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     

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.     

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     

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.     

Effect of processing conditions on shrinkage and warpage and morphology of injection moulded parts using microcellular injection moulding

Abstract

This research investigated the effects of processing conditions on the shrinkage and warpage (SW) behaviour of a box shaped, polypropylene part using conventional and microcellular injection moulding. Two sets of 2^6–1 fractional factorial design of experiments (DOE) were employed to perform the experiments and proper statistical theory was used to analyse the data. After the injection moulding process reached a steady state, moulded samples were collected and measured using an optical coordinate measuring machine (OCMM), which had been evaluated using a proper repeatability and reproducibility (RR) measurement study. By analysing the statistically significant main and two factor interactions, the results show that the supercritical fluid (SCF) content (nitrogen in this case, in terms of SCF dosage time) and the injection speed affect the SW of microcellular injection moulded parts most significantly, whereas hold pressure and hold time have the most significant effect on the SW of conventional injection moulded parts. Investigation of the cell morphology indicated that fine and dense cells are not critical for achieving excellent results regarding SW. Also, the present study quantitatively showed that, within the processing range studied, a reduction in the SW could be achieved using the microcellular injection moulding process.     

Degradation depth profiles and fracture of UV exposed polycarbonate

Abstract

Photodegradation depth profiles have been determined in 3 mm thick polycarbonate injection moulded bars using molecular mass measurements and an attempt made to relate the results to the fracture behaviour in uniaxial tensile tests. Unstabilised samples were compared with similar bars containing a commercial stabilising system. Although the presence of stabiliser restricted molecular mass degradation to a thin surface layer, with little degradation indicated beyond 0·1 mm from the exposed surface, the bars made from stabilised polymer embrittled at least as quickly as those made from unstabilised material, for which molecular mass degradation was evident at least up to 0·4 mm from the exposed surface. The as moulded compressive residual stresses near the surface of unstabilised polycarbonate bars increased in magnitude on UV exposure during the period in which embrittlement developed and may have partially countered the eects of molecular degradation, whereas little varition in the residual stress distribution with exposure was observed in stabilised polycarbonate.     

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.     

Low cost ceramic moulding composites: materials and manufacturing technology

Abstract

Low cost ceramic moulding compounds/composites are composed of inorganic metal silicates, fillers and different types of chopped fibre reinforcements, such as glass fibres. This research investigated manufacturing process of a series of ceramic moulding compounds that could be moulded at modest temperature (～150°C) and pressure (0–8 MPa). This manufacturing process replicated a process of the fibre reinforced polymeric moulding composites or dough moulding compounds. The conventional polymeric matrix of the dough moulding compounds has been replaced by a soluble metal silicates based on an inorganic system which can be compounded with fibres, minerals or synthetic fillers and hardening additives, to produce a thick paste or a doughy substance, so called ceramic dough moulding compounds. The ceramic dough moulding compounds demonstrated a good handle ability and can be adjusted by the viscosity of the matrix through the use of various amount of fillers and additives. The mechanical properties of the compounds have been tested and additional formulation changes have been introduced to maintain desirable processing characteristics.     

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.     

Glass fibre orientation within injection moulded automotive pedal: Simulation and experimental studies

Abstract

Modelling and measurement of the fibre orientation distribution within a reinforced thermoplastic automotive clutch pedal are presented. Products have been manufactured on an injection moulding machine equipped with accurate process monitoring. The fibre orientation distributions at selected points within the pedal were measured using a fully automated, polished section/reflection microscopy image analysis system. Simulation was undertaken using commercial software with a fibre orientation prediction algorithm. The predicted fibre orientation distributions are compared with experimental results in order to assess the accuracy of the software in both thin and thick (～5 mm) sections of the pedal.     

Injection moulding process assessment by energy monitoring

Abstract

The demand for consistent and enhanced injection moulded product quality continues to rise. Modern injection moulding machines come with extensive control and measurement systems for machine control which are generally not available to the user. However, it is desired to actually measure variables that are closely related to the process itself. Sensors normally used to do this are typically invasive. However, as a result of the breadth of application of injection moulding machines, there remains a desire by the process user to be able to monitor some aspects of the process preferably in a non-invasive manner, in order to monitor, control and improve the output. In this paper, the potential for process and production measurement on injection moulding machines using non-invasive, energy measurements is presented. In addition, a comparison of electric and hydraulic injection moulding machines is given, that highlights the benefits of energy measurement to the process user. Finally, it is concluded that energy monitoring can provide a 'fingerprint' of a process, and hence can be used for product quality control.     

