Rapid prototyping technology uses liquid photosensitive polymer (epoxy resin or/and acrylic resin, etc.) as raw materials and uses ultraviolet laser scanning to form a layered entity. Stereo Lithography Apparatus (SLA) is one of the key equipment for rapid prototyping technology application. It has the characteristics of high production accuracy and high utilization rate of raw materials, and can be used to manufacture high-precision entities with complex shapes. This work explores the application of SLA rapid prototyping technology in the manufacture of room temperature vulcanized silicone rubber (RTV silicone rubber) seals.
1 Technical principle
SLA rapid prototyping is divided into three steps: pre-processing, layered stacking free molding and post-processing. Pre-processing is the CAD design of the finished product according to the drawings or objects provided by the user, that is, three-dimensional modeling. If possible, optical scanning or CT scanning of the actual object can be performed to directly obtain the three-dimensional model of the finished product, and then output the three-dimensional model in STL format. File (triangular patch file), and on this basis, layer (thickness 0.05~0.5 mm) slice processing and support editing processing (similar to adding processing fixtures, but the support and the mold are formed at the same time).
Layered stacking free prototyping is the core of rapid prototyping, including cutting and stacking. The principle is shown in Figure 1. The specific process is: the ultraviolet laser scans each layered section of the CAD design point by point, so that the resin thin layer in the scanned area undergoes photopolymerization reaction and cross-links and cures; when the thin layer is cured, the workbench moves downwards, just before Place a new liquid resin layer on the cured resin surface, while the scraper performs horizontal reciprocating motion to keep the liquid level level, and then scan and cure a new layer; the newly cured resin layer is firmly bonded to the previous cured layer. Repeat this until the body is formed. SLA rapid prototyping technology can be used to make resin molds, which are suitable for the production of low-volume, high-precision room temperature (low temperature) casting rubber products.
2. Application in the manufacture of RTV silicone rubber seals
2.1 Mold manufacturing:
2.1.1 CAD design
First, use a vernier caliper, inner diameter micrometer and outer diameter micrometer to accurately measure the size of the damaged rubber seal and the installation shaft. According to the elasticity and shrinkage of the rubber material used, determine the interference between the seal hole and the shaft to be 0.15~0.2 mm .
After completing the seal survey and mapping, the CAD design of the mold is carried out. The principle of CAD design is to construct the mold with the least amount of material. The CAD model of the rubber seal mold of the laparoscopic surgical instrument is shown in Figures 4 and 5 (the end faces of the 6 pouring holes are uniformly distributed as the non-assembly surface), and the wall thickness of the mold cavity is only 0.5mm.
2.1.2 Forming
After completing the CAD design of the mold, the 3D model file in STL format is transferred to the SLA rapid prototyping machine, and the SPR-3010 photosensitive resin used for molding (the formula is: bisphenol A epoxy resin 83, ethoxylated hexanediol diacrylate 17 , 1,4-methylcyclohexane divinyl ether 4.8, diaryl hexafluoroarsenate sulphur tin salt 7.2, benzoin dimethyl ether 4.8) water resistance, chemical resistance, dimensional stability and toughness are good , High strength, wide applicable temperature and humidity range, and can be used in the production of auto parts, aviation parts and medical equipment.
Use RPData data processing software to determine that the mold model is placed flat on the worktable with the gate upward, and then the 3D model is sliced in layers with a slice thickness of 0.2 mm, so that the triangle patch file is converted into a 2D layer. The outline information of the piece. In order to prevent the warpage and deformation of the mold during the molding process and ensure the stability of mold making, the support design of the model must also be carried out. After that, 10 uniforming worktables were produced according to the two specifications of molds. Finally, run the RPBuild production software, that is, turn on the laser and zero the working liquid level, perform layer-by-layer laser scanning and solidification and stacking to complete the mold production. The SLA rapid prototyping process parameters of this seal mold are: resin temperature 30℃, laser power 160 mW, laser wavelength 355 nm, light intensity 1.9 X 100000 cd, filling scanning speed 2 200 mm. s-1, filling vector spacing 0. 1mm. The forming mold is washed with acetone to remove grease and support, and finally the forming mold is irradiated under an ultraviolet lamp for 2 hours to further crosslink and cure.
2.2 Manufacturing of seals
Two-component addition molding RTV silicone rubber is used to cast the seals of the laparoscopic surgical instrument. The formula of component A is: light-terminated polydimethylsiloxane 100, white carbon black 6.3, lead oxide 12.5, methyl silicone oil 6.3; the formula of component B is: ethyl silicate 119, tin dialkyl disulfate 6.3. After components A and B are evenly mixed in a ratio of 10:1, slowly pour the mixture into a container that has been placed in 20 rapid prototyping molds (in order to prevent the mixture from overflowing the container when vacuuming, the height of the container is designed to be the height of the mold 3 ~ 4 times), the mixed liquid is injected into the cavity through the mold pouring hole. Thereafter, the container was placed in a vacuum box, and air and moisture bubbles in the mixed solution were removed under the condition of a vacuum degree of 8.5×10-6 Pa. After venting, take out the container from the vacuum box and vulcanize it in an electric heating box at 60°C for 4 hours. Finally, the vulcanized product is taken out of the oven, the thin layer of the mold is peeled off by hand, and the convex part of the gate is lightly polished with sandpaper to make the end surface flat and smooth, and the finished seal is obtained. After testing, the performance of the seal meets the requirements, and it was put into use immediately. Practical use shows that the performance of the seal has reached the level of the original accessories.
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3 Conclusion
SLA rapid prototyping technology is suitable for the production of low-volume, high-precision room temperature (low temperature) cast molding rubber products. Compared with traditional technology, its production cycle is shortened by 60% ~70%, and the production cost is reduced.