Obtaining proper draft for injection molding

I’m modeling what could be categorized as a canister or container with a lid that is held on by latches (3) that are retained by “ears” (one on each side of the latch). The ears will be a part of the injection molded cylinder and therefor require a draft angle of 1 deg. So far I’ve been inserting a profile of the ears, extruding them and then inserting a rectangular plane, tilt it to the proper angle, extrude it a sufficient distance, moving it to the proper location and then using Boolean difference to cut the little wedge off of the ears.

This seems a little “brute force” to me. I*'m new to 3D so my mind isn’t fully adapted yet., but there should be a simpler method.

Thanks, in advance for helping this 69 year old “newbie” :slight_smile:


KMAC, sounds like you are getting it done. Brute force or not.there is a command called taper extrude. It may help. You can give it a draft angle. Use flip if its not going in the direction you like.
Most people can help better if you can supply a picture or file to look at. Maybe you xan find some close examples of models on youtube that have some of the same ideas that you need. Good luck, Mark

Unfortunately the taper extrude is in the wrong plane for that purpose, but thanks for the heads up, I can use taper extrude on another part.


KMAC, can not you extrude tapered on any plane? You can also reverse the taper by typing in say -5 for a draft angle. You can extrude drafted on odd angles by drawing a hint line, and then choosing “direction” for an option, and choosing both the start and the end of your hint line as the vector/direction towards you want to extrude to.

If things get really really strange, you can always loft things if you have the beginning curve, and come up with a second curve that describes how big your object will be at the end of the extrude, well, loft.

Is 1 degree enough. Are you allowing for shrinkage? : )

Hi there, Im also doind my first design for injection molding. When you solid-extrude tapered, you get an option in the command line Direction, and with that you can set any direction to the tapered extrusion. But… you want the tapers square to the mold faces don’t you?

You folks are obviously way ahead of me. I only started with Rhino in February of this year.

I’m learning a lot from you.

The 1 degree is what the molding company requested.

It’s a little complicated. I’ve attached a quick model of the part (ear). I inserted the profile and extruded it. I’ve incorporated the draft in the profile. Now I need it in the opposing faces. The part line will be in the middle of the face shown in the front view. The draft needs to be 1 deg up & down in the opposing faces shown in the Right view. I’ve also attached an earlier version (KMAC-1)(saved small) to give you an overall view which should help some.

Thanks again.

ear.3dm (47.9 KB)

KMAC-1.3dm (1.56 MB)

See the response and models just sent.


dont understand how the mold works if the cyan surface is the parting surface, you have a very serious problem with the red surfaces. Also, how will the holes in the ears be made? Side actions make the molds very expensive.

A couple things you should know if you want a good result.

First of all don’t trust the Rhino solid modeling tools. There is nothing the solid modeling tools can do that you can’t do with surface tools. The difference being that you can make sure you get it right with the surface tools while the solid modeling tools will get it wrong more often than not and you have no way to prevent those errors. .

Secondly, you need to learn to avoid small tangency errors in your curves. You can use the GCon tool to check tangency. There is no reason your lines and arcs can’t be tangent here. Extrude taper works well with tangent lines and arcs and not so well if you make lines and arcs with small tangent errors.

There are only 4 curves segments you need to extrude tapered.
Explode the curves before extruding and then trim and join manually. After you made one side you can use mirror to make the other. You also need to make a planar cap and make sure the whole part (cap and tapered sides) intersects cleanly with the main body.

Here is what I came up with : Draft_added.3dm (516.7 KB)

I didn’t trim and join the parts together because you probably want to add draft to the front face first.

Thanks for your input.

There also is a revolve command. It will revolve curves around a central axis( that means interior and exterior curves). Could come in handy —Mark

It just occurred to me that maybe you wanted the draft going the other way. I assumed the reference direction for draft was the front cplane Z axis. If instead you want the direction to be the top cplane z-axis then you don;t need to use tapered extrude at all. You just extrude straight the curves with the draft included. The other sides can be made with planes rotated 1 degree.

You have the draft going the wrong way on your outer curves and you still need to be careful about maintaining tangency on the lines and arcs.

Here is the part with draft going the other directionDraft_added2.3dm (578.8 KB)

I appreciate all of the input and advice. I suspected that it was a mistake to send an old model and it was. I did I so that you could see the ears in context. The question was an easier method to get draft in the sides of the ‘ears’. I haven’t seen a clear answer to that question. Please refrain from analyzing the old model and address the issue.

Please understand, I have a way to do it, but I thought that there should be a simpler method. Not having heard one, I guess I will go back to ‘brute force’ which someone pointed out is acceptable if it works and it does. J


What is the issue?
The old model provides some clues as to what you are trying to accomplish.

Seeing what the end result is supposed to be would help with figuring out what it is you want.

Thanks, I’ll give it a look.

Good stuff thanks.

I really do appreciate the constructive comments and suggestions. As an update, I have submitted models of 4 products to the molding company and they all passed. :>)