Multi-stage firing strategy – Reduces short shots and burnout

Injection speed isn't always better the faster it is, nor should it be maintained at a constant speed. The speed profile determines the stability of the melt leading edge and in-mold shear heat: too slow, and it will freeze and stagnate, causing short shots and flow marks; too fast, and it will have high shear and introduce air, causing burnt ends and spray marks. We'll provide a user-friendly, on-site approach to setting "multi-speed" injection, focusing on fixed fill time, stable injection speed, and pressure within limits. We offer a parameter adjustment process to help you quickly minimize short shots and burnt conditions.

Three principles

1. Maintain a steady injection speed: Try to keep the speed close throughout the filling process to avoid sudden changes in speed that could cause air entrainment.
2. The adjustment difference for each mold should not be too large: the variation in filling time for each cavity in the same mold should be controlled within ±3%, and the yield will be significantly improved.
3. There should be a safety margin in the pressure: leave a margin of 10-20% for the peak injection pressure; if the pressure has already reached the upper limit, going faster will only amplify the defects.

First, establish a baseline value: create a single segment before dividing into multiple segments.

Step A | Single-segment speed test

Fill the mold with the product at a single speed until it is almost full (excluding holding pressure), and ensure that the filling time falls within the following recommended range. Then observe the pressure and appearance.

Suggested fill time (for initial reference only):

• Wall thickness ≤ 1.0 mm: 0.3–0.8 s
• 1.0–2.0 mm: 0.8–1.5 s
• 2.0–3.0 mm: 1.2–2.5 s
• ≥3.0 mm: 1.5–3.0 s

Once we obtain this speed that is "runnable, has reasonable pressure, and has an acceptable appearance," we will use it as the baseline speed Vb.

Step B | Segmentation Strategy

• Section 1 (Initial 10–15% volume): 0.3–0.5×Vb, intended to prevent spraying and whitening at the gate.
• Second stage (main filling 70–80% volume): 0.9–1.1×Vb, maintaining leading edge stability.
• Section 3 (end 10–15% volume): 0.6–0.8×Vb, reducing end compressed air and charring.
• When there are thin ribs/long flow paths, a second 2.5 stage can be added: slightly increase the flow rate (+10–20%) before reaching the thin area to help pass through the bottleneck.

*The most stable switching condition is achieved by adjusting the screw position; if the cavity pressure is visible, the starting point of the third segment can be aligned with the position where the pressure is about to rise.

Material differences: Vb selection and final velocity

Defect Countermeasures

Objectives: Stable filling time, pressure not exceeding peak, and OK appearance.

• Short shots (especially on the thin ribs/far end)
  1. Increase the speed of the second segment by 10–20%.
  2. Add a short burst of speed before it reaches the thin ribs.
  3. Adjust the mold temperature by +5–10°C or increase the gate size by one level, then confirm.
• End charring/scorch marks
  1. The third segment starts earlier, and the speed drops to 0.6–0.7 × Vb.
  2. Add end exhaust (or replace the exhaust steel).
  3. Lower the material temperature by 5–10°C; if that doesn't work, switch to V/P earlier.
• Jetting/Serpentine Pattern
  1. The first stage should be reduced to 0.3–0.4 × Vb, or changed to a small rounded corner guide.
  2. Let the molten adhesive "attach to the mold" first, then accelerate to the second stage.
• Flow marks/ripples
  1. The first segment is slightly faster (+10–15%) to make the leading edge more stable.
  2. Increase the mold temperature and add radius (R) to the gate connection area.
• Rough edges accompanied by scorching
  1. Deceleration at the end of the range + earlier entry into the third stage;
  2. Check the mold clamping and parting surfaces;
  3. The pressure limit is lowered by 5–10% to avoid over-compressing the air.

Monitoring indicators

• Fill Time: Variation ≤ 3%.
• Peak injection pressure: 10–20 TP3T margin.
• Cavity pressure: The main filling should be smooth and flat, and there should be no sharp peaks or sudden surges at the end.
• Repeatability of weight per mold confirmed: target for short cycle parts ±0.2–0.4%.
• V/P switching buffer: Retain a small amount of padding (e.g., 3–8 mm, depending on the machine) to prevent backflow.

The core of multi-stage velocity is not "the more the better," but rather stabilizing the leading-edge velocity with the fewest possible stages. In practice, you only need to:

1. First, use a single segment to determine the appropriate filling time and pressure margin.
2. It can be divided into 3 sections according to "slow start, fast run, and then slow down".
3. Fine-tuning for defects
4. I always keep an eye on three things: Fill Time, Peak Pressure, and Weight Variation.

Following this approach, trial molding issues can usually be significantly resolved in a short period of time; subsequently, structural optimization through gate and venting will lead to more stable production. In short, the speed curve is the metronome of molding; by maintaining a stable pace, quality and yield will naturally follow.

Want to learn about injection molding monitoring and key injection parameters?
Further Reading :Key Technologies for Injection Molding Quality Monitoring – In-Mold Pressure and Temperature Sensors