plastic part of a medical device

EGTM89 Polymers: Properties and Design           Assignment 2014 / 2015

This assignment is worth 25% of the module. It is an individual assignment and normal plagiarism rules apply.

The submission should be done as a Word file with a limit of 2000 words, plus associated diagrams and graphs. You can submit an Excel file of data analysis if you wish, but you should aim to explain all your analysis methods within the Word file.

You should submit the assignment via Turnitin on the Blackboard site by 5pm on Thursday 23^rd April.

 

Overview

This assignment relates to a plastic part of a medical device. It is approximately in the form of a closed cylinder which is internally-pressurised by a liquid. It needs to be transparent, be able to be sterilised at 120°C for a short time before use and to be able to be injection-moulded with accurate dimensions. The cylinder is 10cm long and has a diameter of 4cm. It will be under an internal liquid pressure at 37°C for a desired lifetime of 2 years. The internal pressure will be 746 kPa,. The design proposal is to use Polycarbonate as the preferred material.

 

 

Part 1 (worth 20% of the assignment).

Consider whether you agree with the choice of polycarbonate as the material. You should do a short review of the typical properties of polycarbonate, give your opinion of whether it meets the material requirements and whether there are any potential problems.

 

Part 2 (worth 25% of the assignment).

A series of creep tests have been performed on the selected grade of polycarbonate. These have been done in tension at different temperatures from 20°C to 70°C, for a period of 1 week each. The data from these tests is given in the accompanying Excel file, as compliance versus time.
You should determine the shift factors on the time axis that will bring these curves together as a single curve (using the time-temperature superposition method). You should then plot the shift factors as a function of temperature and then use these to construct a “master curve” at 37°C.
Part 3 (worth 20% of the assignment).

It is known that polycarbonate may develop crazes if the tensile strain exceeds 0.5%. Use your master curve to determine the wall thickness required to avoid the potential for crazing. Assume the material behaves as a linear viscoelastic material over the range of stress used for the application. Assume polycarbonate has a Poisson’s ratio of 0.35.

 

Part 4 (worth 20% of the assignment).

After you have calculated the wall thickness, you are informed that “physical ageing” should be taken into account. Do some information gathering to find out about physical ageing and describe how this may affect the creep behaviour.

 

Part 5 (worth 15% of the assignment).

It is known that the creep tests were started 2 weeks after moulding of the test pieces from high temperature and that the components will start to be used 2 weeks after moulding. Use this information to recalculate the required wall thickness for the component including the effects of physical ageing (you will need to use the “effective time theory”).

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