Classification of 3D printing materials
1. according to the physical state of the material classification
Can be divided into liquid materials, sheet materials, powder materials, filamentary materials, etc.
2. According to the chemical properties of the material classification
According to the different chemical properties of materials can be divided into resin materials, paraffin materials, metal materials, ceramic materials and their composite materials.
3. According to the classification of material forming method
According to the different molding methods can be divided into: SLA material, LOM material, SLS material, FDM material, etc.
Liquid material: SLA, photosensitive resin
Solid powder: SLS
Non-metal (wax powder, plastic powder, laminated ceramic powder, laminated sand, etc.)
Metal powder (laminated metal powder)
Solid state sheet: LOM
Paper, plastic, ceramic foil, metallic platinum + binder
Solid state filaments: FDM
Wax filaments, ABS filaments, etc.
Second, the basic properties of 3D printing materials
1. 3D printing general requirements for material properties.
Facilitate rapid and accurate processing of prototype parts.
Rapid prototyping parts should be close to the final requirements, should try to meet the requirements for strength, stiffness, moisture resistance, thermal stability performance, etc..
It should facilitate the subsequent processing process.
2. Requirements for material properties for different application goals.
The four application targets of 3D printing: concept type, test type, mold type, and functional parts, have different requirements for molding materials.
The concept type does not require high material molding accuracy and physical and chemical properties, and mainly requires fast molding speed. For example, for photosensitive resin, lower critical exposure power, larger penetration depth and lower viscosity are required.
The test type has certain requirements for strength, stiffness, temperature resistance and corrosion resistance after molding to meet the test requirements. If used for assembly testing, the molded part is required to have certain accuracy requirements.
Mold type requires materials to adapt to specific mold manufacturing requirements, such as strength and hardness. For example, for the prototype for disappearing mold casting, the material is required to be easy to remove, with less residue and ash after ablation.
Functional parts, on the other hand, require materials with good mechanical and chemical properties.
Third, 3D printing light-curing molding materials
1, 3D printing light-curing material applications
Making various resin samples or functional parts for structural verification and functional testing.
Making fine parts.
Making parts with transparent effect.
master moulds for rapid moulds, turning various rapid moulds.
Replacing the vanishing mold in investment casting to produce metal parts.
2、Light-curing molding resin needs to have the following characteristics
Low viscosity, which is conducive to the molding resin faster leveling, to facilitate rapid molding.
Curing shrinkage is small, curing shrinkage leads to parts deformation, warping, cracking, etc., affecting the accuracy of molded parts, low shrinkage resin is conducive to molding out of high-precision parts.
High wet-state strength, high wet-state strength can ensure that the post-curing process does not produce deformation, expansion and interlayer peeling.
Small dissolution rise, the dissolution rise of the wet state molded parts in the liquid state resin causes the part size to be large.
Less impurities, no odor in the curing process, less toxicity, conducive to the operating environment.
3, the composition of light-curing molding resin and curing mechanism
The photosensitive resin applied to SLA technology is usually composed of two parts, namely photoinitiator and resin, where the resin consists of prepolymer, diluent and a small amount of additives.
When the photoinitiator in the photosensitive resin is irradiated by the light source (specific wavelength of ultraviolet light or laser) to absorb energy, free radicals or cations will be generated, and the free radicals or cations will activate the monomer and active zwitterion to generate polymer cured products by cross-linking reaction.
4、SLA resin shrinkage deformation
Resin shrinkage will occur during the curing process, usually the line shrinkage rate of about 3%. From the perspective of polymer chemistry, the curing process of photosensitive resins is the process of transformation from short small molecules to long-chain macromolecular polymers, and its molecular structure changes greatly, so the curing process of shrinkage is inevitable.
Explained in terms of polymer physics, in the liquid state between the small molecules for the van der Waals force distance, while the solid state of the polymer, the structural units between the covalent bond distance, covalent bond distance is much smaller than the distance of the van der Waals force, so the liquid state prepolymer curing into a solid polymer, will inevitably lead to the volume of the part shrinkage.
Although the resin has been polymerized during the laser scanning process, but only part of the polymerization is completed, there are still parts in the liquid state of the residual resin is not cured or not fully cured (scanning process to complete partial curing, to avoid deformation caused by full curing), part of the strength of the part is also obtained in the post-curing process, therefore, the post-curing process to complete the polymerization of the resin inside the part to improve the part Therefore, the post-curing process is essential to complete the polymerization of the resin inside the part and improve the final mechanical strength of the part. During post-curing, the polymerization of the uncured resin within the part occurs and volume shrinkage produces uniform or non-uniform deformation.
Unlike the deformation during the scanning process, post-curing deformation occurs when the finished part is made up of thin layers bonded to each other by a certain spacing of scan lines within the layers, with uncured resin between the lines and faces, shrinkage stresses and constraints between each other, and temperature stresses caused by cooling from processing temperature (generally higher than room temperature) to room temperature. However, the already cured part has a restraining effect on the post-curing deformation, slowing down the post-curing deformation.
