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Key Technologies to Improve the Performance and Precision of Binder Jetting Metal Parts

2023-03-16 10:19:57Source of article:EasyMFG

  Key Technologies to Improve the Performance and Precision of Binder Jetting Metal Parts

  The dimensional accuracy of current binder jetting (BJ) metal parts is generally within ±0.1mm/100mm, and the highest can reach ±0.05mm. The roughness is between 3~5μm, and the highest can reach 2μm, but it will sacrifice a certain printing speed. Compared to selective laser melting (SLM) technology, BJ has a significant advantage in terms of roughness. Currently, BJ parts have a density of >99%. In the past, BJ was often described as having low accuracy and poor performance.

  This article is divided into three sections:

  I. Analysis of Influencing Factors

  Seven main factors affect the accuracy of BJ technology, including:

  1)Mechanical precision

  2) Motion control

  3) Data accuracy:Data accuracy affects part accuracy because the data is generally dot matrix data, described by resolution. The higher the resolution, the higher the accuracy, but the larger the data processing and storage content. Additionally, higher resolution means more binder droplets and volume. If the binder volume is too high, it can seriously affect the printing process.

  4) Inkjet precision:Inkjet positioning accuracy is a unique precision factor of BJ technology. The size and speed of binder droplets affect positioning accuracy. Larger and heavier droplets increase accuracy but can impact the powder bed, resulting in a rough surface. Adjusting the appropriate droplet size and speed reduces the impact.

  5) Binder droplet penetration:Binder droplet penetration directly affects the accuracy of the powder bed boundary and even the lower surface. If controlled well, the entire surface of the part will be smooth after depowdering.

  6) Shrinkage distortion:BJ technology involves sintering, and uniform shrinkage is easy to compensate through software. Non-uniform shrinkage will significantly affect printing accuracy, and this problem can be solved by considering powder uniformity, density, and shape retention.

  7) Nozzle defect

  Nozzle clogging is an ongoing issue in BJ technology. Many manufacturers are committed to solving nozzle clogging and oblique spraying.

  EasyMFG has a whole set of processing methods to address the factors affecting accuracy, including:

  1. Online data processing: Solves the problem of large data volumes and low memory usage. Different resolutions can be used for contour surfaces and internal entities to facilitate data processing and reduce storage content through partitioned data processing, transmission, and printing.

  2. Waveform control: Realizes stable inkjet and reduces the influence of ambient wind speed on the binder droplet motion curve. Adjusting the binder droplet to a suitable inkjet speed reduces the impact of the powder bed.

  3. Permeation control: Achieves similar accuracy and roughness on all sides (top, bottom, left, and right) of the part.

  4. New powder spreading method: Compatible with powders of different shapes and diameters and improves the uniformity and density of powder spreading.

  Clog-tolerant printing: Allows for nozzle clogging as much as possible.

  Factors Affecting Part Performance

  The performance of parts can be affected by various factors. Here are four main factors that can affect the performance of parts:

  1) Sintering activity of powder: The sintering activity of powder is a crucial factor in determining the final part performance. The activity of the powder can affect the density and strength of the part after sintering.

  2)Density of the green part: Generally speaking, the higher the density of the green part, the smaller the shrinkage rate, and the lower the possibility of deformation. Therefore, it is essential to control the density of the green part during the manufacturing process.

  3)Binder residue: The amount of binder residue left in the part after sintering can affect its performance. The smaller the residue, the better it would be. Even if there is residue, it will not have any significant effect on the performance of the part.

  4)Sintering furnace cleanliness: The cleanliness of the sintering furnace can also affect the performance of the part. Different materials may require different sintering furnaces.

  Corresponding solutions for these factors are:

  1) High-density powder spreading method: It is crucial to use a compacting mechanism to achieve high-density powder spreading.

  2) Low-residue binder: Using a binder with little residue during high-temperature decomposition can minimize any negative effects of the residue.

  3) Grain refinement additive: Adding a grain refinement additive can effectively slow down the growth of grain at high-temperature sintering, resulting in improved part performance.

  4) High cleanliness sintering furnace: Using a sintering furnace with very little volatile substances in the furnace chamber and heating components can help maintain a clean environment, resulting in better part performance.

