This article introduces common 3D printing technologies, compares their resolution characteristics and factors affecting resolution, and provides methods for evaluating resolution, comprehensively understanding the resolution of different 3D printing technologies
Common 3D printing technologies
3D printing technology is also known as additive manufacturing, and the international standards organization (ISO) divides it into seven general types. Here are some common 3D printing technologies:
- Material extrusion technologyThe material is extruded through the nozzle, usually the material is plastic filament, melted and extruded by the heating nozzle, and the printer places the material on the construction platform according to the software planning path, cools and solidifies to form the object.
- Fused Deposition Modeling (FDM) : This is the most common type of material extrusion technology, a market worth billions of dollars and with numerous machines. It starts with a digital model, feeding the filament on the spool into the nozzle of the extrusion head. The nozzle heats and softens the filament, and the layers are connected to form components. Depending on the geometry of the model, support structures may need to be added. Support can be removed later, and support made of partially soluble materials can be removed by solution dissolution. The material range of this technology is wide and the price is relatively low, ranging from $100 to high, but the material properties (strength, durability, etc.) are usually low and the dimensional accuracy is not high.
- 3D bioprinting : This is a special material extrusion technology where organic or biological materials (such as living cells and nutrients) are combined to create a three-dimensional structure similar to tissue. It can be used for tissue engineering, drug testing and development, and regenerative medicine therapy. Although the actual definition is still evolving, the working principle is similar to FDM. The material is discharged by a needle (bio-ink) to create a printed layer. The bio-ink is mainly composed of active substances such as cells. In the carrier material, collagen acts as a molecular scaffold to provide support.
- Construction 3D Printing : Mainly used in the construction industry. It involves the use of ultra-large 3D printers to extrude building materials such as concrete. The printer is often a gantry or robotic arm system. This technology has been used to build multiple 3D printed houses in the US and Europe, and is of great significance for building habitats on the moon and Mars using local materials. It is expected to change the construction industry to reduce labor demand and construction waste.
- Barrel polymerization technology (resin 3D printing)The light source selectively solidifies (or hardens) the photosensitive polymer resin in the barrel. After the first layer is cured, the construction platform moves slightly in a certain direction, and then the next layer is cured and connected to the previous layer. After printing, the object is cleaned and the mechanical properties of the component are enhanced after curing.
- Stereolithography (SLA) : This is a common barrel polymerization technique. By using ultraviolet laser (355nm or 405nm) as the light source and using a galvanometer system to control the laser light spot scanning, the laser beam outlines the shape of the first layer of the object on the surface of the liquid resin, then the production platform descends a certain distance (between 0.05-0.025mm), and then the cured layer is immersed in the liquid resin, and the final solid printing is completed by repeated operation. It is suitable for printing high-precision and high-life prototypes. Although the price is relatively expensive and there is a problem of slow printing speed, the cost performance is relatively high.
- Digital Light Processing (DLP) : First developed by Texas Instruments. The principle is to use a projector to solidify the photosensitive polymer liquid layer by layer to create a 3D printed object. The slicing software slices the model into thin slices, and the projector plays slides. Each layer of image produces photopolymerization reaction solidification in the thin area of the resin layer, forming a thin layer of the part. The forming table moves one layer, and the projector continues to play the next slide to process the next layer. The printing accuracy is very high, up to the micron level, and the printing speed is fast. It is suitable for manufacturing small precision parts, dental models, jewelry, etc., but the projection size is limited, mainly used for printing small-sized objects.
- Liquid Crystal Display (LCD), also known as Mask Stereolithography (MSLA) : An open-source technology that emerged in 2013, based on the principle of liquid crystal panel imaging. It uses optical projection to filter out infrared and ultraviolet rays through the red, green, and blue primary color filters, and then projects the three primary colors through three liquid crystal panels to synthesize projection imaging. However, this technology requires high-power ultraviolet light. LCD screens themselves are afraid of ultraviolet rays and are prone to aging. They also need to withstand tests such as heat resistance and high temperature heat dissipation. The core components have a short lifespan and are suitable for personal and small model production.
- Material spraying technique
This technology sprays materials onto the construction platform through an injection head, which can be in liquid or powder form. The injection head can precisely control the injection amount and position of the material, thus forming a high-precision model.
