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

Abstract

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

An experimental matrix design to optimize the weldline strength in injection molded parts

The forming of weldlines wherever polymer flow fronts meet is one of the problems that confound the overall success of injection molding technology. In this study, an L'18 experimental matrix design based on the Taguchi method was conducted to optimize the weldline strength of injection molded thermoplastics. Experiments were carried out on a 4.1 oz injection molding machine. A plate cavity with an obstacle at the center was used to create a weldline. After molding, the weldline strength of the parts was measured with a tensile tester. Of the factors selected in the main experiments, melt temperature and mold temperature were found to be the principal factors affecting the weldline property of injection molded thermoplastics. Weldline strengths of injection molded parts increase with the size of the obstacles.     

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.     

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.     

Weldline integrity of reinforced plastics: Effect of filler shape

The paper is a preliminary report on glass fiber and glass flake orientation in the weldline zone of injection molded reinforced polypropylene. Two types of weldlines were studied using simple shape molds: one where two melt streams meet head-on and become immobilized, the other where the weldline formation is followed by additional flow. In the weldlines of the first type, which are characteristic of current standard molds used to test weldline strength, the anisometric particles are almost perfectly oriented perpendicular to flow. As a result, the weldlines of this type tend to be weak and brittle. In the second case when the weldline is formed by merging of melt streams separated by an insert, particle orientation in the weldline area remains different from other areas of the sample for long distances from the insert. It is shown that in molds where the weldline formation is followed by laterally expanding flow the change of particle orientation is faster than for unidirectional flow.     

Performance prediction of weldline structure in amorphous polymers

The presence of a weldline generally reduces the mechanical strength of injection molded parts. A typical remedy to eliminate the problem of weak weldline structure has been to increase the melt temperature. This, however, is not an acceptable solution in some situations. A general solution to the weak weldline problem requires an in-depth understanding of the thermomechanical history of the injection molding process. A theoretical model for the strength of weldlines is presented that provides a comprehensive physical insight of the bonding process at the weldline interface. The model is based on the self-diffusion of molecular chains across the polymer-polymer interface and the frozen-in orientation that remains parallel to the interface. Both factors are analyzed separately and then superimposed to predict the strength of weldlines from known processing conditions and geometry. Experimental results show good correlation with predictions.     

Weldline strength in glass fiber reinforced polyamide 66

Presence of weldlines introduces an element of uncertainty to the performance of injection molded parts. Weldlines are particularly problematic in reinforced plastics because, unlike molecular orientation in neat polymers, the flow induced fiber orientation does not relax. This paper deals with the structure and mechanical behavior of weldlines in glass fiber reinforced nylon 66, a plastic known for excellent fiber-matrix adhesion. Two molds were used to generate weldlines: a double gated tensile sample shaped cavity in which the weldline is formed by a head-on collision of melt fronts flowing in opposite directions and a film gated rectangular plaque with a circular insert in which the weldline formation behind the insert is followed by additional flow. In both cases the weldline zone is several millimetres wide: in the plane where the melts fronts have met fibers are oriented parallel to this plane (random-in-plane in the double-gated cavity and unidirectional in the cavity with insert). The transition zone between the weldline plane and the rest of the sample is characterized by an increased presence of microvoids. Weldline tensile depends little on the fiber concentration and on the sample shape or thickness: values close to the matrix strength are found: in samples without weldlines strength increases with the fiber content. However, in instrumented impact penetration test during which the material is subjected to multiaxial loading, the weldline effect appears negligible.     

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.     

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.     

Effects of filler contents and processing parameters on the weldline strength of compression molded natural rubber

The presence of weldlines in rubber products is regarded as a one of the most undesirable phenomena, since it results in poor mechanical properties. Compression molding of large or complicated products are prepared by multiple charges, which produces weldlines once the melt fronts are joined by the impingement flow. In this work, the effects of curing systems, processing parameters, filler types, and contents on weldline strength of compression molded natural rubber (NR) were investigated. Furthermore, the effects of curing systems on the aging properties of NR vulcanizates were studied in details. The results show that an increased amount of calcium carbonate does not affect the weldline strength. However, the difference in tensile strength between weldline and nonweldline specimens becomes larger with the high loading of silica and carbon black. In addition, for the factors selected in the experiments, clamping pressure, and curing system were found to be the principal factors affecting the weldline property of vulcanizates. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Weldline strength in injection molded HDPE/PA6 blends: Influence of interfacial modification

