Gas-assisted Injection Molding Research Articles

Effects of Processing Method and Parameters on the Wall Thickness of Gas-Projectile-Assisted Injection Molding Pipes.

Gas-Projectile-Assisted Injection Molding (G-PAIM) is a new injection molding process derived from the Gas-Assisted Injection Molding (GAIM) process by introducing a projectile to it. In this study, the short-shot method and the overflow method of both the G-PAIM and GAIM processes were experimentally compared and investigated in terms of the wall thickness of the pipes and its uniformity. The results showed that the wall thickness of the G-PAIM molded pipe was thinner and more uniform than that of the GAIM molded pipe, and the wall thickness of the pipe molded by the Gas-Projectile-Assisted Injection Molding Overflow (G-PAIM-O) process was the most uniform. For the G-PAIM-O process, the influence of processing parameters, including melt temperature, gas injection delay time, gas injection pressure, melt injection pressure and mold temperature, on the wall thickness and uniformity of the G-PAIM-O pipes were studied via the single-factor experimental method. It was found that the effects of gas injection delay time and gas injection pressure on the wall thickness of the G-PAIM-O pipes were relatively significant. The wall thickness of the pipes increased with the increase in gas injection delay time and decreased with the increase in gas injection pressure. The melt temperature, melt injection pressure and mold temperature had little effect on the wall thickness of the G-PAIM-O pipes. In general, the wall thickness uniformity of the G-PAIM-O pipes was slightly affected by these processing parameters.

Experimental analysis of fluid‐assisted injection molding of pipe fittings with different angles by short‐shot method

AbstractBased on the self‐assisted injection experimental platform, experimental studies on gas‐assisted injection molding (GAIM), and water‐assisted injection molding (WAIM) of 2 curved pipe fittings by adopting short‐shot method were carried out. UDF model was constructed for numerical simulation analysis. The influence rules of auxiliary medium and bending angle on the terminal morphology, inner wall surface quality at bending angle, medial and lateral residual wall thickness, variation range, and residual wall thickness deviation rate of short‐shot fluid assisted injection molding (SSFAIM) bending samples were compared. Meanwhile, the influence mechanism was investigated. The following experimental findings were obtained. SSFAIM had secondary penetration, while water produced multiple penetrations. There are multiple vacuum shrinkage pores in the unpenetrated area at the end of the SSWAIM sample, as well as serious shrinkage depressions on the surface. The shape of the penetration front of water is arc‐shaped with many penetration holes. The shape of the penetration front of the gas at the end of the SSGAIM sample is “pointy.” Compared with the SSWAIM sample, the residual wall thickness of SSGAIM has a narrow distribution range when the bending angle is 0°. With the increase of the bending angle, the “foaming” phenomenon of the inner wall surface quality at the bending angle of the SSGAIM sample becomes less and less obvious. Moreover, the deviation of the inner and outer residual wall thickness of SSGAIM is more obvious than that of SSWAIM due to the increase of bending angle.

Gas Assisted Injection Molding of an Industrial Part: 3D Simulation Provides Virtual Molding Trials

Abstract Techniques developed for three-dimensional simulation of gas assisted injection molding of thermoplastics are here extended to a complex industrial application, a car door handle, providing uniquely detailed insights into the molding process. Handling of the three stages of the process – plastic filling, and primary and secondary gas penetration – is described in terms of the boundary and initial conditions applied. Predictions for plastic filling patterns are compared successfully with molding results, and provide detailed forewarning of potential problems due to weld surfaces and air-traps. Particular emphasis is placed on accurate modeling of secondary gas penetration, which is also validated against practical results. A further novel feature is the export of an STL model of the gas core, providing a CAD model of the finished molding for subsequent in-service analysis.

Reducing the Burn Marks on Injection-Molded Parts by External Gas-Assisted Injection Molding.

A burn mark is a sort of serious surface defect on injection-molded parts. In some cases, it can be difficult to reduce the burn marks by traditional methods. In this study, external gas-assisted injection molding (EGAIM) was introduced to reduce the burn marks, as EGAIM has been reported to reduce the holding pressure. The parts with different severities of burn marks were produced by EGAIM and conventional injection molding (CIM) with the same molding parameters but different gas parameters. The burn marks were quantified by an image processing method and the quantitative method was introduced to discuss the influence of the gas parameters on burn marks. The results show that the burn marks can be eliminated by EGAIM without changing the structure of the part or the mold, and the severity of the burn marks changed from 4.98% with CIM to 0% with EGAIM. Additionally, the gas delay time is the most important gas parameter affecting the burn marks.

