Rob Firman details how extruded polystyrene insulation (XPS) is made, and explains what gives it its unique combination of qualities.

COMPARED to other commonly specified ground floor insulation types, extruded polystyrene insulation (XPS) has better loadbearing performance and is more robust when subjected to the rigours of being used onsite. It therefore provides a strong and stable base for floor slabs, screeds, and floor finishes, helping achieve a high standard of workmanship from the complete build-up.

It also has a better tolerance to moisture, meaning it can be installed above or below the damp-proof membrane (DPM) without any negative impact on its thermal performance. So, in addition to a well-finished floor, using XPS helps the floor to deliver its intended thermal performance for the long term.

Given these advantages over other insulation types, we thought it would be interesting to detail how XPS is made. What gives XPS its unique combination of qualities? Providing insight into manufacturing processes gives designers, specifiers, and installers a better understanding of the products being selected and used within floors.

It’s also another way in which manufacturers can be responsible and transparent, explaining what they do and how they do it, as part of their contribution to a more intentional and sustainable construction industry.

The manufacture of XPS insulation involves a two-stage extrusion process. Following that, the extruded polystyrene foam is calibrated and profiled to create finished insulation boards. A recycling stage takes offcuts from the process and turns them back into polystyrene pellets, for use in later production runs.

What raw materials are used to make XPS insulation?
At our XPS manufacturer’s production facility, there are several bulk storage tanks that each contain virgin polystyrene and the recycled material amassed from elsewhere in the production process. Because offcuts and waste can be recycled in this way demand for raw polystyrene is kept to a minimum.

This commitment to using recycled material is part of Polyfoam XPS’s policy of zero waste to landfill from the production process.

Another storage tank contains additives used in the foam mix, and the final one contains the gas blowing agent. For every production run, raw materials are measured and combined in a blender to give the correct ‘recipe’ for the product being made.

From the blender, the mix feeds into the extruder – effectively a giant screw that breaks down and mixes the raw materials.

Understanding the ‘extrusion’ of extruded polystyrene
The first of the two stages of extruding XPS insulation involves heating, mixing and gas injection. The virgin and recycled polystyrene included in the initial ‘recipe’ is in crystal form, and must be melted.

Initially, heat is applied from an electrical heating source. Further heat is generated by the shearing action of the extruding screw against the polystyrene crystal. The blowing agent is then injected into the melted polystyrene at high pressure, and the full process creates the closed cell structure of the end product.

Without its closed cell structure, XPS insulation would not have the thermal performance, compressive strength and moisture resistance that makes it such a versatile and robust product.
Polystyrene foam now enters the extrusion die head. Different die heads are used to create different products. This is the second stage of extrusion, where the ‘melt’ is cooled using chilled water, ready to exit the die. Careful control ensures it is cooled to the correct temperature and pressure.

Turning extruded foam into finished XPS insulation boards
Polystyrene foam comes out of the die lips into the vacuum chamber, which is a reinforced concrete tube sealed at the top end by a mechanical seal, and at the bottom end by a water seal created by a water pond/reservoir.

The foam begins to expand, not unlike hair mousse or shaving gel coming out of an aerosol can. Calibrating bars control the spread of the foam to give the required thickness and width. A conveyor carries the product down the vacuum chamber, through the water reservoir and into the profiling station.

Here the foam is cut and profiled into insulation boards, including giving the board edges a lap joint or square edge as required. The size of the machinery determines the maximum dimensions of the product, and will be different for each production line. At the time of writing, Polyfoam XPS can manufacture products up to 3,100mm in length and 1,220mm in width.

During the cutting stage, any offcuts enter the recycling station and are re-densified. The recycling station effectively acts as a mini extruder, melting the offcuts and removing the gas. After being chopped into pellets, the recycled material is stored ready to go back into the start of the manufacturing process.

After cutting, each board has a unique code printed on it for traceability. The boards then enter a quality control process to confirm they are in specification. Finally, they are stacked, wrapped, and packaged.

Different products are stored in different areas of the warehouse. Every manufacturer will have their own approach. At Polyfoam XPS, our digital system means we know which products are where in our large internal capacity warehouse, allowing orders to be made up quickly while also ensuring stock rotation.
www.polyfoamxps.co.uk
Rob Firman is technical and specification manager at Polyfoam