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

Abstract

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     

Investigations into surface visual quality of gas-assisted injection moulded polypropylene parts

Abstract

Although gas-assisted injection moulding (GAIM) provides many advantages compared with conventional injection moulding (CIM), its applications are limited to surface visual quality studies. In the present study, polypropylene plate parts designed with gas channels having five different types of cross-section but with same cross-sectional area were gas-assisted injection moulded. In addition, various plate thickness parts designed with semicircular gas channels of different radius were also moulded. The surface visual quality of GAIM parts was investigated via gloss and chromatics measurements. The 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 investigated. The effect on the surface visual quality of gas channel with fillet design and cavity surface with texturing treatment was also examined. 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 processing conditions significantly affect surface visual quality. Gas channel design of semicircular cross-section (shape A) provides a better surface visual quality than the other designs. In addition, in order to obtain best surface visual quality, the ratio of equivalent radius to plate thickness should be approximately equal to 2.3. Alternatively, surface visual quality can be improved by texturing treatment on the cavity surface of the core-side. The present study provided part design guidelines for choosing the most effective gas channel design to achieve the best surface visual quality.     

Study of mouldability in gas-assisted injection moulded fibre-reinforced Nylon parts

Abstract

Investigation of the characteristics of gas penetration in gas-assisted injection moulding (GAIM) of parts plays an important role in the successful application of gas-assisted injection moulding. Although these have been proposed by various investigators, quantitative rules based on well designed experiments on fibre-reinforced plastic materials have not been reported previously. Additions of glass-fibre bring about rather dramatic changes in material viscosity and heat conductivity, etc. In the present paper, spiral tube moulds with uniform diameters of 8 and 10 mm, respectively, and a plate mould of 3 mm thickness with gas channel design of semicircular crosssection were moulded using glass-fibre reinforced Nylon resin. The effects of fibre content, tube diameter, and processing parameters, including gas pressure, delay time, and injection stroke on gas penetration characteristics and mouldability for fibre-reinforced Nylon parts were investigated experimentally. It was found that the coating melt thickness decreases with increasing gas pressure, until the gas pressure reaches a critical value, when the coating melt thickness becomes relatively constant for the spiral tube. Meanwhile, using a longer delay time for gas injection will increase the skin melt thickness. The hollowed core ratio increases with increasing diameter of spiral tube. Alternatively, the hollowed core ratio of GAIM plate and spiral tube parts increases when the content of glass-fibre is increased. However, when the fibre content is over 10 wt-%, it shows less influence on the hollowed core ratio for plate parts. Furthermore, although the area of the moulding window decreases with increasing content of fibre, they all show good mouldability. From these results, one can provide an empirical formula to CAE simulation designers and part/mould designers for GAIM fibre-reinforced Nylon parts to achieve suitable mouldability and accurate CAE simulation.     

Effects of processing parameters on formation of sinkmarks on injection moulded parts

Abstract

The effects of different processing parameters on sinkmarks, which occur on the surface of injection moulded parts, have been investigated. An L'18 orthogonal array design based on the Taguchi method was conducted to minimise the sinkmarks of injection moulded thermoplastic parts and the relative significance of each processing parameter on the sinkmarks was considered. The polymeric material used was polypropylene and a plate cavity with various ribs was used for moulding. Experiments were carried out on a reciprocating injection moulding machine. After moulding, sinkmarks on the surface of moulded parts were measured using a profile meter. For the factors selected in the main experiments, the size of the gate, the melt temperature, and the width of the rib were found to be the principal parameters affecting sinkmark formation on injection moulded polypropylene. In this way experimental investigation can help in the understanding of the formation mechanism of sinkmarks, so that steps can be taken to optimise the surface quality of moulded parts.     

Characteristics of mould separation and its effects on qualities of thin wall injection moulded parts

Abstract

Precision injection moulding of thin walled parts has become an important concern in the computer, communication, and consumer electronics plastics industry. Previous studies in precision injection moulding control focus on the injection screw and the associated operations. In the present study, the influence of relevant parameters including injection speed, melt temperature, mould temperature, filling-packing switchover, and packing pressure on the mould plate separation under different clamping pressure were investigated as part of precision moulding control. A two cavity tensile test specimen mould equipped with four linear variable displacement transducers across the parting surfaces of the mould was used. A computer based monitoring system was built to detect the mould separation signals. Mould separation can also be identified from part weight and thickness variation and exhibits relevant correspondence with them. It was found that owing to the high injection speed required for thin wall moulding, mould separation is not negligible. In all situations, mould separation decreases with increasing clamping pressure. As melttemperature andmouldtemperatureincrease,mouldseparationincreases, resulting in an increase in part weight and thickness. Similarly, when packing pressure and injection speed increase, mould separation also increases. Earlier switchover from filling to packing can decrease mould separation as well as part weight and thickness. Among all the parameters studied, packing pressure exhibits the greatest influence on mould separation and on the associated weight and thickness change. This influence also becomes larger when the moulded part becomes thinner, owing to the larger injection moulding pressure. PRC/1746     

