Plastics,Rubber and Composites: Macromolecular Engineering

The aim of the present study was to determine whether electron beam (E-beam) radiation has any depth dependent effect on the mechanical and degradation properties of a typical degradable polymer used for medical applications. Any such effect could be utilised to tailor the properties of degradable polymers for a specific application. The material investigated in the present study is a form of polylactide (PLA). Thin layers of the material were compression moulded and arranged at various depths within a framework, with acrylic spacers separating them. The samples were then dosed with 40 kGy of E-beam radiation. Following irradiation the samples were subjected to accelerated aging through hydrolytic degradation in phosphate buffer solution at 70°C. Gel permeation chromatography testing showed that samples within a certain distance of the irradiated surface had consistently lower molecular weight than deeper samples throughout degradation. Corresponding to this, the shallower samples also had lower tensile and flexural strengths than the deeper samples, though no significant variation in modulus was recorded. The results of this study indicate that E-beam radiation can produce a depth dependant variation in mechanical and degradation properties of PLA. The potential therefore exists to modify the through thickness properties of a degradable polymer in such a way as to improve its suitability for a specific medical application.

Tensile tests with constant crosshead speeds ranging from 5 to 200 mm min⁻¹ were performed on Nylon 12 in the temperature range, 25 to 140°C. A modified form of the constitutive equations derived by Drozdov and Yuan were used to describe the time dependant response of Nylon 12 at isothermal deformation and small strains. Reasonable agreement was observed between the experimental data and the numerical calculations. A relationship between the materials constants derived for the uniaxial stress strain behaviour and temperature was found to exist. This is shown to satisfactorily calculate the constants required to accurately predict the stress strain behaviour of Nylon 12 at a specific strain rate and temperature.

Acrylic bone cement is weakened by its porosity, which promotes the formation of microcracks, which contribute to major crack propagation and ultimately failure of the cement mantle. Bone cement mixing techniques play a significant role in determining the quality of bone cement produced. A high degree of porosity is found to exist in cement that is inadequately mixed. Current commercial bone cement mixing systems allow for the preparation of the bone cement under the application of a vacuum in a closed, sealed chamber by means of a repeatable mixing action. These mixing systems are perceived to be repeatable and reliable by orthopaedic community. In this paper, the quality of bone cement mixed using an operator independent bone cement mixing system was compared with that of cement prepared using commercially available devices. The results of the investigation highlighted that cement prepared using the automated, repeatable mixing regime that is operator independent demonstrated consistently better physical and mechanical properties in comparison with cement mixed using proprietary cement mixing devices. Furthermore, Design of Experiments software established the optimal factors that influenced the physical and mechanical properties of PMMA bone cement.

The present paper describes trials that were carried out on a conventional and a metallocene linear low density polyethylene to determine the effect of bubble content on the impact performance of rotationally moulded parts. Internal mould pressurisation was applied at different levels and at different internal air temperatures during the moulding cycle. The quantity of bubbles in the different mouldings was determined and related to impact performance at room temperature and at −40°C. Variations in dynamic mechanical properties were also highlighted.

A systematic study of mould fouling during elastomer injection moulding is presented. Moulding was undertaken principally using nitrile rubber compound and purpose built tooling with interchangeable mould inserts. Rubber metal interactions between treated and untreated Stavax steel surfaces were characterised by a variety of analytical techniques to study factors influencing the onset of mould fouling and its origins. Preliminary simulation and supporting experimental investigation suggested that the design and location of the gate influenced both mould filling pattern and the position of rubber deposition within the cavity.

The present paper describes the results of an investigation into the modelling of plug assisted thermoforming. The objective of this work was to improve the finite element modelling of thermoforming through an enhanced understanding of the physical elements underlying the process. Experiments were carried out to measure the effects on output of changes in major parameters and simultaneously simple finite element models were constructed. The experimental results show that the process creates conflicting and interrelated contact friction and heat transfer effects that largely dictate the final wall thickness distribution. From the simulation work it was demonstrated that a high coefficient of friction and no heat transfer can give a good approximation of the actual wall thickness distribution. However, when conduction was added to the model the results for lower friction values were greatly improved. It was concluded that further work is necessary to provide realistic measurements and models for contact effects in thermoforming.

Angioplasty balloons are a class of medical device used to clear clogged arteries. They are manufactured via a process similar to that of injection stretch blow moulding. At present the forming of angioplasty balloons is something of a black art. When a new balloon is being developed, the process parameters and tube dimensions are determined through a mixture of trial and error and experience. The present paper describes the first phase in the development of a finite element simulation of the process which will ultimately be used to optimise the design and manufacture of these devices. A data acquisition system has been developed that measures the temperature and the tension/force in the tube during forming as well as the displacement applied to the tubing to initiate the forming process. Additionally a high speed video camera has been used to visualise the process. The balloon can be seen to form within 0·03 s with an average strain rate of 2000 s⁻¹. Based on the data supplied from the data acquisition system a process simulation has been developed which replicates the formation of the balloon as seen on the high speed video camera. A series of biaxial tests of Nylon 12 in the temperature range between 60 and 120°C and strain rates between 1 and 32 s⁻¹ show that the behaviour of the material is sensitive to temperature but is not dependent on strain rate. Further work is required in terms of the development of a suitable material model capable of capturing this behaviour.