The continuous quest for weight reduction has focused our technological developments on Magnesium alloys.
Magnesium is the eighth most abundant element in the Earth’s crust and the fourth most common element in the Earth. Magnesium is the lightest of all structural metals. A unique blend of low density, high specific strength, stiffness, high electrical conductivity, high heat dissipation and absorption of vibration makes the magnesium best suitable to support daly activities of humans in almost every aspect of human’s activity. God-given to sustain a higher quality of life.
When combined with easy machining, molding, forming and very simple recycling, magnesium can be the material of the future. Compared with plastics, which had huge growth over the last 50 years, magnesium improves on many qualities, without downsides. Magnesium does not adversely affect wildlife and humans, and is readily available.
Semi-solid metal injection molding is a molding process that involves filling a mold with the metal in a partially molten state in which globules of solid are homogeneously dispersed in the liquid. The four semisolid metal casting processes thixomolding, thixocasting, rheocasting and stress induced melt activation are very complex and highly maintenance intensive. The cold chamber die-casting, the hot chamber die-casting and other processes such as vacuum die-casting, did not fulfill end user expectations for quality, simplicity, energy savings, operator’s safety, economy and environmental sustainability
This is where we can come in and further develop and improve the thixomolding process. We start with mechanically stressed chips and feed these to chips to a pre-conditioning unit and pre-heating processor called MAXIfeedstock™. The chips are heated with precise temperature control of heat with precise temperature control in our vertical injection molding machine called MAXImolding!™.
We call this process solid-to-solid (S2S™) molding because we start with solid chips, produce semi-solid slurry and end with solid net-shape parts out of mold. We use very small amount of Argon to prevent oxidation of chips.
When chamber is cold (cold chamber), the die casting machine superheated metal (above 650°C, 1.200°F) is ladled into cooled shot sleeve between shots and plunger is not immersed in molten liquid. When chamber is hot (hot chamber), die casting machine metal is superheated (above 650°C, 1.200°F) and plunger is immersed in molten liquid. Both machines use blanketing (cover) gas like SF₆, SO₂, N₂/0,2% SF₆, N₂/0,25% R-134a or dangerous, corrosive N₂/1,5% SO₂ that produces 1.177kg of SO₂ per year.
Using 400 Nl/h of N₂/0,2% SF₆ causes 950 t/a of CO₂ equivalent pollution. Using 400 Nl/h of N₂/0,25% R-134a causes 41 t/a of CO₂ equivalent pollution to be added to the Earth‘s atmosphere. 400 Nl/h of dangerous, corrosive N₂/1,5% SO₂ produces 1.177kg of SO₂ per year. Sulfur hexafluoride (SF₆) has global warming potential 23.900 times greater than CO₂ and an atmospheric lifetime of 3.200 years and only one pound of SF₆ has the same global warming impact of 11 tons of CO2.
Many governments around the world either have limited or banned usage of SF₆ completely. Our S2S™ process uses a very small amount of Argon (24Nl/h) to prevent oxidation of chips. This reduces the costs up to 80%.
S2S™ – New Magnesium Semi-Solid Metal Injection Molding Process
Thixomolding is very similar to plastic injection molding in machinery, tooling and process fundamentals. Designers with experience in plastic injection part design are typically familiar with designing Thixomolded products.
Dr. Frank Czerwinski has been working for a decades in the research fields of „Semi-Solid Metal Metallurgy“ and improvements of „Magnesium Injection Molding Machines“. In 2008 he published his work in a book „Magnesium Injection Molding“. In 2.10 Origin and Progress of Magnesium Molding Dr. Frank Cherwinski wrote: „Metal injection molding represents a hybrid, combining features of both fundamental routes (chipping and melting, A.S., see the classification in Fig. 2.28). Similar to the rheo-route, it consists of a single step, but as with the thixo-route it starts from a solid feedstock. One-step processing is possible due to the specific structure of a coarse particulate feedstock, called chips, granules or pellets, created during their manufacturing. The cold-deformed structure of mechanically comminuted chips is similar to a thixoforming billet, produced by the stress induced melt activation (SIMA). Similarly, the structure of rapidly solidified granules with fine dendritic forms is the same as that created in thixoforming billets by magneto hydrodynamic (MHD) stirring or grain refining. As a result of the unique microstructure, the particulates transform into thixotropic slurries under the influence of heat. It is of interest that this microstructure, deliberately created during the manufacturing of billets for thixoforming, is obtained here as a side-effect of comminuting bulk ingots into small particulates. Although injection molding belongs to the family of semisolid technology, some phenomena that take place during processing at high temperatures are unique, and so mechanisms cannot be directly adopted from other semisolid techniques. In addition to benefits common to the semisolid family, the injection molding technique offers unique advantages: environmental friendliness with no SF₆ requirement, no melt loss due to a closed processing system, an ability to mold thinner parts and more complex shapes, and the capability to apply the unique ways of slurry distribution to the mold“ (Magnesium Injection Molding, ISBN-13:978-0-387-72399-0, 2008).
