Switchover Point Optimization — How Do You Set the V-P Switchover to Prevent Over-packing, Under-packing and Dimensional Variation in Your Injection Molded Parts?
Last updated: 2026-09-03 · Reading time: about 6 min
The V-P switchover (velocity-to-pressure, or transfer point) is the instant the machine stops filling by speed and starts packing by pressure. Set it at 95–99% cavity fill and packing finishes the shot cleanly; too early sinks and undersizes, too late flashes and oversizes.
See also related article.
Key facts
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Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. See also related guide.
What are the switchover methods — and how do they differ?
Four triggers, ranked by how directly they measure what is happening in the cavity.
| Method | How it works | Accuracy | When to use |
|---|---|---|---|
| Screw position (stroke) | Switches at a fixed screw position | ±0.5 mm | Most common; simple and reliable |
| Injection pressure | Switches at a set injection pressure | ±10 bar | When cavity pressure is consistent |
| Cavity pressure | Switches at a set cavity-sensor pressure | ±5 bar | Most accurate; needs cavity sensors |
| Part weight (adaptive) | Auto-trimmed to hold a target part weight | ±0.1% | High-precision parts; closed-loop |
Screw position is the default because it is simple and reliable — but it is blind to viscosity. A new resin batch fills to a different level at the same stroke. Cavity pressure keys off what is actually happening in the cavity, so a viscosity change does not move the fill level. Adaptive weight closes the loop on a load cell or check-weigher and holds part weight within ±0.1% against viscosity, mold-temperature and ambient drift.
How do you optimize the switchover — step by step?
A short-shot study finds the point by measurement, not by guess. Four steps.
- Start late, so the cavity is over-full. Set the switchover past the point where the cavity is full, so the part over-packs — flash, high ejection force, dimensional oversize. Record the screw position, the peak injection pressure and the part weight.
- Walk the switchover earlier in steps. Reduce the stroke by 1–2 mm per step and record position, peak pressure and weight at each one. Both weight and peak pressure fall as the switchover moves earlier — less material packed, less melt compressed.
- Find the cushion point. Keep moving earlier until the part weight drops sharply. That drop is where the cavity stops being full. The position just before it is the cushion point — the cavity is about 95% full, and this is the earliest safe switchover.
- Set 0.5–1.0 mm past the cushion point. That lands the cavity at 97–99% full at switchover, so packing completes the fill and compensates shrinkage without over-packing. Verify with 20–30 shots: weight within ±0.3%, no flash or sink, dimensions in tolerance.
The cushion point is your floor. It is the switchover position where the cavity is about 95% full — the last point before part weight falls off a cliff because the cavity is no longer full. Set production 0.5–1.0 mm past it, toward the late side, so every shot lands at 97–99% fill. Go any earlier and each shot risks under-packing, sink and undersize.
“Most of the flash and sink I see on a new mold traces back to one number — where the machine transfers from velocity to pressure. We develop the process with a short-shot study to find the cushion point, set the switchover just past it, and record the weight and cavity-pressure data, so the setting is documented rather than guessed.”
— Project Engineer, aCatMould
Frequently asked questions
What if the switchover is not consistent shot to shot?
If the switchover position wanders more than ±0.5 mm, the cavity fill changes every shot, and so do weight and dimensions.
Weight swings ±0.5–1.0% instead of the ±0.1–0.3% of a consistent process.
Three usual causes: a worn check ring leaking melt past the screw tip (replace every 100,000–500,000 shots, by resin abrasiveness); slow or inconsistent hydraulic valve response (service the spool and seals); and viscosity variation between batches (move to cavity-pressure switchover, which keys off the cavity, not the stroke).
How do you verify the switchover is optimal?
Part weight first: run 20–30 consecutive shots and weigh each. Variation should stay within ±0.3% of the average (±0.3 g on a 100 g part); larger means the process is inconsistent — check the ring, the valve, or move to cavity pressure.
Then cavity pressure and dimensions. On a sensor-equipped mold the switchover should hit the same cavity pressure every shot, within ±5%. Measure critical dimensions on 5–10 parts — within drawing tolerance and ±0.05 mm on a 100 mm feature; outside that, move the switchover earlier or later.
Can the switchover drift during production — and how often do you check?
Yes. Mold temperature drift changes the fill, so check once the mold stabilizes (after 30–60 minutes of running). A resin batch change moves viscosity (MFI varies ±15% between batches), so re-check after each batch — expect a 0.5–2.0 mm adjustment.
Check-ring wear makes the switchover inconsistent, so check every 50,000–100,000 shots (weight variation is the tell), and ambient temperature swings with the seasons, so re-check at each season change. Switchover is a maintained process window, not a one-time setup.
Key takeaways
- The V-P switchover sets cavity fill before packing — aim for 97–99% full, 0.5–1.0 mm past the cushion point, to avoid under-packing (sink, undersize) and over-packing (flash, stress).
- Trigger it by screw position (common, ±0.5 mm), cavity pressure (accurate, ±5 bar) or adaptive weight (best, ±0.1%); cavity-based methods ignore viscosity drift.
- Verify with weight ±0.3%, cavity pressure ±5% and dimensions ±0.05 mm, and re-check after a batch change, mold-temperature stabilization and every 50,000–100,000 shots.
The switchover point is the single most consequential number in the process — it decides whether the part over-packs into flash and stress or under-packs into sink and weak welds. The short-shot study turns that number from a guess into a measurement: start late, walk earlier, find the cushion point, set just past it. If your parts vary dimensionally, flash or sink, check the switchover first — it is usually the root.
Sources
RJG Inc — scientific (decoupled) molding and V-P switchover training
Plastics Technology — switchover and process control practice
Beaumont Technologies — mold filling and switchover analysis
Kistler — cavity-pressure-based switchover and process monitoring
Society of Plastics Engineers — injection molding process engineering
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet