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Advances on Injection Molding of Polymers

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A special issue of Polymers (ISSN 2073-4360). This special issue belongs to the section "Polymer Processing and Engineering".

Deadline for manuscript submissions: closed (30 September 2020) | Viewed by 53620

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Prof. Dr. Giuseppe Titomanlio

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Guest Editor

Department of Industrial Engineering, University of Salerno, Fisciano (SA), Italy
Interests: polymer processing; flow-induced crystallization and effect of crystallinity on rheology; morphology evolution during polymer processing; injection molding simulation
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Dr. Vito Speranza

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Guest Editor

Department of Industrial Engineering, University of Salerno, Fisciano (SA), Italy
Interests: molding; effect of thermomechanical history on final structure of polymeric materials; analysis and numerical simulation of the injection molding process of thermoplastic materials; polymeric materials characterization; atomic force microscopy
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Dr. Sara Liparoti

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Guest Editor

Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fiscian, SA, Italy
Interests: advanced Injection molding; polymer structure development during injection molding; advanced characterization of the mechanical properties within injection molding objects; tuning of surface topography and properties by injection molding; rheology of hydrogels; supercritical fluid based processes
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The simplest description of the injection molding process is: molten materials are injected into a cool cavity where they solidify. In reality, the process is very complex. Nevertheless, it allows us to produce finished objects of any geometry by a single shot. For this reason, the injection molding process is very diffuse, and more than 30% of all plastics are processed by injection molding machines. New developments of the process occur continuously in order to favor the production of parts for a particular use and/or realized with special materials. For instance, in the last few decades, standard injection molding was adapted to realize very small parts due to the rapidly growing and evolving demand for micro- and nanostructured devices with a high level of reproducibility at a small cost.

Among the parameters that identify the quality of a part, the dimensional tolerance and stability of moldings are very important and may well run out of control. Furthermore, the moldings’ final properties are related to their bulk and surface morphology distributions, which in turn are determined by processing conditions.

Thus, this Special Issue will welcome contributions that clarify (by either modeling or correlations) the effects of processing conditions on the quality and, above all, the structure and properties of the moldings. Furthermore, contributions on advanced topics, such as micro-molding, or special techniques, such as gas injection molding, will be welcome. Another relevant topic is the production of high valuable parts obtained combining polymers with other materials, such as for powder injection molding.

Prof. Giuseppe Titomanlio
Dr. Vito Speranza
Dr. Sara Liparoti
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Polymers is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

Injection molding
Injection molding modelling
Morphology structuring and characterization of moldings
Relationships between processing conditions and final properties of the moldings

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Published Papers (12 papers)

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Research

15 pages, 6533 KiB

Open AccessArticle

Multi-Scale Simulation of Injection Molding Process with Micro–Features Replication: Relevance of Rheological Behaviour and Crystallization

by Sara Liparoti, Vito Speranza, Roberto Pantani and Giuseppe Titomanlio

Polymers 2021, 13(19), 3236; https://doi.org/10.3390/polym13193236 - 24 Sep 2021

Cited by 6 | Viewed by 2083

Abstract

The possibility of tailoring key surface properties through the injection molding process makes it intriguing from the perspective of sustainability enhancement. The surface properties depend on the replication accuracy of micro and nanostructures on moldings; such an accuracy is enhanced with cavity temperature. The simulation of the injection molding process is very challenging in the presence of micro and nanostructures on the cavity surface; this does not allow for the neglect of phenomena generally considered not to influence the overall process. In this paper, a multiscale approach was proposed: in the first step, the simulation of the overall process was conducted without considering the presence of the microstructure; in the second step the outputs of the first step were used as an input to simulate the replication of the microfeature. To this purpose, a lubrication approximation was adopted, and the contribution of the trapped air, which slows down the polymer advancement, was accounted for. A modification of the viscosity equation was also proposed to describe the rheological behavior of isotactic polypropylene at very low temperatures. Concerning the microcavity filling simulation, the modification of the viscosity description at low temperatures consistently describes the process, in terms of polymer solidification. Concerning the replication accuracy, it increases with the cavity surface temperature, consistently with the experimental observations. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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12 pages, 3831 KiB

Open AccessArticle

Stress Whitening as an Observation Method of Residual Stress in MABS Polymer Material through the Example of Holding Pressure in an Injection Molding Process

by Paweł Brzęk and Tomasz Sterzyński

Polymers 2020, 12(12), 2871; https://doi.org/10.3390/polym12122871 - 30 Nov 2020

Cited by 3 | Viewed by 3421

Abstract

The effects such as warpage, dimensional instability and environmental stress corrosion, due to the presence of residual stresses in polymeric products, are strongly dependent on injection molding conditions. The holding time and holding pressure belongs to most important processing parameters, determining the dimensional stability and properties of injected goods. A new procedure based on a visualization technique was applied, where the levels of residual stresses of the samples were estimated. The experiments were performed for samples produced of translucent methacrylate acrylonitrile butadiene styrene (MABS), a commodity polymer with a high transparency, necessary for the optical visualization of the stress whitening. The samples produced by injecting molding were deformed to a constant elongation, to observe the dependent stress whitening effect subsequently used to evaluate the stress distribution. It was found that depending on the value of the injection holding pressure, various levels of residual stress and its distribution may be observed in MABS samples. These measurements conformed that the applied optical method is an easy-to-perform technique. The possibility to detect the residual stresses over the whole cross-section of the transparent product, without the necessity for local stress determination, is another significant advantage of this investigation procedure. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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15 pages, 3887 KiB

Open AccessArticle

Processing Optimization for Metal Injection Molding of Orthodontic Braces Considering Powder Concentration Distribution of Feedstock

by Chao-Ming Lin, Jhih-Jyun Wu and Chung-Ming Tan

Polymers 2020, 12(11), 2635; https://doi.org/10.3390/polym12112635 - 10 Nov 2020

Cited by 18 | Viewed by 3692

Abstract

Metal injection molding (MIM) utilizes a compound consisting of metal powder particles and a binding agent as the feedstock material. The present study combines MIM mold flow simulations with the Taguchi method to clarify the individual and combined effects of the main MIM process parameters on the metal powder concentration distribution in the final sintered product. The results show that the molding process should be performed using a short filling time, a high melt temperature, a low packing pressure, a low mold temperature, and a small gate size. Given these process settings, the powder concentration uniformity and phase separation effect are significantly improved; giving rise to a better aesthetic appearance of the final sintered product and an enhanced mechanical strength. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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14 pages, 3589 KiB

Open AccessArticle

An Experimental Study on Color Shift of Injection-Molded Mobile LGP Depending on Surface Micropattern

by Mooyeon Kim, Junhan Lee and Kyunghwan Yoon

Polymers 2020, 12(11), 2610; https://doi.org/10.3390/polym12112610 - 6 Nov 2020

Cited by 2 | Viewed by 2514

Abstract

In the display industry, the LCD backlight unit (BLU) module is variously used in mobile phones, notebook computers, monitors, and TVs. The light guide plate (LGP), which is one of the core parts of a BLU, is getting bigger and thinner consistently. Conventional injection methods and injection processes like injection compression molding (ICM) are becoming more complex and harsher with high-speed injection at high mold and melt temperatures. These approaches lead to a change in physical properties and a decrease in optical properties such as yellowing and color shift in injection-molded parts. In the present study, an injection molding experiment was conducted to understand the effect of surface patterns and major injection process conditions like mold and melt temperatures on the color shift in injection-molded mobile LGP. Optical properties obtained by the direct and total transmittance and CIE xy chromaticity diagram for injection-molded mobile LGP were measured with a UV–visible spectrophotometer. From the measurement of patternless LGP, it was found that total or direct transmittance was not affected by molding process variables. It was also found that yellow shift,

Δ E (x y)

, occurred as much as 0.00111 ± 0.00014, and a color shift angle,

Θ (x y),

of 43.05 ± 2.44° was recorded in CIE coordinates for all nine experimental cases. From the measurement of total transmittance of patterned LGP,

Δ E (x y)

and

Θ (x x)

were found to be almost the same as those of patternless LGP for the locations of low and medium density of the pattern for the LGP, T1 and T2. The measured data of direct transmittance of patterned LGP showed that additional yellow shift due to scattering caused by surface micropattern. Interestingly,

Θ (x y)

of patterned data remained 43.05 ± 2.44°, which was almost the same as that found in the case of patternless LGP. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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17 pages, 5762 KiB

Open AccessArticle

Investigation on the Coupling Effects between Flow and Fibers on Fiber-Reinforced Plastic (FRP) Injection Parts

by Chao-Tsai Huang and Cheng-Hong Lai

Polymers 2020, 12(10), 2274; https://doi.org/10.3390/polym12102274 - 3 Oct 2020

Cited by 9 | Viewed by 3869

Abstract

Glass or carbon fibers have been verified that can enhance the mechanical properties of the polymeric composite injection molding parts due to their orientation distribution. However, the interaction between flow and fiber is still not fully understood yet, especially for the flow–fiber coupling effect. In this study, we have tried to investigate the flow–fiber coupling effect on fiber reinforced plastics (FRP) injection parts utilizing a more complicated geometry system with three ASTM D638 specimens. The study methods include both numerical simulation and experimental observation. Results showed that in the presence of flow–fiber coupling effect, the melt flow front advancement presents some variation, specifically the “convex-flat-flat” pattern will change to a “convex-flat-concave” pattern. Furthermore, through the fiber orientation distribution (FOD) study, the flow–fiber coupling effect is not significant at the near gate region (RG). It might result from the strong shear force to repress the appearance of the flow–fiber interaction. However, at the end of filling region (ER), the flow–fiber coupling effect tries to diminish the flow direction orientation tensor component A₁₁ and enhance the cross-flow orientation tensor component A₂₂ simultaneously. It results in the dominance in the cross-flow direction at the ER. This orientation distribution behavior variation has been verified using a micro-computerized tomography (micro-CT) scan and image analysis technology. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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22 pages, 8531 KiB

Open AccessArticle

Quality Prediction for Injection Molding by Using a Multilayer Perceptron Neural Network

by Kun-Cheng Ke and Ming-Shyan Huang

Polymers 2020, 12(8), 1812; https://doi.org/10.3390/polym12081812 - 12 Aug 2020

Cited by 71 | Viewed by 6296

Abstract

Injection molding has been widely used in the mass production of high-precision products. The finished products obtained through injection molding must have a high quality. Machine parameters do not accurately reflect the molding conditions of the polymer melt; thus, the use of machine parameters leads to erroneous quality judgments. Moreover, the cost of mass inspections of finished products has led to strict restrictions on comprehensive quality testing. Therefore, an automatic quality inspection that provides effective and accurate quality judgment for each injection-molded part is required. This study proposes a multilayer perceptron (MLP) neural network model combined with quality indices for performing fast and automatic prediction of the geometry of finished products. The pressure curves detected by the in-mold pressure sensor, which reflect the flow state of the melt, changes in various indicators and molding quality, were considered in this study. Furthermore, the quality indices extracted from pressure curves with a strong correlation with the part quality were input into the MLP model for learning and prediction. The results indicate that the training and testing of the first-stage holding pressure index, pressure integral index, residual pressure drop index and peak pressure index with respect to the geometric widths were accurate (accuracy rate exceeded 92%), which demonstrates the feasibility of the proposed method. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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11 pages, 4565 KiB

Open AccessArticle

Environmentally Efficient 316L Stainless Steel Feedstocks for Powder Injection Molding

by Berenika Hausnerova and Martin Novak

Polymers 2020, 12(6), 1296; https://doi.org/10.3390/polym12061296 - 5 Jun 2020

Cited by 7 | Viewed by 3201

Abstract

In this study, environmentally convenient highly metal powder filled feedstocks intended for powder injection molding is presented. The composition of 60 vol % 316L stainless steel gas atomized powder feedstocks containing semicrystalline waxes: acrawax or carnauba wax and paraffin wax, combined with polyethylene glycol and modifier, was optimized to provide defect-free parts. Rheological as well as thermogravimetric analyses supported with scanning electron microscopy and metallography were employed to set up optimum conditions for molding, debinding and sintering. The performance of the novel feedstock was compared with currently available polyolefines-based materials, and results showed an efficiency enhancement due to the substantially lower (about 100 °C) mixing and molding temperatures as well as a reduction of debinding and sintering times at the simultaneous achievement of better mechanical properties in terms of elongation and tensile strength, in comparison to the mass production feedstock. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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17 pages, 4530 KiB

