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`Polymer Optics: A manufacturer’s perspective on the factors that
`contribute to successful programs
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`William S. Beich*a, Nicholas Turnera
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`aG-S Plastic Optics, 408 St. Paul Street, Rochester, NY 14605
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`Copyright 2010 Society of Photo-Optical Instrumentation Engineers. This paper was published in Polymer Optics
`Design, Fabrication, and Materials, edited by David H. Krevor, William S. Beich, Proceedings of SPIE Vol. 7788, and
`is made available as an electronic reprint with permission of SPIE. One print or electronic copy may be made for
`personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means,
`duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the
`paper are prohibited.
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`Polymer Optics Design, Fabrication, and Materials, edited by David H. Krevor, William S. Beich,
`Proc. of SPIE Vol. 7788, 778805 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.861364
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`Proc. of SPIE Vol. 7788 778805-1
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`Polymer Optics: A manufacturer’s perspective on the factors that
`contribute to successful programs
`
`William S. Beich*a, Nicholas Turnera
`aG-S Plastic Optics, 408 St. Paul Street, Rochester, NY 14605
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`ABSTRACT
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`Precision polymer optics is a key enabling technology allowing the deployment of sophisticated devices with
`increasingly complex optics on a cost competitive basis. This is possible because of the incredible versatility that
`polymer optics offers the designer. The unique nature of injection molding demands a very disciplined approach during
`the component design and development phase. All too often this process is poorly understood. We will discuss best
`practices when working with a polymer optics manufacturer. This will be done through an examination of the process of
`creating state-of-the-art polymer optics and a review of the cost tradeoffs between design tolerances, production
`volumes, and mold cavitation.
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`Keywords: Optical fabrication, injection molding optics, polymer optics, plastic optics, optical systems design
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`1. INTRODUCTION
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`Polymer optics is a key optical technology enabling a wide array of sophisticated devices. Because these types of optics
`are made of plastic and through the process of injection molding many options exists for providing customized solutions
`to unique engineering and product problems. However, the tremendous flexibility available to the designer is at once a
`bonus and a burden. It’s a bonus because of the potential for creative problem solving. The burden comes from not
`understanding how the optics are made, how they’re toleranced, and how alternative solutions may accomplish the goal-
`albeit with a different design.
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`While many options are available the challenge for designers is to understand the manufacturing process behind these
`solutions so that they can design their programs to leverage the technology. Without this level of understanding the
`designer may not achieve an optimal solution. Or, as is sometimes the case, the design team may go away thinking that
`a polymer optic is not an appropriate solution after all. We call this not knowing what you don’t know. From a
`manufacturer’s perspective many times we have encountered programs where we were given a small glimpse of what the
`engineering team was trying to achieve. This is often presented as a set of disembodied specifications for a particular
`optic. Frequently this comes in the form of a request to substitute the existing expensive glass substrate for a ‘cheaper’
`plastic one. It’s not unusual to hear something like, “the specs are on the drawing, just substitute the word acrylic for the
`word BK-7.”
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`While this approach sometimes works, more often than not the challenges in making polymer optics a commercial
`success are completely ignored. The glass-appropriate specifications, which are completely wrong for plastic, result in
`either a no bid or an optic that works but could have been customized for plastic to work even better.
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`It is our belief that given the challenges and opportunities, designers are well served by getting the manufacturer
`involved early on in program discussions, since it is the optimal time to insert manufacturability expertise. To that end
`we will discuss the polymer optics manufacturing process and examine the best practices to use when working with a
`polymer optics manufacturer.
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`*wbeich@gsoptics.com, phone 585-295-0278; fax 585-232-2314
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`2. WHAT ARE POLYMER OPTICS
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`Polymer optics are precision optics that are made of thermoplastics. Materials such as acrylic, styrene, Topas, Zeonex,
`and Ultem are examples of thermoplastics. In most instances they are made by a process called injection molding.
