Introduction
As an elastomer, the rubber material is characterized by high elasticity, so it is used effectively as a sealing material. It is used in sealing indoors, such as buildings, cars, boats, submarines, etc., from the external environment, including smoke, dust, air, or water. For example, window seal strips are used to seal aluminum and PVC windows. Car windows are sealed against dust, wind, smoke, etc. using car window seal strips. Boat and submarine window seals are considered highly efficient seals as they are used to seal windows and doors against pressurized water as well. Examples of these seals are shown in Figure 1.
Windows and doors have a variety of dimensions and shapes, so sealing strips are manufactured as long strips and cut to the required dimensions. The polymer extrusion process is used mainly in the manufacturing of these sealing strips as it is used to produce long products with specified shapes. In order to increase sealing performance, seals are provided with inner metallic supports. The suitable manufacturing process for these modified seals is the polymer coextrusion process.
Rubber Extrusion
In the rubber extrusion process, Figure 2, rubber pellets are fed into a barrel through a hopper or a feeding system. A rotating screw, placed inside the barrel, pushes the pellets forward towards a fixed die of a specific shape, similar to the required product shape. This motion produces friction between the pellets themselves and between the pellets and the solid surfaces of the barrel and extruder producing a large amount of heat. This heat in addition to the input heat from the outer heaters softens the rubber pellets. As the softened product leaves the die, it is cooled down through a cooling system, and then collected using pull rolls. The main elements of the extrusion machine are the feeding system, barrel, screw, heaters, die, cooling system, and pulling system.
Feeding System
The feeding system of rubber pellets is determined according to the production rate. In low production rates, manual feeding is utilized through a hopper, Figure 3, where pellets are fed manually periodically. While in high production rates, an automatic feeding system is used, Figure 4, where the pellets are fed continuously.
Barrel
The barrel, Figure 5, is the container where the extrusion process takes place. The feeding system is attached on one side and the die on the other side. It is surrounded by heaters which help in softening the rubber pellets. The barrel is manufactured using materials of high wear and corrosion resistance, such as hardened 45 steel, 40Cr, ammoniated steel, 38CrMoAl, etc.
Barrel Heaters
Most barrel heaters are band electric heaters. There are several types, Figure 6. Cast heaters are cast aluminum split cylinders with insulated resistance wires embedded inside. The simplest type of heater is the mica band heater. They contain coated resistance wires sandwiched between mica insulation with a steel enclosure for support. Ceramic heaters are used in applications with high heat requirements. Both cast aluminum and mica band heaters need tight contact with the barrel as they will burn out if they cannot transfer their heat to the barrel.
Heaters are provided by thermocouples to ensure that the required temperature has been attained. It works continuously till the measured temperature is the same as the specified temperature, then it is switched off automatically. In some cases, the barrel is provided by a cooling system to decrease the rubber temperature if its temperature increases extensively or if the extruder must be shut down quickly. Barrels can be cooled using air or liquid. Barrels that are air-cooled have blower units and a plenum chamber under the barrel at each temperature zone. The blower is turned on by the barrel temperature controller when an upper setpoint is reached. Barrels that are cooled by liquid circulate the fluid through coils cast into the heater blocks, or through separate cooling coils. These can be closed loops, re-using the same liquid, or open loops. For the aim of safety, barrel and heaters are insulated using insulation jackets, Figure 7.
Extrusion Screw
The rotating Archimedes’ screw, Figure 8, is considered the “heart” of the extruder and has a significant influence on product quality, throughput, and efficiency of the production plant. In extruders, rubber pellets are plasticized by converting friction into heat through the influence of screw design. To achieve the highest possible plasticizing capacity, it is necessary to generate as much friction as possible. Typically, extruder screws can be divided into three main zones:
- Feed (conveying) zone: in the area, the core diameter of the screw is small, so that the material can flow into the screw channel and transported axially, and be compacted at the same time without strong friction.
- Compression (transition or plasticizing) zone: in this region, the core diameter of the screw increases continuously, decreasing the free volume, therefore friction is increased, and pressure is built up. The essential plasticizing process takes place within this zone
- Metering (pumping) Zone: In this section, the screw core diameter is maintained at the maximum value, where the material is homogenized and ejected from the die.
Common extruder screw materials are made of 45 steel, 40Cr, ammoniated steel, 38CrMOAl, 34CrAlNi, 31CrMo12, etc. It is plated with a layer of chromium or nitride to improve its corrosion resistance and wear resistance.
Extrusion Die
An extrusion die is a precise and specific tool made by cutting an opening through a blank of alloy steel. The shape of the die opening should match the desired cross-section of the product. Because of the viscoelastic nature of rubber and the associated flow behavior, it is not simple to design a die that will produce a smooth extrudate with the desired product dimensions. Rubbers tend to swell when passing through the extrusion die, causing an increase in dimensions.
