While spa pools can all look quite similar at first glance, not all machines are made the same. There’s no better example of these subtle yet significant differences than the shell of a spa pool – the water-tight vessel that keeps the H2O inside your spa.
Two seemingly identical shells can be made of totally different materials, which can ultimately lead to significant differences in everything from comfort to longevity. In this guide we’ll take a closer look at spa pool shells to discover what they are, what they’re made of, and why the material matters.
What is a spa pool shell?
The shell of a spa pool is the vessel that holds the water. It generally sits inside the spa pool cabinet, although there are spas that are cleverly constructed from a single piece of material, such as the Hot Spring Freeflow range.
Up until the 1970s spa pool shells were generally made from timber, though things began to change when it became clear that wood isn’t a particularly long-lasting material when it is in constant contact with water.
Today spa pool shells are made from materials that are designed to handle constant moisture and spa pool chemicals, that will last longer while being exposed to the elements, and that can easily be sculpted into beautiful, comfortable forms.
Why does the material of the spa pool shell matter?
The material of your spa pool hard shell has a greater impact on your ownership experience than you might realise. Here’s why material choice matters:
1. Strength and longevity
The shell of a spa pool has a challenging task: it is the face that your spa pool presents to the world, and has to be strong enough to withstand whatever the sun and Mother Nature throws at it, as well as the bumps, scrapes and presence of people in and around the spa.
2. Beauty and comfort
The shell needs to pair this strength with beauty, as most buyers will want their spa pool to be a showpiece of their backyard. And we can’t forget the importance of comfort – the carefully crafted seats of your spa should perfectly cup the body of anyone who soaks.
3. Functionality
Finally, the spa pool shell needs to be perfectly functional. It needs to feature stain resistant, easy-wipe surfaces that make it easy to clean and maintain. It needs to be good at retaining the heat of the water to ensure the spa pool is as energy efficient as possible. And it needs to be capable of being fitted with all the features, from jets to lights to control panels, that make a soak in a spa pool such a luxurious experience.
With all these characteristics in mind, what should your spa pool shell be made from?
Acrylic spa pool shells
Most hot tubs are made from acrylic sheet. It is a strong, stiff plastic. It exhibits glass-like qualities, clarity and brilliance but at half the weight and many times the impact resistance of glass.
There are two types of acrylic sheet, cast acrylic sheet and closed cell acrylic sheet.
First decisions
The first decisions are the most critical. What type of acrylic sheet and what type of moulding machine? If your first decision is based on price, every subsequent decision is a compromise to try and overcome the first bad decision.
1. Cast acrylic
This is made in an open mould. Liquid acrylic is poured into the sheet mould. It spreads out in the mould and begins to cure. The chemical reaction forms tiny bubbles of gas that work their way to the surface of the sheet. This leaves tiny porous holes in the final acrylic sheet.
The down side of this process is that the tiny holes allow dirt and bacteria to enter the acrylic sheet.
The upside of this process is that the sheet is cheap so it is used by most spa pool manufacturers.
2. Closed cell acrylic
The resin is mixed in a hopper. The liquid acrylic is poured out between rollers and the rollers roll out all the tiny gas bubbles.
The downside is that this is a much more expensive manufacturing process.
The upside is that dirt and bacteria cannot enter the closed cell acrylic sheet.
Sheet thickness
Cast acrylic sheets used by most hot tub manufacturers are mostly 3.2mm in thickness.
The downside of 3.2mm acrylic sheet is that the sheet becomes thinner and thinner, the further down the mould it is drawn.
The upside of 3.2mm sheet thickness is that the acrylic sheet is cheap.
Closed cell acrylic co-laminated sheet is used to manufacture the Hilife™ range of hot tubs. It is 6.9mm thick and is comprised of closed cell acrylic with an ABS backing. ABS is an opaque thermoplastic which is very tough and durable.
The downside is that the sheet is much more expensive.
The upside of 6.9mm co-laminated sheet is the final moulded shell has much thicker spa walls and foot well. Also co-lamination doubles the impact resistance of the sheets.
Moulding machines
Most manufacturers use a compact moulding machine that looks like and works in a similar way to a pizza oven. The acrylic sheet is raised to moulding temperature in the oven. Then the sheet is pulled out hot and placed onto a cold mould. It is then clamped to a mould and a vacuum applied to draw the sheet into the hot tub mould.
Hot Spring moulding machines are purpose built for the precision moulding of acrylic sheet. The machine is as big as a home living room. Both the sheet and the mould are placed in the machine.
When a vacuum is applied, the sheet is drawn against a hot mould. This means that there are no stress areas in the sheet and there is a mirror-like surface to the finished moulding. The moulding machine is also zone heat controlled. That means that the thickness of the shell being formed can be controlled. More material can be put where more strength is required.
The moulds
Most spa manufacturers make vacuum forming moulds mostly from resin with smaller amounts of aluminium powder.
Hot Spring makes its moulds from mostly aluminium with smaller amounts of resin. Some moulds are 100% aluminium. The major advantage of this type of mould construction is that the mould can be heated up in the moulding machine along with the acrylic sheet. This also means that there are no chill spots when the sheet contacts the mould so no localised stress areas.
The mirror finish of the aluminium moulds gives a mirror finish to the vacuum formed spa shell. The single colour sheet shows no defects. Competitors often cover the problem of moulding defects by using acrylic sheet with two strata colours.
The downside is that Hot Spring aluminium moulds can cost up to 20 times the cost of a resin mould.
The upside advantages are that there are no chill stress areas and there is a mirror finish which allows solid colours to be moulded.
Rotomoulded plastic spa pool shells
Begin with plastic powder. Insert it into a mould. Heat it until it melts. Rotate the mould until a smooth, even coating of plastic is applied to every surface. Congratulations – you’ve just created a rotomoulded spa pool.
As seen in the Hot Spring Freeflow collection, rotomoulded spas combine the shell and cabinet into a single piece. Featuring a dull matte finish rather than the gloss of acrylic, they are comparatively cost-effective and lightweight. While very durable, they do lose out to acrylic in terms of energy efficiency, and initial savings may be vacuumed up by higher power bills over time.
The Hot Spring difference
At Hot Spring our spa pool shells are built to last, no matter whether you choose our most premium or most cost-effective option. From our rotomoulded Freeflow range to our market-leading Highlife collection – which also happen to be New Zealand’s most energy efficient spa pools – you can expect your spa pool shell to look great and work even better for years, if not decades, to come.
“The best thing I have ever purchased! Hot Springs spas are the best quality out there, I wouldn't go for any other brands, they just don't live up to Hot Springs” – Angela Besant
Ready to experience the Hot Spring difference? Visit your local dealer today.
