Furnace Hot Zones and Charging Equipment for Metal Injection Molding (MIM) Process
Metal Injection Molding (MIM) combines the technologies of plastic-injection molding with metal-powder sintering in order to produce small metal parts with the same shape complexity and efficiency as plastic-injected parts. The MIM industry continues to grow at a significant rate of about 15% annually. MIM parts are used in many diverse products including watch timepieces, textile, orthodontics, firearms, automotive, medical, dental, electronic devices and generally wherever precision dimensional products are required.
The production process is comprised of four main steps. Initially, feedstock is prepared by mixing fine metal powders with different types of thermoplastic binders. The feedstock is then injected into an engineered mold to form the “green” unsintered part. Next, the binder is removed chemically and thermally, and finally the parts are sintered at high temperature in vacuum or partial gas pressure to densify the metal parts.
MIM Sintering-Furnace ConceptsThe furnace requirements in the MIM sintering process are very demanding with respect to temperature range – 1200-1600°C (2192-2912°F) – temperature uniformity, environment (from vacuum over partial pressure of different gases to pressurized cooling), cleanliness (debinding, catalytic agents, no carbon contamination allowed) and furnace durability resulting from process binder contamination and production requirements that ultimately impact process costs (high yield, dense packing of the parts).
Since MIM is a steadily growing market, furnace manufacturers have developed unique concepts for their sintering furnaces. Most of the concepts utilize molybdenum (moly) all-metal hot zones, molybdenum heating elements and molybdenum carrier and handling hardware.
In the sintering process, both batch and continuous furnaces are used depending on the production volume, product size, material and quality requirements. Continuous furnaces may be pusher or walking beam by design. Both concepts are used to automate for increased production. Initially, continuous furnaces were used by the automotive industry, but today an increasing demand for these furnaces comes from MIM part producers requiring greater production.
In order to be able to have an integrated and automated production flow, a sintering furnace is often combined in the same production line with a continuous debinding oven. In this configuration a catalytic binder-removal process can increase productivity.
Some batch-furnace processes include a combined debinding and sintering operation in the same furnace chamber, which requires lengthy furnace cycle times reducing sintering productivity. To increase furnace sintering capacity, a separate debinding operation may be useful to improve production.
Sintering occurs either in vacuum, nitrogen, hydrogen, argon or mixtures of these atmospheres. Continuous furnaces are operated with a small overpressure, while batch furnaces often operate in partial pressures down to 10 mbar or less in order to support the thermal debinding.
Molybdenum Products in MIM Sintering FurnacesMolybdenum materials are very successfully used in high-temperature sintering furnaces for MIM. These materials are of fundamental importance for the operational performance of the furnace and the economy of the process. The introduction of MLR – a lanthanum-oxide-doped molybdenum – to the MIM industry has improved the performance of heating elements, hot zones and product carriers. Figure 1 shows furnace components typically made of refractory metals.
In continuous furnaces, serpentine moly and tungsten heating elements consisting of solid rod or braided-wire cable are used. The heating elements are secured by molybdenum hooks or held in ceramic shoulders. In order to reduce the carburizing effect from binder residuals, tungsten rod may be preferred for some applications.
The loading assembly in continuous furnaces (Fig. 2) consists of a bottom carrier component and several stacked intermediate plates that are spaced by alumina or moly rods. The bottom carrier may be a moly plate of various thicknesses typically 0.5-0.75 inches (12.7-19.05 mm) for a pusher continuous furnace. Ceramic may also be used as pusher plates. These assemblies, or boats, are often designed having holes for better gas flow. Figure 3 presents two examples of MIM carrier devices. An important consideration is that carrier-plate thicknesses may be reduced when using MLR.
In batch furnaces the all-metal moly hot zone consists of the shield pack, heating elements and hearth assembly. Some furnace concepts use a MIM box inside the hot zone in order to achieve better control of the gas flow and greater temperature uniformity during sintering. At the same time, the MIM box might also work as a multi-tier rack with supports for product-carrier plates. The design and material selection of the MIM box should maintain dimensional integrity to provide for equal gas distribution during operational cycles.
MIM furnace manufacturers and companies who engineer and manufacture replacement hot zones have experience in design of MIM boxes that will perform and operate efficiently in all operating conditions. Figure 4 shows a MIM box that was installed in an existing furnace as an OEM replacement. The rigid double-wall structure is constructed using MLR material, which enables the MIM part producer to increase the load volume of the furnace and reduce sintering costs.
Charging plates either slide into a multi-tier-rack MIM box or are stacked between spacers. Each plate thickness is between 0.04-0.125 inches (1-3.17 mm), and alumina powder may be used to prevent MIM parts from adhering to the moly plates.
The evaporation of residual binders inherent in the “green” unsintered parts may diffuse hydrocarbon gases that could be retained in the sintering-furnace hot zone. The high sintering temperature and possible contamination from binder residuals lead to wear of the hot zone and the carrier support devices. MIM box and carrier performance will be improved with use of more advanced materials like MLR molybdenum alloy.
An example of a redesigned hot-zone replacement using alternate materials like MLR is shown in Figure 5. The deformation of the charge plates (left) had reduced the loading capacity of the furnace, having a negative impact on the dimensional tolerance of the MIM parts. A new hot zone and charge carrier were designed, fabricated and installed by PLANSEE (right). The implementation of the new hot zone into the existing furnace took place without any changes to the vessel or power supply.
MLR - An Advanced Doped-Molybdenum MaterialTo meet the ever-increasing demands of the thermal-processing industry, it became necessary to develop specific advanced molybdenum alloys and use tungsten- and tantalum-based alloys, wherever required, because of their enhanced high-temperature properties.
PLANSEE originally developed the MLR molybdenum alloy, which is delivered in a recrystallized condition. Lanthanum-oxide particles and a sophisticated thermo-mechanical dispersion treatment during production lead to an extremely stable microstructure with elongated grain structure and jagged grain boundaries. As a result, the creep resistance is excellent to temperatures of 1850°C (3362°F), and MLR retains its ductility after exceeding these operating temperatures.
MLR is employed whenever temperatures of 1250°C (2282°F) are exceeded, whenever dimensional stability at high temperatures is required or whenever brittle fracture after recrystallization must be avoided. It is the ideal material for mechanically loaded furnace parts such as plates, boats, rails, grids and racks.
The micrographs in Figure 6 demonstrate the impact of various temperatures up to 2000°C (3632°F) on the microstructure of pure moly, TZM and MLR. Pure molybdenum is already recrystallized after holding at 1000°C (1832°F) for one hour and shows heavy grain growth at higher temperatures. TZM is more stable and is recrystallized after one hour at 1500°C (2732°F). MLR is delivered in a recrystallized state and does not show fundamental microstructural change even after exposure to 2000°C (3632°F) for one hour. This provides the most stabilized grain structure and improved sag-resistant properties.
Summary - MIM-Box Component SelectionThe demanding conditions and heavy wear in MIM sintering furnaces require the replacement of furnace components such as heating elements, complete hot zones, MIM boxes and carrier plates. At that time, there is an opportunity to evaluate the condition of the existing hot zone and furnace hardware for alternate design and material solutions. More advanced materials like MLR may be selected for longer and better service performance when replacement hot zones are required or for critical boat and product carriers. IH
For more information: Dr. Bernd Kleinpass is global business segment manager for PLANSEE USA LLC, 115 Constitution Blvd., Franklin, MA 02038; tel: 508-918-1335; fax: 508-553-9762; e-mail: firstname.lastname@example.org; web: www.plansee-usa.com
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