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9 . 1 I n t r o d u c t i o n

If plastics are processed by injection molding, deviations of the dimensions of the

molding from the dimensions of the cavity cannot be avoided. These deviations from the

nominal size are summarized under the term shrinkage.

9 . 2 D e f i n i t i o n o f S h r i n k a g e

In the injection-molding technique, shrinkage is the difference between an arbitrary

dimension in the cavity and the corresponding dimension in the molding with reference

to the cavity dimension.

S =

1 C

~

1 M

100% (9.1)

^c

Of course, this definition is not unambiguous (Figure 9.1)

[9.1].

On one side, the dimensions of the cavity change from thermal expansion (0

—>

1) and

mechanical loading during operation (1

—>

2 ), on the other side, the effect of time on the

dimensions of the molding has to be taken into consideration (2 - ^ 5).

One distinguishes the demo lding shrinkage (point 3), which is measured imm ediately

after the molding has been ejected, and the processing shrinkage (point 4). The

processing shrinkage is measured after storing the molding in a standard climate for 16

hours [9.2]. In this context the cavity dimension has to be determined at an ambient

temperature of 23

0

C ± 2

0

C .

9 S h r i n k a g e

D

i

m

e

n

s

i

o

n

o

f

m

o

l

i

n

g

Figure 9.1 Dimen sional changes as a

function of time [9.1]

0 Dimension in cold mold,

1 Dimension in hot mold,

2 Dimension in mold under clamping

force and holding p ressure,

3 Dimension of molding after dem olding,

4 Measuremen t of processing shrinkage

(DIN 16901),

5 Dim ension after storage

After-shrinkage AS)

hrnkage after demolding SD)

Processng shrinkage PS)

Total shrinkage TS)

Timet

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After extended storage another dimensional change may occur from the effect of

temperature changes and especially from post-conditioning. It is called post-shrinkage

(4

—>

5). This ch ange is caused by relaxation of residual stresses, re-orientation and

post-crystallization in crystalline materials. Except in crystalline materials, it is

negligibly small, though. The sum of processing shrinkage and post-shrinkage is called

total shrinkage. If additional dimensional deviations from moisture absorption or

higher temperatures of use have to be taken into account at the time of acceptance,

post-treatment and conditions of measurement have to be negotiated between molder

and customer. In addition, one can distinguish shrinkage in dependence on the direction

of flow (Figure 9.2). Radial processing shrinkage is shrinkage in the direction of flow,

tangential shrinkage is that perpendicular to the direction of flow.

The difference in processing shrinkage is the difference between radial and tangential

shrinkage and is a measurement of the anisotropy of the shrinkage. The shrinkage in

thickness is measured as section thickness, but it is usually not of interest in prac tice. For

measuring, any kind of mechanical or optical instrument can be used, but a possible error

from the measuring force should be taken into account for soft materials.

If the dimensions in Equation (9.1) are replaced by the volumes of cavity and

molding, one talks about volume shrinkage [9.3].

Figure 9.2 Frozen mod el [9.7]

Contour shrunk

Mold contour

Plastic core

Frozen frame

ound

length

Figure 9.3 M agnitude of shrinkage

depending on direction of flow

S

R

Radial shrinkage,

S

T

Tangential shrinkage,

S Shrinkage difference

(9-2)

V Specific volume of the material (Figure 9.7)

Longitudinal and volume shrinkage are related to one another but because of anisotropy

(dependency of shrinkage on direction), linear shrinkage cannot be calculated from

volume shrinkage. Another problem is the impossibility to measure volume shrinkage.

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Tolerances are closely related to sh rinkage but also to the nature of the particular plas tic.

Close tolerances can only be expected with precise machine and mold control. Therefore

they are beyond any action the mold maker can take. He has to try, however, to meet the

required dimensions assuming normal processing conditions. If they can only be met

under extreme conditions, the results are not in the center of the tolerance range. Then

this range can easily be exceeded.

