Ehsan Aghajari - Department of Chemical Engineering, Faculty of Engineering,Arak University
Taghi Miri

The use of plastics in everyday applications and devices is growing rapidly. There are many different established methods for forming plastics, including blow molding, injection molding, thermoforming and extrusion. These methods can be employed based on the type of products and their expected functionality. One of the relatively newer methods in polymer forming is gas-assist molding, which was developed to overcome the limitations of conventional injection molding. Just recently water-assist inject introduced and become a hot topic within the plastics industry because of its new abilities in molding hollow or partly hollow components. Both gas and water assist methods produce hollow parts. Therefore, weight of the part and amount ofpolymerused is reduced. Studies show that water-assist injection moldings have shorterprocesscycle time than the gas-assist systems as they can directly cool the mold. The other advantage is that water assist system can be used to produce large diameter products with an even wall thickness distribution. However, water-assist produces a lower degree ofcrystalline as well as having a higher fingering defect. The main parameters affecting the water fingering are water pressure, water injection delay time and short shot size. Shrinkage, warpage and internal foaming phenomenon (usually occur when we use gas assist injection molding) reduced in the water-assist compared to the gas-assist injection molding. Furthermore, water-assist injection molding enables us producing components that have both thin and thick sections. A molding technique such as water-assist requires special knowledge, therefore, usually takes some time to move into the industries. In this paper, we briefly introduce both gas and water assist methods and then make a comparison detailing their advantages and shortfalls in terms of cost of the materials/equipments, cooling time, weight and quality of the products and the complexity of the geometry that can be achieved

Injection Molding, Gas-assist Injection Molding, Water-assist Injection Molding