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`There are some exceptions to this. For example, some large area plastic optics, such as Fresnel lenses, are often made
`using compression molding. We will confine our discussion to optics made using the injection molding process. The
`technology was pioneered by companies such as Eastman Kodak, Polaroid, and U.S Precision Lens.
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`Today, in addition to being manufactured in the United States, polymer optics are made in Europe and in Asia, by
`companies such as Jenoptik in Germany and Nalux in Japan.
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`2.1 Where are they used, why would you want to use them
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`The number of devices and instruments that use these types of optics continues to grow. In short, any application that
`calls for an optical component, be it for imaging, scanning, detection, or illumination is a candidate for using a polymer
`optic. Some limitations on use will be discussed below.
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`A partial listing of devices that are in the market place today employing polymer optics would include: barcode scanners
`(both linear-1D laser scanners and matrix- 2D bar code imagers), biometric security systems, medical devices, document
`scanners, printers, light curtains, light guides, cameras and mobile imaging, smoke detector optics, automated sanitary
`valve systems, and laboratory equipment such as spectrometers and particle counters. All of these and more have
`benefited from using precision polymer optics. Polymer optics are also found in certain telecommunication products and
`commonly used to replicate micro structured surfaces such as microlens arrays, Fresnel lenses, refractive-diffractive
`optics, and some types of gratings. They are increasingly being used in LED illumination applications.
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`2.2 How are they made: the manufacturing technology
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`Polymer optics are manufactured by injection molding thermoplastics into optical forms. The key ingredients for
`production are molding resins, the molds, and injection molding machines.
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`2.2.1 Thermoplastics
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`As noted above, the principle molding thermoplastics are acrylic, styrene, polycarbonate, cyclic-olefins polymers (such
`as Zeonex and Zeonor, manufactured by Zeon Chemicals), Cyclic-olefin co-polymers (Topas, manufactured by Topas
`Advanced Polymers), and other specialty resins such as Ultem., Radel, and Udel. All of these materials are
`thermoplastics, which means they are plastics that can be heated and cooled repeatedly. This category of polymer is
`different from the optical grade thermoset plastics, which, once cured, are not able to become molten again. The
`manufacturers of these materials publish data related to their mechanical, thermal, and optical properties. Optical
`designers need to understand how these materials behave so that they can arrive at appropriate solutions.
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`Table 1. A brief summary of some of the key characteristics of the most important optical thermoplastics.
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`2.2.1.1 Light Transmission
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`Most optical plastics have high clarity in the broad band visible portion of the spectrum. For example, acrylic and some
`grades of Zeonex have transmission properties of about 92%. Materials such as polycarbonate have lower transmission,
`but higher impact resistance. The table below summarizes the transmission characteristics of the most commonly used
`optical polymers.
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`Graph 1. Transmission characteristics of optical polymers.
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`2.2.1.2 Index of Refraction and abbe value
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`The range of available indices of refraction is quite narrow when compared to that available for glass. Acrylics and COP
`materials behave more like crown glass types (having abbe values in the mid 50s) with an index of refraction of about
`1.49 and 1.53 respectively. On the other hand styrene and polycarbonate behave more like flints (with abbe values in the
`low to mid-30s) and having an index of refraction of about 1.59.
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`2.2.1.3 Transition Temperature, Coefficient of Thermal Expansion, H2O uptake, and dn/dt
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`When compared to glass, plastics have a much lower transition temperature (it’s not unusual to see maximum continuous
`service temperatures of under 130-degrees C.) They also have a much higher coefficient of linear expansion (about an
`order of magnitude higher). Plastics will exhibit a change in index of refraction relative to temperature; the
`thermoplastic dn/dt is fairly large (about 20 times that of glass) and negative1. Most thermoplastics (with the exception
`of COP and COC materials) will absorb water, which will cause the lens shape to change dimensionally. For example,
`acrylic will absorb approximately 0.3% water over a 24-hr period. During the same period, a COP or COC material may
`absorb only 0.01%.
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`Plastic generally is lighter in weight than glass, so depending on the glass type alternative, using a polymer optic can
`significantly reduce the weight in a system. Finally, it should be noted that polymers are not nearly as hard as glass.
