Microcellular Injection Research Articles

Establishment of Gas Counter Pressure Technology and Its Application to Improve the Surface Quality of Microcellular Injection Molded Parts

Abstract The purpose of this study is to establish the Gas Counter Pressure (GCP) technology in combined with microcellular injection molding process. The application of gas into the mold cavity during the melt injection period provides a counter force against the melt front advancement, restricting the foaming process during the melt filling stage. Various gas counter pressures from 0 bar up to 300 bar and gas holding times of 0 to 10 s after filling completion were employed for investigating their relevant effect. It was found that when gas counter pressure is greater than 100 bar, foaming does not appear in the skin layer if no holding time is applied after melt-filling. As gas counter pressure increases, thickness of the solid skin layer without foaming also increases. Employment of holding time after melt-filling also assists foaming restriction in the melt core area. If a holding time of 10 s is applied combined with a counter pressure greater than 100 bar, foaming is completely restricted resulting in a transparent PS sample part. The part surface of those subjected to foaming-restriction in the skin layer also appears to be smooth and glossy. For the black PS parts molded via microcellular injection combined with gas counter pressure, it appears the same gloss quality as that achieved by conventional injection molding.

Study of Microcellular Injection Molding with Expandable Thermoplastic Microsphere

Abstract Injection molding with expandable thermoplastic microspheres (ETM) containing blowing chemicals is capable of fabricating lightweight, dimensionally stable plastic parts while using less material. This paper presents the study of microcellular injection molding of low density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS) parts with various ETM contents. It was found that the molded parts exhibit relatively better surface quality than conventional foamed parts. The microcellular morphology and cell density of the fractured cross-sectional surfaces were characterized using a scanning electron microscope (SEM). As reflected by the testing results, the cell microstructure – such as cell size, cell density, and a layered structure with a foamed core sandwiched by skin layers – play an important role in the weight reduction, surface quality, and mechanical properties. A smaller cell diameter and a thicker skin layer help to improve the surface quality and tensile properties of the injection molded parts with ETM. Finally, an appropriate ETM content has a positive effect on cell microstructure and weight reduction, whereas too high a concentration of microspheres adversely affects the tensile properties and surface quality.

Morphology, mechanical, thermal and rheological behavior of microcellular injection molded TPO-clay nanocomposites prepared by kneader

Thermoplastic olefin elastomers (TPO)/montmorillonite (MMT) nanocomposites prepared by kneader was used in this study. The organoclay TPO nanocomposites were then injection molded by conventional and microcellular methods. Nitrogen was used as the blowing agent. The effect of organoclay content on the mechanical/thermal/rheological properties of the TPO-clay nanocomposites was investigated. The results showed that the mechanical properties (tensile, impact, and wear resistance) increased as the clay content increased. Cell size decreased as the clay loading increased. The addition of MMT into TPO also improved the thermal stability of the TPO/clay nanocomposites. The XRD results showed that the nanocomposites having an intercalated layered structure. Rheological results showed the viscosity is clay loading dependent and shear thinning effect takes place at high shear rate.

A novel method for improving the surface quality of microcellular injection molded parts

Microcellular injection molding is the manufacturing method used for producing foamed plastic parts. Microcellular injection molding has many advantages including material, energy, and cost savings as well as enhanced dimensional stability. In spite of these advantages, this technique has been limited by its propensity to create parts with surface defects such as a rough surface or gas flow marks. Methods for improving the surface quality of microcellular plastic parts have been investigated by several researchers. This paper describes a novel method for achieving swirl-free foamed plastic parts using the microcellular injection molding process. By controlling the cell nucleation rate of the polymer/gas solution through material formulation and gas concentration, microcellular injection molded parts free of surface defects were achieved. This paper presents the theoretical background of this approach as well as the experimental results in terms of surface roughness and profile, microstructures, mechanical properties, and dimensional stability.

