Sung W. Cha

The Relationship of Mold Temperatures and Swirl Marks on the Surface of Microcellular Plastics

Abstract Thanks to the various advantages such as lighter weight, reduced sink mark, and short cycle times, foam injection molding is being widely used for production of pallets and footwear. However, the properties of the foamed parts are somewhat decreased and result in swirl marks on the surface. So this technology is not suitable for thin and elegant products such as electronic items. Many researchers are targeting toward enhancing the strength by reducing the size of the bubbles and removing the swirl mark on the surface. In this article, we analyzed the cause of surface swirl marks as a function of mold temperature and then proposed the solution to eliminate surface trouble of foaming injection molding.

A Mold Surface Treatment for Improving Surface Finish of Injection Molded Microcellular Parts

Micro-cellular foaming technology draws attention due to the enhancement of mechanical strength, which has been considered as a weak point of current plastic foaming technology. Foam materials produced by this technology offer improved consistency and homogeneity of cell structure, which can result in products with superior properties and uniformity(1). Thus it is widely used for commercial purposes and its market is now growing significantly. However, since the foamed injection parts have swirl marks on its surface, this technology has limited uses such as interior products, in spite of its diverse merits. In this paper, we propose surface treatment of the mold as a way to remove the swirl mark. We injected simple shape specimens with PP and PC/ABS materials and measured the surface quality values such as roughness and gloss. Also we researched the foaming characteristics of both treated and untreated surface using SEM analysis.

Study on the Accuracy of Injection Molded Plastic Gear with the Assistance of Supercritical Fluid and a Pressurized Mold

Plastic gears are lighter and less noisy compared to steel gears, and they can be easily shaped into diverse forms by the injection molding process. For this reason, plastic gears are widely used in industry. An extensive amount of research has been conducted on gear materials and methods of shaping plastic gears for durability and for broader applications. In this article, the focus is on producing a plastic gear shaping method incorporating the injection molding process. The most important factor influencing the durability of a plastic gear is its accuracy. Unlike steel gears, the dimensions of plastic gears are subject to considerable change during formation, and this is due to certain conditions during the injection molding process and to the mold structure. The main causes of size variability here are the high viscosity of plastic and the shrinkage ratio of the resin. In our study, a supercritical fluid was used to reduce the viscosity of the plastic, and a pressurized mold was used to control the shrinkage ratio of the resin. Thus, production of an improved, more highly accurate plastic gear was achieved.

Combined Effects of Chemical and Microcellular Foaming on Foaming Characteristics of PLA (Poly Lactic Acid) in Injection Molding Process

Foaming technology used in the plastic production process makes it possible to reduce costs and lighten the products, which is why this technology is widely used. Foaming technology is generally categorized as involving either a “Chemical Foaming Process” or a “Physical Foaming Process,” the latter also referred to as a “Microcellular Foaming Process” (MCP). In both processes, gas particles dissolved inside a polymer create cells. For the chemical foaming process, chemical blowing agents are mixed with plastic pellets, whereas for MCPs, gas is directly supplied to a polymer melted inside the barrel of an injection molding machine. The cell morphology changes when either the chemical foaming process or the MCP is applied in an injection foaming process. This study aims to compare and analyze the change of the cell morphology in such a case. In particular, Poly Lactic Acid (PLA) is used in the experiment. In this study, the foaming characteristics of a biodegradable material (PLA) and a convectional material – Acrylonitrile Butadiene Styrene (ABS) – are compared and analyzed. The analysis of the experiment results is done based on an analysis of the foaming ratio and a comparison of the cross-section of the cell morphology of specimens under different experimental conditions. For the former, a weight comparison was made, while for the latter, a scanning electron microscope (SEM) was utilized.

Bubble Nucleation and Growth in Microcellular Injection Molding Processes

Bubble nucleation and growth are the key steps in polymer foam generation processes. The mechanical properties of foamed polymer are closely related to the size of bubbles created inside the material. Thus, it is necessary to study how to improve mechanical strength by producing extremely fine bubbles inside polymer resin. We developed a theoretical framework to help produce uniformly distributed microcellular bubbles and experimentally verified the theoretical analysis results using an injection molding machine modified to make microcellular foaming products.