In addition, one has to differentiate between dimensions connected to the mold and

those not connected to the mold. Dim ensions con nected to the mold are those which are

determined by duplicating one mold part (Figure 9.4 top ). Dim ensions not connected to

the mold are generated by the interaction of parts mov able toward s each other (stationary

Figure 9.4 Mo ld-related (top)

and not-mold-related (bottom)

dimensions [9.2]

Direction

o f

slide movement

D

i

r

e

c

t

o

n

o

f

m

o

l

d

c

l

a

m

p

i

n

g

It is possible to make an assumption of shrinkage from volume shrinkage of

thermoplastics in the following method e:

The shrinkage in direction of thickness H of an injection molded part

S

H

(0.9 - 0.95) S

v

The shrinkage in the direction of length L

S

L

= (0.05 - 0 . 1 ) S

v

For crosslinking polymers exists a special standard for shrinkage (see source [9.4]).

9 3 T o l e r a n c e s

The question of attainable tolerances is a cause of many complaints and, in extreme

cases, has to be the subject of negotiations between molder and customer. Tolerances

should never be closer than required for the perfect functioning of the part in use.

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Figure 9.5

Practical tolerances on dimensions of articles molded from ABS (Courtesy of the

Society of the Plastics Industry)

and movable mold

half,

slides) (Figure 9.4 bottom). This differentiation takes into

account the lower accuracy, which results from movable mold components; they do not

have exactly reproducible end po sitions.

Standards for tolerances are given in the form of tables in the Plastics Engineering

Handbook [9.11]. These tables (an example is shown in Figure 9.5) were developed by

the Society of the Plastics Industry, Inc. and are based on data obtained from

representative material suppliers and molders.

S T A N D A R D S A N D P R A C T I C E S O P

PLASTICS CUSTOM MOLDERS

Engineer ing and

Technical Standards

A B S

NOTE:

The Commercial values shown below represent common production tolerances at the most economical level.

The Fine values represent closer tolerances that can be held but at a greater cost.

Plus or Minus in Thousands of an Inch

imensions

(Inches)

Drawing

Code

A = Diameter

(see Note #1 )

B = Depth

(see Note # 3)

C = Height

(sec Note # 3)

D=BottomWall

(see Note # 3)

E = Side Wall

(see Note # 4)

F = Hole Size

Diameter

(see Note #1)

G = Hole Size

Depth

(seeNote#5)

Draft Allowance

per side

(see Note # 5)

Flatness

(see Note #4 )

Thread Size

(class)

Concentricity

(sec Note #4)

Fillets, Ribs,

Corners

(see Note #6)

Surface Finish

Color Stability

6 000 to 12.000

for each additional

inch add (inches)

0 000 to 0.125

0.125 to

0 250

0 250 to 0 500

0 500 Over

0 000

to

0 250

0 250 to 0 500

0 500 to 1.000

0 000

to 3.000

3.000 to 6 000

Internal

External

(T.I.R.)

(see Note #7 )

(see Note #7)

Comm. ±

.003

.004

.003

.002

.002

.003

.004

.003

.004

.005

2°

.015

.030

1

1

.009

.025

F i n c ±

.002

.002

.002

.001

.001

.002

.002

.002

.002

.003

1°

.010

.020

2

2

.005

.015

REFERENCE NOTES

1 — These tolerances do not include allowance for aging

characteristics of material.

2 - Tolerances based on

wall section.

3

—

Parting line must be taken into consideration.

4 — Part design should maintain a wall thickness as nearly

constant as possible. Complete uniformity in this

dimension is impossible to achieve.

5 — Care must be taken that the ratio of the depth of a

cored hole to its diameter docs not reach a point that

will result in excessive pin damage.

6 —These values should be increased whenever com -

patible with desired design and good molding

technique.

7

—

Customer-Molder understanding necessary prior to

tooling.

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1

Take next higher tolerance group

for

unfilled, crystalline thermoplastics and wall thicknesses more than

4

mm.

Pertinent information concerning tolerances may also be received from the British

Standard BS 4042. The following data are provided with reference to the German

Standard DIN 16 901.

