Gas-assisted Injection Molding Parts Research Articles

A novel hierarchical crystalline structure of injection-molded bars of linear polymer: co-existence of bending and normal shish–kebab structure

The hierarchy structures and orientation behavior of high-density polyethylene (HDPE) molded by conventional injection molding (CIM) and gas-assisted injection molding (GAIM) were intensively examined by using scanning electronic microscopy (SEM) and 2D wide-angle X-ray diffraction (2D-WAXD). Results show that the spatial variation of crystals across the thickness of sample molded by CIM was characterized by a typical skin–core structure as a result of general shear-induced crystallization. Unusually, the crystalline morphologies of the parts prepared by GAIM, primarily due to the penetration of secondary high-compressed gas that was exerted on the polymer melt during gas injection, featured a richer and fascinating supermolecular structure. Besides, the oriented lamellar structure, general shish–kebab structure, and common spherulites existed in the skin, sub-skin, and gas channel region, respectively; a novel morphology of shish–kebab structure was seen in the sub-skin layer of the GAIM parts of HDPE. This special shish–kebab structure (recognized as “bending shish–kebab”) was neither parallel nor perpendicular to the flow direction but at an angle. Furthermore, there was a clear interface between the bending and the normal shish–kebab structures, which may be very significant for our understanding of the melt flow or polymer rheology under the coupling effect of multi-fluid flow and complex temperature profiles in the GAIM process. Based on experimental observations, a schematic illustration was proposed to interpret the formation mechanism of the bending shish–kebab structure during GAIM process.

Role of gas delay time on the hierarchical crystalline structure and mechanical property of HDPE molded by gas-assisted injection molding

The relationship among the processing parameters, crystalline morphology, and macroscopic properties in injected molded bar becomes very complicated due to existence of temperature gradient and shear gradient along the sample thickness. To enhance the shear strength, gas-assisted injection molding (GAIM) was utilized in producing the molded bars. The aim of our research was to explore the relationship between processing conditions and the spatial variation of the hierarchy structure as well as the mechanical properties of high-density polyethylene (HDPE) obtained via GAIM. In our previous work [Wang L, Yang B, Yang W et al (2011) Colloid Polym Sci 289:1661–1671], we found that the enhancement of the gas pressure can remarkably increase the degree of molecular orientation in the HDPE samples, which turns out to improve the mechanical performances of GAIM parts. In this work, the hierarchy structure, orientation behavior, and mechanical properties of molder bars under different gas delay time were investigated using a variety of characterization techniques including rheological experiments, scanning electron microscope, tensile testing, differential scanning calorimetry, and two-dimensional wide-angle X-ray scattering. Moreover, the temperature field during the short shot stage of GAIM process was simulated using an enthalpy transformation approach. Our results indicate that these properties were intimately related to each other, and with prolonged gas delay time, GAIM samples with higher degree of orientation and improved mechanical properties were obtained.

Morphology and mechanical property of high-density polyethylene parts prepared by gas-assisted injection molding

The crystal morphology, melting behavior, and mechanical properties of high-density polyethylene (HDPE) samples obtained via gas-assisted injection molding (GAIM) under different gas pressures were investigated. Moreover, the non-isothermal crystallization kinetics of HDPE under different cooling rates was also studied. The obtained samples were characterized via differential scanning calorimetry, two-dimensional wide-angle X-ray scattering (2D-WAXS), tensile testing, dynamic mechanical analysis (DMA) and scanning electron microscopy techniques. It was found that the properties were intimately related to each other. Macroscopically, the flow-induced morphology of the various HDPE samples was characterized with a hierarchical crystalline structure, possessing oriented lamellar structure, shish–kebab structure, and common spherulites in the skin, sub-skin, and gas channel region, respectively. The 2D-WAXS results demonstrated that the degree of orientation of the high gas pressure sample was larger than that of the low pressure sample at the corresponding layer. The tensile testing results of GAIM parts showed that the mechanical properties of the GAIM parts were improved with an increase of the gas pressure. Furthermore, the DMA was utilized to obtain the dynamic mechanical properties of the GAIM samples, and the results indicated that significant improvement of the orientation was observed with an increase of the gas pressure.