- Adhesive spray technique
Powder materials are bonded together by spraying adhesive to form a three-dimensional object. This technique can use a variety of powder materials, such as ceramic powder, metal powder, etc.
- Directed energy deposition technique
Mainly used for printing metal materials. Metal materials are melted and deposited layer by layer through high-energy beams (such as lasers or electron beams) to form three-dimensional objects. This technology can manufacture complex metal parts with high strength and accuracy.
- Sheet lamination technique
Layer by layer, materials (such as paper, plastic sheets, etc.) are stacked and bonded together to form a three-dimensional object. This technique is suitable for manufacturing large models with low precision requirements.
The resolution characteristics of different 3D printing technologies
- Fused deposition modeling (FDM)
In terms of resolution, FDM technology is constrained by various factors and has limited accuracy. The nozzle diameter has a significant impact on resolution. A larger diameter nozzle makes the printed lines thicker. For example, when the nozzle diameter is 0.4mm, the extruded material width is relatively large, making it difficult to construct fine structures. In addition, the axial movement accuracy of XYZ also plays a big role. If the axial positioning is not precise enough, deviations are prone to occur when stacking layer by layer. The bonding force between layers is lower than that of photosolid materials, and the upper layer material may squeeze the lower layer material. Improper handling can cause printing problems such as distortion, delamination, and undershrinkage, which in turn affects resolution. For example, when printing a model with a vertical thin column structure, there may be cases where the overall resolution is affected by tilting or the connection between layers is not tight.
- Stereolithography (SLA)
SLA technology has high resolution. It constructs the shape of each layer of an object by accurately scanning the surface of liquid photosensitive resin with ultraviolet laser. Its accuracy can reach the micron level because the laser light spot can control the irradiation area very finely. For example, when printing micro models or parts with fine details, SLA can clearly display the details, just like printing some fine-grained artwork ornaments, which can accurately print the fine texture. Moreover, the layer thickness of SLA technology can be set relatively small, such as some devices can achieve a layer thickness setting between 0.025-0.05mm, which makes the vertical resolution higher.
- Digital Light Processing (DLP)
The characteristic of DLP is high precision, which can achieve a minimum light spot size of ± 50 microns. Due to the use of digital micromirror components to project product cross-sectional graphics onto the surface of liquid photosensitive resin, the curing accuracy of each layer is very high. Compared with SLA technology, DLP has advantages in light spot error control. The high power of SLA lasers can easily lead to large molding light spot errors, while DLP is easier to achieve micron-level accuracy. At the same time, the resolution of DLP technology is not greatly affected by the proximity of the projector to the resin surface, and can still achieve accurate curing. However, due to its working principle, the projection size is limited, which indirectly affects its resolution performance for large objects, mainly suitable for printing small-sized and high-precision parts.
- Liquid crystal display (LCD)
LCD technology has certain limitations in terms of resolution. In principle, although its core is the imaging principle of the LCD panel, due to the need for less ultraviolet light to pass through, and in order to ensure solidification and molding, the performance requirements for the LCD screen are very high. In actual use, the LCD screen is prone to aging and damage, which is not conducive to long-term stable maintenance of high resolution. In addition, the imaging principle of LCD may have uneven light at the edge, which affects the resolution of the final printed object edge. For example, when printing models of some slender structures, the edge part may have a more obvious rough feeling.
- 3D bioprinting
3D bioprinting resolution is in the process of continuous development and improvement. Since bioprinting faces organic or biological materials (such as living cells), the technical difficulty is relatively large. At present, bioprinting still needs to improve the resolution when constructing more complex and fine vascular networks and microscopic tissue structures. This is because in addition to accurately placing biological materials in spatial positions, bioprinting also needs to consider the maintenance of biological characteristics such as cell activity and growth. Therefore, in terms of resolution performance, compared with traditional printing technologies mainly based on non-biological materials, the accuracy of microscopic fine structure construction is relatively low, but it can meet the accuracy requirements of building block, simple tubular and other structures in current tissue engineering and other fields, and the technology is constantly breaking through and developing.
