Author Topic: Plastic for lining aluminum  (Read 1752 times)

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Online blueskull

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Plastic for lining aluminum
« on: September 15, 2018, 04:52:49 pm »
A quick question for the enclosure/machining gurus here.

I'm designing an aluminum enclosure for a gadget, and in the process, I need some square holes. Since I can't get perfect square holes from machining, I would just dog-bone the holes and line the aluminum surface with a 1/32" plastic that's been laser cut to exact size.

The question is, which plastic to use?

Here are some more info about my requirements:
1. Adhere to aluminum with obtainable adhesive, preferably reworkable glues (no epoxy please) or double sided tapes.
2. Cutting laser: fiber, I will be using Popolu laser cutting service.
3. Surface finishing: doesn't matter, I will sandblast it.
4. Bendable, 90 degrees on 1/32" thickness, C-shape from flat sheet.
5. Durable, shouldn't scratch easily (matte surface alleviates this, but still the harder the better).
6. Can be engraved with clear and crispy text at 6pt.
7. Can be cleaned with IPA and soapy water.
8. Available in dark grey or black.

Any suggestions? Thanks in advance.
 

Online NiHaoMike

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Re: Plastic for lining aluminum
« Reply #1 on: September 15, 2018, 05:26:51 pm »
How thick is the aluminum? AvE once showed how to drill square holes through thin metal, where "thin" is up to a few mm for aluminum.
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Online blueskull

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Re: Plastic for lining aluminum
« Reply #2 on: September 15, 2018, 05:42:52 pm »
How thick is the aluminum? AvE once showed how to drill square holes through thin metal, where "thin" is up to a few mm for aluminum.

I have other uses for the lining. I need to conceal an antenna, and I also need to provide isolation to connectors from chassis.
 

Online blueskull

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Re: Plastic for lining aluminum
« Reply #3 on: September 15, 2018, 06:58:23 pm »
I'm thinking Noryl, is that a good idea?
 

Offline Barryg41

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Re: Plastic for lining aluminum
« Reply #4 on: September 15, 2018, 11:28:07 pm »
I made plastic for a living. We made aluminum can liner base resin. It was LDPE at a 3MI with 3 to 5 % vinyl acetate. It is very clear, some companies use it for hot glue. We made lot's of it. But they made many other grades for different types of can lining. HTH
 
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Online Cerebus

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Re: Plastic for lining aluminum
« Reply #5 on: September 15, 2018, 11:58:32 pm »
How thick is the aluminum? AvE once showed how to drill square holes through thin metal, where "thin" is up to a few mm for aluminum.

I suspect you're talking about broaching, AvE has certainly covered it. Broaching works for huge thickness of metal and high accuracy - those gun barrels that aren't directly forged are broached so we're talking about potentially material 'thickness' measured in whole metres. You can broach arbitrary shapes to pretty much arbitrary accuracy. It's a fantastic technique. The downside of broaching is making the tooling and having suitable machinery to pull the broach through the workpiece. For small broaches and low thicknesses it's pretty easy to manufacture the tooling on a lathe, sometimes with a little help from a milling machine, and improvise in the absence of a dedicated machine for pulling the broach through.
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Offline coppercone2

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Re: Plastic for lining aluminum
« Reply #6 on: September 16, 2018, 12:25:16 am »
clickspring on youtube shows how to easily make a broach useable for aluminum and brass

otherwise you would need to put a bit of work on it with a shaper or even just files.
« Last Edit: September 16, 2018, 12:36:02 am by coppercone2 »
 

Online Cerebus

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Re: Plastic for lining aluminum
« Reply #7 on: September 16, 2018, 12:32:37 am »
The question is, which plastic to use?

Here are some more info about my requirements:
1. Adhere to aluminum with obtainable adhesive, preferably reworkable glues (no epoxy please) or double sided tapes.
2. Cutting laser: fiber, I will be using Popolu laser cutting service.
3. Surface finishing: doesn't matter, I will sandblast it.
4. Bendable, 90 degrees on 1/32" thickness, C-shape from flat sheet.
5. Durable, shouldn't scratch easily (matte surface alleviates this, but still the harder the better).
6. Can be engraved with clear and crispy text at 6pt.
7. Can be cleaned with IPA and soapy water.
8. Available in dark grey or black.

