Injection molding Machine

Injection molding Machine

Definition: -

Injection molding is nowadays the most popular method to produce 3-dimensional parts of different kinds of polymeric materials. It is a fast process and is used to produce large numbers of identical items from high precision engineering components to disposable consumer goods. It is a suitable method for thermoplastics, thermosets, thermoplastic elastomers, elastomers, short fiber and particulate filled polymers. Today, more than one-third of all thermoplastic materials are injection molded and more than half of the polymer processing equipment is for injection molding. Injection molding is a production method for large series. Injection molding can also be used to manufacture parts from aluminium or brass.

EQUIPMENT: -

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.

MOULD: -

Mold (Tool and/or Mold) is the common term

used to describe the production tooling used to

produce plastic parts in molding.

Traditionally, molds have been expensive to

manufacture. They were usually only used in

mass production where thousands of parts

were being produced. Molds are typically

constructed from hardened steel, pre-hardened

steel, aluminium, and/or beryllium-copper

alloy. The choice of material to build a mold is

primarily one of economics. Steel molds

generally cost more to construct, but their

longer lifespan will offset the higher initial

cost over a higher number of parts made Pre-hardened steel molds are less wear resistant and are used for lower volume requirements or larger components. The steel hardness is typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining. These are by far the superior in terms of wear resistance and lifespan.

Typical hardness ranges between 50

and 60 Rockwell-C (HRC). Aluminium molds can cost substantially less, and when designed

and machined with modern computerized equipment, can be economical for molding tens or even hundreds of thousands of parts.

Beryllium copper is used in areas of the mold

which require fast heat removal or areas that

see the most shear heat generated. 

MACHINING: -

Molds are built through two main methods:

standard machining and EDM machining.

Standard Machining, in its conventional form,

has historically been the method of building

injection molds. With technological

development, CNC machining became the

predominant means of making more complex

molds with more accurate mold details in less

time than traditional methods.

The electrical discharge machining (EDM) or

spark erosion process has become widely used

in mold making. As well as allowing the

formation of shapes which are difficult to

machine, the process allows pre-hardened

molds to be shaped so that no heat treatment is required. Changes to a hardened mold by

conventional drilling and milling normally

require annealing to soften the steel, followed

by heat treatment to harden it again. EDM is a

simple process in which a shaped electrode,

usually made of copper or graphite, is very

slowly lowered onto the mold surface (over a

period of many hours), which is immersed in

paraffin oil. A voltage applied between tool

and mold causes erosion of the mold surface

in the inverse shape of the electrode.

COST: -

The cost of manufacturing molds depends on a

very large set of factors ranging from number

of cavities, size of the parts (and therefore the

mold), complexity of the pieces, expected tool

longevity, surface finishes and many others.

THE PROCESS: -

1. Clamping: -

An injection molding machine consists of

three basic parts; the mold plus the clamping

and injection units. The clamping unit is what

holds the mold under pressure during the

injection and cooling. Basically, it holds the

two halves of the injection mold together.

2. Injection: -

During the injection phase, plastic material,

usually in the form of pellets, are loaded into a

hopper on top of the injection unit. The pellets

feed into the cylinder where they are heated

until they reach molten form (think of how a

hot glue gun works here). Within the heating cylinder there is a motorized screw that mixes

the molten pellets and forces them to end of

the cylinder. Once enough material has

accumulated in front of the screw, the injection process begins. The molten plastic is inserted into the mold through a sprue, while the pressure and speed are controlled by the screw.

3. Dwelling: -

The dwelling phase consists of a pause in the

injection process. The molten plastic has been

injected into the mold and the pressure is

applied to make sure all of the mold cavities

are filled.

4. Cooling: -

The plastic is allowed to cool to its solid form

within the mold.

5. Mold Opening: -

The clamping unit is opened, which separates

the two halves of the mold.

6. Ejection: -

An ejecting rod and plate eject the finished

piece from the mold. The un-used sprues and

runners can be recycled for use again in future

molds.