Over the past two to three decades, metal injection molding (MIM) technology has been producing increasingly complex parts for a wide variety of different industries.
With the growing demand for high-quality parts with low geometric distortion and high material properties, the MIM process has been integrated into production lines in a wide range of industries, such as automotive, medical devices and cellular phone manufacturing. High power density areas such as modern automotive engines, powertrain and mechanical engineering require small, compact mechanical systems that offer greater potential for innovation and higher productivity, while complex MIM parts offer advantages such as reduced assembly times for mass-produced products such as laptops and cell phones.
In order to meet the evolving needs of the industry in terms of technical requirements and specifications, it is important to explore the scope for growth in the accuracy and efficiency of MIM process equipment. Current limitations such as mechanical and chemical properties of the part and optical appearance limitations are caused by the following:
1) Uneven shrinkage (geometric distortion)
Uneven mixing of powders and raw materials;
Density fluctuations caused by injection and/or first degreasing stage;
Uneven temperatures in the sintering furnace.
2) Chemical decomposition and discoloration
Inaccurate process gas management;
Redeposition of binder in the second degreasing stage;
Residual sinter furnace contaminants.
In addition to these technical limitations, the competitive market environment shifts cost pressures to part manufacturers. As a result, more profitable and technologically sophisticated production equipment and materials are critical to moving the MIM industry forward.
In addition to the high cost of sourcing raw materials such as fine-grained metal powders, polymer binders and off-the-shelf injection molding materials, high-temperature sintering is one of the main cost drivers in the MIM process. The investment and operating costs of a degreasing sintering furnace are key to the competitiveness of a coral part manufacturer. In addition, the selection of the most suitable furnace type for the specific production situation is a prerequisite for success in the MIM industry.
II. Applicability of different furnace types
Irrespective of tailor-made, highly specialized systems, the majority of sintering furnaces on the market can be divided into cyclic vacuum furnaces and continuous atmosphere furnaces. Brown parts after injection molding and catalytic/degreasing contain residual polymers, and both types of furnaces offer the option of removing the polymers thermally.
On the one hand, if relatively large parts with identical or similar shapes are to be mass produced, it is appropriate to utilize a continuous atmosphere furnace. In this case, short cycle times and high sintering capacities result in favorable cost yields. However, in small and medium-sized production lines, this type of continuous atmosphere furnace with a minimum annual capacity of 150 – 200 t, high input costs and large size is not economical. Moreover, continuous atmosphere furnaces require longer downtime for maintenance, reducing production flexibility.
On the other hand, cyclic vacuum sintering furnaces have outstanding degreasing and sintering process control technology. The previously mentioned limitations, including geometric distortion and chemical decomposition of finished MIM parts, can be effectively addressed. One solution is through a sophisticated gas control system, where a laminar flow of process gases flushes out the volatile binder material. In addition, by reducing the capacity of the hot zone, the vacuum furnace provides excellent temperature uniformity up to lK. Overall, the vacuum furnace’s excellent atmosphere cleanliness, adjustability of process parameters, and low part shock make it the technology of choice for producing high quality parts, such as medical devices. Many companies are faced with fluctuating order profiles and need to produce parts of different shapes and materials, and the low investment and high cycle time flexibility of the vacuum firing furnace will work in their favor. Running a group of vacuum furnaces not only provides a surplus of production lines, but also allows different processes to be run simultaneously.
However, some specialized vacuum sintering furnaces with these technical advantages are limited by the small available capacity. Their poor input/output ratio and low energy efficiency means that the cost of sintering a part may more than offset the cost savings in other MIM process steps.
Vacuum Furnace Requirements for the MIM Industry
An important factor in the ability of a vacuum sintering furnace to operate cost-effectively is economical process gas and power consumption. Depending on the gas type, these two cost elements of the sintering process can account for up to 50% of the total cost. In order to save on gas consumption, an adjustable gas flow partial pressure pattern must be implemented while keeping the degreasing and sintering process free from contamination. To reduce power consumption, heat loss is reduced by creating hot zones with optimized heating elements. In order to achieve these design points and to keep development costs within reasonable limits, a modern, resource-efficient vacuum sintering furnace utilizes hydrodynamic calculation tools to find the most optimal air and heat flow patterns.
Depending on the weight of the sintered parts and the residual polymer content, the binder will collect to varying degrees on peripheral parts (e.g. exhaust pipes, pumps and hot zones), which can lead to long downtimes for manual cleaning and routine maintenance. With a net material weight of 400 kg (furnace volume > 1000 L) and a binder content of 3% to 4%, up to 15 kg of polymer will be removed in the degassing phase. Even so, most of the outgassing (>95%) should be collected at a specific condensation point (e.g. binder collector or wax separator). Door-to-door cycle times will increase by more than 2 hours due to decontamination and manual cleaning efforts. In this way, an inefficient, poorly designed vacuum sintering furnace will reduce operational performance by 15%. MIM manufacturers will consider more advanced equipment with automated cycle cleaning systems to minimize maintenance and keep unplanned breakdowns at a very low level.
Fast-growing MIM companies need to be able to flexibly plan their production capacity and respond quickly to changing market demands, but long lead times for production equipment will slow the company’s growth. Often, equipment manufacturers start production when they receive an order, rather than stocking key components and critical raw materials in their warehouses. When a MIM company receives an additional rush order, the 4-8 month lead time for new equipment can be a bottleneck for the MIM line. Until recently, leading vacuum furnace manufacturers introduced the concepts of lean manufacturing and standardized production. As an example, Harber Metal has reduced the lead time of their newly introduced vacuum sintering furnace to less than 3 months through modularization and standard parts design.
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