Breaking the "Valley of Death" in Small-Batch Lens Injection Molding: Solutions for Annual Demands of Only a Few Hundred Lenses
Preface: A Difficult Cost Balancing Act
In the optical instrumentation industry, we frequently encounter this dilemma:
A high-end medical device priced at hundreds of thousands of dollars sells only a few hundred units annually, yet it requires a custom aspherical lens.
Theoretically, the unit cost of an injection-molded lens drops to just a few dollars once mass production begins. However, when the order quantity is merely 300–500 pieces, the mold quotation can reach as high as 100,000 RMB. Clients often question: "Is the price artificially inflated due to the low volume?" They may even suggest cutting costs by lowering mold base standards or using ordinary steel.
The high mold cost is essentially an investment in the performance of the optical system. Blindly compressing costs at this stage will lead to rework or even project failure later, with consequences far exceeding the initial savings. Therefore, it is necessary to understand how this mold cost is calculated.
1. Why Are Molds So Expensive? Where Does the Money Go?
High-precision optical lens molds are complex systematic projects, far from being simply "two blocks of iron with a cavity." The main components of mold costs are as follows:
1.1 Design and Analysis: Hidden Intellectual Costs
Even if the client provides drawings, the mold manufacturer must perform deep secondary development:
Structural Design: Due to the stress sensitivity of optical lenses, gate locations, cooling channels (to ensure uniform cooling and prevent deformation), and ejection systems (to prevent surface damage and ejection distortion) must be meticulously designed.
Mold Flow Analysis (Moldflow): Software simulation of plastic flow, packing, and shrinkage is mandatory to predict defects like sink marks and air traps in advance.This phase accounts for approximately 10% of the total cost. It primarily covers the labor of technical personnel, including mold designers, injection molding technicians, and often project managers and optical engineers. Skipping this step exponentially increases the risk during trial runs.
1.2 Mold Base: The Foundation of Precision
The mold base supports all core components. The market offers generic brands costing a few thousand yuan, versus top-tier brands like LKM (Longji) costing tens of thousands. The reasons for using top-tier brands include:
Precision Assurance: Optical molds require closing errors controlled at the micron level. Top-tier brands offer superior plate parallelism and guide pin fit accuracy compared to generic alternatives.
Material Stability: The steel undergoes strict ultrasonic testing and heat treatment, resulting in low internal stress and no deformation over long-term use.
Risk Control: Although the mold base accounts for only about 20% of the cost, inferior bases can lead to flash, misalignment, or even catastrophic mold damage, causing losses far greater than the savings.
Fig1LKM Mold Base
1.3 Mold Core: The Heart of Performance
The mold core directly determines the lens surface profile accuracy and is the core cost driver:
Expensive Equipment: Requires ultra-precision CNC lathes (e.g., Moore, Precitech) or imported high-speed milling machines, entailing high depreciation costs.
Complex Processes: Single-point diamond turning (SPDT) cannot directly machine steel; the steel must first be nickel-plated. High-speed milling requires subsequent mirror polishing.
Premium Materials: Imported mold steels (e.g., ASSAB S136, NAK80) are commonly used for their excellent hardness and polishability.
Cost Share: Machining and material costs account for 40%–50% of the total price, representing hard expenditures.
1.4 Trial Molding and Modification: The Hidden Cost Center
Completing the mold machining is just the beginning; the machine trial is where costs peak:
Machine and Labor: Precision injection molding machines are expensive to operate, and trials disrupt normal production. Collaboration among injection technicians, mechanics, and QC staff is required.
Material Loss (Critical Item):
Drying: Optical-grade PMMA/PC requires drying for over 4 hours.
Material Change and Screw Purging: When switching brands or grades, the screw and barrel must be thoroughly cleaned. New material is used repeatedly for purging until no impurities remain; the resulting mixed material is scrapped, leading to significant raw material waste.
Trial-and-Error Consumption: Defective parts and residual melt generated during debugging are typically non-recyclable.
Modification Cycles: Due to fluid-structure interaction and thermal expansion/contraction, actual shrinkage rates often deviate.
Dimensional deviation → Welding and re-machining.
Surface defects → Adjusting cooling channels or process parameters.
Often, 1–2 rounds of modification are needed to achieve qualification. Modification costs often exceed the initial machining fees.
In summary, the high mold fee corresponds to: Senior design + Top-tier mold base + Ultra-precision equipment depreciation + Premium steel + High-frequency trial material loss.
2. Solving the Dilemma: The Dual Strategy of "Soft Molds" and "Shared Mold Bases"
For small-batch demands of a few hundred pieces, the industry has two mature solutions: "Soft Mold Injection Molding" and "Shared Mold Base Reuse."
2.1 Solution 1: Soft Mold Injection Molding (Most Widely Applicable)
Traditional hard molds pursue a lifespan of 500,000 shots, requiring quenched steel and complex heat treatment. However, small batches only need stable production for 500–2,000 shots.
Soft Mold Strategy:
Material Optimization: The mold core uses aluminum alloy (7075) or pre-hardened steel (P20/718), eliminating the need for quenching. This speeds up machining and reduces costs. The mold base, however, should still use top-tier brands like LKM to ensure closing precision and flash control.
Consistent Process: Injection molding processes, materials, and inspection standards remain identical to mass production.
2.2 Solution 2: Shared Mold Base Reuse (Cost Reduction & Speed)
Professional optical manufacturers often stock common specifications of top-tier mold bases (for standard plano-convex or bi-convex lenses). If a new product's dimensions match existing inventory (or require only minor adjustments), there is no need to purchase a new mold base. The client only pays for the new mold core machining and adaptation debugging, saving both the mold base cost and the procurement lead time.
