Polymers are giant molecules, or macromolecules, built from repeating smaller chemical units, forming the foundation of materials commonly known as plastics. Raw polymers, often delivered as pellets, powders, or viscous liquids, are not directly usable for manufactured goods. Polymer processing is the necessary series of manufacturing steps that converts these raw materials into functional, finished products with a defined shape and structure. This transformation involves three fundamental stages: heating the material to a fluid state, forcing it into a desired shape, and finally, cooling or curing it to lock that shape permanently.
Defining the Transformation: Goals of Polymer Processing
Polymer processing is a controlled transformation aimed at meeting specific performance and production requirements. Mechanically, the primary goal is to achieve a precise three-dimensional shape and dimension while optimizing the material’s physical properties, such as strength, stiffness, or surface finish. Economically, the objective is to enable highly efficient mass production, which significantly reduces the cost of individual parts.
The approach to processing is fundamentally decided by the polymer’s thermal behavior, separating them into two major classes. Thermoplastics soften and melt when heated, allowing them to be repeatedly shaped and solidified without chemical change. This makes them highly recyclable and easy to process in high-volume methods. In contrast, thermosets undergo an irreversible chemical reaction called cross-linking upon initial heating or curing, forming a rigid, three-dimensional network structure. Once cured, thermoset materials cannot be melted or reshaped, offering superior stability and resistance to high temperatures.
Shaping Polymers Through Molding Techniques
Molding techniques are used to create discrete, individual parts, often featuring complex geometries, by forcing the fluidized polymer into a closed cavity.
Injection Molding
Injection molding is the most common method for producing intricate, high-volume parts. A reciprocating screw melts and homogenizes the polymer, which is then forcefully injected at high pressure into a cooled mold cavity where it rapidly solidifies into the final component. The closed mold typically consists of two halves, and the process is characterized by a rapid cycle time, making it suitable for items ranging from bottle caps to automotive dashboards.
Compression Molding
Compression molding involves placing a pre-weighed charge of polymer, often a thermoset compound, directly into the open, heated mold cavity. The mold is then closed under high pressure, forcing the material to flow and fill the cavity while heat initiates the irreversible curing reaction. This method is frequently used for thicker parts that require high strength, such as electrical components and dinnerware, because the pressure eliminates internal voids.
Blow Molding
Blow molding is a method specifically designed for manufacturing hollow objects like bottles, containers, and fuel tanks. The process begins by creating a hollow tube or preform, called a parison, through extrusion or injection molding. The parison is clamped within a cooled mold, and compressed air is injected into the tube, inflating the softened polymer against the interior walls of the mold cavity. The air pressure causes the material to conform precisely to the mold’s shape, which includes features like neck threads, before it is cooled and ejected as a finished, hollow product.
Shaping Polymers Through Continuous Extrusion
Extrusion is a continuous, high-volume manufacturing process used to create products with a constant cross-sectional profile, such as pipes, tubing, films, and window frames. Raw polymer pellets are fed into a hopper and drop onto a rotating screw housed within a heated barrel. The turning screw serves multiple functions, conveying the material forward while its rotation and the barrel heaters melt the polymer into a uniform, viscous fluid.
The melted polymer is continuously pushed through a shaping tool called a die, which has an opening corresponding exactly to the desired profile. Unlike molding, the die is an open structure that allows the material to flow out continuously, rather than being contained in a closed cavity. The resulting polymer strand, film, or profile is then cooled, often using a water bath or air blowers, to solidify the material and lock in the shape. The speed of the screw rotation and the take-off speed are precisely controlled to maintain consistent dimensions and wall thickness throughout the run.