Open AccessArticle

Shrinkage and Warpage Minimization of Glass-Fiber-Reinforced Polyamide 6 Parts by Microcellular Foam Injection Molding

by Youngjae Ryu, Joo Seong Sohn, Chang-Seok Yun and Sung Woon Cha

Polymers 2020, 12(4), 889; https://doi.org/10.3390/polym12040889 - 11 Apr 2020

Cited by 25 | Viewed by 6491

Abstract

Shrinkage and warpage of injection-molded parts can be minimized by applying microcellular foaming technology to the injection molding process. However, unlike the conventional injection molding process, the optimal conditions of the microcellular foam injection molding process are elusive because of core differences such as gas injection. Therefore, this study aims to derive process conditions to minimize the shrinkage and warpage of microcellular foam injection-molded parts made of glass fiber reinforced polyamide 6 (PA6/GF). Process factors and levels were first determined, with experiments planned accordingly. We simulated designed experiments using injection molding analysis software, and the results were analyzed using the Taguchi method, analysis of variance (ANOVA), and response surface methodology (RSM), with the ANOVA analysis being ultimately demonstrating the influence of the factors. We derived and verified the optimal combination of process factors and levels for minimizing both shrinkage and warpage using the Taguchi method and RSM. In addition, the mechanical properties and cell morphology of PA6/GF, which change with microcellular foam injection molding, were confirmed. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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16 pages, 7030 KiB

Open AccessArticle

Visualization Analysis of Defibration and Distributive Uniformity of Fibers in Long-fiber-reinforced, Injection-Molded Resins

by Sai Ma, Xiaobin Wu, Shigeru Owada and Hidetoshi Yokoi

Polymers 2020, 12(3), 727; https://doi.org/10.3390/polym12030727 - 24 Mar 2020

Viewed by 2821

Abstract

The pursuit of mechanical strength in the injection molding of long-fiber-reinforced resins continues to pose major challenges, namely (1) improvement of fiber defibration and fiber distribution and (2) suppression of fiber breakage during the molding machine’s plastication process. In the present study, a new defibration and distribution evaluation mold is developed to quantitatively evaluate the defibration and distribution of long fibers in nozzle-injected resin. A quantitative analysis method using this evaluation mold is proposed for visualizing and observing long-glass-fiber-reinforced resin up to 30 wt % and long carbon fiber-reinforced resin up to 10 wt %. The method, based on the intensity of light transmitted from a backlight source, is also used to evaluate areas of undefibrated fiber pilling and for evaluating the influence of molding conditions on fiber defibration and uniform distribution. The results clarify that fiber distribution non-uniformity can be reduced by improving the concentration adjustment procedure for the dry blending of high-concentration pellets. Additional results show that fiber defibration and distributive uniformity can be improved by applying high back pressure. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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19 pages, 6428 KiB

Open AccessArticle

Influence of Heat Sink on the Mold Temperature of Gypsum Mold Used in Injection Molding

