Noob questions

[Nixx]

Hy everyone,
I have some noob questions for you and I would really apreciate your help.
1) I want to know if it's OK to preheat the parts at 100 degrees C (I'm coating them WCoCr)
2) Which is the recomended finished thickness for the powder I use? I under stand that it's not ok to spray more then 0.3 mm thickness, or cracks will show up.
3) Powder keeps building up on the walls of the air nozzle and affects the shape of the flame footprint. Instead of staying round, during the spraying process it gets different shapes. Also some powder builds up on the face of the injector mouthpiece. Because of this, after each coating, I have to open up the gun and clean the air nozzle and the injector. My guess is that this is not ok to happen. What should I do to prevent this from happening again?

HELP and more questions will be asked since I'm new to all of this.
Thank you!!!

[Gordon]

Hi Nixx

1) Yes for ferrous and nickel based alloys. No for aluminium and magnesium alloys.

2) This depends very much on the powder grade, equipment and spray parameters used. 0.3mm should be safe with WCoCr for most, but some can achieve much thicker.

3) Sounds like you need to optimise spray parameters for your particular powder and equipment. If you are using the powder recommended for use with your equipment, I would contact the supplier as it should not cause these problems.

[Nixx]

2) This depends very much on the powder grade, equipment and spray parameters used. 0.3mm should be safe with WCoCr for most, but some can achieve much thicker.

I'm using powder from Stark (-15um +45um). from what i've read this seems to be a good quality powder, right?
So I think 0.3 mm thickness won't be a problem, but how much should I take off after grinding. 0.05 - 0.1 mm, is it too much? what if the coating is too thin after that?

[djewell]

How much material you remove during grinding is determined by the surface finish you need to achieve. I generally apply 0.13mm extra thickness when coating parts so I have enough material to grind and end up with the right dimension. In other words, add your grinding stock to your desired coating thickness.

[Nixx]

what is the right way to measure the thickness right after you finish spraying?
I tried to measure with a caliper. the problem is that the next day after the part cooled down I noticed a 0.08 mm difference.
To measure the thickness with an elcometer, I have to wait between 20-40 min fot the part to cool down at about 40 degrees C so it won't melt the tip of the elcometer.
Is there a better and faster way to measure the thickness?

[djewell]

You don't have to wait for the part to cool when using the elcometer if you do not hold the tip on the surface too long. When using the caliper to measure thickness, you can measure your part after the pre-heat to determine your initial dimension and then measure the coated part to determine coating thickness. If you have multiple parts to spray, you could spray several of them to determine the "hot" dimension necessary to cool to the correct dimension. On the other hand, if you did not allow your part to get real hot, it would not contract much, and your measurement would be closer to the final value at room temperature.

[Nixx]

My boss wants to use a 500x50x2 mm sheet to spray a test sample to validate the process for spraying a part that has 70mm diameter. The sheet will be attached to a 70mm pipe and sprayed as it rotates. I think that the sheet is too big and it will be a waste of powder. Once again I turn to you people for advice because i don't have anybody else to ask for help. Please tell me what kind of test samples should I use (what kind of material) and what size.
Thank you

[Nixx]

One more question. After I spray the flat strip for testing, our laboratory makes the following measurments: thickness, hardness, microhardness and adhesion. I know there is one more test called "3 point bending test" where the flat strip is bent, on a round bar, at 90 degrees and after that you have to observe the cracks.
Do you know if there are any standards for this test, because I don't how to interpret the degree of cracks.
Can you suggest a software that determines the porosity percentage?
Thank you!!

[djewell]

I don't understand why you would spray a flat coupon this way. As the pipe rotates, the spray angle will change from -35 degrees to +35 degrees. The coating will be different at the edges than in the center. Also, the thickness will be different at the edges compared to the center. Where will you evaluate the coating quality? How can you trust any of your test results in this case? If the pipe were significantly larger, I could possibly go along with the procedure. Why can you not spray the sample in a flat ladder pattern where the spray angle is consistent? It will certainly be more efficient and use less powder.

[Nixx]

I don't understand why you would spray a flat coupon this way. As the pipe rotates, the spray angle will change from -35 degrees to +35 degrees. The coating will be different at the edges than in the center. Also, the thickness will be different at the edges compared to the center. Where will you evaluate the coating quality? How can you trust any of your test results in this case? If the pipe were significantly larger, I could possibly go along with the procedure. Why can you not spray the sample in a flat ladder pattern where the spray angle is consistent? It will certainly be more efficient and use less powder.

I totally agree with you. After spraying a flat strip like this, the results were the same as the ones you described. (to be more precise, the upper part was thicker and the lower part was thiner)
I was asked to validate the procedure (by validating i mean we want to see if the RMP and the lead are ok, because the gas pressures are preset from the control panel) before spraying the actual parts, and my boss insisted to do it like this, because he wants to do the 3 point bending test.
Now the "sample", if I can call it a sample , is a mess; please tell me how could i validate the procedure for round parts?
Thank you for your patience!!

[Brent]

I have had problems with this also, if your shop is like mine you are set up to only spray rotating parts. I have no equipment to spray flat parts other than by hand which is hard to maintain constant standoff and traverse speed. I have had to submit various samples for bend and adhesion test to date. What I came up with is mounting the pieces on a plate rotating parallel to the chuck face. Probably not ideal also but at least the angle of the gun doesn't change sometimes you have to work with what you have

[djewell]

You can do the 3 point bend test when you spray in a ladder pattern. You can match the traverse speed to the rotational speed of the part (V=r*omega) for your validation.

Your gun manufacturer should publish guidelines which describe how to spray. Theses include optimal surface speed, stand off, and spray pattern diameter. From these, you can calculate the RPM of the part. For every rotation, the lead should advance 1 spray diameter.

[Stephen Booth]

Hi Nixx, what spraying equipment are you using, I assume HVOF?

What controller, gun, brand etc?

BTW Starck is a very good powder, one of the highest quality, and most consistent suppliers of powder...

[Nixx]

Hi Nixx, what spraying equipment are you using, I assume HVOF?

What controller, gun, brand etc?

BTW Starck is a very good powder, one of the highest quality, and most consistent suppliers of powder...

We have 2 equipments: Termika3 and SX-5000

[Parat Tech]

I have had problems with this also, if your shop is like mine you are set up to only spray rotating parts. I have no equipment to spray flat parts other than by hand which is hard to maintain constant standoff and traverse speed. I have had to submit various samples for bend and adhesion test to date. What I came up with is mounting the pieces on a plate rotating parallel to the chuck face. Probably not ideal also but at least the angle of the gun doesn't change sometimes you have to work with what you have

I think for flat parts you can use X-Y-Z manipulator, which is an economic solutions of robotic.

Thanks

Hari