The relationship between 3D printing technology and the mold industry

3D printing, also known as additive manufacturing technology, is a method of constructing three-dimensional objects based on three-dimensional CAD model files, using materials such as powder, silk or flakes to construct three-dimensional objects through “layered manufacturing and layer-by-layer stacking”. technology. The currently widely used 3D printing molding processes mainly include:

• 1. Selective Laser Sintering (Selective Laser Sintering, SLS)
• 2. Selective Laser Melting (SLM)
• 3. Direct metal laser sintering (Direct metal Laser Sintering, DMLS)
• 4. Stereo Lithography Apparatus (SLA)
• 5. Fused Deposition Modeling (FDM)
• 6. Layered Object Manufacturing (Laminated Object Manufacturing, LOM)

Different types of processes have advantages in applications in different fields.

The application of 3D printing technology in the mold industry is mainly divided into three aspects:

(1) Directly make prototypes

The above-mentioned 3D printing processes can produce prototypes, but the precision, strength and surface quality of the produced prototypes are different. This is also the most common application method of 3D printing technology at present;

(2) Indirect manufacturing of molds

is to use 3D printed prototypes to rebuild molds through different process methods, such as silicone rubber molds, plaster molds, epoxy molds, sand molds, etc.;

(3) Direct manufacturing of molds

It is to use SLS, DMLS, SLM and other 3D printing processes to directly manufacture soft molds or hard molds.

The advantages of 3D printing technology

• (1) 3D printing technology can achieve “zero” waste of production materials in the production process. The production process of 3D printing technology is to print layer by layer according to the three-dimensional design of the parts. Compared with the traditional “subtractive” processing, it achieves “zero” waste of production materials.
• (2) The use of 3D printing technology can speed up the progress of product development. 3D printing technology has changed the way designers think, they will think according to the different parts of the load-bearing and force-bearing parts.
• (3) The use of 3D printing technology can greatly shorten the production cycle. From design to production, 3D printing technology eliminates the process of process design and verification in the traditional processing process, shortens the production cycle, and can adjust the production batch in time according to market demand.
• (4) The use of 3D printing technology can greatly reduce the human resources in the design and production process.
• (5) 3D printing technology can be used to manufacture molds with special structures, such as conformal cooling molds. This is difficult to achieve with traditional manufacturing methods, and it is also a highlight of the application of 3D printing technology in the mold industry. The conformal cooling mold has many advantages, which can improve the cooling efficiency of the mold, make the cooling of the product tend to be uniform, and improve the product quality and production efficiency.

Traditional mold manufacturing process

The traditional mold manufacturing process is to review the order items after receiving the order, formulate the production schedule after the review, and then perform 3D software revision, mold flow analysis, parting line and feed point determination, and finally feedback to The customer finalizes the draft, and only after the customer is satisfied can the drawing of the parts for manufacturing be determined, and then the processing flow can be prepared. The processing flow is shown in Figure 1. It can be seen from Figure 1 that using the traditional mold manufacturing process to process a qualified mold requires more manpower and material resources, and a longer production cycle.

Mold manufacturing process using 3D printing technology

The process of using 3D printing technology to directly manufacture molds is shown in Figure 2 (take the SLM process as an example), which can be divided into three stages: pre-molding preparation, SLM molding and post-molding processing. Pre-molding preparations include 3D modeling of the mold model, STL format conversion, adding support structures, determining process parameters, and performing data processing such as layered slicing; the SLM molding stage is automated processing with less manual intervention and only needs to work on the SLM equipment The condition is monitored to ensure the normal operation of the equipment; post-forming processing includes picking up parts, cleaning powder, sandblasting, surface grinding, polishing and other processing. The following specifically describes the process of using the SLM process to manufacture molds.

pre-molding treatment

(1) Model design

Model design is the first step of mold manufacturing, which directly determines the shape characteristics of the mold. For example, conformal cooling of injection molds. Not only the cooling effect needs to be considered when designing, but also the limitations of the processing technology and the mold combination used. The cooling effect should take into account both cooling efficiency and cooling quality. The arrangement and structural characteristics of the cooling channel need to be optimized, the design principles and methods of the cooling channel, etc.; the limitation of the processing technology is mainly for the forming characteristics of the SLM process. Treatment of certain features to ensure that the mold will not cause loss of features during molding and manufacturing, such as tiny features, suspended structures, etc.; conformal cooling injection molds are more economical and practical mold combination methods are inlaid.