- Architectural 3D printing
The resolution is relatively low. Architectural 3D printing faces the construction task of large building structural components, usually using materials with larger particles and poor fluidity such as concrete. The concrete material extruded from the nozzle is difficult to achieve fine control like small printed parts when stacked. For example, when printing walls, the layered thickness is often at the centimeter level. Although it can meet the structural requirements at the architectural scale, such as maintaining the basic shape and integrity of the house, it is difficult to achieve high resolution in micro and more accurate structures such as architectural decorative patterns. Its accuracy is more reflected in the complete construction of the building structure at the macro scale and the reasonable layout between larger components.
Comparison of resolutions of different 3D printing technologies
Directly comparing the resolution data of different 3D printing technologies is difficult because resolution may be written in various forms such as dots per inch (DPI), z-axis layer thickness, pixel size, beam spot size, and nozzle diameter, and these parameters are helpful for comparing the resolution of the same type of 3D printer, but not suitable for comparing different 3D printing technologies.
However, in the light-curing technology, the minimum light spot size that DLP technology can achieve is ± 50 microns, and the SLA can reach ± 100 microns, which are comparable data contents that can be referenced. For example, in terms of layer thickness, some FDM printers have a layer thickness of around 0.1-0.3mm, while some devices like SLA can reach a layer thickness between 0.025-0.05mm. There can be a rough comparison at the physical level.
- Small and fine models printed with SLA (such as prototypes of electronic product shells) can achieve high precision in surface smoothness and detail restoration, such as smoother and clearer edges around small circular holes and slender structures.
- Tiny precision parts printed using DLP (such as small pendants in jewelry) can exhibit extremely high detail, and their precision can make small patterns and other details clearly visible.
- When FDM prints some large functional prototypes (such as the preliminary proofing of large mechanical parts), obvious layer patterns and relatively rough surfaces will be shown on some edges and slender structures.
- Building 3D printing constructs large structures at centimeter-level thickness, with almost no fine details from a microscopic perspective, but can construct macroscopic complete building structures.
Factors affecting the resolution of 3D printing technology
- Printing material
For different printing materials, material properties such as viscosity and fluidity have a significant impact on resolution.
In FDM printing, the diameter of the material wire used is a key factor affecting resolution. 3D printing commonly uses wire with a diameter of 1.75mm or 3mm. The larger the material diameter, the thicker the lines formed during extrusion, limiting resolution. For example, objects printed with PLA material with a diameter of 3mm will be rougher than those with a diameter of 1.75mm.
In the photocurable printing technology, the type of photosensitive resin (such as photosensitive resin with different characteristics such as hardness and transparency) will affect the effect of light penetration and curing, and have an impact on resolution. For example, high-transparency resin helps light to penetrate evenly, which can better form fine structures and improve resolution.
- Equipment construction and accuracy
The manufacturing and assembly accuracy of the 3D printer itself, as well as the vibration during operation, will affect the final printing accuracy.
Taking FDM printers as an example, if the rigidity of the printer frame structure and the materials used is insufficient, errors may occur in the XY plane during the printing process, affecting the positioning accuracy between layers. If the xy plane error is large, the relative positions of the layers during the printing process will be difficult to stack accurately. For the z-axis, such as a light-cured 3D printer, the z-axis is controlled for up and down printing, relying on the wire rod and guide rail to drive. If there are assembly accuracy problems or lack of lubrication between the wire rod and guide rail, resulting in significant friction, the movement of the printing platform on the z-axis will not be stable and accurate enough, resulting in uneven thickness or misalignment between the printed layers, which will affect the resolution.
The accuracy of the 3D printer nozzle is a key factor in FDM printing. The nozzle diameter affects the width of the filament extrusion. Small-diameter nozzles can achieve finer printing, but they may also cause problems such as uneven filament output and affect resolution.
- Print parameter settings
Printing speed is a variable that affects resolution in different printing technologies. If the printing speed is too fast, whether it is the filament extrusion of FDM printers or the resin solidification of photocurable printers, it may cause the material to not have enough time to accurately shape.
For example, in FDM printing, if the speed is too fast, the extruded filament will be pulled to the next position before it cools and sets, which will cause the shape to be inaccurate.
For photocurable printers, if the speed is too fast, the resin may not cure completely or uniformly.
