Plastics are ubiquitous today because of the amazing properties they possess. Some plastics are hard and shatter-resistant, some are soft and flexible, some are light yet tough, and yet others are good insulators and have low thermal conductivity. These varied properties make their applications very popular. In fact, plastics make possible many things that are not directly recognized as plastics, like paints, or adhesives. The range of items and parts manufactured by using plastics is quite exhaustive; with each day bringing in another way in which plastic can be used. Plastics have effectively permeated every aspect of modern day life, from cell phones to the simple grocery bag, and from medical equipment to remote control. Plastics are used in industries as diverse as aerospace, automobile, medical, and industrial, to electronics and food. 3D Printing and 3D Printers, one of the latest disruptive manufacturing technologies, is all set to explore more ways in which plastics can be used. In aerospace and automobile engineering, plastics are increasingly replacing metal parts, in medicine, single use disposable syringes are now de-facto standard for injections, in electronics plastics are used as electric connectors, for telephone handsets, and much more.
The industrial term for a plastic is ‘polymers’, and the raw plastic pellets are called resins. 'Poly' means many and 'mero' means part in Greek. ‘Polymers’ thus refer to a class of compounds that have multiple repeating units. Polymers are substances composed of macromolecules, which are nothing but very large molecules. The process of formation of polymers by joining the structural units is known as polymerization. While the layman uses the terms ‘plastics’ and ‘polymers’ interchangeably, there is a slight technical difference between the two. A plastic is an organic polymer, available in some form of resin or some form derived from the basic polymerized resin. Plastics are a subset of polymers, and are manufactured by additives that impart the required quality to the end material. Because of their large molecular size, polymers possess unique chemical and physical properties. These properties begin to appear when the polymer chain is of sufficient length—i.e., when the molecular weight exceeds a certain threshold value—and becomes more prominent as the size of the molecule increases.
Manufacturing Process
Plastic materials are the result of the combination of carbon elements reacting with oxygen, hydrogen, nitrogen, and other organic and inorganic elements. These polymers have the ability to change into a liquid (melt), and are capable of being formed into shapes by the application of heat and pressure. Plastics are a family of materials, not a single kind of material. The main processes for manufacturing plastic include plastic extrusion, injection moulding, blow moulding and vacuum forming. In the recent past, 3D printing has emerged as an important plastic manufacturing process; especially for sectors that require lightweight components or for parts with complex geometries. However, plastic injection moulding still rules the roost, especially for parts / objects that need to be manufactured in large quantities.
For the purpose of commercial manufacturing, the plastics are broadly sub-divided into two categories - thermoplastic resins and thermosetting resins. Thermoplastic resins consist of long polymer molecules, each of which may or may not have side chains or groups. The side chains or groups, if present, are not linked to other polymer molecules (i.e., are not cross-linked). One of the most important properties of thermoplasics is that they do not cure or set under heat as do thermosets. When heated to a suitable temperature, they become fluid and can be forced or transferred under pressure from a heated cavity into a cool mould. Once cooled in the mould, thermoplastics harden and take the shape of the mould. No chemical change usually takes place during this shaping process. Thermosetting plastics on the other hand, are cured or set into a permanent shape once heated to a suitable temperature. Curing or cross-linking is an irreversible chemical reaction, and usually occurs at a higher temperature. Some thermosets though set even at room temperature. On the whole, thermoplastics tend to be tougher and less brittle as compared to thermosets. With the progress of technology though, the demarcation between thermoplastic and thermoset processing has become less distinct. For thermosets, processes have been developed which make use of the economic processing characteristics or thermoplastics. Today, modified machinery and molding compositions have become available to provide the economics of thermoplastic processing to thermosetting materials.
The Plastic Injection Moulding Process
The plastic injection moulding process works by injecting molten raw plastic material (typically melted liquid resin) into a closed mould under high pressure. The liquid is formed by melting resin pellets by heating it in a barrel. It is then compressed and pushed forward with the help of a rotating screw in the barrel. The molten plastic then enters the mould cavity via gates and hot runners, and is allowed to cool rapidly. Once it solidifies, it is ejected from the opened mould.
The main components of an injection-molding machine are the injection unit which melts the moulding material from the feeding hopper and forces it into the mould through the screw, the clamping unit which opens the mould and closes it under pressure, the ejection unit and the electric and hydraulic unit that controls the machine.
What is Mouldflow Analysis?
There is no doubt about it – plastics have changed our world since they were commercially introduced in the early 1900’s. As plastics manufacturing technology continues to develop, the number of plastic products, particularly those replacing metals, has been on the rise. Resin manufacturers create new formulations with improved properties and moulds and dies are highly optimized for maximum part performance. However, plastic product failure is still a concern, and can be caused by improper design, improper manufacturing (including processing and assembly), and improper use of the end product. Since plastic injection moulding still remains one of the most popular processes of manufacturing plastic today - because of its capability of being mass manufactured - a thorough engineering analysis of the process is crucial. Designing of proper processes and choosing the right material is not an easy task, and this is where mould flow analysis plays a major role. In the next article, we will learn more about what mould flow means, what the challenges are in designing proper mould flow process, and how software like 3D Timon can be of help in mould flow analysis.