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The Types Of Polymers In Plastic Injection Molding


Injection molding is a highly productive and efficient method for manufacturing plastic products. The injection molding cycle involves injecting molten plastic into a mold and then cooling it down to form a solidified part.

Thermoplastic polymers are more common than thermosets in injection molding. Thermoplastics are plastics that can repeatedly melt or soften when heated and solidify when cooled, making them a recyclable material. Excess materials from the previous molding cycle are re-crushed and added back to the injection chamber along with the primary pellets, but their addition is limited to a maximum of 30% bulk material, as this may degrade the original physical properties of the plastic.

In contrast, thermoset plastics can only be formed once after initial exposure to heat due to the crosslinking of its polymer chains. In this case, the molten mass of thermosetting materials must be immediately transferred to the mold in order to avoid settling of screws and valves, which can lead to damage to the injection unit. However, these plastics are valued for their strength and rigidity. They are extremely resistant to high temperatures.

#1 The Types Of Resins Used In Injection Molding

The following are some of the most commonly used materials in plastic injection molding:

1.Acrylonitrile Butadiene Styrene (ABS)

Acrylonitrile Butadiene Styrene (ABS)
Acrylonitrile butadiene styrene (ABS) is an opaque and amorphous thermoplastic. It is known for its lighter weight, stiffness, and resistance to impact, heat, and corrosive chemicals. Due to its low melting point, ABS consumes less thermal energy and is easily processed in an injection molding machine. ABS plastics are used in automotive parts, sports and entertainment equipment, and piping materials. The famous Lego toy is made from this material.

2.Polycarbonate

Polycarbonates are transparent thermoplastics that contain carbonates in the structure of their polymer chains. They are known for their strength, rigidity and impact resistance. Applications for polycarbonates include eyeglass lenses, bulletproof glass, automotive components, and containers.
Polycarbonates are transparent thermoplastics that contain carbonates in the structure of their polymer chains. They are known for their strength, rigidity and impact resistance. Applications for polycarbonates include eyeglass lenses, bulletproof glass, automotive components, and containers.

3.Nylon

Nylon is a thermoplastic made from polyamides. It is strong, flexible and resistant to impact and chemicals. It is sometimes reinforced with fiberglass to increase tensile strength. It is used where low friction is required. Nylon has a high melting point, making it an alternative to metals in high temperature environments, but it is also highly flammable. However, nylon is one of the polymers that is difficult to mold due to its hygroscopicity, shrinkage, and tendency to outgassing at high temperatures.
Nylon is a thermoplastic made from polyamides. 
It is strong, flexible and resistant to impact and chemicals. 
It is sometimes reinforced with fiberglass to increase tensile strength. 
It is used where low friction is required. 
Nylon has a high melting point, making it an alternative to metals in high temperature environments, but it is also highly flammable. 
However, nylon is one of the polymers that is difficult to mold due to its hygroscopicity, shrinkage, and tendency to outgassing at high temperatures.

4.Propylene

Propylene is a flexible, strong and fatigue resistant semi-crystalline thermoplastic. Propylene is also a good electrical insulator. It is used in packaging materials, automotive parts, and household and office products. Despite its semi-crystalline nature, propylene has a low melt viscosity, so it can easily flow out of the injection chamber, making molding easier.

5.Polyethylene

Polyethylene comes in several types that vary in density; they are low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE) and ultra high molecular weight polyethylene (UHMWPE). Polyethylene is also a thermoplastic. All types of polyethylene are lightweight, have good chemical resistance and are impervious to liquids and gases. Higher density polyethylene has higher tensile and flexural strength and toughness, but has lower elongation and is brittle at low temperatures. Polyethylenes are used in a wide range of applications, including packaging materials, medical devices, rigid containers, and bulletproof vests.

6.Liquid Silicone Rubber

Liquid silicone rubber (LSR) is a common synthetic thermoset resin. It is a "liquid rubber" solution with a relatively low viscosity, which makes it easier to get into the mold cavities. Silicone is ideal for molding parts with tight dimensional tolerances. It is also UV resistant. Applications for LSR include materials for automotive parts, thermal insulation and medical devices, as well as baby bottles that can withstand high temperatures during sterilization or autoclaving.
Liquid silicone rubber (LSR) is a common synthetic thermoset resin. It is a “liquid rubber” solution with a relatively low viscosity, which makes it easier to get into the mold cavities. Silicone is ideal for molding parts with tight dimensional tolerances. It is also UV resistant. Applications for LSR include materials for automotive parts, thermal insulation and medical devices, as well as baby bottles that can withstand high temperatures during sterilization or autoclaving.

#2 Injection Molding Heaters

Depending on the type of material from which the products will be made, you may need heating elements with different characteristics. Ring heaters of the injection molding machine cylinder differ in type and maximum heating temperature. Micanite clamp heaters , for example, are capable of heating up to 350 C, and ceramic ones – up to 500 C.

Spiral heaters and cartridge heaters also have different electrical characteristics, they differ not only in shape and size. Cartridge heaters, for example, can be made either by placing coils in grooves in a ceramic insulator, or the coil can be wrapped around a core in cases of high power cartridge heaters.

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