Injection Molding Technology

Material From Dr. Piyawit Khumphong (MTEC)

Injection Molding System

Tpe of Mold for Injection Molding

Two-piece mold

Three-piece mold

Flow Channels for Molten Plastic

Sprue Runner Gate Cavity

Sprue

Runner

Gate

Cavity

Cooling System for Injection Mold

Injection Molding Cycle

1. Filling 2. Repeat Filling

(Packing) and Cooling

3. Mooo ooooing 4. Product

ejecting 5. Mold closing

Temperature and Pressure Setting

Unmolten

Tem

p (T

)

Pressure (P)

Degrade

Flash

Short shot Tg + 100Tm + 50

Temperature Setting for Injection Molding Machine

• Depending on Screw (Diameter, D)

1D 1.5D 3D

D

Shot size

Viscous heat ====> shear heating

Mold Filling

Fountain flow

Surface

Center

Skin - core morphology

Birefringence microscopy of HDPE

• Short Shot• Discoloration • Air traps• Burn marks, Black specks-black streaks• Brittleness, Fish Eyes• Flow marks, Ripples• Flash, Jetting, Silver Streaks, Sink Marks• Shrinkage and Warpage• Weld Lines and Meld Lines• Overpacking• Flow into thin section

Defect Problems in Injection Molded

Products

• Air traps >>> Burn marks

Problems that arrived from poor mold design

Front View

Top View

Burn marks Black specks-streaks

Brittleness Fish Eyes

Flash Jetting

Silver Streaks Sink Marks

Flow marks

Ripples

Shrinkage and Warpage

Weld Lines and Meld Lines

Sandwich-Molding Overmolding

Simultaneous

Sequential

1. Coinjection2. Gas assisted

1. Monosandwich2. Airpress III

1. Core-Back-Technique2. Sandwich-Overmoulding3. Insert-Moulding*4. Outsert-Moulding*5. Textile-Overmoulding*6. In-Mould-Labeling*

1. Turntable2. Transfer3. Overmoulding-Stackmould

Single Cavity

Multi-Cavity

Multi-component molding

Sandwich molding

Viscosity ratio

Viscosity ratio = viscosity 2nd melt / viscosity 1st melt

Desired structure

Sandwich molding

Barrel A

Barrel B

Connec ting

pipe

Needle

Simultaneous Technique

Sequential Technique

core material (B)

skin material (A)

Monosandwich molding

core material (B)

skin material (A)

skin material (A)

core material (B)

Monosandwich molding

Door Handle

Monosandwich molding

Compatibility

ABS PA6 PP PE PS PET PC

ABS + ? - - * + +

PA6 ? + - - - ? *

PP - - + + - - -

PE - - + + - - -

PS * - - - + - -

PET + ? - - - + +

PC + * - - - + +(- ) : No adhesion, (*) : Poor adhesion, (+) : Good adhesion, (?) : N/A or Depend on Condition

Depend on

• Chemical bonding• Inter-diffusion

>>>>Time & Temp

PP-PP under 110 °C >>> Poor adhesionABS-ABS under 130 °C >>> Poor adhesion

Compatibility

Corrects:Warpage

Shrinkage

Section change

Surface aesthetics

Developments:

Cold Gas Injection

Water Injection

Gas Assisted Injection Molding

Airpress III

1 2

3 4

Bottle Crades

Airpress III

OVERMOLDING

Overmolding can be ...

multi colour technique

(e.g. buttons for key boards)

multi material technique

(hard and soft, e.g. for integrated seals)

in-mold assembling

(e.g. hinges, air vents ...)

Core – Back Technique

Transfer - Technique

ABS Tray with anti-slip TPE material

Tooth Brush with soft-feel TPE material

TurnTable Technique

Tube shoulder for thoothpastewith barrier layer from PET and over-molded skin from PP

Stack-Turning-Technique

process phase 1process phase 2

preform

Injection unit 2

Main injection

unit

Stroke 3

2nd injection unit

Stroke 2

Assembling

Stroke 4

Demolding

Stroke 1

1st injection unit

Stack-Turning-Technique

Vertical turn with the Cube-Technique

Production cost reduction about 25%

4 sides of the cube usable

Siemens M35

Stack-Turning-Technique