Heat treatment is a key step in the application process of 3D printing. So far, regardless of which 3D printing process is used, it involves several methods to varying degrees, such as powder cleaning, annealing, post curing, unsupported, polished, sandblasted, and colored. Heat treatment is also an important step in the application process of 3D printed parts, and can take various forms depending on the expected results, materials used, and preferred technology.
In additive manufacturing, the produced parts are affected by different temperatures and undergo more or less important heating and cooling stages depending on the selected printing process. These stages have a direct impact on parts that accumulate residual stress. Heat treatment after printing is crucial as it not only eliminates defects but also affects the mechanical properties of the parts, such as bending resistance, ductility, and even final hardness.
01. Sintering
Sintering is one of the heat treatment methods that can be used in additive manufacturing, strictly speaking, it is more of a critical step in the manufacturing process rather than a post-treatment method. In fact, this is an indispensable and important step when using adhesive spraying or other indirect 3D printing methods. By using these two processes, green parts containing inorganic or organic binders can be obtained, which are currently removed through thermal degreasing methods, and then metallurgical bonding of metal particles can be achieved at higher temperatures.
The sintering furnace is an important equipment in this process, and currently vacuum or atmosphere sintering furnaces are commonly used. The former avoids oxidation of parts at high temperatures through high vacuum, while the latter prevents oxidation of parts through reducing atmosphere. During the sintering process, the temperature is usually set slightly lower than the melting temperature of the material used. The sintering process can reduce the porosity of the part and increase its hardness. In addition to being suitable for metal printing, this heat treatment process is also suitable for ceramic printing.
Shrinkage and deformation are important issues that must be addressed in this stage. During the sintering process, the removal of the binder gradually occupies the empty space of the powder material, which will reduce the size of the original part. At present, it is common to ensure the final size of the parts by pre enlarging the dimensions and simulating them, while also requiring certain post-processing techniques.
02. Polymer post curing
At present, photopolymer 3D printing also requires post curing to improve its final performance, which is a hardening or curing stage. Resin is composed of several monomers that are not connected together in liquid state. When exposed to a UV source, these identical monomers will bond together to form the desired components. However, after printing, some areas may not achieve optimal cross-linking, which poses a risk of damaging the overall resistance of the printing component. This is where the hardening step comes into play, as it will fully complete the crosslinking reaction.
After the part is created, it will be cleaned to remove excess non photopolymerized resin. Then the parts can be placed into suitable machines - typically, resin 3D printer manufacturers provide curing stations. This is a UV chamber that can harden printed components and give them their final properties. This reduces its vulnerability and lowers the risk of damage. Curing can also fix the color of the resin, making it safer to handle. Parts have stronger strength over time, which is crucial in many fields.
The type of resin used or the size of the parts will directly affect this heat treatment step. In most cases, the larger the part, the longer the curing time required.
03. Annealing in 3D Printing
This heat treatment method exposes 3D printed components to a certain temperature, which depends on the materials used. After heating, gradually cool to increase its strength. This is a widely used technology for metals. Some plastics can also undergo this annealing, such as PLA and PETG. For other thermoplastic materials, such as ABS, it is not suitable because heat often causes excessive warping or deformation.
04. Metal Heat Treatment Methods
Stress relief annealing is one of the commonly used heat treatment methods for metal 3D printing, and residual stress is an inevitable product of rapid heating and cooling, which is an inherent characteristic of laser powder bed melting technology. During the process of stress annealing, the internal stress will gradually disappear or decrease, accompanied by the rearrangement and refinement of the material's crystals. This can improve the plasticity, toughness, and ductility of the material, while reducing its hardness and strength. Thereby improving the overall performance of the material.
In addition, tempering strengthening can be used to 3D print metal parts, which is an operation of heating the parts to very high temperatures and rapidly cooling them, which will affect the microstructure of the parts; Quenching usually requires the use of liquid to cool the parts.
Hot isostatic pressing is another extremely important step that cannot be skipped in both aerospace and medical applications. Hot isostatic pressing (HIP) is the process of placing a product in a sealed container, applying equal pressure to the product in all directions, and simultaneously applying high temperature. Under the action of high temperature and high pressure, the product can be sintered and densified. Through the process of hot isostatic pressing, defects can be eliminated, and the microstructure and mechanical properties can be improved.
3D printing is only one aspect of this manufacturing method, and this viewpoint has been accepted by most users. Especially the steps related to post-processing or heat treatment are crucial in the application process of parts, involving equipment, temperature, and time settings, which directly affect the final performance of the parts.
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