Is Going Micro Worth the Effort?
Micro is a common buzzword, but is it really a growing industry and should you consider becoming a part of it? In this article, an expert explains the potential profits and pitfalls along with the hardware needed to carry out this unique form of manufacturing.
By Donna Bibber President
Micro Engineering Solutions
Charlton City, MA
Micromolding is a niche technology used to mold tiny features on larger parts and microscopic parts for micro devices. In its early stages of growth in the mid-1990s, micromolding began to thrive as a viable new niche technology for a select few micromolders.
Although there is no standard definition of micromolded components, most define them as having one or more of these attributes:
- Fractions of a plastic pellet or weighing fractions of a gram,
- Having wall thickness of less than 0.005" (0.127 mm),
- Having tolerances from 0.0001 to 0.0002" (0.0025–0.0050 mm),
- Having geometry seen only by using a microscope.
Many new products exist today because of the introduction of micromolding. In the early stages of US-based micromolding, only two or three micromolding machines were available that were considered small enough shot-size machines. Today, there are more than 30 micro-molding machines available.
A worldwide study in 2005 of Micro Manufacturing by the World Technology Evaluation Center Inc. (WTEC; Baltimore, MD) in association with NIST, NSF, DOE, and the Naval Research Academy, among others, described the value of micro manufacturing to the US in these terms:
- It is an enabling technology for the widespread exploitation of nanoscience and nanotechnology developments, bridging the gap between the nano and macroworlds.
- It is a disruptive technology that will completely change our thinking as to how, when, and where products will be manufactured, e.g., on-site, ondemand in the hospital operating room, or onboard a warship.
- It is a transforming technology that will redistribute manufacturing capability from the hands of a few to the hands of the many—micromanufacturing becomes a cottage industry.
- It is a strategic technology that will enhance the competitive advantage of the US through reduced capital investment, reduced space and energy costs, increased portability, and increased productivity.
The medical market represents a huge potential with OEMs spending $4 billion annually on new products, most of which are micro-sized components with microsized features that lend themselves to minimally invasive techniques for orthopaedic, cardiology, and neurology applications. Medical device companies comprise a bulk of micro-molded components and assembly opportunities. Instruments for these procedures must have components that are smaller, lighter, with superior physical properties than those that are available using conventional machining and molding methods.
Micro-molding systems can minimize the risk of failure to OEM component manufacturers by finding solutions to these challenges:
Modeling micro components. There remains a limited understanding of the fundamental physics at the microscale. It is necessary to develop reliable and scalable models. Although there has been work performed in this area, much more research is required to perfect the modeling software, materials specifications, reliability models, and simulation models for micro-component manufacturing.
Simulation models such as mold-flow analysis, finite-element analysis, and design-of-experiments analysis are commonly used to troubleshoot micro-molding processes. Because the features and parts are microscopic, these types of analysis are critical as the fast, visual-feedback loop of human intervention used in conventional or macromolding is not feasible or economical in micromolding.
Part handling/static. Part handling can be challenging given the sizes of micro-molded components. Many micromolders use edge-gated runners to carry their parts from one location to another and many are used as part of the automation process. If parts cannot be edge-gated, customized end-of-arm tooling, vacuum systems, reel-to-reel take-up equipment, and blister packs are utilized accordingly. Static electricity is a micromolder nightmare. Parts as small as dust can easily be lost if proper grounding of part-collection systems, robotics, packaging, and inspection systems are not performed. Static guns, wands, air curtains, and grounding mats are commonplace in micro-molding facilities.
Production of micro-mold tooling requires the scaling of processes involved. Many technologies exist including laser machining, chemical milling, EDM/wire EDM, ultrasonic machining, ion machining, CNC machining, electrochemical machining, and photochemical machining.
All methods are useful, but X-ray lithography and ion-beam machining are not commonly used in mold making today. They will gain in importance as micro machining grows in worldwide acceptance. A drawback to high-speed milling is that high heat is generated that may promote micro cracks in the components leading to steel failure.
Rutgers Engineering (New Brunswick, NJ) is studying the effect that preheating materials with laser beams prior to and during the milling process has on reducing fracture from the high-speed spindle action.
Ion-beam machining is another technology that can create some of the smallest micro structures. This is a process by which submicron features and 100-nanometer radii can be created by bombarding a solid tool blank with ion beams. This technique can also be coupled with chemicals to make the removal process faster.
EDM, a widely used process for stress-free machining, is slow and requires two steps; one to make the electrode and one to burn the electrode in the tool. Wire EDM is a fast process that can create radii down to 0.0005" (0.01 mm) with its smallest wire, but for the most part it is limited to through shapes and 2-D machining.
