10 Ocak 2016 Pazar
The use of stereolithography rapid tools in the manufacturing of metal powder injection molding parts
Introduction
In the competitive market, organizations have been using technologies that aid the achievement of rapid new product development with desirable quality and costs (Jacobs, 1992 & 1996). Considering complex geometric parts, two technologies deserve attention from the production sectors: Rapid Prototyping (RP) and injection molding. Rapid prototyping techniques are important due to their ability to reduce development time, to identify early errors in the project, to achieve a better communication within the project development team and to evaluate the functionality of the product along with many other benefits using physical prototypes/models. Others applications and evolutions from rapid prototyping are Rapid Tooling (RT) and Rapid Manufacturing (RM). In the case of rapid tooling, it is possible to build tools, such as molds, to produce pre-series of components by injection molding in a period shorter than one week. Stereolithography (SL) is one of the most versatile rapid prototyping techniques. This technology is capable of delivering fast and accurate 3D objects with a wide range of resins for many different uses (Wholers, 2001).
Powder Injection Molding (PIM) competes well with technologies such as casting, machining and forming when the components produced are small, complex and have tolerances within a narrow range. However, PIM has a high investment cost, because it needs an infrastructure with ovens, injection molding machines, a relatively expensive feedstock and a high accuracy mold, the latter two also influencing directly the final production cost of each component.1
Rapid tooling might possibly be used with powder injection molding to provide low-cost prototype molds. However, Hemrick et al (2001) and Gomide (2000) have described a particular behavior of PIM parts molded in SL tools. These authors affirm that irregularities in the mold surface have caused a strong attachment of the molded part to the mold. As a result the part broke in the mold opening and ejecting stages. Nevertheless, the injection molding parameters and mold design influence over the failures in the injection molding of mixtures of powder and binders were not completely described.
This paper describes case studies performed to evaluate the injection molding of stainless steel 316L metal powder in rapid molds produced by stereolithography. To clarify, a review about stereolithography with its rapid molds and powder injection molding is presented.
Stereolithography Tools for Injection Molding
Stereolithography is used to manufacture three-dimensional objects by means of photo polymerization of a resin by the energy delivered through an ultraviolet laser beam. A basic sketch of the process is shown in Fig. 1.
Basically, every rapid prototyping process starts with a CAD (Computer Aided Design) system. In the CAD, a three-dimensional object that represents the product is designed. The model is translated into a common language known as STL which is a triangular shell mesh of the object. This STL file is imported into the CAM (Computer Aided Manufacturing) of each process and is sliced into thin layers. Each layer represents one step of the process in the rapid prototyping machine. The CAM system also generates paths and manufacturing parameters according to the material and machine that is going to be used to build the prototype. Later, in the stereolithography machine, data from the CAM is loaded to commence the manufacturing process. The ultraviolet laser beam, as shown in the Fig. 1, is driven by galvanometric mirrors that scan the vat surface which contains a liquid photosensitive resin. The laser radiation activates a polymerization process and the resin hardens forming a solid layer of the three-dimensional object. After finishing one layer the platform dives and the liquid resin spreads over the solid layer. As the resin is too viscous, a blade passes through the surface of the liquid resin leaving a gap between the solid layer and the blade. The scanner starts to solidify a new layer attaching the new layer to the previously made layer. Layer-by-layer the object is manufactured by scanning selectively the laser beam over the resin surface. This short description is the original conception from 1988 developed by 3D Systems who developed the first RP commercial process. Furthermore, there are many variations in other stereolithography processes that use the photo-chemical principle to make and attach layers (Jacobs, 1996; 3D Systems, 1998).
This technology presents as an advantage its speed in manufacturing complex parts directly from the computer and it may be used in many different applications such as: prototypes for functional testing, bio-models for surgery planning, models for aerodynamic tests and rapid molds for injection molding. Using rapid molds, pre-series of parts are obtained in record time with the injection molding material chosen to manufacture the final parts. So, tests may be extended to a wide range of applications before expending time and effort on a definitive hard tool. This technology can be used to evaluate the mold analyzing its injection gates, ejection pins, split-lines, etc.
On the other hand, according to Dickens (1999), SL molds can mold from 20 to 500 parts of polypropylene which is considered very low compared to the worst metal molds that easily mold over 100.000 shots. The average life of the mold depends basically on the part complexity, injection molding material and procedures to set up the injection molding parameters in the injection molding machine. Most of the resins that are used to build SL molds have low mechanical properties above 70oC. Also the resins have low thermal conductivity (for example the SL resin DSM Somos 7110 k=0,2W/m.k). Thus, they are weak after they receive the thermal energy from the injection molded material that is injected usually above 180oC. When the mold is open and the part is ejected some features of the mold can break due to the ejection forces caused by the contraction of the part material over the mold cavity. To decrease early failures of the mold the injection molding parameters must be different from those used in metal molds. Figure 2 compares graphically the differences in their main parameters. Notice that the injection molding cycles are longer for SL molds due to their low thermal conductivity. As a result the injected material can show different mechanical properties caused mainly by different degrees of crystallization achieved (Segal & Campbell, 2001).