The setting of layer thickness is closely related to resolution. As mentioned earlier, in SLA, if the layer thickness is set at 0.025mm, higher vertical resolution can be obtained than when it is set at 0.05mm. Fill rate is important in FDM printing. If the fill rate is set too high, the extrusion of the wire material during the printing process will increase, which will compress adjacent material layers, causing edge deformation and affecting resolution. If the fill rate is too small, it may cause insufficient internal support, resulting in overall structural deformation and destruction of resolution effect.
- Characteristics of the model itself
If the model has a dangling structure or a very complex internal structure, it is a challenge for printing resolution. For example, printing a model with a large number of concave and slender channels requires some special strategies for different printing technologies to complete, and if not handled properly during this process, it can easily lead to a decrease in local resolution.
Taking light-curing printing as an example, if there is a lack of proper support in the printed hanging part, the resin may deform before it is fully cured, resulting in a decrease in the shape accuracy and surface accuracy of this part. For FDM printing, supporting materials are required for hanging structure printing. If the removal process of supporting materials is not detailed enough, it will also strain the solid surface of the model and affect the resolution. In addition, the accuracy of the 3D model itself in the original design. If the triangular surface in the STL file (a commonly used standard file format for 3D printing) is large, the converted printed model will have a lower resolution performance on the surface, because the size of the triangular surface can approximately reflect the surface accuracy of the object.
- Printing environment
In 3D printing, temperature is an important factor.
For example, in FDM printing, the nozzle temperature determines the adhesion performance, stacking performance, wire flow rate, and extruded wire width of the material. If the temperature is too high, the material tends to be liquid, the viscosity coefficient decreases, the fluidity increases, and the extrusion is too fast to form a precisely controllable wire. If the temperature is too low, the material viscosity increases, and the extrusion speed slows down, all of which will affect printing accuracy.
Similarly, the temperature in the molding chamber affects the thermal stress of the formed parts. High temperature helps to reduce thermal stress, but the surface of the parts is prone to wrinkling. Low temperature increases the thermal stress of the formed parts due to sudden cooling of the filament extruded from the nozzle, which can easily cause warping and deformation of the parts, resulting in a decrease in printing resolution. In photocurable printing, if the ambient temperature is uneven or exceeds the optimal curing temperature range of the resin, the curing effect of the resin will also deteriorate, resulting in a lower final printing resolution. Environmental humidity also has an impact on printing. High humidity can affect the dryness of certain printing materials, especially some moisture-sensitive powder materials or photocurable resin printings that are not fully cured.
How to evaluate the resolution of 3D printing technology
Some traditional methods evaluate resolution through specific data indicators such as dots per inch (DPI), z-axis layer thickness, pixel size, beam spot size, and nozzle diameter. However, these indicators are only applicable to comparing similar 3D printers, and evaluating resolution for different types of 3D printing technology is not comprehensive or accurate. The most effective evaluation methods are:
- Visual inspection test : directly use the naked eye to view the printed finished parts. Pay attention to the sharpness of the edges and corners. For example, when printing a cube model, observe whether the edges of the cube are neat and smooth; check the minimum detail size, such as when printing objects with small patterns or inscriptions, check whether these small details are clear and distinguishable; pay attention to the quality of the side walls, such as whether the side walls present a uniform and flat state; at the same time, check the surface smoothness to see if there are obvious layer patterns or uneven material accumulation. This method can visually evaluate the resolution as a whole.
- Using a digital microscope : This is an auxiliary means to judge resolution more finely. A digital microscope can magnify tiny details and can be used to capture the details of printed parts for comparison in many aspects. For example, when printing microscopic models or small parts with extremely high precision requirements, the fineness and boundary clarity of the microstructures printed by different 3D printing technologies can be more clearly seen through a digital microscope.
- Comparison of Model Accuracy and Design Effect When printing functional test models, it is necessary to ensure that the molded parts can accurately reflect the design effect. If there are obvious defects or partial deficiencies in the molded parts relative to the design, it indicates insufficient accuracy or resolution, and the accuracy of the functional test results will also be affected. For example, when designing a mechanical part model with precise internal structure and size requirements, assembly testing is performed after printing. If the parts cannot be accurately assembled (due to insufficient printing resolution), it indicates that there is a problem with the resolution. To evaluate the resolution of 3D printing technology, multiple factors need to be comprehensively considered, and the quality of a 3D printing technology resolution can be relatively accurately judged from multiple aspects.
