Testing for Aluminium Oxide 4

[KatRawlings]

Hiya everyone,

I was just wondering if anybody knows how I can test for the presence of aluminium oxide? We have been shot blasting plastic with an aluminium oxide abrasive, so bits of this abrasive will be left on the surface of the plastic. We believe that we can remove this aluminium oxide using acetic acid (white vinegar). However, it is essential that there is no aluminium oxide left on the surface of the plastic. We therefore need some way to test for this.

Thanks

Kat

 

[MikeHalloran]

Well, now you've got everyone curious.
Plastics are ordinarily used where the geometry is complex enough to make machining prohibitively expensive, so you put the expense in a mold and make the plastic parts from the mold by means of a relatively primitive process...



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

Even still, it might be easier, overall, to make the part as an assembly of several parts, and allow the individual components to be molded.

Additionally, machining can entail using other tools that may allow you to get the surface properties you desire directly on the machine. It would seem to me to be cheaper to switch out a machine tool bit than to install the part on another machine, blast it, and then clean it, and the inspect it.

As I mentioned, if you tell everyone the entire problem, there may be another solution. By holding back you potentially deprive yourself of all the help you could be getting.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[KatRawlings]

I am not meaning to hold back, we are under confidentiality aggreements with the company we are producing the part for so I can’t really go into too much detail. It is a 1m disc with lots of circles cut into it and thin ribs/veins that lead into the centre. It has really small tolerances so maybe this is why it isn’t moulded, I’m not sure. It is definitely not going to be possible to make the parts separately and join them together anyway because there are over 100 of these veins/ribs.

It is not a cheap piece of equipment so we are trying to find the best way to make it, not the cheapest. We basically need some sort of way to remove machine marks and to deburr this plastic while not effecting tolerances. It also needs to not contaminate the part. For example, we could shot blast with a plastic media, but the results just aren’t as good as they were with aluminium oxide.

 

[unclesyd]

One of them brain things hit me, we used the SS White machines to finish some small and large parts. If I remember correctly the process is very easy to control and can be automated. The first link is Airabrasives (SS White) while the second is to information on the suction type blast cabinets.
There are also many things to test like liquid blast with Al2O3 or others.
Continue blasting as you are and followup with a second blast media for a 2 stage process..
Change the angle of attack of the nozzle for your existing equipment.
If you can get access to a good microscope , preferably a metallurgical, you can probably be able to see the Al2O3 by varying the light.

http://www.airbrasive.net/

http://cmvblasting.com/index_ing.html

 

[IRstuff]

Well, what is the intent of the shot blasting; i.e., what exactly are you trying to achieve?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[KatRawlings]

Sorry for any confusion. My orignial aim of posting on this forum has now changed due to information from elsewhere and also posts on here. This is where we are currently:

We are shot blasting to remove machine marks and to deburr so that we have a smooth, even finish across the entire 1m diameter sample. We have achieved this surface finish with aluminium oxide and the customer is happy with this.

However, due to it being for a medical application we cannot use aluminium oxide. This is why I was asking about testing for aluminium oxide and removing aluminium oxide.

It appears as though there will still be traces of aluminium oxide whatever we do to the samples, so it doesn't look like we will be able to use aluminium oxide at all. The problem now is therefore different to the problem in my original post; we now need to find a different technique to give us the same surface finish. One possibility is therefore to blast with a different abrasive that wouldn't need to be removed before the part is used in the medical industry. Our main areas of focus are therefore currently dry ice blasting, soda blasting and blasting with acrylic media. If anybody has any tips for any of these 3 areas or any suggestions of alternative techniques that would be brilliant.

Has anyone used dry ice blasting or soda blasting for plastics?

 

[IRstuff]

Perhaps the machine tool bits need to be changed for something different. We diamond-turn various materials for lenses, and it used to be a requirement to hand polish to remove the machine marks. However, improvements in the cutting technology have minimized those requirements.

Have you consulted with others about using a different tool or cutting process that will not tool marks?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[btrueblood]

Are the "veins" a hollow structure running parallel to the large dimension (diameter) of the disk?

If not, small grooves and small ribs in the surface of a part, no matter how large, are certainly moldable. Even Teflon compression molding will give very good tolerances and surface finishes.

 

[Compositepro]

Have you considered flame polishing? It will give you a glossy rather than matte finish, but it remove tool marks and leaves no residue. You can find videos of the process on You-Tube.

 

[KatRawlings]

@Btrueblood: It is small grooves and ribs in the surface but we are a plastics engineering company and we are not capable of molding so we will definitely not be molding this part. I do not know the reasons that this company has chosen us to make their parts rather than a molding company but I am sure they have thought about this. Even if they haven’t then it is not my place to lose business for my company by suggesting they go elsewhere for these products. I have simply been asked to investigate improving the surface finish of these parts after the current process has been carried out.

@Compositepro: Thanks, I will have a look into flame polishing.

@IRstuff: Yes I have consulted with others about using a different tool or cutting process and this is not going to happen. We have a good process in place and the customer is happy with this. We just need a way to improve the surface finish at the end of the process. We have had some good results after shot blasting with plastic media so it is possible that we will use this.

 

[KatRawlings]

@Compositepro: I have had a quick look into flame polishing and it appears as though it is mainly used on acrylic. We are working with polypropylene and I can't seem to find any information on flame polishing polypropylene. Also, it appears as though a high level of skill is necessary to use this technique and to avoid causing damage to the plastic. Due to this parts being made in a long, complex, expensive process, I would not want to risk damaging one. Thanks for the suggestion though, it's always good to think about new ideas.

Kat

 

[HDS]

Can you solvent polish PP?

 

[unclesyd]

Here are some other ideas. it would be not problem if the part was not circular.

Brush finishing is used for plastics

http://www.curtinhebert.com/deburri...aw_deburring&gclid=CPLR6PS41aYCFUHt7QodiCtpGw

Here is a different media you may want to look at. There is a lot of good information on other media.

http://finishingassociates.thomasne...deburring-polishing-compounds-/zinc-base-shot

blasting-abrasives-briteshot-reg-#s

You may have to cut and paste the links.

 

[KatRawlings]

Thanks Unclesyd.

 

[unclesyd]

Just wanted to keep you busy. The lead for this approach was TV program that showed a flour sifter and since we had a 18" vibrating pad lap there ought to be bigger ones.

Here is another approach that you may want to investigate if the quantities are high enough. The Covington Reciprol Laps go to 36" and it looks like it would not be a problem to enlarge the pan to accommodate your part. Very little additional weight is involved.

http://www.covington-engineering.com/rociprolaps.htm

http://www.lortone.com/laps.html

http://www.diamondpacific.com/moreproducts.html

I polished to mirror finish about 2,000 CS 1" dia disks. on our machine.

 

[KatRawlings]

@unclesid: I think our parts will be too large to use the technique mentioned above. They are between 1m and 2.5m diamter. Thanks anyway.

 

[KatRawlings]

If anybody is interested I have started a new thread:

http://www.eng-tips.com/viewthread.cfm?qid=290817&page=1

I have done this because over time the title of the thread was becoming less and less relevent and we now have a new problem. Thanks for help and advice everyone.