The following is a transcript from micro mold optics engineer Rick Brown of Accumold, during a webinar presentation for Photonics.com.
My name is Rick Brown. I am the senior sales engineer for optical molding at Accumold. My background is in injection molding. I’ve been in the business for 47 years. I’ve been actively involved in optical molding for most of my career. I started in the late ’70s working for Polaroid and Kodak, and we don’t hear much about those companies anymore, but they did a lot of leading-edge work back in the day.
Since joining Accumold in 2005, they have grown to a 130,000 square foot facility with a fully fortified building and a staff of over 350. Accumold is a net exporter, shipping all over the world from its Ankeny, Iowa facility which runs 24 hours a day, seven days a week.
My responsibilities today consist of working with customers on design and implementation of their ideas, from whiteboard drawings to finished design to complete optical sub-assemblies and all the way to production parts.
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I’m going to start by answering the question, “What is micro molding?” Well, there really isn’t a textbook answer but here is how we’ve come to define micro molding. It’s one of three things. So, it’s parts that are micro in size, parts that measure one centimeter or less, even down to sub-millimeter, or it’s parts that are micro in features. There are often larger parts that need extremely small details to function, such as micro-fluidics, complex geometries, or thin wall applications. Or last, it’s parts that are micro in tolerances. It’s common for tolerances to be less than 25 microns, and in many cases, two to three micron tolerances are required, and even down into sub-micron when you talk about things like peak to valley tolerance, lens profiles.
What is optical molding? Well, we’ve identified micro molding, but what is optical molding? Well, micro optical molding is one of the biggest challenges for the injection molders today. Simply put, you have all the challenges of micro molding but with the added challenges of the very tight tolerances and surface finishes required for molding lenses. There are many applications for optical molding, we’ll touch on a few of them today.
Plastic VS Glass in Micro Molding
But first, I want to answer the question, “Plastic versus glass?” So, why would you choose plastic when polished glass provides better surface finishes, better clarity and in many cases, better peak to valley tolerances? Well, in recent history there’s been a significant increase in the popularity of plastic as a manufacturing material, rather than glass.
As the market expands and the demand for modern optical applications increases, polymer-based injection molded optics are now found in a broad range of products. The increasing sophistication of micro-injection molding technologies, as well as dramatic developments in materials, means that today complex miniature and previously fragile micro-optics are now robust and can withstand even the harsh environmental requirements of the consumer electronic industry, including IR reflow and thermal shock.
Working in plastic also introduces enormous design freedom. Injection molding is the only viable manufacturing technology to use when low-cost, high-volume production runs are required. A single tool is capable of mass-producing plastic micro-optic parts, including necessary surface finish. And, if process controls are rigorous enough, 100% accurate replication is possible, run after run. This is not the case when manufacturing in glass.
In addition, mounting and mechanical features, such as spacers and flanges, can be integrated into plastic optical components, thereby removing the need for additional components and assembly considerations. Unique cosmetic functional filter and other properties can also be added to enhance or improve the properties of the raw molded polymer, which again cannot be done with glass.
Shown are some of the common applications that the micro molder deals with. These molded lenses can be anything from imaging optics, prisms, light pipes, diffusers, multi-lens arrays for data com couplers, optical sub-assemblies for sensors, and an array of other applications and can include non-optical mechanical components for optical systems.
What I’m showing here are some of the legacy products that we can show. Unfortunately, some of the latest generation stuff is protected by IP, so that I’m not able to show. But let me describe a little bit of what we’re looking at here. The lens shown in the middle of the slide with a blue background is a great example of a multi-lens data com connector. This part has 12 lenses, they’re 250 micron diameter on 250 micron centers with a non-accumulative tolerance of plus or minus two microns in pitch. We’ve been able to prove a repeatability of half a micron on the lens pitch. The lenses on the far right are part of a four-element imaging optic for endoscopic surgery procedure. The parts on the far left are an example of a non-optical mechanical part for an optical system. And lastly, you can see an acrylic lens prism, which is part of a control panel for stadium lighting.
Micro-optical molding allows the design engineers the freedom to include spherical and aspheric lenses, plano surfaces, freeform shapes, and can include secondary operations, such as reflective and anti-reflective coatings. It’s not uncommon to mold aspheric lenses with a base radius of 25 microns or less.
Most molded optical projects require a peak to valley tolerance of around one micron with a surface finish of 80 to 100 angstroms. Some critical imaging optics can require a peak to valley tolerance of 200 nanometers with surface finishes of 20 angstroms and positional tolerances as tight as plus or minus one micron, repeatable to a half a micron.
This slide shows a little of the history of micro-optical molding that I’ve been part of. The photo in the middle shows the progression of parts that have been produced for the data com industry, starting with single channel ROSA/TOSA couplers and maturing to multi-lens arrays. Next generation parts are much smaller, more complex, and require a tighter tolerance.
Photo lithographic and fused silica inserts for molding diffractive and other microstructures are possible as well, as you can see in the photo on the left. The photo on the right show’s parts with a variety of markets, from night-vision goggles for the military to lenses for medical endoscopic surgeries.
So as you can see, micro-optical molding is some of the tightest tolerance and most challenging molding in the industry today but allows the design engineer the greatest amount of freedom.
What about materials?
Well, there are literally thousands of thermoplastics available to the injection molder today and hundreds, if not thousands, of optical grade materials. Common materials such as polycarbonate, polystyrene and PMMA are available. Also, the more highly engineered materials, such as the COPs, COCs, the PEIs like Ultem and Extem, and many more. There are now IR-clear resins available that can survive the JEDEC standard three times 260C reflow.
Resin manufacturers have developed high index of refraction materials. Add to this list custom-blended materials such as application-specific attenuated Ultem, and you have much for the design engineers to consider. The parts in the center picture are the same part, but shot with the Ultem at different attenuation levels.
So another challenge to the micro-optical molder is their quality system. The extreme tolerances required and constantly changing markets require the molder to make a significant investment in equipment and personnel, and to constantly look for new and improved ways and equipment that can keep up with the market. White light interferometers, confocal microscopy, MTF testers are a few of the very specialized equipment that is available today. These manufacturers are constantly making improvements to their equipment to increase their capability.
For years, we’ve produced data com couplers that have very steep lenses. We could only measure the center of the lenses as the shape quickly got too steep for the equipment. The newest generation interferometers have larger numerical apertures and longer focal lengths, allowing us to measure the entire lens.
Add to this custom software to make the correction for aspheric shapes and we have made significant advancements in the area of lens metrology, and they are all continuing to refine and improve the equipment.
Micro Molding for Wearable Devices
Let’s talk about some of the wearable devices. I’m going to touch on three areas. I’m going to start with fitness applications. So the days of wearing a fitness tracking device that only counted your steps seems like ancient history. Today’s fitness devices and smartwatches do so much more. Not only do they track your steps, but they connect to GPS to track your location, mileage, and even elevation gain. They include accelerometers and gyroscopes to detect movement, and on the back of the watch, the part that touches your wrist, there’s sensors for many features such as heart rate and blood oxygen levels. Future models will include sensors for blood pressure, blood glucose, and many more applications. Many of these sensors require molded optics. Injection-molding these tiny optical sensors is the best way to produce these at the very high volumes required and at a competitive price. The small size and tight tolerances make these optics extremely challenging.
The part on the left-hand view is part of a first-generation optical proximity sensor system. The challenge here was over molding the two lenslets, representing transmit and receive channels into a housing with separation of the two channels and holding the tolerance on the pitch between the two lenses. I believe it was somewhere around a five micron tolerance on the pitch. These parts also required materials that are IR-clear and can survive reflow. So this is a first-gen part. The newer generation sensors that we’re producing today are some of the highest-volume parts being produced by micro molders, with volumes in the hundreds of millions. As you can see by the image on the right, there are several sensors on today’s smartphones with many more in future generation devices.
Optics Micro Molding for Medical
Next I’m going to touch on medical applications, and there are many applications for medical wearable devices but I’m going to focus on what is perhaps the fastest-growing market, continuous blood glucose monitoring. All the major medical device manufacturers and many smaller companies have devices on the market, or are working on next-generation devices to monitor blood glucose levels.
Constantly monitoring the patients’ glucose levels helps the patient avoid the high and low spikes that are very dangerous to the diabetic patient. Many of these devices pair to your phone or watch for reporting, giving the user and doctor a clear view of the patient’s condition at all hours. Some of them pair with a pump for insulin delivery.
So most, if not all, of these devices use micro-optics of some kind. These molded optics present their own challenges, one of which is the sheer size of the market. Finding ways to produce these parts and/or sub-assemblies in the hundreds of millions per year is a challenge that the micro molder is facing today.
Many of these devices have complex assemblies requiring over-molding and other assembly steps that must be considered. The part on the left is a first-generation design of an extremely small light pipe used as part of a sensor system for a wearable monitor. The part has an exit window that is 50 microns by 100 microns and required an aluminum reflective coating over the entire part, except for the input and output windows. The picture here shows the part on the runner, so that large slug of plastic you see is actually just the runner. But look at the bottom of the screen and you can see the very small part there that has not been singulated yet.
Another project, that I can’t show but will briefly describe, uses a ring of molded lenses with an AR coating. Then these lenses are over-molded into a housing, then this over-mold is placed into a third mold along with a series of glass filters and a final over-mold is performed. So there are many other designs being developed, some of them even more complex than these. All have one thing in common, they’re looking for ways to eliminate the pinprick that all diabetics go through. This is one market that will have immediate positive benefits to the many millions of people worldwide that suffer from this disease.
Micro Molding for Emerging Technologies
I’m going to talk about emerging applications a little bit, and there’s a lot of markets these days that I would call emerging applications, but I’m going to touch on one that I’m most familiar with, and that’s augmented reality.