`Many different scales are used to measure hardness. One scale that is readily grasped is Moh’s ordinal scale of mineral
`hardness. With talc at the softest (1) and diamond at the top of the scale (10), most plastics come in at around 2
`(absolute hardness of about 3), equal to gypsum. It is clear that polymer optics must be protected in whatever system
`they are used.
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`2.2.2 Molds
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`The mold used to manufacture polymer optics can be thought of as a sophisticated three dimensional steel puzzle that
`has two main features: (1) the cavity details along with the core pins (also known as optical inserts or nubbins), and (2)
`the frame (sometimes called the base) that houses the cavities and inserts. The figures below illustrate the basic concept
`of the mold. The complexity of the mold is a function of the complexity of the element being molded. One of the key
`advantages of using polymer optics is the ability to combine optical and mechanical features into one platform. So,
`depending upon the nature of the mechanical features being considered the mold itself can take on additional
`complexity.
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`The mold is mounted into the molding press. One side of the mold is mounted to the fixed side of the press; the other
`side is mounted onto the moveable platen within the press. During the molding process, the two mold halves are
`clamped together under high pressure. The molten resin is injected into the mold by the press and the melt moves
`through the channels in the tool to the cavities. The cavities fill with the resin and take on the shape of the cavity detail.
`Once the plastic has cooled to an appropriate temperature, the mold opens and the optics are removed.
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`The mold is built to the negative of the final part. Thus if the final optic has a convex surface the optical insert will be
`concave. The mechanical features of the part have to be drafted (tapered) so that they will not be trapped in the mold
`after the resin has solidified.
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`All thermoplastics shrink as they cool. In general, the shrinkage is approximately 0.5% to 0.6%. It is important that the
`shrinkage be taken into consideration when determining the final dimensions of the mold. If the mold is made to the
`final drawing specifications the part will be too small. One needs to make the mold wrong, if you will, to make the part
`right. Usually molds are built steel-safe, which allows mold adjustments to be done by removing steel.
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`With the advent of sophisticated CNC lathes most optical inserts are diamond turned from nickel-plated steel. This
`method makes it possible to create on and off axis aspheric surfaces and allows the optical molder the flexibility of
`adjusting the inserts for shrinkage after initial molding trials have be done.
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`Figure 1. Mold Cross Section
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` Figure 2. Mold-parting line photos
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`The following is a brief list of some of the key features generally found in molds along with a brief description of their
`intended function2.
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`1. Sprue Bushing. Provides means of entry into the mold interior.
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`2. Top (A-side) plate. Portion of the mold mounted on the stationary side of the press.
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`3. Guide Pins. Maintains proper alignment of the two halves of the mold.
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`4. Bottom (B-side) plate. Side opposite the “A” side, sits on the moveable platen of the molding press.
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`5. Ejector Mechanism System. Used to eject rigid molded elements from the cavities.
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`6. Ejector Housing. Houses the ejector system.
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`7. Runner System. System of channels in the mold face used to convey molten plastic from the sprue to the
`cavities.
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`8. Vents. Structures that allows trapped gas to escape.
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`9. Gates Region of the mold that controls the flow of molten material into the cavities.
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`10. Optical inserts (sometimes referred to as nubbins). Pins within the mold that have been deterministically
`ground and polished against which the optical surface forms during the molding process. These surfaces can be
`steel or a non-ferrous alloy.
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`2.2.3 Molding Machines
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`Molding machines are used to hold the mold and to melt and inject the plastic into the mold. The figure below shows
`the basic features of a molding machine.
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`Figure 3. Schematic of a molding press.
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`The molding press has a clamp unit on one end and the injection unit on the other. The mold is hung in the middle
`region as shown. The clamp unit is used to keep the two mold halves together as the molten resin is being injected. The
`molding cycle begins. The moveable platen closes against the fixed platen (closing the mold). An appropriate amount
`of force is used to hold the mold closed during the injection cycle. The injection unit, consisting of a feed hopper,
`reciprocating screw, and barrel, picks up an amount of pelletized resin from the hopper. It is the job of the injection unit
`to melt the resin and to push it into the mold through the sprue bushing. The reciprocating screw turns within the barrel.
