Much has been written about the Injection process relative to mold filling, in fact there are numerous companies that derive their income teaching this science such as RJG Incorporated, Injection Molding Solutions in the USA and others abroad. This process of determining the injection velocity during fill, pack and hold to produce a qualified part assumes that the plasticaing screw and screw tip can both provide a melt quality necessary to flow uniformly into the mold and that the screw tip (NRV) will shut off quickly and maintain shutoff through the full stroke of the screw. Precise displacement of molten polymer fluid can only occur if both of these criteria are met but the focus of this dissertation is on the second component of the equation, the NRV. Briefly, the screw tip is a non-return valve (NRV) that allows molten polymer flow in one direction and shuts off in the other direction. There has been much written about the NRV and there has been virtually thousands of designs over the last 65 years proposed to improve the timely and complete shutoff so that the mold can be filled with accuracy and repeatability.
The principals at Md Plastics Incorporated have been focusing on these plasticating unit components for over 27 years with the quest for perfecting the perfect NRV. Our philosophy has been to develop a quantum change in the function of the design while considering the constraints that are prevalent on all Injection machines. Afterall, the sliding ring, ball check and poppet designs have been tweaked and redesigned over and over again during this period with only minimal improvement. Not unlike many inventions, it takes numerous tries and endless hours of development before an idea is proven and worthy of the test that the market will bring; our latest “MDP” performance design is such a design and worthy of the continued discussion.
A very accomplished custom Midwest Injection Molding Company sought to find a solution to poor part quality, frequent NRV changes that were application specific and a longer lasting component. They have been using a Industry Standard “Three Piece Sliding Ring” Free Flow design for a majority of their standard processes and a QSO poppet design for gas assist processes. A series of tests were conducted on a 90mm diameter “MDP” performance sliding ring design for a Mitsubishi Molding Machine for the evaluation.
The first test was conducted with a 20% Glass filled 12 MI PP material, the results are as follows:
