Optical lenses, light guides, beam splitters, aspherical lenses, and other plastic optical components are core functional parts in optoelectronic, automotive, security, and medical imaging equipment. Unlike ordinary plastic parts, optical components have strict requirements regarding birefringence, light spot distortion, light transmission uniformity, image clarity, and these indicators are largely determined by residual stress during molding.

 

Many factories encounter very typical pain points: Immediately after demolding, dimensions are acceptable and appearance is scratch-free, but after 24-48 hours, warping, dents, and localized deformation appear; or light transmission inspection reveals bright and dark stripes, light spot shifts, and stress lines; after assembly, slight deformation due to heat causes focal length and optical path shifts, resulting in large-scale rework and scrap. More challenging is that often root cause of problem lies not in injection molding machine parameters, but in inherent flaws in mold structure design, cooling, injection, venting, and ejection processes. Simply adjusting pressure, speed, holding pressure only provides temporary relief and cannot eradicate stress defects, resulting in persistently high rework costs. In a certain automotive optical lens project, before mold optimization, rework and scrap due to residual stress accounted for 61% of overall defects; after systematic optimization of mold, residual stress-related defects were significantly reduced, and rework costs decreased by 53%.
This article starts with formation mechanism of residual stress and explains how precision optical injection molds can control and reduce residual stress from source, thereby stabilizing optical quality, across the entire chain from mold design, processing, temperature control, injection, demolding, and trial molding verification.

Residual stress in injection molding is mainly divided into two categories: flow residual stress and thermal residual stress. Optical materials such as PC, PMMA, COC, COP are inherently rigid and experience uneven cooling and shrinkage, making stress more likely to be frozen inside part.
Residual Stress During High-Speed Melt Filling: Shear forces forcibly stretch and orient polymer chains during high-speed melt filling. If cooling rate is too rapid, molecular chains solidify before they can relax and spring back, creating internal stress. Shear stress is most concentrated near gate, corners, thickness transitions, and weld lines where melt converges, making these areas prone to stress marks. An excessively small gate, excessively fast injection speed, abrupt changes in runner cross-section, and forced sharp turns in flow path all amplify shear stress.
Residual Thermal Stress (Thermal Stress): Uneven cooling rates at different locations in plastic part, with surface layer cooling and solidifying first and inner layer shrinking later, create asynchronous internal stress due to compression and tension. Uneven localized mold temperatures, significant thickness differences, poorly designed water channels, and localized hot spots all contribute to thermal stress. In semi-crystalline and highly transparent optical materials, thermal stress easily transforms into visible stress spots and birefringence.
Many engineers habitually rely on injection molding processes to brute-force stress problems: reducing injection speed, decreasing holding pressure, increasing mold temperature. However, there are limits to process adjustments. Excessive speed reduction can easily lead to short shots and weld lines; excessively high mold temperatures will lengthen molding cycle, causing a sharp drop in production efficiency. To fundamentally improve this, solution must be addressed at mold end.
A common misconception in industry is that residual stress can be resolved simply by adjusting injection molding process. Most of stress in optical components is inherently caused by mold structure. Imbalanced injection, uneven cooling, abrupt changes in wall thickness, and unreasonable ejection forces can all result in high residual stress in parts, even with process optimized, making it difficult to maintain stable mass production yields.