Residual orientation in injection‐moulded plaques of atactic‐polystyrene I: The effect of processing conditions

Injection moulded polymer articles often have residual macromolecular or crystalline orientation which can have a significant impact on the optical and mechanical properties of the moulded article. Small angle neutron scattering (SANS) was used to measure the molecular shape and orientation of deuterated blends of injection moulded polystyrene. For ～1‐mm‐thick mouldings of uniform rectangular cross‐section, the eccentricity in the SANS pattern gave a direct measure of the residual molecular orientation over the length scale ～100–1,500 Å. The residual orientation was found to vary significantly with injection moulding conditions with comparative residual orientation decreasing with decreasing mould fill‐time, and increasing with mould thickness and moulding temperatures. The orientation was found to be a minimum in the centre of the mould and highest near the surface and the average orientation at a particular position in the mould was found to be strongly correlated with the volume of material deposited as a solid skin layer during injection moulding. POLYM. ENG. SCI., 58:1322–1331, 2018. © 2017 Society of Plastics Engineers     

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.     

Physically based dynamic model for control of cavity pressure in injection moulding of amorphous plastics

Abstract

Cavity pressure plays an important role in determining the quality of injection moulded articles. However, the non-linear and time varying behaviour of the process causes difficulties in implementing an efficient control system, since these generally employ linear time invariant design strategies. Conversely, advanced control methods such as self tuning control do not provide physical information about the process. A physically based model has been developed. This model is used to control cavity pressure during the filling and packing phases. Experimental results indicate that this control yields accurate results for the injection moulding of polystyrene. Different control algorithms with different tuning criteria were implemented on a microcomputer to control the cavity pressure. The results indicate that this method is superior to the classical empirical control approach.     

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.     

Gas assisted injection moulding: Finite element modelling and experimental validation

Abstract

Gas assisted injection moulding of a rectangular cross-section tensile test specimen has been modelled using a finite element implementation of the pseudo-concentration method, in which the velocity components and pres sure are interpolated as primitive variables. Two-dimensional and three- dimensional modelling formulations are presented and in each case wall thickness predictions are compared with actual measurements taken from specimens obtained from computer monitored gas assisted injection moulding production. Two different boundary conditions were used for the injection of gas into the cavity.

Initial mechanical testing is used to compare the apparent elastic modulus of the solid material in gas assisted injection moulding specimens with that in solid polymer specimens.     

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.     

Aqueous colloidal processing of PLZT ceramics near the morphotropic phase boundary

Abstract

Abstract

Single phase PLZT powder was fabricated using the mixed oxide approach, with a composition (6/60/40) lying on the rhombohedral-tetragonal (morphotropic) phase boundary. Aqueous suspensions showed an IEP of pH?9·7 in the absence of a polyelectrolyte surfactant. Inductively coupled plasma optical emission spectrometry analysis demonstrated considerable leaching of Pb²⁺ and La³⁺ ions under acidic conditions. The addition of an ammonium salt of poly(methacrylate), or PMA-NH₄, decreased the IEP to pH?5-6. Suspensions were readily stabilised at basic pH using low PMA-NH₄ concentration (<0·1?wt-% of solids). Concentrated suspensions exhibited shear thinning behaviour under low shear rates, with a transition to shear thickening at modest rates (i.e. 100?s^?1). Excessive shear thickening imposed a maximum solids loading of 46?vol.-% on concentrated suspensions, which related to the somewhat ‘flake-like’ particle morphology. Slip casting resulted in green and sintered (1250°C for 30?min) densities of 56±1% and 98·3±0·6% of theoretical, respectively.     

Quantifying part irregularities and subsequent morphology manipulation in stereolithography plastic injection moulding

Abstract

The direct use of moulds produced by stereolithography (SL) provides a rapid tooling technique which allows low volume production by plastic injection moulding. The greatest advantage of the process is that it provides parts that are the same as those that would be produced by metal tooling in a fraction of the time and cost. However, work by the authors demonstrates that the parts possess different characteristics to those produced by metal tooling. This knowledge defies the greatest advantages of the SL injection moulding tooling process – the moulded parts do not replicate parts that would be produced by metal tooling. This work specifically demonstrates that a different rate of part shrinkage is experienced and subsequently investigates the mechanisms in SL tooling that induce these different part properties. The work culminates in different approaches to modifying the moulding process which allow the production of parts whose key morphological characteristics are closer to those that have been produced from metal moulds.     

Process improvement in preparation of ball valves

Abstract

The industrial application of ceramic injection moulding has been limited to small components because of the associated binder burnout problems. Injection moulding may improve the manufacture of alumina ball valves for the process industries by reducing costly and potentially damaging machining operations. However, these components are too large to be produced using conventional thermoplastic binders. Aqueous injection moulding allows most of the binder system to be driven off as water vapour, cutting process time, reducing burnout difficulties, and causing less environmental pollution. The manufacture of ball valve blanks from -alumina was carried out using a die cavity of 25·4 mm outside dia. Improved processing parameters were efficiently identified using orthogonal arrays as described by Taguchi. The following factors were evaluated for their influence on defects: binder content in solution, die temperature, injection velocity, injection time, barrel temperature, injection pressure, hold pressure, and residence time in the mould. Binder content in solution was found to be the most influential factor. The defects observed included cracking, poor make up, and distortion. Ball valve blanks of significantly improved quality were successfully manufactured under the preferred conditions.