Overmo(u)lding a PC part with PC

[jacksonm]

I have a PC (MAKROLON 2205) Lightguide which I want to mould an electronic enclosure around using PC (Lexan 920).
Is this possible due to sofening temps being similar?
What are the 'rules' about overmoulding and material softening temps?
If it only just works what will the results be like?

 

[patprimmer]

Makrolon and Lexan are both Polycarbonate, and although they are different grades and made by different manufacturers, but should melt at similar temperatures.

Check out the web sites for Bayer and for GE to get specific details.

You will need to dry the light guide before over moulding.

The mould and machine will be very expensive and it will not be feasible unless you intend to mass produce the part.

Regards

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[jacksonm]

The melt temperature are almost identical.
I already have a mould tool at our subcontract moulders, presently the part has a hole for the light guide to be inserted into but I want to mould it in for DFMA and sealing reasons.
We will be making around 10k part per year.

 

[patprimmer]

OK

Now we know your serious and not a hobbyist looking to do one part.

Do you want to over mould with fusion between the parts, or do you want to enclose with clearance, or with a tight fit without damage to the light guide surface.

We can only offer useful advice if you tell us exactly what you are trying to do.



Regards

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[jacksonm]

Thanks, I'm quite serious, the parts are for an electric motor drive controller for Fork Lift Trucks etc.

One end of the controller has a PC endcap with a hole through it. The light guide is a solid clear 5mm cylinder with a mushroom dome on one end which transmits light from a SM diagnostic LED. We are currently making batches of 10 but will soon be making batches of 100s then 1000s in 6 months.

I guess I want fusion between the 2 parts to guarantee watertightness (jet washable) but I'll take advice. I'm not overly concerned about losing some light into the endcap plastic (it's black) because so far I've found the light guide to be very efficent even when bonded with black RTV.


Matt Jackson
Product Design Engineer

 

[patprimmer]

If you go to GE's website, they should have data about thermoforming Lexan sheet. It will contain a section on drying. You will need to dry your parts by a similar technique just prior to moulding.

The mould will need to be adjusted until you get a blank off with the insert so as to avoid flash.

The light transmission will be reduced by the disruption to the surface. You can test this by sanding the same area that would be affected.

You can get a reasonable level of bonding with sufficient melt temp and injection speed and pressure, especially if the area to be welded is close to the gate.

Regards

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[deper]

JacksonM-

Using similar materials is really the ideal way to go. You will get the best possible bond between the two parts.

 

[jacksonm]

Well they're pretty similar - both PC, both the same melt temp. Might this be a problem with the lightguide getting melted by the plastic during injection causing it to deform or worse break?

It isn't really the bond I'm worried about it's wether it's even possible.

Matt Jackson
Product Design Engineer

 

[patprimmer]

It might deform, it won't break.

A lot depends on design and dimensions.

It is possible if done right, but there is a lot of chance to get it wrong. PC is a fairly difficult material to process.


Regards

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[jacksonm]

OK. But I don't quite know what else I could have told you, I've 'done my homework' read all the information I can, I've spoken to our sub-contractor and I haven't found the information I was after.

I had some specific questions in my first post which have been partially answered - I suppose.


Matt Jackson
Product Design Engineer

 

[MikeHalloran]

Light guides work because of total internal reflection, which relies on a difference in refractive index between the guide and its immediate surroundings.

I >>>>conjecture<<<< that if you get a good enough joint between the parts to effect a watertight seal, i.e. the resins fuse, the light guide won't work anymore.

You might want to prototype it by fusing simple coupons of black and clear p/c.



Mike Halloran
Pembroke Pines, FL, USA

 

[BradJH]

PC to PC bond is VERY good. It has exceptional weld strength and sealing. I have previous experience in two-shot molding polycarbonate to polycarbonate. There are some basic rules that need to be adhered to but it works quite well.
A slight increase in shrinkage from the primary part may occur. For instance a PC box over-molded with a PC frame will warp.
A light pipe over-molding should be relatively simple and should exhibit good sealing properties. I would really have to see your part though.
Next time you see a semi on the highway with an odometer on the wheel look at the black and clear lens. Check it out closely. I designed the tooling, my company built the tooling and produces the part in production. It used to be an all clear part with a painted bleack outline, but we converted it to two-shot for lean cost reduction principles.

 

[jacksonm]

A bit of feedback.

We have sucessfully moulded the part but initially we had a problem with section thickness. The light guide melted like it was part of the injected material and thus caused the section thickness to be too large which in turn caused some local areas of voiding and mixing/boiling? of the dark and clear material. The light transmission was poor and on inspection there wasn't much continous clear material.

After a modification to the tool to reduce the section in that area, it worked better but still caused some minor voiding and we were still losing some light transmission. Our next solution was to put some sleeving (with a higher melting point than PC)over the light guide to keep the material in a column even if it does melt which also stops the clear material 'wetting out' and losing light in to the black material. There was a few mm of unsleeved light guide at each end to get a good bond to the black material and so you can't see the sleeving. It now works really quite well and will be used in production in the next few weeks.

Thanks to those that helped.

Matt Jackson
Product Design Engineer

 

[patprimmer]

Thanks for the feedback.

It has been an interesting project with various tidbits coming from all involved. Just what this site is about I think.

Regards

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