Many times, the best choice is a combination of these micro-machining methods. The challenge facing many micromolders and moldmakers is how to pull all these machining techniques together to create a robust steel mold that will withstand the 30,000 psi (207-MPa) injection pressures and wear over time with abrasive materials. Micromolds can have core pins as small as 0.003" (0.076 mm), less than the diam of a human hair, but they still need to depreciate over 5–7 years as do conventional molds. Consequently, the maintenance costs for micromold spares must be considered in the program's capital justification program.
There is no question that the barrier to entry in micromolding is sourcing the micromold itself. It is an absolute requirement to have an extremely accurate micromold with very little core-to-cavity error across the parting lines of the mold halves. In many cases of thin-walled components and/or tiny feature components, the cavity-to-core error can be no more than 5µm.
Without this degree of accuracy, the parts will not fill uniformly and premature polymer freeze off will prevent parts from filling the entire cavity. Since there is very little shrinkage relative to the size of the components during and after molding, what you have in steel is what you will have in plastic.
An unconventional approach with conventional equipment is one way to create tiny geometric shapes in steel. One out-of-the-box approach to create 3-D shapes is being developed at Folch Laboratory at University of Washington (Seattle) called micro-tunable molds. These microstructures, many of which are impossible to produce with other processes, have been produced in this manner. The basic concept is creating features (cavities with elastomeric membranes) that can be individually deformed (or tuned) by selective pressure application. Many replica cavities can be created from one mold with this process.
Properly sized machines. It isn't uncommon to see micro-molded components that have sprue and runner systems amount to 75% or more of the total shot. Many molders try to enter the micro-molding market using larger machines. Scaling the machines to the proper shot size is important to achieve proper process control with microscopic components. Molding parts in a conventional manner is not recommended due to thermal stresses inflicted on the small shots inside the barrel.
The WTEC study of micromanufacturing revealed a broad spectrum of processing methods and equipment now in use in micro-scale manufacturing. It was observed that many micro-scale components/products are being manufactured using existing macroscale or reduced-size precision manufacturing processes and equipment. This approach is, however, exposing the difficulties related to the smallest unit of amount of material removal, addition and forming per cycle, and achievable precision.
Metrology/inspection techniques. Inspection techniques in measuring very small micro-molded parts require customized vises, tweezers, and fixturing. Inspecting steel measurements usually provides a flat, robust surface that can be measured with non-contact means or in some cases contact measurement. These same surfaces that make the molded components can be used to certify the dimensions much closer in repeatability and reproducibility than attempting the same corresponding measurement in the micro-molded components. It is not uncommon for the first article inspection to consume as much time if not more time than the entire moldmaking and micro-molding project combined.
Validation. Component manufacturers and micromolders require similar inspection machines with identical fixtures to validate tolerances in microcomponents. Gage R&R from client to vendor requires duplicate fixturing and exact methods of inspection technique to repeat the results to near-micron tolerances. Only a few sources of inspection equipment exist that are capable of measuring to submicron tolerances. Extremely clean, HEPA-filtered air-controlled rooms are necessary for repeatability of measurements. It is also common in macro components and specifically with medical devices to insist on 1.33 Cpk or better with respect to performance to drawing dimensions and tolerances. When the gage R&R and operator R&R are considered 1.33 Cpk on 0.0001" (0.0025-mm) tolerances may be a mathematical impossibility.
Testing/standardization. In the macroworld of conventional molding, a lot of R&D has gone into tensile testing, Izod impact bars, and spiral flow molds—all great tools for prediction and theory on mold flow and physical properties of macrocomponents. These standards are not applicable to micromolding because materials are subjected to an extra element of shear and extreme injection pressures and velocities in micro molding changing the viscosity of the materials. All the rules of general-purpose molding and predictability values are also changed.
In addition, the polymer-skin properties of many materials are of critical importance since there is virtually no wall thickness to these parts. Several laboratories and universities are working with ASTM and NIST to investigate alternative standards for micro-sized parts that would be useful for prediction, verification, and validation for microcomponent manufacturing.
Future microtechnology trends. A glimpse into the future of micromolded components reveals a new level of possible manufacturing techniques. Current micro-molded components are made using top-down methods by taking steel away to make the tooling that creates the parts. New methods in the future will build micro-molded components from the bottom up—building them layer upon molecular layer. Manufacturing will be done in all new factories featuring cleanroom-gowned personnel, robotic handling of components, and untended manufacturing.
Examples of microfactories were found by the WTEC study at the Fraunhofer Institute (Stuttgart, Germany). Two approaches to microfactories are in development. One is a modular approach with multiple sections for assemblies and the other is a table-top approach for miniature to microcomponents.
This article was first published in the December 2006 edition of Manufacturing Engineering magazine.