Volume 26, Issue 3

The main shrinkage and dimensional stability impact factors are processing temperature and pressure. For that reason, the pvT-behaviour is often used to estimate plastic part shrinkage. As there are still differences between predicted and actual part dimensions, a closer look was taken at these influencing factors. During processing, temperature changes occur due to heat conduction to the mold walls, and the pressure varies in the filling stage and holding time along the flow path. However, these pressure gradients are typically not taken into account in plastics processing. Hence, the influence of pressure was examined in detail and further investigations on pvT-behavior of polymers were performed. For that purpose, fundamental examinations of the behavior of amorphous thermoplastics during cooling and compression were made. These include analysis in the different phases and at different compression speeds as well as variation of the pressure-temperature-cycles and their succession. It was found that the specific volume is not defined by one value of p and T, but is dependent on the pathway of the process. That applies for both thermal expansion and compressibility. The adiabatic compression heating was also examined and a numeric solution was found to easily adapt the pvT-results on the base of compressibility measurements. These findings are discussed by means of the free volume theory. Additionally, a choice of these investigations was performed with semi-crystalline thermoplastics. It was found, that compression heating is even more important for these polymers because it can superpose crystallization heating at appropriate parameters. Furthermore, it can be seen that the density, morphology and lamellae thickness is influenced by pressure. In the outlook of this paper, the impact of these results on the injection molding process is discussed in detail.

Injection molding with expandable thermoplastic microspheres (ETM) containing blowing chemicals is capable of fabricating lightweight, dimensionally stable plastic parts while using less material. This paper presents the study of microcellular injection molding of low density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS) parts with various ETM contents. It was found that the molded parts exhibit relatively better surface quality than conventional foamed parts. The microcellular morphology and cell density of the fractured cross-sectional surfaces were characterized using a scanning electron microscope (SEM). As reflected by the testing results, the cell microstructure – such as cell size, cell density, and a layered structure with a foamed core sandwiched by skin layers – play an important role in the weight reduction, surface quality, and mechanical properties. A smaller cell diameter and a thicker skin layer help to improve the surface quality and tensile properties of the injection molded parts with ETM. Finally, an appropriate ETM content has a positive effect on cell microstructure and weight reduction, whereas too high a concentration of microspheres adversely affects the tensile properties and surface quality.

This paper contributes with the analysis of the case of a complex three-dimensional shape that was moulded in a standard steel mould and an alternative hybrid mould with steel cavity and epoxy-based composites cores. The mouldings were produced in semicrystalline and amorphous polymers. Pressure and temperature were recorded at two locations in the cavity. The processing conditions were simulated. The effect of the processing was observed on some dimensions of the part that were used for assessment of the shrinkage and warpage. The results suggest the importance of predicting the regime mould temperature when low thermal conductivity materials are used in moulding blocks. Results also evidenced that in hybrid moulds with resin blocks the shrinkage of the mouldings is a consequence of the specific polymer shrinkage and the deformation of the non-metallic core deformed by the processing pressure. The warpage occurring on these parts is also larger in comparison with conventional steel moulds.

To eliminate surface defects and improve the quality of molded parts, increasing the mold temperature is one of the solutions. Using high mold temperature can eliminate weld lines, reduce molding pressure, residual stress and clamping force and improve part surface quality. However, with the increasing of mold temperature, the cycle time will also be increased. Hence, people have paid the attention to mold temperature control technologies. Among them, the variotherm molding technologies, including Rapid Heat Cycle Molding (RHCM), Induction Heating Molding (IHM), and Electricity Heating Mold (E-mold), are some effective methods. Although those variotherm technologies have been proposed, how does the external or internal heating source affect the injection molding process and the final product? The true function and the efficiency study of each technology still remain vague. Hence, in this paper, we have systematically conducted various technologies, including Conventional Injection Molding (CIM), RHCM, IHM, and E-mold, by using true 3D transient cool technology. Through the inside mechanism investigation at various moments in time, the functions and the heating-cooling efficiency for each technology can be visualized. Besides, experimental study and verification of IHM are also performed.

