OK, then you will have to bear with me as typing is not my strong thing.
Lets start off with talking about plasma cutters that are HF from the basic to the more advanced.
Note: Most HF plasma cutter (but not all) have a built in gap between the electrode and the cutting tip. This is like a spark plug gap.
Note: All electrodes (negative) are isolated from the outer threads (positive) on all plasma cutters HF as well as blow back units.
Now first all HF plasma cutters do NOT have a pilot arc. Some have to have the cutting tip touched on the metal (positive) to complete the circuit to start the arc for cutting. This is because the cutting tip is isolated from the electrode by a swirl ring and a retaining cup that has no metal in it like ceramic or other non conductive material. Also some must be kept in contact with the metal or the arc will go out. There are some that can be lifted off the metal and still cut as long as its close to the metal to maintain the arc.
Then we have the pilot arc HF plasma cutters. These will have a cutting tip that is (positive) by some means. These have a positive wire from the plasma cutter to the outer threads of the torch. The cutting tip is screwed into the torch making contact or the cutting tip is on a swirl ring and the retaining cup has a metal liner when screwed on connecting the cutting tip and threads. When the trigger is pulled there is a spark between the electrode and cutting tip starting the pilot arc. The pilot arc on a lot of the cheaper plasma cutters will stay on continuously as long as the trigger is pulled as well as the hole time it is cutting. This is one reason why HF consumables do not last very long.
Now there are plasma cutters that use a non HF or blow back torch that are HF start. The 2 most common torch’s used are the Thermal Dynamics SL60 and the Trafimet S45. They are timed to blow back just before the HF is activated giving the pilot arc.
Note: Most large commercial CNC tables actually are HF plasma cutting units because they are more reliable then blow back torch’s.
Now, I see that its most likely it will not be long till you will have HF plasma cutters that it will be harder to tell them from blow back units other then the consumables they use. With the advancement with electronics a lot of Tig welders do not use HF to start the arc. They are using electronic to start the arc and I believe it will be used on plasma cutters soon.
We will get into plasma cutters that are not HF.
Other then some HF plasma cutters that use the blow back torch’s listed above use a torch that has a moving electrode. This is by a air activated plunger that pulls the electrode (negative) back away from the cutting tip (positive) creating a small spark starting the pilot arc. Some years back on blow back plasma cutters, just like HF units the pilot arc stayed on the hole time it was cutting and there was no post air flow this caused consumable life to be very short.
Today with the advancement in electronics most of the trouble with the older blow back plasma cutters are gone. The ones still being used today The PT/IPT, S45, X45, etc. if kept clean and lubed they will still work just fine. Also years back you could rebuild most blow back torch’s unlike today in most cases you have to replace the complete torch head. Torch’s like Thermal Dynamics SL torch’s and Hypertherm’s new torch’s have no moving parts to go bad.
HF plasma cutters work fine in most cases. But for better consumable life as well as less electronic interference with other things like computers, CNC controllers, etc. or used around sensitive electronic’s around hospitals or people with pacemakers the blow back plasma cutters work better.
This is not everything but should cover most answers being asked about the difference between the HF and blow back plasma cutters.
Questions?
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