Breakable without causing a bloodbath! 1

[RathanDog]

Hi All,

I'm trying to build a prototype box that has a clear, breakable front. It's going to be approx the size of a shoe box sitting on its long side.

The trouble I'm having is tracking down a plastic that is breakable but won't shatter everywhere or leave sharp edges (the owner is going to need to be able to reach in through the smashed hole). The only alternative I can think of is somehow scoring the plastic so that it naturally breaks along set lines, but I'd like to avoid the visible lines that this would result in.

I don't suppose anyone has any suggestions do they, I feel like I have a great idea but am falling at this last hurdle!

Any help at all would be much appreciated.

 

[dgowans]

Could you go with your score lines, but hide them by texturing/sandbasting an area that's inside the score lines? For example, texture an area that's inset from the score lines by 1/4" or so, out as far as needed.

 

[KENAT]

Look at how things like fire alarms and related emergency supplies do it? Not totally sure they don't leave sharp edges but you'd hope.

KENAT,

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[insideman]

I think you could design your box, not to break but to come loose. For example, the "breakaway" panel could be mounted on studs withe slightly enlarged heads so when the panel was struck it would free itself from the studs. This would have the further advantage of being reusable.

Let me know if you would like help in detail designing this.

 

[Compositepro]

Break-away glass that is used in movies where actors jump though windows or have bottles broken over their heads are made by casting brittle resins like DER 669. This is a solid epoxy resin that normally would be melted and mixed with a curing agent to make a strong plastic. Without being cured the resin is weak and the edges of broken pieces are will not cut.

 

[patprimmer]

If it is a reasonably thin film, it will break in a ductile manner. PET seems a likely candidate if it can be made weak enough.

Regards
Pat
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[btrueblood]

I had a thought similar to Pat's - a thin-ish film of polyethylene or similar. The boxes I've seen all have glass panel fronts, and sharp edges I would guess are of secondary concern if one is trying to reach for the fire alarm pull inside for "real reasons" (vs. hoaxing).

 

[ornerynorsk]

Is a clear film peelable closure an option? Much safer and more user friendly, no need for the hammer on a chain, etc. Just my 2 cents.

 

[RathanDog]

Hi Guys!

Sorry for the slow reply, have been back up in sunny old North Yorkshire for a few days without access to t'internet.

Thanks for all the suggestions, I had no idea there would be so many people offering advice. Warms the cockles.

I am basically trying to recreate an 'In case of emergency break glass' box but one that could be used without immenent death justifying lacerated fingers! Because of this, there certainly does need to be the satisfaction involved in actually smashing the thing, and I positively don't want it to be reusable because that removes the incentive not to open the box!

The resin and the PET film seem like the most suitable options, so I'll be investigating those further when I get back down to the big smoke next week.

Thanks again everyone for your interest, and I'll keep you posted if and when a suitable solution is confirmed!

p.s. that's certainly not to say I'm not open to more ideas! ;-)

 

[HDS]

PET can be fairly tough to break. Maybe you could design something that deforms but never breaks when smashed and falls off. I am thinking of a 5 sided box that springs open around the opening when smashed.

If that doesn't work you could put a stress concentration in the corners that makes it split. Your hand never goes through the hole full of sharp edges.

Don't discount glass. Car windows have this same issue where you don't want to get decapitated after going through the windshield.

 

[Pud]

Suggest you have a look at BS5839. It might have to be glass for some reason:

http://www.screwfix.com/prods/11193..._-Fire Alarm Breakglass Spare Glass Pack of 5

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[dwightdixon]

You could also make the box by injection molding an outer housing from high impact PP or PET and then pull back the slide and overmolding a secondary incompatible material to form a window in the resulting cavity. When you hit the window it dislodges but does not form sharp edges. Then they can reach through to get whatever they are after. There are several clear material incompatible combinations you could try. You want some bonding at the edge. This design would require only one tool but you would have to process it on a two material overmolding machine. Many are available as injection molding houses in the US and I would presume also in the UK.