`It is fluted allowing it to trap the material between the heated chamber wall and the screw. The chamber wall is the
`bearing surface where sheer is applied to the resin as it is being advanced towards the mold. Once the molten material
`accumulates at the end of the screw it is injected at an appropriate speed and pressure into the mold. This causes the
`material to flow into the mold to fill the cavities. The molding machine provides complete control over this process,
`governing the size of the shot, injection speed, injection pressure, backpressure, cushion, and other critical variables that
`will determine the final outcome of the optic. After an appropriate cooling time, the moveable platen moves away from
`the fixed platen, and the mold opens. This allows the optics (still attached to the runner system) to be removed. After
`the shot is removed, the cycle starts over again.
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`Other equipment is often found along side the molding machine. For parts that require a large amount of material, auto
`loading hoppers are used to feed material into the machine. Also, the thermoplastics must be dried before being fed into
`the injection unit. It is common to see desiccating equipment located near the press for this purpose. Once the molding
`cycle is completed it is desirable to promptly remove the shot so that the entire molding process may be repeated with
`regularity. To aide in this, a robotic arm is frequently used to ensure that the removal is done on time. This enables the
`entire process to go into a steady state. Depending on the nature of the program, additional automation or end of arm
`tooling may be required to remove of the parts from the press, degate them from the runner, and package them into trays
`for final shipment. Degating is the process whereby the optical elements themselves are removed from the runner
`system.
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`3. TYING IT ALL TOGETHER
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`As noted above, it is important that the designer has a basic understanding of the manufacturing process and of the
`limits of size and tolerances that might be expected of the finished optics. In general terms, overall shape and
`tolerances of the optic will drive cost and manufacturability. There are some general guidelines: thicker parts take
`longer to mold than thinner parts. Optics with extremely thick centers and thin edges are very challenging to mold.
`Negative optics (thin centers with heavy edges) are difficult to mold. Optics with very tight tolerances may not be
`manufacturable at all in a one cavity mold, much less in a mold with more than one cavity. There are some other
`general tolerances that can describe the limits of fabrication in an ideally designed optic.
`
`Attribute
`Radius of Curvature
`EFL
`Center Thickness
`Diameter
`Wedge (TIR) in the Element
`S1 to S2 Displacement (across the parting line)
`Surface Figure Error
`Surface Irregularity
`Scratch-Dig Specification
`Surface Roughness (RMS)
`Diameter to Center Thickness Ratio
`Center Thickness to Edge Thickness Ratio
`Part to Part Repeatability (in a one cavity mold)
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`Table 2. Rules of thumb.
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`Rules of Thumb Tolerances
`± 0.50%
`± 1.0%
`± 0.020mm
`± 0.020mm
`< 0.010mm
`< 0.020mm
`(cid:1) 2 fringes per 25.4mm (2 fringes = 1 wave @ 632nm)
`(cid:1) 1 fringes per 25.4mm (2 fringes = 1 wave @ 632nm)
`40-20
`(cid:1) 100 Å
`< 4:1
`< 3:1
`< 0.50%
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`Two things should be observed here. (1) Even with the rules of thumb, it is very difficult for the experienced optical
`molder to communicate all of the things that the designer should look out for. There are simply too many variables to
`consider without expert guidance. In this regard we might say it is not unlike consulting with a doctor on a medical issue
`or with a lawyer on a legal matter. One might have a general idea of the issues from one’s own reading or researching
`on the internet; however, expert assistance is needed to answer deeper questions. And (2), what is not discussed here is
`how the rules of thumb interact with one another or how a change in one area will impact another. Rules of thumb are
`quick generalizations. They are useful for initial discussions, but the rules can quickly break down as the limits of size,
`shape, thickness, materials, and tolerances are encountered. It is impossible to publish an exhaustive list of possible
`interactions between all of these variables. The main reason for consulting with the optical molder is that a good optical
`molder will bring years of experience to the table.
