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Recently, different authors agree that the way to optimize the flow in polymer melt is by changing the viscosity. There are several chemical products that help the above-mentioned, but these products change the molecular properties of the polymer. Recent nonchemical approaches use vibration techniques in the manufacturing process. The problems of the aforementioned approaches reside in the necessity of major modifications to the injection machine or the development of a new machine. The novelty of this research is to provide a mechanical apparatus to modify the viscosity of melted polymers by inducing vibrations without sacrificing the polymer properties in the injected parts, and which do not imply major modifications to the machine or molds. The developed apparatus is a simple mold accessory that makes easy for the end user the retrofitting of existing injection machines and it reduces the manufacturing costs. The improvement on the injection efficiency is tested with high-density polyethylene, reaching an improvement of 30% when excited at frequencies around 3 Hz.7390
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Keywords Injection：Rehology; Vibration mold.
Introduction
Some investigations show that the injection molding process has gained importance due to the great number of parts manufactured by this mean [1, 2]. Covering this need implies new technologies that in some cases result in nonconventional injection process. Nowadays, there are numerous chemical products that allow the injection process to be improved. However, some of them, such as sliding, lubricants, viscosity suppressors, etc., affect the molecular features of the polymer melt including weight, whose change affects the mechanical properties of the end products [3–5]. Due to the above-mentioned issues, a nonchemical method that modifies the polymer viscosity, while the molding is done without sacrificing the polymer properties, is particularly acceptable. Recent trends are seen by using vibration techniques in a manufacturing process [6–8]. For instance, Liu [9] analyzed the behavior of the polyethylene under vibration in extrusion by using a device that ove rimposes and measures the vibration at the same time, making necessary the modification of the machine in order to mount it. In the same way, Ibar [10] developed a method for controlling the viscosity of molten polymers with a vibration exciter that is mounted on the injection screw of the injection machine, which implies major modifications on the last part of the injection machine barrel. On the other hand, Yin and Qu [11] improved the metal powder injection (MIM) using vibration through a vibrating device mounted on the piston rod, with a special vibration-assisted injectionmolding machine developed for this purpose. Similarly, Yan et al. [12] developed a special extruding machine in which they induce the vibration through the machine screw. The problems of the aforementioned cases reside in the necessity of major modifications to the injection machine or the development of a new machine in some cases, additionally to the complexity of the designed apparatus. Therefore, it is particularly desirable to have an apparatus that overimposes the vibration without major modifications to the machine, and if possible, to think of this apparatus as an add-on accessory that could be mounted with minimal effort in both new and existent machines. The novelty of this research is to provide a mechanical apparatus to modify the viscosity of melted polymers without sacrificing the polymer properties in the injected parts, and which do not imply major modifications to the machine or molds. Furthermore, the developed apparatus is a simple mold accessory that makes easy for the end user the retrofitting of existing injection machines and it reduces the manufacturing costs.
Methodology
Background
The rheological properties of polymer are critical for the design of plastic manufacturing. A188888mong the persity of variables that can be used to change the melt-polymer behavior, vibration [9, 13] is desirable because it does not affect the molecular features of the polymer melt.