injection molding final

7/29/2019 Injection Molding Final

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Injection Molding Technology

Learning Outcome:

Develop an understanding of the injection molding process and

relevant factors governing the design of the design of injection

mould tools.

References:

1. Modern Plastic Handbook, (2000), Edited By Charles A Harper, McGraw Hill.

2. Injection Molding Handbook, D.V. Rosato, Chapman & Hall.

3. Plastics Engineering (1998) R.J. Crawford, Butterworth-Heinemann

4. Plastics Material and Processing By A.B. Strong.

5. Tool Knowledge 01, Injection Mold (Fundamentals) Pak Swiss Training Centre (PCSIR)


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Over View:

Injection Molding Technology

Products and Industry

The process Cycle

Tools Details

Developments in Injection molding processes


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Injection molding is a versatile process that can produce small

parts that weight in fraction as well as large parts that weight in kg.

During this process, molten plastic is forced (injected) into a mold

and cooled until the melt solidifies.

When the part is cooled sufficiently, the mold is opened, the part

is ejected from the mold, and the mold is closed again to repeat the

cycle.

Injection Molding Process


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Injection molding permits mass-production, high-precision, and

three-dimensional virtual net shape manufacturing of plastic parts.

While there are many variations on the basic process, 90 percent

of injection molding occurs with thermoplastic resin.

Injection molding requires an injection-molding machine, a mold,

and ancillary equipment such as material-feeding and conveying

equipment, dryers, mold temperature controllers, chillers, and

robotics and conveyers.

The material-feeding and conveying equipment and the dryers are

common to most thermoplastic manufacturing processes, while the

robotics and conveyers automate the molding process.

Injection Molding Process


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Products Market Sectors

Automotive.

Aerospace / Defence.

Computers, telecoms & optical disks.

Construction.

Medical.

Packaging.

Appliances.

Home & Leisure.


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Products - Automotive


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Products Telecoms and Computers


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Products Medical:


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Injection Molding Machine

Injection-molding machines have three components:

1. The injection unit.

2. The clamping unit.

3. The Mold.

The injection unit plasticates and injects the polymer melt.

The clamping unit supports the mold, opens and closes the mold,

and contains the part ejection system.

Typically different injection units can be assembled with the same

clamping unit.


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1. The Injection Unit:

The injection unit brings the nozzle into contact with the sprue

bushing of the mold, generates contact pressure between the

nozzle and sprue bushing, melts the plastic material, injects the

molten material into the mold, and builds up packing and holdingpressure.

1.1 Types of Injection Unit.

Several types of injection units have been used.

i) Single Stage Reciprocating Screw

ii) Single Stage Plunger Unit

iii) Two Stage Screw/Plunger Machine or Pre Plasticizing Machine


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i. Single Stage Reciprocation Screw:

In single stage reciprocating screw injection unit, the screw rotate to

plasticate the polymer and moves linearly to inject the melt. Theinjection unit has two major component, the plasticizing unit and a

sled. The plasticizing unit melts and inject the resin while the sled

translate the plasticizing unit.

The plasticizing unit consists of a hopper , feed throat, barrel, screw,screw drive motors and nozzle. Single stage reciprocating operates like

a extruder. During injection , the screw is driven forward by a hydraulic

piston or motor. Injection molding barrels are shorter than extruder

barrel with low L/D ratio.

The injection unit (Sled) retracts away from the mold along its rail.

This permits purging of the material from barrel when changing the

resin type in the machine or getting rid of degraded material.


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Single Stage Reciprocation Screw


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ii. Single Stage Plunger Unit:

Single stage plunge unit melts the polymer by conduction from the

barrel walls and then inject the melt using a hydraulic plunger.

These unit provides limited shot size, poor shot size control, poor

melting and mixing, non uniform heating of the melt, long resistance

time and high pressure losses.


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iii. Pre Plasticizing Machine:

Both the reciprocating screw and the plunger injection system

combine the melting of the resin and the injection of the resin into asingle system. When extremely fast cycle are desired, the melting of

the resin is separated from the injection function.

This is done by attaching the barrel in which melting is done either

by screw or ram to a second barrel that contain a plunger and a

injection chamber. This barrel is called pre plasticating cylinder and

the entire arrangement is called pre plasticizing machine.


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Pre Plasticizing Machine


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2. Clamping Unit:

The clamping Unit supports the mold, hold the mold, closed during

injection, open and closes the mold as rapidly as possible, provide

space for part protection.

Types of Clamping Unit:

i. Hydraulic Clamping.

ii. Toggle Clamping.


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i. Hydraulic Clamping.

A clamping unit actuate by a hydraulic cylinder that is directly

connected to the moving platen. Direct fluid pressure is used toopen and close the mould and to provide clamping force to hold the

mold during injection.


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ii. Toggle Clamping:

A Clamping unit with toggle mechanism directly connected to the

moving platen. A hydraulic or some mechanical force device isconnected to the toggle system to exert the opening & closing force

and hold the mold close during injection.


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3. Mold:

Injection Mold provides a passage way for molten plastic to travel

from the barrel to the mold cavity. It gives final shape to the part,cools the material and eject the final part. They also must

withstand the forces of injection and ejection, transfer motion.

