The art of producing synthetic turf for sports is grossly underestimated and incorrectly compared to the production of plastic household products. In this article, Jacki Stephen of Imajica Consulting focusses on the machinery to be used in the production of high-end, durable and consistently performing synthetic turf products for sports.
The final quality and performance of synthetic turf is determined by a variety of parameters. This starts with using the correct mixture of raw materials, all the way through to the right machinery and production settings. Jacki Stephen explains.
So, you’ve decided you want to make your own grass yarn, whether that’s because of problems in your supply chain, your rock solid belief that it can’t cost as much to produce yarn as you have been quoted, or a business decision to bring everything in-house. At a trade fair for plastics and rubber, you see lots of bright shiny extrusion lines, hence you place an order – and you think you are done, right?
You’ve just bought yourself a white elephant, and the quickest route to white hair or hair loss.
How do you get it right?
The first question to be asked is: what is it that you want to make? Sports yarn, landscape yarn, slit film, monofilament, texturized, knit-de-knit, twisted, spiral wrapped, PP, PE, bi-component, unicolour, bi-colour, tri-colour – the decisions are endless, but important.
Essentially an extrusion line is a simple thing. They all have the same sections, starting with a dosing system to feed the raw materials in, a barrel and screw to melt, mix and transport the materials to a die, and then there is a drawing section and a relaxing section. Once your raw material has gone through all the necessary parts and steps, you end up with something that looks and feels like yarn.
Whether you have good or bad yarn is entirely dependent on the raw materials, machine set-up, and the process settings applied.
Extrusion is a dark art
I often say extrusion is a dark art and I am not kidding.
It starts with the dosing unit. You’ll need a continuous feed gravimetric type to achieve colour consistency. Also, I would suggest getting six hoppers. “Six!!!” I hear you shout. “Why so many?” Well, you’ll be surprised to learn that yarn is not made from just colour (we call it masterbatch, combi-masterbatch, if you want to be strictly accurate, but we are amongst friends here) and polymer.
It is common to blend polymers. This is for several reasons. Firstly, the majority of polymer producers do not make specific polymer for grass yarns, so we need to adapt commercial grades to get the performance we need. Secondly, getting all the characteristics you are looking for from one grade is unrealistic. Blending allows you to make a custom grade for your requirements.
Additionally, you may want or need to add additives such as antibacterial, flame retardants, recycled content, process aids, anti-oxidants etc., etc.
Don’t screw the screw
Then there is the screw, a component on which you could write a whole article about its design.
Most machines are supplied with a barrier type combi-screw, one which is able to be used for polypropylene and polyethylene. Less commonly used is the adiabatic screw, but this can be a little aggressive for the types of materials used in grass manufacture. In simple terms, a screw has several sections, firstly a feed section whose length varies, depending on the type of material and the texture (i.e., powder vs pellets) of the raw material.
Then we have a transition section: this is where the material is melted by means of heat and shear. The length of this section should match the melting rate of the resin being used. Then we have the metering section, which builds pressure to overcome the resistance of the die.
Lastly, we have the mixing section, which is located near the tip. These can have very varied designs, from shallow “V” shapes to an alarming pineapple shape. Achieving the best screw design for your yarns requires the input of an expert who will need to know the raw materials you are using.
Dying to die
Once the material has passed through the screw and has been thoroughly melted and mixed and is at a consistent viscosity and at the right pressure, it is time to make our way to the die. This can be a coat hanger type die for slit film, or a monofilament die (sometimes called a spinneret die) for extruding into filaments.
For a slit film die, you will need a flexible die lip, and you will have a different die lip for PP and PE. These lips are not really interchangeable.
For monofilament, you will need to define the materials, the finished yarn shape and dimensions, as well as the ballpark draw ratio so that the cross section is milled correctly into the die plates.
Generally, the filaments/sheets are extruded into a water bath (also known as a quench tank). Though sheets can also be cooled on a chill roller, this is a bit more old school but useful for products that require high tensile properties. It is better for softer more flexible grass yarns to be cooled in a water bath.
The cooling of the molten material is quite critical. It is the first part of the process and where you can start to impart finished properties into the yarn. Bath temperature as well as the gap between the die and the water all plays a part in your finished result.
The second step once the yarn leaves the bath is drawing. This is where the yarn is stretched. The stretching is done via godets and an oven (hot air, steam, or water). By stretching the yarn under these conditions, we give the yarn strength. The more we stretch, the more tensile strength the yarn will have. You want to optimise your drawing to give the right balance of strength and elasticity – part of the dark art!
Once you have drawn the yarn, it is now completely unstable. If you were to test the fibre, you would find that, under heat, it would shrink right back to its original form.
A good stretch
So, what has to happen now is to make it happy and comfortable in its new drawn-out shape. We do this by relaxing (also called annealing) the yarn.
This is done via heat and speed over godets and through hot air ovens and is a delicate process. Too much heat and speed and the yarn will become tight like banjo strings. Altering the elongation and the shrinkage, not enough heat, or the wrong speed, and the yarn will be irregular and unstable along its length.
The more elastic the materials you are using, the longer the relaxing zone you should have available to you. This will make processing so much easier. Balancing the process is not easy, and avoiding issues such as slubs and tip curl while optimising tensile, elongation and shrinkage to get the performance required for FIFA quality yarns, in terms of durability, resilience, and softness, takes skill, patience, and a lot of effort.
There is not a one size fits all option when it comes to producing grass yarns. Sports yarns use highly specialised materials, machinery and processes combined to produce the right product for your requirements – and these will be unique to you.
Jacki Stephen has close to 30 years experience in the production of synthetic turf yarns for sports. She is member of the Technical Committee for outdoor sports surfaces of the European Standards Committee as well as the EMEA Synthetic Turf Council.
The views and opinions expressed in this article are those of the author and are not necessarily endorsed by the publisher.