Improving Surface Roughness of Polypropylene and Measuring Surface Roughness

[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

[Gordon]

Hi Kat

to the Surface Engineering Forum.

First, I would like to distinguish differences between shot blasting and grit blasting. Though both processes roughen a surface and can provide the same measured Ra value, the actual surface textures/profiles can be very different. Shot blasting uses spherical media, while grit blasting uses angular sharp edged grit. Shot or spherical media essentially peens or mechanically deforms the surface only leaving relatively smooth rounded indentations, while grit blasting has additional cutting action due to sharp edges and produces a 'sharper' more angular etch to the surface. This aspect can be very important to subsequent surface engineering processes, for example in thermal spray coatings there is a huge difference in bonding properties between grit and shot blast prepared surfaces, grit blasting being by far the best.

For changing the roughness the major contributor will be the grit/shot size, obviously finer grit will give finer finish. Other parameters will be blast pressure, nozzle diameter and stand-off distance. Adjustment of all these parameters hopefully will enable you to achieve your goal. One further thought, with polypropylene the temperature may also play a part.

There are many small portable hand-held surface roughness testing devices out there, which I think would be good for quick on the spot testing, particularly if you can verify results later on better less portable equipment. Just google "portable surface roughness testers" or similar. I don't have enough knowledge to recommend any specific model/manufacturer/supplier, so I will leave that to other members to chip in

Hope that helps

[MichiganMan]

0.8 microns is not much. Is there a +- that you have to work with?

That seems like it might be more consistent with a lapping process than an abrasive blast. I admittedly have little experience working with Ra this low, but is anyone else thinking that 0.8 micron might not be possible by abrasive blast?

[kat.rawlings]

Gordon, thank you for your post, it was very helpful. So I have used both shot blasting and grit blasting. We have been able to improve surface roughness considerably and we found that the best abrasive so far has been aluminium oxide 180/220. I am therefore thinking it might be worth using a finer grade of aluminium oxide to see if we can improve the finish further. We have also been playing around with things like the nozzle size and the blast pressure. If the pressure is too low then the surface finish will not be significantly affected and if the pressure is too high then the surface finish will become rougher. It is therefore important to find an optimum pressure for all blast media used.

MichiganMan, he Ra has to be 0.8 microns or better than this. Not sure lapping would work because the part has ribs and grooves leading from the edge of the circle to the centre. The ribs are 3mm thick and the grooves in between the ribs are as small as 5mm near the centre of the disc.

These ribs and grooves are also causing problems when trying to find a portable surface roughness tester because the cheaper ones cannot be used in these grooves. We have found one here for £2055 + £20 delivery + VAT:

https://www.spectrum-metrology.co.uk/surf25.htm

However, we would prefer to hire one for a week than pay out this amount of money. Does anybody know of any cheaper models that might work? Or anyone who would offer hire service?

[kat.rawlings]

I have sorted out surface roughness testing now as we can take the samples to a local company and they will measure them for us. However, we are still struggling to achieve a surface finish of 0.8 microns (Ra) so does anybody have any new suggestions? Are there any chemicals that might work on polypropylene?

Thanks

[Stephen Booth]

Hi Kat.Rawlings,

If the Ra is the only Surface Parameter being measured (not Rz, etc.) then you might consider contacting to 3M for

Scotch-Brite™ Radial Bristle Brushes

Scotch-Brite Bristle Abrasives Range (clipped from website)

Available in a range of small sizes for polishing detailed parts or small areas, bristle disc brushes can be used in-line on CNC machines
Larger bristle brushes can be used on a bench motor for cleaning, finishing, polishing and light deburring

Maybe they will have a grade that would work. If this was a metal component, this would be good choice, unless you need to have blasting for another reason.

(link dead)

[pulaunias]

I am not absolutely sure what your problem is. I guess what you want is to make 0.8 micron roughness on PP and sandblasting makes too rough profiles.

If you cannot find mechanical ways to realize this; try chemically etch it.

I studied oxidizing ultrahigh molecular weight polyehtylene with a mixture of KMnO4 and concentrated sulfuric acid as an attemtp to improve its bonding with adhesives. I found it produced micron-to sub-micron-scale roughness on the surface.

You can try it with PP as well.

---

I will try attach electron micrographs here.

[pulaunias]

left image: as-received ultrahigh molecular weight polyethylene (UHMWPE, generally smooth)
right image: UHMWPE after treatment with KMnO4/H2SO4 (see submicron granular surface texture)

I imagine the Ra might be tuned by temperature, concentration, time....

