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MIG gas preflow and postflow control how long shielding gas runs before and after the arc. Set them too short and air can contaminate the weld at startup or as the puddle cools. Set them too long and you waste gas, slow the job, and may disturb the weld area with an unnecessary gas stream.
What preflow and postflow do
Preflow starts shielding gas before the arc. It gives the gas time to displace air around the contact tip and work area before welding begins. Postflow keeps gas flowing briefly after the arc stops, shielding the hot crater and wire end while they cool.
Many basic MIG machines have no separate preflow or postflow setting. Gas starts when the trigger is pulled and stops when it is released, with any delay determined by the machine’s control design. On machines with adjustable timers, the settings are usually in seconds. Check the manual: some machines apply them differently in two-step and four-step trigger modes.
Practical starting settings
For ordinary MIG work with a cylinder, regulator, and sound torch, start with about 0.2–0.5 seconds of preflow and 0.5–1.5 seconds of postflow. These are starting points, not universal requirements. A short preflow is often enough to establish coverage; postflow generally needs to be longer because it protects the hot weld after the arc ends.
If your machine lets you adjust only postflow, try 1 second and inspect the weld. Increase it in small steps if the crater oxidizes or the wire end is visibly exposed while hot. If the weld starts cleanly without a pop or porous spot, there may be no reason to add more preflow.
Use the gas flow rate specified for your process and setup as well. A common starting range for short-circuit MIG with a small-to-medium nozzle is roughly 20–30 cubic feet per hour (CFH), but nozzle size, joint access, draft, gas mix, and manufacturer guidance matter. A longer timer cannot compensate for a leak, clogged nozzle, poor ground, or insufficient flow.
Adjust for the job
| Situation | Preflow starting point | Postflow starting point | What to watch for |
|---|---|---|---|
| General mild-steel work indoors | 0.2–0.5 s | 0.5–1.0 s | Clean starts and a protected crater |
| Short tack welds | 0.2–0.4 s | 0.5–1.0 s | Gas use can add up across many tacks |
| Longer welds or hotter settings | 0.3–0.5 s | 1.0–1.5 s | More heat may keep the end crater hot longer |
| Drafty area or difficult joint access | Test around 0.5 s | Test around 1.0–1.5 s | Fix drafts and torch position before simply increasing flow |
These ranges are not a substitute for a welding procedure or the machine’s recommended settings. Aluminum and stainless steel can be more sensitive to contamination, but simply adding seconds does not guarantee sound shielding. Keep the nozzle close enough to the work, use the correct gas and consumables, and protect the arc from moving air.
Diagnose bad shielding before changing timers
Porosity, soot, or a rough, oxidized weld start can point to poor shielding, but several faults can look alike. Check that the cylinder valve is open, the regulator is set correctly, hose connections are tight, and the nozzle is not packed with spatter. A leak can waste gas and leave the torch short of shielding even when the flowmeter looks normal.
Hold the torch over a scrap piece and pull the trigger briefly without striking an arc. Listen for gas and confirm that it starts before the wire begins feeding if the machine supports preflow. Do not use a flame to check for leaks; use an appropriate leak-detection solution. If flow seems weak or inconsistent, inspect the liner and gas hose and follow the machine manual.
Too much flow can also cause trouble. Excessive gas velocity can pull surrounding air into the shielding stream, especially with a long stickout or awkward torch angle. If you see porosity, do not immediately turn the regulator higher. Check for drafts, torch distance, nozzle condition, and the correct flow rate first. A welding MIG gas flow meter can help confirm flow at the torch when the regulator reading alone is misleading.
When to buy a timer or flow-checking tool
If the machine already has adjustable preflow and postflow, use its controls before buying accessories. If it does not, a separate gas timer is usually unnecessary for routine shop MIG; consistent technique and a properly functioning gas system matter more. For frequent critical work, an upgrade to a welder with adjustable gas timing may be worthwhile, but compare the feature against the cost and the machine’s duty cycle, output, and service support.
A basic regulator is fine for many hobby and repair jobs if it is compatible with your gas cylinder and maintains stable flow. If yours is damaged, hard to read, or inconsistent, look for a MIG regulator with a flowmeter that matches your gas and cylinder connection. Avoid buying a more expensive regulator just for its appearance; reliable, repeatable flow is the useful feature.
Tune the settings on scrap
Make several short beads on clean scrap of the same material and thickness as the job. Keep wire speed, voltage, gas flow, stickout, and torch angle constant while you change only one timer setting at a time. Start with the ranges above, then inspect the beginning and end of each bead for pinholes, oxidation, or an unprotected crater.
If the start is clean but gas use is high during repeated tacking, reduce preflow in small increments and test again. If the end is discolored or porous, add a little postflow and check that you are holding the torch in place over the crater as the gas continues. On machines without adjustable timing, finish the weld with a brief pause and maintain torch position until the arc and gas stop. That simple habit is often all a basic MIG setup needs.