Solving the problem of low cavity filling rate in high-speed packaging injection molding

High-Speed Packaging Injection Molding

2025/12/03 By le zhan

high-speed packaging injection molding 1-7

Low cavity fill rate is a significant challenge in high-speed injection molding. For products such as thin-walled containers, caps, or blister packaging, even a cycle time reduced to 3-5 seconds can lead to a 5% decrease in output and a 20% increase in scrap costs. The primary reason for this is the mismatch between injection speed and pressure profiles, rather than injection molding machine malfunction or poor resin quality. High-speed packaging injection molding demands extremely high precision: the plastic flow rate must be fast enough to fill narrow cavities before cooling, yet not so fast as to cause air bubbles or missed areas. Therefore, the key to solving this problem lies in mastering the “balance” between speed and pressure—these two curves must be coordinated to ensure smooth resin injection into each cavity.

The Importance of Cavity Fill Rate in High-Speed Packaging Injection Molding

In high-speed injection molding for packaging, a “good enough” fill rate is far from sufficient. Unlike automotive or industrial parts (where cycle times can exceed 30 seconds), packaging production lines rely on output: over 10,000 parts per hour, operating 24/7. Low cavity fill rate disrupts this rhythm in three ways, thereby increasing costs.

First, scrap costs skyrocket. A 10% drop in fill rate means 10% of the product becomes scrap in every production run—melted, injected, and then discarded resin. For a production line using $2 per kilogram of PP resin with a daily output of 500 kilograms, 10% scrap results in a $100 daily loss, or $3,000 per month. Second, output plummets. High-speed packaging injection molding lines are calibrated to specific output targets. If one-fifth of the cavities cannot be filled, a 25% increase in cycle time is needed to reach the daily quota—this reduces the production line’s output from 15,000 units per hour to 12,000 units per hour.

Third, mold wear intensifies. When plastic cannot fill the cavities evenly, the injection molding machine compensates by increasing pressure, but this leads to uneven stress on the cavity and core. The most effective way to control fill rate is to adjust the injection molding machine’s speed and pressure profile.

The Root Causes of Low Fill Rate in High-Speed Packaging Injection Molding

Before adjusting the curves, you need to identify the root causes. Low cavity fill rate in high-speed injection molding for high-speed packaging typically stems from four common issues; speed and pressure curves are the root causes in most cases.

First, this occurs when the injection molding machine‘s speed and pressure are out of sync. This can result in speeds that are too fast or too early, too slow during cycles, or pressures that rise too late. Second, there are material viscosity issues. Resin with too high a viscosity flows slowly, while too low a viscosity will burn or flow out prematurely. We need to check if the resin melt flow rate meets the mold requirements. Next, a gate that is too narrow, an uneven runner system, or excessively sharp corners can all restrict resin flow. If the fill rate is low only in new molds, the problem is likely in the mold design. Finally, there are calibration errors in injection molding machines. Worn servo motors, faulty pressure sensors, or outdated firmware can all reduce curve accuracy.

The Root Causes of Low Fill Rate in High-Speed Packaging Injection Molding

Injection Speed Curve Optimization

In high-speed packaging injection molding, speed is not a single value, but a curve: the speed at which the screw moves during each filling stage. The goal is to achieve a mold filling speed that is both fast enough to prevent cooling and smooth enough to avoid bubbles or jetting.

Stage 1: Gate Filling (Slow Start)

The first 10-20% of the cavity volume (gate area) requires a slow, controlled speed. If the speed is too high during this stage, “jetting” will occur—the high-speed plastic stream impacts the mold wall, hindering subsequent flow after cooling. For a 50 mm cavity, set the screw speed to 100–150 mm/s for the first 5–10 mm of stroke (maximum speed 250 mm/s).

Stage 2: Cavity Filling (Medium Speed)

Once the gate is filled (10-20% of the stroke), the speed is increased to maximum. This is the origin of the name “high-speed packaging injection molding”—rapid flow ensures the plastic reaches all corners of the cavity before cooling. For thin-walled parts (0.5-1mm thick), use a speed of 200-300mm/s; for thicker parts (1-2mm thick), use a speed of 150-200mm/s. Topstar’s servo-driven injection molding machines allow speed adjustment in 0.1 mm/s increments, enabling fine-tuning for each part.