Rheological characterization of polystyrene melts dissolved with supercritical nitrogen fluid during microcellular injection moulding

AbstractThere are several benefits of using the supercritical fluid microcellular injection moulding process. The part weight, melt temperature, viscosity, moulding pressure, shrink/warpage, and cooling/cycle time are all significantly reduced. The purpose of this study is to investigate the rheological behaviour of PS melt dissolved SCF of nitrogen during Microcellular Injection Moulding process applied with Gas Counter Pressure (GCP) technology. The application of gas into the mould cavity prior to the melt filling provides a counter force against the melt front advancement, restricting the foaming process during the melt filling stage. A slit cavity is designed to measure the pressure drop of polystyrene mixed with 0.4wt% supercritical nitrogen fluid under different mould temperatures (185°C, 195°C, and 205°C), injection speeds (5, 10, and 15 mm/s) as well as counter pressures (0, 150, 300 bars). It was found that melt viscosity is reduced by up to 30% when GCP is increased from 50 to 150 bar as compared to conventional injection moulding. The non-nucleation mixture melt obtained by using a GCP of 300 bar has 32~49% lower viscosity. In addition, the glass transition temperature, Tg, was found to be reduced from 96 °C to 50 °C when the applied GCP is 300 bar.

Comparisons of Microcellular PHBV/PBAT Parts Injection Molded with Supercritical Nitrogen and Expandable Thermoplastic Microspheres: Surface Roughness, Tensile Properties, and Morphology

Solid and foamed tensile bars made of poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/ poly (butylenes adipate-co-terephthalate) (PBAT) blends with a weight ratio of 45/55 were prepared via conventional and microcellular injection molding processes, respectively. To fabricate the degradable PHBV/PBAT foamed parts, two kinds of blowing agents, namely, supercritical nitrogen (SCF N2) as the physical blowing agent and expandable thermoplastic microspheres (ETM) as the chemical blowing agent, were employed. Various properties were investigated and compared in terms of surface roughness, mechanical tensile properties, and cell morphologies. Through surface roughness comparisons using a 2D surface roughness analyzer and a 3D white-light interferometer surface profiler, microcellular injection molded parts with ETM exhibit a better surface quality. The tensile property results show that PHBV/PBAT–N foamed with SCF N2 have a longer strain-at-break. The microcellular morphologies and sandwich-like multilayer structure on the fractured cross-sectional surfaces were characterized by using a scanning electron microscope (SEM). As shown by the test results, the cell microstructures - such as cell size, cell density, and multi-layered structures with a foamed core sandwiched by skin layers - played an important role in the surface quality and mechanical properties. The rather interesting and unusual cell morphology of PHBV/PBAT–N explains why the Young's modulus and ultimate strength are lower.

The Microcellular Injection Molding of Low-Density Polyethylene (LDPE) Composites

This paper reports the study of microcellular injection molding of low-density polyethylene- (LDPE) based composites. The effects of adding nanoclays and polymer additives in LDPE as well as rheological property of materials on the cell morphology, mechanical properties and surface properties of microcellular injection molded LDPE based composites are presented. For the microcellular injection molding process, when 3 wt% of nanoclays are added into LDPE-based polymers, the cell morphology can be significantly improved due to the nucleating effects resulting from the broad interface areas between polymer and nanoclays. Also, the addition of low melt flow LDPE into high melt flow LDPE could achieve smaller and denser bubbles in the polymer matrix than neat high melt flow LDPE.

Topics of Foam Product Made by Microcellular Injection Molding Using Super Critical Fluid

Topics of Foam Product Made by Microcellular Injection Molding Using Super Critical Fluid

Modeling and Characterization of the Relationship between Cell Size and Mechanical Behavior of Microcellular PP/Mica Composites

Abstract The microcellular foamed polypropylene (PP)/mica composites were prepared through chemical microcellular injection to investigate the relationship between the mechanical behavior and the cell size. The mechanical behavior model of the microcellular foams is built based on the elastic stress/strain field. The mechanical behavior of the microcellular composites coincides well with the theoretical model. Under loading the large and non uniform cells are in a state of plane strain, which leads to low tensile and impact strengths, whereas small and uniform cells are in a state of plane stress, which contributes to the transition from brittle to tough fracture behavior in the microcellular PP/mica composite.

Effect of organoclay on the mechanical/thermal properties of microcellular injection molded PBT–clay nanocomposites

An organically modified montmorillonite (MMT) was compounded with polybutylene terephthalate (PBT) in a twin-screw extruder. The organoclay PBT nanocomposites were then injection molded by conventional and microcellular methods. Nitrogen was used as the blowing agent. The effect of organoclay content, organoclay size (8 and 35 μm), and speed of the screw (80 and 100 rpm) on the mechanical and thermal properties were investigated. The results showed that when the MMT content was 1.0 wt.%, the nanocomposites have maximum tensile strength, wear resistance, and cell density. Moreover, the larger the particle size, the greater the tensile strength. The screw speed during compounding also affected the mechanical strength. The higher speed of the screw increased the tensile strength of the nanocomposites. The addition of MMT also helped the thermal stability of the PBT/clay nanocomposites. The WXRD results showed that when MMT loading is 1.0 wt.%, the nanocomposites have maximum d-spacing structure. TEM results showed that MMT is well dispersed on the nanocomposites at a MMT loading of 1.0 wt.%.