Parts in the post-curing process also to produce deformation, the experimental measurement of parts after curing shrinkage accounted for 30% to 40% of the total shrinkage.
6, the development of SLA materials
(1) SLA composites
SLA light-curing resin by adding nano-ceramic powder, short fibers, etc., can change the strength of the material, heat resistance, etc., to change its use, there are already directly available as a tool of light-curing resin.
(2) SLA as a carrier
SLA light-cured parts as a shell, which is filled with functional materials, such as bioactive substances, high temperature, SLA ablation, the manufacture of functional parts.
(3) Other special performance parts, such as rubber elastic material.
Fourth, 3D printing powder sintering molding materials
Theoretically, all powder materials that can be bonded to each other after heat or powder materials covered with thermoplastic (solid) binder on the surface can be used as SLS materials.
However, to be really suitable for SLS sintering, the powder material must have good thermoplasticity (solid), certain thermal conductivity, and certain bonding strength after the powder is laser sintered; the particle size of the powder material should not be too large, otherwise it will reduce the quality of the molded parts; and the SLS material should also have a narrow "softening - curing" temperature range, which When the range is large, the accuracy of the parts will be affected.
Broadly speaking, the basic requirements of 3D printing laser sintering process for molding materials are.
Good sintering properties for rapid and accurate prototyping without special processes.
For prototypes used directly as functional parts or molds, mechanical and physical properties (strength, rigidity, thermal stability, thermal conductivity and processing properties) to meet the requirements of use.
When the prototype is used indirectly, it should be conducive to quick and easy subsequent handling and processing processes, i.e., the interface with the subsequent process should be good.
(1) use: sintering to make wax models, precision casting metal parts.
(2) traditional molten die precision casting wax (alkane wax, fatty acid wax, etc.), its melting point is low, at about 60 ℃, short firing time, no residue after firing, good adaptability to molten die casting, and low cost.
(3) But there are the following disadvantages.
Sensitivity to temperature and high melt fluidity during sintering, which makes molding not easy to control.
Poor molding accuracy, with a wax mold dimensional error of ±0.25mm.
Low strength of wax mold, which is difficult to meet the requirements of castings with fine and complex structures.
The preparation of powder is very difficult.
2, polystyrene (PS), polycarbonate, engineering plastics (ABS)
Polystyrene (PS) is a thermoplastic resin, melting temperature of 100 ℃, after heating can be melted, bonding, after cooling can be cured molding, and the material moisture absorption rate is very small, only 0.05%, shrinkage rate is also small, its powder after modification, can be used as laser sintering molding materials.
Sintered molding parts have the following functions by different post-treatment processes: First, combined with the resin dipping process to further improve its strength, it can be used as prototype parts and functional parts. Second, after the immersion wax post-treatment, can be used as fine casting wax mold, through the investment mold precision casting, the production of metal castings.
3、Nylon powder (PA)
Powder particle size is small, making models with high accuracy, used for CAD data verification; because of sufficient strength can be functional verification.
Sintering temperature - powder melting temperature of 180 ℃.
Sintered parts do not need special post-treatment, that can have 49MPa tensile strength.
(3) Other: nylon powder sintering rapid prototyping process, the need for higher preheating temperature, the need for a protective atmosphere, high equipment performance requirements.
4, laminated sand powder, laminated ceramic powder materials
(1) laminated sand similar to the foundry laminated sand, using thermosetting resin, such as phenolic resin covered zirconium sand (ZrO2), quartz sand (SiO2) method of production. Using the laser sintering method, the prototype made can be directly used as a casting sand (core) to make metal castings, where zirconium sand has better casting performance, especially suitable for casting of non-ferrous alloys with complex shapes, such as magnesium, aluminum and other alloys.
Material composition: quartz sand or zirconium sand coated with phenolic resin, with a particle size of 160 mesh or more.
Applications: quartz or zirconium types (cores) for sand casting.
Application examples: sand casting and core making, suitable for single piece and small batch sand casting metal casting production, especially suitable for metal casting which is difficult to be realized by traditional manufacturing technology.
(2) Clad ceramic powder
Similar to the production process of clad sand, the clad ceramic powder can be Al2O3, ZrO2 and SiC, etc. After laser sintering and rapid molding, combined with the post-treatment process, including degreasing and high temperature sintering, it can be quickly manufactured for precision casting shells and then cast metal parts.
Can also be directly manufactured engineering ceramic parts, sintering and then hot isostatic pressing treatment, the final relative density of parts up to 99.9%, can be used for oil bearings and other wear-resistant, heat-resistant ceramic parts.
The method of manufacturing metal functional parts with SLS has indirect method and direct method, among which the indirect method is faster, more accurate, the most mature technology and the most widely used.
