SME TECHNICAL PAPER (note: figures removed)

Society of Manufacturing Engineers
One SME Drive, Dearborn, MI 48121
313-271-1500
www.sme.org

Boney A. Mathew

Mathson Materials Technology, Inc.
1845 Thunderbird Street
Troy, MI 48084, USA

ABSTRACT

Traditional applications for metal injection molding or ceramic injection molding have been small net shaped parts exhibiting design and economic benefit over wrought or cast components. A new water based agar binder system for metal and ceramic injection molding has been developed that offers the ability to produce large thick parts in range of 1 to 2 kg which are competitive with those produced by investment casting. Other significant advantages of this system are a clean and rapid debind, and the ability to mold into soft tooling. As a result, savings are achieved not only by eliminating machining and other processing steps but also by reduced development and inventory costs. This technology is applied to a variety of automotive applications such as, EGR System, Exhaust System, Oxygen Sensors, Electrical Components, Engine Components, etc. Many of these applications require demanding mechanical properties. For an example, the average tensile properties of the 17-4PH stainless steel alloy is compared against wrought and cast properties, tensile properties were essentially equivalent to wrought and exceed typical cast properties. Due to the relative low molding pressure and temperature required for water based agar binder, soft tooling may be employed in place of hardened tools for production runs less than 10,000 parts. Primary materials using water-based agar binding systems are Stainless Steel, Alumina, Zirconia, Steatite and other metal alloys. This injection molding process allows economical competition with large net shaped process techniques such as Investment Casting.

The importance of this paper is that it allows designers and engineers to utilize Ceramics and Metal Alloys Injection Molding Technology to combine several machined or cast components into one large cost effective system eliminating several manufacturing steps.

INTRODUCTION

The injection molding for metal and ceramic powders are production technologies that are experiencing rapid growth worldwide. This growth is mainly in the USA, Europe and Asia. Ceramic Injection Molding (CIM) and Metal Injection Molding (MIM) is a manufacturing technique for making complex and precision components. Ceramic and Metal Injection molded components compete in the market place against parts made using various processes. Investment Casting, Slip Casting, Machining and Powder Forging are examples of the competitive processes. An important part of CIM and MIM is the binder in the feedstock technology and removal of binder from the part after injection molding. One binder technology currently established is the polyacetal-based binder. The polyacetal based binder systems uses a catalytic debinding utilizing Nitric Acid as depolymerization Catalyst.1. New development in binder technology such as aqueous based agar binder systems for metal and ceramic offers the ability to produce large thick parts in the range of 1 to 2 kg which are competitive with those produced by Investment Casting.

THE AGAR BASED AQUEOUS BINDER SYSTEM

This binder is based on agar, a polysaccharide derived from seaweed, which is water-soluble. Metal or ceramic powder is mixed with the water, agar and a gel strength-enhancing agent having the form of a borate compound such as calcium borate, zinc borate, etc. to form feedstock pellets that can be injection molded into components2.

The use of such gel forming materials in combination with a gel strength-enhancing additive substantially reduces the amount of binder (typically 2-3 wt %) needed to form a self-supporting article. Thus complex components produced from agar with gel strength enhancing borate compounds imparts high strength and deformation resistance. It is critical to have adequate part green strength in order to handle components without damage after Injection Molding Process.

Typically, 55% by volume (18-wt. %) water is added to the mixture, which performs the dual function of being a solvent and a carrier for agar containing mixtures 3. The mixture may also contain a variety of additives, which can serve any number of useful purposes. For example, dispersants may be employed to ensure a more homogeneous mixture. Biocides may be used to inhibit bacterial growth in the molding compositions, in particular if they are to be stored for a long period of time4.

The metal powder used is typically under 20 micrometers and is made by gas or water atomization is shown in Figure 1. The feedstock with a uniform composition of high solid loading is desirable in the production of Injection Molded parts. The higher solids loading feedstock imparts lower shrinkage when the molded parts are dried and fired, facilitating dimensional control and reducing tendency for cracks during sintering process, in turn higher yields of acceptable product with lower scrap rates.

Figure 1. Water and Gas Atomized 17-4 PH Stainless Steel Powders

The solid loading of the feedstock is near 92-wt % (61 vol. %), the balance consisting of water. The feedstock is produced by passing mixture composition directly through a twin screw extruder and cutting the extrudate into pellets as it exits the die. Alternate method of feedstock production is by mixing the mixture composition in a sigma blender and then shredding the blended mixture into pellets.