Improving surface quality of microcellular injection molded parts through mold surface temperature manipulation with thin film insulation

AbstractMicrocellular injection molding offers many advantages such as material and energy savings, reduced cycle times, and excellent dimensional stability. However, typical surface characteristics of microcellular injection molded parts—such as gas flow and swirl marks and a lack of smoothness—have precluded the process from being used for applications where surface appearance is important. This article presents an insulator‐assisted method that has been shown to improve the surface quality of microcellular injection molded parts significantly. By incorporating a thin film (75–225 μm) of polytetrafluoroethylene (PTFE) insulator on the mold surface, the polymer melt–insulator interfacial temperature can be manipulated and can be kept high enough during mold filling to reduce or eliminate swirl marks on the surface. The experimental results in terms of surface roughness and surface profile of conventional and microcellular injection molded parts with and without the insulator film are discussed. Thermal analyses of the corresponding microcellular injection molding experiments were performed to elucidate the correlation between film thickness, interfacial temperature, and the surface quality. The effect of insulator on the cooling time increase is also analyzed and presented. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers

Processing and characterization of microcellular PHBV/PBAT blends

Solid and microcellular specimens made of poly (3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV)/ poly(butylene adipate-co-terephthalate) (PBAT) blends with three different weight ratios (i.e., PHBV/PBAT = 98.5/1.5 (Y1000P), 45/55 (5010P), and 30/70 (6010P), respectively) were prepared via conventional and microcellular injection molding process, respectively. Various properties such as miscibility, cell morphology, thermal, and mechanical properties were investigated. Compared with the microcellular specimens, the solid specimens showed better miscibility. Increasing the PBAT content led to higher specific toughness and strain-at-break, but lower specific modulus and specific strength for both solid and microcellular specimens. In addition, increasing the PBAT content increased the average cell size, but decreased the cell density of the microcellular samples. Finally, the crystallinity of PHBV in the PHBV/PBAT blends decreased significantly with increasing PBAT content for both solid and microcellular specimens. POLYM. ENG. SCI., 50:1440-1448, 2010. © 2010 Society of Plastics Engineers.

Bubble growth in mold cavities during microcellular injection molding processes

Bubble nucleation and growth are the key steps in polymer foam generation processes. The mechanical properties of foam polymers are closely related to the size of the bubbles created inside the material, and most existing analysis methods use a constant viscosity and surface tension to predict the size of the bubbles. Under actual situations, however, when the polymer contains gases, changes occur in the viscosity and surface tension that cause discrepancies between the estimated and observed bubble sizes. Therefore, we developed a theoretical framework to improve our bubble growth rate and size predictions, and experimentally verified our theoretical results using an injection molding machine modified to make microcellular foam products.

Optimization of Microcellular Injection Molding Parameters

Abstract Microcellular foamed polymeric products have found various applications depending upon their cell morphology. The microcellular injection molding process parameters have effect on the cell size, cell density and cell distribution. In order to develop desired cell morphology for a particular application, the set of injection molding parameters is required to be optimized for achieving a correct process window. In this paper an effort is made to study the effect of various injection molding processing parameters on cell morphology. Series of experiments were conducted for each of the processing parameter and the samples were observed under Scanning Electron Microscope (SEM). The trends of cell size and cell density with respect to the processing parameters were qualitatively analyzed. The results are useful for the processors who are interested in building up an understanding of this novel process and also to enable optimization of the set of processing parameters so that the desired microstructure of a foamed polymer can be obtained.

A Numerical Simulation of Full-shot Microcellular Injection Molding

The microcellular injection molding process is classified into short-shot process and full-shot process. Most current studies focus on short-shot process. In this paper, an algorithm for numerical simulation of full-shot microcellular injection molding is presented. The polymer-gas mixture in the cavity is treated as a macroscopic single-phase fluid. The unit cell model proposed by Amon and Denson is employed to model the cell growth, and the microscopic model of the cell growth is coupled with the macroscopic flow of polymer melt to describe the molding process. Governing equations involving the flow field and bubble growth are derived and solved iteratively. The effects of process parameters such as cell density and gas concentration on cell size are also investigated.

