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Special solution for automotive parts injection molding of PMMA thick-walled headlights

2025/09/01 By le zhan

Thick-walled PMMA car lights(1)

For the automotive parts injection molding of thick-walled PMMA headlights in automotive components, Topstar utilizes a direct injection molding machine equipped with a patented center-clamping mechanism. Specifically, this method enables two-shot moulding of 35 mm thick headlight lenses, significantly improving product quality and optical performance while reducing cycle time. The direct injection molding machine systematically addresses these issues by ensuring uniform clamping force distribution at the center of the mold plate, achieving near-perfect mold parallelism and fit, and providing a controlled injection profile. These features work together to effectively eliminate air entrapment, reduce undershot, and produce a dense, void-free polymer structure within the light-guiding geometry.

Material Properties of Thick-Wall PMMA for Automotive Parts Injection Molding

PMMA offers excellent optical clarity, high light transmittance, and good weather resistance—properties highly valued in headlight optics and decorative lenses. However, PMMA has low thermal conductivity and is sensitive to moisture and shear forces, requiring careful upstream control. Thick-walled PMMA parts, such as 35mm lenses, exacerbate several phenomena. Long filling times lead to air entrapment, temperature gradients that create residual stresses, and slow cooling that can lead to differential shrinkage and sink marks.

Key material considerations include the drying strategy, melt temperature window, and shear history. PMMA is somewhat hygroscopic; even small amounts of moisture can cause microvoids when injected into thick sections. Therefore, the desiccant must be continuously dried to the manufacturer’s specified dew point. Melt temperature must be tightly controlled. Similarly, the screw design must strike a balance between sufficient plasticization and low shear to avoid localized degradation and dark spots caused by burnt polymer. Furthermore, mold temperature control is essential. Uneven cooling can lead to localized shrinkage and internal stresses, which scatter light and reduce optical uniformity.

Automotive parts injection molding 2

Core Injection Molding Machines in Automotive parts injection molding

The architecture of the injection molding machine determines the achievable process window for demanding automotive parts injection molding applications. Direct injection moulding machines with patented centre clamping structures offer the mechanical stability and process repeatability necessary for producing thick-walled PMMA headlamps.

The center clamping of a direct injection molding machine evenly distributes clamping force across the entire platen. In large headlight moulds, off-centre clamping can lead to slight tilting, misalignment, and fluctuations in cavity pressure. These can lead to flow line variations, insufficient packing, and air entrapment. Topstar’s patented center clamping technology concentrates and balances force in the center of the mold while supporting the outer areas with a system of guide rods and platens. This ultimately eliminates micro-gaps that can trap air. Another unique advantage of direct injection molding machines is their robust, low-flow hot runner manifold and rigid injection platform, which minimize the melt’s travel distance from the screw to the mold cavity. This reduces pressure loss, maintains injection inertia, and enables more precise speed-pressure conversion.

Large injection molding machine 1-8

Targeted Process Control Strategies

Injection molding process control is crucial for translating machine performance into product performance. For automotive parts injection molding, especially thick-walled PMMA parts, the optimal filling and packing strategies differ significantly from those for thin-walled thermoplastic parts. The goal is to fill the cavity while minimizing shear and air entrapment, then apply sufficient holding/compression pressure to eliminate shrinkage and voids during part solidification. High injection pressure, combined with a low, constant injection speed, produces a dense, bubble-free melt front. High pressure ensures that the molten polymer penetrates all thin-walled sections and undercuts, helping to squeeze out trapped air through vents or vacuum ports. Consequently, controlled and constant filling maintains laminar flow and a consistent pressure distribution.

Topstar’s direct injection molding machines, equipped with a forward-travel platform and rigid control system, enable close coordination of the injection, compression, and clamping phases. Furthermore, a tapered, multi-step holding pressure profile reduces residual stresses, followed by a gradual reduction in pressure to prevent overstressing thick-walled sections. An integrated cavity pressure sensor empirically determines the V/P switching point. This sensor feedback enables closed-loop control, ensuring optimal switching even during long-term production runs despite changes in mold or resin parameters.

Targeted Process Control Strategies 1

Precise Pick and Place with backend Automation

In the backend of automotive parts injection molding production, Topstar integrates a six-axis robot into its automated production cell, providing deterministic handling during the injection molding process, which is critical for thick-walled PMMA headlights.

Six-axis robotic handling improves repeatability. After the first shot, semi-finished lenses are very fragile and often thermally unstable. The six-axis robot securely grasps the part, performs a drying or temperature stabilization pause, and places the part into the mold for the second shot with micron-level positioning accuracy. Furthermore, the robot’s motion trajectory can be adjusted to avoid dynamic stress or markings on the part surface. Automation goes beyond pick and place; cameras and machine vision systems can inspect the semi-finished product surface for bubbles, flow marks, or contamination before the second shot. Defective parts can be automatically classified as rework or scrap. In addition, synchronizing robots and presses via deterministic I/O or fieldbus reduces cycle times and ensures accurate handovers at optimal temperature and dimensional conditions.

Injection Molding Solutions for High-Quality Thick-Wall PMMA Headlights

Producing high-quality, thick-wall PMMA headlights for automotive applications requires systematic production planning, including material specifications, dedicated molds, direct injection molding machines, precise process control, optical-grade injection platforms, and automated robotic handling. Topstar’s approach seamlessly integrates these elements. A patented centre clamping mechanism ensures mould parallelism and uniform force distribution. A forward-moving injection platform and dual-exit hydraulic cylinders guarantee shot integrity. A six-axis robot enables precise two-shot workflows with minimal manual intervention. These technologies, combined with stringent drying and melt control, as well as cavity pressure-based closed-loop control, address the most challenging defects in automotive parts injection molding, while reducing cycle times and increasing production output.

Common Problems in Injection Molding Thick-Wall PMMA Headlights

Q1. Why is center clamping crucial for thick-wall PMMA headlights?
A1: Center clamping balances clamping force across the mold, maintains platen parallelism, and minimizes micro-gaps. This uniform contact prevents air entrapment and ensures consistent cavity pressure.

Q2. How does the injection-compression process help reduce optical defects in thick parts?
A2: The injection-compression process fills the cavity in a controlled manner with the mold partially open, then compresses to the final thickness. This reduces shear forces and flow marks, improves surface replication, and enables higher final packing pressures, thereby eliminating voids and sink marks.

Q3. What role do six-axis robots play in the automotive parts injection molding?
A3: Six-axis robots enable precise and repeatable handling of semi-finished parts between injections, ensuring accurate positioning of the second shot.

Q4. What process parameters can prevent yellowing and black specks in PMMA?
A4: Key parameters include melt temperature control, strict resin drying, low shear during plasticization, appropriate screw design, and maintaining a stable injection profile.

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