FEM of Gas-Assisted Injection Molding Based on 3D Model

The gas-assisted injection molding (GAIM) process is so complicated that increasing reliance has been placed on CAE (Computer Aided Engineering) as a tool for both mold designers and process engineers. In this paper, a 3D theoretical model and numerical scheme is presented to simulate the GAIM process, in which an equal-order velocity-pressure formulation method is employed to eliminate the pressure oscillation. In addition, the whole flow field including the gas and melt regions is calculated using a uniform momentum equation with the viscosity of gas raised to a certain order of magnitude, and a 3D control volume scheme is employed to track the flow front of the melt and gas. Finally, the validity of the model has been tested through case studies and experimental verification.

Symulacje wtryskiwania grubościennej wypraski z zastosowaniem programu Moldex3D

Simulation of thick-walled part produced by standard injection molding (IM), gas-assisted injection molding (GAIM) and microcellular injection molding (MuCell) for two plastic types: ABS and POM using Moldex3D R15 software. It was found high reducing mass and reducing cooling times and decreasing sink mark, volumetric shrinkage and warpage deformations of parts analyzed by GAIM and MuCell compared to IM.

Influence of Diameter on the Templated Crystallization of Polyethylene/Carbon Material Fiber Composites under Intense Shear Flow.

In this study, carbon fibers (CFs) and carbon nanofibers (CNFs) were introduced into polyethylene (PE) and intensive shear flow was imposed on the melt during the melt second flow caused by gas penetrating the melt during the gas-assisted injection molding (GAIM). The effect of fiber diameter on crystalline morphologies of obtained composites was deeply studied. The results revealed the fact that no matter whether CFs or CNFs were introduced into the PE matrix or not, the orientation degree of PE lamellae would increase during the melt second flow. Hybrid shish-kebab structures were difficult to be formed in GAIM CF/PE composites and only a few oriented PE lamellae overgrew on the local surface of CFs. However for GAIM CNF/PE composites, the formation of hybrid shish-kebab structures was clearly observed in the entire thickness. The structural diversity was mainly ascribed to the difference of CF diameter, showing obvious size-dependent effect.

Temperature and shear rate distributions of gas‐assisted injection molded (GAIM) polypropylene and the application in forecast of cooling time

ABSTRACTAn umbrella handle product of polypropylene molded by gas‐assisted injection molding (GAIM) was studied from both aspects: theoretical modeling and simulation as well as in situ temperature measurement. The simulation was primarily through the use of the commercial software Moldflow (version 6.1) coupled with enthalpy transformation method (ETM) in an attempt to investigate the shear rate and temperature fields during GAIM process. A four‐parameter model (FPM) was used to nonlinearly fit the temperature decays during the GAIM cooling stage on the basis of a three‐parameter model (TPM) raised previously in our group. The FPM showed perfect fitting effect as well as presented fairly acceptable cooling time (tc) prediction in comparison to experimental data, which could better reflect the nature of crystalline polymers during melt crystallization process. The understanding of the shear rate and temperature fields would be of practical importance to the further research on relationship of “processing–structure–property” as well as the optimization of cooling parameters for industrial GAIM operations of crystalline polymers. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47390.

Foaming Mechanism of Polypropylene in Gas-Assisted Microcellular Injection Molding

In our recent study, it is reported that gas-assisted microcellular injection molding (GAMIM) is a promising method to fabricate plastic foams with good surface quality, high weight reduction, and improved mechanical performance. To clarify the foaming mechanism, the crystallization behaviors of polypropylene in both conventional injection molding (CIM) and gas-assisted injection molding (GAIM) were studied. It is found that GAIM can significantly promote melt crystallization and refine crystals. To explore the improvement mechanism of crystallization, a numerical model was developed to simulate the filling and cooling processes of CIM and GAIM. The simulation results show that the much stronger shear field and the faster cooling rate lead to the refinement of crystals in GAIM. Finally, a crystallization-driven cell nucleation model was proposed to explain the improved foaming behavior of polypropylene in GAMIM. This paper provides a deep understanding of the foaming behavior and hence benefits the foaming process design.