Simulation of Gas-Assisted Injection Molding of High-Density Polyethylene: The Role of Rheological Properties and Physical Fields on the Crystalline Morphology

The shear and temperature fields in the filling process during gas penetration of gas-assisted injection molding (GAIM) of two kinds of high-density polyethylene (HDPE) with different molecular weights (Mw) were simulated numerically via Moldflow 6.1. The simulated results indicated that HDPE with higher Mw showed a relatively low shear rate in both the outer and inner zone of GAIM parts as compared to that with lower Mw, while the shear rate of the inner zone of the parts of the two materials were lower than that of the outer zone, which has a lower temperature and shorter time for crystallization. Considering the remarkable difference between their viscosities, a model of the shear thinning and recovering effects on crystallization during the GAIM process is proposed. Through the simulation, along with the analysis of their viscosities and rheological properties, the influence of molecular weight and shear and temperature fields on two special kinds of crystalline morphology (i.e., shish-kebabs and banded spherulites) during GAIM were interpreted.

Banded spherulites of HDPE molded by gas-assisted and conventional injection molding

Crystal morphologies of high density polyethylene (HDPE) with low molecular weight obtained by gas-assisted injection molding (GAIM), conventional injection molding (CIM), and spontaneous cooling, respectively, were studied by scanning electronic microscopy (SEM). It is found that banded spherulites are generated in the inner zone of GAIM parts and the outer zone of CIM parts but are absent in quiescent parts. According to the results, the representative morphologies of crystal change with gradual increment of instantaneous flow field in crystallization from non-banded spherulite to banded spherulite and then to oriented lamellae. This morphological evolution indicates that banded spherulites could be induced by flow field with certain intensity, which is confined by both an upper critical value and a lower one.

Core out geometries in water channels of water assisted injection moulded parts

In water assisted injection moulded parts, structural ribs are usually designed to reinforce the parts as well as to serve as water channels. Water penetration in the channels results in improved mouldability and minimises sinkmarks. However, water leaves a rib core void, which reduces part rigidity. The present study examines the core out shapes of channels in water assisted injection moulded parts. The effects of channel geometry as well as processing conditions such as water injection delay time, water temperature, water pressure, etc., on the core out shapes were investigated. For the parameters selected, melt temperature, short shot size and water injection delay time were found to be the principal factors affecting the core out shapes of moulded parts. The core out geometries in water assisted injection moulded parts were compared with those in gas assisted injection moulded products. Water penetrated the parts with higher core out ratios, which are the ratios of the cross-sectional area of the water (gas) channel to the area of the water (gas) bubble in moulded parts, than gas. The core out geometry of water assisted injection moulded parts mainly resembles that of the water channel, while the void shape of gas assisted injection moulded parts tends to be more round shaped. Channels with ribs on top exhibited the highest bending strengths. Furthermore, a numerical simulation of the temperature distribution in the channels was completed to better interpret the water core out behaviour in water assisted injection moulded products.

Morphology of gas‐assisted and conventional injection molded polycarbonate/polyethylene blend

AbstractThe skin‐core structure of the gas‐assisted and conventional injection molded polycarbonate (PC)/polyethylene (PE) blend was investigated. The results indicated that both the size and the shape of the dispersed PC phase depended not only on the nature of PC/PE blend and molding parameters, but also on its location in the parts. Although the gas‐assisted injection molding (GAIM) parts and conventional injection molding (CIM) part have the similar skin‐core structure, the morphology evolution of PC phase in the GAIM moldings and the CIM moldings showed completely different characteristics. In the section perpendicular to the melt flow direction, the morphology of the GAIM moldings included five layers, skin intermediate layer, subskin, core layer, core intermediate layer as well as gas channel intermediate layer, according to the degree of deformation. PC phase changed severely in the core layer of GAIM moldings, as well as in the subskin of CIM moldings. In GAIM parts, PC phase in the core layer of the nongate end changed far more intensely and aligned much orderly than that in the gate end. The morphology of PC phase in the GAIM part molded with higher gas pressure changed more severe than that in the GAIM part molded with lower gas pressure. In a word, PC phase showed more obvious fibrillation in the GAIM moldings than that in the CIM moldings. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3069–3077, 2006

Study of the Bending Properties in Gas-assisted Injection Molded Fiber-reinforced Nylon Parts

Glass fiber-reinforced Nylon plate parts designed with gas channels having five different types of cross section but with the same section were gas-assisted injection molded (GAIM). Effects of glass fiber content and geometrical factors introduced by various section shapes and the associated dimensions of gas channels on bending properties of GAIM parts were investigated via a bending test. Test results were also compared with those of conventional injection molded parts. Based on the measured results, it is found that gas-assisted injection molded parts show better bending properties including flexural strength, absorbed energy and bending stiffness than conventional injection molded parts. Alternatively, bending performance of GAIM parts increases when the content of glass fiber is increased. However, the fiber content of 15% has pretty good efficiency of bending performance. Meanwhile, for five gas channel designs, both gas channel designs attached with top rib (shapes D and E) show the higher bending stiffness and maximum bending load, correspondingly. So, generally speaking, these two gas channel designs provide the best enhancement in bending performance, however, parts with semicircular and rectangular gas channel designs (shapes A and B) can absorb more bending energy than the other designs. Alternatively, the flexural strength shows only slight influence from gas channel design, the deviation from average values less than 10%. The present study provides part designers with a design guideline for choosing the most effective gas channel design and fiber content to achieve a specific objective of part structural performance.

