Micro Molding Challenges
4/3/12: As miniature molded parts approach micro or nano in size, several challenges exist to molding them in a production environment.
This paper explores some of these challenges such as polymer property changes induced by shear stress through near micron-sized gates, humidity control for extremely small shot sizes, and integrating macro to micro technologies to produce near micron level geometry in precision, micro mold components.
New mold manufacturing technologies exist to remove variation from micromolded features and further work is being done with scientific micromolding process control, both of which are critical to long-term micromolding process capability.
Introduction
The need for micro-injection molded parts or micromolding, we must consider the need for tiny
microscopic features as well as parts. MEMS (Micro Electrical Mechanical Systems) has created a
need for tiny circuitry insert molded or insert and overmolded onto a silicone chip. MEMS are
devices containing extremely small mechanical elements, which are usually integrated together with
electronic processing circuitry. To define this type of mechanism, so small that one cannot
distinguish them with the naked eye. These devices are one market that is taking shape for
microscopic plastic moldings. Our conventional injection molding methods do not fit in theory or
practice for parts with part weights of 0.00012 grams. These problems require us to rethink how we
manufacture in conventional methods. Micromolding is not conventional molding and this paper
describes the differences and challenges that micromolders face and offers some corresponding
challenges that have been overcome and some that have yet to be overcome.
Other growth markets for micromolding include:
LED’s-Front-lighting & Back-lighting for Liquid Crystal Displays, DVD Pickup Lenses, Microfluidics- Lab on a Chip, Bio Drug Discovery i.e.: Micro Titer Plates w/ 384 Well, 1536 Well, Future 3456 Well for Pharmaceutical, Drug Discovery, DNA Analysis
Medical- Ophthalmic Lenses, facial Implants, microsurgery catheters, syringes, Pipettes & Cuvettes, Dosing Pens, Blood Analysis, Subcutaneous, Intravascular, Biodegradable PLA Applications
Electronics-Micro sensors, Insert Connector Applications, Miniature Electronics, Fiber-optic Ferrules & Connectors
Nanotechnology- Nano sized parts are 0.00000001 inch and are in the near future for molding and the products that are moldable today are in the Micro range or 0.00001 inch range in size.
Challenges exist for micromolders because there is very little information that exists in theory and practice for micromolding. Many micromolders that have developed tricks and tips either are not willing to freely share knowledge due to confidentiality reasons or will not share knowledge since it is their edge over their competition, which keeps micromolders busy when other molders are not busy.
From the initial design stages, manufacturing and plastics engineers alike are paving the way by developing new technologies or using existing technology in a way they never did before in order to manufacture micromolded components. For example, even dimensioning micro parts on drawings or CAD, usually in thousandths of an inch must be converted to millimeters and/or microns, a paradigm shift for many engineers.
Other challenges include:
•Inspection techniques – Inspection techniques in measuring very small micromolded parts requires customized vises, tweezers, and fixturing. Non-contact optical inspection systems are readily used for inspecting thin walls and microscopic features on miniature parts. 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 select few sources of inspection equipment exist that are capable of measuring to sub-micron tolerances and extremely clean and heap-filtered, air controlled rooms are necessary to the environment needed for repeatable measurements.
•Material Plasticizing -Extremely consistent raw material plasticizing has recently been re-introduced by injection molding machine manufacturers. Precision plungers down to 0.060 inch have been added as a second stage to the micromolding process to create plasticating independence from injection. Because of this phenomenon, each pellet of raw material sees the same L/D screw processing window and travels the full length of the screw. This creates less material gassing into the mold and allows the screw to act independently thus it can do the work it was intended to do, which was to create uniform material melt conditions.
•Material feeding – Most micromolding screws diameters are in the 14mm range and have not gone smaller due to the material resin pellet sizes. Any screw diameter smaller than this today cannot transition the pellets properly over the flights of the screws causing screw slippage and subsequent degradation.
•Shot size generally too large for micro parts – Micromolded parts exist today that are 520 parts per single plastic pellet in size.[1] These parts, .00012 grams in weight have runners that are thousands of times larger than their parts. This phenomenon holds true due to their runner having to register some known percentage of shot size in the micromolding machine so that some level of process control from shot weight to residence time.
•Static electricity Issues – Static electricity is a micromolders 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 micromolding facilities.
•Part handling – Part handling can be challenging given the sizes of micromolded 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.
Theory & Definitions
Things behave substantially differently in the micro domain. Forces related to volume, like weight and inertia, tend to decrease in significance. Forces related to surface area, such as friction and electrostatics, tend to become large. And forces like surface tension that depend upon an edge become enormous. It takes awhile to get one’s micro intuition sorted out. — An ant carrying many times its weight or a water bug walking on the surface of a pond are just two manifestations of this different micro world.[2]
Largely debated often misrepresented, true micromolding is defined as molded parts that are fractions of a pellet, and/or weighing fractions of a gram, and/or having wall thickness of less than 0.005”(0.127mm), and/or having tolerances of 0.0001-0.0002” (0.025-0.050mm). In most cases, micromolded components are a combination of some or all of these attributes.
