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Home / How Can Ultra-Low-Speed Injection Reduce Flow Marks in Automotive Headlamp Lenses?

How Can Ultra-Low-Speed Injection Reduce Flow Marks in Automotive Headlamp Lenses?

2026/10/09 By le zhan

How Can Ultra-Low-Speed Injection Reduce Flow Marks in Automotive Headlamp Lenses

Ultra-low-speed injection molding technology can reduce flow marks on automotive headlight lenses. It works by ensuring the melt front flows smoothly through optically sensitive areas, preventing visible marks caused by sudden changes in flow velocity, shear force, and pressure. TopStar’s electric injection molding machines utilize servo direct-drive control technology combined with a high-pressure, low-speed coordinated injection process, ensuring stable melt delivery even at injection speeds as low as 0.1 mm/s. For transparent PC or PMMA optical parts, this precise control reduces flow instability while maintaining sufficient cavity pressure, improving surface-quality consistency and production yield.

Why Are Flow Marks Common in Automotive Headlight Lenses?

Automotive headlight lenses place extremely high demands on injection-molding surface quality. Materials such as PC and PMMA offer high transparency, precise optical surfaces, and excellent mold surface replication capabilities.

These optical properties make even minute molding defects highly visible. Subtle changes in melt front behavior may manifest after molding as differences in gloss, streaks, ripples, or flow marks. On polished headlight lenses, these defects not only affect appearance but may also compromise optical performance. Part geometry further complicates molding. Automotive lenses often feature complex designs, including long flow paths, wall thickness transitions, lettering, ribs, optical structural features, and curved surfaces. If engineers do not properly control the mold design and processing conditions, the melt may leave flow marks as it passes over lettering or other localized mold features.

Therefore, automotive headlight lenses tend to develop flow marks because the melt front moves unstably.

Why Are Flow Marks Common in Automotive Headlight Lenses

How Does Ultra-Low-Speed Injection Improve the Surface Quality of Automotive Lenses?

In molding optical lenses, such as automotive lenses, ultra-low-speed injection is critical when the melt must flow through sensitive areas with minimal, repeatable speed variation.

Electric injection molding machines commonly use ultra-low-speed injection because their electric servo drives can control screw movement directly, without relying on hydraulic valves or hydraulic response times. Topstar’s TEII series of electric injection molding machines utilize servo direct-drive motors that maintain smooth operation even at extremely low speeds. Their low-damping injection unit is designed for precise metering and stable melt delivery.

This matters because sudden acceleration or deceleration can change flow-front velocity, local shear rates, and pressure distribution.

The same engineering principles apply to automotive lens production:

Injection molding machines should control melt flow characteristics based on cavity geometry rather than using a single, constant speed.

Topstar’s electric injection molding platform offers ultra-low-speed precision control down to 0.1 mm/s. However, this does not mean that the entire filling process for PC or PMMA headlight lenses must be conducted at 0.1 mm/s. The 0.1 mm/s figure demonstrates the machine’s precise low-speed control capability, enabling engineers to set controlled multi-stage speed profiles that allow for slower, smoother screw movement in sensitive areas.

Ultimately, this enables:

Control of melt-front velocity as it flows through critical optical areas, preventing sudden shear or pressure changes that could cause visible defects on the finished lens surface.

How Does Ultra-Low-Speed Injection Improve the Surface Quality of Automotive Lenses

How Does High Pressure Compensate for Slow Filling Speeds?

Ultra-low-speed injection molding presents a critical issue:
Because the melt flows slowly, it takes longer for heat to transfer to the mold walls. This can increase viscosity and flow resistance; if pressure is insufficient, it may also cause flow stagnation or incomplete surface replication.

Therefore, ultra-low-speed injection molding must be paired with pressure control.

A study conducted by NLM on PC optical lenses clearly demonstrates this relationship. “In this specific lens and mold configuration, injection pressures below 60 MPa result in noticeable flow marks; conversely, excessively high injection and holding pressures intensify shear forces and lead to material degradation.” The results indicate that lens molding requires a controlled pressure range and cannot rely solely on reducing the injection speed. Reference: https://pmc.ncbi.nlm.nih.gov/articles/PMC11360770/?utm_source=chatgpt.com

Topstar’s electric injection molding machines meet this requirement by coordinating high pressure with low speed. Its servo system combines position, speed, and pressure control, allowing the screw to move slowly while generating enough pressure to overcome cavity resistance and ensure continuous advancement of the melt front.

The entire process can be understood as follows:
Reduced controlled speed → Reduced sudden flow disturbances → Sufficient pressure to maintain melt flow → Stable cavity filling.

Therefore, ultra-low-speed injection molding machines require not only motors capable of low-speed movement but also precise, coordinated control of pressure and position to maintain smooth motion.

How Does High Pressure Compensate for Slow Filling Speeds

Reducing Flow Marks on Automotive Headlight Lenses Using Ultra-Low-Speed Injection Molding

Ultra-low-speed injection molding reduces flow marks on automotive headlight lenses by ensuring that the melt front flows smoothly and predictably within critical optical areas. This process uses precise, low-speed melt delivery to stabilize cavity filling, preventing visible streaks and gloss variations caused by sudden changes in speed, shear force, or pressure.

For PC and PMMA lenses, the key isn’t merely “reducing injection speed.” Instead, the injection molding machine must combine extremely low screw speeds with sufficient pressure to keep the melt advancing continuously and prevent stagnation or premature cooling. Topstar’s electric injection molding machines meet these process requirements through servo direct-drive technology, precise low-speed control down to 0.1 mm/s, and coordinated high-pressure and low-speed control functions.

The underlying mechanism can be summarized as follows:

Ultra-low-speed control → Smoother flow at the melt front → Reduced shear forces and pressure spikes → More stable cavity filling → Fewer visible flow marks.

For automotive headlight lenses, this is the core value of ultra-low-speed injection molding: it improves surface consistency by controlling melt flow within the cavity, not just by adjusting injection speed.

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