We field this question daily. Here are the quick and dirty answers:
- For a sharp edge and a steep wall, hold the nozzle close, or 0.025”- 0.060” from the target.
- For greater coverage, hold the nozzle farther away, or 1″-3” from the target.
Now, if only all applications were that simple…
What is Spot Size?
Before I dive further into the variables that impact this decision, let’s define “spot size.” Spot size is the affected region or the pattern that the blast stream erodes from a targeted surface. Simply put, the spot size is the etched or cut area surrounded by overspray. Overspray has no meaningful impact; rather, it is just a “frosting” on the surface.
Image of eroded spot and overspray area from a 0.018” nozzle at a distance of 0.5″ shown with and without the circles delimiting the spot size and overspray area. The black circle defines the limit of the eroded spot, or the limit of the “spot size”, and the red circle defines the limit of the overspray.
Check out the reference chart below under ‘Dig Deeper’ section at the bottom that specifically matches spot size and nozzle distance.
The Nozzle is #1
The nozzle size, shape, and placement ultimately determine spot size, regardless of the media used or the velocity of the abrasive stream. Nozzle distance and spot size have a pretty simple relationship: the closer the nozzle is to the target, the smaller and sharper the spot size. The farther away the nozzle is held from the surface, the larger and less defined the spot size. You get the idea.
0.030″ standard round nozzle at a distance of: 0.005″/ 0.025″/ 0.05″/ 0.25″/ 0.5″ & 1″ from target. Each bore blasted for 2 minutes. As nozzle backs away, the rate of material removal increases, but the bore becomes less defined.
Nozzle shape has a trickier relationship with spot size. Round nozzles are the workhorse nozzles. They suit most applications, and the spot size from a round nozzle tends to remain circular at any distance. However, the spot size from rectangular nozzles gradually opens into a circular blast pattern as the nozzle pulls away from the target. Thus, rectangular nozzles are better for precise cuts at close distances (see image below).
Spot size from rectangular nozzles close to the target (L) and farther away (R). The spot size from the rectangular nozzle opens into a circular shape as the nozzle pulls away.
But not too close! Abrasive particles ricochet between the nozzle and the part IF the nozzle is any closer than 0.025” to the target. Cut speeds dramatically decrease and overspray increases. Basically, holding the nozzle too close slows the jet stream and makes a mess.
Blasted spots circled in red show effect from holding nozzle too close (or 0.005″ from the surface.)
Why Hold the Nozzle Farther Away?
Applications that require broad regions of texture or material removal benefit from greater nozzle distance. Abrasive particles spread can over a wide area at this distance, reducing particle collision. Greater distance allows the use of a larger orifice, increasing abrasive density in the airstream, which results in faster part processing.
The jet tends to be most effective if the end of the nozzle tip is 1″-3″ from the targeted part. Abrasive particle velocity increases after it leaves the nozzle, reaching equilibrium with the air stream at about 1”, and begins to slow around 2″-3”.
Nozzle distance creates uniform textured finish on a silicon wafer.
Do Hi/Performance Nozzles Make a Difference?
YES! The internal geometry of Comco’s Hi/Performance nozzle speeds up particle velocities by 30%, providing increased focus to the jet stream and reducing overspray. (Note the differences in bore depth and profile in the image below.) Increased focus allows an operator to reduce blast pressure and process parts faster. Plus, Hi/Performance nozzles have a lifespan 3-5x longer than a standard nozzle.
Comparison of results from 0.030″ nozzle, standard (top) and Hi/Performance (bottom).
Nozzle distance (L-R): 0.005″ / 0.025″ / 0.05″ / 0.25″ / 0.5″ and 1″. Each bore blasted for 2 minutes.
Comparison of the spot sizes between standard and Hi/Performance nozzles with the same I.D. The 0.030” nozzles were shifted up by 0.1” and the 0.046” nozzles were shifted by 0.2” for ease of viewing.