Any suggestions? Thanks in advance.

Barring simultaneous flexibility and scratch protection (see below) that list sounds like good old perspex (acrylic) would do the job.

Acrylic is already quite hard, harder than Noryl, but for ultimate hardness you can get hard coated perspex that is surprisingly scratch resistant but the coating would crack, flake or craze on significant bending. If you're talking in sensible quantities you might be able to get it bent and then hard coated. All depends on whether you're asking for "bendable" as you stated or flexible - i.e. one time bending to shape for which acrylic is fine or repeated bending (e.g. a live hinge).

It's all a question of what counts as "shouldn't scratch easily" in your view - even uncoated acrylic has quite a hard surface (> 130 Rockwell). By comparison Noryl is around 120 Rockwell, so is acetal (Delrin), Nylon around 110 and polystyrenes 100. Hard coated acrylics are pretty hard but it's difficult to make a quantitative assessment as they are usually quoted against the pencil hardness scale (yes, really, typically 6H) and I can't find a sensible conversion chart anywhere. The same kind of hard coatings used on acrylic are used on plastic spectacle lenses, so you can probably get a feel from experience with that.
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Online blueskull

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Re: Plastic for lining aluminum
« Reply #8 on: September 16, 2018, 12:42:39 am »
Acrylic is okay on other aspects, but fails badly on chemical resistance. It fogs with even alcohols.
The bendable part is easy, just thermoform it. I can machine aluminum, so I guess that's not the biggest issue.
R120 is hard enough. When I say hard, I mean not as soft as water bottles.
 

Online Cerebus

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Re: Plastic for lining aluminum
« Reply #9 on: September 16, 2018, 01:11:48 am »
Depends what you mean by chemical resistance. Acrylic is fine with IPA, I've been cleaning it with that for years to no bad effect, which is the harshest thing you've asked for thus far. If you're going to move up to esters, ketones and chlorinated solvents then you're pushed back to the softer alkane like polymers like PTFE and polythene. Those are pretty tough in everyday use but are a sod to glue without specialist surface pre-treatments and don't like being machined (i.e. engraved).
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Online blueskull

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Re: Plastic for lining aluminum
« Reply #10 on: September 16, 2018, 01:34:45 am »
Depends what you mean by chemical resistance. Acrylic is fine with IPA, I've been cleaning it with that for years to no bad effect, which is the harshest thing you've asked for thus far. If you're going to move up to esters, ketones and chlorinated solvents then you're pushed back to the softer alkane like polymers like PTFE and polythene. Those are pretty tough in everyday use but are a sod to glue without specialist surface pre-treatments and don't like being machined (i.e. engraved).

Many sources, like this: http://www.sdplastics.com/acryliteliterature/1554BChemResistanceChart.pdf

Have stated that acrylic doesn't like pure ether and i-propyl alcohols.
 

Offline coppercone2

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Re: Plastic for lining aluminum
« Reply #11 on: September 16, 2018, 01:36:31 am »
can plastic glued to aluminum develop cracks because of temperature cycling?
 

Online blueskull

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Re: Plastic for lining aluminum
« Reply #12 on: September 16, 2018, 01:46:07 am »
can plastic glued to aluminum develop cracks because of temperature cycling?

Most plastics have REALLY high ultimate elongation, sometimes 50% or more. And Noryl has a CTE that's only 12% lower than aluminum -- pretty good matching.
 

Online Cerebus

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Re: Plastic for lining aluminum
« Reply #13 on: September 16, 2018, 01:58:36 am »
Depends what you mean by chemical resistance. Acrylic is fine with IPA, I've been cleaning it with that for years to no bad effect, which is the harshest thing you've asked for thus far. If you're going to move up to esters, ketones and chlorinated solvents then you're pushed back to the softer alkane like polymers like PTFE and polythene. Those are pretty tough in everyday use but are a sod to glue without specialist surface pre-treatments and don't like being machined (i.e. engraved).

Many sources, like this: http://www.sdplastics.com/acryliteliterature/1554BChemResistanceChart.pdf

Have stated that acrylic doesn't like pure ether and i-propyl alcohols.

