Injection Molding

Injection molding is a process in which a polymer is heated to a highly plastic state and
forced to flow under high pressure into a mold cavity, where it solidifies. The molded
part, called a molding, is then removed from the cavity.

https://www.youtube.com/watch?v=eUthHS3MTdA
https://www.youtube.com/watch?v=RMjtmsr3CqA

It is ideal for producing high volumes of plastic parts due to the fact that several parts
can be produced in each cycle by using multi-cavity injection molds.

Some advantages of injection molding are

high tolerance precision, repeatability, large material selection, low labor cost,
minimal scrap losses, and little need to finish parts after molding.

Some disadvantages of this process are expensive initial tooling investment and
process limitations.
INJECTION MOLDING MACHINE

CONSISTS OF TWO BASIC PARTS :

a. Injection Unit
b. Clamping Unit

Injection unit – melts and delivers polymer melt, operates much like an extruder
 Injection unit of molding machine consists of barrel fed from one end by a
hopper containing plastic pellets

 The screw inside the barrel performs the following functions,

a. Mixes and heats the polymer
b. Acts as a ram to inject molten plastic into the mold

 Non – return valve near tip of screw prevents melt flowing backward

 During the molding cycle the ram retracts to its original position
 The Clamping Unit performs the following functions,

 Holds two halves of mold in proper alignment with each other

 Keeps mold closed during injection by applying a clamping force to resist injection force

 Opens and closes the mold at the appropriate times in molding cycle

TYPES OF CLAMPING UNITS:

 Toggle

 Hydraulic
Examples of injection molded items are cabinets for computers, computer screens, and
computer keyboards, parts of plastic model or hobby kits, plastic bottle caps, brake and
engine parts, reflectors and headlights, circuit breakers, toaster oven panels, power tool
and lighting housings, and even tableware.
Clamping Unit

As part of the injection molding machine the clamping unit holds the mold in place and
closed during injection of the molten polymer and enables opening and closing of the
tool in order to release the part. The clamping unit contains a mechanism to operate the
ejector plate in tool.

The clamping units main function is to open and close the mold along with ejecting the
parts.

The two most common types of mold clamps are the are the direct hydraulic and the
toggle clamps.
Toggle Clamp Mechanism

https://www.youtube.com/watch?v=eOr-lpny13U

A hydraulic cylinder or electric actuator moves a crosshead forward and through the
toggle links pushes the moving platen in the direction of the stationary platen.
 • The design of crosshead and toggle links make
the moving platen move fast in the first part of
the closing.

• The platen speed decreases and force

increases as the toggle mechanism is extended
fully.

Advantages of the toggle clamp are closing speed and increased tool safety due to the
low speed just prior to full closure.
Hydraulic Clamping Mechanism

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Direct Hydraulic Lock is a system where the moving machine platen is driven by a
hydraulic piston arrangement which also generates the required force to keep the
mould shut during the injection operation.
Mold

A mold is a hollow metal block into which molten plastic is injected to from a certain fixed
shape.
 https://www.youtube.com/watch?v=wScUAoCciGo

Mould simply consists of two halves commonly referred to as the moving (core) half and
fixed (cavity) half.

The sprue bush is profiled with a radius to match up with the injection unit nozzle so that
material can be directly transferred from the injection unit through to the mould cavity.

In the case of a single impression (cavity) mould, the sprue may feed directly onto the
component.

in the case of a multi impression mould, the sprue feeds onto a runner system machined
into the tool face that acts as a transfer system to the cavity for the molten material.
hot runner systems

runner feed system is constantly molten and therefore not ejected at the end of the
cycle. Instead the molten material remaining in the hot runner system after injection of a
component forms part of the next shot.

There are two types of hot runner systems:

Insulated runner molds

The insulated effect of the plastic combined with the heat applied with each shot
maintains an open, molten flow path.

Heated runner systems

There are two designs: internally heated and externally heated.

Hot Runner Type Advantages

Less complicated design.

Insulated Less costly to build.

Improved distribution of heat.

Internally Heated

Improved distribution of heat.

Externally Heated Better temperature control.
Cold runner systems

Advantages
-Comparatively cheaper to produce and maintain

Disadvantages
-Cycle times are slower than hot runner systems
-Plastic waste from runners (particularly if they cannot be reground and recycled)

Hot runner systems

Advantages
-Potential faster cycle times
-Eliminates runners and potential waste

Disadvantages
-More expensive molds to produce
-Higher maintenance costs and potential downtime
-May not be suited to certain thermally sensitive materials
Gates
https://www.youtube.com/watch?v=uazs9xQmbsY

Every injection mold design has to have a gate, or an opening through which the molten
plastic is injected into the cavity of the mold. The type and size of gate plays a very
significant role in the process of injection molding

Gates vary in size and shape depending upon the type of plastic being molded and the
size and shape of the part as well. Obviously, larger parts require larger gates, or even
several gates.