Even though they present low mechanical properties, stereolithography molds can reproduce injection molding parts with fine details in less than 4 days without the high costs of a definitive tool (Gomide, 2000). Consequently it may aid engineering teams to evaluate their projects, avoiding error detection in later phases of new product development.
Powder Injection Molding
PIM technology is a combination of powder metallurgy with injection molding of thermoplastics. As a result it is possible to manufacture complex parts with metals, ceramics & composites. The hard particles of ceramics or metals are mixed with a binder system that covers the particles. This binder system is made usually of thermoplastic, wax and additives that allow the mixture to be molten and injected inside a mold in a way similar to that performed for thermoplastics alone. A basic sequence of the powder injection molding process is presented in Fig. 3.
The process starts from choosing a combination of powder and binder system. The powder will be responsible for giving the proper mechanical, thermal, electrical and chemical properties to the manufactured part. The appropriate binder system will transport the particles to the inside of the mold and hold the shape, the part impression. After mixing, homogenizing and granulating the mixture it is placed in the feed system of the injection molding machine. The material is heated in the barrel by heater bands and shear caused by the rotation of the screw. After the material becomes molten it is injected inside the closed mold, applying a holding pressure to compensate the material contraction after it cools down. When the part is strong enough to be ejected the mold is opened and ejection pins extract the part from the mold obtaining a "green" part. The green part undergoes thermal and/or chemical processes of debinding to extract the binder system before the final thermal treatment to sinter the part. Sintering is responsible for achieving the optimal physical/chemical properties of the part material. The debinding and sintering treatments cause a 15-25% contraction in the part depending on the powder; binder, proportions and their applications. The process presented in Fig. 3 is a basic process but there are many variations in each step.
The use of powder injection molding may be employed for economical or technical reasons. In Figure 4 many examples of parts obtained by metal powder injection molding, with simple and complex geometries are presented. There is a wide range of applications for this technology and it may be used to produce parts for industries such as automotive, aerospace, consumer products, medical implants, computers, armaments and etc.
German & Bose (1997) mention as main advantages of this technology well designed and highly complex parts, low costs for large scale production, high precision and repeatability, a high diversity of materials with excellent properties (mechanical, chemical, etc).
Powder injection molding has appeared due to limitations in the conventional uniaxial compaction which does not allow efficient compaction of complex parts. With PIM it is possible to obtain parts with densities higher than 95% of the theoretical value homogenously distributed in the part (German & Bose, 1997).
Methodology
To evaluate the use of stereolithography tools to mold PIM parts two case studies were performed. The first case study was carried out to identify the process difficulties. A second case study, using a more complex geometry considered the results obtained from the first. Conclusions were then drawn to establish the positive and negative points of this application.
Design and Manufacturing of the SL Molds
For the first case study a geometry that can be considered simple in relation to the injection molding process was designed. The geometry was easy to mold and eject with constant thickness. For the second case study a more complex geometry was designed with a welding line caused by injection material flow around a core that made the ejection difficult. These geometries with the injection gate position and welding line are presented inFig. 5.
The dimensions of the case study 1 specimen were chosen without any concern for the final dimension of the parts obtained. The percentages of material contraction after the injection molding, after the debinding and after the sintering were therefore not incorporated into the CAD design. On the other hand, for the second case study the specimen was designed considering a volumetric contraction of 21,5% which is considered standard for the chosen material.
To design the mold for each case study, design guidelines for stereolithography molds (Gomide, 2000; Cedorge et al, 1999) and design rules for powder injection molding (German & Bose, 1997) were considered. To aid the ejection of the part from the mold ejector pins were distributed along the cavity surfaces. It is very important to homogeneously apply the ejection forces of the pins to the part surface because the green part is very weak even to handle. Also a draft angle of 1,5o was used in the first case study and 1o in the second. Both CAD mold designs can be observed in Figs. 6 and 7.
The molds were built in a stereolithography machine model SLA-250/30A with the resin DSM Somos 7110 using a standard building strategy with a layer thickness of 150mm. After building the molds they were cleaned with isopropyl alcohol and post cured inside an ultraviolet chamber for 1 hour. This procedure is standard for parts using this resin but it is possible to obtain extra cure with heat treatments. To avoid dimensional distortions no post finishing process was applied to the mold surfaces. As a result the staircase effect reported by Ahrens et al (2001) which is caused by the layer-by-layer manufacturing was reproduced in the molded parts. For economical reasons the molds as seen in Figs. 6 and 7 were designed in shell format. This approach saved resin and machine use hours. For this reason the molds needed a backfilling procedure to increase the mold strength. In case study 1 a polyester based resin (Massa Plástica Anjo) in-house filled with iron powder to diminish contraction was used. For the second case study an epoxy based resin with aluminum powder was used (Vantico Renshape RP 4036 resin & RP 1500 hardener – heat resistant casting system). To monitor the temperature, a K type thermocouple was placed in the back of the SL mold shell in this case study (Fig. 7). Figure 8 shows the mold of study 2 placed and adjusted in the bolsters ready for injection molding.