Surface Visual Quality of Gas-Assisted Injection Molded ABS Parts with Various Gas Channel Designs

Gas-assisted injection molding (GAIM) provides many more advantages than conventional injection molding (CIM), but its applications are limited to the surface visual quality problem. ABS plate parts designed with gas channels having five different types of cross-section but with the same section area were gas-assisted injection molded. Furthermore, various plate thickness parts designed with semicircular gas channel of different radii were also molded. Surface visual quality of GAIM parts was investigated by gloss and chromatics measurements. Effects of processing parameters and geometrical factors, introduced by part thickness, shape, and associated dimensions of gas channels on glossy difference and chromatic aberration of GAIM parts were studied. Influence of mold with etching treatment on surface visual quality was also examined. Basically, it was found that glossy difference is very sensitive to the degree of crystallinity whereas gas channel residual skin melt thickness plays a dominant factor for chromatic aberration. The surface visual quality shows a strong dependence on processing conditions. In general, gas channel design of semicircular cross section (shape A) provides better surface effect than the other designs in visual quality. Meanwhile, to obtain best surface visual quality, the ratio of equivalent radius to plate thickness should be ranged from 2.2 to 2.4. Alternatively, the surface visual quality can be improved by etching treatment on mold core-side. The results of this research may provide part design guideline for choosing the most effective gas channel design to achieve the best surface visual quality.

Parameters affecting the full‐shot molding of gas‐assisted injection‐molded parts

AbstractFull‐shot gas‐assisted injection‐molding has the advantage of eliminating the switchover mark that usually occurs on the surface of short‐shot gas‐assisted molded parts. The purpose of this report was to study the effects of processing parameters on the moldability of the full‐shot gas‐assisted injection‐molding process. Experiments were carried out on an 80‐ton injection‐molding machine equipped with a high‐pressure nitrogen‐gas injection unit. The materials used were general‐purpose polystyrene and polypropylene. A plate cavity with a gas channel of various geometries (trapezoid, semicircle, and rectangle) across the center was used to mold the parts. After molding, the lengths of gas penetration were determined. The hollowed core ratio by the gas was also determined. A numerical analysis was carried out to find out the temperature distribution of the polymer melt inside the gas channel. It was found that the sink mark of molded parts decreases with the length of gas penetration. Molded parts using trapezoidal gas channel had the longest gas penetration length. In addition, a thermal contraction model was proposed to predict the gas penetration volume inside the parts. Good agreement was reached between the experimental data and the calculated result. © 2003 Wiley Periodicals, Inc. Adv Polym Techn 22: 1–14, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10036

An empirical study of gas penetration in full-shot gas-assisted injection moulded parts

Full-shot gas-assisted injection moulding has increasingly become one of the most important methods used to produce plastic components. It has the advantage of eliminating the switchover mark, which usually occurs on the surface of short-shot gas-assisted injection moulded parts. This paper is devoted to an investigation of the effects of different processing parameters on the length of gas penetration in full-shot gas-assisted injection moulded parts. The first part of this report shows how the gas penetration of moulded parts is optimized. An L'18 experimental matrix design based on the Taguchi method was conducted to investigate the processing factors that affect the length of gas penetration in full-shot moulded parts. The second part of this paper identifies the relative significance of each processing parameter on the gas penetration of moulded products. The materials used were general-purpose polystyrene and polypropylene. Experiments were carried out on an 80 ton injection-moulding machine equipped with a high-pressure nitrogen gas injection unit. For the factors selected in the main experiments, melt temperature, gas injection delay time and gas hold time were found to be the key processing parameters affecting the length of gas penetration in full-shot gas-assisted injection moulded parts. In addition, the sink mark of full-shot moulded parts decreases with the length of gas penetration. Bending strength of full-shot gas-assisted injection moulded parts is higher than that of short-shot moulded parts.