The feedstock is injection molded to the desired shape, typically around 85o C melt temperature with a molding pressure in the range of 150 psi to about 800 psi. At injection molding temperature of 85o C the feedstock is relatively fluid and is easily injection molded into a net shape part. Compared to plastic injection molding, water based agar system is molded at lower melt temperature and injection pressure.

Once the feedstock temperature is cooled in the mold to near room temperature, the green part can be ejected from the mold. Typically injection molding cycle time is in the order of 30 seconds depending on the part size. The viscosity of the feedstock is similar to that of unfilled Nylon 6 during injection molding process, as shown in Figure 2. Due to the relative low molding pressure and temperature required for water based agar binder, soft tooling may be employed in place of hardened tools for production runs less than 10,000 parts. 3D systems have developed SLA rapid tooling fabricated directly from electronic files of a part. A polymeric tool can be grown in a matter of hours at a relatively low cost. Using water-based agar feedstock prototype parts can be molded in a SLA tool.

Figure 2. Aqueous Based Agar Binder 316L Stainless Steel Viscosity Comparison with Unfilled Nylon 6 Polymer

The green part is left open in the ambient air for approximately 1 hour for drying. No separate debind step is required, as is necessary for traditional MIM feedstock’s. For water based feedstock debind step is incorporated into first one (1) hour cycle of the sintering process. During the sintering process, the binder pyrolizes in the sintering furnace and carbon is removed during the sintering cycle. The sintering is typically performed in Hydrogen medium for metal alloys for carbon control, and air is typically used as a medium for ceramics sintering. Sintering temperatures in the range of 1300o – 1400o C are typically employed for stainless steel alloys, and sintering temperatures in the range of 1600o – 1700o C are typically employed for Ceramic material such as Alumina. Batch or continuous sintering furnaces can be employed for debinding/sintering based on production volume requirements. The schematic of ceramic and metal injection molding process using the water based agar binder system is shown in Figure 3.

Figure 3. Schematics of Ceramic and Metal Injection Molding Process using Water Based Agar Binder System

MICROSTRUCTURAL AND MECHANICAL PROPERTIES

The mechanical properties in 17-4PH Stainless Steel is defined by uniform formation of martensite, which requires control of carbon during sintering process. Due to the low level of binder in water based agar feedstock, as well as its relative ease in removal during sintering process, carbon can be tightly controlled. The typical specification of 17-4PH stainless steel is below 0.07 wt % carbon. Carbon Analysis in sintered components with a cross sectional thickness of 19 millimeters manufactured from water based agar system; carbon level is measured at .01% wt %. The core and near surface of the component show a uniform formation of martensite, demonstrating the tight control of carbon achieved through the use of water based agar feedstock system (5).

Many of the automotive applications require demanding mechanical properties. Figure 4 lists the average tensile properties of the 17-4PH stainless steel alloy6. The samples consisted of two batches, UnHIPed and HIPed, which were heat-treated to the H1025 condition. The 9.52 millimeter thick tensile bars employed in the study had an average sintered density of 98.5 + 0.5%. The tensile properties of water based agar system 17-4PH stainless steel were equivalent to wrought and exceeded typical cast properties. It was found that HIPed samples had improved tensile properties due to 1-2% reduced porosity compared to UnHIPed material (7).

Figure 4. Average 17-4PH H1025 Stainless Steel Tensile Data

DIMENSIONAL CONTROL

Parts molded from water based agar system; there is an approximate 2% shrinkage during drying and 17% shrinkage during sintering process. This shrinkage is repeatable resulting in control in dimensional tolerances to about 0.5% to a 3-sigma production yield. Tighter tolerances can be obtained for critical dimensions through the use of setters during the sintering process. Figure 5 shows the current dimensional tolerance limits of Ceramic and Metal Injection Molded Components (8).

Figure 5. Current Dimensional Tolerance Limits for Ceramics and Metal Injection Molded Components

AUTOMOTIVE APPLICATIONS

The water based agar binder system is available in a variety of stainless steel alloys such as 17-4PH, 316L, 410 etc. as well as tungsten and nickel based super alloys such as GMR 235, Hastelloy X etc. Additionally, ceramics such as Alumina, Zirconia and Steatite can be manufactured using water based Agar binder system.

