Injection molding
From Plastics Wiki, free encyclopedia
Injection molding (or Injection Moulding) is a manufacturing technique for making parts from plastic material. Molten plastic is injected at high pressure into a mould, which is the inverse of the desired shape. The mould is made by a mouldmaker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection moulding is very widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. It is the most common method of production, with some commonly made items including bottle caps and outdoor furniture.
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Types of Injection Molding
- Reaction injection molding
- Liquid injection molding
- Gas assist injection molding
- Co-injection molding
- 2-Shot Injection Molding
- Fusible core injection molding
- Rapid injection molding
Injection molding equipment
Injection molding machine
Injection molding machines, also known as presses, hold the molds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can generate. This pressure keeps the mold closed during the injection process. Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations.
Horizontal or vertical machines
Injection molding machines can fasten the molds in either a horizontal or vertical position. The majority is horizontally oriented but vertical machines are used in some niche applications such as insert molding, allowing the machine to take advantage of gravity.
Clamping unit
Machines are classified primarily by the type of driving systems they use: hydraulic, electric, or hybrid. Hydraulic presses have historically been the only option available to molders until Nissei introduced the first all electric machine in 1983. The electric press, also known as Electric Machine Technology (EMT), reduces operation costs by cutting energy consumption and also addresses some of the environmental concerns surrounding the hydraulic press. Electric presses have been shown to be quieter, faster, and have a higher accuracy, however the machines are more expensive. Hybrid injection molding machines take advantage of the best features of both hydraulic and electric systems. Hydraulic machines are the predominant type in most of the world, with the exception of Japan.
Injection mold
Considerable thought is put into the design of moulded parts and their moulds, to ensure that the parts will not be trapped in the mould, that the moulds can be completely filled before the molten resin solidifies, to compensate for material shrinkage, and to minimize imperfections in the parts, which can occur due to peculiarities of the process.
Injection process
Injection Molding Cycle
The basic injection cycle is as follows: Mold close - injection carriage forward - inject plastic - metering - carriage retract - mold open - eject part(s)
The molds are closed shut by hydraulics or electric, and the heated plastic is forced by the pressure of the injection screw to take the shape of the mold. Some machines are run by electric motors instead of hydraulics or a combination of both. The water-cooling channels then assist in cooling the mold and the heated plastic solidifies into the part. Improper cooling can result in distorted molding or one that is burnt. The cycle is completed when the mold opens and the part is ejected with the assistance of ejector pins within the mold.
Molding trial
When filling a new or unfamiliar mold for the first time, where shot size for that mold is unknown, a technician/tool setter usually starts with a small shot weight and fills gradually until the mold is 95 to 99% full. Once this is achieved a small amount of holding pressure will be applied and holding time increased until gate freeze off has occurred, then holding pressure is increased until the parts are free of sinks and part weight has been achieved. Once the parts are good enough and have passed any specific criteria, a setting sheet is produced for people to follow in the future.
Molding defects
Injection molding is a complex technology with possible production problems. They can either be caused by defects in the molds or more often by part processing (molding)
History
In 1868, John Wesley Hyatt became the first to inject hot celluloid into a mold, producing billiard balls. He and his brother Isaiah patented an injection molding machine that used a plunger in 1872, and the process remained more or less the same until 1946, when James Hendry built the first screw injection molding machine, revolutionizing the plastics industry. Roughly 95% of all molding machines now use screws to efficiently heat, mix, and inject plastic into moulds.
Process: Injection molding cycle Reaction injection molding, Reinforced reaction injection molding, Liquid injection molding, Gas-assist injection molding, Coinjection molding, 2-shot injection molding, Fusible core injection molding, Rapid injection molding, Micro injection molding
Equipment: Injection molding machine(Vertical, Horizontal), Injection molding press, Injection mold (Hot runner, Cold runner), Injection unit (Injection ram, Injection screw), Clamping unit (Toggle , Hydraulic, Hydromechanical), Plunger injection molding, Injection molding machine manufacturers, Types of injection molding machines
Terms: Injection pressure, Injection rate, Clamping pressure
See also: Plastic thermoforming • Blow molding • Plastic welding • Plastic extrusion • Rotational molding