The Injection Molding Procedures
Stainless steel 316L was used as the powder and the binder chosen to be used in the experiments was that presented in Table 1. The 316L was selected due to its economical importance indicated by German & Bose (1997).
After assembling the bolster in the injection molding machine (Arburg Allrounder 320S 50T), procedures to adjust the injection molding parameters were taken.
For case study 1, the injection molding procedure started using safe parameters to avoid early failures to the stereolithography mold. This meant that low pressures, low clamping forces, low injection speeds, no holding pressures and lowest temperature to decrease viscosity of the feedstock were used. These parameters were changed gradually until successfully injection and extraction of parts from the mold without perceptible defects. Between each shot the impression was analyzed and a demolder (PVA, poly vinyl alcohol) was applied to the surface to reduce the ejection forces.
The parameters obtained from the first case study were used to indicate those for the second. Gradually the values were changed to make it possible to mold and eject parts considered good quality. After the 10th part obtained no demolder was used. To help the mold to cool down an air stream was used between each shot.
Debinding and Sintering Treatments
The best 19 parts obtained from case study 1 and 30 from case study 2 were debinded and sintered. The treatments were performed in the production line of Steelinject (Lupatech Industries Group). A description of the debinding and sintering is shown in Table 2.
Measurement Procedures
Concerning the dimensional control, the dimensions from the molds before and after the injection molding were taken. The parts were measured after the molding and sintering. Figure 9 shows the dimensions designed in CAD for case study 1 where contraction was not considered. The same dimensions were measured on obtained parts.
Figure 10 presents the target dimensions for sintered parts and the dimensions measured in the mold for the case study 2. The parts chosen for measure were taken in cycles when the injection molding process was considered stable (after the initial adjustments) with constant time, speed and pressures. Also in case study 2, the mass of each measured part was taken after the molding and after the sintering treatment.
Furthermore, as previously described, the temperature of the cavity was monitored using a K type thermocouple connected to a monitoring system (Picolog TC08, Picoteck Technology) with readings taken every 5 seconds.
Results
The parts obtained presented a good superficial quality after molding and sintering. Nevertheless, in case study 1 the incorrect adjustment of the ejection pins caused marks on the part that can be seen in Fig. 11. The parts obtained from case study 2 presented another defect. An excessive flash (thickness of 0,4mm) occurred due to the imprecise adjustment of the cavity closing. Later adjustments of the molds reduced the flash thickness to below 0,15mm. Figure 12 shows green and sintered parts obtained in case study 2 without flash.
Some of the most important injection molding parameters used to mold the parts in the case studies are presented in Table 3. The main divergences between the case studies are the holding pressures that were applied with more efficiency in case 2..
The cooling time measured in case study 2 was longer compared to case study 1. One of the main reasons for this was the stability of the injection molding cycles and the control of the temperature between each shot. Using the thermocouple it was possible to estimate better the right time to open the mold and to eject the part. The minimum time necessary to cool down the mold before starting another injection cycle was also precise.Figure 13 presents the monitoring results from the injection molding cycles for case study 2. The graph shows temperature variation over time for a thermocouple position of 0,5mm from cavity surface, pointing out the maximum temperature peak being 49oC. Additionally, it was possible to obtain the total time for each cycle which ranged between 60 and 360 seconds due to the cleaning and cooling of the cavity.
The dimensions from case study 1 reveal that the percentage contraction of the parts after the sintering was 21,56%. This is a value close to the theoretical value, considering the measurement errors, for the mixture of powder and binder system used (refer to Table 1). In Table 4 the dimensional percentage deviations compared with the CAD target dimensions for the final parts of case study 2 are presented. It can be observed that the mold dimensions change after the injection cycles. This occurs because the stereolithography resin is not completely cured after the standard building and post-processing procedures. During the injection cycles the combination of pressures and temperatures help to cure the resin but unfortunately change its shape. The final average results for the sintered parts present a discrepancy in dimension "C" which is related to the part thickness. This error is caused by the excessive flash mentioned previously which added an extra thickness to the parts. However, the results indicated that it is possible to optimize the mold design and finishing processes in order to overcome this problem.
Analyzing the mass variation of the parts it was possible to evaluate the pores percentage. Considering the CAD model for 100% dense stainless steel 316L and the mass average of the sintered parts the pore percentage was almost the same as those for parts molded in steel molds (3,65%).