Functions of gate system:

To control the volume and direction of molten plastic flow in the injection mold.

To enclose the molten plastics in the mold until it solidifies, and to block backflow to the
runners.

To generate heat by viscous dissipation due to shear at the narrow gate portion, and thus
to raise polymer temperature so that no flow mark and weld line.

To make it easy to cut off the runner, and to simplify post- treatment of products.
The tab gate is easy to manufacture, maintain, and process, so most commonly used.

It consists of a trapezoidal block milled into the parting line on an exterior surface of the
part.

It typically leaves the largest vestige of the all gate types,

its position at the parting line affects only the edge of the part, where it usually does not
interfere with function or cosmetics. Its position usually makes it easy to trim off,
A tunnel, or submarine gate,
is more complicated and requires a much higher degree of skill to add to the mold.

This type of gate is a cone that is added below the surface of the part, thus the name:
submarine gate. It also has the appearance of a tunnel.

This is used when there cannot be much evidence of the gate left on the part.
Hot Tip Gates

The hot tip gate is positioned near the center of a part rather than at the edge. This
reduces the distance resin must travel to fill the mold.

The hot tip gate is often used on round or domed parts to achieve radial flow rather
than the linear flow of a tab gate.

Because it is a small gate, a hot tip can become plugged if used to inject resins with
high glass fill content.
Problems associate with injection molding

Sink marks is caused by localized shrinkage of the

material at thick sections without sufficient
compensation when the part is cooling.

Suggested Solutions

Increase pack and hold pressure to get more molecules in the cavity.

Increase the gate size let more molecules enter before the gates freeze.
Too much flash

Flash is that thin layer of plastic that flows

outside of the cavity where the two halves of
the injection mold meet.

Causes Suggestions

• Injection pressure too high

• Lower injection pressure
• Injection speed too fast
• Make smaller clearance
•Material flow too soft
• Lower mold temperature
• Clearance of mold surface
• Mold temperature too high
Short Shot

Short shots occur when cavity in an injection

mold do not completely fill.

Causes
•Insufficient amount of material injected
•Insufficient injection fill speed
•Insufficient pack pressure
•Insufficient injection time
•Unbalanced multiple cavity mould
•Foreign material clogging nozzle
•Runners, gates, sprue or vents too small
Quality reduction as a result of weld lines and trapped air

Weld lines occur when two or more melt streams unite in the mould. This happens, for
example, when the melt has to flow around a mould insert or when parts are gated at
several points.

In addition, different wall thicknesses in a part can also lead to separation of melt fronts
and so cause weld lines.
A weld line looks like a slight break or crack and occurs as a result of the flow pattern of
hot plastic.

When plastic fills a mold cavity, at some point it meets itself and forms a weld line.

The easiest place to find a weld line is near a molded hole. Weld lines are weak points
that cannot be avoided. The best mold designers will place weld lines in an area where
they do the least harm.

If the injection-molded hardware you are using keeps breaking in the same place, chances
are it is at the point of a weak weld line.
Voids

Voids appear as small bubbles or blisters

within the moulding and are typically created
by entrapped air, gas, or perhaps a vacuum.
Excessive shrinkage
Excessive shrinkage is a characterized by moulded parts which appear smaller than
the intended dimensions.

Excessive shrinkage can be caused by:

Cure time too short

Pack pressure too low
Mould or stock temperature too high
Insufficient injection pressure
Runners or gates too small
Poor part design, varying wall thickness
 Reaction Injection Molding (RIM)

similar to injection molding except

thermosetting polymers are used, which
requires a curing reaction to occur within the
mould.

Injection molding is the process of forcing melted

plastic into a mold.

With reaction injection molding,

two liquid components are mixed in a high- or
low- pressure mixing head and pumped into a
mold.

Epoxy , polyurethane/polyester hybrids,

https://www.youtube.com/watch?v=TWEfbGH-Ewc
Gas Assist Injection Molding

Gas-assist injection molding is a process that utilizes an inert gas (normally nitrogen)
to create one or more hollow channels within an injection-molded plastic part.

At the end of the filling stage, the gas (N2) is injected into the still liquid core of the
molding. From there, the gas follows the path of the least resistance and replaces the
thick molten sections with gas-filled channels.

Next, gas pressure packs the plastic against the mold cavity surface, compensating for
volumetric shrinkage until the part solidifies. Finally, the gas is vented to atmosphere
or recycled.

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Co-Injection Molding
Processes of combining different material qualities and properties in one component.

www.youtube.com/watch?v=w9yH-0cx-f4

www.youtube.com/watch?v=aR9AY-XapF8
Bi-Injection Molding