Table 1 – Historical semi-solid process steps are compared and presented in the table below.
|PROCESS||FEED MATERIAL||CONDITIONS IN THE PROCESSING BARREL||INVENTIVE STEPS COMPARISONS|
|Rheocasting (Flemings ‘544)||Semi solid metal slurry produced in separate slurry producer||Constant temperature no shear||2-step process – requires slurry producing machine|
|Thixocasting (Flemings 1976)||Partially remeleted SSM slugs||Constant temperature – No shear||2-step (requires SSM billets or Ingots)|
|Thixomolding (Bradley ‘589)||Metal pellets||Heating and shear||1- step|
|Rheomolding – Cornell wo95/34393||Liquid metal||Cooling and shear||1-step|
|S2S™- MAXImolding!™ (Stone ‘386)||Comminuted chips||Fast Heating – No shear||1-step|
The adopted process does not require screw rotation. It is not necessary to shear the chips, produce semi-solid slurry that contains spherical solid particles in the melt. The generation of thixotropic slurry that contains spherical solid particles in the melt can be done in the thermal mass reactor under the sole influence of heat. Precise temerature control of chips fed under gravitation from the top to the bottom of the reactor is only what is required.
MAXImolding!™ – Vertical Magnesium Semi-Solid Injection Molding Machine
MAXImolding!™ is an advanced development of magnesium injection molding machines. It is based on the thixomolding technology (injection molding of a thixotropic, semisolid magnesium alloy like AZ91D) and avoids disadvantages of thixomolding with enviromentally un-friendly, high energy consumption hot and cold chamber High Pressure Die Castings (HPDC) machines.
Most products made from magnesium today are being made by die casting. Magnesium casting was left behind in the technology curve. However new semi-solid metal injection molding machines offer much improved processing capability. For example, MAXImolding!™ does not require melting pot of overheated molten magnesium during casting that requires SF₆ or SO₂ cover gas (GHG pollutant) and does not need energy to maintain molten metal in overheated molten state. Rather solid magnesium particulates can be used. Chips are fed into magnesium main processor that (by adding heat) converts the solid magnesium into semi-solid slush of material. This material structure flows like water and yet is at temperatures often 200°C below current Magnesium processing temperatures resulting in significant energy advantages. This semi-solid magnesium alloy is injected into a permanent mold, very similar to plastic molding processing. Our process is fully enclosed, solid to solid part injection molding machines can be operated in ordinary manufacturing plants.
The simplicity of the process makes it easy to operate in any regions of the world without need for extensive training. The vertical orientation saves floor space and improves material flow. Fully automated part removal is easily integrated by simple robotic devices. Magnesium solidification in the mold releases captured heat via an innovative patented cooling process and we recycle this heat by pre-heating input magnesium chips. This energy savings alone allows part costs to be competitive in the market compared to any other magnesium process known today. Carbon credits generated by this technology could be suitably marketed and sold as a carbon shares on the open market to those who need to operate plants with high CO₂ output.
We took the above development even further. We took horizontal Magnesium Injection Molding Machine and oriented it vertically. We replaced barrel and screw with new high thermal mass reactor in a form of a revolver cylinder. We drilled eight smaller processing cavities for AZ91D-chips entering from the top, four on each left and right side and larger one in the middle of cylinder for injection piston. Heaters are placed around the cylinder and nuzzle from the top to bottom for uniform and precise heating. Lower platen with mold opens downwards.
With replacement of the extruder (screw + barrel) with a high thermal mass reactor assemby including heaters, stuffer cylinders/stuffer rods, injection cylinder/injection piston, injection slurry collection chamber with direct acting valves and nuzzle, the horizontal magnesium injection thixomolding machine changed into a vertical Semi-Solid Metal Injection Molding Machine (SSM-IMM) called MAXImolding!™. Much more efficient and less maintenance intensive.
In MAXImolding!™ the stock material (AZ91D chips) is then re-heated to the desired forming temperature and injected into the mold while in the semi-solid state. The semi-solid metal mixture (slurry) is produced on demand and injected directly into the mold. This greatly reduces the total cycle time and improves part quality while at low energy content.
