B. Haworth

Optimum sintering region for laser sintered nylon-12

It is well known that the mechanical properties of laser sintered Nylon-12 improve with increased applied energy. However properties can reach a maximum after which, the application of additional energy can a lead to a decline in part properties. It is thought that the reason for this decline is that the additional energy causes polymer chain degradation or other changes in molecular structure. This paper aims to use thermogravimetric analysis (TGA) to investigate the thermal degradation of nylon-12 and explain the deterioration of mechanical properties when high energy density conditions are applied during processing. The key findings are the application of modelling methods to predict the temperatures achieved during laser sintering of nylon-12. It is shown that temperatures in the laser sintering machine can achieve levels above 300°C. At these temperatures, TGA data show that mass loss occurs and could cause mechanical property breakdown. This practical work coupled differential scanning calorimetry and TGA as a means of identifying thermal transitions in the material. The term ‘stable sintering region’ is proposed as a novel concept for the laser sintering community, and can have implications for better understanding of how process parameters can affect parts built in the machine. In addition, the concept could be used in the material selection process when screening potential new polymers for the process. One limitation of laser sintering, compared to other polymer processes such as injection moulding, is the limited understanding of the connection between machine parameters and part properties. This work aims to improve that understanding by discussing the pattern of thermal behaviour, including degradation, seen in polyamide exposed to high laser parameters.

Shear viscosity measurements on Polyamide‐12 polymers for laser sintering

Purpose – Laser sintering kinetics and part reliability are critically dependent on the melt viscosity of materials, including polyamide 12 (PA‐12). The purpose of this paper is to characterise the viscosity of PA‐12 powders using alternative scientific methods: constrained boundary flows (capillary rheometry) and rotational rheometry.Design/methodology/approach – Various PA‐12 powders were selected and characterised by both techniques. Measurement of molecular weight was also carried out to interpret the viscosity data.Findings – Results demonstrate conventional pseudoplastic flow in all PA‐12 materials. Zero‐shear viscosity has been quantified by rotational rheometry; a notable observation is the striking difference between virgin/used PA‐12. This is interpreted in terms of molecular weight and chain structure modifications, arising from polycondensation of PA‐12 held at the bed temperature during laser sintering.Research limitations/implications – Accurate zero‐shear viscosity data provide scope for us...

Melt-state elongational rheometry of mineral-filled polyethylene

Abstract A free-surface elongational rheometer has been further developed to study the melt state deformation and rupture behaviour of high-density polyethylene (HDPE) polymers, and corresponding elasticity characteristics. An upgraded, PC-interfaced data collection system has been introduced, appropriately calibrated to provide rapid and accurate data collection when tests are carried out at high extensional strain rates; good agreement has been obtained between experimental and theoretical displacements, over a range of strain rates (0.1–6.0 s −1 ). Deformation (extensional stress–strain) response of HDPE is shown to be sensitive to molecular weight, with low molar mass grades suffering melt rupture at Hencky strains between 2.0 and 3.0. Incorporation of inorganic calcium carbonate filler increases transient-state extensional viscosity and rupture stress, but reduces the extensional strain at failure. The degree of elastic recovery is dependent upon the total elongational strain developed during the test; calcium carbonate-filled HDPE melts exhibit lower recovery at a given strain, but elasticity is enhanced when fatty-acid coatings are used to assist filler dispersion in the compound.

Extensional flow characterisation and extrusion blow moulding of high density polyethylene modified by calcium carbonate

AbstractFree surface elongational flow properties, including transient state stress growth and melt rupture data, have been measured on a range of calcium carbonate filled high density polyethylene (HDPE) compounds, using a Rutherford elongational rheometer operating at constant strain rate in the range 0·1–6·0 s-1. Results show that polymer molecular weight has a strong influence on both tensile stress growth and melt rupture data; in creasing elongational strain rate decreases the stress growth coefficient, in all compounds. At any specific elongational strain rate, the stress growth coefficient increases with volume fraction of particulate additives and also when using additives of fine particle size, as a result of packing fraction effects. Acid coatings added at super-monolayer levels influence the viscosity of highly filled compounds, by internal lubrication.Dynamic extrusion data obtained by experimental blow moulding of calcium carbonate modified HDPE materials show that modifications to melt stat...

Elongational rheometry of thermoplastic PET in the rubbery region

Abstract Recent advances in processing have witnessed the development of products shaped from amorphous poly(ethylene terephthalate) (PET) by controlled stretching in the temperature region immediately above the glass transition. This paper describes how deformation and rheological data relevant to these processes may be derived from a uniaxial specimen-end-separation Elongational Rheometer, which was originally developed to study the tensile flow characteristics of polymer melts. The operating principles and use of the rheometer are presented, together with details of the various testing modes which are feasible. Some typical results are shown, illustrating test output data for a PET resin of intrinsic viscosity 0·73 dl/g. The steady uniaxial extension data illustrate the sensitivity of various viscoelastic properties (and therefore processing characteristics) to temperature, strain and rate effects, in the processing range 80–110°C.

