[PPT] - Polymer Engineering Polymer Engineering (MM3POE) (MM3POE) PowerPoint Presentation

SLIDE 1

Injection Moulding 1

Polymer Engineering Polymer Engineering (MM3POE) (MM3POE)

http://www.nottingham.ac.uk/~eazacl/MM3POE

INJECTION MOULDING INJECTION MOULDING

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Injection Moulding 2

Contents Contents

Principles of injection moulding Reciprocating screw machine

Moulding sequence Machine features

Injection mould design Mould filling calculations

Filling pressures Clamping forces Filling times

Component design for injection moulding

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Injection Moulding 3

1. Introduction
1. Introduction

Principles of Injection Moulding:

Melting : Thermoplastic material (granules/pellets) heated to melt polymer Melt Transport & Shaping : Polymer melt is forced through a nozzle into a closed mould Stabilisation : Component cools in relatively “cold” mould prior to ejection

Main Advantages:

Automation & high production rates Manufacture of parts with close tolerances Versatility in moulding wide range of products

eg. appliance housings,washing up bowls, gearwheels,

fascia panels, crash helmets, air intake manifolds

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Injection Moulding 4

1. Introduction
1. Introduction

Plunger Type Machines

Fig. 4.30

Plunger type injection moulding machine

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

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Injection Moulding 5

1. Introduction
1. Introduction

Plunger Type Machines

Fig. 7.3 Injection moulding machine with screw preplasticator unit

N G McCrum et al (1997) Principles of Polymer Engineering, Oxford Science Publications.

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Injection Moulding 6

2. Reciprocating Screw Machine
2. Reciprocating Screw Machine

Machine Features:

Feed hopper Screw/plunger

DRIVE

Heater bands CLAMPING UNIT Nozzle Back flow check valve

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Injection Moulding 7

2. Reciprocating Screw Machine
2. Reciprocating Screw Machine

(a) Mould closes & screw (not rotating) injects melt into mould.

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Injection Moulding 8

2. Reciprocating Screw Machine
2. Reciprocating Screw Machine

(b) Screw maintains pressure until material sets at the gate.

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Injection Moulding 9

2. Reciprocating Screw Machine
2. Reciprocating Screw Machine

(c) Screw (rotating) draws material from hopper & plasticises it. Back pressure forces screw back until shot volume reached.

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Injection Moulding 10

2. Reciprocating Screw Machine
2. Reciprocating Screw Machine

(d) When moulding has set, mould opens & part is ejected.

Animation from: www.bpf.co.uk

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Injection Moulding 11

2. Reciprocating Screw Machine
2. Reciprocating Screw Machine

Injection moulder with nanocomposite tensile specimens (inset) Hopper Tool Screw & ram Injection moulder with nanocomposite tensile specimens (inset) Hopper Tool Screw & ram

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Injection Moulding 12

2. Reciprocating Screw Machine
2. Reciprocating Screw Machine

Screws Screws: Similar to extruder screws

Length/Diameter ratios 15 15-

25

25 Compression ratios 2.5 2.5-

4.0 : 1

4.0 : 1 Injection pressure up to 200 200 MPa MPa

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Injection Moulding 13

3. Injection Mould Design
3. Injection Mould Design

backing plates backing plates guide pin guide pin locating locating ring ring sprue bush sprue bush

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Injection Moulding 14

3. Injection Mould Design
3. Injection Mould Design

ejector ejector pin pin shoulder shoulder screw screw

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Injection Moulding 15

3. Injection Mould Design
3. Injection Mould Design

cavity cavity runner runner sprue sprue cooling cooling channels channels vent vent channel channel

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Injection Moulding 16

3. Injection Mould Design
3. Injection Mould Design
Fig. 7.31 Feed system of multi-impression mould

N G McCrum et al (1997) Principles of Polymer Engineering, Oxford Science Publications.

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Injection Moulding 17

3. Injection Mould Design
3. Injection Mould Design

Aalborg Universitet, Denmark http://www.aaue.dk/bm/mfg/

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Injection Moulding 18

3. Injection Mould Design
3. Injection Mould Design

Gate Gate: Narrow constriction at entrance to cavity

cavity (impression). Incorrect gating can lead to problems during flow:

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Injection Moulding 19

3. Injection Mould Design
3. Injection Mould Design

Gate Gate: Narrow constriction at entrance to cavity

cavity (impression). Incorrect gating can lead to problems during flow:

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Injection Moulding 20

3. Injection Mould Design
3. Injection Mould Design

Typical process conditions: Typical process conditions:

Process is non-isothermal as mould & barrel are at different temperatures:

