High Green Strength Research Articles

Ink development for the additive manufacturing of strong green parts by layerwise slurry deposition (LSD‐print)

AbstractObtaining dense fine ceramics by the binder jetting additive manufacturing process is challenging. A slurry‐based binder jetting process, such as the layerwise slurry deposition (LSD‐print) process, can enable the printing of dense ceramic parts. This work describes a procedure to develop and qualify a suitable ink to manufacture silicon carbide green parts by LSD‐print. Not only the printability but also the compatibility of the ink with the powder bed and the effect of the binding agent on the properties of the green parts are considered. Both aspects are important to obtain high green strength, which is necessary for printing large or thin‐walled parts. Characterization methods, such as rheological and surface tension measurements, are applied to optimize three selected inks. The interplay between ink and powder bed is tested by contact angle measurements and by comparing the biaxial strength of cast and additively manufactured specimens. Out of the three binding agents tested, a polyethyleneimine and a phenolic resin have a high potential for their use in the LSD‐print of silicon carbide green bodies, whereas a polyacrylate binding agent did not show the required properties.

On amine treated polyoxymethylene (POM) blends with low formaldehyde emission for metal injection moulding (MIM)

When polyoxymethylene (POM)—a common polymer used in metal injection moulding feedstock—is exposed to heat and oxygen during compounding, it can be easily decomposed, releasing undesired gaseous formaldehyde products. In order to reduce the formaldehyde emission from POM, amine treatment was performed. The effectiveness of propylamine at different concentrations and its role as a formaldehyde scavenger was studied via the UV–vis Spectrophotometry, Fourier Transform Infra-red, Thermogravimetric Analysis, Scanning Electron Microscopy, and melt flow index. The results proved that a simple amine, such as propylamine, is a promising candidate for scavenging formaldehyde in POM. It is also demonstrated that the best concentration of propylamine is 2 wt.% (POM-PA2) with a minimum formaldehyde emission of 1.44 mg/L. Further, when used in formulating metal injection moulding feedstock (MIM), the POM-PA2 reveals good rheological properties and high green strength. These advantages make the modified polyoxymethylene (POM-PA2) a promising binder system for MIM feedstock.

Penetrated surface characteristics of cement - mixed sand in powder bed 3D printing

ABSTRACT Powder bed 3D printing is an additive manufacturing process that combines powder particles by selectively depositing a liquid activator on a powder bed. Sand molds for high-resolution casting can be realized as a high-quality green body via powder bed 3D printing. In this study, we investigated the effect of vertical penetration of aqueous droplets on the quality of the green body formed from a cement-mixed sand powder. The cement was hydrated by aqueous droplets and acted as a binder between the sand particles. The penetration ratio of the prepared powders was calculated from the penetration depth and spread diameter. Evidently, the penetration ratio and penetration resolution determined the strength and surface resolution of the green body. The results showed that an increase in the silica sand grain size led to a desirable penetration resolution with a relatively high green body strength at the same cement content. Additionally, the green body made from the sand with a particle size of 70 μm showed a relatively high green body strength and an excellent green body surface resolution even at 5% cement content, and it was determined that the green body strength and resolution depend on the penetration pattern.

Optimization of spray freeze drying parameters for spark plasma sintering of transparent MgAl2O4 spinel

Magnesium aluminate spinel (MgAl2O4) is considered as one of the most important spinels possessing desirable mechanical properties, with a wide range of applications at high temperatures. Powder granulation by spray freeze drying can lead to better powder flow and distribution in the mold, resulting in a specimen with high green density and strength. Accordingly, this study aimed at investigating the influential parameters on the granulation process of spinel powders via the spray freezing drying method. At first, spinel powders with various weight percents of 20, 25, 30, 35 and 50, and PVA binders with 0, 1, 2 and 8 wt% were investigated to prepare the optimal solution. Finally, the granules were sintered using the SPS method. The obtained bodies were tested by X-ray diffraction, SEM, FE-SEM, Viscosity and UV–Vis. The results showed that the optimal amount of the binder to obtain uniform granules in this process was 3%. Also, 35 wt% of solid loads represented the optimum amount for slurries, forming granules with the size range of 10–50 μm. By powder granulation, density experienced an increase of about 20%, leading to the growth of more than 15% in the in-line transmission for the IR range.

