The shell of a spa pool is without a doubt the most valuable spa component. Unlike any other element of your spa, the shell cannot be replaced.
Because of the extreme importance of the spa shell we ensure that we produce them out of high quality, durable materials to avoid damage. While some spa shells are created using a sheet of acrylic on top of layers of fibreglass, others use a sheet of acrylic with a sheet ABS plastic fused to the back. ABS acrylic is used on car bumpers, proving its durability.
Let’s discuss these two processes further to build a more solid understanding of the benefits and disadvantages of each.
Fibreglass backed acrylic shells.
In this particular process, the first layer is acrylic. Acrylic is used widely by most spa manufacturing companies because of its versatility and availability in a broad spectrum of colours. Firstly, the acrylic must be moulded to the correct shape, this is achieved by heating it to approximately 180 degrees Celsius, then suctioning the heated acrylic into a moulded vacuum. Various vacuums exist for this process but the best is those that provide ‘zone controlled heating’. Zone controlled heating allows different areas of the spa mould to be heated to different temperatures. As acrylic is heated it thins, therefore having the ability to heat control different areas of the shell creates a thicker, more durable shell.
In the moulding process, not all thinning can be avoided. This is why the acrylic must be layered with one or a number of layers of fibreglass in order to be strong enough to hold the 1500 litres of water that your average spa holds.
There are two main ways in which fibreglass can be applied. The cheapest way for a company to achieve a fibreglass backing is by simply spraying a layer of general purpose resin, mixed with glass fibre, directly onto the back of the acrylic. This process is not the most efficient as acrylic has a smooth finish and the fibreglass does not adhere well to the surface. Over time, because of this loose bond between the acrylic and fibreglass coating bubbles can form in-between the two surfaces. The formation of bubbles creates a space for water to reside resulting in irreparable leaks.
However, these bubbles and leaks can be avoided prior to the coating process. The use of a waterproof and acrylic fusible product named Vinyl Ester resin can be applied directly to the acrylic as a first coat. Although Vinyl Ester is not useful on its own, when layered between the acrylic and fibreglass coating it ensures a permanent bond.
The next step is for the shell to be placed into a temperature and humidity controlled cured oven, this completes the bonding of the layers. This process is particularly crucial for spas which are manufactured in extreme weather environments e.g., very hot, very cold or humid conditions. Without this curing process taking place, the chance of delamination of the shells layers leading to leaks and irreversible damage is increased.
The final layer is created when another layer of resin is applied, typically produced with a ceramic powder within to further develop the strength of the shell. This step completes the 4 layers found in the shell of a quality spa pool. When spas are created with less than 4 layers it will be made evident through the shorter warranties that accompany the spa pool as its lifespan will be dramatically shortened. When purchasing your spa pool, we urge you to ensure the spas shell construction follows the above details.
ABS backed acrylic shells
The use of ABS backed acrylic means applying vinyl ester is no longer necessary. Although ABS based acrylic has proven strength, fibreglass is still sometimes added in order to ensure it maintains its strength alongside supporting the spas water and bather load.
An alternate option to backing the ABS is to fill the space between the spa shell and cabinet with foam. Although using foam filling does provide excellent insulation, this process can cause issues in relation to the maintenance of your spa, these are outlined within the insulation section of this document.
When choosing a spa pool, ensure its shell is created from either ABS backed acrylic, or uses vinyl resin and is oven cured. A quality spa pool will always use one of these two processes. Your salesperson should be able to detail the particular one used on the spa you are looking at.