Our team of senior professional engineers has over 100 years of combined experience in industrial environments. We are dedicated and motivated to impact on the way magnesium products are used, and reduce the energy content of the final product.
Table 2 – MAXImolded parts have several advantages compared with die-casting and thixomolding
How MAXImolding!™ works:
- Chipped raw material (chips, pallets, granules) is added in the preparation/heating/Ag-flooding unit called MAXIfeedstock™ that is attached to the entry port of semi-solid metal injection molding machine called MAXImolding!™
- An inert gas, Argon, is added to the system to prevent oxidation
- Chips are cleaned and heated up to 200°C
- A robotic arm micro-sprays the mold with a lubricant to prevent parts from sticking and the mold is closed
- Raw material is heated in the reactor to a semi-solid state in the processing units while being transported from the processing units to the slurry collection chamber (slurry temperature for Mg alloy AZ91D 480-580°C)
- Slurry is transferred in front of the injection piston („First In – First Out“)
- Slurry is injected into the heated mold (175-300°C) through nozzle, using a high speed (1 – 12,0 m/s), high pressure (15,000 PSI, 1.000 bars) hydraulic injection unit
- The mold opens, parts are taken out of the mold, placed on conveyor and cooled down
Applications of the MAXImolding!™ technology
- Potential replacement of permanent mold parts to eliminate machining and finishing
- Pressure tight parts such as master brake cylinders, fuel rails, air conditioner compressor housings etc.
- High strength parts such as engine mounts, tie rods etc.
- Wear resistant parts such as compressor piston, brake drums, gear shift levers etc.
- Large surface area parts such as doors, motor and luggage hood, roof
Advantages of the MAXImolding!™ technology
- Molding of high integrity net-shape complex parts with highest mechanical performance at low cost
- Molding of parts with ultra large thin areas using multi injectors
- Molding tighter tolerance, while process consistency is high
- Higher yield, faster cycle time, inject directly into cavity
- Most efficient energy usage at approx. 2/3 KJ per each part’s gram net weight
- Most efficient material usage with precise melt dosing
- Extended mold life, 200°C lower melt temperature during injection
- Very low porosity (less than 0,5%), smooth mold filling with no air entrapment favouring part soundness
- Fewer secondary operations, less labour
- Reduced solidification shrinkage
- Heat out of mold recovery, mold atomizing spray cooling
- Parts recycling by AZ91D alloy melting at 4% of the energy originally used
- Best worker safety, no fire hazards
- A single, dedicated 1-step molding process reduces the number of process steps
- Fully enclosed clean process, no emission of dangerous gases in the atmosphere
- Production of semi-solid metal slurry is directly on machine – on demand
- Small machine footprint vs. huge horizontal casting machines
- Using innovations from the plastic and magnesium injection molding as well as x-ray technology to produce stronger and more intricate part at lower costs
MAXImolding!™ – Less energy and material usage
A recent study (*) done by Dr.-Ing. Andreas Lohmüller, Dr.-Ing. Michael Hilbinger and Dr.-Ing. Martin Franke, all from Neue Materialien Fürth GmbH, Germany, comparing a number of currently used processes for energy and material consumption for finished engine bracket (net weight 430g, red line), shows the great advantage of the thixomolding, especially with hot runner seen on the chart below. That means reduction of energy consumption by around 60% and material usage by around 50% in comparision to cold-chamber die casting (CC-DC).
The new molding machine reduces estimated energy consumption by approx. 30% compared to a 220t-JSW thixomolding machine with hot runner as seen on chart above.
MAXImolding machine can do even more, like molding parts consisting of different layers, use different feedstock composition in different area of part, multi-injector usage for molding parts of large surface area etc. With chips pre-conditioned and pre-heated, precise melt and mold temperature control, mold atomizing cooling and mold heat recycling, even better results can be achieved.
Applications of MAXImolding!™
MAXIfactory™- a fully automated self-learning digital molding factory
The idea that we can produce quality parts through an inspection process that consists of sorting “good” parts from “bad” is inadequate, resulting in poor quality products and high production costs. Through a lifetime of experience in automotive wheel inspection at Jacobsen X-ray in Canada and Germany, we have come to realize that there is a better way to use inspection, as an integral part of the injection molding process rather than through separate, post-production inspection.
In 2006, we first published a well-received presentation on the subject of efficacy of die-casting titled “How to improve productivity in wheel casting plants using real-time x-ray in the loop?” at the SAE 2006 World Congress & Exhibition in Detroit, Michigan. We re-iterated well-known ideas promoted by Dr. W. Edwards Deming to build quality by preventing defects – by analysing and fine-tuning light alloy wheel manufacturing process with Mr. Yamamoto at UBE factory in Sarnia, Ontario, Canada with specific focus on the cause of the defects. To accomplish this, we first used manual x-ray picture feedback to set up the casting machines to produce fewer defects in the first place.
