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. 2024 Aug 16;16(16):2318.
doi: 10.3390/polym16162318.

Study of Injection Molding Process to Improve Geometrical Quality of Thick-Walled Polycarbonate Optical Lenses by Reducing Sink Marks

Jiri Vanek  1 , Martin Ovsik  1 , Michal Stanek  1 , Jan Hanzlik  1 , Vladimir Pata  1
Affiliations

Study of Injection Molding Process to Improve Geometrical Quality of Thick-Walled Polycarbonate Optical Lenses by Reducing Sink Marks

Jiri Vanek et al. Polymers (Basel). .

Abstract

This study investigates the challenges and potential of conventional injection molding for producing thick-walled optical components. The research primarily focuses on optimizing process parameters and mold design to enhance product quality. The methods include software simulations and experimental validation using polycarbonate test samples (optical lenses). Significant parameters such as melt temperature, mold temperature, injection pressure, and packing pressure were varied to assess their impact on geometric accuracy and visual properties. The results show that lower melt temperatures and higher mold temperatures significantly reduce the occurrence of dimensional defects. Additionally, the design of the gate system was found to be crucial in minimizing defects and ensuring uniform material flow. Effective packing pressure was essential in reducing volumetric shrinkage and sink marks. Furthermore, we monitored the deviation between the predicted and actual defects relative to the thickness of the sample wall. After optimization, the occurrence of obvious defects was eliminated across all sample thicknesses (lenses), and the impact of the critical defect, the sink mark on the planar side of the lens, was minimized. These findings demonstrate the substantial potential of conventional injection molding to produce high-quality thick-walled parts when these parameters are precisely controlled. This study provides valuable insights for the efficient design and manufacturing of optical components, addressing the growing demand for high-performance thick-walled plastic products.

Keywords: injection molding; optical lenses; polycarbonate; process evaluation; quality improvement; thick-walled parts.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
Schematic plan of the experiment.
Figure 2
Figure 2
Visualization and dimensions of designed test sample variants: (A) 12.5 mm; (B) 16.5 mm; (C) 20.5 mm.
Figure 3
Figure 3
Experimental mold with a detailed view of the cavity layout relative to the sprue retainer.
Figure 4
Figure 4
Volumetric mesh definition for imported 3D model.
Figure 5
Figure 5
Meshed cooling system and injection mold block.
Figure 6
Figure 6
Procedure for scanning the surface of the test samples and evaluating sink mark depth.
Figure 7
Figure 7
Designed modifications of gate geometries: (A): point gate; (B) sub-gate; (C) three-point gate; (D) fan gate; (E) film gate type 1; (F) film gate type 2.
Figure 8
Figure 8
Thickness of the frozen layer as a fraction of the part thickness, captured 6 s after injection (red = solid; blue = melt): (A): point gate; (B) sub-gate; (C) three-point gate; (D) fan gate; (E) film gate type 1; (F) film gate type 2.
Figure 9
Figure 9
Designed gate system relative to the injection-molded test sample.
Figure 10
Figure 10
Visualization of mold cavity filling.
Figure 11
Figure 11
Simulation results indicating sink mark depth.
Figure 12
Figure 12
The depth of sink marks predicted by Cadmould calculated for all three lens thicknesses, two gate thicknesses, and multiple melt and mold temperature settings.
Figure 13
Figure 13
Evaluated maximal deformations ((A): X direction, (B): Y direction, (C): Z direction).
Figure 14
Figure 14
Deflection calculated for all three lens thicknesses, two gate thicknesses, and multiple melt and mold temperature settings.
Figure 15
Figure 15
The time required to reach ejection temperature.
Figure 16
Figure 16
Time to reach ejection temperature calculated for all three lens thicknesses, two gate thicknesses, and multiple melt and mold temperature settings.
Figure 17
Figure 17
Images of test samples from the initial test series: (a) severe material yellowing; (b) noticeable flow marks on the surface.
Figure 18
Figure 18
Images of test samples after partial correction of process parameters: (a) persistent flow lines; (b) air bubbles.
Figure 19
Figure 19
Manufactured test samples (lenses) with thicknesses of 12.5 mm, 16.5 mm, and 20.5 mm without visible defects.
Figure 20
Figure 20
Evaluation of sink mark depth (SMD) in relation to lens thickness (LT).
Figure 21
Figure 21
The effects of all factors and their combinations including Pareto charts: (a) effect of factors for 12.5 mm lens; (b) Pareto chart for 12.5 mm lens; (c) effect of factors for 16.5 mm lens; (d) Pareto chart for 16.5 mm lens; (e) effect of factors for 20.5 mm lens; (f) Pareto chart for 20.5 mm lens.
Figure 21
Figure 21
The effects of all factors and their combinations including Pareto charts: (a) effect of factors for 12.5 mm lens; (b) Pareto chart for 12.5 mm lens; (c) effect of factors for 16.5 mm lens; (d) Pareto chart for 16.5 mm lens; (e) effect of factors for 20.5 mm lens; (f) Pareto chart for 20.5 mm lens.
Figure 22
Figure 22
The effects after reducing insignificant factors and their combinations, including Pareto charts: (a) effect of factors for 12.5 mm lens; (b) Pareto chart for 12.5 mm lens; (c) effect of factors for 16.5 mm lens; (d) Pareto chart for 16.5 mm lens; (e) effect of factors for 20.5 mm lens; (f) Pareto chart for 20.5 mm lens.
Figure 22
Figure 22
The effects after reducing insignificant factors and their combinations, including Pareto charts: (a) effect of factors for 12.5 mm lens; (b) Pareto chart for 12.5 mm lens; (c) effect of factors for 16.5 mm lens; (d) Pareto chart for 16.5 mm lens; (e) effect of factors for 20.5 mm lens; (f) Pareto chart for 20.5 mm lens.
Figure 23
Figure 23
Response optimizer results for all test sample thicknesses.
Figure 24
Figure 24
Measured data summary for 12.5 mm thick lens.
Figure 25
Figure 25
Evaluation of measured and simulated data for 12.5 mm thick lens: (a) measured values versus simulated results; (b) analysis of average measured value and simulation outcome.
Figure 26
Figure 26
Measured data summary for 16.5 mm thick lens.
Figure 27
Figure 27
Evaluation of measured and simulated data for 16.5 mm thick lens: (a) measured values versus simulated results; (b) analysis of average measured value and simulation outcome.
Figure 28
Figure 28
Measured data summary for 20.5 mm thick lens.
Figure 29
Figure 29
Evaluation of measured and simulated data for 20.5 mm thick lens: (a) measured values versus simulated results; (b) analysis of average measured value and simulation outcome.
Figure 30
Figure 30
Comparison of measured data with ideal linear trend and simulation data.
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