Visualization of counter pressure mechanism in gas-assisted injection molding process

Gas-assisted injection molding (GAIM) refers to injecting gas into the short shot melt during the filling stage. Compressed gas is used as the medium to push the melt and to provide the packing pressure. In GAIM, the hollow area and penetration length are the main factors that will affect the quality of molded parts. This study has applied a Gas Counter Pressure (GCP) mechanism and has discussed the effect of GCP in the GAIM process with in-mold visualization of this complex molding flow. This study introduces a counter pressure mechanism in a thick paper-clip-shaped cavity design. The flow field under different counter pressure conditions is observed by high-speed photography, the fiber orientations are analyzed with SEM, and the affected penetration length and hollow area are measured relatively. The experimental results show that when the GCP is applied to GAIM, although the hollow area is reduced, the penetration length will be increased, so as to make the quality of molded part more uniform and reduce the shrinkage. And a quantitative measuring method of two-stage penetration time span is proposed to get more in-depth discussion about the interactions between GCP and GAIM.

A novel gas-assisted microcellular injection molding method for preparing lightweight foams with superior surface appearance and enhanced mechanical performance

This paper proposed a novel gas-assisted microcellular injection molding (GAMIM) method by combining the gas-assisted injection molding (GAIM) with the microcellular injection molding (MIM). Firstly, under the assistance of the high-pressure gas from the GAIM, the amount of the polymer melt required for fully filling the mold cavity is reduced in the GAMIM compared with that in the MIM, thus leading to a high weight reduction of the foamed part. Secondly, the high-pressure assisted gas from the GAIM can dissolve all the cells generated in the melt filling stage back into the polymer melt, thus improving the molded part's surface appearance by eliminating the surface sliver marks. Thirdly, the secondary foaming process in a steady state triggered by releasing the high-pressure assisted gas makes the foamed part have a fine cellular structure and a compact solid skin layer, which can help to enhance the part's mechanical properties. In order to verify the effectiveness of the GAMIM, comparison experiments of the MIM and the GAMIM were conducted. The results demonstrate that the GAMIM can not only significantly increase the weight reduction, but also greatly improve the surface appearance and mechanical properties of the foamed part.

Simulation of dynamic gas penetrations on fingering behaviors during gas-assisted injection molding

Abstract The gas-assisted injection molding (GAIM) process is often adopted to compensate for shrinkage and improve part quality. However, gas penetrations consisting of primary and secondary penetrations will usually induce “fingering” behaviors, in which gas penetrates unevenly inside the parts and may lead to significant reductions in part stiffness. Due to the dynamic instability and complex material properties of fingering behaviors, it is desirable to be able to predict the behaviors with the help of simulation tools. In order to simulate the fingering behaviors, it is necessary to calculate the gas penetrations with a sharp interface between melt and gas, and to incorporate the compressibility of materials with high accuracy. In this study, we present a three-dimensional finite volume method that considers the non-Newtonian compressible flow with a high-resolution interface scheme for simulating the fingering behaviors during the GAIM process. A thin plate case is done to validate the accuracy and the capability of our proposed method to predict the sharp interface for primary and secondary gas penetrations. Then, we present a fish-bone plate with branches to investigate the gas penetrations on fingering behaviors through the dynamic simulation results and the specific volume changes on a pressure-volume-temperature diagram.

Strong shear-driven large scale formation of hybrid shish-kebab in carbon nanofiber reinforced polyethylene composites during the melt second flow.

The formation of a hybrid shish-kebab (HSK) structure with different degrees of lamellar orientations was first observed in the solution crystallization of polyethylene (PE) in the presence of carbon nanofibers (CNFs). In this study, PE crystal lamellae were periodically decorated on the surface of CNFs and were aligned approximately perpendicular to the long axes of the CNFs, forming aligned hybrid shish-kebab nanostructures. More importantly, the fascinating structure was directly formed in all regions of the injection molded bars of HDPE/CNF composites, via a gas-assisted injection molding (GAIM), instead of the shell-core structure. In the GAIM process, an intense shear was imposed onto the melt during the melt second flow and drove PE chains to orient along the axes of the CNFs. Then the entropy penalty for PE chains deposited on the CNF surface was drastically decreased. Although the attractive van der Waals interactions were weak, the oriented PE chains could successfully adsorb on the CNF surface due to the decrease of the entropy penalty, therewith the underlayer coating was formed along the axis based on a two-dimensional mode for early nucleation on the CNF surface. Subsequently, subglobules appeared on the ordered structure, which could be regarded as the crystal nucleus. Finally, the oriented PE chains began to epitaxially grow from the subglobules with a folded-chain shape to decrease the polymer surface energy and grew perpendicular to the CNFs long axis, abiding by the "soft epitaxy" crystallization mechanism regardless of strict lattice matching.