The purpose of this study is to establish the Gas Counter Pressure (GCP) technology in combined with microcellular injection molding process. The application of gas into the mold cavity during the melt injection period provides a counter force against the melt front advancement, restricting the foaming process during the melt filling stage. Various gas counter pressures from 0 bar up to 300 bar and gas holding times of 0 to 10 s after filling completion were employed for investigating their relevant effect. It was found that when gas counter pressure is greater than 100 bar, foaming does not appear in the skin layer if no holding time is applied after melt-filling. As gas counter pressure increases, thickness of the solid skin layer without foaming also increases. Employment of holding time after melt-filling also assists foaming restriction in the melt core area. If a holding time of 10 s is applied combined with a counter pressure greater than 100 bar, foaming is completely restricted resulting in a transparent PS sample part. The part surface of those subjected to foaming-restriction in the skin layer also appears to be smooth and glossy. For the black PS parts molded via microcellular injection combined with gas counter pressure, it appears the same gloss quality as that achieved by conventional injection molding.

Sub-μm computed tomography (sub-μm-CT) was used to determine the fibre orientation and fibre length distribution in long glass fibre filled polypropylene. For data evaluation two different concepts based on the application of a sequence of different filters were applied. The first concept is based on segmentation by binarisation using a global threshold, followed by a detailed analysis of regions where fibres are touching. The second concept is based on analysis of the original gray value image. For each voxel the local fibre orientation is determined by calculating the Hessian Matrix and analysing its Eigen values. The effectivity of the two data analysis concepts in determining orientation and length was investigated. For this purpose the algorithms were applied to specimens with four different levels of fibre content: 1, 5, 10 and 30% by weight. To quantify the level of error in fibre determination, a minimum and average probability for correct fibre determination were estimated. The results show a strong dependence of the level of error on the fibre content. Whilst the determination of fibre orientation is not significantly affected, determination of fibre length distribution is significantly influenced by fibre content. For samples with fibre content above 5%, concept 1 does not produce correct representations of all fibres. In particular, problems arise if the fibres are touching. Concept 2 delivers much better results and represents most of the fibres correctly even at higher fibre content levels and for touching fibres. This was proven by using artificial CT-data sets generated by CT-simulation and by systematic comparisons. A practical application of the CT-evaluation pipeline is presented for glass fibre reinforced rings produced by injection-moulding and extrusion. For both samples the orientation tensors are calculated and the orientations of the fibres are visualized in three dimensions by colour coding.

Injection molding of long fiber-reinforced thermoplastics is a well-established method in automotive industry to produce high quality structural parts in mass production without the need of further finishing, where the performance is strongly dependent on the fiber length. It is known that the processing parameters have to be carefully chosen as they directly influence the final fiber length and can therefore have a negative effect on the resulting mechanical properties of the part. Particularly in terms of impact behavior, the fiber length is seen as a key factor. The aim of the present work is to quantify the effects of the injection molding parameters on the impact behavior of long fiber reinforced compounds. In a first step the resulting fiber length of injection molded glass fiber reinforced polypropylene is analyzed as a function of the injection velocity, holding pressure, revolution speed and back pressure. The single steps of fiber length analysis are carefully investigated in terms of feasibility and accuracy, in order to assure that a reproducible and reliable test method is employed. Subsequently, the impact behavior of the PP-GF compounds is quantified by falling dart experiments, and the results are then correlated to the fiber length and the processing parameters to establish a fundamental processing-structure-property relationship. The investigation of the fiber length analysis shows that a high pyrolysis temperature leads to embrittlement of the glass fibers and a decrease of tensile strength of the single fibers. This may consequently lead to incorrect characterization of the fiber length due to fiber breakage. The results of the statistical investigation of the processing parameters indicate a significant influence of the back pressure on both impact energy and fiber length.

Micromolding with microscale surface features and thin-wall plates of polycarbonate polymer was performed with heat insulator molds using zirconia ceramic. Effects of process parameters on processability and surface replication of the molded optical parts such as anti-reflective plates were evaluated. Calculation results show the interfacial layer of molded parts with a heat insulator mold shows 20 K higher than a conventional mold. The state of replication and higher-order structure of molded parts were analyzed using a scanning electron microscope and a polarizing optical microscope. Using a heat insulator mold, the flow length is increased, and the state of surface replication of molded parts is better. Due to the better surface replication, plates using the insulator mold show higher light transmission due to the less light reflection at the surface of plates. The molecular orientation of molded parts decreased using this mold, as it did in molding conditions of higher mold temperatures and injection temperatures.