1. Gating System Optimization: Reducing Localized High Shear. Gate is narrow channel through which melt finally passes, and it's also location with the highest shear stress. An unreasonable gate will almost certainly result in significant stress concentration around it.
Prioritize Large Cross-Section, Short-Flow Gating: Avoid extremely fine gates (less than 0.3mm) for optical components. If conditions permit, use fan-shaped gates or flat side gates to increase feed cross-sectional area, reduce shear rate of melt passing through gate, minimize forced stretching and orientation of polymer chains. For aspherical microlenses where only point gates are possible, gate diameter needs to be appropriately enlarged, and gate transition zone lengthened to avoid abrupt cross-sectional shrinkage.
Avoid optically effective area for gate placement. Area near gate is a stress-prone zone with strongest birefringence. Gates should not be placed on imaging surface or light-transmitting working surface; prioritize edge or non-optical areas for injection.
Achieve complete runner balance in multi-cavity optical molds. If a mold has multiple cavities, a naturally balanced runner system must be used to ensure that flow length, turns, and cross-section of each cavity are completely consistent. Asynchronous filling of cavities forces some cavities to undergo high-pressure feeding, adding additional residual stress.
All runner corners should have rounded transitions. 90° right-angle turns in runner system cause severe melt shearing and eddies. All corners should have rounded transitions to reduce flow disturbance.
2. Cooling System: Uniform Temperature Control to Eliminate Thermal Stress. Temperature difference is the biggest source of residual thermal stress. Optical mold cooling cannot simply aim to "cool down"; it must ensure a uniform temperature field throughout cavity.
Water channels should be as close to cavity as possible and arranged at equal intervals. Distance between water channels and cavity surface should be kept consistent, with a tolerance control of ±0.3-0.5mm, to avoid hot spots formed by one side cooling rapidly while the other side cools slowly. Prioritize water channel coverage for curved areas of lens and thick-walled sections.
Prioritize parallel water channels; avoid long series loops. A large temperature difference between inlet and outlet of a series water channel results in uneven cooling on one side of mold, leading to inconsistent shrinkage rates on both sides of plastic part, which easily causes warping and residual internal stress.
Targeted Enhanced Cooling for Thick Areas: For locally thickened areas of part, such as central boss of a lens, slow heat dissipation and a large internal-external cooling difference can easily lead to internal residual stress. Conformal water channels and beryllium copper inserts can be used to assist heat dissipation and balance cooling rate between thick and thin areas.
Independent temperature control for different mold zones: Separate temperature control for moving mold, fixed mold, and gate areas. Optical components typically require high and stable mold temperatures to reduce surface rapid cooling and stress buildup. Overall mold temperature fluctuation control within ±1℃.
Optimization goal: Maximum temperature difference on cavity surface during molding ≤2℃.
3. Optimization of product wall thickness and mold cavity structure for smooth transitions: Drastic changes in wall thickness are high-stress concentration points. Mold must be optimized for wall thickness in advance using DFM (Design for Manufacturing).
Aim for uniform wall thickness, using beveled radius (R) transitions between thick and thin sections, avoiding abrupt right-angle steps. Significant differences in melt cooling rates at locations of abrupt wall thickness changes result in delayed shrinkage in thicker areas, generating substantial internal stress, manifesting as distorted light spots and dark lines.
Add transition fillets to lens root and flange corners: Corner radius (R) should not be too small; enlarge fillets within structural limits to prevent melt from being forcibly compressed at sharp corners, creating shear stress.
If product drawings cannot be modified, mold cooling and injection schemes must be specifically compensated, with priority given to cooling thicker areas.

 

4. Venting Optimization: Avoiding Additional Stress Caused by High-Pressure Trapping
Many people are unaware that trapped air can also induce residual stress. When gas cannot escape from cavity, melt is forced to compress and circulate under high pressure, causing a sharp increase in local filling resistance and generating additional stress on molecular chains under high-pressure shear. Key points for venting in optical molds:
Uniform venting should be provided at melt end, parting surface, and lens edge flange location; Ventilation grooves should generally not be provided on effective optical surface to prevent vent marks; Ventilation groove depth should be strictly controlled, generally 0.015-0.025mm for PC/PMMA to prevent flash; Number, depth, and location of venting grooves in each cavity of a multi-cavity mold should be completely consistent.
Short-shot testing should be conducted to check whether material flow front is synchronized and whether there is localized air trapping. For areas with air trapping, priority should be given to improving venting, rather than simply increasing injection pressure.
5. Ejection and demolding structure to reduce mechanical stress during demolding. Parts are still in a high-temperature, relatively soft state. If ejection force is uneven or localized stress is too high, residual mechanical stress will be directly generated. This may not be visible to naked eye, but optical testing will show that birefringence index exceeds standard. Principles of optical mold demolding design:
Use multi-point balanced ejection to maximize the ejection contact area. Prioritize ejector pins + push plates and ring ejection, avoiding hard ejection by small-diameter single-point ejector pins to prevent localized compressive stress.
Avoid ejection positions on optical working surfaces. Ejector pin marks are not only cosmetic defects but also create compressive stress zones at corresponding locations, affecting optical performance.
Ensure sufficient draft angle. Draft angle for optical curved surfaces and flange surfaces should not be too small to prevent plastic part from being scratched and pulled by cavity sidewalls during mold opening, generating surface stress. Polish surface of high-gloss mirror cavities to reduce demolding friction resistance.
Uniform Cavity Mirror Surface Polishing Texture Direction. Irrational polishing texture direction increases demolding friction. Optical cavities must be polished circumferentially or along demolding direction, avoiding messy, intersecting polishing patterns.
6. Controlling Micro-Stress in Mold Steel, Mirror Surface Processing, and Insert Structure. Cavity surface quality and insert assembly pre-stress will also be transferred to plastic part.
High-purity mirror steel is preferred for optical molds. STAVAX, S136 ESR, and other electroslag remelted mirror steels have fewer impurities, higher polishing limits, and can achieve scratch-free mirror surfaces. Scratches and pits on cavity surface will cause micro-tensile stress on the surface of plastic part.
Insert Assembly Must Avoid Excessive Interference Stress. Lens cavities extensively use insert structures. If inserts are pressed in too tightly or have excessive pre-stress, slight deformation of cavity after thermal expansion of mold will generate uneven stress inside part. Strict control of insert fit clearance is essential, and steel with a matching coefficient of thermal expansion should be selected.
Reduce direct EDM machining of optical surfaces. EDM produces a surface alteration layer, requiring extensive polishing and easily leaving micro-stress. High-speed milling followed by mirror grinding is preferred.