Experimental study of weldline caused by flow lead effect in gas assisted injection moulded parts

Gas assisted injection moulding has become one of the most important methods for the manufacture of plastic products. However, there are several unsolved problems that confound the overall success of this technique. The weldline caused by the flow lead effect in the polymer melt is one of them. In this report, an L′18 orthogonal array design based on the Taguchi method has been conducted to investigate the flow lead induced weldlines in gas assisted injection moulded parts. A plate cavity with a gas channel on the side was used for moulding. Experiments were carried out on an 80 t reciprocating screw injection moulding machine equipped with a high pressure gas injection unit. After moulding, the depth of the weldline was measured. For the factors selected in the main experiments, melt injection temperature and mould temperature were found to be the principal factors affecting the weldline depth of gas assisted injection moulded parts. Experimental investigation of a gas assisted injection moulding problem can help in better understanding the weldlines caused by flow lead effects, so that steps can be taken to optimise the surface quality of moulded parts. PRC/1755

Variations in surface gloss on parts made by gas assisted injection moulding

Gas assisted injection moulding has proved to be a breakthrough in moulding technology for thermoplastic materials. However, there are still unsolved problems that limit the overall success of this technique. The aim of this work was to study the phenomenon of gloss variations occurring across the surfaces of gas assisted injection moulded parts. Experiments were carried out on an 80 t injection moulding machine equipped with a high pressure, nitrogen gas injection unit. The materials used were pigmented acrylonitrile/butadiene/ styrene and polypropylene. A plate cavity with a gas channel across its centre was used to mould the parts. Various processing parameters were varied: melt temperature; mould temperature; melt filling speed; short shot size; gas pressure; and gas injection delay time. After moulding, a glossmeter was used to determine the effects of these processing parameters on the surface gloss profiles of the parts. A roughness meter and scanning electronic microscope were also employed to characterise the surface quality of moulded parts. In addition, a numerical analysis of the filling process was carried out to help better understand the mechanisms responsible for the phenomenon of surface gloss variations. It was found that the surface gloss difference occurs mainly in the transition area between channel and plate in the moulded parts, which might be the result of the shear stress gradient in the polymer melt during the filling process. Surface roughness of moulded parts might also be another factor resulting in the gloss difference problem. PRC/1720

Gas-assisted injection molding: the effects of process variables and gas channel geometry

Polystyrene parts with different rib geometries but having the same aspect ratio were molded using gas-assisted injection molding (GAIM). The process variables that have an influence on the gas bubble distribution, residual wall thickness (RWT), fingering formation and mechanical properties were explored. The test results revealed that there is an inherent relationship between gas fingering and gas bubble penetration that has consequences on part strength. Shot size and delay time are the most dominant factors affecting the gas penetration, fingering formation, RWT and mechanical properties of the GAIM parts. The effects of rib geometries are also discussed. Computer simulation of the GAIM process was carried out using Moldflow, a commercial software. The outcomes predicted by simulation are compared with the experimental results. Based on the results, some useful design guidelines are suggested.

Investigations on the Impact Performance of Gas-Assisted Injection Molded Polystyrene Parts

The effects of geometrical factors, introduced by part thickness and various gas channel designs, including semicircular gas channel and rectangular gas channels with different geometry, on the impact properties of gas-assisted injection molded (GAIM) polystyrene parts were investigated via impact energy measurement. Test results were also compared with those of conventional injection molded (CIM) parts and plate parts without gas channel. It was found that gas channel design results in part impact reinforcement and gas-assisted injection molded parts show better impact performance than conventional injection molded parts. Generally speaking, impact strength of GAIM parts increases when the size of the gas channel is increased. Meanwhile, for five gas channel designs, both gas channel designs attached with top rib (shapes D and E) show the higher impact energy and impact value. These two gas channel designs provide the best enhancement effect in impact performance. The present investigation provides a design guideline for choosing the most effective gas channel design to achieve the part impact performance.

Mechanical properties of gas-assisted injection moulded PS, PP and Nylon parts

Abstract PS, PP and Nylon plate parts designed with gas channels having five different types of cross-section but with same section area were gas-assisted injection moulded (GAIM). Mechanical properties of GAIM parts were investigated via tensile and bending tests. Effects of part thickness, shape and associated dimensions of gas channels on tensile and bending properties of GAIM parts were examined. It was found that maximum tensile load and ultimate tensile stress show only slight influence from gas channel design and part thickness except Nylon parts which exhibit significant dependence on part thickness due to degree of crystallinity. Gas channel design, introducing additional moment of inertia, results in part structural reinforcement. Part stiffness and maximum bending load basically increases linearly with the moment of inertia and the section modulus of the plate, respectively. Gas channel design attached with top rib (shapes D and E) show the best effect of structural reinforcement. For brittle PS parts, plates with semicircular gas channel (shape A) exhibit maximum flexural strength. PS parts with rectangular gas channel design (shape B) can absorb more bending energy than the other designs. The present study provides part designers with a design guideline for choosing the most effective gas channel design to achieve a specific objective of part structural performance.

