Poor Blasting Results

[Pugly]

I'm trying to improve my blasting setup as I seem to get poor and very slow results. I have to hold the gun super close to remove even crusty old paint or to clean up rusty car parts.

I'm a novice to the world of home blasting (have used blasting cabinets at work before with infinitely better results) so wonder if someone could look over my setup and point out where I'm going wrong;

- Wolf Dakota 14CFM compressor (90L tank)
- Clarke 1/2" Air Filter / Regulator connected to end of tank (bypassing original reg) set to 80psi
- 15m of 10mm bore air line all on 1/4bsp fittings
- PCL XF quick release at the hose end
- Clarke SB30 blasting cabinet & gun (bought second hand so not sure how old/abused the gun is - guessing it's fairly old as the cabinet has seen better days)
- 80-120 grit Aluminium Oxide Powder

Any pointers would be appreciated.

 

[Wallace]

How big is the blast nozzle? If it's over 3 or 4mm it will be slow as your compressor is a bit small for shot blasting.

 

[W.olly]

As above ^^^^^^

I am not all up on it myself but after just blasting a GRP boat i used two compressors each being 11 and 14 CFM 3HP size, 50l each.

Done the job well enough with a 2 and 2.5mm nozzle.

you may need bigger out put compressor and nozzle to match.

 

[brightspark]

I presume u mean the blasting gun is a suction type and not a pressure pot. don't expect a lot of a suction gun because its not going to happen pressure pot with 3mm jet and a 14cfm, comp just about cuts it

 

[rtbcomp]

If you have a suction gun the size of the nozzle you can see does not affect the air usage - it's the air nozzle inside the gun that determines the air consumption.

But as said so many times, the Clarke et al blasting guns are not up to much and you're infinitely better of with a pressure pot.

That said, some types of paint are tough as old boots, but rust should clean off easily enough.

 

[Pugly]

Thanks for the info gents.

Not sure what size the nozzle is (rookie - sorry!), will check next time I'm at the unit.

It's a suction gun so sounds like that may be one of the issues.

 

[Wallace]

If you have a suction gun the size of the nozzle you can see does not affect the air usage - it's the air nozzle inside the gun that determines the air consumption.

Makes a big difference on mine once the ceramic has worn or you try a larger one as the grit velocity drops.

 

[rtbcomp]

Makes a big difference on mine once the ceramic has worn or you try a larger one as the grit velocity drops.

You have the same amount of air & grit going through a bigger hole, so the velocity drops.

 

[Wallace]

But, as said, it doesn't affect the air consumption.

It affects the venturi and hence media pickup, concentration and velocity - but not the amount of air...

Granted it doesn't affect air consumption but does affect the gun operation dramatically as you say.

You have the same amount of air & grit going through a bigger hole, so the velocity drops.

Yes with a blast pot but not with a vacuum gun as if the nozzle velocity drops so does the vacuum to lift the grit and you deliver less.

 

[rtbcomp]

...

Yes with a blast pot but not with a vacuum gun as if the nozzle velocity drops so does the vacuum to lift the grit and you deliver less.

Not convinced, it's the velocity of the air from the air nozzle that causes the venturi effect to lift the grit, this will stay the same (or maybe increase slightly) as the ceramic nozzle wears.

 

[brightspark]

the more wear on the blast nozzle on a suction gun the less vacuum andn suction of media. the air nozzle remains constant as it doesn't wear. I cast my own blast gun and machined the nozzles and found the optimum size for air requirement and grit size

 

[Wallace]

Isn't the ceramic acting as the Venturi creating a pressure drop to lift the media? Surely if the Venturi increases in diameter for the same flow then the pressure drops reducing the vacuum lift?

 

[rtbcomp]

The venturi is immediately after the air nozzle, the media gets sucked up the tube and mixes with the air and gets expelled through the grit nozzle.

If the grit nozzle wears the same amount of grit goes through a bigger hole and slows down, as I said in post #9.

 

[Wallace]

The Venturi on both my guns is the ceramic and the air nozzle is directly behind. The pickup tube is quite a way back from the air nozzle forming a mixing chamber.

Is this the type of gun you are describing where the pickup tube is in front of the air nozzle?

 

[brightspark]

i made a couple of guns 40 years ago home cast in aluminium and used a worn out casting as the pattern the jets are made of carbon steel hardened and the end caps turned and machined on lathe and miller out of brass bar . quite a nice project for first time machining and didn't cost anything .its jetted for a 14cfm comp and 1mm blast media and runs at 90psi

 

[Wallace]

Top job! Blast nozzle looks similar to a Guyson part I have here somewhere.

 

[brightspark]

Top job! Blast nozzle looks similar to a Guyson part I have here somewhere.

possibly

 

[Wallace]

 

[Dave H2]

My rubbish suction gun works much better with the pickup tube restricted by a piece of old hose - seems to stop the gun choking.

 

[rcx132]

i made a couple of guns 40 years ago home cast in aluminium and used a worn out casting as the pattern the jets are made of carbon steel hardened and the end caps turned and machined on lathe and miller out of brass bar . quite a nice project for first time machining and didn't cost anything .its jetted for a 14cfm comp and 1mm blast media and runs at 90psi

Very impressive Brightspark!

@Pugly when I got my cabinet it was frustratingly slow, I spent half the evening getting my wifes frying pan blasted. Then I discovered the pressure pots. If I tell you they are five times faster that would be no exaggeration! I did a battery tray from my car the other day and it was done literally within a few minutes... then a brake drum.... done in 3 min, then my nozzle wore out...

I'm trying to get my home blasting rig set up now better now as I had some problems, the extractor was sucking up my expensive media, and the pressure pot hose is too stiff. Waiting for some oil hose to arrive and some mesh to keep the media from going up the extractor. Also awaiting a tungsten nozzle. The ceramic nozzles are ok on a suction gun but wore out after minutes with the pressure pot.

I don't know why people say 14 CFM is not enough. It was definitely enough for my suction gun, not sure about the pressure pot but seemed ok. I upgraded to two compressors shortly after getting my pot so it's hard to tell.