ChETE 3 Shaping Plastics - Injection Molding

Injection uses a reciprocating screw which can move back and forth

What is the main difference between extrusion and injection?

Injection molding

Leading method to create closures

Shot

Injection molding requires a powerful extruder with capability to inject a precise amount of resin called a

• Filling

• Cooling

• Mold opening

• Ejection

Injection molding process

1. Extuder section/Injection process

2. Clamp section

Parts of an Injection Molding Machine

ram-screw type

Reciprocating screws are also called

Clamp force

Which should be greater, clamp force or injection force?

flash

Excess material attached to a molded product

short shot

If there is not enough injection force and the material does not completely fill the mold

Clamping force tonnage is related to the surface area of the part.

The greater the area, the higher the injection force (meaning even higher clamping force)

What determines the clamping force?

Based on their L/D ratio

How are extruder sizes named?

They are dependent on injection capacity and clamp force

How does the size of injection molders determined?

mold

A hollow form or matrix for giving a particular shape to something

cavity

the hollow or female part of the mold

core

the positive or male part of the mold

Cavity and core

Two main parts of the mold

Cavity

Mounted on the machine in a stationary position

Core

Mounted on a sliding frame so the mold can open or close

Clamp section

What houses the mold of the product?

Runners

Channels that distribute molten plastic to various cavities in a multicavity mold

Exactly the same

Runner distance from the sprue to gate

Gate

Small opening through which the molten plastic enters the mold cavity

Cooling channels

Cools the molten polymer so that it solidifies. Usually water/water-glycol

Ejector plate/Knockout pins

Help physically eject the molded part.

Sink mark

Characteristic mark left at injection molded parts surrounded by a slight depression

Undercuts

are features in the part design that requires additional parts of the mold to be moved before the part can be ejected.

• Unscrewing the core

• Stripping closure off the core

• Collapsing the core

Ways to remove closures from molds