[kat.rawlings]

Hi Kat.Rawlings,

If the Ra is the only Surface Parameter being measured (not Rz, etc.) then you might consider contacting to 3M for

Scotch-Brite™ Radial Bristle Brushes

Scotch-Brite Bristle Abrasives Range (clipped from website)

Available in a range of small sizes for polishing detailed parts or small areas, bristle disc brushes can be used in-line on CNC machines
Larger bristle brushes can be used on a bench motor for cleaning, finishing, polishing and light deburring

Maybe they will have a grade that would work. If this was a metal component, this would be good choice, unless you need to have blasting for another reason.

(link dead)

Thanks Stephen, I will have a look into this and contact 3m.

---

left image: as-received ultrahigh molecular weight polyethylene (UHMWPE, generally smooth)
right image: UHMWPE after treatment with KMnO4/H2SO4 (see submicron granular surface texture)

I imagine the Ra might be tuned by temperature, concentration, time....

Thanks Pulaunias, very helpful. You are correct, we need a 0.8 micron roughness on PP and sandblasting is improving the roughness but it is still too rough. I have considered using a chemical but I didn't know what chemical it would be best to use. This could solve the problem

Thanks.

---

left image: as-received ultrahigh molecular weight polyethylene (UHMWPE, generally smooth)
right image: UHMWPE after treatment with KMnO4/H2SO4 (see submicron granular surface texture)

I imagine the Ra might be tuned by temperature, concentration, time....

Concentrated sulfuric acid reacts with KMnO4 to give Mn2O7, which can be explosive. This doesn't sound good!

[Gordon]

Hi Kat

Interesting thread you have started . Going back to grit blasting, have you tried finer blasting abrasives?

I have achieved the sort of finish you are after with blasting (on metals mind you) using alumina based powders (-45/+5 μm and -25/+5 μm). Note the surface finish on the surface before blasting needs to be finer to start with.

[kat.rawlings]

Hi Kat

Interesting thread you have started . Going back to grit blasting, have you tried finer blasting abrasives?

I have achieved the sort of finish you are after with blasting (on metals mind you) using alumina based powders (-45/+5 μm and -25/+5 μm). Note the surface finish on the surface before blasting needs to be finer to start with.

We have ordered some of a finer grade of aluminium oxide so we will test this when it arrives and I will let you know how it goes. I am also trying to get a sample of a natural abrasive (rather than having to purchase 25kg) to see if this would work but I am struggling. I am going to carry out more testing over the next few weeks varying things such as pressure and using a combination of abrasives so I will let you know how this goes.

I am also willing to consider alternative techniques for grit blasting. Does anybody know of any chemicals I could use on polypropylene?

[pulaunias]

I agree that H2SO4 and KMnO4 are both highly oxidative and unpleasant. However, it is just fine to mixt them; they don't produce Mn2O7; the mixture is not explosive.

Bad news about chemical etching is that they always use STRONG oxidants (meaning dangerous), especially for relatively inert materials such as PP, because weaker oxidatns are simply not reactive enough. There are oxidants that are super strong and totall safe--plasma; but that usually requires vacuum and often not practical.

Blasting with finer grits should be the best option.

[kat.rawlings]

I agree that H2SO4 and KMnO4 are both highly oxidative and unpleasant. However, it is just fine to mixt them; they don't produce Mn2O7; the mixture is not explosive.

Bad news about chemical etching is that they always use STRONG oxidants (meaning dangerous), especially for relatively inert materials such as PP, because weaker oxidatns are simply not reactive enough. There are oxidants that are super strong and totall safe--plasma; but that usually requires vacuum and often not practical.

Blasting with finer grits should be the best option.

Well I guess I shouldn’t believe everything I read in Wikipedia. I’ll have to have another look into using these chemicals. We can try blasting with finer grits anyway and see what happens. Thanks for your help.

[loriolo]

I've had experience about finish bronze pieces for aero industry, and we obtained a good and homogeneous finish of about 0,4-0,5 µ Ra, by grit blasting the surface with vegetable sand (0,5/1 mm broken hazelnut shell), there are a lot of grit blast media, like vegetable, several kind of stone (like pumice), plastic, etc..
Regards

[kat.rawlings]

I've had experience about finish bronze pieces for aero industry, and we obtained a good and homogeneous finish of about 0,4-0,5 µ Ra, by grit blasting the surface with vegetable sand (0,5/1 mm broken hazelnut shell), there are a lot of grit blast media, like vegetable, several kind of stone (like pumice), plastic, etc..
Regards

Thanks Loriolo, I am looking into purchasing some natural abrasives but I am struggling to get hold of less than 25kg for trials. It might be worth us considering purchasing a 25kg bag. Thank you.