Spray Gun Nozzle Pressure
#11
I have a LVLP spray gun that I'm very satisfied with. I have read that LVLPs as well as HVLPs are designed to have no more than 10psi at the nozzle. Can someone explain that to me? Maybe I misunderstood what I read. Does that mean that no matter what the pressure is coming into the gun, it will be only 10 psi at the nozzle? I was recently spraying some paint and changing the incoming pressure seemed to matter unless it was just my imagination.
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#12
(07-26-2022, 05:48 PM)Willyou Wrote: I have a LVLP spray gun that I'm very satisfied with. I have read that LVLPs as well as HVLPs are designed to have no more than 10psi at the nozzle. Can someone explain that to me? Maybe I misunderstood what I read. Does that mean that no matter what the pressure is coming into the gun, it will be only 10 psi at the nozzle? I was recently spraying some paint and changing the incoming pressure seemed to matter unless it was just my imagination.

I think 10 psi max at the tip is the law in some states.  For my Qualspray gun, the literature says it will be 10 psi at the tip when the inlet pressure is 29 psi.  Tip pressure will vary with incoming air pressure; nothing new there.  

John
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#13
(07-26-2022, 07:47 PM)jteneyck Wrote: I think 10 psi max at the tip is the law in some states.  For my Qualspray gun, the literature says it will be 10 psi at the tip when the inlet pressure is 29 psi.  Tip pressure will vary with incoming air pressure; nothing new there.  

John
Thanks. So, once your inlet pressure goes above 29 psi, does the tip pressure go up by equal amounts; i.e., 11 psi at the tip for 30 psi inlet and so on?
Does the tip pressure gradually increase up to 10 as the inlet increases to 29 or is it at 10 when the inlet is 10 and stay there until the inlet reaches 29?
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#14
(07-26-2022, 08:48 PM)Willyou Wrote: Thanks. So, once your inlet pressure goes above 29 psi, does the tip pressure go up by equal amounts; i.e., 11 psi at the tip for 30 psi inlet and so on?
Does the tip pressure gradually increase up to 10 as the inlet increases to 29 or is it at 10 when the inlet is 10 and stay there until the inlet reaches 29?

Only your gun maker can answer that question as it will vary by design.  But I can tell you that it varies in a fairly linear fashion from 10 to 29 and I would expect it to do so from there up as there is no regulator internal to the gun.  It flow determines the pressure and the flow is predetermined by the passage sizes.

The 10 is not a state thing but a federal spec, just a way to specify a true HVLP gun.   There is a later version, High Transfer Efficiency.  HTE isn't bound by pressure alone but instead measures the percentage of the material that leaves the gun that is actually transferred to the intended surface.
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#15
(07-26-2022, 08:48 PM)Willyou Wrote: Thanks. So, once your inlet pressure goes above 29 psi, does the tip pressure go up by equal amounts; i.e., 11 psi at the tip for 30 psi inlet and so on?
Does the tip pressure gradually increase up to 10 as the inlet increases to 29 or is it at 10 when the inlet is 10 and stay there until the inlet reaches 29?

Tip pressure changes with inlet pressure.  I doubt it's linear though; most pressure/flow problems aren't.  For sure there is some upper pressure beyond which flow, and therefore, pressure at the tip, will not increase.   

John
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#16
Thanks for the help. I only spray occasionally and always do a test panel or two before doing the project. But, it helps to know a little more about how the equipment works.
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#17
The innards of every gun are different. Cheap guns have lots of turbulence and air and fluid friction inside the gun impeding flow. Not so much pressure, but flow. Nicer guns like SATA for instance have chemically polished insides. Polished air ports on the air cap and polished spray tip orifices etc etc. They get real efficient at atomizing and delivering air and product. A cheap gun and a high end gun with the same size needle and tip and spray cap and the same pressure at the inlet will deliver different flow rates and atomization from the tip at the same pressure.

If you want your LVLP or HVLP conversions guns to spray great, take off the regulator and get a regulator that you can crank up to about 40-45 psi. It won't be legal in some states but the gun will lay out a better finish, atomize better and deliver more product... so you can work a lot faster.
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#18
(07-27-2022, 09:52 PM)Snipe Hunter Wrote: The innards of every gun are different. Cheap guns have lots of turbulence and air and fluid friction inside the gun impeding flow. Not so much pressure, but flow. Nicer guns like SATA for instance have chemically polished insides. Polished air ports on the air cap and polished spray tip orifices etc etc. They get real efficient at atomizing and delivering air and product. A cheap gun and a high end gun with the same size needle and tip and spray cap and the same pressure at the inlet will deliver different flow rates and atomization from the tip at the same pressure.

If you want your LVLP or HVLP conversions guns to spray great, take off the regulator and get a regulator that you can crank up to about 40-45 psi. It won't be legal in some states but the gun will lay out a better finish, atomize better and deliver more product... so you can work a lot faster.
Are you talking about the regulator that hangs off of the end of the handle? My guns did not come with one, but I have added one that allows higher settings. I think they are Iwata knock-offs and based on my very limited experience, they do a very good job. One has a 2.0mm tip and I recently sprayed some water based enamel (not latex) that took about 4 minutes to drain through a #4 Ford cup. I only had to thin it about 10-15%. I used a pressure setting a bit higher than 45.
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#19
(07-27-2022, 10:23 PM)Willyou Wrote: Are you talking about the regulator that hangs off of the end of the handle? My guns did not come with one, but I have added one that allows higher settings. I think they are Iwata knock-offs and based on my very limited experience, they do a very good job. One has a 2.0mm tip and I recently sprayed some water based enamel (not latex) that took about 4 minutes to drain through a #4 Ford cup. I only had to thin it about 10-15%. I used a pressure setting a bit higher than 45.

What was the viscosity after you thinned it?  If you are using a gravity feed gun anything higher than around 100 seconds with a 2.0 mm N/N isn't going to work very well no matter what pressure you set it at.  

Higher pressure will give better atomization but at the expense of more overspray and problems when spraying inside a cabinet.  

If you want to be able to spray high viscosity products well you should add a 3M H/O PPS cup to your gun, with its own regulator.  It is a pressurized cup system that forces the finish through the gun.  The gun still needs to be able to atomize it efficiently, but the pressure feed from the H/O cup will do so using a smaller diameter N/N and lower air pressure.  

John
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#20
(07-28-2022, 12:11 PM)jteneyck Wrote: What was the viscosity after you thinned it?  If you are using a gravity feed gun anything higher than around 100 seconds with a 2.0 mm N/N isn't going to work very well no matter what pressure you set it at.  

Higher pressure will give better atomization but at the expense of more overspray and problems when spraying inside a cabinet.  

If you want to be able to spray high viscosity products well you should add a 3M H/O PPS cup to your gun, with its own regulator.  It is a pressurized cup system that forces the finish through the gun.  The gun still needs to be able to atomize it efficiently, but the pressure feed from the H/O cup will do so using a smaller diameter N/N and lower air pressure.  

John
No. Four min + was the viscosity out of the can (DTM water based enamel). I didn't check the viscosity after thinning, but it didn't take as much as I thought it would. I might have been able to refine it a bit, but it was an outdoor project and quality of the finish wasn't hugely important. Out of the gun it had just a tad of orange peel, but it laid flat after it dried.

Thanks for the suggestion. I don't do a lot of spraying with thick material. I mostly spray WB Poly or shellac and I can do those out of the can with no thinning (1.5 or 1.3 tip).
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