Description of Tooling Equipment And Processes
The following methods are used for creating microscopic features or entire cavities in micro molds:
“Top Down” Methods “Bottom Up” Methods
Laser Machining Genetic Code
EDM-WEDM Complexity Theory
Ultrasonic Machining Self-assembly
Ion Machining Biological Cell
CNC Machining Proteins
Chemical Milling DNA and RNA
Photochemical Milling
Electrochemical Machining
“Top Down” and “Bottom Up” are commonly used terms in micro and nanotechnology to describe the way in which the steel is constructed. For example, for most micro features in steel, “top down” methods or methods that reduce the size of the steel during machining. For “bottom up” methods, “growing” steel from the molecular level up is necessary to make features as small an sub-micron and nano sized.
EDM (Electrical Discharge Machining) is a commonly used method of producing micromold
cavities. EDM is a non-contact method that uses an electrode (mirror image of resulting
steel) made from a electrically conductive material (in most cases graphite or copper). EDM cavities
are produced in heat-treated, hardened steel producing tight-tolerance components without the need
to heat treat after machining, causing no distortion to the workpiece. EDM works well for very hard
materials and the electrode never touches the steel, it’s a non-contact machining method, therefore
no stress is introduced to the steel during this process.
In some cases, the electrode is produced and burned in the same setup using an orbit path of the electrode to wear away the electrode shape, allowing the component setup to stay in the same machine, therefore not violating the tooling setup by removing components and placing them back in again. Wire EDM (WEDM) is very similar to conventional EDM, except the wire acts as the electrode. Wires down to 0.001” (0.025mm) can burn through shapes straight or tapered at 30 degrees. In either case, EDM is a versatile method for manufacturing micromold cavities. Its challenges include longer machining times and scanning electron microscope surface quality. In most cases this surface quality is not necessary.
Description of Micromolding Equipment And Processes
Many choices exist today for micromolding machines. In the past year, several have been introduced using the screw over plunger technology. The following table is a select few of these machines available for micromolding:
| Battenfeld | Boy | Ferro-matik Milacron | Nissei | Nissei | Nissei | Sodick | Sodick | SodickVertical | Sumitomo | Toshiba | |
| Model | Micro-system | 129-11 | Baby-plast | HM7-C | HM7 Denkey | AU3 | TR05EH | TR20EH | TR20EHV | SE18S | EC5-0.1A |
| Tonnage | 5.6 | 3.1 | 6.6 | 7 | 7 | 3 | 5.5 | 22 | 22 | 18 | 5.5 |
| Inj Cap (oz) | 0.04 | 0.21 | 0.13 | 0.21 | 0.21 | 0.11 | 0.16 | 0.16 | 0.16 | 0.21 | 0.24 |
| Inj Cap (cm3) | 1.1 | 6.1 | 3.8 | 6.2 | 6.2 | 3.1 | 4.5 | 4.5 | 4.5 | 6.2 | 7.1 |
| Inj Psi | 36,259 | 26,106 | 38,425 | 25,388 | 25,600 | 28,814 | 39,841 | 39,841 | 32,429 | 29,000 | |
| Screw Dia. (in) | 0.55 | 0.55 | 0.55 | 0.55 | 0.55 | 0.6 | 0.6 | 0.6 | 0.55 | 0.55 | |
| Inj Piston Dia (in) | 0.197 | n/a | n/a | n/a | n/a | 0.47 | 0.5 | 0.5 | 0.5 | n/a | n/a |
| Max Daylight (in) | 11.8 | 11.8 | 5.51 | 8.7 | 8.7 | 11.8 | 13.8 | 13.8 | 9.8 | ||
| Footprint (in) | 73x81x95 | 90x38x63 | 35x18x26 | 68x24x63 | 54x20x61 | 75.5×24.4×57.4 | 97×33.9×86.3 | 51.2×43.3×112.9 | 89x24x57 | 71x31x55 |
References
Gode, S. M., Orman, T. P., & Carey, R. (1967). Writers and writing. New York: Lucerne Publishing.
MacDonald, S. E. (1993). Words. In The new encyclopedia Britannica (vol. 38, pp. 745-758). Chicago: Forty-One Publishing.
Wilson, J. C. (2001). Scientific research papers. In Stewart, J. H. (Ed.), Research papers that work (pp. 123-256). New York: Lucerne Publishing.
http://mmadou.eng.uci.edu/LivingBook/webter7.htm BioMems, Dr. Marc J. Madou
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