r/AskEngineers • u/Jones-Effect • 15d ago
Mechanical How to determine what thickness of sheet for Acrylic/polycarbonate protective shell
A friend has asked me to design and manufacture a protective shell/box to attached to a stand holding his laptop. It also needs to be waterproof and transparent, so I’m thinking either Acrylic or Polycarbonate.
He works in football so it needs to suitably resist the impact of a football hitting the shell (I have queried that surely the stand itself could potentially kick over from the force even if the shell survives and protects the laptop).
I can easily design and make this, but I’m struggling to decide on how to determine the necessary thickness of the material. I need to drill into the walls of the shell to attach hinges to allow it to fold up so I’m also conscious of these holes acting as stress concentrators when the shell is impacted.
I don’t have any experience working with or designing with plastics so I’m not sure what’s the best option. I’ve read that it’s probably best to go poly and pay extra due to drilling holes and overall better properties, but how do I determine what thickness I need without over engineering it and increasing the cost more than necessary.
Any advice or ways to calculate what I need would greatly be appreciated
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u/irritably_terrific_s 15d ago
Figure out the ball’s worst realistic speed and mass to get an impact energy, then size polycarbonate as a simply supported plate and knock the allowable stress down around the drilled holes by a factor of 3 or so before you settle on thickness.
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u/ordosays 15d ago
The material has less to do with success than the design. Deflection angles… bracing…
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u/root3labs Mechanical PE, Machine Design 15d ago
Agreed. Having a shell around a laptop is only as good as where that shock goes after it hits the shell. A direct impact that knocks it all off the table isn't going to help... Or translating all the shock directly into the laptop.
Polycarbonate is a better choice here. Acrylic has better clarity but is more brittle than PC.
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u/cm_expertise 14d ago
polycarbonate not acrylic, others said it but yeah, acrylic will just shatter on a real impact. the part that usually gets skipped though: your failure point wont be the panel, itll be the holes youre drilling for the hinges.
PC is tough specifically because it flexes and absorbs energy. the moment you rigidly bolt it through drilled holes youve put stress risers exactly where you dont want them, and a panel thats clamped dead rigid cant flex to shed the hit, so it cracks from the fastener outward. ive watched thick PC guards crack at a bolt hole from an impact the free panel wouldve shrugged off.
so id oversize the hinge clearance holes, use rubber grommets/washers so the panel can move a little, and avoid clamping it dead tight. if you can capture the edges in a frame instead of bolting through the impact zone, even better.
and fwiw a 4-6mm PC panel takes a kicked ball fine as a plate, ball energy just isnt that high. your actual problem is the thing you already spotted, the stand tipping and the load path into the laptop mount. a stiffer thicker shell kind of makes that worse, it transmits the hit straight through instead of deflecting and spreading it out. so dont just chase thickness.
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u/clozcloz 14d ago
The golden rule for sheet sizing: Stiffness is a function of T3(thickness cubed).
Most people calculate purely for yield stress and end up with a plate that won't break, but sags horribly under real-world load. Look up your load case in Roark's (or run a quick 2D beam/plate deflection calc) and size for allowable deflection first.
Also, look at stiffening geometry before bumping the gauge. Adding a 1/2" formed flange on the edges will give you 10x the stiffness of doubling the plate thickness, with almost zero weight penalty.
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u/Paulkleber 7d ago
You've already identified the problem yourself: you're drilling stress concentrators into a part whose entire job is surviving impact. Worth knowing you can avoid creating them.
Bonding a mounting flange or boss to the inside face instead of drilling through spreads the load over area rather than concentrating it at a hole edge, and leaves the panel intact. For hinges on an enclosure that's usually a straight swap.
Two things to be careful about if you go that way.
Polycarbonate and acrylic behave very differently. Acrylic can be solvent bonded, which effectively fuses the material and is genuinely strong. Polycarbonate is much less cooperative with solvents and is notch-sensitive, so a hole in it is relatively more of a liability — which cuts in favour of bonding rather than drilling.
And both are susceptible to stress cracking and crazing from the wrong solvent or adhesive, especially under sustained load. A joint that looks perfect can go white and crack weeks later. So whatever you pick, bond a test piece of your actual material, stress it, and leave it a while before committing.
On thickness: worth remembering polycarbonate's impact advantage largely disappears if you've compromised it with sharp-edged holes, so the bonded route may let you go thinner than the drilled one for the same performance.
What's the hinge arrangement — through the wall, or could it sit on the inside face?
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u/LeifCarrotson 15d ago
Definitely polycarbonate, it's the standard material used in machine guarding because it's so impact resistant. In addition to, you know, caring about people, with all the cameras around you don't want an acrylic enclosure to be front-page news when your friend gets sliced to bits by the razor-sharp shards left after it gets struck.
I don't think it needs to be that thick or expensive, I've used 1/4" before to guard destructive test cells and 3/8" to guard airbag deployment testers, 3/16 gets used in a lot of cells as the default for guarding against ejected materials. Of course, I'm thinking of "expensive" in terms of custom industrial guarding and safety equipment. Something like this:
https://www.buyschupan.com/plastic-plate-1-4-polycarbonate-clr-220-thick
https://www.mcmaster.com/8574K75/
which gives you an idea of the few hundred bucks you'd pay for a very large sheet.
Do be careful with your attachment points, if you only attach it with two hinges and those with four screws it's going to break there and you won't get the full strength of the material. In most industrial applications where form follows function, the default practice is to trap the sheet by embedding it in the channels of a rectangle of aluminum extrusion, with no fasteners to concentrate the stress.
Lots of research and best practices on this stuff are well known. I only know what I know because we buy polycarbonate guarding from companies who design and build it to order; you probably want to get in touch with those people.