5.1 Indirect sintering molding.
(1) Principle of indirect sintering molding. Polymer is used as the binder. Due to the low softening temperature, good thermoplasticity and low viscosity of the polymer, the polymer is wrapped on the surface of the metal powder or mixed with the metal powder material in some form by the wrapping process, and the polymer becomes molten by laser heating during SLS molding and flows into the metal powder particles to bond the metal powder together. In the green part, there are both metal and polymer components. The blanks are subjected to thermal degradation, secondary sintering and post-treatment with metallization to become pure metal parts.
Among the materials used in the indirect method, the structural materials are metals, mainly stainless steel and nickel powder, and the polymers are mainly thermoplastic materials.
There are two types of thermoplastic polymer materials, one is amorphous and the other is crystalline. The arrangement of molecules in the molecular chain of amorphous materials is disordered, such as PC materials; the arrangement of molecules in the molecular chain of crystalline materials is ordered, such as nylon (nylon) materials. Both of these thermoplastic polymers can be used as binders in SLS materials.
Since amorphous and crystalline materials each have different thermal properties, they also determine different SLS process parameters.
Polymers are present in the molding material in two main forms, a mechanical mixture of polymer powder and metal powder, and a homogeneous coating of polymer on the surface of the metal powder particles. There are various methods to cover the polymer on the surface of the metal powder, such as the thermoplastic material can be made into a solution, diluted and mixed with the powder, stirred, and then dried; the polymer can also be heated and melted, sprayed in the form of mist, and covered on the surface of the powder.
In the case of the same mass fraction of polymer and metal powder, the strength of the cladding powder after sintering is higher than that of the mechanically mixed material.
At present, the most applied molding materials are mainly cladding metal powder.
(2) Indirect method sintering molding process
Process parameters: laser power, scanning speed, scanning spacing, powder preheating temperature.
Formed blanks must be post-treated to become dense metal functional parts. There are generally three post-processing steps: degradation of the polymer, secondary sintering and metal infiltration. These three stages can be carried out in the same heating furnace with a protective atmosphere of 30% hydrogen and 70% nitrogen.
Degradation of the polymer
The degradation heating is done in two different temperature holding stages, first the billet is heated to 350°C for 5 h and then it is heated to 450°C for 4 h. In both temperature stages the polymer decomposes and its products are a variety of gases which are removed by means of an extraction system on the heating furnace. By degradation, more than 98 % of the polymer is removed.
When the polymer is mostly degraded, the metal powder is held between the grains by only a residual bit of friction between the polymer and the metal powder, which is very small. To maintain the shape, a new linkage must be established between the metal powder grains, which is done by heating the blanks to a higher temperature and creating the linkage by diffusion. The heating temperature is determined according to the material; for RapidSteel110, it is heated to about 1000°C and held for 8h.
Infiltration of metal
The molded part after secondary sintering is porous and not very strong, the way to improve the strength is to infiltrate metal. After the melting of the metal with lower melting point, under the action of capillary force or gravity, it fills all the voids in the molded part through the interconnected holes in the molded part, so that the molded part becomes a dense metal part. Metal infiltration is carried out in a controlled atmosphere or in a vacuum. In a controlled atmosphere, the infiltrated metal must flow in one direction, so that the air in the interconnected pores can leave the molded part; if it infiltrates in multiple directions, the gas in the molded part will be sealed in the body, forming pores and weakening the strength. If the molded part is placed in a vacuum chamber to infiltrate metal, as there is no air in the molded part, the molded part can be immersed in the liquid metal and the metal liquid will infiltrate from all around at the same time, which is fast and short.
(3) Indirect sintering rapid forming parts process characteristics
Indirect sintering of metal parts with SLS system is faster and can manufacture metal parts with complex shapes, mainly used for rapid manufacturing of injection and die-casting molds. The disadvantages of the indirect method are the limited accuracy of the parts, the limited compensation due to the volume shrinkage during the degradation and secondary sintering process, and the long post-processing time.
To solve these problems, research is conducted in the following two aspects: improving the binder, infiltrating non-metallic materials, eliminating the degradation and secondary sintering process, so that the blanks do not pass the heating, so that the molded parts have high precision, short manufacturing cycle, low cost, and can meet the requirements of the mold with short service life.
5.2 Direct sintering molding
Compared with indirect sintering, the direct sintering process is significantly shorter and does not require the complicated post-processing stage in indirect sintering. However, a higher power laser is necessary to ensure the direct melting of the metal powder during the direct sintering process.
Therefore, the choice of laser parameters in direct sintering and the control of the melting process of the sintered metal powder material is the key in sintering and forming. The laser power is an important factor in the laser direct sintering process. The higher the power, the higher the energy density in the laser range of action, the more fully the material is melted, while the more material is involved in the sintering process, the larger the size of the melt pool formed, the larger the powder.