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Plastic Part Design Expert: you?

design-engine

design-engine

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Lecure on what it takes to be a Plastic Part Design Expert

I am giving a lecture Tuesday night on what it takes to be an expert at Plastic part design.

Curious as to what you guys would list. Might be Ill add your thoughts to my talk.

Here is my short list

1. use parameters to control shell thickness and rib thickness so when the 20 changes trickle down from marketing and such you can handle the changes w/out having to be so attention to detail and make every change yourself. With parameters and relations it updates all automatically.

2 Use top down design to delineate between parts

3 user top down design to delineate between co injected rubber portions and harder plastic options.

4 use of surfacing to create draft when Draft feature doesn't work

5 you know the difference between core and cavity tools and that plastic shrinks and can shrink around the core that's why you need ejector pins to push product off the core. Plastic can shrink out of the cavity thus you might be able to get away with no draft in that situation.

6 surfacing Can you really be an expert at Plastic part design w/o being a surfacing user?

What would you add to the list for being a plastic part design expert?

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Edited by: design-engine
 
more criteria

- can shell anything (with surface offsets)

- can round anything (with surfaces)

- draft anything

- can burn plastic to determine it
 
Determining the right plastic to use.


Why that particular plastic is better than another.


The list goes on...
 
I don't think surface knowledge is a must, although many
plastic parts will require it. There are certainly lots
of plastic parts that don't require surfacing knowledge.

Understanding the difference between the draft hinge and
the draft direction is a big one.

Understanding the basics of how a mold works is big too.
Directly injecting the plastic (hot tip) vs. runners,
types of ejection (pins, blades, sleeves, etc), types of
undercut actions (lifters, slides, etc) and where the
steel goes when they activate (lifters need room to move
to clear the undercut), limits on core dept (for
cooling), etc.
 
I would say there are four main topics one must master to make it all happen at injection molded part design. When you design a part if you can keep it simple there may be no risk, but if the application is not straight forward you better know what your doing.

1) Materials
2) Mold design and mold materials materials (understand what can be done and what it may cost you, also what are the risks in terms of mold maintenance and life)
3) Engineering (GD&T, Stackups, calculations for loading, calculations for strain, time based creep and recovery, stress etc)
4) Cad (strategies and methodologies to design the part and detail the critical features)
Prepare to iterate if you want perfection and only work with a mold house that will work with you to make it happen.

If you only know #4 and your mold house is in China you better be a good talker when things start to go sideways. Document all communications.

Doug is right on, because if you go to mold and all you can do is draw the part you may be in deep trouble when you have to review and approve the mold layout. Even worse if you fail to make a usable part after tooling your reputation could be circling the bowl and you will have to defend yourself. Even after successful defense it will leave a mark.
Best thing to do is never let it get that far.

Ask me how I know.... (I just got done with a project that has 18 molded parts)
It took 10+ sample runs for our mold house to just meet the print, never mind if I made a mistake.
Lessons learned, best learned by lumps. Don't be scared just make sure to cover your bases.


Chris
 
The talk is over (last night) BTW but Chis got me thinking. An even higher level of expertise would involve a group of parts that have to fit and work together... there is the potential of parts tolerances binding as in a switch that has to work in extreme temperatures. Ansys can be used to determine fit problems.

any engineers on the board trying to do this type of tol verification using Ansys to save money before going to tooling?
 
Hi Bart,

Hope your talk went over well.

I did not see this earlier as I don't follow forums that much. I am concerned with higher performance parts
and not throwaway items which have their own set of things to think about.

Agree with pretty much all comments but there is so much more to it. Laughingly I will try to capture some of this
in bullet points:

You have to think about what
you are designing as a plastic part in the first place; You should never
design a 3D model then say "wonder if I can make this from
plastic?" Source of many a failed part/design and a large
source of the generally poor popular conception of plastics.Plastics selection. I think understanding at least the
family of plastics the part you are using for your design is
important. All plastics are most
definitely not equal and their properties affect your design before it
gets near tooling.Understanding your
tooling. If you think you can do a design and get three tooling
quotes then take the lowest price AND get what you want then Fantasy Land
is for you. Maybe because I started as a toolmaker I hold this so
high but MANY problems relate to issues that are designed into the part.Related to this is an understanding
of tooling methodologies. These
have changed markedly over the years. Widespread use of wire cut EDM, and computer
controlled high speed machining are the two most profound. The later affects being able to machine
pre-hardened material as well as the standard material and EDM electrodes
while the former allows for inset fits that used to make grown men cry.Also tooling related is gate
position and type. How is the
material going to fill the part and what effects does that have to both the
plastic flow and end propertiesDraft is already mentioned
and all comments look OK. Regarding surfacing for draft I think this
is essential and as part of this VSS will become a dear friend. Good
point Bart about differences in draft requirements for cavity and core but
it is more subtle than that with depth of draw, draw profile and edges
being some factors.Fillets. So much here. Yes the sometimes difficulty of making
them in the first place but take a step back from there. What needs to be filleted and why? I would argue that everything that is
not a split line, shut edge or must be sharp for some particular reason
should be filleted. Often this was
covered back in the drafting board days with a note saying
 
muadib3d said:
I checked the link and it failed to work.
Click to expand...
<UL>
<LI>Material selection (like polypropylene or polyethylene - check with your material supplier for the proper grade).</LI>
<LI>Gate location (flow must go across the hinge).</LI>
<LI>How the part is handled after molding (the hinge should be actuated several times right after the part is ejected to help orient the material).</LI>[/list]


[url]http://machinedesign.com/article/care-and-feeding-of-living- hinges-0819[/url]


[url]http://www.efunda.com/designstandards/plastic_design/hinge.c fm[/url]


[url]http://www.plasticstoday.com/imm/articles/design-polypropyle ne-part-design-part-2-living-hinges [/url]


Other good references


Dow Polyolefins


Glenn Beall's By Design series


As Mr Brent Drysdale mentioned sharp corners are the enemy. I agree that the old note about breaking corners is obsolete and the reason is because it will do little to relieve molded-in stress if it's less than 1/4 of the wall thickness


View attachment 5693


And knowing and understanding where the gate location is important because the plastic should ideally flow from thick sections to thin. On the other hand, uniform wall thickness is also important. A little trick if you have constant walls is to make the area immediately around the gate 10% - 20% thicker to faciliate flow. If you have to have thick and thin, use ramps and radii at the transistions.


Plastics are key to future prosperity


And finally just read this and this


that's all I have to say
 
I think I mentioned this some time ago;


If you are going to Taiwan, etc., for your tools, make sure they use the appropriate steels, and, appropriate heat treatment. If your parts do not require slides, then, this is less important, however, if you have a slide in your mold, you BETTER have the right steel combinations. Ever here the term gall?


We had some tools come from over there that were not "right". 2000 shots. Should have been good for over 100,000 shots! Yea, we saved a lot of money....
 
Dear all

I think china has a huge potential towards supplying plastic mold products

Professionals coming out of china are establishing new factories around the world

thanks
 
Folks!

I thought this discussion was on aspects of Plastic Part Design and in
particular how this relates to using ProE?

Our company uses suppliers from all over the world and these
include the USA, Taiwan and China.
Surely the only relevant thing as a designer is have a detailed knowledge
of who your suppliers are and what they can do for you?

If you work with one supplier and they have cosseted you
with your parts so that you don
 
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