Tailoring crystalline structures and mechanical properties of injection-molded polyethylene by tuning the relaxation time of molecular chains

The relaxation times of commercial polymeric materials are generally very short and most of the oriented molecular chains tend to relax or disorient during practical processing. In the present work, the relaxation time of high-density polyethylene (HDPE) was greatly prolonged by subjecting HDPE pellets to a certain dose of electrical irradiation. Gas-assisted injection molding (GAIM), with a strong shear flow during the secondary melt flow process, was utilized to prepare the molded bars. Macroscopically, the sub-skin layer of virgin HDPE sample was characterized by typical shish-kebabs, whereas random lamellar dominated in the gas channel region. With the enhancing of the relaxation time of polymer materials, the gas channel zone of PE-10 sample exhibited regularly aligned oriented lamellae. A high and homogeneous orientation structure in PE-10 sample led to remarkable mechanical enhancement from 36.2 and 931.2MPa of virgin HDPE sample to 52.5 and 1251.4MPa of PE-10 specimen for tensile strength and modulus, respectively. More importantly, under the same processing condition, all the skin, sub-skin and gas channel zones of PE-25 specimen exhibited shish-kebab crystals owing to the further increasing relaxation time. With this large scale of homogeneous orientation structure, PE-25 sample exhibited a further reinforcement to 66.8 and 1715.7MPa for the tensile strength and modulus, respectively. Our findings demonstrated that the flow-induced crystalline structures of injection-molded bars can be tailored by tuning the relaxation time of polymer materials and it provides a new method to self-reinforce polymer products by controlling the crystalline structures.

Numerical Analysis and Evaluation of Process and Geometry Specific Transient Temperature Fields for a New Variation of Gas-Assisted Injection Molding

Abstract Two-Stage GITBlow, a special injection molding process developed by Polymer Processing Paderborn, combines the advantages of gas-assisted injection molding and blow molding. This is achieved by producing a preform with gas-assisted injection molding, which is then inflated into a larger cavity in the same mold. The inflation behavior is heavily influenced by the temperature distribution over the cross section of the preform. The preform is not rotationally symmetric and features functional geometries that are not inflated during the second process stage. Therefore, the temperature development during the first stage, i.e. the gas-assisted injection molding, as well as cooling and handling of the preform, is essential for the subsequent inflation. If certain thermal conditions are met, inflation and the creation of a homogeneous wall thickness is improved. The development of the necessary temperature profiles for homogeneous inflation is heavily dependent on the geometry of the gas chamber. Due to the preform's geometry, characteristic material accumulations are formed during the gas injection. These create local temperature hotspots that act as heat sources during the temperature equalization before inflation starts. Simulations were conducted to predict the gas chamber geometry and its influence on the temperature distribution and development during the process. The combination of two different simulation approaches creates a new method to analyze GITBlow and similar processes. Simulation results were verified with extensive experimental studies.

Tuning Crystalline Morphology of High‐Density Polyethylene by Tailoring its Molecular Weight Distribution for Coupling with a Secondary Flow Field

Designing a material from the chain architecture to achieve a tailored crystalline morphology in a molding condition is a grand challenge. Here, the crystalline morphology of high‐density polyethylene is tuned by tailoring its molecular weight distribution for coupling with the secondary flow field under gas‐assisted injection molding (GAIM). The selected N‐PE and the tailored B‐PE have similar weight‐average molecular weights (Mw), but the latter possesses a broader molecular weight distribution (MWD). Although a weaker flow field is triggered due to the slightly slower melt advance rate in the cavity, B‐PE is capable of better coupling with secondary flow field in comparison with N‐PE, which causes much more oriented crystals in molded parts, such as shish kebab, and shows higher orientation behaviors in the corresponding zones. These significant results provide an important step to explore the coupling of chain architectures and secondary flow field for designing desired crystalline morphology.

