coining pmp to achieve optical transparency 1

[shoobaka1]

Is it possible to coin polymethelpentene to achieve an optically transparent,flat component that is only 0.010" thick ? If so, what molding parameters would help achieve this "warp free" part ?

 

[patprimmer]

I am not sure what you mean by coin, but TPX is certainly hard to mould warp free. It has considerably different shrinkage along the flow vs across the flow.

If you can mould rectangular plates with full side gates, then cut the part you want from that plate you might get a flat part.

It is 20 years since I moulded TPX, but if you need, I might be able to dig up some old data, re effects on warpage of thickness, melt temp, mould temp, injection speed and pressures at various points in the fill and pack.

You might also be able to extrude and calander a thin sheet that can be cut to shape

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[shoobaka1]

Thanks for the responce...I am a new member and wasn't sure if anyone looked at this web site.

The part is a 3" x 5" x 0.060" tray that is used for labratory diognostic equipment. There are six "windows" that are located in thr center of the part. (side by side). They measure 1" x 1/2" and are only 0.010" thick. I was unable to use conventional injection molding techniques due to the weld line that appeared in the center of each window. This 0.010" thickness was also very difficult to fill. I added a mechanical "coining" feature to the window areas and am now able to produce this optically transparent thin walled window easily. These windows need to be extremely flat. I have a slight warp in this area that I think is being cuased by the displacement of material during the coining process. I have very little experience compressing plastic as this process is doing. I don't understand the cause of this warp and therefor am not able to counteract it. The material supplier has not been able to help either.

I am getting very close to achieving acceptable product. Any advice would be helpful.

 

[patprimmer]

Have you tried anealing, by heating past the glass transion point, or maybe to the softening point, then cooling slowly, like overnight.

You might be able to jig it while you aneal.

TPX is partly crystaline, so shrinkage is very dependant on degree of crystalisation, which is very dependant on cooling rate.

The bonds that hold the crystals together will form as the material cools. The opertunity to bond at particular positions will be dependant on the pressure involved.

Coining obviously will induce stress. The degree of stress will depend on the temperature it is done at.

I think you have a very difficult problem to solve.

I guess you need the olefin type chemical properties together with transparency and temperature resistance..

A couple of long shots.

Warning. These might be going bravely where no man has gone before, to explore new moulding and toolmaking techniques. They could be very expensive failures.

Could you use 0.010' thick sheet, and overmould it.

Could you do a 2 shot process with a long cooling time between shots, and a very hot mould on the thin section. The tool would be very complicated to get filling and full edge gating on the windows.

Could you use 0.010" sheet, and 0.050" sheet, cut the windows in the 0.050" sheet, then weld them together. I would expect a problem cleaning the edges of the windows as the weld will not be smoot and complete at the edges.



Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[shoobaka1]

What a resource!!!

I have tried overmolding a film and the cosmetics were not acceptable. The warp that I am seeing now is minimal. I think that I am going in the right direction. The material supplier recommeded high mold temps. The other direction is what is yielding better results. Currently, I am playing with a hot stamp machine to use as a make shift annealing process that adds pressure to keep the part flat. My hope is to NOT have to use a seconday operation though.

 

[shoobaka1]

Here's another tid bit of info. I have a 2.5 " fan gate that feeds this part. The four windows in front of this gate are flat. The two window at the ends of the part have the warp. The flow front encompasses the entire part almost evenly across it. Obviously, the ends that are not in front of the gate are slightly slower to fill. All six windows are coined similtaneously after they have passed this area of the cavity. Mold is at 45 deg. F. I am entertaining the idea of gating the entire length of the part to remove the variation in flow.

 

[patprimmer]

I would certainly make the gate the entire width, and probably about 0.020" thick with a short land.

I would have thought a mould temp of 80 deg C was more appropriate.

A hotter mould will allow more uniform pack across the cavity.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[shoobaka1]

I'll try to conclude this thread with good news... In the next few weeks.

Thanks for the advice

Regards,

Ryan Heidenfeld
Plastics Process Engineer

 

[shoobaka1]

Here's an update...

I have moved this component to a larger tonnage molding machine. The thin walled areas varied in thickness. This was the cause of the warp in 0.010" windows. By coining these features at a higher pressure, I was able to keep them at a uniform thickness. The tooling for this component uses a mechanical coining mechanism that it actuated when the mold is fully closed. The lower tonnage press did not bring the window feature all the way to it's mechanical limits.

Any info on "dipping edges" for use in injection compression molds ???