Conclusions
Gomide (2000) & Hemrick et al (2001) affirm that the powder and binder system mixture sticks to the stereolithography molds causing difficulties in ejecting parts from the mold. Nevertheless, the adherence of the mixture to the cavity surface can not be completely explained by a single phenomenon. As in metallic molds, the injection molding process in SL molds presents many similarities although the values differ.
As in metallic molds, too high a holding pressure or too long a cooling period can make the ejection of parts difficult. They may cause the part to break or to crack. A well designed ejection system can diminish these problems. The holding pressures applied to injection molds made with the stereolithography resin are low due to the low mechanical properties of DSM Somos 7110 resin at high temperatures (Hopkinson et al, 2000). Ascertaining the correct cooling periods before opening and ejecting the part is therefore very crucial to the success of the process. A long cooling period leads to high contraction of the mixture (~1%) and it becomes too fragile to eject from the mold. If the mixture is not solid enough the part will deform in the mold opening. The injection and holding pressures are vital when injecting powder parts using stereolithography molds. Finding the correct pressure adjustments is a complex process because there is a minimum value to mold the part without defects and a maximum value that will not cause the part to excessively adhere to the mold cavities. It is also necessary to avoid pressures which are too high, in order to prevent early failure of the mold.
The superficial quality of the mold also plays an important role in the ejection of the part. In both cases studies no kind of surface treatment was applied. For the designed specimens this was not a problem because they were planar. However, for more complex parts it may be necessary to use techniques such as sanding, polishing and electroplating to achieve better surface qualities. It is also important to note that the machine used had a building resolution of 150mm and in new models it is possible to build 20mm layers giving a better surface quality and higher precision.
The injection speed used in the evaluated geometries was equal to those recommended for metallic molds (German & Bose, 1997 e Haupt & Walcher, 1998). The speed can not be too high otherwise it will cause flow jetting. Low speeds would not cause problems because the resin works as an insulator and it is unlikely that flow freezing would occur. When injection pressures are too low it is necessary to compensate with higher injection speeds.
The results obtained from the dimensional analysis, especially from the second case study, have proven satisfactory. Excluding the dimension affected by the excessive flash the dimensional errors were under 1.7% when comparing the target dimension to the final sintered part. Many authors (Kulkarni, 1996; German & Bose, 1997) affirm that tolerances for the powder injection molding process must be close to ±0,3%. Nevertheless, David (1998) affirms that for industrial applications tolerances of ±1% are more realistic. Considering that the use of stereolithography molds is mainly for design evaluation purposes inaccuracies around 2% are acceptable. Moreover, there are new resins for stereolithography with excellent mechanical properties even at high temperatures. Also, there is a new ceramic filled resin specifically for building injection molds.
The time to obtain a mold for stereolithography can be considered the greatest advantage of its application.Table 5 shows the time expended to obtain 100 green parts in case study 2. It is important to note that to obtain the final sintered parts more than 68 hours are necessary to debind and to sinter them. This is inherent to the powder injection molding process.
The time to build the molds was 16 hours, using a laser power of 13mW (over time HeCd lasers units lose their power). With a new laser power of 40mW it is possible to build the molds in 9 hours, in the same machine. Using new machines with a laser power of 200mW and a faster recoating system the time can be reduced to 3 hours.
The period necessary to cure the backfilling material is also relative because as with the rapid prototyping machine it can be performed at the end of the day to get the objects ready for the next morning.
This work proves that it is possible to obtain powder injection molded parts from stereolithography molds. Despite the low complexity of the specimens it is possible to gain important information relevant to performing injection molding of more complex parts.
Powder injection mouldingBE
Plastic components have been produced by injection moldingAE for years. Therefore the complexity of the components have increased steadily. Through the development of special processes, such as the multi-component injection molding, but also the micro-injection molding, completely new possibilities of plastics processing result are possible. In addition to plastics, since years the powder injection moulding of metallic and ceramic materials are now well established. With this procedure complex components can be made out of metal and ceramic far cheaper than by using other production possibilities.
The powder injection molding is divided into four production steps:
The powder injection molding is divided into four production steps:
First, through a combination of plastic, wax and metal or ceramic powder a material is generated which can be processed by injection moulding. This material is generally named - feedstock - . By custom machine and tool technology, this feedstock can be injected, similar to plastic injection molding. The moulded part is so-called green part. The next step is the release of the plastic and wax component by the debindering process. The result is called brown part. The last step is the sintering of the brown part. In this process the individual particles are merged together, the brown part shrinks and becomes an compact component. This sintering shrink depends on the proportion of powder in the feedstock: normally between 20% and 30%. This final component has similar properties to the base material.
If you have further questions on the powder injection have a look at our other explanations. If you have specific project ideas please contact us. We will try to clarify your questions or give you an appropriate contact person to you.
