Injection moulding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mould cavity where it cools and hardens to the configuration of the cavity. After a product is designed, usually by an industrial designer or an engineer, moulds are made by a mould maker (or toolmaker) from metal, usually either steel or aluminum, and precision-machined to form the features of the desired part. Injection moulding is widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars.
1847 Jons Jacob Berzelius produces first condensation polymer, polyester, from glycerin (propanetriol) and tartaric acid
Jons Jacob Berzelius is also credited with originating the chemical terms “catalysis,” “polymer,” “isomer,” and “allotrope,” although his original definitions differ dramatically from modern usage. For example, he coined the term “polymer” in 1833 to describe organic compounds which shared identical empirical formulas but which differed in overall molecular weight, the larger of the compounds being described as “polymers” of the smallest. According to this (now obsolete) definition, glucose (C6H12O6) would be a polymer of formaldehyde (CH2O).
The first man-made commercial plastic was invented in Britain in 1861 by Alexander Parkes. He publicly demonstrated it at the 1862 International Exhibition in London, calling the material he produced “Parkesine.” Derived from cellulose, Parkesine could be heated, moulded, and retain its shape when cooled. It was, however, expensive to produce, prone to cracking, and highly flammable.
In 1868, American inventor John Wesley Hyatt developed a plastic material he named Celluloid, improving on Parkes’ invention so that it could be processed into finished form. Together with his brother Isaiah, Hyatt patented the first injection moulding machine in 1872. This machine was relatively simple compared to machines in use today. It worked like a large hypodermic needle, using a plunger to inject plastic through a heated cylinder into a mould. The industry progressed slowly over the years, producing products such as collar stays, buttons, and hair combs.
The industry expanded rapidly in the 1940s because World War II created a huge demand for inexpensive, mass-produced products. In 1946, American inventor James Watson Hendry built the first screw injection machine, which allowed much more precise control over the speed of injection and the quality of articles produced. This machine also allowed material to be mixed before injection, so that colored or recycled plastic could be added to virgin material and mixed thoroughly before being injected. Today screw injection machines account for the vast majority of all injection machines. In the 1970s, Hendry went on to develop the first gas-assisted injection moulding process, which permitted the production of complex, hollow articles that cooled quickly. This greatly improved design flexibility as well as the strength and finish of manufactured parts while reducing production time, cost, weight and waste.
The plastic injection moulding industry has evolved over the years from producing combs and buttons to producing a vast array of products for many industries including automotive, medical, aerospace, consumer products, toys, plumbing, packaging, and construction.
Injection moulding is used to create many things such as wire spools, packaging, bottle caps, automotive dashboards, pocket combs, some musical instruments (and parts of them), one-piece chairs and small tables, storage containers, mechanical parts (including gears), and most other plastic products available today. Injection moulding is the most common method of part manufacturing. It is ideal for producing high volumes of the same object. Some advantages of injection moulding are high production rates, repeatable high tolerances, the ability to use a wide range of materials, low labor cost, minimal scrap losses, and little need to finish parts after moulding. Some disadvantages of this process are expensive equipment investment, potentially high running costs, and the need to design mouldable parts.
Examples of polymers best suited for the process
Most polymers, sometimes referred to as resins, may be used, including all thermoplastics, some thermosets, and some elastomers. In 1995 there were approximately 18,000 different materials available for injection moulding and that number was increasing at an average rate of 750 per year. The available materials are alloys or blends of previously developed materials meaning that product designers can choose from a vast selection of materials, one that has exactly the right properties. Materials are chosen based on the strength and function required for the final part, but also each material has different parameters for moulding that must be taken into account.Common polymers like epoxy and phenolic are examples of thermosetting plastics while nylon, polyethylene, and polystyrene are thermoplastic. Until comparatively recently, plastic springs were not possible, but advances in polymer properties make them quite practical. Among such applications are buckles for anchoring and disconnecting outdoor-equipment webbing.
With injection moulding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly moved forward by a screw-type plunger, the plastic is forced into a heated chamber, where it is melted. As the plunger advances, the melted plastic is forced through a nozzle that rests against the mould, allowing it to enter the mould cavity through a gate and runner system. The mould remains cold so the plastic solidifies almost as soon as the mould is filled
Injection moulding cycle
The sequence of events during the injection mould of a plastic part is called the injection moulding cycle. The cycle begins when the mould closes, followed by the injection of the polymer into the mould cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw.This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mould opens and the part is ejected.
Optimal process settings are critical to influencing the cost, quality, and productivity of plastic injection moulding. The main trouble in injection moulding is to have a box of good plastics parts contaminated with scrap. For that reason process optimization studies have to be done and process monitoring has to take place. First article inspection of internal and external geometry including imperfections such as porosity can be completed using Industrial CT Scanning, a 3D x-ray technology. For external geometry verification only a Coordinate-measuring machine or white light scanner can be used.
Tolerances and surfaces
Moulding tolerance is a specified allowance on the deviation in parameters such as dimensions, weights, shapes, or angles, etc. To maximize control in setting tolerances there is usually a minimum and maximum limit on thickness, based on the process used. Injection moulding typically is capable of tolerances equivalent to an IT Grade of about 9–14. The possible tolerance of a thermoplastic or a thermoset is ±0.008 to ±0.002 inches. Surface finishes of two to four microinches or better can be obtained. Rough or pebbled surfaces are also possible.
|Moulding Type||Typical [In]||Possible [In]|
Lubrication and cooling
Obviously, the mould must be cooled in order for the production to take place. Because of the heat capacity, low cost, and availability of water, water is used as the primary cooling agent. To cool the mould, water can be channeled through the mould to account for quick cooling times. Usually a colder mould is more efficient because this allows for faster cycle times. However, this is not always true because crystalline materials require the opposite: a warmer mould and lengthier cycle time.
The power required for this process of injection moulding depends on many things and varies between materials used. Manufacturing Processes Reference Guide states that the power requirements depend on “a material’s specific gravity, melting point, thermal conductivity, part size, and moulding rate.” Below is a table from page 243 of the same reference as previously mentioned that best illustrates the characteristics relevant to the power required for the most commonly used materials.
|Material||Specific gravity (g/cm3)||Melting point (°F)||Melting point (°C)|
|Epoxy||1.12 to 1.24||248||120|
|Phenolic||1.34 to 1.95||248||120|
|Nylon||1.01 to 1.15||381 - 509||194 - 265|
|Polyethylene||0.91 to 0.965||230 - 243||110 - 117|
|Polystyrene||1.04 to 1.07||338||170|
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