Engineering Plastics Categories
Across the broad spectrum of plastics, there are two main categories: thermosets and thermoplastics. The primary difference between the two being how they respond to applied heat. Simply put, thermoplastics will melt before they chemically degrade and thermoset will never melt once formed. The reason for these differences lies in how the polymers are formed. Thermoplastics are made up of polymers that lie in a single dimensional strand. Much like a strand of pearls, at high temperatures the polymers thaw out (i.e. relax) and can be remodeled into multitudes of shapes and patterns. Thus, heat is the medium that can continually soften, form and shape this class. An advantage is making a complex shape but a disadvantage when the wrong thermoplastics is chosen and steamed in a microwave to create a deformed container. The good news however is that the choice of thermoplastics and the temperature they melt at is quite extensive. Thermosets are formed from a chemical reaction where a starting polymer reacts with other chains to create a three-dimensional network that, once reacted, can no longer be melted by heat. A great idea when making a speedboat or a stealth bomber. When working with either thermoplastics or thermosets it is possible to use multiple forming processes but the designs need to accommodate a chemical reaction for one and a high pressure melt pumping for the other.
Thermoplastics as a base for a Successful Product
There are literally thousands of thermoplastics available and when one considers the variety of blends, additives, fillers and reinforcements the choices are limitless. The key starting point then is design. Consider the microwave container above. The desire is to have the look, feel, performance, heat resistance and of course the cost to have a successful product in the marketplace. Regardless of which thermoplastics forming process is used the key first step is for a designer to translate the customer’s final product desire into the proper plastic type, special additives to achieve the look and feel, physical design of the part for the engineering performance and function; and all the while making sure that the key geometries of the part support the manufacture of the final part with a cost that will ultimately allow the part to compete and be profitable.
In the production of thermoplastic parts there are four main process types: Injection, Extrusion, Blow Molding and Thermoforming. For each of these the starting point is the same. The base plastic (either in pellet, bead or powder form) is place into a holding bin or hopper which feeds into a heated barrel. At this point a variety of colors, additives, filler and fibers can also be added to create a unique set of properties and characteristics. However much like baking a cake, if the recipe is followed consistently, the results will be the same time and again. Inside the barrel is a flighted screw that will move (or pump) the plastic along the barrel as the plastic goes through various phases of softening, melting and finally massing to be finally ready to move into one of the four processes. If allowed to simply pump out of the end of the barrel, the material at this point would look very much like a solid piece of rope.
Plastic Injection Molding
In Injection molding, the molten material above masses into a holding chamber at the end of the screw. When filled to the predetermined level, the molten mass is pressed (“injected”) into the mold forming the shape of the final desired product. Since this process is performed at high temperatures and pressures, injection machines and molds are typically high strength and high power units. Unlike the other three processes, injection molding can be employed with virtually all of the thermoplastic resins available. While it is limited to single solid parts, the complexity of the molds to allow for moving slides, cores and pins can support a broad array of shapes and functions In order to create parts via the process of injection molding, they must be very carefully designed. Once the design for the part is complete, the molds are made by a mold maker (or toolmaker). Molds are usually made out of either steel or aluminum and precision-machined to form the features of the desired part. Mold temperature control also becomes a very key feature for controlling high speed production in injection molding (with cycles typically measured in seconds). The temperature of the mold is colder than that of the melted plastic to promote quick production cycles but to also control the dimensions and appearance of the finished parts. The contrast between the high temperature and pressure at which the plastic is injected and the coolness of the mold results in products that can have very detailed surface finishes from high gloss to highly detailed wood grains.
Injection molding allows complex shapes to be molded at high production rates while maintaining good dimensional stability. It is for that reason that injection molding is one of the most popular forms of processing plastics.
Extrusion molding is a direct collection of the molten rope described above. The key value in extrusion is that a shaped metal die is attached to the end of the barrel. Thus as the molten plastic exits through the die, it takes on the die shape and if cooled quickly enough it holds this shape. The most recognizable extrusion product is a soda straw where a tube is formed. Extrusion production is continuous and thus works very well for long continuous strands of product be they soda straws, medical tubing, window gaskets or home siding. Since it is continuous and the finished product needs to be continuously identical, the ability to maintain the final shape is key. The end product can be cut and finishing touches given or rolled up and shipped directly from the machine. Films, sheets, and pipes are common products that are produced through extrusion molding.
Blow molding is a plastic processing method that can combine extrusion and injection molding. The most direct blowing method is extrusion where a tube is extruded between two halves of a die. When the tube reaches the end of the die set, they close. This closes off the end and allows the top to have air injected. The air forces the still molten plastic up against the cold die walls forming a hollow container (such as a milk bottle). A more high volume technique first injection molds a parison (which will look much like a small test tube). This tube is then warmed and placed in between the two cold halves where again they close and air is injected to force the tube walls out to hit the die walls again forming a hollow container but at much higher production speeds (typical for water bottles)
Often viewed as a variation on extrusion, this process is extremely important to the Packaging industry as a means to achieve thin wall containers and is thus deserved of treatment as a unique process. Thermoforming begins with the Extrusion of a flat sheet. The sheet is then placed over a field of machined shapes (typically low pressure, easily machined materials such as aluminum). The sheet is heated to a softening temperature then drawn down typically with a vacuum to “stretch” the soft sheet over the shape. In this manner a thin sheet can be pulled into the shape of an even thinner wall container (such as yogurt cups). The formed sheet is then removed to a trimming station where the final shapes are either cut or punched out of the base sheet.
Plastics have an excellent surface finish without added operations beyond what is required to convert the plastic from a raw material into its final shape. However, additives can be introduced into the plastics to improve their properties, reduce the cost, improve their moldability, impart color and even add aromas and scents. Additives can be classified as fillers, plasticizers, coloring agents, or lubricants. Fillers are added to improve the strength or to decrease the cost of the plastics. Plasticizers are used to increase and control the flow of plastic during molding and/or make the plastic more flexible. Since plastics are usually clear or opaque, coloring agents are added in order to give the parts the color that is needed frequently to match the customer’s exact tint and look. Lubricants improve plastics’ moldability and facilitate removal of parts from the molds but also can change the surface finish and feel of the finished part.
Originally valued as a light weight means to achieve highly reproducible, mass produced objects replacing heavy metals, clays, glass, etc., plastics were often seen as “cheap” and less durable. However plastics have quickly evolved through a myriad of chemical types, additives and reinforcements to provide an engineering selection class of their own, while still offering the high speed production. It is now quite typical to see an assembly such as the coffee maker on your kitchen counter contain dozens of specific types of plastics and additives to provide a specific property all within a tight specification assembly that provides good electrical, food contact, engineering and aesthetics all in a complex unit that can sell for a very attractive price on the shelf..
Coupled with this broad array of plastic performance is an equally stunning array of high technology equipment in the plastics fabrication. Where highly precise computer driven injection presses tie in with multi axis robotics and automated assembly and packaging lines to support very accurate, very high speed production available in very few other materials.
As we move into the future the ability of plastics to be produced from naturally based materials, and to be recycled and reformed into other uses allows for a wise environmental partnership.
If we start with the customer’s product desires and design our choices to that focus we have a broad array of tools that will allow us to use plastics and optimize to a successful finale.