If you have further questions on the powder injection have a look at our other explanations. If you have specific project ideas please contact us. We will try to clarify your questions or give you an appropriate contact person to you.
Powders for Metal Injection Moulding
MIM offers diversity: from nickel-iron to titanium, copper and advanced superalloys
In MIM, particle size distribution is the most important powder characteristic as this determines the sinterability and surface quality of the final product. As a general rule, the finer the powder the better.
Common powders for MIM
Carbonyl iron and nickel powders
Fortunately for MIM technology, very fine carbonyl iron and nickel powders were already in industrial production when the process was developed.
These powders have spherical particles, which is beneficial for good flowability, with particle sizes of less than 10 µm, an order of magnitude finer than the common atomised and reduced iron powders used in uniaxial powder compaction (Fig. 4).
Even special grades with silica coated powder particles were developed for MIM to improve the flowability.
Carbonyl iron powder has excellent sintering properties. The small particle size provides a high sintered density, excellent strength and surface texture quality in the final part. The uniform spherical particle shape gives high dimensional accuracy.
Many ferrous MIM alloys are based on carbonyl iron and nickel. Powders are blended with elemental or master alloy powders to achieve the desired alloy composition.
Fig. 5 Gas atomised 90% 16 μm 17-4PH metal powder produced by
Sandvik Osprey, UK
Sandvik Osprey, UK
Water and gas atomised powders
Prealloyed powders are predominantly (but not exclusively) used for high alloy steels, nickel and cobalt base alloys. Alloy powders are usually gas atomised and have a spherical particle shape.
In some cases water atomised powders with irregular particle shapes are also added.
With the growing demand for fine metal powders, manufacturers have improved their atomising techniques. Today specialists offer many kinds of high alloy steel powders with less than 10 µm average particle size.
There are a number of well established companies supplying atomised powders for MIM, including producers in Europe, North America and Asia.
- See more at: http://www.pim-international.com/aboutpim/sintering#sthash.WENjPqOV.dpuf
AN OVERVİEW THE METAL INJECTION MOLDING PROCEES
A simple concept, a complex process
The idea to plastify powdered raw materials with the help of thermoplastic additives and subsequently use injection moulding to form complex components was first developed for ceramic components.
Fig. 2 The metal injection moulding process
(Courtesy IFAM, Germany)
(Courtesy IFAM, Germany)
The principal technological problems that had to be solved before the MIM process could be industrialised included:
- Production of a homogeneous powder-binder mix with a high metal powder loading and sufficient viscosity for injection moulding
- Development of economical binder removal processes providing shape retention
- Sintering to high density and dimensional accuracy.
The key steps
Preparing the feedstock
The primary raw materials for MIM are metal powders and a thermoplastic binder. The binder is only an intermediate processing aid and removed from the products after injection moulding. The properties of the powder determine the final properties of the MIM product.
The blended powder mix is worked into the plastified binder at elevated temperature using a kneader or shear roll extruder. The intermediate product is the so-called feedstock. It is usually granulated with granule sizes of several millimetres, as is common in the plastic injection moulding industry.
Feedstock can either be purchased "ready to mould" from a number of international suppliers, or it can be manufactured in-house by a MIM producer if the necessary skills and knowledge are available.
Injection moulding
The ‘green’ MIM parts are formed in an injection moulding process equivalent to the forming of plastic parts. The variety of part geometries that can be produced by this process are similar to the great variety of plastic components.
Binder removal
The subsequent binder removal process serves to obtain parts with an interconnected pore network without destroying the shape of the components. The types of binder removal processes applied are further explained later in this introduction. At the end of the binder removal process there is often still some binder present in the parts holding the metal powder particles together, but the pore network allows to evaporate the residual binder quickly in the initial phase of sintering, at the same time as sintering necks start to grow between the metallic particles.
Sintering
The sintering process leads to the elimination of most of the pore volume formerly occupied by the binder. As a consequence, MIM parts exhibit a substantial shrinkage during sintering. The linear shrinkage is usually as high as 15 to 20% (Fig. 3). If required, sintered MIM parts may be further processed by conventional metalworking processes like heat treatments or surface treatments in the same way as cast or wrought parts.
For certain applications, such as the automotive, medical and aerospace sectors, Hot Isostatic Pressing (HIP) can be used to completely remove any residual porosity. As MIM parts are typically small, this can be relatively cost effective for critical components.
(Courtesy BASF AG, Germany)
A dynamic $1.5 billion-plus global industry
Over the following pages you can find information on MIM markets, the history of the technology, how the process works, what materials can be processed, and a selection of case studies from around the world.
Fig. 1 Regional concentration of MIM applications.
