Prosthetics - Snap Moulds, part 2! Polyurethane positive/core

Now that a silicone mould has been made and prepped, the next step is to create the polyurethane positive/core for the snap mould. "Polyurethane is a plastic material, which exists in various forms. It can be tailored to either be rigid or flexible". (polyurethanes, no date). According to Dalchem, polyurethane is made of two components, those being urethane and polymers. These polymers are bonded together by urethane groups which then creates a versatile solution that can take many different forms. (Dalchem, no date). Polyurethane has many different uses, such as insulation of fridges and freezers, car parts, mattresses, tyres and even shoe soles. (Polyurethanes, no date). 

The polyurethane used was Axson F40 polyurethane resin. This resin has a quick setting time, a low shrinkage rate (meaning it won't shrink as it sets) and high abrasion resistance. (Neillsmaterials, no date). A catalyst is required for this, so isocyanate F40 was used, as per manufacturer's instructions. The mixing ratio for this is for every 100g of resin, 20g of catalyst must be added. This works out to be a 20% ratio of catalyst. For example, 200g of resin needs 40g of catalyst, 50g of resin needs 10g of catalyst and so on. 

If the silicone mould is small enough, it can just be filled with the polyurethane and left to set. However, the student's would have required quite a lot of polyurethane to fill it, therefore brush coats were done instead. This method reduces the quantity of polyurethane but still uses what is needed. Three brush coats were applied to ensure that the polyurethane was thick enough. Each brush coat was 100g of resin, and as seen in figures 2 and 3, these coats do give a good outer layer. As the name suggests, this was applied with a brush, and to ensure that the resin did not set within the bristles, ruining the brush, it was cleaned in acetone after each layer.

                                                      Figure 1 - Polyurethane brush coats (Brown, J. 2020)

Figure 2 - Polyurethane brush coats (Brown, J. 2020)

Figure 3 - Polyurethane brush coats (Brown, J. 2020)

Once the third layer had set, another 100g of resin was mixed, but this time poly fibre was added to create a thick paste. This paste was then used to ensure the keys were completely filled as there is a chance they could snap later on if they are not thick enough. The paste also covered inside the perimeter, creating a rough surface.

Figure 4 - Polyurethane with poly fibre (Brown, J. 2020)

The poly fibre layer was added because this will be filled with plaster. This is the alternative to completely filling the mould with the resin. As polyurethane sets very smooth, plaster will not bond or connect to it, meaning it will very easily come apart. The paste serves as a way to thicken the resin layer but also gives the plaster something to grab on to.

Figure 5 - Core filled with plaster (Brown, J. 2020)

Figure 6 - Core filled with plaster (Brown, J. 2020)

Figure 7 - Core filled with plaster (Brown, J. 2020)

After the plaster has set, the cores can be removed from the mould. Any jagged edges around the base or keys can be sanded away to make it smooth. One part of the snap mould has now been created and the sculpt can now be transferred from the main life cast to these cores.

Figure 8 - Polyurethane core, right eye (Brown, J. 2021)

Figure 9 - Polyurethane core, left eye (Brown, J. 2021)

Figure 10 - Polyurethane core, mouth/lower face (Brown, J. 2021)

When working with chemicals, it is very important to make sure to read and understand the safety data sheets. As this was a two part mixture, there is a safety sheet for both the resin and the catalyst.

Figure 11 - Isocyanate F40 catalyst safety sheet (Brown, J. 2021)

Figure 12 - Polyurethane resin safety sheet (Brown, J. 2021)

Looking at these figures, the resin itself is not overly harmful to work with, unless released into the environment. However, the catalyst is where precautions need to be made as it is carcinogenic, harmful if inhaled and can cause damage to organs over prolonged or repeated exposure. Therefore, the coverall, vapour respirator, safety goggles and gloves used in fibreglassing, were used again in this process to ensure the student was fully protected.

The student found this process to be quite simple. The mixing ratio was a lot easier as it is set out in grams, rather than percentages like with fibre glassing. This made it a lot less confusing to work out how much catalyst to add. It was also reminiscent to working with silicone in the way of adding poly fibre to thicken it up. They definitely kept in mind that they needed to make sure enough poly fibre was added, and achieving the correct paste consistency. 

-Jess

References:

• Axson F40 | SikaAxson. (no date) Available from https://www.neillsmaterials.co.uk/product/f-40/ [Accessed 24/01/2021].
• What is polyurethane? (no date) Available from https://dalchem.com.au/how-to/what-is-polyurethane [Accessed 24/01/2021]. 
• What is polyurethane? (no date) Available from https://www.polyurethanes.org/en/what-is-it/ [Accessed 24/01/2021].

Bibliography:

• Construction hazardous substances: Isocyanates. (no date) Available from https://www.hse.gov.uk/construction/healthrisks/hazardous-substances/isocyanates.htm [Accessed 24/01/2021].
• Debreceni, T. (2019) Special Makeup Effects for screen and stage: Making and applying prosthetics. 3rd ed. Abingdon, Routledge.
• F 40 Isocyanate. (no date) Available from https://www.jacomp.fi/wp-content/uploads/2019/10/fascast-f40-1-gb.pdf [Accessed 24/01/2021].
• Polyurethane applications. (no date) Available from https://polyurethane.americanchemistry.com/Applications/#:~:text=Polyurethane%2C%20mostly%20in%20the%20form,more%20durable%2C%20comfortable%20and%20supportive. [Accessed 24/01/2021].
• Polyurethane resin guide - everything you need to know. (2020) Available from https://resin-expert.com/en/guide/polyurethane-resin#:~:text=Generally%20speaking%2C%20polyurethanes%20are%20synthetic,a%20wide%20range%20of%20possibilities. [Accessed 24/01/2021].
• Product data sheet. (no date) Available from https://industry.sika.com/dms/getdocument.get/47754dda-e69a-453a-bf5d-92bbd67a7730/pds-SikaBiresin-F40-(F40-1)-en.pdf [Accessed 24/01/2021].
• Safety data sheet. (no date) Available from https://docs-emea.rs-online.com/webdocs/15ed/0900766b815ed217.pdf [Accessed 24/01/2021].

Figures:

• Figure 1, 2, 3, 4, 5, 6, 7 - Brown, J. (2020) York College.
• Figure 8, 9, 10, 11, 12 - Brown, J. (2021) York College.