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  • Overmoulding Silicone/TPE onto a Printed Circuit Board

    Discussion in 'Plastic moulding' started by harryhutton, Jul 3, 2014.

    1. harryhutton

      harryhutton New Member

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      Hey guys,

      I'm a student trying to figure out how to manufacture a piece of wearable tech - wondering if anyone here might point me in the right direction.

      The product is a wristband coated in TPE. What would be really nice is if I could overmould the TPE directly onto the PCB.
      I have been in contact with a couple of companies, and their response has been a) to overmould a PCB, one must go for a low pressure, low heat solution. This plastic is usually quite rigid - not the sort of flexibility suitable for a wristband. b) silicone / TPE requires high pressure, high heat. This more or less blasts the components off the PCB.

      Hence its tricky. However, I took apart a Nike Fuelband (http://i.imgur.com/yhZkoJi.jpg) - and it seems they have managed it. They have used some sort of TPE (I think but I can't tell), and they have overmoulded it directly onto the PCB - I was able to peel it right off the surface (perhaps some sort of coating was used?)

      How did they do it?
       
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    3. VJmech

      VJmech New Member

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      Hi Harry

      Normally to protect the board they will go with a LPM mold which uses a LPO resin .

      But i guess the picture what ur showing is look like a shrink tube .

      We can made a custom made shrink tube to ur size of the PCB (0.2 - 0.5 mm thickness) which is made of PET material (good wear resistance).

      Once u insert on to ur PCB apply a hot air around it, now the PET tube will shrink and cover the PCB tightly .

      The same principle we used in the wiring .

      And i dont think it is a over mould, because if it is over mould it may be difficult to remove as u have done. some plastic definitely would have stuck on the PCB.

      And iam so curious what board u have used, it is the Flexible PCB or FR1 or paper phenolic board?
       
    4. srdfmc

      srdfmc Well-Known Member

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      Do you have a video when you took the band apart ?

      Was there any sticking btw the PCB components and the mold ? Or was there nothing ?
       
    5. harryhutton

      harryhutton New Member

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      there was no sticking and no material left on the PCB - it was able to peel right off. it was interesting though as some of the overmould TPE is quite firmly stuck to some of the rigid plastic structural parts on the lower section (hence why the sides have been cut and ripped in the photo).

      This makes me think there may have been some sort of coating.

      @VJMech - Flexi PCB :) I am quite sure it is overmoulded, I don't think it is a shrink tube.
       
    6. bdeuell

      bdeuell Member

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      The difference in adhesion may be caused by the different base materials and surface finished, i.e. the overmolding material bonded to the plastic components but not the circuit board.

      I suggest you Google overmolding PCB and look for a material that fits your requirements rather than trying to use a conventional injection molding material.

      Alternatively you could look at using a two part epoxy type material. I have used products from http://www.freemansupply.com/ to produce low volume elasomeric components but was not attempting to overmold a circuit board.

      On another project I was developing a product with a potted circuit board. We had problems with the battery losing contact with the board (button cell type with spring contacts to the board). My recommendation is to solder all components directly to the board. Keep in mind the potting/overmolding material will subject your components to all kinds of forces they typically don't see in an open air design.
       
    7. MichaelCreighton

      MichaelCreighton Member

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    8. PeterB

      PeterB Member

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      TPEs are thermoplastic elastomers. So they are generally mouldable by heating to whatever the particular material requires. They are not a single compound, but come from different suppliers in many differing materials and grades.

      If this band has actually been injection moulded in TPE, then the degree of heat which the PCB needed to accommodate/resist, will depend on the initial melt temperature of the material used, its thickness or volume and the cycle time of the process. The component population may have been selected on the basis of heat resistance too. If there are low-temperature TPEs around, then they might not be much use if they re-melt on your product when in hot climates! After all, they remain thermoplastic all their lives.

      To discover if that sample band is TPE, put a heated metal blade onto it. If it melts it is probably TPE. If it doesn't then it must be a type of thermoset material.

      It may be more likely that this material is a silicone rubber. They are effectively thermoset materials and they allow you to look at both hot and cold processes. Silicone rubbers come in a wide range of hardness/softness and are usually very heat and chemically resistant too. They are more expensive than TPEs, but in small products the material content isn't usually critical to the overall pricing.

      The other advantages are that they can be chemically and biologically inert. Hence their use in medical implants.

      Liquid silicones can be injection moulded and it is the heated tool that completes the curing process to provide a faster cycle time and allow high volume manufacture. That might prove problematic for sensitive pcb components.

      There are other grades which work with compression moulding processes, more suited to lower volumes, which may also use a heated tool to speed up the curing. Choice of catalyst will affect the curing cycle time. So a low-temperature process may be possible if you enquire with the material suppliers.

      Other grades are twin-pack and used in potting and casting processes. These can be low temperature or cold, depending on the required cure time. The materials are often used in prosthetic and film prop manufacture. So they can be moulded around the body at comfortable temperatures.

      I'd have a good look at these if you can, as I'm fairly sure that is what you will need.


       
      Last edited: Sep 19, 2014

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