Data for Europe and North America relates to
2010, Asia 2008. As presented by Keith Murray,
Sandvik Osprey Ltd, at the EuroMIM Open
Meeting, October 10, 2010 and published in
PIM International, Vol.4 No.4, December 2010
Data for Europe and North America relates to
2010, Asia 2008. As presented by Keith Murray,
Sandvik Osprey Ltd, at the EuroMIM Open
Meeting, October 10, 2010 and published in
PIM International, Vol.4 No.4, December 2010
Whilst this is focused on MIM, much of the information can also be related to CIM.
Powder Injection Moulding (PIM), which encompasses Metal Injection Moulding (MIM) and Ceramic Injection Moulding (CIM) is a major manufacturing technology with estimated sales of more than $1.5 billion.
MIM parts production accounts for around 90% of the market for PIM products, which is seeing annual growth of between 10-20% worldwide.
PIM is a truly global business, with more than 450 parts producers located worldwide. According to recent data, Asia is the world’s largest PIM producing region by sales, followed by Europe and North America.
Applications for MIM
Many objects we encounter in everyday life such as cars, mobile phones, watches, domestic appliances, cameras and power tools contain MIM parts.
So many MIM applications have been published to-date that it is impossible to give a comprehensive overview, but a number of application examples are provided later in this section that demonstrate the strengths and potential of this technology.
Regional market variations
The differences in MIM markets by world region is quite distinct with, in general terms, Asia dominated by IT and electronics, North America by medical, orthodontic and firearms applications, and Europe by automotive and consumer products.
- See more at: http://www.pim-international.com/what_is_metal_injection_moulding/introduction_to_metal_injection_molding_MIM_PIM#sthash.FJPHevQH.dpuf
Master debinding curves for solvent extraction of binders in powder injection molding
This study successfully extends the master sintering curve concept to model removal of polymeric binders in powder injection molded bodies. In this demonstration the focus is solvent debinding. Master debinding curves (MDC) were used to estimate the activation energy for debinding. Binder removal curves were developed for solvent extraction of polyethylene glycol (PEG) from injection molded shapes made from silicon nitride doped with yttria and spinel (Si3N4 5% Y2O3 5% MgAl2O4). The extraction for different shapes incorporated a shape parameter in the standard master curve equation, giving activation energy of 12.4 kJ/mol extraction of PEG. Master debinding curves were also developed for wicking debinding. The analysis showed lower activation energies for solvent extraction and wicking compared to thermal debinding. The lower activation energy suggests easier removal by solvent or wicking versus pyrolysis. The study provides a unifying quantitative framework for comparing and predicting the effects of material, process and geometry on binder removal. The master debinding curves can be utilized in design and optimization of binder removal.
Graphical abstract
This study successfully extends the master sintering curve concept to model removal of polymeric binders in powder injection molded bodies by solvent extraction and wicking processes. The extraction for different shapes incorporated a shape parameter in the standard master curve equation. The analysis showed lower activation energies for solvent extraction and wicking suggesting easier polymer removal compared to thermal debinding.
This invention relates to processes for compacting metal powders. It is more particularly concerned with the injection molding of articles from metal powders.
It is conventional to produce articles of metal powders, particularly of high performance alloys, sometimes called "super alloys", by filling a die with powder mixed with a binder, compacting it under pressure to produce a self-supporting green compact, so-called, ejecting the compact from the die and then sintering the compact so provided. The binder is volatilized or burned out before or during sintering. Such processes are limited in that density gradients through the article are difficult to eliminate. Density gradients in conventionally produced parts arise from particle-to-particle and particle-to-die-wall friction, and bring about non-uniform shrinkage in the sintered part. Because of this, conventionally produced articles seldom have length-to-diameter ratios greater than 2:1. Furthermore, those processes are not readily adapted to provide undercut parts or to produce articles having cored aperatures. Coring in conventionally produced articles is limited to the pressing direction. Transverse coring interferes with particle flow during the die filling and compaction.
Injection molding of plastics is widely employed. In such processes, because of the fluid-like flow of the material, density gradients are avoided. In injection molding withdrawal cores through the mold cavity may be positioned in virtually any direction. It would be advantageous to produce articles of metal powder by injection molding, and for such process thermo-plastic and thermo-setting resins would appear to be suitable binders. However, in order to make the metal powder flow to fill a die cavity, the entire void volume of the metal powder must be filled with some plastic medium. As the tap densities of metal powders range from about 50% to about 65%, depending on the particle size, configuration and the method of production, the volume of plastic medium incorporated would be considerable, and it must be largely removed to produce articles of densities approaching the as-cast density of the metal. Conventionally bindered and dry compacted metal powders are pressed to green densities ranging from about 60% to 70% of as-cast densities. The volume percent of pores, which are interconnecting throughout the article, provide adequate escape passage for burn-out gases. Molded articles produced by injection molding as above described are non-porous, however, and it is very difficult if at all possible, to burn out the plastic medium without blistering or cracking the article.