After mold processing, it's not enough to just measure dimensions and appearance; a specific residual stress verification is essential.
Short-shot filling test: Observe flow front of material, noting any sharp turns, flow splitting, or lag, to determine if shearing is excessive.
Constant Temperature Aging Test: After placing molded sample at room temperature for 24h/48h, re-measure warpage and deformation. Immediate demolding compliance followed by deformation after a period indicates excessive residual stress.
Polarized Light Stress Detection (Birefringence): Using a polarizing light detector, internal stress distribution of part can be visually observed: whether obvious stress stripes appear around gate, corners, and thickness transition areas.
Process Window Preliminary Test: Slightly adjust mold temperature and injection speed, observing changes in optical quality. If even slight fluctuations in parameters immediately cause stress defects to exceed limits, it indicates inherent mold deficiencies, an excessively narrow forming window, and high risks in subsequent mass production.

After a period of mold use, many previously acceptable optical components begin to experience a resurgence of stress defects. Common causes include: Wear and roughening of cavity mirror surface, increasing demolding friction resistance; Clogged venting channels by decomposition and precipitates, leading to localized air trapping; Scale buildup in water system, reducing cooling efficiency and causing uneven mold temperature; Weared ejector pins, resulting in unbalanced ejection force.
Maintenance Measures: Regular mirror surface polishing to prevent minor scratches on cavity surface; Clean all venting channels after each mold run; Regularly descale water system to ensure unobstructed cooling flow; Inspect ejection mechanism to ensure balanced ejection force.

 

Q1: What is relationship between residual stress, birefringence, and light spot distortion?
Residual stress within plastic will cause uneven distribution of material's refractive index. When light passes through part, light path is deflected, directly manifesting as light spot distortion, bright and dark stripes, and blurred imaging—what optics industry commonly refers to as stress patterns and excessive birefringence.
Q2: Can residual stress be reduced by modifying an existing mold?
To address root cause of problem, optimization options include: enlarging gate, improving venting, adjusting cooling system, and adding rounded corners. If the overall cooling layout is severely unbalanced or wall thickness is a structural flaw, mold modification is limited, and redesigning mold is more cost-effective in long run.
Q3: Can increasing mold temperature completely eliminate residual stress?
Increasing mold temperature promotes molecular chain relaxation, which can alleviate stress, but it lengthens molding cycle and reduces efficiency. Simply relying on high temperatures cannot compensate for stress caused by uneven mold cooling or excessive gate shear. Mold structure optimization is fundamental solution.
Q4: What are differences in mold solutions for different optical materials like COC, PMMA, and PC?
PC has high viscosity and is shear-sensitive, so controlling gate shear is crucial; PMMA is brittle and prone to cracking due to demolding stress; COC material shrinkage is extremely sensitive to temperature, so uniform cooling is paramount.
Conclusion: For residual stress management in precision optical components, process is a remedial measure, but mold is fundamental solution. Six key mold factors—gate shearing, uneven temperature field, abrupt changes in wall thickness, trapped air, demolding stress, and cavity machining quality—jointly determine basic stress level of a part. If mold has inherent defects, subsequent processing relies on costly annealing and stringent process windows, resulting in high rework losses. By systematically optimizing mold and directly controlling residual stress at molding stage, scrap and rework rate of optical components can be significantly reduced, post-processing steps can be minimized, stable mass production of optical parts can be achieved.