McDonalds effect: SLA parts can be too accurate...
Updated: Jul 6, 2021
I bought a burger at McDonalds, and it looked nothing like the menu image.
I’m sure you’ve also encountered this effect at many fast food restaurants.
Marketing companies do this all the time to help sell products.
They spend hours preparing the models for the photoshoot.
They want the parts to look as perfect as possible.
But when burgers are made at high volume and low cost, quality suffers.
This effect also happens in product design and engineering.
I see this a lot in die-casting or injection molding part designs.
Injection molding and die casting require high pressures and temperatures.
These can cause quality issues on final parts.
Cooling rates, shrinkage, warp, and underfilling are failure modes that affect part quality.
But these issues don’t show up on 3D printed parts.
3D Printed parts look perfect, especially with high-res resin printing technology!
I designed a plastic enclosure for a device.
My CAD model had a small designed latch feature.
I couldn’t make the feature any bigger because of the wall thickness of the plastic.
To validate the design, I ordered a few 3D printed samples of a plastic part that was designed to be injection molded.
The SLA printed parts looked amazing. The locking feature worked well.
I inspected the parts and they matched the CAD model.
The client was thrilled. I was happy.
With the confidence of the high-res SLA printed sample, I ordered the injection-mold.
But I didn’t realize my mistake until later…
Six weeks later, the first sample parts came back.
The parts didn’t fit together. The small injection-molded feature had shrunk, sunken, and warped.
This made the latching feature non-functional.
After talking to the vendor, it would be difficult to control dimensional tolerance due to radial shrinkage.
The feature was poorly designed for injection molding.
I always expect issues when ordering the first injection-molded samples.
This was not the issue.
The issue was that I put too much faith in my SLA printed sample.
I had selected a design that couldn’t easily be manufactured with molding.
Design for Injection Molding
When printing parts that are stand-ins for injection-molded parts, remember:
Flat printed parts won’t necessarily be flat after molding
Small features that will resolve in SLA printing won’t necessarily fill inside an injection mold
Thick sections in 3D printed parts will create thick sections and sink in molded parts.
Knit lines in molding create weak zones. 3D printed parts don’t show knit lines.
Don’t assume that just because it looks good in printing that it will work in production.
Design your parts with your final production method in mind.
I recommend using an FEA simulation tool like Autodesk Moldflow or Moldex3D to approximate the mold flow/shrinkage, etc.
Solidworks also provides tools like Thickness and Flow analysis for measuring wall and feature thickness.
Let me know if I can help you work through any prototype engineering challenges.
Make it a great week!
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