Microstructural and Physical Properties of Yttria Stabilized Zirconia (YSZ) Prepared by Ceramic Injection Moulding (CIM)

Ceramic injection moulding (CIM) is a near net shape process to produce smaller and intricate parts at a competitive cost. However, fine particle size (nano scale) used for such injection moulding process generally leads to agglomeration, higher binder content and critical dimensional shrinkage which results in defects on the sintered components. This study extensively investigates the characteristics of Yttria Stabilized Zirconia (YSZ) for CIM process. YSZ parts were moulded by CIM process that utilized a multi-component binder system using palm stearin (PS) and polyethylene (PE) in 60:40 (vol %) ratio. The powders were characterized using particle size analyzer, pycnometer density and scanning electron microscopic (SEM). The binders were characterized by the pycnometer density, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Powders loading were choosen by the critical powder volume percentage (CPVP) through oil absorption test. YSZ powder was mixed with the binders at different powder loadings ranging from 58 to 60 vol% based on CPVP. The parts were injected moulded, thermal debound, pre-sintered and sintered. The microstructure, green strength and hardness of the sintered part at different powder loadings were investigated. A large porous region was clearly observed at 58 vol% compared to 60 vol%. All samples were sintered at 1350°C, with the highest green strength and hardness were 13MPa and 357.42 HV respectively was given by the sintered part at powder loading of 60 vol.

Compositional effects on cure kinetics, mechanical properties and printability of dual-cure epoxy/acrylate resins for DIW additive manufacturing

Interest in 3D printing of thermoset resins has increased significantly in recent years. One approach to additive manufacturing of thermoset resins is printing dual-cure resins with direct ink write (DIW). Dual-cure resins are multi-component resins which employ an in situ curable constituent to enable net-shape fabrication while a second constituent and cure mechanism contribute to the final mechanical properties of the printed materials. In this work, the cure kinetics, green strength, printability, and print fidelity of dual-cure epoxy/acrylate thermoset resins are investigated. Resin properties are evaluated as a function of acrylate concentration and in situ UV exposure conditions. The acrylate cure kinetics are probed using photo-differential scanning calorimetry and the impacts of resin composition and UV cure profile on the acrylate extent of conversion are presented. Continuous and pulsed UV cure profiles are shown to affect total conversion due to variances in radical efficiency at different UV intensities and acrylate concentrations. The effects of acrylate concentration on the kinetics of the epoxy thermal cure and the final mechanical properties are also investigated using dynamic mechanical analysis and three-point bend measurements. The glass transition temperature is dependent on formulation, with increasing acrylate content decreasing the Tg. However, the room temperature shear moduli, flexural moduli, strength, strain-to-failure, and toughness values are relatively independent of resin composition. The similarity of the final properties allows for greater flexibility in resin formulation and in situ cure parameters, which can enable the printing of complex parts that require high green strength. We found that the in situ UV print intensities and exposure profiles that are necessary to achieve the best print quality are not, in most cases, the conditions that maximize conversion of the acrylate network. This highlights the importance of developing optimized resin compositions which enable complete cure of the acrylate network by promoting acrylate dark cure or thermal cure.

Influence of rice husk on the properties of fiber-reinforced silicon sol shells used in investment casting process

Herein, short carbon fibers were incorporated in silica sol slurry to enhance the strength of silica sol shell for investment casting. The content of carbon fibers, with a length of 3.0 mm and diameter of 7.32 µm, was varied from 0 to 1 wt. % of the refractory powder mass. The influence of fiber content on rheological properties was systematically investigated. The results showed that the apparent viscosities of slurry with different amounts of fibers decreased with increasing shear rate. The apparent viscosity of as-prepared slurry increased with increasing fiber content and a maximum value of 149.25 × 10−3 Pa·s was attained at the fiber content of 0.4 wt. %, which corresponded to an increase of 22.6%. The high shear viscosity of the slurry initially increased with the increase in carbon fiber content, followed by a gradual decrease. Similarly, thixotropic loop area of the as-prepared slurry, with 0.4 wt. % carbon fibers, exhibited maximum value of 239.41 Pa·s −1, corresponding to an increase of 265%. Moreover, when the fiber content exceeded 0.6%, the gas permeability was significantly increased, which can be attributed to the high fiber content, the occurrence of agglomeration and clustering. To improve the permeability and reduce the residual strength of the shell, rice husk was added in the back slurry. The rice husk modified fiber-reinforced shell exhibited higher green strength and lower residual strength. When the content of rice husk was 1.0%, the residual strength decreased by 29.10%. The porosity and permeability of shell after roasting has improved.

