Bottom of expansion tank
This is the cover of plastic water tank for a branded automotive company. The material is PP. The part features with a long BOSS and round undercut of 1mm. The BOSS structured with a round pin on the cavity followed by forced demolding. In most shops, slides are usually used to form the feature but this is a waste of money considering the quantity of the products. So we use insert to make the BOSS. Since undercut is to assembly the water tube, we have to keep tight control on the undercut measurement. After forced demolding, the size of would become smaller. So we have to check the dimension of of undercut after trial and then use mirror EDM to machine until we get the right size. This process usually has to been repeated over 2-3 times.
For the water tank, the key technology is the part must be flat without any deform and assembly holes must be aligned perfectly. If there is any warp, the two parts could not be vibrated-welding together and water leaks. In order to solve the issue, firstly we have to run mold flow to do a pre-warp analysis so that the warp degree could be calculated and gate position decided. Mold flow is only an assistant tool in theory. Sometimes, mold flow result differs greatly with actual result. According to our years of experience in making water tank injection molds, we know where to put the gate and what size to better control the even flow of the plastic based on the reading of Mold Flow. Secondly, water line is very important. It is better to have as much cooling as possible and independent water lines for the cavity and core. It is easy to adjust the warp by changing the hot or cool water at trial stage. We accumulate rich experience in making tank injection molds at a low cost without compromising quality and know how to get a perfect flat part through reasonable design plus with the adjustment of molding parameter. Though a lot of mold makers could build injection molds for plastic water tanks, but not everybody could make a perfect part without warp. This is not a question of machining, but a technology requires rich experience.