Applicable Scenarios: Small-batch pilot production of standard optical elements and common aperture lenses.
2.3 Core Prerequisite: Rigorous Optical Tolerance Analysis
Whether choosing a "soft mold" or a "shared mold base," it is not a simple matter of swapping materials or reusing old frames. It must be preceded by rigorous tolerance analysis from an optical engineer.
Elastic Deformation: Injection pressures range from tens to hundreds of tons. Since aluminum's elastic modulus is far lower than steel, it is prone to micro-deformation under high pressure. Compensation must be calculated in advance; otherwise, the surface profile accuracy (PV value) will exceed limits.
Thermal Expansion: Aluminum's coefficient of thermal expansion is roughly twice that of steel. Under high temperatures, aluminum cores expand significantly more, meaning the shrinkage compensation algorithm differs entirely from that of steel molds.
Therefore, detailed evaluation by the optical engineer (the designer of the lens used in the project) is essential. Only through rigorous analysis can we ensure that "low cost" does not equal "low quality."
3. Haoge Optics in Action: Strategies for Cost Optimization and Rapid Delivery in Small-Batch Lens
3.1 Case Background
A well-known IVD manufacturer was developing a new product: a portable analyzer. It required a custom φ30mm aspherical condenser lens with a surface profile accuracy (PV value) within 1 micron and surface roughness within 20 nanometers, using a specific grade of material. The project was in the late R&D stage; other components (circuitry, structure, software) were already debugged. The client expected lens production to be completed within 15 days. The initial demand was 200 pieces, with unknown annual volume.
3.2 Specific Challenges
Technical Dilemma:For a φ30mm lens, a PV value within 1 micron constitutes ultra-precision injection molding. The mold core must be machined using a single-point diamond lathe or an ultra-precision grinder.
Directly turning an aluminum core with a diamond lathe is fast, but it may not guarantee surface roughness within 20 nanometers.
Using an imported high-speed mill to machine a steel core followed by mirror polishing achieves excellent roughness but struggles to guarantee a PV value within 1 micron.
Fig2.Aspherical Lens
Material Dilemma:
The client specified a specific grade of PMMA from a German brand, a top-tier optical material with excellent optical performance, heat resistance, and surface hardness. However, this material required sourcing from abroad. Given the tight timeline and minimal usage volume, procuring this material was difficult.
Lead Time Dilemma:
A 15-day deadline is extremely tight for the mold industry, requiring a sequence of tasks: mold design, mold base procurement, mold base machining, core machining, assembly, and injection molding.
3.3 The Solution
A project team comprising optical engineers, mold designers, and production managers was formed to analyze these challenges.
Parameter Re-evaluation: Based on years of experience with similar instruments, our optical engineers determined that this lens was critical in the optical path, serving as both the field stop for the imaging system and the aperture stop for the illumination system. Further analysis revealed that the lens had low optical power, meaning requirements for surface profile accuracy were not as stringent as initially thought. However, requirements for surface roughness were extremely high; otherwise, illumination across the image plane would be highly uneven. After full communication with the client's optical engineers, the final specifications were set to: Surface PV value < 10 microns and Surface roughness < 12nm.
Material Selection: Through discussion, it was confirmed that the actual requirements for the lens within the instrument were high transmittance, high temperature resistance, and low haze. We ultimately selected a specific grade of Mitsubishi PMMA regularly stocked by Haoge Optics. While the hardness of the Mitsubishi material was slightly lower than the client's original choice, hardness was not a critical requirement for this internal component.
Manufacturing Execution: We utilized a stocked LKM mold base. The mold core was made from Japanese Daido NAK80 mirror-finish steel, machined using a Mikron high-speed mill, and finally mirror-polished.
Fig3. Aspherical Lens Mold Core
Results:
First article delivered in 12 days; 200 lenses delivered in 13 days.
Final metrics: Surface PV value of 6–8 microns, surface roughness of 10nm, fully meeting instrument requirements.
Cost Breakdown: Core cost: 8,600 RMB; Trial & Production: 2,800 RMB; Testing: 1,500 RMB; Others: 2,000 RMB. Total: 12,900 RMB.
Long-term Value: The client successfully passed validation with this batch. One year later, upon mass production, they directly opened a precision steel mold based on accurate pilot data, achieving a seamless transition.
4. Conclusion: Small-Batch Production is a Critical Validation Stage for Product Maturity
For high-value instruments, completing a small batch of a few hundred lenses is key to product success. Forcing a full steel mold creates an excessive cost burden. Blindly pursuing special materials or non-standard processes can lead to delivery delays or cost overruns.
There is only one correct approach: Choose a supplier who understands small-batch production and engage in deep technical upfront communication. Engineers from both sides must re-evaluate specifications: eliminate "excessive indicators" that unnecessarily increase costs, address "insufficient indicators" that hide potential risks, and avoid material selections that jeopardize delivery schedules.
A common misconception must be corrected: Many project managers without an optical background view optics as a "niche and simple" standard component, habitually adding it to the project only after circuitry and structure are finalized. This "hardware-first, optics-later" sequence is often the biggest pitfall. Due to a lack of early systemic consideration, teams are often forced to overhaul completed structures or even circuits later to accommodate optical performance. This passive reactive rectification is the true culprit behind rising costs and delays.
Small-batch production is never a compromise; it is the most critical trial-and-error phase in iteration. For demands of a few hundred lenses, the breakthrough lies not in desperately squeezing the budget, but in matching—matching the most appropriate process, tolerances, materials, and mold life. After all, ensuring every dollar of the budget delivers maximum value is the hallmark of professionalism for engineers and project managers.
Danyang Haoge Optics
丹阳浩格光电科技有限公司