  II. Analysis of Key Technologies

  1. Print Data Processing Software (RIP)

  Printing data processing software is crucial for the success of BJ Metal 3D printing. This software converts sliced contour data into bitmap data that the printer can accept. Several functions are essential for printing data processing software:

  1) Region-based data processing: For large-size and high-precision data, the volume of data can be enormous. Suppose the part size is 700mm x 700mm, and the resolution is 1200 x 1200 DPI. In that case, the single-color bitmap data reaches 1.02GB, and the full-color bitmap data reaches 3.06GB. If only one layer of data can be calculated and stored at a time, the memory usage during computer operation usually doubles, making the computer overwhelmed. Region-based data output can gradually output printing data in a region-based manner during the printing process, which means that data processing is done while printing. This approach achieves "small memory processing of large data," enabling larger-size and higher-resolution data processing, allowing the printer to print normally.

  2) Online data processing: Offline data processing does not allow for real-time calculation and sending of printing data in a region-based manner. This problem may not be exposed in small-size machines, but for large-size and high-precision printing, online real-time data processing is required to ensure normal computer operation.

  3) 3D printing has extremely high requirements for part size accuracy, and the RIP algorithm used in the traditional inkjet printing industry uses interpolation to improve printing resolution, which can improve color fidelity. However, the interpolation method cannot guarantee the physical position accuracy of the bitmap. Full mapping data processing technology maps high-precision bitmap data to every physical position actually sprayed by the nozzle without any data error.

  2. Fine Binder Volume Adjustment Technology

  Controlling the size and shape of ink droplets is usually necessary, but adjusting ink droplets is far from enough. The amount of binder needs to be controlled as well. BJ metal printing is very sensitive to the amount of binder; too much or too little binder per layer will affect the printing quality. EasyMFG adopts waveform modulation technology to control the ink droplet size of the print head. Then, it distinguishes the outline area (accuracy description) and filling area (intensity and binder volume description) for spray dot adjustment to adjust the spray points, achieving fine binder volume control.

  3. Accurate Contour Reproduction Technology: Internal Offset Contour Algorithm

  BJ metal printing is divided into two areas in the printed spraying area: outline and fill, both of which are obtained from the vector data sliced from the model. The inkjet amount of the outline area will directly affect the horizontal penetration of the binder, causing the part size to be larger.

  The width of the outline: Setting a certain width for the outline will result in the "outward shift" phenomenon beyond the actual outline area (as shown in the red area in the figure on the right), directly causing the part to "overflow," and the size to be larger. At the same time, the outline may appear too thick, or adjacent outlines may "touch" and lose details.

  Inner offset contour algorithm: The traditional contour offset algorithm compensates for the horizontal binder penetration and pen width offset, but may lose contour details due to the inherent defects of the offset algorithm, resulting in unclear or incorrect details in the part. The inner offset contour (optical plate compensation) algorithm only "widens" the inside of the outline based on the outline vector data, without affecting the contour data. The final contour data is shown in the black area on the right.

  4. Adaptive Blocking Technology

  Printing defects caused by clogged nozzles can lead to poor local inkjet, which can affect the part's performance. However, with print control technology, clogs can be avoided by adjusting the data printed by each nozzle during each print using the RIP algorithm. The technology also randomly rearranges the nozzle positions of the current layer and the upper and lower layers during printing, which disperses the defects inside the part instead of accumulating in one location, effectively avoiding poor local inkjet caused by nozzle blockage, which can lead to fracture.

 

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  Printing effects at blockage 40% are observed.

  5. Fully Automatic Powder Feeding and Spreading System

  The M400Pro uses a fully automatic powder feeding and spreading system that features high powder compatibility, high-density powder spreading, uniform powder spreading, and an integrated "powder valve" developed in-house, which ensures free powder transmission shutdown. The entire powder spreading mechanism uniformly spreads the powder over the entire working surface and then compacts it.

 

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  High-Density Shaped Powder

  The M400Pro uses high green metal powder with reduced particle size, which increases the sintering activity and the loose packing density to ensure fluidity. Additionally, the spherical shape of the high shaped metal powder is retained by combining water-gas combined atomization powder and gas atomization powder, which improves the powder flowability for easy spreading.

  Low-Residue Environmentally Friendly Water-Based Binder

  The M400Pro uses a binder that is environmentally friendly and has little residue during high-temperature sintering. The binder's penetration rate needs to be controlled within a certain range, and different binders are generally used for different powders.

  III. New Equipment that Integrates Various Technologies

  The M400Pro, a third-generation BJ printing equipment from EasyMFG, integrates various technologies to create a system that is highly automated and closed-loop. The equipment has not been officially launched yet.