Pressure Loss Range in Hot Runner Injection Molding

Pressure loss in hot runner injection molding refers to the pressure decay that occurs as molten plastic flows through the hot runner system—including the sprue bushing, manifold, nozzles, and gates—due to friction with the channel walls, structural resistance at flow restrictions, and internal shear within the melt. This parameter directly impacts the ability to fill the mold cavity completely and maintain consistent part quality. In practical production, this loss is not a fixed value but varies based on system design, material properties, and processing parameters. The industry-recognized typical range for this pressure loss is 5–35 MPa, with distinct distributions depending on the specific application scenario. Effectively managing this range is crucial for ensuring stable and efficient mass production.

I. Loss Ranges and Application Scenarios

1. Low Loss Range (5–15 MPa)

This range primarily applies to single-point hot runner systems, commonly found in single-cavity molds or small-to-medium volume multi-cavity molds (≤4 cavities). These systems feature a simple structure with short flow paths (typically ≤300mm) and larger diameters (8–12mm), minimizing resistance. For low-viscosity materials like PE and PP, pressure loss typically stabilizes between 5–10 MPa. For medium-viscosity materials such as ABS and PS, the loss increases slightly but usually remains below 15 MPa, which is manageable for most standard injection molding machines.

2. Medium to High Loss Range (15–35 MPa)

This range is characteristic of complex multi-cavity systems, especially those with ≥8 cavities designed for high-volume production. These systems require intricate manifold designs to distribute melt to multiple points, resulting in long flow paths (500–1000mm) and smaller channel diameters (4–6mm) in the branch lines. High-viscosity engineering plastics like PC, PMMA, and PPS experience significant resistance, leading to pressure losses of 25–35 MPa. Even low-viscosity materials in such systems will see losses of 15–20 MPa. Additionally, valve-gate systems introduce extra resistance compared to open-gate systems, typically adding an additional 3–5 MPa due to the flow restriction created by the valve pin.

II. Core Factors Influencing Pressure Loss

1. Runner System Design

The physical geometry of the runner is the foundational factor. Industry data confirms that runner diameter and length have the most significant impact. Pressure loss is inversely proportional to diameter; reducing the diameter by 1mm can increase loss by 5–8 MPa. Conversely, loss increases with length, rising by 4–6 MPa for every 200mm increase. Furthermore, the number and angle of turns affect resistance; sharp right-angle bends create 20% more local resistance than smooth radii, making rounded transitions essential in complex layouts.

2. Melt Properties

Melt viscosity is the key material factor. Low-viscosity materials flow more easily, resulting in losses that are 10–15 MPa lower than those of high-viscosity materials. Temperature plays a critical role in regulating viscosity; within the safe processing range, increasing the melt temperature by 20°C can reduce pressure loss by 3–5 MPa. The material's shear sensitivity also matters; shear-thinning materials experience a more significant drop in viscosity at higher injection speeds, which can help mitigate pressure loss increases during fast fills.

3. Processing Parameters

Injection speed has a direct correlation with pressure loss. Higher speeds increase shear rates and friction, elevating pressure loss. While slower speeds reduce loss, they risk premature solidification, leading to short shots or sink marks. The holding pressure phase also influences efficiency; typically set at 60%–80% of the filling pressure, it must be balanced to avoid exacerbating resistance in the runners.

III. Control, Optimization, and Technical Trends

1. Runner Structure Optimization

Designers should prioritize minimizing flow path length. For multi-cavity molds, a balanced, symmetrical layout ensures even distribution, keeping pressure differences between branches within 5 MPa. Selecting appropriate diameters (6–12mm range) based on part size and material, and utilizing generous radii (R≥3mm) for all bends, are fundamental strategies to reduce localized resistance.

2. Processing Parameter Matching

Setting the optimal melt temperature—usually 15–30°C above the material's melting point—balances flowability with thermal stability. Implementing a multi-stage injection profile is also effective: a slow initial speed to avoid shock, a medium speed for bulk filling, and a slow final speed to minimize shear heating and pressure spikes at the gate.

3. System Selection and Technological Advancements

Matching the nozzle type to the material is key; open nozzles suit low-viscosity materials, while precision valve gates are better for high-viscosity resins or cosmetic parts. Current industry trends leverage CAE simulation software during the design phase to predict pressure distribution accurately, significantly improving first-shot success rates. Intelligent PID temperature control systems maintain stability within ±0.3°C, reducing viscosity fluctuations. Additionally, advanced surface treatments like DLC (Diamond-Like Carbon) coatings on runner walls can lower shear stress by up to 18%, further optimizing pressure transmission efficiency.

In summary, the core pressure loss range in hot runner systems is 5–35 MPa. By combining scientific structural design, precise process tuning, and advanced technologies, manufacturers can control this loss within a reasonable window. This ensures complete cavity filling while preventing flash and mold damage caused by excessive pressure, thereby supporting efficient and stable mass production.