It is an object of my invention to provide a process of injection molding an article of metal powder adapted for sintering without blistering or cracking. It is another object to provide such a process which will produce articles having a length-to-diameter ratio greater than 2. It is another object to provide such a process employing a plastic medium which flows during injection molding, but which when heated becomes sufficiently viscous to hold the metal powder together in the shape of the die so that it can be ejected therefrom. It is still another object to provide a process employing a plastic medium comprising a suitably modified binder dissolved in a solvent which evaporates prior to sintering of the compact. Other objects of my invention will appear in the course of the description thereof which follows.
My process comprises mixing the metal powder with a plastic medium comprising an organic binder and modifiers, where required, dissolved in a solvent, the organic binder having the property of dissolving in the solvent at room temperature, but of decreasing in solubility at a moderately higher temperature. The mixture of powder and plastic medium in such proportion to have the properties of a fluid is injected under pressure into a closed die which is heated and maintained at a constant temperature at which the plastic medium increases in viscosity. The resulting compact is then held together by the plastic medium so that it can be ejected from the die. The heated die causes rejection of some of the solvent and further oven heating of the ejected compact volatilizes the solvent, leaving a network of pores in the compact and a film of binder in contact with the powder particles. When the compact is sintered, the organic binder volatilizes or sublimes and escapes through the pores of the compact before the powder coalesces so that a dense article results, free from blisters or cracks.
The solvent which I prefer is water and the organic binder I prefer is methyl cellulose, which is soluble in cold water but becomes less soluble in hot water, that is, at temperatures of about 170° to 190° F. The viscosity increase during injection molding is caused by the rejection of water molecules from the surfaces of the long thread-like polymer molecules. During this part of the process some of the solvent is rejected from the compact.
Modifiers are required to promote mold release and complete healing of interfaces within the molded part to prevent drying cracks from forming. A combination of glycerin and boric acid has been found to accomplish this. Both are water soluble and boric acid is soluble in glycerin. Glycerin is a well-known plasticizer for methylcellulose, and enhances mold release. The plasticization enhances the interface healing, but it is not completely effective without the boric acid, thus, it is a synergistic combination.
The plastic medium and solvent combination above described is effective over a considerable range of variations of content of its components. For optimum results, with atomized powders, the solvent should comprise around 60% by weight of the plastic medium composition.
The maximum green density of the molded and dried part is dependent on the tap density of the metal powder being used. A clean, dry, inert gas-atomized, -325 mesh powder of the composition above mentioned has a tap density of about 63% to 65% of as-cast density. If the 35% to 37% void volume of that powder is filled with plastic medium and the powder is injection molded, the green density of the molded article will be comparable to the tap density of the powder.
In carrying out my process I dry blend methyl cellulose powder with the metal powder. Glycerin and boric acid are put into solution in the water, which is warmed, and that solution is added to the mixed powder. The resulting plastic mass is injected at room temperature into a closed die which has been heated to about 190° F., and is subjected to a pressure of about 4 tons per square inch on the injection cylinder. When the metal powder is -325 mesh atomized powder the resulting compact has a green density of about 64% of as-cast density. The compact is dried for a few hours at about 220° F. to 250° F. and then exhibits a transverse rupture strength of about 2400 pounds per square inch.
As I have mentioned, the above drying of the compact vaporizes water, leaving the remainder of the plastic medium as a continuous film around the metal powder particles and a considerable volume of interconnected pores throughout the compact. The compact is then sintered in a reducing atmosphere or vacuum, the heating causing substantially all of the continuous film to vaporize and escape through the pores before sintering causes the metal powder grains to coalesce. It should be mentioned that the boric acid broadens, and lowers, the sintering temperature range for certain alloys such as the super alloys.
A typical plastic medium for -325 mesh atomized metal powder, expressed in percentage of the weight of the metal powder is:
______________________________________Methyl Cellulose 2.0%Glycerin 1.0%Boric Acid 0.5%Water 4.5%______________________________________
Desirable ranges of plastic media for atomized powder of from -30 to -325 mesh sizes are:
______________________________________Methyl Cellulose 1.5 to 3.5%Glycerin 0.25 to 2.0%Boric Acid 0.1 to 1.0%Water 4.0 to 6.0%______________________________________
Ball milled powder of -325 mesh can also be injection molded using plastic media as above described. However, because of the higher surface area and irregular shape of the ball milled particles, about twice the weight of solvent is required to wet particle surfaces in order to obtain a workable mix. The green densities of the resulting parts are in the range of 48 to 50% of as-cast density.
Mechanical properties of conventionally pressed and sintered bars and test bars made by the method of my invention hereindescribed and sintered are tabulated below.
The super alloy had the following nominal composition, in percentage by weight:
______________________________________Cr W C Ni Si Fe Mn MO Co______________________________________27.0- 3.5- 0.90-31.0 5.5 1.40 3* 1.5* 3.0* 1.0* 1.5* Bal.______________________________________ *Maximum
Column A is the average value of three lots of the alloy of -325 mesh atomized powder conventionally pressed with 3% polyvinyl alcohol binder and sintered. Column B is an identical powder injection molded by my process hereindescribed with the typical plastic medium hereinbefore set out.