The occurrence of surface roughness in gas assist injection molded nylon composites

Gas assist injection molding has increasingly become an important industrial process because of its tremendous flexibility in the design and manufacture of plastic parts. However, there are some unsolved problems that limit the overall success of this technique. The purpose of this report was to study the surface roughness phenomenon occurring in gas assist injection molded thermoplastic composities. The materials used were 15 % and 35% glass‐fiber filled nylon‐6 composites. Experiments were carried out on an 80‐ton injection molding machine equipped with a high‐pressure nitrogen‐gas injection unit. Two “float‐shape” axisymmetric cavities were used. After molding, the surface quality of molded parts was measured by a roughness meter. Various processing variables were studied in terms of their influence on formation of surface roughness: melt temperature, mold temperature, melt filling speed, short‐shot size, gas pressure, and gas injection delay time. Scanning electronic microscopy was also employed to characterize the composites. It was found that the surface roughness results mainly from the exposure of glass fiber in the matrix. The jetting and irregular flows of the polymer melt during the filling process might be factors causing the fiber exposure.     

Investigations into moulding window of gas assisted injection moulded polystyrene plates with various gas channel designs

Abstract

The design of the gas channel plays an important role in the successful application of gas assisted injection moulding. Although empirical guidelines for gas channel design have been proposed by the various equipment suppliers, quantitative rules based on well designed experiments have not been reported previously. To investigate the effects of geometry on gas penetration for two plate thicknesses, transparent polystyrene (PS) plates designed with semicircular gas channels of differing radii and with rectangular gas channels of differing width to height ratios have been produced using gas assisted injection moulding. Moulding windows and criteria for gas penetration were also chosen so that design rules could be defined quantitatively. The mouldability index was also classified into five levels (excellent, good, fair, poor, and bad) based on the relative areas of the moulding windows. From a plot of mouldability index against R eq , the ratio of equivalent gas channel radius to plate thickness, it was found that to obtain an appropriate moulding window (i.e at least a fair mouldability index) R _eq should be greater than 2. Gas channels with a semicircular cross-section provide better mouldability than those with rectangular cross-sections of the same cross-sectional area. For the same equivalent radius, the ratio of width to height in rectangular gas channels also affects the mouldability index. The present investigation provides part designers with preliminary quantitative design/moulding guidelines for choosing the effective gas channel design that allows the parts to be moulded within an appropriate moulding window, so that the uncertainty in both simulation and process control can be overcome. A methodology for the establishment of guidelines for quantitative design of gas channels is also proposed.     

Optimisation of bubble size in qc rotationally moulded parts

Abstract

Rotational moulding is one of the most important methods of manufacture of hollow plastic products. However, there are several unsolved problems that confound the overall success of this technique, including surface pin holes and internal bubbles of moulded parts, caused by inappropriate mould design and processing conditions. In this report, an L′18 experimental matrix design based on the Taguchi method was conducted to optimise the bubble size of rotationally moulded parts. Experiments were carried out on a laboratory scale biaxial rotation moulding unit. The polymeric material used to mould the parts was linear low density polyethylene. After moulding, the size of the bubbles on the surface of moulded parts was characterised by an image analysis system. For the factors selected in the main experiments, the cooling conditions and the particle size of the material were found to be the principal factors affecting the bubble size of rotationally moulded thermoplastics. In addition, mould pressurisation helped decrease the size of the bubbles. The bubble size of moulded parts was not affected by the water content of the polymeric powder, but increased with the viscosity of the materials.     

Experimental investigation and numerical simulation of cooling process in water assisted injection moulded parts

Abstract

This report investigated, both experimentally and numerically, the cooling process in water assisted injection moulded parts. Experiments were carried out on a laboratory developed water assisted injection moulding system, which included an injection moulding machine, a water pump, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The resin used was semi-crystalline polypropylene. The in-mould temperature of the polymeric materials during the cooling process was measured. A transient heat transfer finite element model was adopted to simulate and predict the temperature variation within water assisted injection moulded products. Simulated results matched well with the experimental data. Experimental investigation and numerical simulations of a water assisted injection moulding cooling process can provide an improved understanding of the influence of water related parameters on the cooling process of water assisted injection moulded parts.     

