Extreme Challenge: Solving Center Voids in a 36mm Thick-Wall PMMA Lens ( 5:1 Wall Ratio)

In injection molding industry, producing thick-wall optical components has always been a significant challenge.

Recently, our team tackled a project: Ø100mm Biconvex PMMA Lens.

●Technical Specifications:

●Material: PMMA (Acrylic)

●Geometry: Biconvex Lens

●Dimensions: Ø100mm

●The Challenge: Edge thickness: 7.4mm vs. Center thickness: 36mm.

●Wall Ratio: Nearly 5:1!

Fig. 1: 3D CAD Mode

In practical molding operations, a wall ratio >2:1 is risky; >3:1 is critical.

 At 5:1, the initial trials were disastrous. Severe central voids and sink marks rendered the lens optically non-functional.

Fig. 2: The Reality Check(Defective Lens)

Next, let’s deep-dive into how Haoge Optics solved this "impossible" void issue.

Diagnosis: Air Trap or Vacuum Void?

It’s easy to mistake this for Air Trap. So, how did we determine it was a vacuum void?

Differentiation Analysis:

Air Trap: Occurs at flow ends; irregular shape; often shows burn marks.

Vacuum Void: Centered in the thickest section ; perfectly round; smooth boundaries.

Root Cause Analysis:

Attributed to PMMA's low thermal conductivity, a differential cooling rate occurred: the 7.4mm outer shell solidified while the 36mm core remained molten. As the core cooled and underwent volumetric shrinkage, internal negative pressure caused the material to form a void.

Our 3-Step Solution (The "Haoge Approach")

1. Process Mastery: The "Time War" of Multi-Stage Packing

Standard packing wasn't enough. We implemented a 4-Stage Packing Profile:

Stages 1 & 2: High pressure to prevent gate freeze.

Stages 3 & 4: using low pressure with extended duration. We extended packing time to 35 seconds, ensuring pressure penetrated the core until full solidification.

Validation: We used the "Weight Method" to lock parameters only when part weight stabilized.

2. Mold Optimization: Cold Slugs & Precision Venting

Added deep cold slug wells to trap the coolest "first shot" material, preventing haze or flow marks.

Enhanced venting channels to 0.02mm depth at parting lines and inserts to eliminate compressed air burns.

3. The Game Changer: Stress-Relief Annealing

Thick PMMA isn't just about bubbles; internal stress is a ticking time bomb (cracking upon alcohol exposure or over time).

Protocol: Immediate post-mold annealing at 75°C for 4-6 hours.

Result: Completely eliminated internal stress, ensuring long-term optical stability and durability.

Fig. 3: Final Product

A Note on DFM (Design for Manufacturing)

While we conquered this challenge, from a DFM perspective, we always advise clients to avoid extreme wall ratios if possible.

Alternative 1: Optimize aspheric curvature to reduce center thickness while maintaining focal power.

Alternative 2: Consider Fresnel structures for lighting applications to drastically reduce volume and cost.

However, at Haoge Optics, our value isn't just saying "the design is bad." It’s about making the "impossible" possible.

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