CHAPTER 4 Plastic

7/23/2019 CHAPTER 4 Plastic

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CHAPTER 4

FORMING ND SH PING

PL STICS COMPOSITE

M TERI L



DEFINITION :

A hollow form for shaping molten

plastic material that is used for

repetitive mass production of

molded parts utilizing properties of

plasticity ductility and fluidity

WHAT IS MOULD?



That can produce molded part with required structure

shape and dimension.

It can produces molded parts that do not need to be

finished and required little secondary processing

Structure with high molding efficiency

Durable structure

Structure with short production cycle and low production cost

1. Mold Requirement



» Injection Molding

» Extrusion molding

2. POLYMER PROCESSING



» Compression Molding

» Blow Molding



» Transfer Molding

» Vacuum Forming



Cavity : space within the mold where the product shaping

takes place.

Channel : Fluid path from sprue runner to cavity

Temperature Control Unit: Cooling lines used to control mold temperature

Ejector Unit : Mechanism to extract the molded part from themold

Mold Fixed Unit : Part of the mold attached to the moldingmachine

3. Mold Component Requirement



2 Plate Mold Structure






Hot Runner Mold Structure





Hot Runner Mold Structure

2. INJECTION MOLD TYPE



2 Plate Mold Parts List



3 Plate Mold Parts List



Hot Runner Mold Parts List



EXPLODED MOLD LAYOUT



2 Plate Mold Operation

3 Plate Mold Operation

Injection Mold Operation



[2 Plate Mold Operation]



[3 Plate Mold Operation]



Sprue

Runner Gate

3. Sprue, Runner and Gate



Definition :

Located at the mold entrance, channels moltenplastic through runner and gate into the cavity.

Sprue



Deinition :

- Path for molten plastic between sprue and gate

Requirement

- Thick and circular cross section

Runner

• Low Fluid resistance• Molten Plastic should not easily cooled• For Pressure transmission• For Heat transmission



Definition- Entrance to the cavity

- Gate Location, quantity, shape and dimension

assert an important influence on molded part

for appearance and molding efficiency.

Requirement

- Cross-section

- Type

Gate

• A large cross section has good filling characteristics but takeslonger to seal and separate gate

• A small cross-section area, there is some problem for fillingcondition with quick solidity

• By resin type, part shape and product requirements.



Direct Gate

Side Gate

Overlap Gate Submarine Gate

Curved Gate

Pinpoint Gate

Gate Types



Gate used to replenish cavity directly from the sprue.

Sprue entrance diameter is dependent on the nozzle holediameter and is made about 0.5~1mm larger than the

nozzle hole diameter.

Direct Gate



Used in many molded products Called a side gate because a rectangular or semicircular

shaped gate is installed at the side of the cavity. Advantage :

* Simple cross-section facilitates machineprocessing

* Applicable with almos all types of polymers

Disadvantage :* A gate mark is left on the molded partexterior

Side Gate



Side Gate



Used instead of the side gate to prevent incidence of flowmarks in molded parts

Overlap Gate



While a side gate is located on the parting line surface of themold, a submarine gate tunnels into the cavity throughsupporting ribs and is also called a tunnel gate.

Submarine Gate



Submarine Gate



Similar to the submarine gate, the tunnel is machineddirectly in the backside mold of the product.

Curved Gate



A circular gate used when the gate is located at the centerof the molding part, it is also frequently used as a multi-point gate

Pinpoint Gate



The Reasons why injection molding defects is occurred

4. Injection Molding Defect Cause

• Injection Molding Condition• Product Shape

• Mold• Injection Molding Machine• Material• Others : men, environment.. etc



Molding Defect Types and Causes

• Weld Line• Flash• Short Shot

• Sink Mark• Void• Flow Mark

• Silver Streak• Black Streak• Jetting Mark•

Warpage• Crack• Cloudy Surface



Weld line is formed when ahead of two molten resin flow withdifferent direction meet finally.

Contact areas is not composed of a complete crystallizationstructure because of difference of resin temperature. So, thinline is formed around holes, among multiple gate.

- Cause due to mold structure :

• - Cause due to injection condition :

Weld Line

No. of Weld Lines (N) = H + (G-1) [H: No.of Holes, G: No. of gates]

Excessive small runner, gate

Excessive distance from gate to the weld Faulty gas ventilation

Low resin temperature or mold temperature

Slow injection speed



Weld Line



Flash refer to the area or mass increase to the molded partthat occurs because excess molten plastic flows into gaps inthe parting line or slide in the core side of the mold.

Flash

- cause due to mold structure :

Defective mold parting surface Occurrence of mold warping

- Cause due to injection condition : Excessive injection pressure and long application period

- Causes due to injection machines : Insufficient clamp force

Poor maintenance management of molding machine



Flash



Resin with high viscosity and low fluidity can prevent

the cavity from being completely filled resulting inparts with short or missing features.

Short Shot

- Cause due to mold structure :

Excessively small sprue, runner, gate

- Cause due to injection condition : Low injection pressure or resin temperature Insufficient injection amount

Slow injection speed Low mold temperature

-Cause due to injection machine : Insufficient molding machine capacity

Insufficient supply capacity



Short Shot



Undesired depressions on the surface of a part. cause when thick sections and outer walls containing ribs and bosses of

a product fails to stabilize due to insufficient cooling and outer layersare pulled towards the center by cooling stress.