______________________________________ A B______________________________________Green Density, % 68.0 65.0Sintered Density, % 98.7 99.5Sintered Hardness, Rc 38-39 41-43Ult. Strength, psi 141,333 146,500Elongation, % 2.6 2.5______________________________________
By the method of my invention hereindescribed I have injection molded a bar 0.75 inch square and 10 inches long with a single injection port located at the center of the bar. This was equivalent to forming two 5 inch long bars at the same time, each having a length-to-diameter ratio of 6.6:1. No non-uniformity in sintering shrinkage was experienced, and no blistering or cracking was observed.
In the foregoing specification I have described presently preferred embodiments of my invention; however, it will be understood that my invention can be otherwise embodied within the scope of the following claims.
Microstructure and mechanical properties of titanium components fabricated by a new powder injection molding technique
A powder injection molding (PIM) binder system has been developed for reactive metals such as titanium that employs an aromatic compound as the primary component to facilitate easy binder removal and mitigate problems with carbon contamination. In the study presented here, we examined the densification behavior, microstructure, and mechanical properties of titanium specimens formed by this process using naphthalene as the principle binder constituent. In general, it was found that tensile strengths could be achieved comparable to wrought titanium in the PIM-formed specimens, but that maximum elongation was less than expected. Chemical and microstructural analyses indicate that this process does not add oxygen to the material,suggesting that the use of higher purity powder and further process optimization should lead to significant improvements in ductility.
Parts are formed from metal powder by mixing the powder with a plastic medium comprising an organic binder dissolved in a solvent in which it is soluble at room temperature but in which it is substantially less soluble at a higher temperature such that the plastic binder becomes viscous at that temperature. Binder modifiers may be incorporated to promote mold release and promote healing of interfaces within the molded part and prevent the formation of drying cracks. The plastic mixture is injected under pressure into a closed die preheated to the above mentioned higher temperature, whereby the rejection of solvent and increase in viscosity of the plastic medium produces a compact sufficiently self-supporting to hold its molded shape and be ejected from the die. The compact is then dried to evaporate the remaining solvent, thus leaving interconnecting pores in the compact for the escape of gases resulting from subsequent burning out of the binder during the sintering operation.
DG-M SERIES POWDER INJECTION MOLDİNG MACHINE
The powder injection molding (PIM) process includes both metal and ceramic powder. In the field of application such as the automotive, the cutting tools, the lock, medical machine, textile equipment and watch of precision industry, etcetera.The importance of powder injection molding development of new area is a wide view in the future.
The special features of PIM are: Run under automatic and keep its precision as well as repeatability.The cost-effectively control under precision.
We offer the first PIM machine to custom since 2005. And we have sold the small amount of PIM machine. The PIM IMM relied on import in Taiwan market before. But now, our PIM machines have been running many years.In proof of DG IMM has occupied the PIM field.
DG offers to adapt special screw geometry and highly wear-resistant barrel modules.
PIM STEPS
Powder Injection Molding | |
 | |
| Powder Injection Molding (PIM) Powder injection molding (PIM) is a manufacturing solution for producing intricate parts in medium to high volumes (10,000 to over 2,000,000 parts annually) using fine (<20 μm) metal or ceramic powders. PIM is capable of transforming complex concepts & designs into high precision, high final properties, and net-shaped products from a wide range of materials such as carbon steels, low alloy steels, stainless steels, low expansion alloys (kovar and invar), tool steels, soft magnetic alloys, super alloys, to conductive materials (copper), and ceramics. PIM is well suited for parts weighing from 0.1gm to 250 gm. Cross sections are typically less than 0.25 in. (6.35 mm). However, parts are not restricted to this combination of mass and cross section. Tolerances are on the order of ±0.3 to 0.5%, albeit specific dimensions can be held as close as ±0.1%. |
| Mixing | |
 | Very fine metal or ceramic powders are mixed with thermoplastic polymer (known as the binder) to form a homogeneous mixture of ingredients that is pelletized and directly fed into a injection molding machine. This pelletized powder-polymer mixture is known as feedstock. |
| Injection | |
 | In this process, the feedstock is heated to melt the binder content in order to form the desired component geometry. The molded part is known as the green part. |
| Debinding | |
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The polymeric binder is removed from green part by thermal heating to approximately 400°C or 752°F. The result is known as the brown part that still contains its original geometry and size.
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| Sintering | |
 | In this process, the brown part is heated to approximately 85% of the material's melting temperature, allowing densification and shrinking of the powder into a dense solid with the elimination of pores. The sintered density is approximately 98% of theoretical. The end result is a net shape or near net-shape metal or ceramic component, with properties similar to that of bar stock. |
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