Changing from Plastic to Metal

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Hi,

We have developed a electronic latch that is currently in a plastic case. We have identified that the plastic is weak and I have been asked if we can diecast the outer case. This would increase the stength of the product plus enable us to keep the product built inhouse rather than buying the case from out side.

My question is apart from the obviouse conducting problems are there any more issues I should consider. One issue I can see is the Motor contacts are very close to the plastic casing infact they can sometimes touch it. I can move the motor slightly away from the case but what distance should this be.

If anyone could direct me towards an online paper or just some information on this I would be most greatful.

Thanks in advance

James

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How many volts are on the motor? What standards do you have to comply with?

Generally going to a metal enclosure gives you EMI shielding and a heatsink, but the downsides can be that it needs to be earthed if mains are present inside and that it will conduct ESD rather than disipate it.

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goujam wrote:
One issue I can see is the Motor contacts are very close to the plastic casing infact they can sometimes touch it. I can move the motor slightly away from the case but what distance should this be.
Can you line the inside of the metal case with an insulating barrier?

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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Sheet metal needs to be plated to prevent rust, etc. You are supposed to avoid hexavalent chromium. Most sheet metal houses know how to comply with RoHS and other regulations. You did not say what the power supply is in the device. It makes life easier if you can use a DC wall adapter in terms of passing regulations. You can obtain adapters that pass the ground through to the negative side of the DC plug.

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Hi,

Thanks for all the replies, the power supply to the latch is between 3V and 16V and can be either battery or power supply. The problem is we dont know where the product will be mounted, we supply just the latch as were a lock manufacture. Many of our customers are locker manufactures of some type but we also have customers for school desks and other applications.

I think the idea of some how insulating the inside of the metal case would be a good idea. I was thinking maybe some kind of plastic sheeting as seen in some PC power suppies.

Im not sure on this but would it be useful for me to ground the metal case to the ground of the power supply to the latch, or would I be better off telling the end user to ground the end product i.e. the metal locker.

It makes things much simpler when using plastic casing lol!!!!

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if the case is going to be die cast in house then it seems like you would be able to have any custom shape you want. if the design requirements allow why not enlarge the box in areas that might be problems or add reinforcement to prevent accidental crushing.

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Garboui,

Yes we can make the case anyshape we want but we have standard fixing holes and thicknesses to stick to. The size of the latch at the moment would mean we can fit our latch into the side of most fileing cabinets, if we increase the size we would make the latch to large.

However using you idea I have just noticed where the two side of the case join together we have a lip. I could ask the mechanicle guy to locally remove this lip thus allowing more space for the two motor connections. This then may allow room for a insulating material to be placed between the terminals and the metal case.

Does anyone know if there is a regualtion to the thickness of the insulating material between the case and the terminals considering the maximum voltage will be 3V at the motor and 250mA.

thanks

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You can find out what the UL code is for the product and then look in the UL docs and see what they require for clearance based on the voltage level. I would imagine for low voltage it probably is not much.

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At 24 volts or less UL is not even an issue, electrically.

Tom Pappano
Tulsa, Oklahoma

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I kind'a liked reinforcing the case approach.

When you add an insulator you add parts, custom sized. OH, now you have to secure the insulator to the case. Glue, rivet, ...? Assembly complexity just increased, as did parts count.

Kids slam lockers. Add a sheet of insulation and you just added a new component that can possibly fail. If the glue fails on the Nth slamming of the door, will it jam the lock permanently closed? Have to fine a good glue for metal and plastic, over the desired temp range... Have to properly clean and prep the parts before you glue them. Have to convince the assembly techs that it really does matter if they clean the part properly, or not...

I'm not a production engr, but I think this is not a trivial decision, even though on the surface it may seem so.

JC

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Thanks for the links alwelch ill look into the links and try to see how we could use something lik this. As were looking at around 12k parts a year id need to get a tool to create the insultation.

DocJC You raise some very good points there about the extra part being a pain in the assembly. If it was purly down to me id stick to plastic case and we still may do were doing strenght testing soon. However being a traditional metal bashing company the mechanicle guys like to use diecast parts, and me being the only electronic guy counts against me lol.

Just one last point the bit where you say the latch would fail and lock permanently well we have this sorted, all our electronic locks have key override. People for some reason dont trust electronics and working for a lock maufacture we decided we would put key overides in. Sorry this doesent help with my problem just bragging about our new latch lol!!!

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Fish Paper insulation stock has been around for years and is pretty tuff stuff. You can mount it with dbl sided adhesive tape which is really strong bonding stuff. Also it could be riveted or screwed to the case but that is more parts and work. The tape is easy. I am not pushing this but it does insulate and is durable.

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Hi James,

I am sure that the complexity of adding, and tracking, a key override is a pain. That said, I've needed to use a key to unlock an electronic lock several times. I would not buy one without such a "feature".

If the motor's mounting bolts (?) go through holes on the insulator, securing them both together, then you avoid the need to glue / rivet / etc. the insulating plate, and you avoid the issue of it becoming dislodged, and possible mechanically obstructing the latch throw lever.

JC

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Packaging tape ... even two layers :lol:

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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valusoft wrote:
Packaging tape ... even two layers :lol:

Cheers,

Ross

Too easy to cut through and short. The fish paper is really hard to cut or scratch through. If you are going to insulate it this is the best way to go.

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would it be possible for the fish paper / plastic insulation to be same size or larger than the circuit board, them make holes in the fish paper matching mounting holes in pcb. (depending on how pcb is mounted) Insulator is then mechanically mounted between both parts and should not come loose, no adhesive necessary.

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alwelch wrote:
valusoft wrote:
Packaging tape ... even two layers :lol:

Cheers,

Ross

Too easy to cut through and short. The fish paper is really hard to cut or scratch through. If you are going to insulate it this is the best way to go.

Yep. I was too fast in reading ... thought it was a once off hobby requirement. Clearly not.

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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Some free advice -- and probably not worth the price.

The fish paper is probably a good route to go -- it will protect the circuit even if there is some damage to the box.

You might also consider providing a method to ground the box to the supply negative. It's low voltage, but it's a simple precaution.

Also, you should mention in your instructions that the supply voltage will be grounded, and that the supply should have some kind of overcurrent protection.

These suggstions are made with product liability in mind. Product liability lawsuits are the reason that user manuals are mainly filled with warnings. Our product liability folks used to say that the reason that there are no stepladders shorter than 2 feet is that they didn't have room for the warning labels.

If you made your product as is, and just changed to a metal box. it would probably be fine. Especially if you moved the motor contacts. However, one fire or whatever that could be linked -- by a lawyer -- to your product would not be a good thing.

Also, please don't just adopt my suggestions without further research. They might not be appropriate for your product classification.

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Thanks for all your responses, its been a great help. I think im going to still try push the product to be made in a plastic case. If we must change then im going to have to look at something like the fish paper between the motor contacts and the case wall. The pcb is mounted on ledge within the case and two pegs keep the PCB from moving. A layer of the fish paper under the PCB should protect me. Im now thinking a link from the PCB to the case would be easy enough to ground the case covering our selfs. But its clear I need to look more into the liability side of the changes!!