What is Extrusion?
Extrusion manufacturing allows producers to shape and produce things with a uniform cross-sectional profile. To make a continuous shape with a homogeneous cross-section, a material, often a thermoplastic or a flexible metal, is driven through a die or an aperture.
Extrusion Ratio
The extrusion ratio is the final extrusion’s starting cross-sectional area / cross-sectional area.
The extrusion ratio is important because it indicates how much mechanical work the machine must perform during the extrusion. Important to note is that when the extrusion ratio is low, a minimal amount of mechanical work has to be performed during the extrusion process.
When the extrusion ratio is high, on the other hand, more pressure is required. This can be solved by adjusting the force that flows through the machines, otherwise specified as the throughput or feed rate.
- You need to dispense more material in your feed throat. Most of the biggest blenders in the industry allow you to dispense up to 1600 kg/h (throughput) in the material.
- You need to adjust the screw speed to ensure a low extrusion ratio.
Why is the extrusion ratio important?
The extrusion ratio is vital for manufacturers because their machines will perform over the years it’s running. It affects not only the degree of deformation and the flow character of the extruded material but also the strength parameters of this process. An extruder, especially the barrel, and die, is subject to wear and tear, and the machine needs manual adjustment to ensure efficient use of material and consistent product quality.
Extrusion Types
Double extrusion (co-extrusion)
Co-extrusion is forming an extrudate composed of more than one thermoplastic melt stream. The process came about because some service demands, particularly from the packaging industry, could not be satisfied by a single polymer, although a combination of polymers could meet them. Co-extrusion was first practiced in cast film production and is now used in blown film and sheet extrusion.
The co-extrusion process is used in many extrusion processes to bring two or more extruder outputs together to form a single multilayer product that has
- Improved performance
- Product differentiation
- Cost savings
- Added functionality
- Reduced handling
- High throughput
Plastic Extrusion
Plastic extrusion is a manufacturing process in which raw plastic (predominantly polymer) is melted and pushed through a die to create a continuous profile. Extrusion manufacturing in plastic produces pipe/tubing, fencing, deck railing, window frames, and sheeting.
The process of creating extruded plastic products starts by dosing and feeding plastic materials, like powder, pellets, and granules, from a hopper, into the barrel of the extruder. After the material is melted and forced through the screw, the molten material is forced through the die. After your extruded product exits the die, it’s shaped and cooled.
Blow molding is exceptionally popular for making other products like plastic bottles. For the plastic toy industry, injection molding is more suitable. Do you want to know the difference? Please read our article: Injection Molding vs. Blow Molding.
Film Extrusion
Plastic film is a thin, continuous material applied widely in the food packaging industry. A thicker plastic material is often called a “sheet.” Plastic film separates areas or volumes, holds items, and acts as barriers or printable surfaces. It’s most commonly used for packaging, confectionary wrapping, photographic film, or even to wrap a complete helicopter for transport.
Tubing Extrusion
Tubing Extrusion. The tubing or extruded tubing process manufactures medical tubing and drinking straws. This process is similar to the regular extrusion process until the die and actual extrusion stage.
Plastic Extrusion Process
Understanding the different processes involved in extrusion is essential for achieving precise and efficient production outcomes. The plastic extrusion process generally consists of the following steps:
Material Preparation
In extrusion, you have the option to use a multitude of polymers. However, materials processing is complicated and requires picking the right material to get to a product that is suitable for selling. You will need a polymer resin, also known as a polymer carrier, for your production. Depending on your desired product, you might consider adding a pigment, among other optional additives like UV absorbers, anti-fracture processing aids, antioxidants, or PET chain extenders.
Synthetic materials commonly used for extrusion processes include:
Polyethylene (PE)
A versatile thermoplastic polymer known for its excellent chemical resistance, durability, and low cost. It is widely used in various applications, including packaging films, pipes, tubes, and wire insulation.
Polypropylene (PP)
Another popular thermoplastic polymer known for its high strength, chemical resistance, and good thermal stability. PP is commonly used in packaging, automotive parts, textiles, and household goods.
Polyvinyl Chloride (PVC)
A widely used thermoplastic polymer known for its excellent electrical insulation properties, durability, and flame resistance. PVC is commonly used in construction materials, pipes, cables, and window profiles.
Polystyrene (PS)
A versatile thermoplastic with good rigidity, impact resistance, and clarity. It is commonly used in packaging materials, disposable cutlery, and insulation panels.