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`What is the best way for the designer to work with an optical molder? Perhaps the best way is to proceed from a systems
`design perspective. Instead of communicating with the optical molder at arms length with a drawing and tolerances
`hoping for the right solution, instead, why not communicate the big picture to the molder so that they can help address
`questions that may not even be in view at the component drawing level. Perhaps the best way to grasp this is to consider
`an example.
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`3.1 Example 1. The effect of design on cycle time and total cost of acquisition
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`The optical molder received the following request for quotation. The element is acrylic, bi-convex, aspheric on both
`surfaces, 75mm in diameter ± 0.050mm, with a 12mm center thickness and a 2mm edge thickness, both toleranced at ±
`0.020mm. The clear aperture extends to within 2 mm of the edge. Power and irregularity are specified at 5 fringes and
`10 fringes respectively. The lens has a S1 to S2 displacement tolerance of ± 0.020mm. The drawing has no provisions
`for a gate location. Volumes are 10,000 pieces per year. Please quote.
`A lens with this description is going to be very expensive. If we use an overhead rate of $120/hr3, and an estimated
`cycle time of 6 minutes, we would have a lens that costs about $12.00. The tight tolerances would likely increase scrap,
`so accounting for yield loss would push the price higher to around $14.00. To mitigate this increase, a typical tactic
`would be to build a higher cavitation tool, but because of the tight tolerances, this lens could never be run in a multi-
`cavity mold. Cavity to cavity variation would increase the power and irregularity errors to a point where not every
`cavity would meet the specification. There is no way to achieve the economies of scale that can be realized by going to
`higher cavitation. If we say that the mold for this lens would cost about $15,000, the total cost of acquisition for the first
`year production would be about $15.50/lens.
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`3.2 Example 2. How a manufacturable design can reduce the total cost of acquisition
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`An alternative approach to example 1 is to look at the system design with guidance from an optical molder, who may ask
`the question: can this thick optic be split into two thinner optics? If the system design were flexible enough to allow a
`two lens solution, then we might see an alternative scenario where two lenses with 3 minute cycle times can reduce the
`total cost of ownership through yield improvements (assuming the two separate lenses have more achievable tolerances,
`which is very likely).
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`The tooling cost for the two-lens solution, which would involve building one mold with two cavities that can be run
`independently, would be about $20,000 – a $5,000 increase over example 1. The graph below, which plots the unit cost
`of examples 1 and 2 (a set of two parts) including amortized tooling, shows that the improvement in yield gained through
`a more manufacturable design results in a breakeven point of around 6,700 pieces. From a cost perspective, if more than
`6,700 pieces are required, it becomes cheaper to have a two lens solution. This savings could increase if the cycle time
`for either lens is less than 3 minutes (which is likely).
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`Graph 2. Unit cost per set with tooling amortized.
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`The economics work out, but the hidden cost of the risk between examples 1 and 2 is not captured. On paper example 1
`is more challenging to manufacture and may lead to unexpected manufacturability issues. A competent optical molder
`would identify those issues as potential red flags so that contingencies can be determined upfront. For example, the
`power and irregularity tolerances are very difficult, and the parts may only barely meet the specification. What is the
`impact to the system if these values exceed the spec? Will it degrade performance? What if the tolerance analysis is
`incorrect and the specs need to be tighter, but that is only discovered after parts have been molded and tested? These are
`a few of the questions an optical molder must consider. Example 2 is expected to have looser tolerances, so many of the
`above concerns are inherently less risky.
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`The inquiry in example one called for an annual volume of 10,000 pieces. If the company has success selling the
`product, the volumes may go up and create a capacity constraint or lower cost requirement. A mold can only produce a
`certain number of pieces per year (around 35,000 pieces for one shift in example 1), and the cost is primarily cycle time
`driven. Since multi-cavity tools help to address both concerns, it is important to consider at the beginning if the lens can
`be made in a mold that has more than one-cavity. In example one above, the answer is no. The tolerances are too tight.
