MIM and the quest for timeless precision

It is generally acknowledged that the Swatch Group, and specifically its manufacturing arm ETA SA Fabriques d'Ebauches, is among the MIM technology leaders. The plant in Grenchen, Switzerland is considered the largest MIM operation in Europe, and it makes more than just Swatch products. It produces MIM parts for other companies in markets such as machinery, automobiles, medical and dental equipment, and many more. 

The Swatch Irony watches (Figure 1) with their MIM stainless steel cases and bezels are frequently referenced when talking about excellent and/or successful applications of PIM technology. Yet when the Irony was being readied for market in 1994, there were two manufacturing processes in the final running: MIM and massive forming. The reasons why MIM won the race are worth looking at even today. 

Steep Launch Ramp 
Nicolas G. Hayek, cofounder, president, and ceo of the Swatch Group, set an ambitious product launch goal for the Irony, so it was decided to develop both manufacturing methods in parallel. Swatch engineers started research on MIM in 1990 by developing a proprietary binder system for the injection molding, thermal debinding, and vacuum sintering of stainless steel powder. The feedstock worked well in production, but the system required thermal debinding cycles of up to four days. The BASF system with its much faster catalytic debinding process was adopted in 1993. 

After extensive molding experiments, flow analysis with different tool and runner designs, and optimization of the gate location, the first small-scale production of Irony cases started in Grenchen during the summer of 1994. Production equipment consisted of a single injection molding machine, two laboratory catalytic debinding furnaces, and a batch vacuum furnace. 

The first Irony case was a close match to the by-then very successful Swatch plastic watch. The cases, which weighed about 3g, were to be made of 316L stainless steel and polished to a mirror finish. 

Since Swatch pays close attention to health and safety issues such as nickel allergy, the cases had to meet very strict corrosion and homologation tests. And like its plastic counterpart, the Irony had to be Swiss-made, shock-resistant, and water-resistant to at least 30m. Dimensional tolerances on the case would be ±.05 percent for critical areas like the bezel and the movement container, ±2 percent for small and/or "nonfunctional" areas, and ±.3 percent for outer dimensions like case width and length. 

Both manufacturing methods were seen as capable of producing the Irony cases in 1994. The question was, which would scale up faster and meet the product launch and cost targets? In the massive forming process, pictured in Figure 2, a stainless steel slug was forged by three blows into a blank that was then further formed by rotary forging at medium temperature. Since burrs from stamping had to be removed and postforming surface quality was too poor for polishing, six-step surface grinding was necessary. Attachment holes for the band were laser cut in the flanges. CNC machining of the bezel seatings and the movement container inside the case came next, followed by drilling of the winding stem hole. Finally, the case was polished and marked by laser. 

Technical issues with massive forming included the following: 

• An unexpected amount of wear and tear on forging tools increased costs. 

• Grinding cycle times were higher than expected due to dimensional variations of the forged blanks. 

• Deburring the laser-cut holes proved difficult to automate. 

• The sharp edges from grinding were difficult to polish without leaving traces. 

The MIM sequence is shown in Figure 3. About 32g of acetal-based precompounded feedstock was injected into a single-cavity mold. Burrs at the parting line were removed manually and the green part's gate location was machined. Catalytic debinding of the green part was performed in a batch furnace, and the brown part then was sintered to 99.2 to 99.5 percent theoretical density in a batch vacuum furnace. No secondary operations apart from polishing were required on the surface of the MIM "blanks." The bezel and container settings and the winding stem hole had to be CNC machined. 

The main technical problems with MIM included the following: 

• Powder-binder separation at the surface and the resulting macro-pores increased polishing time. 

• Dimensional scattering due to feedstock inconsistencies created CNC machining problems. 

• Ovalization of the case during sintering resulting from the eccentrically located battery hole put parts out of tolerance. 

• Lack of a rapid cooling option on the batch furnace caused longer-than-expected sintering cycles. 

Two Winners 
Solutions for all the MIM problems could be found, but the massive forming problems proved much more difficult. After a three-month optimizing phase, it was found that the absolute best-case scenario for massive forming yielded nearly the same manufacturing costs as MIM. 

With MIM production up and increasing in scale, development of massive forming was refocused from stainless steel to aluminum where the major problems could be solved easily and quickly. Full-scale production of competitively priced high-quality Swatch Irony watches started in 1995--with stainless steel cases made by MIM and aluminum cases made by massive forming. 

The launch of the Swatch Irony was very successful and a further surge in demand followed. MIM production at ETA's Grenchen plant was increased. Currently, ETA runs 16 Arburg and Netstal molding machines with clamping forces of 40 to 120 metric tons. Production systems are highly automated, and most of that technology comes from the Swatch Group's own automation specialists, SMH Automation. 

The injection molding and machining operations are designed to run lights out, 24/7. Total annual sintering capacity is more than 200 metric tons. Debinding and sintering can be batch or continuous. 

Dimensional tolerances after sintering are generally held to ±.3 percent for dimensions greater than 12 mm, but this depends on the production line and amount of fine-tuning possible--both factors being influenced strongly by cost limits. Since tolerances of ±.015 mm on some watchcases with 35-mm diameters still cannot be achieved by MIM, secondary CNC machining is still required, as well as fully automated final polishing. 

ETA Advancing 
Apart from continuous processing know-how, the never-ending development of new Irony watches has given ETA engineers a lot of experience in MIM tool design. There are seven different Irony cases and three different bezels currently in production (Figure 4). Accommodating varying production quantities, molds are single-, double-, and four-cavity (Figure 5). 

Inquiries from other companies wanting to make MIM parts have been rising in recent years. The Grenchen plant has supplied watchband clasps and links along with bezels and cases to Swatch Group companies Lascor and Ruedin. Products developed with machinery, automotive, medical, and dental customers have used materials such as low-alloy steels, tool steels, soft magnetic alloys, titanium, and nickel-free stainless steels. 

ETA has identified three factors as the keys to rapid scale-up to high-volume production. First is having in-house detailed technical knowledge of compounding, injection molding, debinding, and sintering. Solid experience in mold design and injection molding of plastics is the base for this development. Second, monetary resources must be available for the molding, debinding, and sintering equipment required to scale up. Third is consistent, top-quality precompounded feedstock with advanced debinding characteristics. Employing this strategy let ETA focus on processing challenges, which in turn enabled the switch to continuous processing. 

ETA today has three key focus points. First, costs for secondary CNC machining and polishing have to be further reduced. The molder plans to do this through improved dimensional control during sintering and better surface quality after. Second, but of equal importance, is reducing the current average of 1 percent postsintering scrap caused by molding defects and low brown strength after secondary debinding. The final point is of particular interest to a volume producer like ETA: Downtime on furnaces for maintenance has to be reduced by eliminating the soiling from secondary binder and reaction products. ETA's current tests say all these targets are achievable.