Polyethylene Terephthalate (PET)
A thermoplastic polymer known for its excellent strength, transparency, and resistance to moisture and chemicals. PET is widely used in beverage bottles, food packaging, and synthetic fibers.
Acrylonitrile Butadiene Styrene (ABS)
A tough and rigid thermoplastic known for its high impact resistance and heat resistance. ABS is commonly used in automotive parts, electronic housings, and consumer goods.
Polycarbonate (PC)
A transparent thermoplastic known for its high impact strength, optical clarity, and heat resistance. PC is commonly used in eyeglass lenses, automotive parts, and electronic components.
Polyamide/Nylon (PA)
A solid and durable thermoplastic known for its excellent mechanical properties, heat, and chemical resistance. Nylon is commonly used in engineering applications, textiles, and automotive components.
These are just a few examples of synthetic materials used in extrusion processes. Many other types of plastics and polymer compounds are available, each with unique properties and applications.
Extruder Setup
Mounting the extruder machine involves careful installation and precise calibration to ensure proper alignment.
The next step is installing the appropriate extrusion die or tooling, a task that plays a pivotal role in determining the final product’s shape and size.
Consideration of screw design is essential, particularly focusing on the cylindrical design of the barrel. This aspect significantly influences accuracy of your material dispersion and contributes to achieving the desired product specifications.
Dosing
Dosing is all about accurately measuring and controlling the precise amount of additives, colorants, or other components to be mixed with the base plastic material.
Manufacturers use dosing equipment or systems to introduce these additional substances into the extruder in a controlled manner.
Want to know more about how we help plastic manufacturers in extrusion dose materials accurately? Visit our website!
Melting
Feeding the plastic material into the extruder barrel using a screw mechanism. Applying heat and mechanical energy to melt plastic creates a homogenous molten mass. Melt extrusion is melting polymer pellets by applying heat and friction through the extruder’s barrel.
The molten polymer is then pushed through a tiny hole or, more often, a “shower head” of holes called a spinneret. As the molten polymer stream moves away from the spinneret, it hardens into a solid fiber. This fiber is pulled and heated repeatedly as it moves along the extrusion line to the end product.
The most significant prerequisite for material choice is that the polymer must melt and be able to handle strong shear forces without losing too much of its molecular weight.
To produce a high-quality product, manufacturing engineers need to ensure correct particle size by establishing the melting rate and incorporating gradual melting by applying heat within the barrel. You’ll need the utmost constant heat to ensure the proper viscosity of your product. If any of these steps are not executed perfectly, it will result in a distorted product.
Feeding the screw
After the material is fed into the screw, a combination of heating and throughput elements pushes the material through the die to form the extruded product.
Alternatively, some manufacturers melt material by heating the die and pulling through the forced material. This technique is also known as pultrusion. Extrusion pushes material through a die, whereas pultrusion pulls the material through. Pultrusion utilizes composites reinforced with long fiber strands, such as carbon, kevlar, or glass.
Homogenization and Pressure Build-up
The extruder continuously mixes and homogenizes the molten plastic using the rotating screw.
Increasing pressure on the molten plastic ensures its proper flow and eliminates air bubbles or impurities.
It’s also possible to eliminate most of the force at which the material is pushed through the die. A common technique in the industry is pultrusion. The disadvantages include that the process is limited to constant or near constant cross-section components, and costs for heated dies can be high. However, pultrusion also has numerous benefits. When using pultrusion, you’ll have a product:
- With high strength.
- With low weight.
- With excellent corrosion and rot resistance.
- With insensitivity to electricity
- With more excellent uniformity of products of the same type.
- With an extremely low number of manufacturing defects in products.
Extrusion speed
The extrusion speed refers to the meters per hour after the molten material is forced through the die. It’s essential to measure because you want to know how much you produce.
The challenge is measuring how much material you use to produce a product. When you can see how much material you used for a certain amount of the final product, you can automate your order systems and save significant costs on material usage.
Moreover, knowing the line speed does not tell you anything about the thickness of your product, which is especially important for hollow products.
Plastic Extrusion Profiles (dies)
In manufacturing precise extrusion shapes, dealing with a professional manufacturer is vital. The die is something you should never save money on. Why? A slight discrepancy will result in faulty production, and a low-quality die will deteriorate quickly.
A popular way to solve this problem in the industry is to use line control.
In essence, line control is a wheel that measures the line speed of your extrusion. To elaborate, it spins so the wheel can measure the approximate thickness of your product within a few meters of the product leaving the die.