`In contrast example two has the benefit of looser tolerances and thinner optics, both of which bode well for the ability to
`expand to multiple cavities. The need to act on higher cavitation tooling may be delayed due to the higher production
`capacity of the 1-cavity molds (due to shorter cycle times), but having flexibility in the decision making process is often
`beneficial.
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`In much the same way that future expectations of multi-cavity molds must be considered upfront, the need for and
`method of prototyping must be mindful of future production methods and requirements. Prototypes are often used to
`provide functional devices to evaluate customer demand and market opportunities, but sometimes they are produced to
`prove that the system will work. Diamond turning is the most direct way of producing prototypes, and the achieved
`tolerances are typically much less than the specification limits. This is both a blessing and a curse, since the end result
`of the prototyping process may verify that a design is functional without testing the tolerance limits. Understanding the
`differences between what is achievable with prototypes (via diamond turning or prototype molding) versus volume
`production is crucial to making sure a design is production capable. The experienced optical molder can help a designer
`navigate through these potential pitfalls.
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`Other system design factors come into play as well. The temptation is to consider these as mundane non-optical issues,
`however, if not addressed correctly these issues can add considerably to the total cost of acquisition. For example,
`where will the lens be gated? Can the mating part be adjusted so that a longer gate vestige can be accommodated? How
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`will the part be packaged? How will the part be handled? Does the optic need some kind of keying feature to help with
`down the road assembly? The answers to these questions can add additional and sometimes significant cost to the final
`component.
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`There are other things to consider: As the design deviates from a conventional on-axis rotationally symmetric optic,
`measuring the part to verify conformance can become a limiting factor. Interferometers are typically used to measure
`flat and spherical surfaces, and contact profilometers are proficient at measuring aspheres. For bi-conic, freeform, or
`off-axis surfaces, a combination of profilometry and CMM (contour measuring machine) inspection can answer many
`inspection questions. There are instances, though, where the design requires an optic be inspected to a level beyond
`which these tools are capable. Functional testers and customized inspection setups can often close the gap, but
`identifying that a gap exists early on in the design process is critical for finding a solution.
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`The point here is that the lens designer may not be thinking of these things when presenting a drawing for bid. He may
`not even be aware that there are larger issues like this to consider. He is probably concentrating on how the lens needs to
`perform in the system and rightly so. But the lens does not exist in isolation. The rest of the system, along with the
`commercial aspects of future production needs, should be addressed up front so that the appropriate tooling set can be
`accounted for. These are the things that the layman may not know that he doesn’t know.
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`Finally, similar to how it is impossible for a designer to work from an exhaustive list of optical molding rules, there are
`other critical to success aspects of the manufacturing process that the molder does not know either. This is where the
`molder’s supplier network comes into play. The tooling supplier identifies risks and makes suggestions about the mold
`in a back and forth process similar to how the molder works with the designer/customer. The same is true about coating,
`diamond turning, or other processes that are needed to make or support the production of the part. As an extension of
`this, it is beneficial for the molder to know many of these things first hand so that as many requirements can be
`determined in the one-on-one discussions with the customer. Finding a molder that has internal capability for diamond
`turning, coating, automation, fixturing, and so forth, will help to streamline the process, manage cost, and improve
`quality.
`
`
`
`
`REFERENCES
`
`[1] Smith, Warren J., [Modern Optical Engineering: The Design of Optical Systems 2nd Ed.], McGraw-Hill, Chapter
`7.5 (2000).
`
`[2] Rosato, D.V., Rosato, D.V., Rosato, M.G., (Eds.), [Injection Molding Handbook, 3rd Edition], Kluwer Academic
`Publishers, Norwell, MA., page 232 (2000).
`
`[3] Schaub, Michael P., [The Design of Plastic Optical Systems], SPIE, Bellinham, WA., Chapter 3.4 (2009).
`
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`Apple v. Corephotonics
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`Apple Ex. 1020
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