That's a list labelled "resistant", "limited resistance" and "no resistance". Acrylic is not a good idea for a bottle to hold isopropanol but, as stated, has limited resistance (i.e. not unlimited resistance). Unless you're immersing it in hot isopropanol, or leaving it in contact for long periods it behaves fine in its presence.

Ether? Probably not a good idea with anything, you'll struggle to find a plastic truly tolerant of ethyl ether. If you're seriously considering ethyl ether as a possible problem then I presume you're doing all the other things that would make electrical equipment safe in an explosive atmosphere? If ethyl ether really is a consideration for you it's the sort of thing you ought to have mentioned up front, not as an afterthought. If on the other hand you're just picking a random solvent from a list to illustrate 'chemical resistance' then you've picked the wrong one, you've picked the chemical equivalent of a DVM class IV protection requirement for an instrument to be used in a mine.
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Offline coppercone2

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« Reply #14 on: September 16, 2018, 02:20:19 am »
lol ether someone is gonna have fun :clap:
 

Online blueskull

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Re: Plastic for lining aluminum
« Reply #15 on: September 16, 2018, 02:24:01 am »
Unless you're immersing it in hot isopropanol, or leaving it in contact for long periods it behaves fine in its presence.

Ether? Probably not a good idea with anything, you'll struggle to find a plastic truly tolerant of ethyl ether.

I'll give it a try. I will order a sheet of acrylic and a sheet of Noryl, and I hope they can arrive by Tuesday.

Ether was never expected to be used. I just included it for the sake of completeness.

I only need it to tolerate at least ONE common solvents, preferably a household one (IPA, EtOH).

I know ketones and ethers are usually not friendly with most plastics, so I was really putting my hopes on alcohol resistivity.
 

Offline alsetalokin4017

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Re: Plastic for lining aluminum
« Reply #16 on: September 16, 2018, 04:54:11 am »
Clear acrylic plastic should never be allowed to contact isopropanol. It does not soften the plastic but it will cause crazing that will eventually lead to cracking, especially if the plastic is stressed at all. I don't even use cleaning products containing iso alcohol on acrylic.

To see what I mean, put a flex load on a piece of acrylic plastic sheet and then wipe the stressed bend with 91 percent isopropyl alcohol. It might shatter immediately, but if it doesn't, let it sit for a few minutes then examine the surface carefully with a magnifier.

There are many references available on the web. Here's one:
http://www.multitechproducts.com/content/Procedures/Aristech-TechBulletin-stress-crazing.pdf
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Offline 2N3055

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Re: Plastic for lining aluminum
« Reply #17 on: September 16, 2018, 11:12:26 am »
At a chance to sound stupid how about FR4 ?  You could sandwich antenna in it too...

For glue, 3M makes double sided adhesive films. There are also spray on adhesives..
 

Online Cerebus

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Re: Plastic for lining aluminum
« Reply #18 on: September 16, 2018, 03:28:17 pm »
Clear acrylic plastic should never be allowed to contact isopropanol. It does not soften the plastic but it will cause crazing that will eventually lead to cracking, especially if the plastic is stressed at all. I don't even use cleaning products containing iso alcohol on acrylic.

To see what I mean, put a flex load on a piece of acrylic plastic sheet and then wipe the stressed bend with 91 percent isopropyl alcohol. It might shatter immediately, but if it doesn't, let it sit for a few minutes then examine the surface carefully with a magnifier.

There are many references available on the web. Here's one:
http://www.multitechproducts.com/content/Procedures/Aristech-TechBulletin-stress-crazing.pdf

It can in theory suffer solvent crazing. In practice, in the 99% of applications that are unstressed it does not. This is because solvent crazing is a phenomenon related to stress corrosion cracking, it will only show up with either cyclic stress or static stress that is near the yield point in some part of the material, close enough to the yield point that the presence of the solvent causes the material to locally fail. It needs cyclic stress in the ongoing presence of the solvent to make it really troublesome. It is not a one time phenomenon except in high stressed material. I have dozens of bits of PMMA that have been frequently cleaned with a 70% isopropanol solution and none of them show cracking or solvent crazing as a result. Front panels on electronics gear are not stressed structural members in normal circumstances on reasonable designs.