Typically, rubber products from extrusion are cooled by pulling through a set of water tanks. The Cooling Tank requires a supply of cooling water. This can be done using several ways. Either directly dispensed to the tank from the plant water or a chilled water system. This could utilize two types of cooling systems:
1) A simple inlet/outlet of the water through the process with no recirculation in the tank.
2) A recirculation system to reduce water consumption from the plant.
The cooling water is then washed across the product as it is pulled through the tank. Some type of support is usually mounted inside the tank to hold the part while it is being cooled by the water. The water is either sprayed on the part from multiple directions to help the uniformity of the cooling or the tank is flooded with water so that the part is immersed in the water as it passes through the tank. Spray tanks generally offer more efficient cooling with the added cost and maintenance of the spray nozzles. Commonly, more than one cooling tank is required to ensure that the part is cooled sufficiently and maintains dimensional stability once it leaves the last tank in the line.
Pulling System
Pulling systems consist of a series of successive pulling rolls, where the rubber strips are collected around for further processing.
Rubber Extrusion Materials
The characteristics of extruded rubber products depend on their chemical and physical properties, such as elasticity, thermal and electrical insulation, and shock-absorbing properties. The commonly used materials are:
Butadiene Rubber (BR): most of the BR produced are used to make tires, while some are blended with other rubbers to improve the final product’s properties. Generally, BR has good cracking, abrasion, and rolling resistance but is prone to ozone degradation.
Styrene-Butadiene Rubber (SBR): it is one of the most important types of rubber which are used to manufacture tires, gaskets, and shoes. SBR is preferred due to its better abrasion, tear, and thermal resistance than natural rubber.
Acrylonitrile Butadiene (Nitrile) Rubber (NBR): It is the most common rubber type when it comes to automotive applications because of its resistance to petroleum-based chemicals such as oils, fuels, and grease. Thus, they are used as excellent gaskets and sealants. However, they have low tensile strength and poor low-temperature performance, which is mitigated by compounding reinforcing fillers.
Ethylene Propylene Rubber (EPM, EPDM): When diene is not used, the result is referred to as EPM and is only curable by peroxide. Both EPDM and EPM have good weathering resistance, good insulating and dielectric properties, excellent mechanical properties both at high and low temperatures, and chemical resistance.
Butyl Rubber (IIR): its low unsaturation enables it to repel most chemicals (gas and liquids) and is highly resistant to aging when vulcanized properly. Because of this property, IIR is used to produce O-rings, gaskets, window and door trims, and hoses.
Halogenated Butyl Rubber (CIIR, BIIR): it improves the ozone and chemical resistance of the material but decreases its insulation capabilities and water resistance when compared to IIR. Regardless, its permeability to moisture and gas is still low compared to other rubber types. Its areas of application are similar to IIR.
Chloroprene (Neoprene) Rubber (CR): it is widely used in the production of weatherstripping seals, gaskets, and hoses because of its inherent chemical inertness.
Fluorocarbon (Viton) Rubber (FKM): it is known commercially as Viton, which is a trademark of DuPont. FKM is generally known to be highly resistant to almost all chemicals, aside from having good mechanical properties. They are perfect to produce cord stocks for making O-rings and other sealing products.
Natural Rubber (NR): it is preferred because of its excellent heat buildup and fatigue resistance compared with other rubbers.
Isoprene Rubber (IR): it has good heat buildup and fatigue resistance, on top of its inherent impact-absorbing capability. Thus, they are well-suited to producing shock absorbers and rubber bushings.
Silicone Rubber: it is characterized by good oxygen, ozone, heat, light, and moisture resistance. However, they are more expensive and have poorer mechanical properties than organic rubber. They are used in many industrial applications, but they are most popularly used in food and drug manufacturing.
Rubber Coextrusion
The rubber coextrusion process, Figure 11, involves the extrusion of multiple materials simultaneously. It can be considered as an upgrade for the typical extrusion process. It allows compatible thermoplastic materials to be fed from multiple extruders into the same feed block and multilayer die assembly. Figure 12 shows dual durometer rubber, where the harder rubber is used for attachment while the softer rubber is used for sealing.
Coextrusion of several layers allows polymers to interact, improving the bond strength between plastic layers in a unique laminate construction. Through the coextrusion process of multilayer thin-film to thick-sheet laminate, constructions can be made having bond interfaces of the highest integrity. It can be used to produce metal-supported rubber, where a metallic insert is added to the rubber products for support.
However, the process produces better rubber products than that which are produced by extrusion, it is considered as a complicated process. The extruded polymer types should be compatible chemically and physically, and the temperatures and the flow rates of the extruded parts should be consistent. The die is complicated as it should be designed to mix the extrudes and produces the required product.
Conclusion
As a summary, sealing strips used in windows and doors of buildings and transportation are made of rubber. Typically, they are manufactured using two main processes, extrusion and coextrusion, where each has been discussed in detail. Extrusion process is used to produce seals made of one material type. Coextrusion is an upgrade of the extrusion process, where multi-layer extrudes can be produced using several extruders. In addition, it can be used in the production of seals with metal inserts.