As you know, virtual reality devices have been on the market for several years now. The current drive is to develop augmented reality glasses and contact lenses. These devices have many applications, from heads-up display for military and industry applications, gaming, video play, et cetera. Imagine a world where a pair of glasses can replace your smartphone. You’d have the ability to stay present in the world around you and at the same time, have all the features of your phone available. Email, text, web browsing, gaming, et cetera. This is a very exciting market with great growth potential, and all the major manufacturers are working on these.
The parts on the left are part of an augmented reality glasses system. Again, this is a first-generation device which is the reason I’m able to show these. So, the larger part has four off-axis anamorphic aspheres with very tight tolerance on peak to valley and RMS roughness. It has a plano surface on the bottom that also has a very tight tolerance on the flatness and surface finish. This part presented a significant challenge to the micro molder, both in hitting the required tolerances and in measuring the parts to prove these were in spec. This is a case where it’s very important for the micro molder to work closely with the end customer to develop systems for measuring the parts.
In this case, the parts were scanned with a white light interferometer, then the raw data was exported into MATLAB where an executable program was run to analyze the scan. It turned out to be a successful conclusion to a very difficult challenge.
The parts on the right are part of augmented reality contact lenses. In this case, the challenge was the very small size and again, a very tight peak to valley tolerance. These lenses have an outside diameter of 600 microns. There are also medical device manufacturers working on glasses and contact lenses for correction of severe myopia without surgery. These devices will stop the progression of myopia. These parts include complicated over-molded operations with extremely tight tolerances.
Design Considerations for Micro Molding
Let’s talk about some of your design considerations and some of the secondary operations and value-add that the micro molder can supply. So there’s much for the design engineer to consider when tackling a new project. My best advice is to engage early with the micro molder that has a strong background in DFM, design for manufacturability, and under one roof, has control of all aspects of the product development process, from design to high-volume manufacturing.
Micro molding projects are, by their very nature, complicated and the fact that many require the repeatable attainment of extremely tight tolerances demands that there is a collective focus on accuracy when looking at micro tool fabrication, molding, validation, and automation processes. Material selection and design for micro molding are also hugely important and must be a primary consideration as time, cost and the frustration of creating a design that needs to be radically altered before it can be manufactured.
As a general rule of thumb when looking at DFM, dimensionally, the micro molder can accommodate parts up to 13 millimeters in the largest dimension and to date, the smallest production part that I’ve worked on is 300 microns by 380 by 800. Now, I’ve worked on smaller parts in development, so I’m not saying that’s the limit on the small end.
General guidelines for design engineers to consider are as follows. Thin wall sections need to be .1 millimeter or thicker and there needs to be a focus on thick to thin wall transitions as well as on wall thickness uniformity. Feature aspect ratios around 6:1 are attainable, although this is highly material-dependent. Gates can get as small as .1 millimeter and ejector pins can be as small as .25 millimeters. It is also vital to have an understanding of how shrink rates will affect the part and to be cognizant of parting line mismatch.
Over-molding and secondary operations are also available to the design engineer and should be considered early in the process. In the case shown here on the right, a two-element imaging optic required a center aperture and an air gap between the lens elements. I was able to come up with a unique solution to over-mold this assembly, greatly reducing the cost of what would have been an expensive assembly procedure.
I’ve recently seen, had a customer show me, patent applications for parts that use a similar design, but the reality is these ideas are decades old. Most cutting-edge ideas are simply a rehash of tried and true designs that have been around for a long time. This is why it’s important to engage with a micro molder that has a long history in the industry and a deep bench.
Material choice is also crucial when planning to over-mold plastic on plastic. There are many other materials that can be over-molded as well. Metal, stamped or etched lead frames, glass, fabric, flex circuits and many more, but when over-molding plastic on plastic, you need to consider reflow. The first shot needs to have a higher temperature than the second shot. It’s very common to mold optics at a much higher temperature than the mean recommended temperature. You don’t want to get that first shot to its glass-transition temperature and get a reflow.
We’ve recently over-molded a lens on a lens, and doing a materials search to find materials that were compatible was a very important part of the process in bringing that to a successful conclusion.
For secondary operations, ultra-sonic and laser welding, snap-fit assemblies, or complete sub-assemblies can add value for the design engineer. Optical coatings of all kinds are available as well. As you can see on the picture on the left, that’s a gold reflective coating on a plastic substrate. I believe that is a Ultem molded coin there. We developed a coin mold for testing materials and coatings. So, aluminum and gold reflective coatings, anti-reflective coatings, beam splitters and coatings for EMI shielding are a few that can be considered.
So, this is an exciting time for this industry. As parts get smaller and tolerances get tighter, the field of qualified micro molders that can meet these new challenges gets smaller as well. There is much more under development, so I’d like to open it up for questions. I’ll do the best I can to answer all of them in the time allowed.
Rick Brown is the senior sales engineer for optical molding at Accumold where he has made a significant impact on growing the optics business. His technical background involving optical injection molding spans 47 years, beginning in the late 70’s for Polaroid and Kodak. He enjoys working closely with his customers on design and implementation of their ideas from white board to production parts.