Table 1: Injection moulding conditions for thermoplastics (after Elias) Polymer TG/oC TM/oC Tpoly/oC Tmould/oC Amorphous polymers PC 150

280 - 320

85 - 120 SAN 120

200 - 260

30 - 85 ABS 100

200 - 280

40 - 80 PS 100

170 - 280

5 - 70 PMMA 105

150 - 200

50 - 90 uPVC 82

180 - 210

20 - 60 Semi-crystalline polymers PET 70 265 270 - 280 120 - 140 PTFE 40 220 220 - 280 80 - 130 PA 6 50 215 230 - 290 40 - 60 POM

82

181 180 - 230 60 - 120 PP

15

176 200 - 300 20 - 60 HDPE

80

135 240 - 300 20 - 60 LDPE

80

115 180 - 260 20 - 60

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Injection Moulding 21

5. Mould Filling Calculations
5. Mould Filling Calculations

Require expressions to calculate maximum

injection pressure injection pressure to fill a part

To design/select injection system To determine clamping force Can obtain reasonable approximation from: Isothermal analysis Mould cavity only Constant flow rate In practice moulding operation can be complex: Non-isothermal & non-Newtonian - hence η = f (T, ) Flow within sprue, runners, gate & mould cavity Injection sequence can be relatively complex

 

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Injection Moulding 22

5.1 Filling Pressures 5.1 Filling Pressures

Injection pressure (Pmin) for given flow rate (Q) can be determined from non-Newtonian flow expressions (see Melt Rheology & Processing Melt Rheology & Processing notes).

Rectangular cavity (depth h):

T L Gate

Pmin

        nTh 1).2Q + (2n h L . C P drop pressure = P

2 n

2

min

Th LQ 12

3 a

 

(Newtonian, ie. n=1)

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Injection Moulding 23

5.1 Filling Pressures 5.1 Filling Pressures

Injection pressure (Pmin) for given flow rate (Q) can be determined from non-Newtonian flow expressions (see Melt Rheology & Processing Melt Rheology & Processing notes).

Rectangular cavity (depth h):

T L Gate

Pmin

        nTh 1).2Q + (2n h L . C P drop pressure = P

2 n

2

min

Th LQ 12

3 a

 

(Newtonian, ie. n=1)

Min. pressure usually not sufficient

Apply additional pressure to:

Compact material
Counteract shrinkage

Hold Hold-

on
n pressure of up to 2 x Pmin

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Injection Moulding 24

5.2 Clamping Forces 5.2 Clamping Forces

T L Gate

PG PX x dx

Rectangular cavity: Force required to clamp element of mould dx: δF = Px δA = Px T dx Total clamping force:

dx T P = F

x L



P L x

P

= P

G x



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Injection Moulding 25

5.2 Clamping Forces 5.2 Clamping Forces

T L Gate

PG PX x dx

Rectangular cavity:

P L x

P

= P

G x



Assuming linear linear pressure distribution:

dx P L x

P

T = F

G L

               2 P L

L

P T =

G

       2 P

P

TL =

G

Therefore: Projected area Projected area Clamping force Clamping force = = x x Mean pressure Mean pressure

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Injection Moulding 26

5.3 Mould Filling Times 5.3 Mould Filling Times

The time to fill a mould is simply:

rate flow volume volume total = t f

eg. for a rectangular mould cavity:

T L Gate

Pmin

Q TLh = t f

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Injection Moulding 27

5. Mould Filling Calculations
5. Mould Filling Calculations

Worked Example - Injection Mould Filling Calculate the minumum gate pressure required to fill a rectangular plaque cavity, 150mm x 25mm x 3mm, with Acrylic resin in one second, assuming the following conditions: (a) Newtonian flow with apparent viscosity ηa = 1000 Ns/m2

r (b) Non-Newtonian flow using Acrylic flow data.

If the gate pressure is 1.5 X this minimum, estimate the mould clamping force required for a double impression mould. Page 11

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Injection Moulding 28

5. Mould Filling Calculations
5. Mould Filling Calculations

Page 13

n = 0.254 C = 5.395 x 104 n = 0.566 C = 9.333 x 103 1 s-1 10 s-1 40 s-1 50 s-1 102 s-1 103 s-1 n = 0.254 C = 5.395 x 104 n = 0.566 C = 9.333 x 103 1 s-1 10 s-1 40 s-1 50 s-1 102 s-1 103 s-1

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Injection Moulding 29

5. Mould Filling Calculations
5. Mould Filling Calculations

Exercise sheet 4,

Qu. 1 – Fig. Q1