Hybrid Fan-Film Gate Design for Overmolding Sealings in Polycarbonate Parts

Abstract

This thesis focuses on the creation of a hybrid fan film gate that facilitates the overmolding of thermoplastic elastomer (TPE) sealings on polycarbonate (PC) Lexan 503R parts using a standard injection molding machine. Primarily, this thesis addresses the need to eliminate the time consuming process of molding the sealings first, physically removing the gates of the molded parts by hand, and then assembling the final product which is an assembly containing a sealing and cover part out of Lexan. This process increases the costs of production and results in more defective parts due to improper bonding or dimensional problems. By being able to perform overmolding effectively with the same rigid parts that are currently used without the need for using complicated two component (2K) machines which are expensive, this thesis aims to increase the rate of production, reduce labor needs, and improve the quality of the finished parts. The hybrid fan film gate utilizes the combination of a traditional fan gate that assists in distributing the melt. It does this with the thin film style gates, which enables complete filling with smoother flow, and allows for easy removal of the gates. Additionally, this system removes the need for cutting holes in the Lexan part at the location of the gates as required with standard overmolding procedures. This could result in simplifying the overmolding process with a standard injection molding machine used by most companies, with the potential to improve the scalability of mass production at relatively low costs. The design aims at developing a hybrid fan film gate with a sprue of diameter 5 mm connected to a 77 mm wide fan opening going into a film gate that changes its thickness with different test designs involving a thickness of 0.02 0.12 mm. This makes the melt flow of materials go through the cavity in two different directions located around the edge of the Lexan part, which is 310 mm in length and has dimensions of 0.8 mm x 0.8 mm. Materials chosen include Kraiburg TC5MLZ TPE (with a hardness of Shore A 50 and a density of 0.9 g/cm³) due to its good bonding on PC Lexan 503R. The rigid part is SABIC Lexan 503R with qualities of being UV stable, with the strength of the material being around 65 MPa. The design of the sealing involved the creation of the shape using SolidWorks with the help of a step file of the Lexan part and manual measurements of the Lexan part. The next step was to make the mold plates in SolidWorks by using HASCO’s own plates that were downloaded from HASCO mold assist. A mold base with dimensions of 218 mm x 296 mm with the material being 1.173 steel was created. The last step was combining the sealing cavity with the mold base. Before the real mold was manufactured at Plastoco OY AB, plastic simulation of the design was done. The simulation tested the design for maximum filling pressure and detecting weld lines and air pockets. In Plastoco, the mold manufacturing involved milling the cavity and gate pattern, four cooling channels of size 8 mm (distributed 15 mm away from the cavity to maintain the cavity temperature at 50°C), as well as spark erosion (EDM) on the film gate and cavity for precise dimension. Also, two different metal inserts were machined and added to the cavity for improving the quality, which were fastened with M5 bolts. Test runs of the mold were performed on an Arburg 420C machine with a maximum pressure of 130 MPa, melt temperature ranging from 200°C to 225°C, and an injection rate of 30 cm³/s, (the same parameters which was used in simulation). The first test run with a 0.02 mm film gate only filled the area of the gate because there was too much resistance. The second test run was with a 0.07 mm film gate that filled one side of the sealing but melted the Lexan part so that it leaked into the cavity. The third test run was with a 0.07 mm film gate and a steel insert added. This resulted in an almost filled sealing but also melted the Lexan part on the side of the inserts. Finally, the fourth test run was with a 0.12 mm film gate with the full length insert; it completely filled the sealing area with even flow in both directions with the easy removal of the gates with no leftovers. However, there were leaks in the sealing because the fitting of the Lexan part and the mold cavity was not perfect. This thesis shows that the hybrid fan film gate system provides an advantageous design over more normally used overmolding gates because it eliminates the need for additional processing after molding. It also removes the need to open up the Lexan part for injection location, which is nowadays commonly done. This design makes overmolding more accessible with standard injection molding machines with the potential of low cost but high quality production. The fit related issues with specific shapes and constraints of the STEP file used suggest that 3D scans are needed. For more accurate results, longer test cycles with various materials can be done. Future experiments may analyze the inclusion of warp analyses during the packing and cooling phases, and test runs with cycles over 1,000.