by Chung-Chih Lin, Kun-Chen Chen and Hon-Chih Yeh

Polymers 2020, 12(3), 701; https://doi.org/10.3390/polym12030701 - 21 Mar 2020

Cited by 5 | Viewed by 4122

Abstract

Gypsum molds have been developed as an alternative for the Rapid tooling (RT) method used in injection molding. However, the poor capability of the heat delivery forces the gypsum mold to operate under a high-risk condition, and distortion of the molded part becomes apparent. The goal is to investigate the effect of a heat sink on the reduction of the gypsum mold temperature and to establish a methodology for the heat sink design. The methodology used the advantage of the electrical circuit concept to analyze the mold temperature. The heat transfer of a mold was modeled using an equivalent thermal circuit. After all the components on the circuit were determined, the heat transfer rate could then be calculated. Once the heat transfer rate was known, the mold temperature could be easily analyzed. A modified thermal circuit considering transverse heat conduction was also proposed, which estimated the mold temperature more accurately. The mold temperature was reduced by 16.8 °C when a gypsum mold was installed with a 40 mm thick heat sink in a parallel configuration. Moreover, the reduction of the mold temperature improved the deflection of the molded part from 0.78 mm to 0.54 mm. This work provides a quick approach to analyze the mold temperature based on the thermal circuit concept. As the cooling system of the mold was modularized analytically, important properties of the cooling system in the heat transfer process were revealed by analyzing the thermal circuit of the mold, for example, the heat transfer rate or the mold temperature. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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35 pages, 34900 KiB

Open AccessArticle

A New Conformal Cooling Design Procedure for Injection Molding Based on Temperature Clusters and Multidimensional Discrete Models

by Abelardo Torres-Alba, Jorge Manuel Mercado-Colmenero, Daniel Diaz-Perete and Cristina Martin-Doñate

Polymers 2020, 12(1), 154; https://doi.org/10.3390/polym12010154 - 7 Jan 2020

Cited by 31 | Viewed by 8557

Abstract

This paper presents a new method for the automated design of the conformal cooling system for injection molding technology based on a discrete multidimensional model of the plastic part. The algorithm surpasses the current state of the art since it uses as input variables firstly the discrete map of temperatures of the melt plastic flow at the end of the filling phase, and secondly a set of geometrical parameters extracted from the discrete mesh together with technological and functional requirements of cooling in injection molds. In the first phase, the algorithm groups and classifies the discrete temperature of the nodes at the end of the filling phase in geometrical areas called temperature clusters. The topological and rheological information of the clusters along with the geometrical and manufacturing information of the surface mesh remains stored in a multidimensional discrete model of the plastic part. Taking advantage of using genetic evolutionary algorithms and by applying a physical model linked to the cluster specifications the proposed algorithm automatically designs and dimensions all the parameters required for the conformal cooling system. The method presented improves on any conventional cooling system design model since the cooling times obtained are analogous to the cooling times of analytical models, including boundary conditions and ideal solutions not exceeding 5% of relative error in the cases analyzed. The final quality of the plastic parts after the cooling phase meets the minimum criteria and requirements established by the injection industry. As an additional advantage the proposed algorithm allows the validation and dimensioning of the injection mold cooling system automatically, without requiring experienced mold designers with extensive skills in manual computing. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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16 pages, 6674 KiB

Open AccessArticle

Effects of Three Different Injection-Molding Methods on the Mechanical Properties and Electrical Conductivity of Carbon Nanotube/Polyethylene/Polyamide 6 Nanocomposite

by Dashan Mi, Zhongguo Zhao and Wenli Zhu

Polymers 2019, 11(11), 1779; https://doi.org/10.3390/polym11111779 - 30 Oct 2019

Cited by 3 | Viewed by 3575

Abstract

Morphological evolution under shear, during different injection processes, is an important issue in the phase morphology control, electrical conductivity, and physical properties of immiscible polymer blends. In the current work, conductive nanocomposites were produced through three different injection-molding methods, namely, conventional injection molding, multi-flow vibration injection molding (MFVIM), and pressure vibration injection molding (PVIM). Carbon nanotubes in the polyamide (PA) phase and the morphology of the PA phase were controlled by various injection methods. For MFVIM, multi-flows provided consistently stable shear forces, and mechanical properties were considerably improved after the application of high shear stress. Shear forces improved electrical property along the flow direction by forming an oriented conductive path. However, shear does not always promote the formation of conductive paths. Oscillatory shear stress from a vibration system of PVIM can tear a conductive path, thereby reducing electrical conductivity by six orders of magnitude. Although unstable high shear forces can greatly improve mechanical properties compared with the conventional injection molding (CIM) sample, oscillatory shear stress increases the dispersion of the PA phase. These interesting results provide insights into the production of nanocomposites with high mechanical properties and suitable electrical conductivity by efficient injection molding. Full article

(This article belongs to the Special Issue Advances on Injection Molding of Polymers)

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