The development of custom solutions for the mixing of MIM feedstocks
While many MIM companies choose to buy commercially available “off the shelf” feedstocks, a significant proportion still manufacture their own—or use both routes. Motives can range from a desire to use a proprietary binder composition for specific applications to simple cost savings. What is critical is that in-house-produced feedstocks are manufactured to the highest quality and with excellent batch-to-batch repeatability.
In this article, Tim Simpson, Sales & Technical Director of Winkworth Machinery Ltd., reviews the process of mixing, extruding and pelletising MIM feedstocks and introduces a specially-developed machine that combines these three operations.
What is MIM?
MIM (Metal Injection Moulding) is a process that merges two established technologies: plastic injection moulding and powder metallurgy. This hybrid technology combines the shape-making complexity of plastic injection moulding with the material versatility of powder metallurgy (PM).
Metal Injection Moulding process
Unlike standard powder metallurgy, which can achieve only 80–90% of its theoretical density, MIM results in 95–98%. This means one can achieve close tolerances and reduce costs by producing small, complex parts over high production runs.
One of the many secrets to successful Metal Injection Moulding production is, however, to have absolute batch-to-batch consistency in your feedstock. If this is not achieved, then defects can occur in moulding as well as distortion in sintering.
The composition of feedstocks for metal injection moulding applications varies according to the de-binding process being used and the requirements of the material to be manufactured. For a feedstock to perform correctly, the distribution of the metal powder particles is critical. To ensure that the binder material coats each individual particle, the feedstock must be mixed and distributed effectively.
When it comes to mixer selection for this process, a kneader-blade (double-arm) type Z-blade mixer is often used. These mixers are renowned for gently manipulating particles throughout the mixing chamber and, after a period of time, achieving a very homogeneous distribution of particles.
Z-blade kneaders are designed for strength and to cope with the stiff phases that mixtures often go through.
The coating of particles typically requires a binder formulation that is solid at room temperature, needs modest levels of heat to melt, and is able to coat metal powders homogeneously. The design of a mixer must, therefore, include the ability to create consistent heat throughout the mixture. This is achieved using a thin-walled jacket on the mixing chamber; heat transfer oil is then circulated inside the jacket, conducting through into the product to achieve a homogenous temperature across the mixing chamber, with the blade design effectively distributing the heat throughout the batch.
Overview of the feedstock mixing process (compounding)
The metal powders are very heavy, but remain free-flowing. The binders used are also free-flowing. Both of these dry, free-flowing materials respond when heated and mixed. As the metal particles heat up, some of the heat will melt the binder. All particles will randomly respond to the transfer caused by contact with the mixing chamber and with each other during the mixing process. This heat transfer melts and liquefies the binder, allowing the surface area of all the powders to be fully coated. The mixing process continues and the mixture then becomes a crumb as the binder melts.
At higher temperatures, the mixture becomes a smooth dough (Fig. 3). If the temperature is allowed to go too high, some binder ingredients may degrade, affecting the ratios of binder to metal powder and ingredients may degrade, affecting the ratios of binder to metal powder. Therefore, temperature control throughout the mixing process is critical. Once the mixture is created, the right consistency is achieved.
The mixture is now ready for discharge. Due to the rapid nature of cooling, which is prevalent with a metal paste due to its thermal conductivity, discharging the mixture also requires precise temperature control, often featuring a controlled temperature reduction of the mass before discharge. Such cooling is essential if immediate pelletisation is to be conducted; otherwise, the mass would be too soft and agglomerate after cutting.
Creating MIM feedstock pellets
After the material has been discharged from the mixer, it needs to be processed into a suitable form. The high-pressure injection moulding machines typically used for MIM are very similar to plastic injection moulding machines. Pellets are loaded into the hopper/chamber and as they are pressed forward along the length of the machine, heat is applied to the injection barrel. The solid pellets re-melt to create an injectable material.
Once inside the injection barrel, heat transfer must occur to achieve a remelt. If particle sizes vary significantly, a risk of poor flow or bigger particles being insufficiently heated occurs and will adversely affect injecting. In order to avoid this, it is essential to achieve a consistent particle size and shape. This consistency of particle size then allows the parameters on the injection moulding machine to be established. Typically, a charge of pellets will require a determined residency period in the barrel to achieve liquification before injecting. Once remelting has occurred, the injection into the component cavity is initiated.
With tilt discharge kneader mixers, before the pelletising of the feedstock can begin, the hot feedstock mass from the chamber is tilt discharged to a cooling table, followed by a fracturing or smashing stage before passing through a granulator. This process requires additional equipment with high-wear parts, while the breaking up of the block of hardened lumps can deliver random results. Quality control, productivity and cost savings are, therefore, best achieved with a hybrid mixer/extruder/pelletiser combined into one system.
The preferred approach is to create pellets directly from the mixing process. These pellets need to be consistent in diameter, length and density, although a consistent pelletised product with such a temperature-sensitive mixture creates a number of engineering challenges.
Discharging through a number of small apertures at a constant rate is necessary and as the product is discharged through the discharge gate or “die plate,” an immediate cropping or cutting of this extrudate of mixture is necessary to create pellets. The typical pellet size is normally 6–8 mm in length. The temperature sensitivity of the mixture requires extreme control of the mixing chamber environment, the discharge environment, the die-plate environment, the cutting environment and the post-cutting separation and cooling environment. These parameters must be managed with a great deal of precision and understanding of the mechanical behaviour of the mixture in order to achieve free-flowing pellets.
