Author Topic: Very small and thin transparent enclosure  (Read 4928 times)

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Offline jmajaTopic starter

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Re: Very small and thin transparent enclosure
« Reply #25 on: April 12, 2019, 09:56:25 am »
That might be a good option for the heatshrink, but I also need to have a stiff option that allows changing the battery and different kind of installation.

I took a quick survey of what I found on my desk. Two scientific calculators from the 80's. Phillips had M2x5 machine screws that seemed to screw into thread inserts. Casio had 2x6.5 mm thread forming screws into plastic. My old Samsung S4 Mini had M1.4x4 machine screws that probably screw into metal frame (didn't open it just took one screw out). MR16 LED "bulb" had M2x4 machine holding the PCB and screwing into metal (aluminium?) frame.

All of these seem to open and tighten just fine.
 

Offline reboots

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Re: Very small and thin transparent enclosure
« Reply #26 on: April 13, 2019, 07:36:26 pm »
Be aware that custom-machined polycarbonate, particularly using screw closures, may lead to serious problems with stress concentration and cracking. Do not allow your machine shop to use any kind of process coolant or lubricant other than water.

I recommend producing a set of prototype enclosures of the final intended design, using material from your expected production supplier, and torture-testing them for as long as possible to make sure that e.g. thermal cycling won't cause stress cracking at the screw threads.

I have had some luck with Plastite trilobular thread-forming screws in molded polycarbonate enclosures.

The calculator enclosures you mentioned are likely made from ABS or similar plastic, which is softer and less prone to stress concentration.
 

Offline jmajaTopic starter

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Re: Very small and thin transparent enclosure
« Reply #27 on: April 15, 2019, 05:15:45 am »
Be aware that custom-machined polycarbonate, particularly using screw closures, may lead to serious problems with stress concentration and cracking. Do not allow your machine shop to use any kind of process coolant or lubricant other than water.

I recommend producing a set of prototype enclosures of the final intended design, using material from your expected production supplier, and torture-testing them for as long as possible to make sure that e.g. thermal cycling won't cause stress cracking at the screw threads.

I have had some luck with Plastite trilobular thread-forming screws in molded polycarbonate enclosures.

The calculator enclosures you mentioned are likely made from ABS or similar plastic, which is softer and less prone to stress concentration.

Are threads cut to PC the most likely problem? Would using a recession for a nut or a thread insert make these problems less likely? This thread insert would be thin enough (even when recessed): https://www.tappex.co.uk/products/brass-threaded-inserts/microbarb

The first enclosure prototype will be made tomorrow. If that seems feasible, the plan is to make ~20 and test them for 6 months before going further.
 

Online Nusa

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Re: Very small and thin transparent enclosure
« Reply #28 on: April 15, 2019, 05:54:52 am »
If you design your case to have interlocking hinges arrangement on opposing sides (or even all four sides), you wouldn't need screws at all. Wire pins through the hinges would keep it closed. Easily removed for battery changes.
 

Offline reboots

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Re: Very small and thin transparent enclosure
« Reply #29 on: April 15, 2019, 01:27:20 pm »
Are threads cut to PC the most likely problem? Would using a recession for a nut or a thread insert make these problems less likely? This thread insert would be thin enough (even when recessed): https://www.tappex.co.uk/products/brass-threaded-inserts/microbarb

I have not used threaded inserts in polycarbonate, so I can't say. I would try oversized holes with minimal barb engagement, just enough to keep the inserts placed. I know some manufactures of polycarbonate enclosures use inserts, but those might be overmolded.
 

Offline jmajaTopic starter

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Re: Very small and thin transparent enclosure
« Reply #30 on: April 16, 2019, 06:56:23 am »
Do not allow your machine shop to use any kind of process coolant or lubricant other than water.

What is the reasoning behind this? Machine shop would like to use the same cutting fluid mixture they use for other stuff. Mainly water, but does include other stuff to make it non-corrosive and better lubricant. Is PC sensitive to these chemicals?
 

Online Ian.M

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Re: Very small and thin transparent enclosure
« Reply #31 on: April 16, 2019, 10:38:37 am »
Do not allow your machine shop to use any kind of process coolant or lubricant other than water.

What is the reasoning behind this? Machine shop would like to use the same cutting fluid mixture they use for other stuff. Mainly water, but does include other stuff to make it non-corrosive and better lubricant. Is PC sensitive to these chemicals?
Very!  If exposed to the wrong solvents or certain oils, Polycarbonate becomes very brittle and then cracks readily and can even craze like broken safety glass just from internal stresses and diurnal temperature cycling.
https://www.theplasticshop.co.uk/plastic_technical_data_sheets/chemical_resistance_guide_polycarbonate_sheet.pdf

A little dish soap can be added to the water to lubricate the cut as well as cool it, but you wont be popular with the machine shop unless they are set up for a lot of plastics work with a separate mill, free from metal cutting fluids, as the dish soap will FUBAR their soluble oil coolant and may wash away too much oil from the mill table mechanism causing it to rust.
 

Online Psi

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Re: Very small and thin transparent enclosure
« Reply #32 on: April 16, 2019, 10:53:19 am »
+1 for heatshink tubing.

Not the normal heatshrink though, you can get special stuff that is thinner than normal heatshrink and sets much harder.
It's the stuff used on RC speed controllers and the like.
I think it's usually called 'thin wall' or 'ultra thin wall'
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline jmajaTopic starter

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Re: Very small and thin transparent enclosure
« Reply #33 on: April 16, 2019, 02:47:30 pm »
Machining seems to be quite challenging. The 6 mm thick part we used as stock bends too easily making very difficult to get the thin walls (0.8 mm) right. One had 0.8 mm, but the next only 0.45 mm, which was way too flexible, but still surprisingly difficult to break. I even hit it with a hammer (not that hard) and it survived. The enclosure appears to be too flexible in total making it probably leak in use. It's now only 4.7 mm thick with 0.6-2.2 mm walls. Probably will try to use ~6 mm total thickness allowing for 1.2-2.8 mm walls and more margin for the O-ring.
 

Offline reboots

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Re: Very small and thin transparent enclosure
« Reply #34 on: April 16, 2019, 04:20:31 pm »
Do not allow your machine shop to use any kind of process coolant or lubricant other than water.

What is the reasoning behind this? Machine shop would like to use the same cutting fluid mixture they use for other stuff. Mainly water, but does include other stuff to make it non-corrosive and better lubricant. Is PC sensitive to these chemicals?
Very!  If exposed to the wrong solvents or certain oils, Polycarbonate becomes very brittle and then cracks readily and can even craze like broken safety glass just from internal stresses and diurnal temperature cycling.
https://www.theplasticshop.co.uk/plastic_technical_data_sheets/chemical_resistance_guide_polycarbonate_sheet.pdf

A little dish soap can be added to the water to lubricate the cut as well as cool it, but you wont be popular with the machine shop unless they are set up for a lot of plastics work with a separate mill, free from metal cutting fluids, as the dish soap will FUBAR their soluble oil coolant and may wash away too much oil from the mill table mechanism causing it to rust.

Seconding Ian.M's explanation, which describes my experience. As as alternative, your machine shop might prefer to machine dry with compressed air for cooling and chip removal. You may still need to wash the parts in detergent to remove any oils from the tooling.
 


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