Micromolding: Small parts, big challenges

Comic book fans might remember The Atom, a superhero that could shrink to subatomic size at will. If his forte had been plastics processing instead of crime fighting, however, he might well have become a micromolder.

Micromolding is a growing part of the plastics processing scene, driven chiefly by the needs of the medical market and the constant demand for ever-smaller electrical parts.

“Demand for micromolding is absolutely increasing,” said Stuart Kaplan, president of Makuta Technics Inc., a molder of so-called microparts.

DEFINING ISSUE

But the process is fraught with challenges, beginning with attempts to define it. “Although there is no universal agreement, the definition for a micropart that is widely used in the industry is a fraction of a plastic pellet, weighing fractions of a gram,” said Donna Bibber, president of Micro Engineering Solutions, a micro manufacturing firm. “They’re parts in which you need magnification to see features or details.”

Micromolders stress the distinction between true microparts and simple miniature parts. “Micromolded parts weigh one gram or less,” said Scott Herbert, president of micromolding company Rapidwerks Inc. “By contrast, miniature parts are between one to four grams.”

According to Bibber, the smallest micropart presently on the market weighs 0.00012 grams, which is 520 parts per pellet, and is designed for minimally invasive surgical procedures. “The part is literally a dust speck,” she said. “In fact, current machinery is not yet capable of making it. Material has to be added to give process control over the shot size in order to manufacture the part.”

Bibber identified micro-EDM, micro- milling and laser machining technologies as perhaps the best methods for making micro molds even smaller in the future. In addition, other processes developed for micro-electrico-mechanical systems (MEMS) and semiconductor markets will enable feature sizes and tolerances for nanometers, she said.

MATERIAL WORLD

As the example of the 0.00012 gram part suggests, micromolding presents a number of challenges to traditional molding concepts and traditional ways of doing business in a molding facility. Simply put, the old manufacturing rules don’t always apply in the microworld.

At the outset, an important decision facing micromolders is which material to use. “Micromolding can change the viscosity of a polymer as it is being squeezed into such a small area, and the material has to be forgiving,” said Bibber. Insiders recommend plastics with a high melt-flow index and low viscosity at processing temperatures, such as polyamide imide, polyether imide PEEK and PPS. “PEEK and reabsorbable polymers are particularly popular in micromolding because the materials are costly and using less material is critical to development costs,” Bibber said.

The relationship between micromolders and their resins is a story in itself. “Some of the specialized resins used for micromolding can cost between US$3,000 and US$22,000 per pound,” said Bibber. Given such prices, the good news is that microparts don’t consume much material. “With some of our parts, a single bag of resin will last an entire year,” said Makuta’s Kaplan. “For this reason, though, material suppliers don’t like us very much.”

TOOLING IS CRITICAL

An even greater challenge relates to tooling. “Tooling is by far the enabling component in the micromolding process,” Bibber explained. “Because microparts can be dust speck size, the tooling that makes them, such as core pins and ejector pins, are literally the size of a human hair.”

Finding moldmakers capable of producing such precise tooling in North America can be difficult. “Ninety-nine per cent of our molds are obtained from Makuta’s sister facility in Japan,” said Kaplan. “The problem in North America is that the equipment needed to do this represents a substantial investment for a moldmaker, while the market that needs to be covered is still not very big.”

One North American machine tool builder that does make molds suitable for microparts is Makino. The company is also tasked with making small features on large parts, such as a die to work with material 0.005 inches (0.13mm) thick or less, according to Lee Richmond, Makino’s micromarket showroom manager.

Typically, micromolding involves two- to four-cavity molds, Bibber said, although the smallest parts involve use of a single cavity. For parts that are the size of a pencil tip and larger, a 24- or 32-cavity mold can be used, she continued.

Hot runner systems designed for micromolding are available from suppliers such as D-M-E Company. D-M-E’s Stellar hot runners, for example, are externally heated and can be used with both commodity and engineering grade resins. The Stellar system is available with standardized multi-nozzle assemblies in configurations of two, three, four, six, eight and 12 drops.

WANTED: THE RIGHT MACHINES

Once the tooling requirements have been satisfied, another hurdle is that only certain injection molding machines are suited to the manufacture of microparts. According to Michael T. Werner, technical sales manager with Toshiba Machine Co., microparts are not recommended on machines larger than 30 tons because of the difficulty in controlling such small shot sizes. “The injection unit capabilities on molding machines with clamping force of less than 30 tons vary from as little as a few grams to 20 grams,” he said.

Also, long residence times and material degradation would occur if micromolding were performed with normal size barrels, which would be oversize for these applications. “If you are trying to make a part that weighs one-third of a milligram, you don’t want a machine that has 150 tons of clamping force and 200mm screw barrel,” Makuta’s Kaplan explained.

Injection molding machines suppliers who have developed or refined machines for micromolding in the tonnage range of 30 and under include Arburg, Battenfeld, Boy, Demag Plastics Group, Nissei, Sumitomo and Toshiba.