These pellets can then be transported in trays, barrels, or through distribution pipework to feed the injection moulding machines.
Winkworth, on instruction from one of the world’s leading MIM companies, has designed and supplied a number of machines for this purpose (Figs. 5 and 6 below).
The earliest machines with automated controls and temperature management had discharge via an extruder screw and a rotary knife against the die plate; this produced an effective result. It was later discovered, however, that a reduced heat transfer to the mixture occurred as the mixer emptied over time. To manage the process during discharge, manual intervention on the temperature of the heating oil was required.
This intervention would vary according to the alloy and powder type being processed, the binder, and the ratios of the two. If the chamber is allowed to cool too much, a significant back pressure is generated during discharge as the mixture’s flowability reduces, leading to solidification. In these circumstances, a detrimental effect on the machine can occur due to the pressures created and the power applied during extrusion.
Innovation in MIM feedstock production
Winkworth has introduced a number of innovations to minimise the risks of high back pressures, increase discharge rates, avoid manual interventions during discharge, maintain separation once cropped into pellets and ensure the continued separation of pellets during cooling—particularly the avoidance of rebinding. Meeting these challenges has resulted in a two-stage process:
An extrusion discharges, unpressurised, into an integrated perpendicular chamber. This chamber is temperature-controlled using an independent heating oil recirculation system.
A vertical hydraulic ram piston is designed to uniformly press the mixture through the die plate. Electrically heated die plates are temperature-controlled.
Mixture discharge rates are about 200 kg in 45 minutes once composition parameters are understood.. Pelletising productivity eliminates the need to monitor mixer chamber temperatures during discharge cycles.
For good pelletising and discharge, the slurry should resemble soft, but not sticky, plasticine. More fluid will cause ripping and irregular pellet shapes. Pellets will be hard to discharge if the knife cutter reciprocation rates are too high. High back pressures may distort the die plate or other mechanical arrangements.
In Winkworth’s system (Fig. 7), a hydraulic ram perpendicular to the mixing chamber harnesses gravity to drop pellets vertically. The pellets are sliced by a fast-reciprocating multi-knife blade. Pellets then fall to the vibrating conveyor below. The conveyor must quickly move pellets away from the drop zone because discharged pellets can rebind if they touch others due to heat retention. The pellets are cooled and separated by a spiral-cooled vibratory elevating conveyor. Additionally, the installation achieves a small footprint. This discharge management method significantly increases mixing time and client productivity.
In addition to mixing and pelletising issues, MIM feedstock solidifies when cooled, thus cleaning between batches or formulations in a cold state (room temperature) is impracticable since the material forms a cold-set structure like brittle toffee—and sticks! The mixing machine’s design must allow for the removal of the blades, the extruder screw, and the mixing chamber and side walls for cleaning at high temperatures.
Cleaning at these temperatures could injure operators and hot working gloves and overalls are required. Critically, the machine must be designed for quick disassembly due to temperature transfer and heat losses. The machine must be openable to allow scraping of the hot mixture, blade removal, and extruder screw removal to an adjacent bench. Hot blades and extruder screws can be carefully cleaned. Several hydraulic-assisted clamps that release on the operator’s command and a pivoting non-drive end plate that opens 90° provide access. Drive stub shafts drive blades and the extruder screw, allowing tool-free removal. The non-drive end plate cleverly retains these axially throughout operation.
During the cleaning procedure, a jib crane on the machine helps handle these hot and heavy (over 20 kg each) component parts. This attention to detail maximises the machine’s mixing and discharge performance and enables for safe, fast, and thorough cleaning during recipe changes, avoiding inter-batch contamination.
Winkworth’s Basingstoke (UK) factory includes mixing test facilities for MIM or CIM feedstocks in 2 litre, 7 litre, and 25 litre models. For “in-house” development or production, we can lease such mixers for long-term testing.
While Z Mixers are one type of mixer used for stiffer products, there are many other types of mixers to choose from. Z blade sizes range from 100litre to 1,800litre. Early discussion with a mixer specialist can save time in finding a solution.
Twin screw and shear roll extruders are also utilised in MIM. Compounding is efficient with the twin screw extruder. The raw material is intensively sheared and transported forward in a heated barrel by two screws. At the end, it is extruded and can then be pelletised. The process is continuous.
A more recent compounding technology is shear roll extrusion. Two parallel heated rolls with helical grooves are rotated and the premixed, homogenous powder-binder feedstock pre-mix is fed on one roll. The slot between the rolls can be adjusted. It is usually 5 to 7 mm wide, so that the entire mass is forced into the grooves. The driving roll is rotated a little faster than the roll carrying the feedstock to increase the shear intensity. Due to the groove geometry, the mass is transported towards the outlet at the end, where it is slit and cut by a rotating wheel.
Metal Injection Moulding is increasingly being used by a diverse range of industries and demand is undoubtedly growing. The ability to produce low-cost, high-integrity components will continue to drive this technology. Developments in mixing and discharge management, such as those described above, can make a significant contribution to the future adoption of MIM approaches. Similar approaches can, of course, be utilised in Ceramic Injection Moulding (CIM) and other Powder Injection Moulding (PIM) applications.