Jetting and weld lines in ceramic injection moulding

Abstract

A multi-thickness mould cavity incorporating replaceable mould face plates of differing thickness was constructed for assessing the integrity of large section mouldings. The jetting phenomenon, which leads to a moulding replete with weld lines, was overcome for 15 mm thick mouldings by adjusting the nozzle and sprue diameters and by changing the angle of impingement of the melt on the opposite face of the cavity. However, for 25 and 35 mm cavities the formation of multiple weld lines during mould filling could not be completely eliminated. The emergence of cracks near the surface in some sintered parts could be traced back to the formation of weld lines in the injection stage.     

Rheology and product properties in injection moulding

The types of flow occurring in the runner, gate and cavity of a cylindrical bar mould have been investigated and the relation between the flow and local modulus of the moulding determined for crystal polystyrene. The results suggest that non-isothermal shear flow was occurring in the runner while the gate displayed the characteristics of shear flow when heated to the polymer melt temperature and a complex type of flow when cold. The cavity was found, under normal injection rates, to fill by a jetting mechanism which often produced surface irregularities in the product and progressive filling from the gate was only observed at low injection rates. Investigation of local moulding properties revealed that while considerable variations within a specimen existed, these were apparently not due to the mould filling mechanism. They have been attributed tentatively to the small opening of the mould which takes place during injection and to cooling strain.     

气体辅助注射成型技术

介绍了消除厚壁制件所出现的缩痕、凹痕缺陷的一种新型塑料成型工艺－－气体辅助注射成型技术的工作原理,技术关键、设备配置及其应用,并重点介绍了内置气体辅助成型和表面气体辅助成型技术。     

气体辅助注射成型充模流动模拟

本文建立了描述气体辅助注射成型充模流动过程的数学模型,并采用有限元/有限差分混合算法进行数值求解,在对移动边界的处理上采用控制体积法对充模过程中的两类移动边界:熔体前沿、熔体-气体边界进行跟踪,从而实现气体辅助注射成型充模过程的数值模拟。通过对一平板带厚筋结构进行数值分析验证了本文给出的理论算法及软件的可靠性。     

Micromoulding: process characteristics and product properties

Abstract

Key aspects of the technology and challenges associated with the micromoulding process are discussed. The apparent shear and extensional viscosity behaviour of a polyacetal at high wall shear rates have been measured using inline capillary rheometry on a commercial micromoulding machine and a larger servoelectric injection moulding machine; the polymer behaved predictably at shear rates in excess of 10⁶ s^-1. Initial moulding trials indicated that a stepped plaque and 0·25 mm thick rectangular plaque mouldings filled in a satisfactory manner, but a thicker plaque cavity exhibited a jetting flow into the cavity.

A data capture system capable of measuring multiple process dynamics at high sampling rates (up to 50 kHz per channel) allowed detailed process measurements taken during moulding of the stepped plaque moulding. Atomic force microscopy of the moulded products showed different surface finishes on each step of the stepped plaque moulding. Topography scans of the 0·25 mm thick rectangular plaque moulding showed that mould surface features with length scales of the order of a few micrometres were well replicated on the moulded product and the quality of the surface finish is dependent on the melt pressure during moulding.     

Injection Moulding: Evaluation of Appropriate Melt Injection Points for the Gas Injection Technique

In gas assisted injection moulding the melt front advancement has a considerable effect on the gas penetration. The evaluation of an appropriate melt filling is an important step to avoid instabilities in the process sequence. Taking a sample moulded part a procedure is presented that enables the part designer to evaluate required melt and gas injection points according to the gas injection technique. Using finite element simulations, different calculations for the melt front advancement lead to the correct gate location.

Presentation of different degrees of filling for the optimised article geometry.     

Computer simulation of the filling process in gas‐assisted injection molding based on gas‐penetration modeling

Gas‐assisted injection molding can effectively produce parts free of sink marks in thick sections and free of warpage in long plates. This article concerns the numerical simulation of melt flow and gas penetration during the filling stage in gas‐assisted injection molding. By taking the influence of gas penetration on the melt flow as boundary conditions of the melt‐filling region, a hybrid finite‐element/finite‐difference method similar to conventional‐injection molding simulation was used in the gas‐assisted injection molding‐filling simulation. For gas penetration within the gas channel, an analytical formulation of the gas‐penetration thickness ratio was deduced based on the matching asymptotic expansion method. Finally, an experiment was employed to verify this proposed simulation scheme and gas‐penetration model, by comparing the results of the experiment with the simulation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2377–2384, 2003     

