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.
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!