An additional issue during injection molding of microparts is positional accuracy. “Conventional machines open and close molds with hydraulic systems that could hold plus or minus a couple of thousandths of an inch,” said Rapidwerks’ Herbert. “Positional accuracy on micromolding machines has increasingly improved for year to year,” he said. “Today, machines such as the Rapidwerks Micro-Molding system work within a few microns, which is critical for repeatability of movement in opening and closing molds, because they are delicate tools and you don’t want molds to slam shut.”

A boost to the micromolding process came through the development of Engel’s X-Melt system, which eliminates the need for a gas accumulator to achieve high-speed injection. The X-Melt uses the energy stored in the pressurized melt to fill the mold without moving the injection ram, and can produce parts from 0.4 to 0.8 grams with wall thickness down to 0.1mm, according to Engel.

PASSING INSPECTION

The challenges don’t end when a micropart has been made, however. Inspection to determine whether it’s the part the molder wanted is critical, and requires precise metrology. In this respect, however, molders can face difficulties even before the examination begins. “For U.S. molders, there is a learning curve in going from inches to metric or microns,” Micro Engineering’s Bibber said. “Additionally, the gauging of the tolerances is a challenge; you have to make sure that what you’re measuring is what your client is measuring.”

Quality control equipment suitable for microparts tends to come from suppliers outside the plastics industry. For example, optical system manufacturer Carl Zeiss IMT Corp. recently made available the F25 measuring machine. According to the company, the F25 has a probe sensor that can be a 0.12mm diameter sphere, which is small enough to access many micromachined part features. Also, the machine delivers a guaranteed measurem
ent uncertainty not to exceed 250 nm.

ROBOTS & REGRIND

Robots are important to the micromanufacturing industry, but there can be problems in obtaining suitable machines. “At Makuta, we make our own automation,” Kaplan said. “It’s difficult to get automation suppliers interested in micromolding because they don’t see it as a big market.”

One supplier that does make robots for handling microparts is Fanuc, which has a line of six-axis tabletop units designed for microparts. The units come with an iRVision integrated robot vision system that tells the robot where the part is. If required, the robot can pick up the part and present it to a camera to check for specific features.

As with so much else in the micro- world, the realities of handling regrind can be different, also. “Obtaining a grinder that can handle micromolded parts can be difficult,” Kaplan said. Kaplan identified Matsui as one of the few auxiliary suppliers to manufacture grinders for microparts.

Kaplan also noted that most of the medical parts that necessitate micromolding in the first place cannot be made from regrind material. This can create an issue around what to do with the regrind, since selling it is unpractical. “Regrind purchasers are only interested if a company has a truckful of material, which can take a micromolder years to accumulate,” he said.

RESOURCE LIST

Arburg Inc. (Newington, Conn.);www.arburg.us; 860-667-6500

Dcube (Montreal); www.dcube.ca; 514-272-0500

Patton Plastics (Mississauga, Ont.); www.pattonplastics.com; 905-568-1133, Ext. 106

Battenfeld Canada (Mississauga, Ont.);www.smsk.com; 905-670-9384

Boy Machines Inc. (Exton, Pa.);www.boymachines.com; 610-363-9121

Barway Plastics Equipment Inc. (Vaudreuil-Dorion, Que.); www.barway.ca; 450-455-1396

Plascom Plastic Machinery Inc. (Toronto); 416-491-8450

Carl Zeiss Inc. (Peabody, Mass.);www.smt.zeiss.com; 978-826-7909

Elliot-Matsuura Canada Inc. (Oakville, Ont.); www.elliotmachinery.com; 905-829-2211

Demag Plastics Group (Strongsville, Ohio);ww.dpg.com; 866-491-1045

Stephen Sales Group (Markham, Ont.); 905-940-5577

D-M-E of Canada Ltd. (Mississauga, Ont.);www.dme.net; 800-387-6600

Engel Canada Inc. (Guelph, Ont.);www.engelglobal.com/na; 519-836-0220

Fanuc Robotics Canada (Mississauga, Ont.);www.fanucrobotics.ca; 905-812-2300

Makino (Auburn Hills, Mich.);www.makino.com; 888-625-4664

Single Source Technologies (Mississauga, Ont.); www.singlesourcetech.com; 905-565-6886

Makuta Technics Inc. (Shelbyville, Ind.);www.makuta.com; 317-642-0001

Matsui America Inc. (Chicago, Ill.);www.matsuiamerica.com; 847-290-9680

Micro Engineering Solutions LLC (Charlton City, Mass.);www.microengineeringsolutions.com; 774-230-3459

Nissei Plastic Industrial c/o En-Plas Inc. (Toronto); www.en-plasinc.com; 416-286-3030

Rapidwerks Inc. (Pleasanton, Calif.);www.rapidwerks.com; 925-417-0124

Sumitomo Plastics Machinery (Norcross, Ga.);www.sumitomoPM.com; 770-447-5430

Plastic Machinery Inc. (Newmarket, Ont.); www.pmiplastics.com; 905-895-5054

Resource Polytec Inc. (Vancouver, B.C.); www.resourcepolytec.com; 604-454-1295

Toshiba Machine Machinery Co. (Markham, Ont.); www.toshibamachine.ca; 905-479-9111