Influence of thermomechanical conditions on structure development and mechanical properties of polyethylene mouldings produced using different moulding methods

Abstract

The properties of moulded plastic products are dependent on the processing technology used in their manufacture and in particular on the structural morphology resulting from the thermomechanical environment imposed on the melt. This paper investigates these phenomena, through the moulding of polyethylene using different methods and the subsequent testing of the mechanical properties of the products. A unified approach is presented to describe the behaviour of the products based on knowledge of the thermomechanical conditions imposed during processing. A linear medium density polyethylene was processed using rotational moulding, compression moulding, and injection moulding in order to achieve different thermomechanical conditions (i.e. shear rates and cooling rates). The processing conditions used were typical of those in common use in the respective industries. The moulded parts were mechanically tested in order to determine the tensile, flexural, and impact properties. These measurements were performed both on samples corresponding to the entire thickness of the moulding and on slices taken from across the section of the mouldings. The mechanical tests were complemented with density measurements, to assess the crystallinity across the wall thickness, and polarised light microscopy, to characterise the mouldings' microstructure. It is shown that the type of dependence of the mechanical performance on the thermomechanical conditions imposed during processing is similar for the three moulding techniques used.     

Integrated compounding and injection moulding of short fibre reinforced composites

Abstract

Composites of high density polyethylene (HDPE) and carbon fibre (C fibre) were compounded and moulded into tensile test bars in compounding injection moulding (CIM) equipment that combines a twin-screw extruder and an injection moulding unit. Two HDPE grades exhibiting different rheological behaviours were used as matrices. The mechanical properties of the moulded parts were assessed by both tensile and impact tests. The respective morphologies were characterised by scanning electron microscopy (SEM) and the semicrystalline structures of the matrices investigated by X-ray diffraction. The final fibre length distribution and fibre orientation profiles along the part thickness were also quantified. The composites with lower viscosity exhibit higher stiffness, higher strength and superior impact performance. Both composites exhibit a three layer laminated morphology, featuring two shell zones and a core region. Interfacial interaction is favoured by a lower melt viscosity that enhances the wetting of the fibre surfaces and promotes mechanical interlocking. The composites display a bimodal fibre length distribution that accounts for significant fibre length degradation upon processing. The dimensions of the transversely orientated core differ for the two composites, which is attributed to the dissimilar pseudoplastic behaviour of the two HDPE grades and the different thermal levels of the compounds during injection moulding. Further improvements in mechanical performance are expected through the optimisation of the processing conditions, tailoring of the rheological behaviour of the compound and the use of more adequate mould designs.     

高技术陶瓷产品的精密注射成型制备技术的应用与发展

讨论了陶瓷粉末注射成型制备精密陶瓷部件这一新技术及其国内外发展状况。先进陶瓷精密注射成型的科学基础是现代高分子精密注塑理论和现代陶瓷制造技术,它将高分子流变学、陶瓷粉体技术、陶瓷工艺学和金属模具精密制造技术结合在一起。该技术突出的优点有:①可净近成型各种复杂形状的陶瓷零部件,使烧结后的陶瓷产品无需进行机加工或少加工,从而减少昂贵的陶瓷成本;②成型制品具有极高的尺寸精度和表面光洁度;③可实现微成型(Micro Injection Molding),制备μm-mm范围内的微型陶瓷零件;④成型过程机械化和自动化程度高,重复性好,便于规模化低成本生产。该技术已用于陶瓷发动机、通讯产业中光纤连接器插芯(Ferrule)、计算机工业中光盘盒磁盘驱动用陶瓷轴承和生物医学用陶瓷制品等精密陶瓷件的制造。随着微注射成型新技术的发展,微型陶瓷部件将应用于环境要求苛刻、结构复杂的MEMS系统。     

Real time diagnostics of gas/water assisted injection moulding using integrated ultrasonic sensors

Abstract

An ultrasound sensor system has been applied to the mould of both the water and gas assisted injection moulding processes. The mould has a cavity wall mounted pressure sensor and instrumentation to monitor the injection moulding machine. Two ultrasound sensors are used to monitor the arrival of the fluid (gas or water) bubble tip through the detection of reflected ultrasound energy from the fluid polymer boundary and the fluid bubble tip velocity through the polymer melt is estimated. The polymer contact with the cavity wall is observed through the reflected ultrasound energy from that boundary. A theoretically based estimation of the residual wall thickness is made using the ultrasound reflection from the fluid (gas or water) polymer boundary while the samples are still inside the mould and a good correlation with a physical measurement is observed.