The surface quality of injection molded parts depends on the processing conditions, the cavity surface structure, the runner system, the used polymer and the part geometry. Different replication of the cavity surface structure to the molded part influences its surface function and visual appearance. A descriptive model of the replication process has been developed, taking into account the thickness of the frozen layer during injection. The expected dependencies were proven by systematic injection molding tests with geometric surface micro structures, having depths of 45 or 100 μm and widths from 400 to 2000 μm, that were replicated into high-crystalline polypropylene (HCPP) and polycarbonate (PC). The volumetric structure replication ratio increased with rising mold temperature, melt temperature and cavity pressure. As expected, the mold temperature was dominant for very small micro features. The RMS roughness, determined by confocal microscopy and atomic-force microscopy, was found to be a suitable replication parameter for draw-polished to mirror-finished stochastic cavity surface structures. An abrupt change in wall thickness decelerated the flow front velocity, thus decreased the replicated polymer surface roughness and increased the surface gloss. Moreover, a several micrometer high “surface step” remained, due to the different thickness-dependent shrinkage. This step always ascended from the thicker to the thinner part area. The replication of a mirror-finished mold surface into HCPP was dominated by morphological effects. Local micro shrinkage differences led to micro sink marks, which affected the surface gloss much more than the mold surface structure.

In temperature cycle injection moulding (TCIM) of thermoplastics, the mould cavity surface is heated rapidly to a temperature close to the glass transition or crystalline melting point of the resin before melt injection, and then cooled after injection is complete. A range of important benefits of the process are listed, among which is surface improvement, and results are presented that quantify in detail the improvements achhieved. Weldline dimensions and surface roughness are determined using white light inteferometry, with a scale of inspection below 5 nm. For a qualitative comparison of the surface finish, photography and stereomicroscopy are used. Weld lines on conventional ABS/PMMA parts are up to 17 μm deep and 70 μm wide, hence clearly visible, whereas they are not detectable on TCIM parts. Surface roughness, Ra, on these parts is found to be 37 nm for conventional parts, reducing to 20 nm using TCIM. Surface roughness is compared for conventional and TCIM mouldings in chemically foamed ABS, foamed PP with and without talc, and long-fibre-glass filled PP. For the conventionally produced foamed parts, Ra is approximately 1500 nm. Using TCIM, Ra reduces to 30 nm for ABS, 70 nm for unfilled PP and 130 nm for PP with talc. Visually, this corresponds to a change from a heavily patterned, striated appearance to a uniform glossy surface. The parts in long-fibre (11 mm) filled PP show a reduction in Ra from 1600 nm to 150 nm using TCIM. The conventionally moulded parts have a rough, pitted surface; the TCIM parts are smooth and glossy.

Delay in fiber orientation evolution based on the Folgar-Tucker model with a slip parameter correction has been proposed as a simple alternative to improve predictions of fiber orientation in injection molded parts. Predictions based on this model and model parameters fitted to data from simple shear experiments were compared with experimentally determined fiber orientation in a center-gated disk. Three methods of fitting to simple shear data were assessed to obtain the isotropic diffusivity and the slip parameter. The model parameters and orientation data evaluated at the entry, lubrication and near-end-of fill regions in a center-gated disk for 30 wt.% short glass fiber-filled polybutylene terephtalate (PBT) were obtained from earlier efforts in our laboratory. Simulation results based on the Folgar-Tucker model with the slip correction using customarily assumed inlet orientation being random and experimentally measured at the gate disagreed with measured orientation values at certain positions along the disk. However, improvement in the prediction of orientation due the slip correction was found at the core and transition layers in the lubrication region. The use of inlet conditions washes out quickly in the absence of the slip correction and induced a general reduction of orientation towards the center of the sample causing underestimation of orientation at the entry and lubrication region. Model predictions combining the slip correction and experimentally determined orientation at the gate are in agreement with the experimental data for the core layers near the end-of-fill region.