Last updated: 2026-09-04 · 5 min read

Injection Speed Profiling — How Do You Set the Injection Speed at Each Stage of the Fill to Eliminate Defects and Optimize the Cycle Time?

Injection speed profiling divides the fill into 4 stages — each at a different speed to match melt front behavior. A single-speed fill causes jetting at the gate, burn marks at last fill, and flash at the parting line. The standard profile: slow through the gate → fast fill → slow at 90–95% fill → packing transition. Each stage targets one defect mode.

Key Facts

Four stages: Stage 1 — slow through gate (10–30 mm/s, prevent jetting); Stage 2 — fast through runners and cavity (80–150 mm/s, fill before freeze); Stage 3 — slow at 90–95% fill (20–50 mm/s, prevent burn marks); Stage 4 — transition to packing pressure at 95–98% fill.

The switchover point (Stage 3→4) is the most critical parameter: switch at 95–98% cavity fill by position (not time). Switch too early → sink marks and voids. Switch too late → flash, overpacking, stuck parts.

Setup method: short-shot studies at decreasing fill positions (90%, 85%, 80%, 70%) to map where the melt front is at each position. The speed profile is then set to match the flow condition: slow where the melt front is near a restriction, fast where it is in open cavity.

Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. The related guide covers adjacent topics.

What Is the Standard 4-Stage Profile and What Defect Does Each Stage Prevent?

Four stages, each targeting one defect:

Stage	Speed	Purpose	Defect if Wrong
1: Gate entry	10–30 mm/s (slow)	Prevent jetting as melt passes through the gate	Too fast: jetting, flow marks, tiger stripes
2: Cavity fill	80–150 mm/s (fast)	Fill the cavity before the melt freezes	Too slow: short shots, cold flow marks, high injection pressure
3: Last fill area	20–50 mm/s (slow)	Let gas escape through vents before melt reaches them	Too fast: burn marks, gas traps, diesel effect
4: Packing transition	Switch to packing pressure at 95–98% fill	Prevent overpacking and flash	Wrong timing: sink marks (early), flash (late)

The gate entry speed (Stage 1) is the most important for cosmetic parts. If the melt enters the cavity at high velocity through a small gate, it jets — the melt pulses rather than flows smoothly, creating flow marks (tiger stripes) on the surface. The fix: slow injection (10–30 mm/s) as the melt passes through the gate, then ramp up to full speed once the melt is in the open cavity.

The cavity fill speed (Stage 2) must be fast enough to fill before the melt freezes. The melt cools as it flows — if the injection speed is too slow, the melt front temperature drops below the flow-stop temperature before the cavity is full, causing a short shot. The required speed depends on the resin (PP flows longer than PC), the wall thickness (thin walls freeze faster) and the flow path length.

How Do You Set the Switchover Point and Optimize the Profile?

The switchover from Stage 3 (slow fill) to Stage 4 (packing pressure) is the most critical parameter in the injection profile. The switchover should occur at 95–98% cavity fill — by position (screw position), not by time.

Run short-shot studies. Reduce the injection position in steps (90%, 85%, 80%, 70% of full stroke) — each shot fills less of the cavity. Examine each short shot to map where the melt front is at each position. This reveals the flow pattern and identifies restrictions, weld lines and last-fill areas.
Identify the 95% fill position. From the short-shot series, find the screw position where the cavity is approximately 95% full (the last vent area is about to fill). Set the switchover point at this position.
Set Stage 3 speed. The speed for the last 5–10% of fill should be slow (20–50 mm/s) — this lets gas escape through the vents before the melt reaches them. If the melt reaches the vents at high speed, the gas is compressed, heated and burns (diesel effect).
Verify with a short-shot series. Run the full short-shot series again with the profile set. Check: no jetting at the gate (Stage 1), no short shots (Stage 2), no burn marks at last fill (Stage 3), clean switchover at 95% (Stage 4).

Two common mistakes. Switching by time instead of position: the melt temperature and viscosity change between cycles — a time-based switchover fills different volumes each cycle. Position-based switchover is consistent regardless of cycle variation. No Stage 1 speed reduction: the melt jets through the gate at full speed, creating flow marks that are visible on cosmetic surfaces — the fix is always to slow down at the gate.

Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. The related guide covers adjacent topics.

Injection speed profiling is not a tuning exercise — it is a defect elimination method. Each stage targets one specific defect: jetting, short shots, burn marks or overpacking. The profile is correct when all four defect modes are eliminated simultaneously.

Key parameter identified — critical variable confirmed against spec.

Tolerance within achievable range — commercial ±0.15mm or tighter as needed.

Material matched to application — resin grade selected for end-use environment.

Process window validated at T1 — parameters locked after first trial.

Inspection method defined — CMM, visual, or functional test specified.

Frequently Asked Questions

How many speed stages do you need?

Four stages is the standard minimum: slow at gate, fast fill, slow at last fill, packing transition. Complex parts may require 6–8 stages — each stage targets a specific flow condition (a restriction, a thick section, a weld line area). The rule: add a stage wherever the flow condition changes — a change in wall thickness, a change in flow direction, or a change in venting condition.

More stages give finer control but increase the setup time. For simple parts (thin walls, single gate), 4 stages are sufficient. For complex parts (multiple gates, varying wall thickness, cosmetic surfaces), 6–8 stages may be needed.

How do you know if the switchover point is correct?

Three checks. Parts with sink marks: the switchover is too early (the cavity is not full before packing starts — the packing pressure cannot compensate for shrinkage). Parts with flash: the switchover is too late (the cavity overfills before the packing pressure kicks in). Parts with dimensional variation: the switchover position is inconsistent between cycles — check that the switchover is by position, not by time.

The correct switchover produces parts that are dimensionally within spec, free of flash, free of sink marks and with a consistent weight (±0.5% part-to-part).

Does the injection speed profile affect weld line strength?

Yes. A higher injection speed at the weld line area increases the melt temperature at the weld (the melt has not cooled as much) — this improves weld line strength. A lower speed allows the melt to cool before the flow fronts meet — the weld line is weaker and more visible.

If the weld line is in a critical area (load-bearing), increase the injection speed in the weld line zone (add a fast stage just before the weld). If the weld line is cosmetic, move the gate location to eliminate or relocate the weld — speed adjustments alone cannot hide a visible weld line.

Key Takeaways

4 stages: slow at gate → fast fill → slow at last fill → packing transition.

Switchover at 95–98% fill by position (not time) — the most critical parameter.

Setup: short-shot studies map the flow pattern, then set speed to match each flow condition.

Each stage targets one defect: jetting, short shots, burn marks, overpacking.

The injection speed profile is set during the mold trial (T1–T2) and locked into the process sheet.
Once locked, the profile should not be changed without a formal process change request — even small changes to the speed or switchover point can introduce defects that are not visible until the parts are in production.