Third Stage: End Filling (Deceleration)

The last 10% of the cavity volume (near the vent) requires a speed reduction. This allows air to escape through the vent, preventing “overfilling” (which can cause a flash). For most packaged parts, you should reduce the speed to 50–100 mm/s when the screw stroke reaches 90% of the total stroke.

Pressure Profile—A Key Complement to Speed

Speed determines the injection rate of the plastic, while pressure ensures that the plastic fills every corner of the mold. A mismatched pressure profile—too high, too low, or at the wrong time—will render even the optimal speed setting ineffective. In high-speed injection molding for packaging, the pressure profile comprises two critical phases: injection pressure (during filling) and holding pressure (after filling). Both phases require precise adjustment to maximize filling speed.

Injection Pressure: Match to Speed, Avoid Over-Insufficiency

Injection pressure should increase with speed to maintain flowability, but must not exceed the mold’s rated pressure (typically 150–200 MPa for packaging molds). For high speeds (250 mm/s) and low-viscosity resins (PP MFR 35), set the pressure to 120–140 MPa. For lower speeds (150 mm/s) and higher-viscosity resins (PET MFR 15), use a pressure of 160–180 MPa.

Holding Pressure: Timing is Crucial

Holding pressure begins when the cavity fill rate reaches 95–98%, forcing the remaining plastic into the narrow gaps. A common mistake in most plants is applying holding pressure too early (impeding flow) or too late (causing plastic cooling). For high-speed packaging injection molding, the hold pressure is triggered when the screw position reaches 97% of the total stroke. Set the hold pressure to 60%–80% of the injection pressure (e.g., 80–100 MPa if the injection pressure is 140 MPa) and hold for 0.5–1 second.

Pressure Decay: Preventing Cavity Collapse

After applying hold pressure, the pressure profile should gradually decay (over 0.2–0.3 seconds), rather than dropping abruptly. This prevents the plastic from shrinking and detaching from the cavity walls during cooling. Topstar injection molding machines automate this process, ensuring consistent filling even during high-cycle operation.

Injection Molding Machines with Optimized Profiles

Adjusting speed and pressure profiles no longer requires guesswork. Topstar injection molding machines have built-in tools that simplify the optimization process even for operators without advanced engineering training. These tools reduce profile adjustments from 4 hours to 30 minutes.

Topstar injection molding machines all feature a touchscreen human-machine interface (HMI) that displays speed and pressure curves in easy-to-read graphical form. You can drag curve points up and down to adjust the speed/pressure at specific screw positions and then save preset values for different parts. Additionally, the HMI displays real-time cavity filling data: which cavities are underfilled, at which stage of injection the problem occurs, and the current speed/pressure changes. For new parts or new resins, use Topstar’s auto-tuning function. Enter the part’s weight, wall thickness, resin type, and mold details; the machine will run 5-10 test cycles, analyze the filling data, and generate an optimized curve.

Injection Molding Machines with Optimized Profiles

Optimize Curves to Improve Cavity Fill Rate

Low cavity fill rate in high-speed injection molding for high-speed packaging is not a “machine problem,” but a “curve problem.” High-speed packaging injection molding is a precision-critical process. Every 0.1 mm/s change in speed, every 5 MPa adjustment in pressure, and every 0.1-second change in holding time will affect the fill rate, and consequently, your output. When your fill rate reaches 98% or higher, you can not only produce more parts, but also earn more profits.

Prev: Injection Mold Machine Screw Selection Guide: Single Screw vs. Twin Screw Selection

Next: The Importance of Intelligent Digitalization in the Future Development of Injection Moulding Machines

Solving the problem of low cavity filling rate in high-speed packaging injection molding

The Importance of Cavity Fill Rate in High-Speed Packaging Injection Molding