Integrated simulations of structural performance, molding process, and warpage for gas-assisted injection-molded parts. III. Simulation of cyclic, transient variations in mold wall temperatures

Whether it is feasible to perform an integrated simulation for structural analysis, process simulation, as well as warpage calculation based on a unified CAE model for gas-assisted injection molding (GAIM) is a great concern. In the present study, numerical algorithms based on the same CAE model used for process simulation regarding filling and packing stages were developed to simulate the cooling phase of GAIM considering the influence of the cooling system. The cycle-averaged mold cavity surface temperature distribution within a steady cycle is first calculated based on a steady-state approach to count for overall heat balance using three-dimensional modified boundary element technique. The part temperature distribution and profiles, as well as the associated transient heat flux on plastic-mold interface, are then computed by a finite difference method in a decoupled manner. Finally, the difference between cycle-averaged heat flux and transient heat flux is analyzed to obtain the cyclic, transient mold cavity surface temperatures. The analysis results for GAIM plates with semicircular gas channel design are illustrated and discussed. It was found that the difference in cycle-averaged mold wall temperatures may be as high as 10°C and within a steady cycle, part temperatures may also vary ∼ 15°C. The conversion of gas channel into equivalent circular pipe and further simplified to two-node elements using a line source approach not only affects the mold wall temperature calculation very slightly, but also reduces the computer time by 95%. This investigation indicates that it is feasible to achieve an integrated process simulation for GAIM under one CAE model, resulting in great computational efficiency for industrial application.

Correlation of gas penetration and permeation to the structural performance of gas-assisted injection-molded parts

The correlation of insufficient gas penetration and gas permeation to structural performance in gas-assisted injection-molded polystyrene parts, designed with gas channels having five different types of cross sections but with the same section area, was investigated via tensile experiments and bending tests. Test results were also compared with those of standard injection-molded parts. Based on the measured results, it was found that for specimens with insufficient gas penetration, under tensile test and bending test the breaking region occurred in the neighborhood of the gas front tip near the gas entrance. This indicates that the effect of the hollow core developed by the gas, resulting in reduction of the cross-sectional area, is larger than the effect of stress concentration or residual stress at the gas front tip. In tensile tests, it is evident that gas permeation and insufficient gas penetration show significant influence on the tensile properties of parts, lowering the ultimate tensile strength. Moreover, in bending tests, the stress concentration or residual stress occurring at the gas front tip of insufficient gas penetration and the surrounding area of gas permeation shows less significant influence on part bending properties. At the same time, the cross-sectional geometry plays an important role in determining bending properties. The present investigation provides part designers design guidelines for understanding the correlation of insufficient gas penetration and gas permeation to structural performance of gas-assisted injection-molded parts. © 1999 John Wiley & Sons, Inc. Adv Polym Techn 18: 303–313, 1999

Study of “gas fingering” behavior in gas‐assisted injection molding

AbstractA common problem encountered in gas‐assisted injection molding (GAIM) is “gas fingering,” i.e., gas bubbles penetrating outside designed gas channels and forming finger‐shaped branches. Severe gas fingering can result in significant reductions in part stiffness. In this paper, nondestructive techniques have been used to quantitatively characterize gas bubble geometry in specially designed GAIM parts. Based on the quantitative measurements, the most crucial process parameters to control gas fingering have been identified through factorial design experiments. More detailed process studies reveal that there is an intrinsic relationship between gas fingering and cross‐sectional gas bubble shape. By examining the effects of gas channel design and material type on the gas bubble shape, some useful gas channel design guidelines have been obtained.

Effect of Gas Channel Design on Bending Properties of Gas-Assisted Injection Molded Polystyrene Parts

Polystyrene (PS) thin plates designed with semicircular gas channel of different radius and with rectangular gas channel of different width to height ratio were gasassisted injection molded (GAIM). Effects of geometrical factors introduced by part thickness, various section shapes and the associated dimensions of gas channels on bending properties of GAIM parts were investigated via a bending test. It was found that gas channel design, introducing additional moment of inertia determined by the geometry of channel section and the hollowed core, results in part structural reinforcement. Bending properties of GAIM parts, including maximum bending load and absorbed bending energy, are enhanced when part thickness and the sizes of semicircular and rectangular gas channel increase. Part stiffness and maximum bending load increases linearly with inertia moment and section modulus of the plate, respectively. Increasing gas channel height has the best effect on the enhancement of bending stiffness. The present investigation provides part designers a guideline for choosing the most effective gas channel design to achieve a specific part bending structural performance.