever molder may not put it anywhere near where you expect it to be.

Don't specify any draft at all on the first issue of the drawing. Take a print to your molding partner and go over it with him in some detail. Work from his markups for the first revision.

Especially for low production volumes, there are mold systems that work better with zero draft, so don't just assume you need it.

When you have mutually agreed on how much draft you need, and in which direction it tapers, make sure the drawing is unambiguous. Exaggerate the taper for effect if you need to. Better; dimension both ends and the angle.

Do not be real quick to change molders. They all do things differently, so you can't just rip a mold out of one guy's machine and install it on another guy's machine and get good parts. The new molder will almost always insist on design changes to adapt the part to his way of doing and thinking. Factor substantial mold change costs into any decision about moving a mold. Lots of standard contracts assert ownership of the mold, but they're largely a waste of time, because molders don't interchange. Did I mention you need to develop a close relationship with one or few molders?

Always mold a part number, revision level, and a date code in a part, and a cavity identifier for multiple cavities. Always start with one cavity and work up.

It's good to have a talented molder, but a design that wins a prize for the molder is hardly ever a winner for the customer. The best molder can help you arrive at a design that makes his life easy, and fulfills your requirements to make your life easy.

Stay away from high performance resins unless you really, really, really need them. Most are hideously expensive, sensitive to process control and therefore subject to mysterious rashes of field failures when the process changes, and some physically erode or corrode the molding machine. Having to buy a new molding screw for every run of parts you buy can get expensive. Ask up front about stuff like that when making material choices.



Mike Halloran
Pembroke Pines, FL, USA

 

[ajack1]

Work in 3D.

 

[ctopher]

MM,
Do you intend to have a vendor create the mold and do the molding for you? Or, are you going to design the mold and pour the mold? I'm not clear.
If the vendor is doing everything, just send them the 3D part and let them at it.
If you are doing the design, you can send a vendor a dwg to check. I have done this before. Most are nice enough to help out.

Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)

 

[MadMango]

Thanks for your input Mike. We have done basic molding before, but usually related to taking simple delrin machined parts (spacers, bushings, etc) and converting them over to injection molding for lower part costs.

We have also done thermo forming or vacuum forming, and rotational modling for simple covers, drainage trays, and close-out panels. Now we are looking into injection molding, and I assume its an entirely different beast as far as drawing requirements are concerned.

Ajack1, yes, 3D is the way to go. I have been communicating with our vendors via PDF drawings and parasolid models. So far, so good.

Are there any "standard" notes that I should be using on drawings? Are there standard notes related to how much flash is acceptable and how it should be trimed? Are there standards for surface textures? Can anyone recommend published standards or specifications that I should be investing in?

[green]"Art without engineering is dreaming; Engineering without art is calculating."[/green]

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[Heckler]

These are typical notes for molds we build both in house and out of house.


NOTE: UNLESS OTHERWISE SPECIFIED.

MATERIAL: MOLDED FROM TRANSPARENT POLYCARBONATE PANTONE COLOR 2728C. COLOR MUST MATCH EXISTING XXXXXXX PARTS.

FINISH: INTERIOR SURFACES TO HAVE A SPI/SPE #3 FINISH.
EXTERIOR SURFACES TO HAVE A MOLD-TECH #11010 FINISH.

EXTERNAL FEATURES DRAFT TO BE 2<MOD-DEG>.
(-) = DRAFT SUBTRACTS FROM DIMENSION.
(+) = DRAFT ADDS TO DIMENSION.

INTERNAL FEATURES DRAFT TO BE 1<MOD-DEG>.
(-) = DRAFT SUBTRACTS FROM DIMENSION.
(+) = DRAFT ADDS TO DIMENSION.

EJECTOR AND GATE LOCATIONS SUBJECT TO APPROVAL
BY XXXXXXX ENGINEERING DEPARTMENT.

Best Regards,

Heckler
Sr. Mechanical Engineer
SWx 2007 SP 2.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
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[ctopher]

Some links:

http://www.emachineshop.com/molding_machines/injection_molding.htm

http://www.protomold.com/designguidelines/

Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)

 

[MadMango]

Heckler, I think that is what I was looking for. Thanks. What does "SPI/SPE" stand for?

Ctopher, we are creating the design, but the molders are making all tooling. Our vendors are sophisticated enough to only need the model, but I still need to generate decent 2D drawings to capture the design and give our QA department something to do and use for FAI. I will check your provided links as well.

[green]"Art without engineering is dreaming; Engineering without art is calculating."[/green]

Have you read faq731-376 to make the best use of Eng-Tips Forums?

 

[dgowans]

SPI/SPE

Society of the Plastics Industry / Society of Plastics Engineers

Or some such nonsense. You can get sample plaques from SPI / SPE which have examples of different surface finishes you can specify on an injection mold/molded part. I don't recall exactly what they are, but the A series are the most highly polished, if I recall correctly.

For more examples of surface finish and different injection molded part design guidelines, I'd recommend going to

http://www.protomold.com

and request that they send you their sample part. It's a pretty unique marketing tool that illustrates different surface finishes and different things to consider when designing an injection-molded part. If you're an experienced plastic part designer a lot of it is old hat, but it's a valuable tool for plastics newbies.

 

[Heckler]

MM,

SPI - The Society of the Plastics Industry. These are guides of typical surface finishes which aid in the communication of what is expected from the mold maker. Sometimes we will just call out a light EDM finish if the part is not critical in appearance. The Mold-Tech is another surface texture specification.

Best Regards,

Heckler
Sr. Mechanical Engineer
SWx 2007 SP 2.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)

(In reference to David Beckham) "He can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. Apart from that, he'