[kat.rawlings]

Hi everyone,

I was just wondering if anybody can offer me any tips or advice for a project I am working on at the moment. We have been carrying out shot blasting using various abrasives to improve the surface roughness of some polypropylene parts that we are making for the medical industry (1m to 2.5m diameters). Various abrasives have been tested such as garnet, glass bead AB 177-297 and 801 AC, brown aluminium oxide 80/100 and 180/220, white aluminium oxide 70, 120 and 180/220, kiln dried sand, blasting grit (came free with the Sealey shot blasting gun), Honite 13 glass bead, Type 2 plastic media (Urea Formaldehyde) and Type 5 plastic media (20/30 and 30/40 acrylic).

White aluminium oxide 180/220 and acrylic blast media gave us the best results. However, when an electron microscope was used to find the surface roughness of some polypropylene samples that had been blasted with aluminium oxide 180/220, we found that the surface roughness was around 2.5 microns Ra, when it needs to be 0.8 microns Ra.

We therefore still need to improve the surface finish considerably. Does anybody have any suggestions of how we can do this? We are currently carrying out more shot blasting testing with aluminium oxide and plastic media. We are also looking into dry ice blasting.

Also, we would like to be able to measure the surface roughness of our samples on-site while we are carrying out trials. I am therefore looking into hiring/buying a surface roughness testing machine. Can anyone recommend UK companies who might be able to help with this?

Thanks

Kat

[kat.rawlings]

A previous thread was started a while ago regarding this project if anybody wants to see suggestions that have already been made:

http://www.chemicalforums.com/index.php?topic=46420.0

Also, please remember that these parts are 1.5-2m diameter so processes suggested need to work on large parts.

People have suggested several times that parts could be molded in the first place to achieve the desired surface finish, rather than being machined etc. However, this is not possible because the cost of molding would be far too high for such large parts when the quantities will be quite low.

[Enthalpy]

To measure surface roughness, one standard procedure is to compare with your fingers the feeling with a set of surfaces of calibrated roughness.

It works very well. Between the samples of calibrated surfaces, you feel the difference very distinctly, and 0.8µm is nothing very smooth to the fingers. The procedure is a standard, and such sets of calibrated surfaces are available commercially for little money.

-----

Your target roughness is very reasonable and can be achieved directly by milling and turning polypropylene. 20 years ago, a roughness of 2.5µm would have been considered very bad, and 0.8µm the beginning of proper work.

Meanwhile mechanical contractors deliver parts with a horrible roughness. I suppose this is a consequence of CNC machining, whose fast control allows to cut at a huge speed. I've seen it with plastics as well as metals.

Especially PP and PE need low cutting speeds that don't melt the plastic, and old-fashioned well-sharpened steel tools, not ceramic tools. Then they give naturally very pleasant surfaces. Contractors use huge cutting speeds because a plastic won't melt the tool.

A low cutting speed is bad for productivity, and your parts are big. To compensate partially, feeds can be huge. They're limited only by the resistance of the part. In trials, I increased milling feeds over 2mm/tooth (!) and the resulting long waves had still a pleasant feeling, so there's no real limit.

[kat.rawlings]

Thanks for your advice.

[kat.rawlings]

Does anybody know of any chemicals I could use to improve the surface finish of polypropylene?

Or are there any other suggestions on how I can achieve this surface finish after machining the product on a CNC router?

Thanks

Kat