- cause due to mold structure : Small sprue, runner or gate Inappropriate mold temperature control

-cause due to injection condition : Low injection pressure Short injection pressure Insufficient control over injection amount

-Cause due to product shape :

Sink Mark



Sink Mark



Causes :

Similar to the sink mark

Void occurs in the molded part itself.

Condition :

When part surface of mold with low temperature issuddenly cooledcontraction of part structure is suddenlycooled,contraction of part’s structure is occurred from the

inside towards the surfaceVoid also be performed closely to the center.

Void



Void



Is a pattern of concentric circles that forms around the gate on the surface. This is happen when low temperature, high viscosity molten plasticity with poor

fluidity gets in touch with the cavity and is forced in a semi-solid state andpattern form perpendicular to the flow direction.

Flow mark can be minimized by dispersing the high viscosity molten plastic fromthe gate when molding occurs.

causes due to mold structure :-Excessive cooling of the gate

cause due to injection condition:-High resin viscosity

-Low mold temperature-Slow injection speed

Flow Mark



Flow Mark



Condition in which silver/white colored line form in the flowdirection. Causes : resin thermal integration, insufficient gatesize, low resin temperature and low mold temperature

Cause due to mold structure :- Small sprue,runner or gate.

- Moisture,oilon mold surface

causes due to injection condition :- High injection pressure/high injection speed- Low resin temperature and mold temperature

- Resin pyrolysis- Air seepage in screw

Silver Streak



Silver Streak



Condition in which a black/brown colored flowpattern form on the molded part.

Causes due to mold structure :

- Small sprue, runner or gate- Moisture, oil on mold surface

Causes due to injection condition :- High injection pressure and high injection speed- Low resin and mold temperature- Resin pyrolysis- Air seepage in screw.

Black Streak



Black Streak



Jetting mark is a condition in which a ribbon shaped flow patternforms on the molded part surface. Inappropriate gate location,insufficient gate size and excessive high injection speed can bethe causes.

Cause due to mold structure :- Gate structure

- Absence or insufficient size of cold slug well

Cause due to injection condition

- Mold temperature

Cause due to injection material High resin viscosity

Jetting Mark



Jetting Mark



Warpage or deformations are mostly cause by differences incontraction rates of the various section of the molded part.

Cause due to mold structure :

- Non uniform cooling

- Inappropriate gate location

- Excessively small gate size

Cause due to injection Molding- Different in mold temperature

Warpage



Warpage



condition where crack occurs on the surface ofmolded parts.

Main Causes : Residual stress, external stress andenvironmental stress

Crack



Crack



What is Injection Molding??

Manufacturing process in which molten plastic is injected into amold, cooled and the resulting molded part extracted

1872 : Pyroxiline molding machine developed

by Hyatt brothers

1878 : Multi cavity mold designed by John Hyatt

1919 : Injection mold press developed 1950 : Reciprocating screw type injection

molding machine developed

INJECTION MOLDINGand HISTORY



Complex shape product molding capability

High productivity / mass production capability

Ease of automation

Labor cost reduction

Most of parts do not need finishing

Resin can be mixed with reinforcing material(glass fiber, carbon etc)

Injection Molding (Advantages)



Mold is expensive

Cost of molding machine is high

High pressure process

Quality of molded parts cannot be determined quickly

Problems can occur if basic knowledge of molding process islimited

Mold process control is not easy

Injection Molding(Disadvantages)



Injection Molding Machine Structure(Injection System)



Injection Molding Machine Structure(Mold System)



Operation Principle (Plasticizing stage)



Operation Principle (Filling Stage)



Operation Principle (Packing Stage)



Operation Principle (Cooling Stage)



Operation Principle (Ejection Stage)



Resin is fed into the molding machine hopper

In the heated cylinder, resin is transported by the screw, melted and mixed

Hot molten plastic is injected into the mold at high pressure

Molten plastic is cooled within the mold

Molded part is ejected and a new injection molding cycle is started

INJECTION MOLDING PROCESS



Injection Molding Process and Defect



Injection Molding Process

I j ti M ldi g O ti



Cycle Time ·Injection Pressure

Injection Molding Operations



Filling stage in which hydraulic or sub motor connected tothe screw drive mechanism pushes the molten plasticthrough the sprue bushing into the cavity

Screw does not rotate but moves forward to allow molten

plastic to enter the mold Duration of this period is defined as the injection time and

screw advance speed is controlled by the re

Injection Molding Process (Injection)



Process in which uniform pressure is maintained at the nozzle duringpacking time.

Process in which extra resin is pushed in at high pressure to compensatefor volume contraction caused by the cooling mold during this stage

Packing time is set until gate solidification

Injection Molding Process (Packing)

Injection Molding Process



Process in which molten plastic within the mold is cooled to thetemperature at which ejection can take place

When solidification starts, mold clamping cylinder pressure is loweredsince the force holding the mold plate together can be reduced. During

this solidification period the injection system enters the plasticizingstage in order to melt the resin for the next cycle. In this stage the screwis rotated using an electric or hydraulic motor

Injection Molding Process(Cooling, Measurement)

Injection Molding process



At about the mold opening completion, the mold comes incontact with the ejector rod which is fixed to the injectorand pushes out the molded part

Injection Molding process(Ejection)

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CHAPTER 4 Plastic