Figure 6 shows a sintered automotive turbo charger wheel made of the nickel alloy GMR2359. Figure 7 shows balanced valve shaft for EGR system using 316L stainless steel. Figure 8 shows automotive exhaust and electrical components manufactured from Alumina and Steatite Ceramics. Figure 9 shows advanced research concepts by Mathson Industries on Exhaust Manifold technology using metal, ceramic and metal or ceramic and plastic Injection Molding solution.

Figure 6. Sintered Automotive Turbocharger Wheels made of the GMR235 Nickel Alloy

Figure 7. Balanced Valve Shaft for EGR System using 316L Stainless Steel

Figure 8. Automotive Exhaust and Electrical Components Manufactured from Alumina and Steatite Ceramics

Figure 9. Advanced Exhaust Manifold Technology using Metal or Ceramic and Metal or Ceramic and Plastic Injection Molding Solution

CONCLUSION

Ceramic and Metal Injection Molding offers the ability to produce large thick parts which are competitive with those produced by Investment Casting. The mechanical property of Injection Molded stainless steel alloy is equivalent to wrought and exceeds typical cast properties. Due to low molding pressure and temperature required for Injection Molding, soft tooling could be used for low production runs. Ceramics and metal alloys Injection Molding can combine several machined or cast components into one cost-effective product eliminating several manufacturing steps.

Producing large parts such as exhaust manifolds in high volumes can create a chain effect. For example the automotive exhaust manifold annual usage in North America exceeds more than 17 million units and each manifold weighs several pounds, the total annual usage of the material will be several million pounds. This will allow raw material suppliers to produce powders at a reasonable cost. The feedstock price will decrease, as the powder price is the largest cost component of the feedstock. This will then increase the potential for part suppliers to produce parts at a competitive price. This will enable PIM technology to produce other applications, which currently cannot be produced due to prohibitive feedstock cost.

ACKNOWLEDGEMENTS

The author would like to thank Cathy Mathew and Jane Salvati of Mathson Industries, Inc. for their valuable suggestions and assistance with this project.

AUTHOR’S BIOGRAPHY

Boney A. Mathew is President & CEO of Mathson Industries, Inc., Mathson Materials Technology, Inc. and Mathson Intellectual Technology, Inc., Troy, Michigan, USA. Mathson Industries is a leading supplier of Ceramics, Metal Alloys, Elastomers, and Plastics Injection Molded Systems and Assemblies, globally. Mathson Materials Technology is a leading materials supplier of Ceramics and Metal Alloys for the Powder Injection Molding Industry. Mathson Intellectual Technology provides Patented New Products and Technology, globally.

Mr. Mathew’s background includes eighteen years in the automotive and plastics industry.

Prior to starting Mathson Industries, Inc., he was a Marketing and Technical Executive for Nyltech North America - Joint Venture of Rhone-Poulenc - France and Fiat - Italy.

He also served as Plastics Manager for Teleflex, Inc.’s Automotive, Aerospace and Medical Company. Mr. Mathew is the inventor and co-inventor of several patents, including “Method of Making Fluorocarbon Coated Composite Braided Hose Assemblies”, “Hose End Fitting Assembly” and “Expanded Fluoropolymer Tubular Structure, Hose Assembly and Method for Making Same”.

He is a member of the Society of Plastics Engineers, Society of Automotive Engineers, APMI and ASEI. He has published and presented twelve technical publications on various plastics, ceramics, metals and composites at various international technical conferences and is the co-author of “Passage to India, A Comprehensive Study on the Indian Automotive Industry”.

Mr. Mathew earned Bachelor of Science and Master of Science degrees in Plastics Engineering from the University of Massachusetts, Lowell, Massachusetts USA.

REFERENCES

1. German, R., & Bose, A., “Injection Molding of Metals & Ceramics”, Metal Powder Industries Federation, 1997, pp. 209-210.

2. Fenelli et al., “Gel Strength enhancing additives for agaroid-based Injection Molding Compositions”, United States Patent Number 5,746,957, May 5, 1998, p.4.

3. ibid., p. 6

4. ibid., p. 6

5. Lasalle, J., “Net Shape Processing of Metals Using and Aqueous – Based Injection Molding Binder”.

6. ibid., p. 3

7. ibid., p. 4

8. German, R., opcit. p. 288.

9. Lasalle, J., opcit., p. 5.