Effects of Repeated Microcellular Foaming Process on Cell Morphology and Foaming Ratio of Microcellular Plastics

Microcellular plastics (MCPs) are manufactured through a batch process comprised of saturation and foaming stages. In the saturation process, gas molecules are dissolved into plastic in a high-pressure vessel. Following the saturation process, micro-cells are formed inside the plastic as the gas-dissolved plastic sample undergoes the foaming process. In this paper, we investigate the effects of repeating the batch process on the formation of MCPs. Because the plastic sample after the first batch process has developed microcells, these pre-existing cells are expected to affect the second round of the batch process. Of particular interest is the effect of repeated saturation at different saturation pressures. Experiments show that repeating the batch process can lead to favorable outcomes in terms of foaming ratio and cell morphology, which are otherwise unattainable particularly with a single batch process.

The Effect of Control Factors and the Effect of CaCO3 on the Microcellular Foam Morphology

Abstract Microcellular foamed plastics by injection molder draw attraction due to light weight, less warp, reduced cycle time, less sink mark, and so on, because of cells nucleated in the polymer matrix during the injection. Although foamed plastics by injection molder have many merits as listed above, there are few studies which factors are influencing on the foam morphology that characterize the quality of foamed plastics due to the complexity of injection molding. We studied the effect of control factors of injection molding and the effect of CaCO3 as an additive for nucleation on the cell morphology according to the design of experiments. We analyzed the effects of each factor by using SEM images of foamed plastics through Minitab software using main effect plot, interaction plot, and ANOVA.

Combined Effects of Saturation Pressure and Gas Desorption on Foaming Characteristics of Microcellular Plastics

The microcellular foaming process consists of the saturation process for dissolving gas molecules into plastic and the subsequent foaming process for cell formation. Foaming characteristics of microcellular plastics (MCPs) such as foaming ratio and cell morphology are largely determined by the saturation conditions, particularly by the saturation pressure. In this study, we investigate the effects of saturation pressure on the foaming characteristics of MCPs, when the quantity of dissolved gas (or the weight gain) is kept constant. Because the weight gain of a specimen is an increasing function of saturation pressure, different desorption times are used in order to maintain the same weight gain across specimens from different saturation pressures. Contrary to the common belief, for specimens with the same weight gain higher saturation pressures lead to lower foaming ratios. A hypothesis for the underlying mechanism and a practical ramification of the phenomenon are discussed.

The Effect of Comonomers on the Foaming Characteristics of Polypropylene

In the process of polypropylene (PP) polymerization, different kinds of PP are produced depending on the differences in the structural changes with the arrangement of comonomers. Each kind of PP has distinct properties, which arise from differences in the fundamental chain bonding structure of each polymer and affect not only the mechanical properties but also the foaming characteristics of each resin. Therefore, the fundamental foaming characteristics require experimental investigation. We examined the effects of PP polymerization on the foaming rate and solubility in the microcellular foaming process.

A study on the effect of porous CaCO3 on micro-cellular plastics as an additive for nucleation

Plastics are widely used in industry, in that they are light and easily manufactured. Much research is being conducted currently to increase the strengths and to reduce the price of the material. Foaming techniques are used to achieve insulation properties, to reduce weight, and to reduce the material cost. Due to their unique properties, foamed plastics are applied to refrigerator, pipe, and insulators. In 1980, Micro-cellular foaming was invented at MIT. By applying the outstanding foaming technique, many small cells are generated in the polymer matrices and micro-cellular foamed plastics show relatively high specific strength. We investigated the role of CaCO3, which is one of the most widely used additives in the plastics industry, for nucleation in view of cell morphology. Porous CaCO3 chemically treated to increase the dispersibility and to lower the density was used in this study. Two experiments were conducted in order to check the role of the additive for nucleation. One is the compound-ability test and the other is for the role as nucleation agents.