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#75 – Cutting Edges & Flat Moulds

July 20, 2021 By //  by Stuart

We covered cutting edges in episode #61 but this one is specifically about cutting edges on flat moulds.

Cutting edges on appliance moulds do the work of separating the fine appliance edge from the flashing and excess, allowing the mould to close properly and achieve the feather thin edge you have sculpted.


To listen to the podcast, you can stream or download from here, or simply subscribe through your favourite podcast app – we are on many, including Spotify, Apple Podcasts, Soundcloud, IHeartRADIO , STITCHER , Luminary and Google Podcasts.


The exact width of the distance between the cutting edge and the sculpted edge varies between artists and techniques, preferences and materials. I have seen many sculpts where folk have had a massive distance between the cutting edge and the sculpt, and this is what prompted this episode.

So what is the optimal distance? Make it too narrow and there isn’t enough mould to provide a stable cutting edge and make it too wide and you have an excessive amount of cap plastic bordering your piece which can cause unusual wrinkling, flatten hair or skin textures and generally make a piece have a larger footprint than necessary.

Personally, I keep my edges tight to the sculpt (maybe 2-3mm away from the edge) and follow the sculpt as closely as possible, like a beach hugging the coastline. In this video, I show you why we do it like that.

In a typical rigid core and mould made from plaster or resin, the ‘cutting’ action of the mould on the core is caused by the thin edge being pressed down using a clamp, strap or weight usually. The appliance material (usually silicone or foam latex) is essentially a liquid and therefore responds to the pressure exerted on it by the cutting edge (assuming the mould is accurate and material strong enough to withstand the closing pressure required).

With flat moulds, however, there is no core being squished into a mould. Instead, typically the back of the mould is left open and wiped clean with a rigid scraper to arrive at a clean border of cap plastic around the appliance. This clean border should have no silicone residue on there, which allows it to be then melted away on the skin later and thus blending into the skin.

Because silicone is therefore compressed as you push down during scraping, softer silicones will compress more than firmer silicones. For this reason, firmer silicones for flat moulds are better with regards to scraping. Knowing that the silicone mould will compress, the cutting edge or margin of clean cap plastic will increase (even a firm silicone mould will compress a little). Anticipating this, I make the cutting edge close to begin with. Watch the video to see what I mean.

Here are the two identical mouds, one cast in soft silicone, one in firm.
Here are the pieces from those moulds. Made exactly the sae way, you see the apliance on the right has wider areas of cap plastic owing to the softer silicone mould compressing during scraping.
Here is an example of making flat moulded pieces, starting out with the sculpts done on a lifecast to ensure the shape and ‘footprint’ of the appliance is the correct size and shape for the eye. The face cast is plaster, painted with acrylic paint to match my plastiline and then prepared with Scopas Parting Agent to enable safe removal of the sculpts later.
Here you can see the appliances soaked off from the head. Immersed in water overnight, the dried Scopas Parting Agent becomes slippery again, allowing the peices to be peeled off the face cast without damagin them.
Next job is to fatten the peices out nto the board, trying to do as little distorion and damage to the sculpt as possible. The edges need blending out and the texture needs finishing off, but essentially all the forms are there in the right place becasue it was done on the face cast to begin with. No guesswork as to where the shhapes should be to look right.
These are the finished appliances with the cutting edge border in place, neatly cut a consistent distance away from the sculpted edge and engled to help removal from the mould. Draft them angles!
Here you can see an illustration of two versions of the cutting edge placement. On the left, you an see how a wider distance will mean a bigger mould and more cap plastic. On the right, closer makes it far neater and we are aware that the appliance, once scraped, will have a slightly wider margin of cap plastic.

The book I was reading which mentioned ‘Stereo Type’ with regards to printing was The Village Carpenter: The Classic Memoir of the Life of a Victorian Craftsman by Walter Rose, published originally in 1937. Check out the Stereotype process on the Wikipedia page.

See what books are freely available at Project Gutenberg https://www.gutenberg.org/.


See what I mean about the unfortunate similarity. I mean, I can read and all but on a blocky, low resolution webcam, it was quite similar.

Many thanks, as always, for your time checking the stuff out. You can email us directly at stuartandtodd@gmail.com or leave us a voice message on our site.

If you enjoy this podcast and got something out of it, would you do us a solid and tell just one more person about us? Send them a link and help us grow!

–Stuart & Todd

To listen to the podcast, you can stream or download from here, or simply subscribe through your favourite podcast app – we are on many, including Spotify, Apple Podcasts, Soundcloud, IHeartRADIO , STITCHER , Luminary and Google Podcasts.

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