Uniquecoat HVAF
 
Uniquecoat HVAF in general and also specifically the Uniquecoat G4 powder feeder is the best on its market.

Being the best and in the spirit of “failure is not an option’” we here have some information and pointers to keep it that way.
This is information for customers that already have the equipment and also information for customers that are looking into purchasing HVAF and/or powder feeders.


G4 powder feeder.

The feeder is developed for feeding cemented carbide powders and metal powders into  a HVAF process.
It can in its standard configuration feed up to 500 g/min (30 kg/h) of WC Co Cr powder.
Feeding these kind of powders (5-30 µm and 15-45µm) into an HVAF process is easy and requires no special attention. The carrier gas flow is 30-40 l/min.
The G4 has caught on to the market and is now used for much more than the normal HVAF thermal spray applications that it was designed for,
One close to normal operation is the feeding of “flash carbide” powders with a PSD of 5-15 µm. These powders is not at all a problem for the G4 feeder but an additional stirrer blade takes away all potential problems. This blade is now a part of the standard delivery of a G4 but has not been so all the time.
If you don’t have this stirrer blade contact Uniquecoat or PTC Innovation to sort that out.
PTC Innovation and many others are now using the G4 feeder to feed fine powders like ceramic Chrome Oxide, Alumina etc. down to a PSD of 5-10 µm.
With the stirrer blade and a vibrator this is not a problem.

The G4 feeder now comes as standard with a pneumatic Vibrator.
The vibrator is mostly to solve problems with powders with low flowability that might want to stick to the surface in the bottom collecting funnel.
The blade is for breaking up bridging powders to let the feeding blade feed the powder through the metering gap.

When you use the G4 feeder for very fine ceramic powders it is of course together with a plasma spray process.
Radial feed Plasma process normally require a fairly low and very precise carrier gas flow. Axial feed Mettech Plasma process could have the same requirement to keep the residence time high in the high enthalpy zone.
When carrier gas flows gets low the demands on the hose transporting the powder increases. We use antistatic plastic hoses.
With very low carrier gas flows (to match the introduction of powder into a radial feed plasma system), the gas velocity in the hose can get so low that problems comes with carrying the powder.
One solution is to use a smaller internal diameter hose to increase the gas velocity for a given mass flow of carrier gas.
Uniquecoat sells a special antistatic carrier gas hose with a OD of 6 mm and a ID of 2.5 mm.
Other generic problems with powder transportation is diameter steps in the hose/feed line. Make sure that the fitting for the hose in the bottom of the G4 feeder has a chamfered entry and not a sharp step going from the collecting funnel into the hose connector. The same in the other end where the powder hose meets some kind of powder injector.

The means of transportation of powder out of a powder feeder into the powder hose and into the torch powder injector is gas flow.
This gas flow is generated by a pressure difference.
Without a pressurization of the powder feeders “hopper” there will be no carrier gas flow.
It is therefore essential that one let the flow controller for the carrier gas flow gas into the hopper long enough so that it is pressurized with a pressure that is determined by the pressure drop over the powder hose for each given carrier gas flow chosen.
Only after that pressure is established it is OK to start actually feeding powder through the powder hose.
In most processes this comes by automatic sequence.
An HVAF torch needs to pressurize the hopper to create gas flow in the powder hose during start up (this is done by maxing out the carrier gas mass flow controller during startup or even having a solenoid valve bypassing the mass flow controller momentarily at startup). Without this flow the powder hose will get burned off from hot combusting gas traveling backwards.
A DJ2600 or DJ2700 HVOF torch will instantly melt the tip of the injector if there is not enough carrier gas flow (any time during ignited operation)
The same is valid for a Mettech Axial 3 Plasma system.
Most radial injectors in a radial injected plasma system will survive the close proximity to the plasma even without flow of carrier gas.
It is on the other hand normal and best practice that the carrier gas will start flowing as soon as the radial plasma torch is started.
This is to be ready for start of powder feed at any time after startup.

In the current version of G4 there is a “soft start” feature that can be turned on where you define at what % of final federate it should start and the ramp time to reach final feed rate. This eliminates the risk of having a “clog” of powder in the exit of collecting funnel if you abruptly start with a very high feed rate.

For very “problematic” powders there might be an idea to initially set a higher carrier gas flow value, start the powder and then dial it down to the setting needed for the process.

If one purchase a Uniquecoat powder feeder as a stand alone unit to pair this with an existing thermal spray system and it Is not operated in a automatic manner it is essential to figure out how to operate ignition of spray system, pressurizing of powder feeder and start of powder in a sequence that will not cause any clogging of the feeder exit or powder hose.

It is also important, purchasing the G4 feeder as a separate item, to communicate to Uniquecoat and/or PTC Innovation as much information as possible on the intended use  and what thermal spray equipment it should be feeding.
With the proper information we can make sure that the G4 powder feeder will work with your process.



The G4 feeder can be purchased with an option of integrated mass flow controllers for carrier gas if it is to be used stand alone in the situation where there is no mass flow controlled carrier gas feed from the thermal spray equipment it should service.
The G4 powder feeder is also available as V4. It then has the same powder hopper and the very solid feeding mechanism but Is mounted on a stand instead of suspended off a load cell in  a cabinet.
It will have a very stabile powder feed rate flow and it will feed all the difficult powders that the G4 can do but it will have, as all volumetric powder feeders, a slight shift in feed rate going from full to empty.

In all cases handling thermal spray powders it is important that they are dried in a low temperature furnace so that moisture will not cause bridging etc. of the powder.

Another problem that can occur while feeding powder is having a gas leak in the powder feeding system. Any kind of leak in the powder feeder, hose from the carrier gas mass flow controller to feeder and powder hose will be detrimental to the force that is supposed to carry powder from the feeder to the injector.
A quick way of checking this is to create a artificial “powder clog” at the end of the hose.
Run carrier gas through the system without feeding powder. 10 l / min or whatever is normal.
Attach a connector to the end of the powder hose that also has a small ball valve.
Close the ball valve and let pressure build up in the system.
When you have 5-6 bar pressure (can be read on the pressure gauge on the feeder hopper), turn off the carrier gas flow.
Pressure should now be trapped and it should not decrease rapidly.
If the pressure holds you do not have a leak and you can release the pressure after a while with the ball valve at the end of the powder hose.
If pressure drops you have a leak in the system and need to find it and repair it.
(listen, leak spray , soap water etc.).

The G4 and V4 powder feeders have no wearing parts.
Over longer time one should check the O rings for wear.

Regarding cleaning of the powder feeder:
Cleaning is very easy and when powder feeder is empty ( dump powder feature in the control system) it is taken apart. Detach the bottom cone and put in a holder.
The big aluminium ring with the metering insert is taken off.
Next the stationary bottom floor with the feeding blade is taken off.
The drive motor and support ball bearing will now be visible.
Please occasionally take the ball bearing out and check the surface underneath, there should be a thing shim lifting the ball bearing a little but no powder in this position underneath the bearing.

The porous ring (metering insert) can over time get clogged up if you run very fine powder.
This will show up as some afterflow when you turn off powder feed. Clean it in a ultrasonic cleaner, then blow air in the gap in between the aluminium body and the porous ring (all around).
If you have the brass porous ring there is now a new porous ring of stainless steel that has higher porosity.
This new ring is less likely to clogg up and is more suitable if you run very fine powders.
Contact Uniquecoat if you want to aquire one of these.

Handling of powder, a trolley system can be purchased with the G4 feeder that enables convenient pickup of the powder hopper from the inside of the G4 cabinet to be moved to a ventilated safe area for disassembly and cleaning.