Injection molding

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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.

1 Types of Injection Molding
2 Equipment
- 2.1 Injection molding machine
  - 2.1.1 Horizontal or vertical machines
  - 2.1.2 Clamping unit
- 2.2 Injection mold
3 Injection process
4 History

Types of Injection Molding

Main article: Types of 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

Main article: 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

Main article: 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

Main article: 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

Main article: Plastic processing troubleshooting

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)

Molding Defects	Alternative name	Descriptions	Causes
Blister	Blistering	Raised or layered zone on surface of the part	Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater
Burn Marks	Air Burn/ Gas Burn	Localized burnt zone (often in the yellow/brown tones)	Tool lacks venting, injection speed is too high
Color Streaks		Localized change of color	Masterbatch isn't mixing properly, or the material has run out and it's starting to come through as natural only
Delamination		Thin mica like layers formed in part wall	Contamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond
Flash	Burrs	Excess material in thin layer exceeding normal part geometry	Tool damage, too much injection speed/material injected
Embedded contaminates	Embedded Particulates	Foreign particle (burnt material or other) embedded in the part	Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection
Flow marks		Directionally "off tone" wavy lines or patterns	Injection speeds too slow (the plastic has cooled down too much during injection, injection speeds must be set as fast as you can get away with at all times)
Jetting		Deformed part by turbulent flow of material	Poor tool design
Silver streaks		Circular pattern around gate caused by hot gas
Sink Marks		Localized depression (In thicker zones)	Holding time/pressure too low, cooling time too low, with sprueless hot runners this can also be caused by the gate temperature being set too high
Short shot	Non-Fill / Short mold	Partial part	Lack of material, injection speeds too slow
Splay Marks	Splash mark / Silver Streaks	Circular pattern around gate caused by hot gas
Stringiness		String like remain from previous shot transfer in new shot	Gate hasn't frozen off
Voids		Empty space within part (Air pocket)	Lack of holding pressure (holding pressure is used to pack out the part during the holding time)
Weld line	Knit Line	Discolored line where two flow fronts meet	Mold/material temperatures set too low (the material is cold when they meet, so they don't bond)
Warping	Twisting	Distorted part	Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the cool side of the tool)

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.