While this approach worked, it required highly skilled people involved with setup of the machine and process. Firstly, the people analyzing x-ray images of the complete part (in our case alloy wheel) and making process adjustments must be experts in interpretation of such inspection findings. Staff must take the knowledge of a quality-inspected part and related this to a set of relevant parameters on the actual part that can be applied to input of the machine to hopefully produce new part with better and more exact quality characteristics. To mitigate these challenges, a further molding/casting system and its control system is shown in Figure below. Example of the production of light alloy wheels displays a novel MAXImolding!™ – a semi-solid metal injection molding machine (SSM-IMM) working together with fully automated ADR x-ray inspection system and using fully automated data feedback from the x-ray machine.
This new process, as well as the materials – available today in the form of the semi-solid metal (SSM) metallurgy – combined with applied mathematics (AI, machine learning, big data mining, SPC etc.) and the use of self-contained fully operated modules based on the new HyperComMachinery™ (**) concept can revolutionise the classical die-casting industry.
The magnesium semi-solid injection molding MAXImolding!™ machine is able to achieve a fully automated molding factory. It will ensure the fast production of good parts, and maintain stable automatic closed loop process control. The factory consists of proprietary robotics and machine vision systems, based on one single unified software. The novelty of this invention is in the way in which an additional set of variables obtained from the inspected parts is generated and used, to allow for iteratively starting a fast automated process. This allows us to obtain good production parts with minimal losses in energy and materials.
A model of a fully automated injection molding process is depicted in the Figure above. The operator is only required to power up the machine, choos the 3D part to be molded and ensure material is available and molded parts are taken away. For example, the molding wheel for a car would follow a basic sequence like this:
The operator would provide power to the semisolid molding machine, follow an initial checklist related to supply of the material, and safety verification, and then press the start button. The new MAXImolding!™ injection molding machine would start molding the first wheel based on proposed initial process parameter data set and, upon completion the wheel is immediately inspected at the x-ray inspection system. The fully automated x-ray inspection system would determine the quality of the wheel and generate the digital model of the part that is now unequally identified for life of the part. If the part is good, all information about this good wheel will be stored in a good wheel data set data base. The same process would be followed for the original production (process) data set, as well as the design 3D model data set. If the wheel is bad, information will be stored in a bad wheel dataset in the database. Assuming that the wheel is bad, the knowledge system will determine which set of parameters need to be adjusted iteratively to improve deficient characteristics of the wheel, and a new set of inputs will be generated in the parameter generator. For example, the inspection may indicate that the wheel is not completely formed, indicating insufficient feedstock has been injected into the mold, commonly referred to as a short shot. The knowledge system determines a longer injection period is appropriate, and iteratively instructs the parameter generator to delay the closing of the nozzle. The next machine cycle is executed, serialized, and passed on to the inspection system. More data from the inspection system is generated which allows for more information and faster convergence to a set of parameters that yields good quality wheels. This process for producing magnesium wheels by closing the loop with data from an inspection system and accelerating convergence with data sets from good wheels and bad wheels as well as digital 3D model of the original design will allow the production of net-shape high integrity wheels with minimal rejects. A fully automated process will generate the necessary information to run the process and maintain high quality production at all times.
This is an intrinsically safe, energy and material efficient, environmentally sound magnesium-molding factory with no emission of gases outside of factory parameters. All molding machines and x-ray inspection machines, as well as proprietary linear robotics and machine vision systems integrated into this semisolid injection molding process, will ensure the fast production of good parts, and maintain stable automatic closed loop process control. The collection of process data for the same or similar parts worldwide (planetary molding) will go via cloud and everybody can use it.
(*) Lohmüller, A.; Körner, C.; Singer, R. F.: Neue Gießtechnologien: Ressourceneffizient und wirtschaftlich. In: R. Neugebauer (Hrsg.): Ressourceneffiziente Technologien für den Powe(*) Lohmüller, A.; Körner, C.; Singer, R. F.: Neue Gießtechnologien: Ressourceneffizient und wirtschaftlich. In: R. Neugebauer (Hrsg.): Ressourceneffiziente Technologien für den Powertrain; International Chemnitz Manufacturing Colloquium ICMC 2012, S. 67 – 81
(**) HyperComMachinery™ concept defines self-contained fully operated modules that flow seamlessly into factory SCADA system according to CEO’s ultimate wish – one software for whole factory!