Evaluation of the combined addition of aluminum lactate and calcium carbonate to alumina-based castables

Calcium aluminate cement (CAC) can induce the development of high green mechanical strength to refractory castables in a short period of time (24 h). However, the production of this hydraulic binder is energy-intensive and releases a large content of carbon dioxide (CO2), which adds to global warming. Thus, aiming to develop an alternative binding system, this study investigated the combined addition of aluminum lactate (0.25-1.0 wt%) and calcium carbonate (3.2 wt%) to alumina castables. The prepared samples were analyzed using different experimental techniques, such as cold and hot modulus of rupture, hot elastic modulus, X-ray diffraction, and apparent porosity. According to the results, the lactate addition to the castables improved their green mechanical strength after drying at 110 °C/24 h (4.89 MPa) when compared to the one containing plain calcium carbonate (1.72 MPa). Furthermore, the carbonate-containing refractories presented cold and hot modulus of rupture similar or even superior to the castable containing cement in a wide temperature range (600-1500 °C).

Design and fabrication of gypsum prototypes based on binder jetting technology

Binder jet 3D printing is an emerging technology in the field of rapid prototyping which has a capability of processing a wide range of materials, including metals and ceramics. The main aim of our project is to design and fabricate gypsum prototypes based on binder jetting technology. The gypsum powder is chosen for the development of green part and Sodium silicate also called as water glass is used as a binder for the gypsum based green part. These materials are low cost, easily available materials and are utilized for developing the gypsum prototypes and hence it reduces the cost of building of binder jetting machine. This paper presents our design modifications from the existing open source binder jetting machine (Oasis 3DP) to make it efficient in terms of cost, build and performance. The modifications are introduced in the build volume, binder extrusion head, support structures and gantry mechanism. With this new design introductions in the powder 3D printing machine, we can develop intricate, complex shaped gypsum prototypes and yield high green part strength. These modifications also unlock the full potential of the binder jetting technology and allow us to vision more in utilizing different types of powder materials like sugar, metallic powder, gypsum and ceramics with suitable corresponding binder.

Printability and advantages of 3D printing mortar with 100% recycled sand

Currently, 3D concrete printing attracts worldwide attention due to its digital-construction advantages, meanwhile, the problems of pollution caused by construction and demolition waste as well as the shortage of river sand become more and more serious. Based on it, this research tries to verify the printability of 3D printing mortar with 100% recycled sand by assessing its fluidity, extrudability, printability window, and green strength. Result shows that, when additional and suitable sodium gluconate is added into the printing mortar with 100% recycled sand, this mortar can not only meet the printability requirements of 3D printing, but also obtain a longer printability window and a higher green strength than the mortar with 100% natural sand, which is beneficial for the practical application of 3D concrete printing.

Synthesis and properties of reactive polyurethane hot melt adhesive based on a novel phosphorus-nitrogen-containing polyol

A novel phosphorus-nitrogen-contaning polyol (BHAPE) was synthesized from formaldehyde solution, 2, 2'-iminodiethanol and diethly phosphite by typical Kabachnik-Fields reaction. Reactive polyurethane hot melt adhesives with hydrolysis resistance and enhanced flame retardant properties (HMPURs) were prepared from poly (1, 4-butylene adipate), poly (carbonate diol), poly (tetramethylene ether glycol) and BHAPE. The bonding strength, thermal stability, flame retardant performance, mechanical properties and hydrolytic resistance of the HMPURs were studied. The results illustrate that the incorporation of BHAPE for HMPURs could cause a higher thermal stability and more char yield. The melt viscosity, Young's modulus, tensile strength and elongation at break of HMPURs increase with the increase of BHAPE content, and then decrease when the BHAPE content reaches 4 wt%.The optimal content of BHAPE is 4–6wt% and the obtained HMPURs have the limit oxygen index (LOI) of 27.1–31.7% and the higher green strength. Meanwhile, introducing polycarbonate diol to HMPUR can effectively improve its hydrolysis resistance.