Fracture mechanics approach to compare laser sintered parts and injection mouldings of nylon-12

In comparison to equivalent parts made by injection moulding, components manufactured by laser sintering are often perceived to offer inferior mechanical properties, yet the evidence for this is rarely based upon systematic research studies. In this paper, attempts have been made to conduct a fundamental study of the fracture behaviour of both injection moulded and laser sintered parts, based upon a modification of a standard technique used to determine linear elastic fracture mechanics parameters. The influence of specimen thickness (in the range 2–10 mm) was also included in the experimental plan, which concentrated upon the testing of single-edge notch bending (SENB) beam specimens. Force-displacement characteristics demonstrated significant plastic deformation in nylon (PA12) specimens, so that the J-integral method was used to obtain quantitative fracture parameters, including the energy requirements for crack growth. Comparisons of this parameter showed thickness-dependent ‘geometry-sensitive’ data for each set of samples. For the injection moulded SENB specimens, energy absorption decreased with increasing thickness, a result attributed to the influence of plane-strain-dominated conditions. In contrast, laser sintered samples exhibited increased toughness as specimen thickness increased towards 10 mm; this could not be explained by density or particle melting data and may be interpreted by changes in molecular structure that occur during the additive manufacturing process.

Elongational rheometry of polyethylene terephthalate/bisphenol-A polycarbonate blends

The elongational deformation properties of polyethylene terephthalate (PETP) and bisphenol-A polycarbonate (PC) blends were determined using a Rutherford elongational rheometer. The effects of temperature (in the thermoelastic region) and strain rate were studied on blends containing up to 50% PC. Addition of low levels of polycarbonate permits the thermoelastic processing of PETP over a wider temperature range. PETP is particularly sensitive to changes in temperature. Over the range studied, the effect of strain rate on elongational deformation is not very marked. However, the deformation temperature changes the strain levels at which strain stiffening occurs, an important observation in respect of processing and optimisation of physical properties of uniaxially oriented products. The effect of the addition of a phenoxy resin on the blend was studied, and although TEM analysis suggests that it did not compatibilise the blend, it did increase the available extension at higher temperatures.

Computational modelling of electrical field intensity for high voltage semiconductor package design

Although plastic packaging has successfully replaced ceramic and metal packages for many high reliability electronic packaging applications (e.g. military and aerospace), hermetically sealed ceramic packages are still the dominant technology for large power devices, such as the thyristors and diodes used in high voltage DC (HVDC) power transmission. With increasing energy requirements of growing economies, the demand for higher operating currents is driving suppliers to use larger diameter silicon devices, therefore requiring bigger and more expensive packages. A switch to polymer packaging in such applications has the potential to provide robust and light weight components at a lower cost. A Finite Element Analysis (FEA) based study aimed at optimising the electrical performance of a polymeric package for such power semiconductors is described in this paper. From the FEA simulations carried out, it was observed that one critical region where the electrical stresses tend to be high, and long-term failure may therefore occur, is in the contact region between the polymer housing and the metal inserts (flanges) which attach the housing to the copper pole pieces of the device. Different design features for the insert edge were studied in order to investigate their influence and to reduce the peak electrical field (E-field) in the critical region. The results showed that the E-field around the contact region decreased as the radii of curvature of the insert ends was increased. A comparison of the E-field magnitude for different insert designs also showed it to be lower around an elliptical insert end compared to circular and straight flange designs. Changes in the depth to which the flange protrudes inside the housing also had a significant effect in the electrical field magnitude in the contact region, whilst variation of other housing design parameters, such as the package thickness and the location of the insert relative to the housing periphery, did not cause the electric field to change significantly.

Melt compounding of rigid PVC formulations with hydrotalcites

Abstract Hydrotalcites, compounds of magnesium–aluminium–hydroxycarbonate, are promoted as environmentally safe materials for costabilisation of PVC products. Commercial grades of hydrotalcites have been added to rigid PVC formulations, containing a range of different stabiliser types, to evaluate their contribution to heat stability and their effect on mechanical properties. Hydrotalcites are confirmed to be effective costabilisers for rigid PVC: static thermal stability (through oven testing) and dynamic thermal stability (via torque rheometry) of PVC compounds are modified by their presence. The extent of change is determined by the primary stabiliser type and the grade of the hydrotalcite. Detailed analysis of mechanical properties has been carried out on compression moulded samples and on extruded compounds. With the former, no significant change in tensile or impact performance was observed, for hydrotalcite levels up to 5 phr. Charpy impact data on extruded PVC have shown significant increase in performance of compounds containing an acrylate modifier when hydrotalcites are used.

The measurement of thin PVdC coatings on PET substrates using fluorescence microscopy

Abstract In this paper, the authors demonstrate how a novel technique based upon autoluminescence of plastics can be used to measure accurately poly(vinylidenechloride) (PVdC) coating thickness profiles on poly(ethyleneterephthalate) (PET) substrates. The procedure uses simple sample preparation and examination methods when used in a destructive mode. Some preliminary work has also shown that luminescence phenomena may also be used to provide coating thickness profiles non-destructively.