Large scale formation of various highly oriented structures in polyethylene/polycarbonate microfibril blends subjected to secondary melt flow

A strong shear flow was imposed on the melt of polycarbonate (PC) microfibril reinforced high density Polyethylene (HDPE) during a secondary melt flow process, i.e. gas assisted injection molding (GAIM). Classic shish-kebabs and typical transcrystallinity were simultaneously observed in the entire thickness of the GAIM HDPE/PC microfibril composites, which were closely related to the strong shear flow that was further amplified and distributed by incorporated PC microfibrils. Interestingly, some nano-sized ultrafine PC microfibril inclined to absorb extended chain bundles to form shish nuclei on its surface first, which subsequently evolved into hybrid shish-kebab superstructures. It was deemed that the induced formation of hybrid shish-kebab superstructures on nano-sized ultrafine PC microfibril was due to the absorbing of extended chain bundles for hybrid shish nuclei with the strong shear flow serving as the driving force. Importantly, large scale formation of these highly oriented crystalline superstructures can bring significant mechanical reinforcement in GAIM HDPE/PC microfibril composite. For GAIM HDPE/PC microfibril composites, its yield strength is increased by 68% and 66%, compared to the GAIM HDPE parts and the common injection molded (CIM) HDPE/PC composites, respectively; meanwhile, the Young's modulus is enhanced by 253% and 17%, compared to the GAIM HDPE parts and the CIM HDPE/PC composites, respectively.

The effects of gas assisted injection molding on the mechanical properties of medical grade thermoplastic elastomers

Gas-assisted injection molding (GAIM) has been demonstrated to have several advantages over traditional injection molding, including a reduction in raw material, cycle time, clamping force and injection pressure. The objective of this study was to determine the effects of GAIM parameters on the manufacturability and mechanical performance of medical grade thermoplastic elastomer injection molded parts for a bladder support device. Gas assist parts used up to 37% less raw material than conventional parts. However, GAIM methods also reduced part stiffness compared with solid parts. GAIM had a limited effect on the time dependent characteristics of creep and relaxation of each material, which are important characteristics for many medical devices. Using gas assist to manufacture flexible medical devices from thermoplastic elastomers provides an opportunity to reduce raw material and processing costs, however, predicting the effect of GAIM on part mechanics varies with each material and will require preliminary testing.

Extension of the orientation region of high density polyethylene molded by gas‐assisted injection molding: control of the thermal field

AbstractWider zones with close‐knit orientation crystals in high density polyethylene (HDPE) parts prepared via the gas‐assisted injection molding (GAIM) process were obtained under high cooling gas pressure. In this study, compressed nitrogen, as a cooling medium, was introduced to retain a high cooling rate of the polymer melt. The high gas pressure leads to fast cooling of the polymer melt, which contributes to the stability of more oriented and stretched chains during the cooling stage. Then many more oriented structures are formed. SEM shows that many more oriented structures and interlocking shish‐kebab structures are achieved in parts under highest cooling gas pressure (P3). The P3 parts possess a higher degree of orientation than the corresponding regions of parts under lowest cooling gas pressure (P1). Moreover, tensile testing indicates that, compared with P1 parts, although P3 parts have lower crystallinity, the mechanical properties are improved because of the wider orientation zone and many more interlocking shish‐kebab structures. Combining the HDPE molecular parameters with the characteristics of the GAIM flow field and temperature field, the stability of oriented or stretched chains and the formation of orientation structures in various zones of the parts were analyzed. © 2014 Society of Chemical Industry

The Complex Crystalline Structure of Polyethylene/Polycarbonate Microfibril Blends in a Secondary Flow Field

A mass of stretched molecular chains in the entire thickness direction of gas‐assisted injection molding (GAIM) composites is induced due to the redistribution and amplification of the shear flow caused by the introduction of polycarbonate (PC) microfibril under intense shear stress. In the vicinity of the PC microfibrils, stretched chains are either absorbed by PC microfibrils with a large diameter to form a transcrystallinity, or captured by ultrafine PC microfibrils to firstly form shish nuclei and finally form hybrid shish‐kebab structures. Typical shish‐kebab superstructures are formed in the zone with the absence of PC microfibrils. In summary, multi­form crystalline superstructures across the thickness direction are successfully obtained by the GAIM. Thus, this work can open a new way for the preparation of high‐performance polymer composites in industrial processing. image