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Prevent flow stagnation and diffusion of low viscosity resins in plastic molding machine

2025/09/24 By le zhan

plastic molding machine 7-5

Low-viscosity resins enable thinner walls, faster filling rates, and finer details in the injection molding process. However, these rheological advantages also present persistent challenges, including flow stagnation, uncontrolled spreading, weld lines, and unpredictable cooling patterns. For plastic product manufacturers, these defects directly lead to scrap, rework, production losses, and customer returns. Therefore, preventing flow stagnation and spreading in low-viscosity resins during plastic molding machine operations is crucial.

Resin Rheology, Temperature Sensitivity, and Causes of Stagnation

Stagnation and uncontrolled spreading are rooted in fluid dynamics. Low-viscosity resins behave like low-resistance fluids, flowing readily under pressure but also responding rapidly to temperature gradients, shear history, and cooling of the mold surface. Stagnation also stems from viscosity and shear thinning. Low-viscosity materials still exhibit shear thinning. In practice, this means that high shear rates at gates and runners thin the material and accelerate flow. However, when flow slows in dead zones or near core pins, viscosity rebounds, leading to localized stagnation.

Furthermore, many low-viscosity resins have a narrow processing window due to temperature sensitivity and temperature gradients. Slight cooling at the gate or along thin ribs can increase local viscosity and promote premature solidification, leading to downstream material stagnation and weld line formation. Conversely, excessively high temperatures can reduce viscosity and increase diffusion and flash. For injection molding, it is essential to obtain rheological data for the resin from the supplier. For each candidate polymer, conduct short-term trials using conservative gate dimensions and moderate injection speeds, monitoring the pressure and screw recovery curves for early signs of stagnation.

Plastic Molding Machine and Screw Selection

The injection unit is your first line of defense. A plastic molding machine designed for high-flow, low-viscosity materials must provide controlled shear, fast recovery, and microsecond pressure control. A suitable screw and drive system can shape the melt state for predictable and stagnation-free filling. For low-viscosity resins, a short aspect ratio, moderate compression ratio, and barrier or distributed mixing elements are recommended to ensure uniform melt temperature and avoid localized overheating. A barrier screw improves melt efficiency while still maintaining a controlled metering zone to prevent excessive thinning.

Screw Selection

Additionally, it’s essential to select a plastic molding machine equipped with high-precision servo drive controls to precisely control injection speed and pressure. Topstar’s all electric injection molding machines offer a faster closed-loop response and tighter pressure control, reducing overshoot that can cause flash and the potential for late freeze-up, which can lead to stagnation. It’s also essential to use a thermally balanced nozzle with a small inner diameter that matches the shot size. This reduces dead volume and avoids cold spots that can cause droop. Hot tips or hot runner systems designed for low viscosities must maintain temperature control within a tolerance of ±1°C.

All electric injection molding machine

Avoid dead zones and adjust the gate geometry of the plastic molding machine.

Mold design determines whether the fluid will stagnate within the cavity. For low-viscosity resins, precise gating, runner balancing, and venting are crucial. The goal is to maintain velocity and eliminate areas where the fluid could unexpectedly decelerate and cool. Therefore, it’s essential to use edge gates or tunnel gates and carefully calculate the gate length to prevent premature freeze-up. At the same time, a wedge-shaped gate with a gradually decreasing cross-sectional area helps maintain gate flow. For thin-walled or long runners, a larger gate cross-section is preferred in individual cavities; however, this can be compensated for by slightly reducing the injection speed to avoid flash. For microfeatures or areas where aesthetics are crucial, the shortest path gate remains essential; use automatic shear control to minimize surface defects.

Additionally, runners should minimize their dead legs. For multi-cavity molds, runner designs should be as short and symmetrical as possible. Flow balancing should avoid excessively long runners or complex, long-arm hot runner manifolds unless the manifold features active temperature control and is optimized for resin viscosity. For hot runners, ensure the manifold channel diameter is appropriately sized to maintain low residence time and stable flow.

Avoid dead zones and adjust the gate geometry of the plastic molding machine

Adopt advanced strategies for process control and stable flow.

Even with well-designed molds and machines, process control can hinder stable production and increase yield fluctuations. Low-viscosity resins require both preventative and reactive control: predicting behavior and reacting within milliseconds to changing conditions. This can be achieved by installing piezoelectric or strain gauge cavity pressure sensors at critical locations. Use pressure-based control for speed-pressure switching instead of fixed-position control. This ensures the plastic molding machine adapts to actual cavity conditions, preventing switching too early, which can cause stalling, or too late, which can cause flashing.

Also, replace blunt step acceleration profiles with smooth S-shaped acceleration profiles to reduce inertial transients and minimize local shear peaks that can alter viscosity. Furthermore, real-time melt temperature monitoring enables the detection of screw and nozzle melt temperature variations, allowing for smooth filling or preventing front-end freeze. Use online melt temperature sensors and integrate them into the mold run sequence. Dynamically adjust barrel zone setpoints based on melt temperature feedback, and configure dwell and recovery times to avoid hot spots. Furthermore, monitor injection volume and screw cushion changes during each cycle of the plastic molding machine. Increased injection volume or cushion deviations may indicate drooling, degraded check valve performance, or inconsistent melt—all precursors to stalling and flashing.

Maintain optimal performance through regular maintenance and the use of auxiliary systems

Continuous prevention of stalling depends on operational discipline and the auxiliary systems running alongside the plastic molding machine. Minor oversights, such as worn check valves, dirty screens, or clogged vents, can cause a rapid reaction to low-viscosity resins. In injection molding, keep raw materials dry and clean. Install a high-efficiency hopper dryer, tightly control the dew point, and use an inline melt filter or screen appropriate for the resin and flow rate. Contaminants or moisture can significantly alter viscosity, causing blockages and diffusion abnormalities.

Also, regularly inspect nozzles for fouling and hot runner manifolds for deposits on runners. Low-viscosity resins can leave behind thin films that polymerize when exposed to heat. Check valves should be replaced regularly. Valve leakage and backflow can cause fluctuations in shot size and potentially create cold lumps, leading to stagnation. Ensure valve bodies have low dead volume geometry and are compatible with the resin’s thermal properties.

Eliminating Stagnation and Controlling Diffusion

Preventing stagnant flow and uncontrolled diffusion of low-viscosity resins in plastic molding machines is a complex issue. It requires selecting the appropriate screw geometry and a responsive injection unit for the plastic molding machine, as well as designing a mold with short, balanced runners, effective gates, and vents. Furthermore, real-time sensors and adaptive controls are utilized to manage the injection profile, ensuring performance is maintained through rigorous maintenance, effective material handling, and strict operator discipline.

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