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Blogs • Hot tub shell forming – the 21st century process By:Daniel Robinson. November 14, 2019

Hot tub shell forming – the 21st century process

A Spa or hot-tub’s main structure is the ‘shell’. This is the inside area that forms the floor through to the seats & up over the edge or the lip as its known.


Typically the ‘shell’ is formed from a single sheet of plastic known as acrylic. The world-leading brand of acrylic is from US manufacturer ‘ICI Lucite®’. They supply so many more industries than you could ever imagine from bathtubs to bulletproof ‘glass’.

Further to only sourcing our acrylic sheets from Lucite®, Just Hot Tubs use the top of the line version that is impregnated with Microban®, an anti-bacterial, mould & mildew inhibitor that stops black mould & bacteria from growing on the surface of the hot tub. 

STRONGER WITH ABS, For much longer than traditionally possible

A Just Hot Tubs hot-tub is different as we layer the back of the shell with a sheet of special ABS plastic.

ABS has a very similar melting point to acrylic & thus this blending of two materials is a great match, a really attractive balance between aesthetically pleasing acrylic top layer & the super strong ABS underneath.

The acrylic & ABS pairing is heated in an ‘oven’ which become as one, a single sheet. The ABS significantly increases the structural integrity of the ‘sheet’ whilst adding very little weight when compared to the norm,  fibre-glass which has to be added later ( we’ll get to this later too).

POPPING THE MOLD, Baking & Popping

This sheet is & then slowly lowered over a ‘mould’. The mould is an upside-down hot tub shaped block with seats & other features hand-carved out of it. When the hot sheet is lowered onto this mould the air is drawn out causing the sheet it to be ‘sucked’ onto the mould, during this critical time the sheet is heated in certain spots to ensure it better stretches.

Once cooled the sheet (now referred to as the ‘shell’) is then ‘popped’ from the mould & thoroughly inspected. From here the shell is moved to the next step – reinforcing.

FIBRE-GLASS VS. ABS PLASTIC, A long way departed from the ‘old school’

At this stage, most manufacturers then stick sheets or slabs of fibreglass onto the back of the shell to increase its strength. Whilst this was acceptable 30 years ago, it is slowly becoming frowned upon for a few reasons.

Fibreglass has very different thermal properties to acrylic, over time as the two materials constantly heat-up & then cool-down they slowly begin to separate.
Given long enough, often only a few years with substandard manufacturing & the signs of failure can be seen. Cracking, crazing & de-lamination of the layers… they shell is literally pulled apart from within causing the hot tub to warp in shape & split around the edges & at more precisely moulded locations.

Just Hot Tubs’s ABS & acrylic sheet combination, created at a much earlier stage makes this occurrence a thing of the past, they quite simply become one layer during the ‘baking’ process.

REINFORCING FOR THE LONG TERM, Protegrity®  for a cleaner, safer environment

In the rigidising booth, after the shell has cooled a third layer is added, this is the final stage that makes up our ‘TriLayer’® shell. 

The layer that we spray on is a liquid material known as Acrylobond®.
This final layer adds a 50% increase in strength over untreated surface & unlike the traditional method of spraying 40lb foam on to the back of the shell is highly consistent, leaves an evenly layered finish that is free of bubbles & critically safe to the sprayer & the environment.

Lets home in on that final point; Acrylobond® is a ‘green’ product, which unlike sprayed styrene-based products (foam), follows the Clean Air Act Amendments of 1990, and is NOT designated as a Hazardous Air Pollutant (HAP) that “may reasonably be anticipated to be carcinogenic, mutagenic & exhibit other adverse health effects”. Acrylobond® really does provide a much healthier working environment.

This ties up the process that we call the ProTegrity® rigidising system – why settle for less than a Just Hot Tubs TriLayer® hot tub shell?

Stronger, cleaner, safer all of this for longer, much longer, than most….