“MDP” Valve Performance Evaluation
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;
The data proves that our design shut off quicker than the Three Piece Sliding Ring design, was more precise and worked very well without the use of melt decompression.
The second test was with a 20% Glass Filled Polycarbonate with an in-mold Gas Assist application. The process requires that the screw bottom out each shot so a proper distribution of gas disperses throughout the part. The results are as follows;
It is important to note that the Poppet Valve was run first with a 145mm stroke, when the “MDP” valve was installed the part weight immediately increased to 1012.4 grams which proves that the “MDP” valve shut off quicker than the Poppet Valve. Since the process dictates that that the screw needs to bottom out, the stroke had to be reduced to 140mm. If a volumetric evaluation is made using 1.2g/cm^ 3 for density of the polymer, the part weight would be 1069.9 grams, proving that the “MDP” valve shuts off quickly and stays shut through the full stroke. The data also supports a more precise operating component.
The third and final test was with a 30% Glass Filled PA 6/6 material. The results are as follows;
The Three Piece Free Flow Sliding Ring design experienced numerous short shots and flashed parts with 15mm decompression and two short shots with 10mm decompression.
Since its inception, the “MDP” performance NRV has proven that it is the most precise sliding ring NRV in the industry virtue of its unique geometry and low pressure drop in applications from 16mm through 215mm in diameter while covering the gamut of resin Viscosity and Injection Velocity.