Plastic Extrusion How Extrusion Works
Taken from Geist Plastics, archived page
Plastic products take many different forms – known as plastic profiles to us – but they all start as raw material called resin. In plastic extrusion, resin beads are melted down, filtered for uniform consistency, pushed through a die cut for the final product shape, and then cooled. It’s a complicated process that takes many forms, but here’s a quick breakdown of how it works
1. Raw plastic is fed into a hopper – along with any colorants or additives – that feeds down into the extruder.
2. The beads flow from the hopper down through the feed throat, which dispenses them onto a large spinning screw operating within a horizontal barrel.
3. The beads pass through the barrel on the screw while being heated to the melting temperature specific to that type of plastic. By the time it reaches the end of the screw, the plastic is thoroughly mixed and has a consistency like thick bubble gum.
4. At the end of the screw, the extruded plastic flows across a screen and a breaker plate, which serve two functions. The screen removes any contaminants or inconsistencies in the plastic, and the breaker plate changes the motion of the plastic from rotational to longitudinal. It’s now ready for the die.
5. The die for an individual plastic profile is more than just a hollowed-out shape of the final product. It’s designed so that the plastic flows smoothly and evenly from the cylindrical profile of the extruder into the final profile shape. Consistency in this flow is critical to achieving an end product with integrity.
Plastic extrusion is used to produce a huge range of products on the market today, from building materials to consumer products to industrial parts. Pipes, tubing, window frames, electrical covers, fence, edging, and weather stripping are just a few of the common items made by plastic extrusion, along with thousands of custom profiles.
Plastics Extrusion Process Choices
Naturally, with so many possible resultant products, the process of plastic extrusion begins with choices. Before getting started, it’s important to know which extrusion process will be needed to create the product profile. There are a number of variations on that process that are used to create plastic products. We’ve already described Profile extrusion, which is what we do here at Geist Plastics. Learn more about Geist Plastics capabilities by clicking here. The other main types of plastic extrusion are listed below
- Blown film extrusion is used to create thin plastic film like that in most plastic shopping bags. The die used is an upright cylinder from which the molten plastic is pulled upward at varying distance and speed – these factors determine the thickness of the film. Compressed air is moved up the center of the film to create a bubble, from which thin sheets can be cut, spooled, or sealed into shape.
- Sheet/film extrusion uses a number of cooling rolls in succession to create even, flat profiles of plastic. This process is often used for profiles that will receive an additive layer or to create sheets that will undergo thermoforming to become yet another new shape.
- Co-extrusion is the simultaneous extrusion of multiple plastic profiles that then pass through the same die to become a more complex product. This process can be used to create layers or products with an outer coating of a different plastic makeup.
Choosing Plastic Types
Perhaps nothing, though, is as important as choosing the right plastic material for each individual profile. Dozens of materials can be extruded, each chosen for a unique set of properties. Polyethylenes, PVCs, polypropylenes, and styrenes vary greatly in molding and cooling temperature, strength, integrity, ability to withstand chemicals or weather, production cost, and more.
For example, PVC products and polystyrene products are often chosen for their low cost and high dimensional stability, but don’t have the temperature resistance or strength for other applications. Polypropylene sheet products have great chemical and moisture resistance. These are just a few of the available types of plastic for the plastic extrusion process – if your product has unique material requirements, there’s a plastic profile for you.
For further information on choosing the right material for your extruded plastic product, take a look at http://www.iplasticsupply.com/wp-content/uploads/2011/07/iplasticsupply-plastic-material-selection-chart.pdf this chart by the International Association of Plastics Distribution which lists many of the most common plastics and their advantages and disadvantages.
Taken from Geist Plastics, archived page