Microwave in line heating to address the challenges of high rate deposition

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Microwave in line heating to address the challenges of high rate deposition

Host Institutions: The University of Bristol, The University of Sheffield, Wrexham Glyndŵr University

Start Date: 1st April, 2019

Duration: 6 months

Lead Investigator: Richard Day

Aims

There are a number of techniques for high deposition rate processing of composites. These include automated tow placement, automated fibre placement, and filament winding. Typically these are limited to 20 kg/hour. Current forecasts for the demand for composites mean that much higher rates are required if processes are to keep up with the demand. In order to increase deposition speeds currently lasers are used to heat the surface of the tow. This has limitations as if the power is increased too far the laser burns the surface of the tow so the rate at which the material can be deposited is limited by this.

In this study potential techniques for increasing the throughput for placement of thermosetting and thermoplastic tows will be investigated in order to increase the rate of two processes – automated tow placement and filament winding. The aim is to produce processes capable of placing 100 kg/hour. Hence this project is complementary to the core project on the technology framework for automated dry fibre placement and fits within the high rate deposition and rapid processing technologies research priority area in the call.

The study has the following objectives:

  1. To develop microwave cavities suitable for in line heating of tows at 2.45 GHz and to couple to an existing 2 kW microwave system
  2. To assess the potential heating rates achievable on narrow static tows in the laboratory using a sealed system.
  3. To investigate designs of microwave choke or screening to ensure that the microwave radiation is contained during processing with conductive fibres.
  4. To conduct simple trials at UoB to assess the potential of the microwave system in enhancing the lay down rate of wide (100 mm) tape.
  5. To consider methods for increasing the power delivered in follow on projects.

Methodology

In the first instance samples of a variety of thermosetting prepreg will placed centrally in a whispering gallery mode microwave cavity coupled to a 2 kW microwave generator and the ends sealed. Fibre optic temperature probes (Opsens) with fast response (<0.01s) will be attached and the heating rate monitored as a function of power up to 250oC (the upper limit of these probes). Similar experiments will be undertaken with thermoplastic composites but different probes will be used for the higher temperature measurements as they need to withstand as high as 400oC for some materials. These measurements will be used to assess the potential heating rates available and the power which will be required in order to yield the required deposition rates. IR thermocouples will also be used to monitor the process.

In the first part of the programme a suitable microwave cavity for rapid heating of wide tows will be explored. A simple solution is to use a TM010 microwave cavity, these have their microwave field concentrated along their central axis of a cylinder, hence are potentially useful for heating tows of prepreg, but since they have a field distribution concentrated axially, the maximum width of the tow could only be of the order of 10 mm. If the cylinder were split along its length as shown in Figure 1 (below) then a much wider tow, such as 100 mm commonly used in processing could be accommodated.

 

In the project a range of potential solutions will be examined, to see if the proposed one is best. The cavity will be designed using analytical methods but the design will be verified using COMSOL at UoS AMRC prior to being manufactured. Likewise the process will be modelled using COMSOL so that the rate limits of the process can be understood.

One of the issues involved in locating a conductive material into a microwave cavity is that it will act as an antenna and irradiate the workplace with microwaves. Similar problems exist with using high power lasers for this purpose as the light is bright enough to cause severe damage to the human body and so the system has to be completely enclosed. Hence there is a potential solution to the leakage. In this work we will consider at this closely to see if there is potential for using a microwave choke or similar system around the deposition area on an AFP head to make the process more readily adoptable.

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