Plastic parts-drafting technique

[magneplanar]

Drafting methodology question:

Do you design plastic parts as a vertical wall object and draft everything, or try to build in as much draft as possible and add drafts on the remaining features?

benefits of the approaches?

alternate approaches?

leave the draft to the toolmaker?

cheers,

M

 

[pjw]

Sometimes you can't use the draft feature and have to build in your draft with features like variable section sweeps/ If you are CNC cutting your moulds then it will be very difficult if not impossible for your toolmaker to add draft later on. You are best adding all necessary draft to your model even if it does add some complication for you. Your finished tool will be of much higher quality regarding split lines etc.


Phil

 

[michael3130]

Each job is different so sometimes I would build draft into my skecthes. Other times, it's best to leave the draft till later, sometimes near the start. There are a lot of different factors that determine when draft should be added when modelling a plastic part.


One thing I would never do though is leave the draft to the tool maker. They will never know the product as well as I do and not know the design intent. Yes, they can advise and tweak if necessary. Also 2 parts joing together in an assembly, the toolmaker may not be aware of this and adding draft from the wrong place could give misalignment between surfaces.

 

[dgs]

I find that it's almost always easier to deal with orthographic (undrafted) geometry and then add the draft where needed. Adding additional features to walls that are off angle get challenging.


If you think about it, you almost always conceptualize a part as undrafted and add the draft to make it mold-able. Why not build it as you think of it? Better captures your design intent in my mind.


There are times where the 'draft' is built into the desired form and times where adding it into the sketch makes the most sense for some reason, but I'd say 95% of the time I add it on later.

 

[design-engine]

I find there is never one way. On the simple parts you might do the verticle wall and draft. Maybe split draft.

On the more sculpted forms you get to use a parting surface and sweep your draft ... or use surfacing....

Who is at the conference? My cell is on my profile. Long beach kinda sux but I have a convertible.

 

[AHA-D]

By all means put draft in your model. It's the only way to find out where surfaces are added or "eaten", where radii work and where they fail. Especially with drafts of considerable depth there can be a lot of difference between drafted and undrafted models. There are also cases where you make use of draft to build plastic parts where front and after of the mould create special features. You can't accurately know where you'll end up if you build the model as being straight.


Alex

 

[davisdony]

What all which is told above is correct I would
like to add some thing more



It will be very good that while you are drafting keep the

minimum material condition on the component and maximum material condition on the cor and cavity




This in return will help you in the later stages,

if you accidentally made some
errors

or

some minor modifications is required



Making corrections by removing material is easier than adding material

 

[magneplanar]

AHA-D

a guy once showed me how easy it was to go to tooling if I built the model with draft. It certainly isn't universal. But I try to do what I can. I do try to draft anything that I don't want the toolmaker messing up. After awhile I feel comfortable building with draft. I have no sold my coworkers on the idea haha.

davisdony,

you sound like someone who has made a few plastic parts and had to get some welding done on the mold. Haven't we all hahaha...thanks for the tip

design-engine,

don't stay in LB with that convertible, malibu is calling, or maybe a bit of the san diego...do you wanna tijuana? hehehe

 

[davisdony]

<div style="margin-left: 40px; text-align: left;">don't stay in LB with that convertible, malibu is calling, or maybe a bit of the san diego...do you wanna tijuana? hehehe





I did not get you a this sentence

What is that about



Keep in touch

</div>

 

[dgs]

I find there is never one way. On the simple parts you might do the verticle wall and draft. Maybe split draft.

On the more sculpted forms you get to use a parting surface and sweep your draft ... or use surfacing....

Who is at the conference? My cell is on my profile. Long beach kinda sux but I have a convertible.

I wish I had known your cell was here, I'd have called you. Oh well, I'll see you tomorrrow.


You got a car? And a convert. too? Lucky! Shuttles and foot power for me.

 

[design-engine]

I'm in the Hilton till Thursday. I speak at 1:45 where I compare Alias workflow to Pro/E workflow and talk about the degree of the curve limitations in pro/E and discuss workarounds in Pro/E

Edited by: design-engine

 

[michael3130]

Bart,


Are any of your talks on YouTube?


Our budget wont allow us to go to Dublin this year let alone LongBeach.

Thanks

 

[Zestje]

Having experience with complex plastic parts (like e.g. mobile phone housing parts) I consider the ONLY correct way to have all drafts and all radii already in your model!

Why?:
To me this is the only easy way to prevent yourself from making geometry which is impossible to achieve with a tool and it also guarantees that the toolmaker makes exactly the geometry you want to have. Believe me, if the geometry is not 100% clear it is likely there will be differences between how you intended it and how the toolmaker made it. Especially e.g. in slider areas the drafts on the geometry might be critical for the tool making. Moreover, you can't be sure the function of the part is still guaranteed when the geometry changes because drafts are added by the toolmaker.
I also advise to add all radii to the part for several reasons. The first is that there is no discussion with the toolmaker when parts have already been produced about how the edges and corners should look. It also means that for edges and corners which do not have rounds I specifically want to have these sharp. This is again a very critical issue for tool making in slider areas.

How?:
Since most of our parts have some kind of freeform design surfaces the first step is to make shell geometry out of these with correct draft angles. Ideally, the design surfaces already have the necessary minimum draft angles, because most of the time these surfaces must be used exactly the way they are and drafts simply change geometry.
Then I work my way through the part adding functional areas -with- drafts going from main functions to details. For example snap hooks can be made by just using surface operations and adding drafts and radii, before turning them into volumes which are melted to the part. Then you can add further radii and drafts for the area, where the snap hook has been joined to the main geometry. The most important advantage is that this will give you a very stable model, because most features for every functional area do not influence the other features already in the part. This specifically goes for drafts, which can have great (unnoticed) influence on already constructed geometry. It also allows for faster and easier changes to the part.

The choice whether to add drafts in sketches or as separate drafts really depends on a combination of experience, common sense and luck...

@davisdony:
Your way of using min/max material condition does not make sense to me. At least not for the parts we make. We provide the toolmaker with the exact nominal geometry we require. Of course, there will be adjustments made to the tool, but not making the nominal geometry would mean that even if the parts functions are all ok, the part will simply not be as accurate to the intended geometry as it can be by making tool adjustments for deviating areas. I also think that min/max material condition will not save you from making tool adjustments most of the time. Errors are seldom corrected just by adding some material.
Only exception in my experience are surfaces I specifially design to be adjusted later. For example flat surfaces which position parts to eachother can simply be adjusted to make a better fit if there is too much air gap between parts.

That's enough for now... have fun!

 

[dgs]

I'm in the Hilton till Thursday. I speak at 1:45 where I compare Alias workflow to Pro/E workflow and talk about the degree of the curve limitations in pro/E and discuss workarounds in Pro/E

Who were you speaking to on Thursday?


My flight out was canceled and I ended up staying an extra night near the Orange Co. airport. They have a flight restriction - no takeoffs after 10 PM. Keeps the beach front property owners happy. My airplane didn't arrive until 9:50, so I was out of luck and had to stay over and catch a 6:45 AM flight on Thursday.


Enjoyed your talk, BTW.