Model SG-100 Plasma Spray Gun
Operator’s Manual
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Issued: 12/11
Page 38
One time constant interaction relates to gun motion. For example,
assume the gun’s linear motion is one second left to right. Assume
also, that powder traverses the hose’s moving portion in one
second. In this scenario, the gun motion can overcome the
powder’s velocity in the hose, resulting in slowed powder flow or
pulsing. The solution lies in changing the parameters which affect
the time constants:
•
Adjust the carrier gas flow, optimizing the spray pattern
and deposit efficiency.
• Change
gun
manipulation
parameters (velocity), if possible.
• Change
the
powder
hose
length, or alter the free length at
the gun end, to change the time constant.
•
Change the powder feed hose’s internal diameter (usually
smaller) to significantly alter the time constant. Decreasing
the I.D. from 3/16" (4.8 mm) to 1/8" (3.2 mm) halves the
time constant.
H. Traverse Speed/Pitch
Generally, a traverse speed of about 1200 in./min. (0.5 m/sec) is a good
rule-of-thumb. Proper traverse speed creates deposits of approximately
.001 in. (25.4 µm)/pass for metallic and coarse materials and .0005 in.
(127 µm)/pass for ceramic and fine materials. Following these guidelines
decreases residual stress (hence bond strength is increased), increases
thickness buildup, and decreases porosity. Adjust traverse to obtain an
appropriate thickness buildup rate in terms of mils/pass.
Pitch or increment is the step or feed of the gun motion associated with
traverse. Generally, pitch is 0.125 - 0.200 in. (3.2 - 5.0 mm)/revolution or
step. Pitch is adjusted to prevent striping or “barber poling.”
I. Substrate Cooling
Cooling jets should be used to provide a stream of gas or dry air at or near
the point of impingement of the powder stream on the substrate. Cooling
air cools the substrate and blows away the unmolten particles and smoke
(overspray). The type of cooling gas used depends on the powder being
sprayed. Argon or nitrogen can be used for most materials that oxidize
readily. With materials such as titanium, which forms a nitride easily,
argon is the better choice. Carbon dioxide can also be used for cooling.
Dry air may be used if oxides are not a concern.