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Casting and molded draft embedded in the solid model

Casting and molded draft embedded in the solid model

If anyone has/had some experience on modeling castings and/or plastic parts, I really would like to know what are the prons and cons of moleling the draft angle, in other words do you include the draft angle in the model? Or it is just shown on the drawing as a note and the solid model is made as being straight surface/cylinder?

I am trying to define the best practice going forward toward the implementation of MBD, so consequently this question came up.

RE: Casting and molded draft embedded in the solid model

It depends, but mostly I'd go with draft. It is necessary to have good conversations with the mold/pattern maker to ensure the best match between the model and the intended part. Other than that it breaks out the same way that other features with tolerances get modeled: is it the max draft, the min, the average of the two, or somewhere in between?

The con is that it can be a lot of work to get the draft right along with creating blends. It can easily add five to ten times the features over the prismatic model. And if the vendor changes there will end up being another version to create.

The pro is that it can give insight into features that may be expensive to mold/cast; and it can uncover some really stupid interferences before the expensive parts make it to the assembly area leaving a lot of unhappy thumb twiddlers looking at the designer who missed some little detail, possibly while he cleans out his desk.

RE: Casting and molded draft embedded in the solid model

Tool makers have enough to do without adding draft to my models. They get the final model. One less cook.

RE: Casting and molded draft embedded in the solid model

Definitely include maximum draft, you cannot run a proper casting or molding sim without it. At a previous employer which owned several foundries we took it a step further by modeling each half of the mold and cores, and linking them via imported surfaces so as changes were made to the model the foundry could easily see the impact and adjust accordingly.

RE: Casting and molded draft embedded in the solid model

I am dealing with moldings that have thin walls and it's really easy to make mistake and create a feature that will turn into nothing with draft applied afterwards.

So, definitely draft included in the model. Unfortunately that means making other decisions like parting lines, etc.

You have to accept the fact that molding models / drawings are very process-heavy.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: Casting and molded draft embedded in the solid model

I agree with all the above, model the draft required to ensure you know what you're going to get from the supplier.

"Art without engineering is dreaming; Engineering without art is calculating."

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RE: Casting and molded draft embedded in the solid model

We model all draft, Fillets, that we want in our Castings, and injection molding. We use Cad verification tools to verify that there is draft, and the part if fully filleted per the split line. We discuss this with the casting supplier while we are designing to make sure we get the proper draft and die split lines accounted for.

RE: Casting and molded draft embedded in the solid model

IF your casting is even remotely complicated, or has thin features or nonlinear shapes, there is no alternative but to model the draft.

RE: Casting and molded draft embedded in the solid model

My friend Chuck and I went to visit a molder who was new to making what was a very old and high volume part for us.
It had thin walls and a very small specified draft and some optical specs, but we hadn't gotten that far.
There was a lot of discussion about the 'hot tip' in the mold, and whether that was the cause of the problem, i.e., the whole bottom pulling out of the generally square-ish cup shaped part on mold separation.

The molder happened to have the actual mold on his desk, and a tiny toolmaker's adjustable square.
With permission, since I had never touched such a square, I noodled around with it, measuring various planes on the mold with it.

After a while, I remarked to no one in particular that the mold seemed to have zero draft, vs. the specified quarter of a degree.

The molder immediately grabbed the square and the mold, and made a phone call to the person who had actually sunk the cavity, and with his toolmaker, also present, they had a heated discussion about whether sinking the cavity included adding draft, or not.
... in front of Chuck and I, their customer's representatives.
It was kind of embarrassing.

We left shortly, and within a day or two we got a few buckets of good parts.

I'm thinking now that modeling the draft in the model might be a good idea.

Mike Halloran
Pembroke Pines, FL, USA

RE: Casting and molded draft embedded in the solid model

Mike, Ding! Ding! Ding! We have a winner!

It also reinforces my suspicion that many suppliers don't actually do inspections, and depend on the fabricators to self-check. Apparently some don't inspect even when there's a problem.

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