3D recycled mortar printing: System development, process design, material properties and on-site printing

3D concrete printing (3DCP) has received worldwide attention in recent years. However, very limited studies have been carried out on the simultaneous research and mutual coordination of printing system, process design and printing material. In this study, a printing system including supporting subsystem, controlling subsystem, feeding subsystem, and extruding subsystem was developed by considering the properties of mortar and the large-size characteristics of 3D mortar printing (3DMP). The process design of 3DMP, such as tool path and printing parameters, was also redesigned to improve the printing qualities. The properties of printing mortars with/without recycled sand were then comparatively studied, followed by a successful on-site printing of a 2.5 m × 2.5 m × 3 m room. Results have shown that replacing natural sand with 25% of recycled sand caused higher green strength and no obvious decrease in hardened mechanical properties. By properly adjusting printing parameters, the printing mortars (with or without recycled sand) can be well-matched with the printing system. This verifies the practicability of the developed printing system as well as the method of partial application of recycled sand in 3DMP. Besides, it is recommended that both real-time monitoring system and continuous printing are necessary to achieve higher printing quality and efficiency.

Machinability of green titanium powder compacts during drilling using HSS twist drills

The poor machinability of titanium-based alloys has until now been addressed by either choosing a suitable tool material, management of the heat generated during machining or by thermally assisted machining. This article reports on a new technique, called green machining, defined as the machining of compacted powders. Commercially pure titanium powder compacts possessing different strengths, called green strengths, were generated by uniaxial compaction and their drilling machinability characterized using the size of breakouts, surface finish of the drilled holes, and cutting forces. The cutting force for drilling sintered compacts was also determined for comparison purposes. The study found that all the green powder compacts could be drilled at high speeds regardless of their green strength. While high green strength, high speeds, and feed rates all affected machinability, feed rate was the main controlling factor. Also, depending on the binder used, cutting speed and feed rate could interact and worsen the machinability. However, compared to sintered compacts, the twist drills used on the green powder compacts did not catastrophically fail, and cutting forces were an order of magnitude less. It was concluded that green machining of titanium is a viable strategy to improve the poor machinability of the alloys.

Vinyl Cis Rubber superior polymer for future trends

Vinyl Cis Rubber (VCR) is cis-1,4-polybutadiene reinforced by highly crystalline fibrous syndiotactic-1,2-polybutadiene (SPB). It has been developed and commercialized by Ube Industries Co.,Ltd. The highly crystalline syndiotactic-1,2-polybutadiene having melting point (M.p.) up to 473K is obtained by using cobalt catalyst system. Fibrous crystalline domains in VCR are effective for the stress-induced crystallization of the rubber matrix. VCR has various improved properties such as high green strength, good processing, large modulus, good flex-crack resistance, etc. The improvement of physical and dynamic mechanical properties depends on the content of syndiotactic-1,2-polybutadiene dispersion in cis-1,4-polybutadiene matrix and rubber compound recipe. For examples; with the same compound recipe, VCR vulcanized significantly improve in modulus up to 80%, increase hardness up to 10%, and improve in flex-crack resistance. With the same hardness of vulcanized recipe, filler can be reduced down to 10-20 part per hundred rubbers consequently energy loss performance can be improved up to 10-18%. This polymer is widely used in eco-friendly tires, high performance tires, out sole of shoe, conveyor belts and other rubber goods.

Innovative in situ photocuring-assisted 3D plotting technique for complex-shaped ceramic architectures with high shape retention

We herein propose a novel type of additive manufacturing (AM) technique, denoted as “in situ photocuring-assisted 3D plotting”, which can rapidly solidify extruded green filaments comprised of ceramic powders and photocurable monomers using UV light during 3D plotting. To accomplish this, the rheological properties and photocuring behavior of the ceramic slurry were carefully tailored, particularly by using a mixture of diruethane dimethacrylate (UDMA) and triethylene glycol dimethacrylate (TEGDMA) monomers as the photocurable vehicle. This innovative approach enabled the favorable extrusion of the ceramic slurry through a fine nozzle with high green strength after photocuring, and thus complex-shaped ceramic architectures with high shape retention could be constructed. As an example, a free-standing helical structure with a circular cross-section was successfully produced even without the use of any supporting materials. In addition, a porous ceramic scaffold with a tightly controlled porous structure could be produced.