Injection mold can be classified as

1. Two Plate Mold

2. Three Plate Mold

The standard two plate mold has two halves. The division between

these two halves is called, the parting.


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Injection MoldTwo Plate Mold


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Cavity and Punch:

A cavity in a mould is a the hollow space to accommodate the molten

plastic injected under pressure into it. It determines the outer shape of the

component and is located mostly on the injection side.

Punch is gives inner shape to the part. Punch is normally located on

ejector side (moving side) Cavity-Punch will be treated as one unit in

respect of application but they are separated by parting line.

In normal case both cavity and punch are used to form component.

Runner:

The runner is expected to offer to the hot plastic a quick flow path. The

main runner starting from sprue is called Primary runner and further

branches are known as secondary runner .


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Number of Cavities:

When a small size molded component is required in a large quantity and a

shorter period of time it is necessary to have more than one cavity in amould.

Some times multi cavity mould is also designed to produce different

component in each cavity, mostly required for a common assemble. This is

applicable only when equal numbers of each component are required andalso in the same color and material. This is called group mould.


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Number of Cavities:


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Arrange of Runners:

Gates:


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Gates:

The small passage which is leading directly into the cavity is called gate.

The selection of best suitable gate is very important.

1. Sprue Gate:

Advantages:

Minimum pressure loss occur due to direct injection.Regular wall thickness can be obtained.

Disadvantage:

Additional labor for removing the sprue is needed.

Sprue mark will always be visible.


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Edge Gate:

Advantage:

Easy removal of runner since the section is comparatively small.

Rather minor gate marks.

Disadvantage:

High injection pressure required since the material flow is poor

due to small section of gate.


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Diaphragm Gate:

Advantages:Uniformity in diameter and strength is assured by using this gate.

Due to uniform filling no weld line will appear.

In view of accuracy very good result can be obtained.

Disadvantages:Extra labor is required for degating, mostly on machine.

Core can be located on one side only, therefore length of hollow

component to be molded must be restricted if accuracy is expected.


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Ring Gate:

Advantages:

Injection take place all around the cavity at the same time and

provide uniform wall thickness and strength.

Disadvantage:

Extra labor is required for degating, mostly on machine.


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Flash Gate:

Advantages:

As the material is injected from one side so the flow of material is in one

direction and no weld line will appear and homogeneous structure and good

mechanical strength will be obtained.

Disadvantage:

Extra labor is required for degating.


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Submarine Gate:

Advantages:

Automatic Production is possible due to automatic degating.

No extra labor cost.

Disadvantages:Relatively high injection pressure is required since material

flow is rather poor due to small gate section.


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Pin Point Gate:

Advantages:

Automatic degating and therefore no extra cost.

Disadvantages:

Most of the time a more complicated mould is required and also the

moulding machines is expected to be well developed, so initial cost are

certainly higher.


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EJECTION SYSTEM

In injection molding the component normally shrinks and grips on

the punch which is generally on the moveable half of the tool. After

the tool opens it is necessary to remove the component from the

punch which demands considerable care. This removal of thecomponent is called Ejection .


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1. Pin Ejector


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1. Pin Ejector


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2. Blade Ejector


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3. Sleeve Ejector


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4. Stripper Plate


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4. Stripper Plate


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5. Disc Ejector


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6. Air Ejection


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7. Chain Ejection


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Slider Mold

1 Pin Ejector:


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1. Pin Ejector:

These are the simplest, cheapest and most common types of ejectors.

Generally cylindrical pins are used for this purpose. Such pins are often

available in the market in different sizes and with one end peeled into a

head (Cylindrical or 60o tapered).

The pins can be stepped or ground flat on one side. The ejector leaves the

impression on the moulding. It should be located as not to damage the

component, moreover the impression are not to disturb its function.

2. Blade Ejector:

The component is trapped between two opposite steel faces of the tool

(cavity/punch). In case we put ejectors on the spots X (certainly cheaper)

the component is not ejected nicely, it is likely to stick at the areas R (for

resistance). The result is bent or torn component. This ejector shape

increases the contact area. In cases like this pin ejectors could easily

damage the component.

3. Sleeve Ejector:


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3. Sleeve Ejector:

This type of ejector is used to eject circular components, with a thin wall

sticking on a core. The core itself is attached to the back plate (ejection

side). The ejector in form of a sleeve is around this core pins ejects the

component all around the edge.

4. Stripper Plate:

The stripper plate is an accurately adjusted plate around the core, being the

parting line. During ejection the plate moves along the core pushing and

removing the component.

5. Disc Ejector:

This mechanism is used basically for moulding house hold utensils or big

enough components (circular). Is consists of a round disc operated by a

centrally located rod. The disc ejects a comparatively delicate moulding atthe base without damaging it (as the contact surface is large). It is cheap to

manufacture and more effective when ejecting deep components with thin

walls.


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6. Air Ejection:

Compressed air enters through the punch plate and pushes the ejector

towards the injection side. The air is of course not flowing continuously,but is regulated by a switch allowing one short blast after each mould

opening .

7. Chain Ejection:

When is not possible to do centre ejection due to the machine design or

the construction of the tool, this method can be used. Chains are always

fixed on the sides of the tool. Provision to adjust the chain slackness

equally on either side is very important.

injection molding final

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