It works like this: By gravimetrically measuring the material usage, the dosing solution can communicate with the rotating wheel (line control) behind your die. The wheel is connected to technology that allows you to communicate with the feeder to dispatch more/less material to get the correct product thickness.
We’re glad to have helped dozens of companies eliminate tons of waste and look forward to working on more innovative solutions to increase efficiency in the extrusion industry.
Shaping and Cooling
After the plastic is hot enough to come out of the die, the heat must be removed without distorting product dimensions. There are three primary coolants — water, air, and a cooled metal surface, such as a roll. Heat-transfer fluids are used where temperatures above 100°C are required, as inside some rolls for sheet lines.
Water Cooling
The thickness and cross-sectional profile depend on the cooling rate when extruding plastic. Filaments are often cooled by running them through a bath of circulating water.
For specific polymers and particular uses, the polymer may be necessary to remain dry. For example, hydrophilic polymers that readily absorb water (hygroscopic) and are later used in extrusion (such as in a 3D printer) may pick up too much water if they are cooled in a water bath (water tanks). In those instances, air cooling only should be used.
Air Cooling
Air cooling in extrusion rapidly cools and solidifies the extruded product after it exits the die. It involves using ambient air or forced air circulation to lower the temperature of the material, allowing it to maintain its shape and structural integrity.
Here are a few requirements for air cooling to work most efficiently:
- Parts must be exposed to the blow-off for an adequate period.
- Air flow must be sufficient to dry all parts equally.
- The conveyor air cooling flow on the blowers should be adjustable.
- Blow-off should result in a clean, dry, better surface.
Although there are various published data for overall heat-transfer coefficients between polymers and the cooling medium, they are usually not specific to your operation and should be determined experimentally.
Metal Cooling
In extrusion, metal plate cooling refers to a method in which metal plates cool and solidify the extruded product. These plates are in direct contact with the extrusion, facilitating efficient heat transfer and enabling the product to solidify rapidly. The metal plates help maintain the extruded material’s shape, dimensions, and mechanical properties, ensuring high-quality end products.
Metal roll surfaces are common for cooling most flat sheets, extrusion coating, and some film. Most common sheets are multi-roll stacks; most are set vertically, and a few are at an angle, even horizontally, and may allow the melt to fall into a nip. Thin films and coatings may have only one big cooling roll, which the plastic contacts as it leaves the die (distance adjustable).
Post Production
Cutting and Finishing
After extruding the product, manufacturers trim and cut the product into the desired shapes or lengths. Mostly, this happens automatically after the material is pushed through the die.
Additional finishing processes, such as cooling, sizing, embossing, or surface treatment, may be applied depending on the specific product requirements.
Take-off and Winding
Most manufacturers are using a take-off system to carefully pull the extruded plastic product away from the die and maintain its shape and dimensions.
Winding is about changing the extruded product onto spools, reels, or other forms of packaging for storage, transportation, or further processing.
Quality Control and Inspection
Quality control aims to conduct regular checks throughout the extrusion process to ensure dimensional accuracy, surface quality, and adherence to specifications.
By monitoring critical parameters like temperature, pressure, and extrusion speed manufacturers can maintain process control and optimize product quality.
Post-Extrusion Processes
Additional post-extrusion processes may be required depending on the specific application, such as printing, cutting, assembly, surface finishes, or further customization.
Packaging and Distribution
After the post-extrusion processes, products are properly packaged to protect them during storage, shipping, and distribution.
Maintenance and Cleaning
Performing routine maintenance and cleaning of the extrusion equipment is imperative to ensure the extruders’ longevity and optimal performance.
Waste Management
Waste management has the aim to promote and focus on environmentally friendly practices, managing and properly disposing of any waste or scrap generated during the extrusion process.
Reference List
https://www.sciencedirect.com/topics/materials-science/coextrusion
https://www.ift.org/news-and-publications/food-technology-magazine/issues/2017/july/columns/processing-extrusion-and-applications-in-food-industry#:~:text=An%20Overview%20of%20Extrusion%20Technology&text=It%20is%20a%20widely%20used,cheese%2C%20bread%2C%20and%20more.
https://www.aluminiumtc.com.au/aluminium-extrusions/#:~:text=Aluminium%20Extrusions%20is%20a%20term,section%20of%20the%20die%20pattern.
https://en.wikipedia.org/wiki/Plastic_extrusion
https://www.sciencedirect.com/topics/engineering/melt-extrusion
https://www.intechopen.com/chapters/55427
https://www.sciencedirect.com/science/article/abs/pii/B9780128153604000043