Effect of Flow Pattern on Energy Consumption and Properties of Copper Powder in the Electrolytic Process

Because of distinctive properties, such as dendritic structure, high green strength, and low oxygen content, electrolytic copper powder has been widely used in aviation, aerospace, national defense industry and other domains. But at present, energy consumption of the electrolysis process in copper powder production is high, and the current efficiency is only about 90%. Therefore,the decrease in energy consumption of the electrolysis process has become the major bottlenecks in the development of the enterprises. In this paper, a new electrolysis cell with different electrolyte inlet arranged on the cell was manufactured. Then, the effect of flow pattern of electrolyte on the current efficiency, energy consumption and properties of copper powder was investigated. The experimental results showed that the electrolytic process had the higher current efficiency, lower energy consumption and smaller copper powders when the flow rate is 0.5l/min in the paralleled inlet and 1.5 l/min in the traditional inlet. Under the optimal conditions, the current efficiency, energy consumption and copper powder size were 99.10%, 712.90kw∙h/t and 47.80um respectively. This means an obvious rise in current efficiency and decrease in energy consumption compared to traditional feeding method.

Effect of fine alumina in improving refractoriness of ceramic shell moulds used for aeronautical grade Ni-base superalloy castings

This paper discusses an improvement in shell refractoriness and dimensional stability of columnar grained (CG) low pressure turbine blade castings made using Ni base superalloy by directional solidification process (DS). Two ceramic shell systems were adopted, namely shell system I and II. Shell moulds were prepared by using ceramic slurries containing zircon flour as a filler material and colloidal silica as a binder. As compared to shell system II (zircon filler with colloidal silica binder and fine alumina), shell system I (zircon filler with colloidal silica binder) showed lower refractoriness. Shell system II showed an increase in the flexural strength both in the green as well as in fired conditions. Shells made from shell system II showed about 13% higher green strength and 55% higher fired strength as compared to shell system I. Shell system II also exhibited superior self sag resistance up to 1625 °C. Moulds prepared from this shell system yielded aeronautical grade casting with high dimensional accuracy even at a metal pouring temperature of 1550 °C. Moulds from shell system I, on the other hand, underwent sagging even at metal pouring temperature of 1500 °C, leading to dimensionally unacceptable castings. The superior performance of shells prepared from shell system II can be ascribed to the presence of fine alumina in the shell.

Influence of sawdust on the properties of the ceramic shell used in investment casting process

The quality of the investment casting (IC) ceramic shell was enhanced using sawdust as an inexpensive and naturally available additive in the ceramic slurries. The important shell properties such as thickness, porosity, permeability and flexural strength were determined. The ceramic shell accommodated a substantial amount of sawdust because of its low particle density and porous structure which resulted in higher green strength and thickness of the shell. The same shell upon firing led to increased shell porosity and permeability with good knockability characteristic. The addition of sawdust in the secondary layers did not affect the composition of the primary layer of the ceramic shell. The mechanical properties of the aluminium-silicon (Al-Si) alloy castings obtained from the sawdust-modified shells were found to be comparatively higher than those obtained from the conventional shells.

Rheological Behavior of Yttria Stabilized Zirconia (YSZ) Feedstock for Ceramic Injection Moulding (CIM) Process

The characterization of CIM feedstock consisting 58, 59, 60vol% of YSZ powder with binder system comprising a palm stearin (PS) and low density polyethylene (LDPE) were studied. The effects of powder loading and temperature (°C) on the rheological behavior of the YSZ were investigated by using Rosand RH2000 Capillary Rheometer. The results showed all the feedstock achieved desirable injection moulding characteristics such as pseudoplastic behavior, flow behavior index (n) less than 1 and low activation energy (E). The rectangular parts were successfully injected moulded at optimum temperature of 170°C with the highest green strength was 12.7 N, obtained from 60 vol% powder loading which correspond to greater density and low porosity of the samples.

Effects of process parameters on nonaqueous gelcasting for copper powder

An approach to fabricate sintered copper with high green strength and high sintered density using nonaqueous gelcasting technology is presented in this study. The effects of various gelcasting processing parameters such as monomer content, monomer/crosslinker ratio, initiator content, dispersant dosage, and temperature on the flexural strength of dried green bodies and the relative density of sintered bodies were studied to obtain better microstructures and properties. The appropriate process parameters obtained for copper gelcasting are as follows: monomer content, 20vol%–30vol% (based on the total volume of reagents); monomer/crosslinker ratio, 10:1 to 20:1; initiator content, 3vol%–4vol% (based on the volume of the monomer); dispersant dosage, 1.5wt%–2.5wt% (based on the mass of the copper powder); and reaction temperature, 65–75°C.