In the document you cite they say:
Quote
To help in preventing and/or minimizing crazing, design engineers recommend that, in general, parts should exhibit no greater than 0.5% or 3,000 psi (211 kg/cm2). Ideally the stress in a finished part should be less than 600 psi (42 kg/cm2) or 0.07%.

They talk about keeping  strain to below 0.5% or a stress of 211kg/cm2 to avoid crazing in the presence of solvents. That's equivalent to a piece of 1/4" sheet 1/2" wide holding the weight of a 70 kg man and still providing an adequate response in the presence of solvents.

In the light of that saying
Clear acrylic plastic should never be allowed to contact isopropanol.
is clearly going a step too far.
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Offline SeanB

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Re: Plastic for lining aluminum
« Reply #19 on: September 16, 2018, 05:38:20 pm »
If you are going to machine it anyway, how about a material like nylon, which can be a glass filled one ( your choice from 10% to 60%, the more the worse to machine, but a lot stiffer, probably around 30% will still be flexible) which is available in black. Can be glued down, and machines well, plus quite resistant to alcohols and a large range of aromatics as well. Cuts with a laser, and in the thickness you are using it can be bent, though you probably will want to laser etch a thinner region to act as a hinge point, which with all of them will be best anyway. Sandblast resistant, and hard wearing. Glues are 3m tapes, superglues and a few other contact adhesives, but you will need to do standard joint prep anyway (degrease, etch either mechanically with sandpaper or chemical etch) for them to have best bonding. But tough and hard wearing, as it is used for bearing materials.
 

Online blueskull

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Re: Plastic for lining aluminum
« Reply #20 on: September 16, 2018, 05:56:40 pm »
If you are going to machine it anyway, how about a material like nylon, which can be a glass filled one ( your choice from 10% to 60%, the more the worse to machine, but a lot stiffer, probably around 30% will still be flexible) which is available in black.

PA6 is not solvent resist enough, and it has bad acid/alkaline resistivity, even to weak ones. Also, PA6 has huge CTE mismatch with aluminum. which may cause bulging (remember, it's a C-shape).
PA66 is better in almost every sense, but PA66 is very UV-unstable. This is supposed to be an indoor product, but usage near windows is anticipated.
 

Online Cerebus

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Re: Plastic for lining aluminum
« Reply #21 on: September 16, 2018, 07:41:45 pm »
If you are going to machine it anyway, how about a material like nylon, which can be a glass filled one ( your choice from 10% to 60%, the more the worse to machine, but a lot stiffer, probably around 30% will still be flexible) which is available in black.

PA6 is not solvent resist enough, and it has bad acid/alkaline resistivity, even to weak ones. Also, PA6 has huge CTE mismatch with aluminum. which may cause bulging (remember, it's a C-shape).
PA66 is better in almost every sense, but PA66 is very UV-unstable. This is supposed to be an indoor product, but usage near windows is anticipated.

If you have a particular chemical/environmental resistance characteristic in mind it would be helpful if you mentioned it explicitly upfront. At the moment you're doing the material science equivalent to the frequent beginner electronics questions we get here where no mention is made upfront of source/load voltage/current/impedance/bandwidth leaving them to be eked out by repeated questioning and then popping up a "the circuit must operate at -40C" on the second page. It's rather frustrating. C'mon, I know you can do better.
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Online blueskull

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Re: Plastic for lining aluminum
« Reply #22 on: September 16, 2018, 07:44:33 pm »
If you have a particular chemical/environmental resistance characteristic in mind it would be helpful if you mentioned it explicitly upfront.

My fault. I forgot to mention UV resistance since I didn't know it, but after McMaster listed it as non-outdoor friendly, I changed my mind and went to find an outdoor friendly one.
Hence the added UV requirement.
 

Offline grifftech

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Re: Plastic for lining aluminum
« Reply #23 on: September 17, 2018, 02:43:33 pm »
truck bed liner
 

Offline coppercone2

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Re: Plastic for lining aluminum
« Reply #24 on: September 17, 2018, 08:13:34 pm »
truck bed liner is the most redneck suggestion I have ever heard

I think because they used it for protecting concrete from spalling due to attack it turned into some kind of southern wonder material, I am surprised I don't see plates made from the stuff.
 


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