Medical tray material selection - comments please 2

[Soupcat]

Folks,

Let me run this material selection problem past you, my customer requires perforated trays for medical purpose. The trays will hold 10 Kg of tablets which have been soaked in methylated spirits. The trays will then be stacked 25 high (bottom tray taking a 300Kg loading) in an oven at approx 70 degrees C for 12 hours. After this cycle the trays will be washed in 85 degrees C water and wiped probably with an Iso-propy alcohol wipe. The trays don't need to be USP or EP. US or European pharmacopea but they do need to be FDA approved which will place limitations on fillers etc. Tray is approx 740mm x 420mm x 50mm high. Wall section is 3.5mm. (1 inch = 25.4mm approx) Perforation holes are 2.1mm x 2.1mm at 6mm pitches outer edge to outer edge. I'm proposing to feed in 4 places using hot tips and an insulated runner system. Product life expectancy is 5 years and service cycling can assumed to be continuous. Please see image below. I have ideas on what material I need to use but would be interested in some of your comments.

Kieran

http://www.moulddesigner.com

 

[insideman]

A quick estimate shows that neat polywarpylene should work. But I don't like the heavy wall...

 

[patprimmer]

70 Deg C for 12 hours, with up to 300Kg load distrubeted over 2,320mm long, 3.5mm wide and 50mm high.
I would expect a severe creep problem with PP, even glass coupled.
At first look, nylons might appear OK, but water absorbtion when being regularly washed at 85 dec C will reduce the creep propewrties down to not a lot more than PP. If you go nylon, 6.6 will do a lot better than 6, and aromatic nylons will do better again.
Noryl might be worth a look, as might Ultem.

An often overlooked polymer in highish temperature loaded situations where FDA and chemical resistance is required is TPX, which is Poly Methyl Pentene.

It is a polyolefin, so has many similar properties to PP, including extreme tendencyto warp or even crack to relieve moulded in stresses caused by differential shrinkage re direction of flow. This is of course worse in thin sections. It has useful properties at well over 100 deg C. Mineral filled FDA grades were avaliable from Mitsui in the 70's and 80's. I sold it into microwave ovenware applications where PC was failing from solvent stress cracking due to fats and oils present when cooking.

Another interesting little point is that, although being fairly crystaline, it is transparent (refractive index of the crystals is identical to refractive index of the amorphose phase).

PC will hydrolyse dureing washing at 85 deg C, so will polyesters Regards
pat

 

[patprimmer]

Another thought, what about Styrene Malic Anhydride. It can be good to Hydrolysis, so long as the water is pure. Regards
pat

 

[Soupcat]

Pat,

What would your thoughts be about using acetal for this application? I appreciate that warp could be a problem given the relatively high shrinkage but I only need 1000 parts of the tool in its lifetime so jigging could be an option. I've also had a material supplier suggest PBT (Crastin)??? BTW the loading is taken up by 20 side support ribs (10 per side although just 6 per side visible on image shown)

Kieran

http://www.moulddesigner.com

 

[patprimmer]

Kieran
The image didn't load on my computer.
I just get a red X where it should be.
Acetal really does not like continued exposure to hot water.
Warpage is only a problem if you have differential cooling, and therefore crystallinity.
If I went Acetal, I would very carefully consider the position of the cooling channels, so as to get a uniform mould temperature, especially at the corners and edges.
Bayer put out an excellent publication re controling this type of warpage, in both nylon and PBT, but same applies to Acetal.
An aluminium tool might also help with costs and uniform cooling for 1000 only parts.
Unfilled PBT will warp at least as bad as Acetal, and will be even worse to hydrolysis. Also, without looking it up, I don't expect that it is as good under long term load at 70 deg C as acetal.
Have you considered TPX. It's probably expensive now, but at an SG of 0.8 vs 1.41, it could be nearly twice the price of Acetal and still be competitive. Also, I think that Noryl is good to hot water and should be OK under long term load at 70 deg C.
This is all of the top of my head as I don't have time for litterature searches right now. The various raw materialsuppliers should be able to provide data so long as you ask the right questions. Keep in mind that some suppliers present test results that are done under circumstances that optimise the results, and they sometimes do not publish indicators of potential problems. For example, some only publish data on nylon, dry as moulded, or do chemical resistance tests with A.R grade chemicals, not comercial grades. eg AR grade paraffin gives good resistance in PC, but commercial paraffin with maybe 1% aromatics (as an impurity), gives very poor results for PC
If you want to e-mail me the image at patprimmer@optusnet.com.au, I will have a look at it for you, if you send me your e-mail, I will also find and e-mail the Bayer data on warpage at corners in semi crystaline materials Regards
pat

 

[buut]

Kiran, the 300 kgs weight is too much for the most plastic materials because of the creep so I think a metal tray coated with PTFE, FEP or PFA could do the job.
If you want to see a tray like this please mail me,
Ernst.Buter@home.nl

 

[patprimmer]

Kieran
I finally have the images, scanned, saved in a reasonably compact format and zipped, but in the meantime, I lost your e-mail address.
Could you please resend it to me at patprimmer@optusnet.com.au
The file contains quite a few pages and is about 8 meg, after compression Regards
pat