Effect of gas counter pressure on shrinkage and residual stress for injection molding process

Abstract

Plastic products are common in contemporary daily lives. In the plastics industry, the injection molding process is advantageous for features such as mass production and stable quality. The problem, however, is that the melt will be affected by the residual stress and shrinkage generated in the process of filling and cooling; hence, defects such as warping, deformation, and sink marks will occur. In order to reduce product deformation and shrinkage during the process of molding, the screw of the injection molding machine will start the packing stage when filling is completed, which continuously pushes the melt into the cavity, thus making up for product shrinkage and improving their appearance, quality, and strength. If the packing pressure is too high, however, the internal residual stress will increase accordingly. This study set out to apply gas counter pressure (GCP) in the injection molding process. By importing gas through the ends of the cavity, the melt was exposed to a melt front pressure, which, together with the packing pressure from the screw, is supposed to reduce product shrinkage. The aim was to investigate the impacts of GCP on the process parameters via the changes in machine feedback data, such as pressure and the remaining injection resin. This study also used a relatively thin plate-shaped product and measurements, such as the photoelastic effect and luminance meter, to probe into the impacts of GCP on product residual stress, while a relatively thick paper-clip-shaped product was used to see the impacts of GCP on shrinkage in thick parts. According to the experimental results, the addition of GCP resulted in increased filling volume, improvement of product weight and stability, and effective reduction of section shrinkage, which was most obvious at the point closest to the gas entrance. The shrinkage of the sections parallel and vertical to the flow direction was proved to be reduced by 32% and 16%, respectively. Moreover, observations made via the polarizing stress viewer and luminance meter showed that the internal residual stress of a product could be effectively reduced by a proper amount of GCP.

CNN-Based Defect Inspection for Injection Molding Using Edge Computing and Industrial IoT Systems

“…Santis et al [16] explored the effects of suppression, time, and geometric constraints on the contraction of semi-crystalline polymers with strain gauges. Chen SC et al [17,18] applied gas backpressure to reduce the shrinkage of parts during injection molding. Qi et al [19] found that mixing of polypropylene copolymers can effectively reduce the molding shrinkage of isletic polypropylene.…”

Fluidity of Rubber and TPE Influenced by Mold Surface Roughness

“…[2] Fig. 1 Material flow in the mold [11] Test Injection Mold for Thermoplastic Elastomers Fig. 2 Testing injection mold for thermoplastic elastomers 1 -insulating plates, 2 -clamping plates, 3 -anchor plates, 4 -guiding elements, 5 -ejector plate, 6 -supporting plates, 7 -cavity plate, 8 -special sprue puller, 9 -lifting eye bolt, 10 -testing plate, 11 -sprue bushing, 12 -cooling nipple, 13 -ejector rod The test injection mold was a simple construction with changeable plates as well as gate size.…”

Injection molding and shear‐induced stress simulation of poly (styrene‐ethylene‐butylene‐styrene) and polypropylene blends

“…3 On the other hand, when the shear flow aligns the macromolecular chains, residual stress may be generated when the orientation freezes to form an unbalanced conformation. 4,5 The release of residual stress may lead to stress cracking, warping deformation or fatigue failure. 6,7 Therefore, controlling the shear flow in the injection molding process is very important to the conformation and properties of the polymer material.…”

“…The shearing effect during the injection process can untie the molecular chain and improve the filling flow 3 . On the other hand, when the shear flow aligns the macromolecular chains, residual stress may be generated when the orientation freezes to form an unbalanced conformation 4,5 . The release of residual stress may lead to stress cracking, warping deformation or fatigue failure 6,7 .…”