Effect of organoclay on the mechanical / thermal properties of microcellular injection molded polystyrene–clay nanocomposites

An organically modified montmorillonite was compounded with polystyrene (PS) in a twin-screw extruder. The organoclay polystyrene nanocomposites were then injection molded by conventional and microcellular methods. Nitrogen was used as the blowing agent. The effect of organoclay content on the mechanical and thermal properties was investigated. The results showed that when the MMT content was 1 wt.%, the nanocomposites have maximum tensile strength, wear resistance, and cell density. Moreover, the addition of organoclay increases the glass transition and decomposition temperature of the nanocomposites. The XRD results showed that the layer spacing of the nanocomposites decreases by comparison with the organoclay. TEM pictures showed that MMT is well dispersed within the PS matrix.

Studies on Polypropylene/Carbon Fiber Composite Foams by Nozzle-Based Microcellular Injection Molding System

A continuous production of microcellular polypropylene (PP)/30% carbon fiber composite was achieved in a newly developed nozzle-based microcellular injection molding system. In this work, a conventional injection molding machine was retrofitted with a nitrogen (N2) gas injection system. A specially designed “injection cum mixing nozzle” was added between the machine barrel and a shut-off nozzle to form a polymer/gas mixture as against barrel injection where the screw and barrel are usually modified for gas insertion. The foamed plastic was formed due to the thermodynamic instability caused by sudden pressure drop in the mold. The effectiveness of foam molding system developed in this study was verified by change of process parameters resulting into variety of cell morphology and cell size. Further, PP/30% carbon fiber composite was injection molded into ASTM tensile test sample with both conventional and foam molding system. These samples were then subjected to scanning electron microscope (SEM) as well as tensile testing to study the effect of injection speed on the cell morphology and tensile properties of foam molded composites. It was observed that cell size decreased and cell density increased with increase in injection speed. The mechanical properties of the foamed composites depend largely on the processing conditions.

Effect of clay and compatibilizer on the mechanical/thermal properties of microcellular injection molded low density polyethylene nanocomposites

In this study, the effect of montmorillonite (MMT) and compatibilizers content on the mechanical/thermal properties of low density polyethylene (LDPE) was studied. Maleated LDPE was compounded in a kneader with different MMT loadings. The maleic anhydride(MA)-grafted LDPE (LDPEgMA) nanocomposites were then molded by conventional and microcellular injection molding process. The effect of MMT and compatibilizers content on the mechanical/thermal properties was investigated. The results showed that LDPEgMA nanocomposites (up to 5 wt.% MMT loading) have better tensile strength and wear resistance than their neat counterparts, using either the conventional or microcellular injection molding process. In addition to the mechanical properties, the LDPEgMA nanocomposites also have better foaming property (i.e., cell size and cell density) than their neat counterparts. The thermal stability of the LDPE material is also improved by the addition of MMT.

The Influence of Microcellular Foaming Process on Electrical Conductivity of Conductive Composites

By compounding ordinary plastics and conductive additives, conductive polymers can possess conductivity. Conductive polymers are diverse, ranging from antistatic to static dissipative and electromagnetic shielding. Usually, carbon fiber, carbon black, nickel-coated carbon fiber copper powder and metal powder etc. are used as conductive additives and conductivity is determined by both the type and contents of the additives. In this research, a microcellular foaming batch process, a microcellular injection molding process and microcellular extrusion process were adopted to diversify the conductive properties of one conductive polymer. The results indicated that changes in the conductivity of conductive polymers are observed in a certain range and the hypothesis that foaming samples have a higher conductivity than non-foaming samples was verified under the specific cell size and foaming condition. We concluded that the optimum cell size and foaming rate exist in conductive polymers for maximum conductivity; we also concluded that the use of the microcellular foaming process is expected to form conductive composites.

Correlation between Molding Conditions and Foam Morphology in Microcellular Injection Molding

In this study, a quantitative analysis of foam cell distribution at the cross section of products in microcellular injection molding was conducted concerning the relationship between the mold conditions and laminar morphology. The following results were obtained; (1) The morphology consists of a surface layer (Skin layer I) with silver streaks, a layer (Skin layer II) with no cells inside, and a foam layer (Core layers I, II, III) with many cells of different size. (2) The morphology changes depending on the molding conditions and cavity position. (3) The core layer domain decreases from the gate to the distal end. (4) Injection conditions greatly affect the thickness of Skin layer II. (5) Maximum filling pressure in the mold affects mainly the core layer of the foam morphology.