Post-processing is a key step in the application process of 3D printing, and so far, no matter which 3D printing process is used, it involves more or less the following methods - powder cleaning, annealing, post-curing, desupporting, sanding, sandblasting, coloring, etc. Heat treatment is also an important part of the 3D printed part application process and can take many forms depending on the desired result, the material used, and the preferred technology.
In additive manufacturing, the parts produced are subjected to different temperatures and go through more or less important heating and cooling phases depending on the printing process chosen. These stages have a direct impact on the part where residual stresses are accumulated. Heat treatment after printing is crucial because it not only eliminates defects but also affects the mechanical properties of the part, such as bending resistance, ductility, and even final hardness.
In this article, the 3D Printing Technology Reference will review and discuss the heat treatment methods such as sintering, metal annealing, hardening (curing), and hot isostatic pressing that exist in additive manufacturing, and understand their respective challenges. These post-processing steps have a specific goal in mind β to reduce the internal stress of the 3D printed part and improve its mechanical properties. Depending on the technique chosen, different results are obtained, but the goal is to influence the residual stresses that may occur during the printing process.
Sintering is one of the heat treatment methods that can be used for additive manufacturing, and strictly speaking, this is more of a part of the manufacturing processCritical steps, not post-processing methodsγIn fact, it's an indispensable part of using binder jetting or other indirect 3D printing. Through these two processes, the green body of the part, which contains inorganic or organic binders, is currently removed by thermal degreasing, and then the metallurgical bonding of the metal particles is continued at higher temperatures.
*: Markforged sintering furnace is an important equipment in this link, at present, vacuum or atmosphere sintering furnace is mostly used, the former uses high vacuum to avoid oxidation of parts at high temperatures, and the latter is to prevent 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, and the sintering process reduces the porosity of the part and increases its hardness. In addition to metal printing, this heat treatment step is also suitable for ceramic printing.
Shrinkage and deformation are important problems that have to be faced in this link. During the sintering process, the removal of the binder allows the powder material to gradually occupy the vacant position, which will reduce the size of the original part. At present, the final part size is guaranteed as much as possible by pre-expanding the size of the part and the first treatment, and it is also necessary to cooperate with certain post-processing means.
Currently, light-curable 3D printed polymers also need to be post-cured to improve their final properties, which is a hardening or curing stage. Resins are made up of several monomers that are not joined together in a liquid state, and when exposed to UV sources, these identical monomers will combine to form the desired part. However, after printing, some areas will not be optimally crosslinked, risking damage to the overall resistance of the printed component. This is where the hardening step comes into play, and it will allow the cross-linking reaction to be done adequately.
*: Once a Formlabs part has been created, it is cleaned to remove excess non-photopolymeric resins. The parts can then be placed into the right machine β typically, the resin 3D printer manufacturer will provide a curing station. This is a UV chamber that hardens the printed part and gives it its final properties. This reduces its fragility and reduces the risk of breakage. Curing also fixes the color of the resin, making it safer to handle. Parts develop stronger strength over time, which is critical in many areas.
Factors such as the type of resin used or the size of the part will directly influence this heat treatment step. Most of the time, the larger the part, the longer it will take to cure.
This heat treatment method exposes the 3D printed part to a certain temperature, which depends on the material used. After heating, it is gradually cooled to increase its strength. It is a technique that is widely used in metals. This annealing can also be done on certain plastics, such as PLA and PETG. It is not suitable for other thermoplastics, such as ABS, because the heat tends to cause excessive warping or deformation.
Some thermoplastics are suitable for annealing (**ultimaker), specifically, annealing is the heating of a material to a temperature above its crystallization temperature but below its melting point. In the case of polymers, this will allow the molecule to self-recombine, improving the stability of the final part. For metals, annealing will have the potential to increase grain size due to their recrystallization. In both cases, the duration of annealing has an impact on the structure of the final part β the longer it is exposed to heat, the greater its structural changes. For example, smaller plastic parts can be obtained after the annealing step, as the heat accelerates their contraction. This needs to be taken into account when making the relevant settings.
Annealing in metal 3D printing will improve the ductility of the manufactured parts and their strength.
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 property of the laser powder bed fusion process. During stress annealing, the internal stress gradually disappears or decreases, accompanied by the rearrangement and refinement of the crystalline of the material. This can improve the plasticity, toughness and ductility of the material, and reduce the hardness and strength of the material. So as to improve the overall properties of the material.
In addition, it is also possible to use tempering to strengthen 3D printed metal parts, which is an operation that heats the part to a very high temperature and cools it quickly, which will affect the microstructure of the part piece;Quenching usually requires the use of liquids to cool the parts.
3D printing for HIP processing (**Bodycote) HIP pressing is another extremely important step, whether it is aerospace or medical. Hot isostatic pressing (HIP for short) is to place the product in a closed container, apply equal pressure in each direction to the product, and apply high temperature at the same time, under the action of high temperature and high pressure, the product can be sintered and densified. After the HIP process, defects can be eliminated and the micromorphology and mechanical properties can be improved.