(2) Add support

There are two main purposes for adding support. One is to fix the molded workpiece on the substrate. This is because during the mold forming process, the powder needs to be evenly and tightly spread on the substrate during powder spreading. With certain shearing force, if the molded parts are not fixed or insufficiently fixed on the substrate, slight displacement will cause the finished workpiece to be staggered. In severe cases, the workpiece may jam the powder spreading device and damage the equipment. Therefore, sufficient support is required to fix the shaped workpiece. The second is to prevent the loss of features during printing of a specific structure, which is mainly for the structure with a large inclination angle.

Adding support is an important task of pre-processing, and it has an important impact on the quality of the workpiece. The support of different processing equipment is different, mainly divided into two types, one is a staggered mesh structure, which is mainly used for supporting larger workpieces with a flat bottom surface; the other is a sheet-like support, which is used for cylindrical surfaces Supports such as non-flat curved surfaces. The minimum support height is the distance from the lowest molding surface to the substrate plane. If it is too high, the total molding height of the workpiece will be too large, and the amount of powder used will increase; if it is too low, it will be difficult to pick up the parts. Comprehensive considerations, Generally choose 3 to 5mm.

(3) Determine the process parameters

The process parameters directly determine the quality of the formed workpiece. Process parameters include powder thickness, laser scanning speed, scanning method, and the spatial position of the workpiece.

post-molding treatment

(1) pickup

After the 3D printing is completed, the printed workpiece is submerged in the powder. When picking up the part, the waste generated by the fusion is removed to prevent the waste from contaminating the powder; then the workbench is raised, and the preliminary powder cleaning is carried out in the processing warehouse. The brush sweeps the unsintered powder that is attached to the surface of the workpiece into the powder recovery cylinder for recycling. Finally, the workpiece and the substrate are taken out together.

(2) Remove support

After taking the part, the workpiece and the substrate need to be separated, usually by wire cutting, sawing, etc. Wire cutting separation takes a long time and is mostly used to separate workpieces with thin-walled features at the support connection. Because the separation method is softer, it will not cause workpiece deformation. When the workpiece is small, the support is less, or the support connection is a solid structure, in order to save separation time, a chisel can also be used to directly remove the workpiece.

（3）Clean powder

The powder cleaning is mainly for the cooling channel part of the mold. You can use a brush to clean the powder directly, or you can use auxiliary equipment such as a vacuum cleaner or a hair dryer to remove the powder that stays in the cooling pipe. The structure of the cooling channel has a certain influence on the difficulty of cleaning powder, such as the diameter and the radius of curvature of the channel.

(4) sandblasting

Sandblasting is powered by compressed air to form a high-speed jet beam to spray the blasting material (copper ore, quartz sand, emery, iron sand, Hainan sand, etc.) on the surface of the workpiece to be treated at high speed, so that the appearance or shape of the workpiece is changed. , To obtain certain performance. For the workpiece formed by the SLM process, sandblasting mainly has the following two purposes: 1) Sandblasting can clean the powder that adheres to the surface of the workpiece, and improve the smoothness and accuracy of the workpiece. The surface of the workpiece will adhere to a small amount of incompletely sintered powder during molding. Although the connection strength is low, it is difficult to remove when the powder is cleaned, so sandblasting is used. 2) Eliminate thermal stress and improve the mechanical properties of the workpiece. During the sintering process of the powder, thermal stress accumulates, and the formed workpiece has a large internal stress. In order to prevent deformation or cracking during use, sandblasting is used to eliminate it.

(5) Other processing

The technical advantage of 　　SLM process lies in forming workpieces with complex and irregular structures inside. The injection mold has cooling channels inside, and various structure types on the outside, such as bosses, concave holes, etc. At present, due to the large surface roughness of the workpiece formed by the SLM process, the accuracy is difficult to control, it is not mature to directly apply the SLM formed mold to plastic molding, especially for plastic parts with high surface finish and precision requirements. Therefore, other traditional machining methods are needed for subsequent processing to meet the requirements of mold accuracy and surface quality.

3D printing technology in the mold application research

At present, in China, the application of 3D printing technology in molds is mostly based on the research of 3D printing technology by various research units themselves, such as Huazhong University of Science and Technology, South China University of Technology, etc., and certain progress has been made.