Rubber Sheeting – How is it Made?
Do you know how rubber sheeting is manufactured?
Whether you do or you have no idea, hopefully this article will provide some new and interesting information. White Cross Rubber are the UK’s leading independent specialised rubber sheeting, reinforced rubber sheeting and rubber coated textile manufacturers. We’ll share some insights about how we and the rest of the industry manufacture our core products.
In general terms there are three main ways in which the primary process of shaping crude rubber compound into precision sheeting is undertaken; calendering, extrusion and press moulding.
Rubber is a thermoset material – once the primary process is complete, it is cured or vulcanised (these terms are interchangeable). This allows it to impart its final and optimum characteristics. This heat cycle can take place in an autoclave (a large oven), a rotocure or a press. They are also used to engineer a specific final appearance or surface finish on the sheeting required by the user.
Regardless of the shaping or curing process, some common principles are at play:
1. prepare the rubber to make it malleable
2. shape it
3. bake it!
You won’t therefore be surprised to hear that the process has some parallels with bread or pastry making; once the dough is mixed it is kneaded to transfer some heat and energy from your hands. This enables it to be rolled out without crumbling, and once shaped it is put in the oven to bake.
Let’s take a look at these processes in more detail.
The calendering process is the most widely used as it is an adaptable and flexible way to manufacture rubber sheeting. Calendered rubber sheeting can be made from a variety of different rubber compounds of different types and hardnesses. It is made in dimensions ranging from 0.1 to 12 mm thick and up to 3 m wide. All of these variations are possible by adjusting the settings on the calender, rather than having to fit different tooling.
For the uninitiated, the calender is like a giant mangle. It has a series of large horizontal parallel steel cylinders or ‘bowls’ that the rubber is fed through. The gap between these steel bowls is accurately controlled and as the rubber is fed through this gap it is formed into a sheet of a designated thickness. This continuous and linear process enables large quantities of rubber sheeting to be manufactured cost effectively.
Calenders come in a variety of types or configurations; the main difference is the number and positioning of the bowls. Certain configurations are optimal for certain types of products; ‘4 bowl inverted L’ calenders are ideal for coating textiles. They are employed in the tyre industry to rubberise tyre chord. ‘4 bowl inclined Z’ calenders enable two rubber plys to be manufactured simultaneously. ‘3 bowl vertical’ calenders are most common and most flexible.
All calenders need to be fed with a supply of well masticated and blended compound to enable them to deliver good results. Therefore, upstream from the calender there is a rubber mill or series of rubber mills (or perhaps an extruder). These machines warm and mix the rubber, lowering its viscosity before it is fed into the calender for forming. Downstream from the calender the sheeting is batched or wound up on a roll. It will have a polythene or textile interleave fed in to it. This will stop the sheeting (which is still in an uncured state) from sticking to itself. This roll can then either be inspected and shipped to the customer or moved to the next stage for vulcanisation.
The calender is king when it comes to manufacturing rubber sheeting. Although care needs to be taken to mitigate the natural tendency for the sheeting to ‘crown’ (thicken) in the middle. As well as this, you must avoid practices that can result in air entrapment or blistering in the sheet. An experienced and reputable rubber calendering company will be able to manufacture almost any specification of rubber sheet required.
The next most prevalent forming process employed is extrusion. An extruder is like a sausage machine in an old-fashioned butchers’ shop. Sausage meat is fed in one end, travels through the machine on a screw as a handle is turned. The meat is forced out the other end through a head that shapes the meat into a sausage. This basic principle can be applied to rubber sheeting but the extruder is somewhat larger and the process and controls more complex.
Rubber compound (usually in a continuous strip format) is fed into the end of the extruder and travels through the barrel on a motorised screw. Whilst in the barrel the rubber is warmed and mixed under high shear before it is forced through the extruder head.
As you might expect, the head used for making rubber sheeting is very different to a sausage machine. It is, in fact similar in shape to the sheet that needs to be made. These ‘flat heads’ are thin and wide and produce sheeting in a range of thicknesses and widths. Such a wide variety as can be produced on the calender is not possible though without changing the extruder head. These are costly to manufacture.
Extruded rubber sheet is typically of high quality and tight dimensional tolerances. A well optimised extruder can achieve excellent output. However, such optimisation can also be a drawback. It is typically only suited to manufacturing a narrow range of products from limited rubber types. For this reason, extruders do not offer the same flexibility as calenders.
The last method of manufacturing rubber sheeting to consider is press moulding. Presses come in a variety of shapes and sizes. They are responsible in some cases for shaping and curing the sheet in a single process. Broadly speaking, presses can be categorised as large and small. Small presses with beds a few square metres or less can be used to produce single rubber sheets every cycle.
A crude sheet or ‘blank’ must first before formed on either a simple calender, a mill or maybe an extruder. This is then presented to the press. A rubber press like any other type of press consists of two plates that can be opened and then closed again once the material to be pressed has been placed inside. In the rubber industry the plates or plattens as they’re sometimes called are heated. Simple tooling can be placed in between the plattens to control the thickness of sheet created. Once the plattens are closed, the blank is compressed to the desired thickness. Additionally, the heat cures the sheet at the same time during a cycle (which might be several minutes long).
Press moulded sheets typically can be made to tight dimensional tolerances. These tolerances are better than calenders or extruders but output is slow and not the same league as other methods. They serve small lot production very well. They are often used for expensive high-grade materials when only small quantities are needed. Large presses, as the name suggests are much bigger – perhaps two metres wide by ten to fifteen metres long. However, the principle is the same. They are usually fed on a semi continuous basis and used to manufacture thick gauge rubber sheeting (12 mm or more) or specialised sheeting products like heavy duty conveyor belting.
For more information about any of our specialised calendered sheeting, reinforced rubber sheeting and rubber coated textiles, please call our Technical Sales Department on 01524 585200 or visit www.wcrp.uk.com.