Engineering and design student insights

Student projects, placements, research and study experiences in the Faculty of Engineering & Design

Posts By: Barrie Dams

Drilling into polyurethane foam

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📥  Department of Architecture & Civil Engineering, Please categorise your post, Postgraduate

Continuing my investigation into the properties of both low and high density polyurethane foams and their suitability for 3D printing, the drilling resistance of test specimens was measured. Interfaces are a crucial element in 3D printing and the aim of the drilling was to discover if there was a change in the density of cured foam at interfaces and moulded boundaries.

Two types of rectangular block specimens were used:

Cut-edged specimens with an interface: The liquid components were poured into a tray in two stages. Enough liquid was poured in to expand and occupy half of the tray volume and once fully cured, a second quantity of foam liquid was poured on top of the cured layer.
Moulded one-layered specimens: the two liquid components were mixed and poured into moulds to expand and cure. Enough liquid was poured in to fully occupy the volume, therefore no internal interfaces were present in these specimens.
Drilling resistance was measured using a Sint Technology Cordless Drilling Resistance Measurement System. The position of the drill bit was linked to a software program to continuously record the force required to advance the penetration of the moving bit through the foam. Specimens were placed into position and clamped as pictured.

drill

The results showed that the material was higher in density at interfaces and moulded boundaries, with the difference being most pronounced in the low density, high expanding foam – up to approximately ten times as dense. Drilling into polyurethane foam was an interesting and entirely new experience, with the drill gliding effortlessly though low density foam and the high density foam putting up a little bit more of a struggle!

 

HIGH DENSITY POLYURETHANE FOAMS

  

📥  Department of Architecture & Civil Engineering, Postgraduate

My PhD project, looking at materials suitable for 3D printing buildings using swarming aerial robots, began with investigating low density, expanding polyurethane foam ‘LD40’ (a) manufactured by the company Isothane. Now it is the turn of high density polyurethane foams, and the studies have used Reprocell 300 (b) and Reprocell 500 (c), commercially available foams manufactured by the same company.

foam

While LD40 is established in the construction industry as an insulating material, the higher density foams are not readily associated with construction. Reprocell 300 is usually found as a substitute for timber in prop and set design along with applications such as balustrades and mantelpieces, whereas Reprocell 500 is primarily used for deep sea buoyancy applications.

Blocks of foam created so that the materials may be tested in line with the British standards, have been primarily made by hand mixing on the high density foams, such as the compressive strength test specimens shown in the figure.

Through trial and error, I arrived at a recipe for making the high density specimens. Firstly, I heat the two liquid components (one resin, one hardening agent) to 30°C, pour together and hand-mix for 90 seconds. The creamy, viscous liquid then turns a darker brown and becomes much less viscous as the mixed liquid heats up and the polymerisation process begins. I then stir again until the 150 second mark, at which point the material expands. A few more careful stirs then follow until I withdraw and the material quickly hardens, becoming solid at 180 seconds and feeling like a block of concrete.

This recipe has produced specimens which exceed the density stated in the manufacturer’s literature, most notably with the Reprocell 500 (specimens average 685 kg/m3). Compressive tests on the 500 have shown strength in excess of 30 MPa, astonishingly competitive with concrete. Flexural tests have also shown strengths indicating the material is competitive with the lower range of timbers. Reprocell 300 has around a third of the compressive strength of the 500.

Reprocell 500 has therefore shown that it has potential to be a structural material. The downside of denser specimens of course is that the material does not expand quite so freely so I am having to use more of the liquid components!

Making 500 specimens with my little syringe device outlined in the previous post has proved to be challenging. Despite experimenting with longer tubing and multiple static mixers, the material is being deposited in its creamy viscous state, and then the darker / hotter / thinner / runnier phase is occurring on the surface of the mat after deposition, leading to lateral spreading.

After the current round of tests are complete, several options will be investigated, most notably whether a catalyst may be applied to speed up the reaction time and the investigation of whether particles can be added to the material to favourably alter its rheological properties – these may include clay nanoparticles, graphene, carbon fibres, or ways to combine the lower density foam with the higher density foam.

Barrie Dams 19/01/2017

 

Straight into the Labs!

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📥  Department of Architecture & Civil Engineering, Postgraduate

I am currently in the second month of my PhD project, which will consist of examining materials suitable for 3D printing lightweight structures. The project will be using swarming, coordinated drones to deposit the material rather than current ground based heavy machinery.

I have dived straight into laboratory work with a small device which represents the amount of material that a single drone can carry. I have enjoyed commencing lab work early and this will help inform my reading by refining the search for suitable literature. Material investigation will focus upon cement pastes and polymers, with the starting point for my work being whether polyurethane foam may play a role in a structural material.

The device consists of two syringes and a 6V DC motor connected to rods moving the plungers up and down the syringes. Polyurethane foam has two liquid components – a resin and a hardener. The figures below show the liquids being drawn into the syringes and then deposited through an arrangement of silicon tubes with an epoxy mixer nozzle into a mould.

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Long established as insulating materials, initial syringe device operation has been carried out using standard density polyurethane foam. I have been using different machines for the first time to characterise the material, with mechanical testing in the structures laboratory, travelling to Chemical Engineering to use the Rheometer and to the Microscopy analysis suite to use the Scanning Electron Microscope and Fourier Transform Infrared Spectroscopy. This will lead on to the investigation of higher density polyurethane foam, which has a density comparable to timber. I look forward in the coming weeks to using the techniques that I have learnt in the first month of my PhD to investigate the higher density foam and determine the structural feasibility of the material.