Most injection molded objects contain defects known as weldlines. This defect may introduce an element of weakness affecting the object's performance. Weldlines are particularly problematic in multiphase materials where the situation may be exaggerated by component mismatch on the two sides of the interface that results in additional weakening when the two components do not adhere well to each other. In addition, weldline behavior is influenced by orientation and morphological effects. This paper deals with relationships between the structure and the mechanical properties in injection molded high density polyethylene polyamide-6 blends. The weldline effect is investigated in detail. Two molds were used to generate weldlines: a double-gated tensile bar cavity in which the weldline results from the meeting of two melt fronts flowing into each other from opposite directions, and a film-gated rectangular plaque mold with a circular insert that divides the melt front in two. Following the recombination of the fronts, there is additional flow as the melt fills the mold cavity. Two preparations containing 75 vol % of polyamide-6 and 25 vol % of polyethylene with and without compatibilizer were studied. In the first case, a compatibilizer was incorporated into the polyethylene prior to compounding with the polyamide-6. In the directly molded tensile bar the minor phase is strongly oriented parallel to flow. Only in the core, which represents about 10% of the sample thickness, do the dispersed phase particles assume spherical shape. The morphology of the weldline is closely related to that of the skin: the elongated structures are oriented parallel to the weldline plane. The effect of the compatibilizer on the mechanical properties (without the weldline) of the directly molded tensile bars is minor: It is overshadowed by the flow-induced morphology. The weldline strength loss is about 40% in the noncompatibilized blend. The introduction of the compatibilizer has restored the material's ability to yield and the properties are close to those measured without the weldline. For the second type mold, the effect of the weldline is less pronounced and the effect of the distance from the insert is negligible. The anisotropy is quite pronounced in the noncompatibilized blend. In compatibilized blends, all tensile properties are unaffected by the presence of weldline, except for the 2-mm-thick plaque in the position close to the insert. The properties in the direction parallel to flow are similar to the type I mold and not affected by the increase of plaque thickness. Consequently one may question the utility of the directly molded tensile specimens in studying various aspects of the mechanical behavior of multiphase materials where the flow-generated structure is very different from that found in “real” injection molded parts. © 1995 John Wiley & Sons, Inc.     

Effects of Processing Parameters on Tensile Strength of Thin-Wall HDPE Parts Injection Molded by Ultra High-Speed Process

This study explores how ultra high-speed processing parameters affect the melt flow length and tensile strength of thin-wall injection molded parts. A spiral shaped mold with a specimen thickness of 0.4 mm and a width of 6 mm was first constructed to test the melt flow length as an index of process capability for ultra high-speed injection molding. It was observed that the flow length increases with increasing injection speed. High-density polyethylene (HDPE) tensile test specimens with different thicknesses (0.6 mm and 2 mm) were also molded for tensile tests. Both single gate and double gates were used to form parts without and with weldlines. Injection molding trials were executed by systematically adjusting related parameters setting including mold temperature, melt temperature, and injection speed. The parts’ tensile strengths were measured experimentally. It was found that tensile strengths of 0.6 mm thick parts both with and without weldlines were higher than those of 2 mm thick parts. The tensile strength of 0.6 mm thick specimens increases with increasing mold temperature, melt temperature and injection speed, whereas tensile strength in 2 mm thick specimens was only weakly dependent on the corresponding processing parameters. Furthermore, 0.6 mm thick specimens with weldlines had tensile strengths lowered about 9.6% compared to parts without weldlines. For 2 mm thick part the corresponding reduction is 4.3%.     

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.     

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.     

Studies of microcellular nanocomposites with supercritical fluid assisted injection moulding process

Abstract

In this study, the microstructure, thermal behaviour and mechanical properties of microcellular nanocomposites were studied. Microcell wall structure and smoothness were determined by the size of the crystalline structure, which, in turn, was based on the material system and moulding conditions. Nanoclay in the microcellular, supercrtitical fluid assisted injection moulding process promoted the γ form and suppressed the α form crystalline structure of polyamide 6 (PA6). In the crystallisation kinetics studies, the Avrami equation and the modified Ozawa equation with the Mo method were used to model and analyse isothermal and non-isothermal crystallisation processes respectively. The existence of nanoclay increased the magnitude of the activation energy for both isothermal and non-isothermal crystallisation processes. This suggests the fast crystallisation process and the small crystalline size for microcellular nanocomposite processing. Interestingly, the dissolved gas lowered the crystallinity of the cores of moulded microcellular parts, but the addition of nanoclay reduced the crystallinity of both the cores and the skins of parts. The collective effect of the dissolved gas and nanoclay acted to shorten the moulding cycle time greatly with a reduction in the overall crystallinity of microcellular nanocomposite parts.     

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.     

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.     

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     

Influence of high injection moulding pressures on the engineering properties of linear polyethylene

Two grades of high density polyethylene, one injection moulding grade and another with a substantially higher molecular weight (melt index $\text{0.1}\text{g}\text{10}\text{min}$) were injection moulded at pressures ranging from 100 to 500 MPa using a modified conventional injection moulding machine. For the high molecular weight grade, improvements were observed in the elastic modulus, the tensile strength at rupture measured in the flow direction, and the unnotched impact strength. These improvements were accompanied by a second high temperature (137°C) melting peak in d.s.c. diagrams. For both grades it was also found that the mould shrinkage decreased and the crystallinity increased with injection pressure.     

Fabrication of coloured zirconia ceramics by infiltrating water debound injection moulded green body

Abstract

Abstract

The authors developed a new technique combining ceramic injection moulding and liquid precursor infiltration, presenting a new strategy for fabrication of coloured zirconia ceramics. The authors’ strategy includes ceramic injection moulding 3Y‐TZP powder using a water debinding binder system, debinding moulded parts in water and drying, immersing debound parts in solutions containing different colouring ions and then sintering and creating coloured zirconia ceramics. The fabricated coloured ceramic bars exhibit a core shell structure, and the thickness of the coloured shell can be tailored by adjusting immersion time and temperature. Using solutions containing different colouring ions, ceramic bars with various colours can be prepared.