Depending on the molding material one can determine several tolerance groups

Table 9.1). Within each group a distinction is made between two grades of accuracy for

entered allowances above and below nominal sizes. For each tolerance group allowances

depending on nominal sizes can be taken from a second table Table 9.2). In this table a

Table 9.1

Coordination

of

tolerance groups with molding materials

[9.2]

Moldings made

of:

Acetal (polyoxyrnethylene)

1

(unfilled), part length: < 150 mm

Acetal (polyoxymethylene)

1

(unfilled), part length: >150 mm

Acetal (polyoxymethylene)

1

(filled)

Acrylic

Diallyl phthalate comp ounds (with inorganic filler)

Polyethersulfone (unfilled)

Polyethylene

1

(unfilled)

Polyethylene terephthalate (amorphous)

Polyethylene terephthalate (crystalline)

Polyethylene terephthalate (filled)

Polyphenylene oxide

Polyphenylene oxide-styrene mixture (filled, unfilled)

Polyphenylene sulfide (filled)

Polypropylene

1

(unfilled)

Polypropylene

1

(filled with glass ribers or talc)

Polypropylene impact copolymer (unfilled)

Polystyrene

Polysulfone (filled, unfilled)

Polyvinyl chloride (without plasticizer)

Poly vinyl ch loride (with plasticizer)

Styrene-acrylonitrile (filled, unfilled)

Styrene-butadiene copolymers

Fluorinated ethylene propylene

Thermoplastic polyurethanes (hardness 70-90 Shore

A)

Thermoplastic polyurethanes (hardness

> 50

Shore

D)

Tolerance groups

For

common

tolerances

140

150

130

130

130

130

150

130

140

130

130

130

130

150

140

140

130

130

130

For dimensions

with allowances

entered in the

drawing

Grade

1

130

140

120

120

120

120

140

120

130

120

120

120

120

140

130

130

120

120

120

Grade 2

120

130

110

110

110

110

130

110

120

110

110

110

110

130

120

120

110

110

110

No information

at

present

130

130

150

150

140

120

120

140

140

130

110

110

130

130

120

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Table 9.2 Coordination of tolerances with tolerance groups [9.2]

Tolerance

group

from

Table 1

Code

letter

1

Range

of

nominal dimensions more than/until)

0

1

1

3

3

6

6

10

10

15

15

22

22

30

30

40

40

53

Common tolerances

160

150

140

130

A

B

A

B

A

B

A

B

±0.28

±0.18

±0.23

±0.13

±0.20

±0.10

±0.18

±0.08

±0.30

±0.20

±0.25

±0.15

±0.21

±0.11

±0.19

±0.09

±0.33

±0.23

±0.27

±0.17

±0.22

±0.12

±0.20

±0.10

±0.37

±0.27

±0.30

±0.20

±0.24

±0.14

±0.21

±0.11

±0.42

±0.32

±0.34

±0.24

±0.27

±0.17

±0.23

±0.13

±0.49

±0.39

±0.38

±0.28

±0.30

±0.20

±0.25

±0.15

±0.57

±0.47

±0.43

±0.33

±0.34

±0.24

±0.27

±0.17

±0.66

±0.56

±0.49

±0.39

±0.38

±0.28

±0.30

±0.20

±0.78

±0.68

±0.57

±0.47

±0.43

±0.33

±0.34

±0.24

Tolerances

for

dimensions with entered allowances

160

150

140

130

120

110

Precision

molding

A

B

A

B

A

B

A

B

A

B

A

B

A

B

0.56

0.36

0.46

0.26

0.40

0.20

0.36

0.16

0.32

0.12

0.18

0.08

0.10

0.05

0.60

0.40

0.50

0.30

0.42

0.22

0.38

0.18

0.34

0.14

0.20

0.10

0.12

0.06

0.66

0.46

0.54

0.34

0.44

0.24

0.40

0.20

0.36

• 0 . 1 6

0.22

0.12

0.14

0.07

0.74

0.54

0.60

0.40

0.48

0.28

0.42

0.22

0.38

0.18

0.24

0.14

0.16

0.08

0.84

0.64

0.68

0.48

0.54

0.34

0.46

0.26

0.40

0.20

0.26

0.16

0.20

0.10

0.98

0.78

0.76

0.56

0.60

0.40

0.50

0.30

0.42

0.22

0.28

0.18

0.22

0.12

1.14

0.94

0.86

0.66

0.68

0.48

0.54

0.34

0.46

0.26

0.30

0.20

0.24

0.14

1.32

1.12

0.98

0.78

0.76

0.56

0.60

0.40

0.50

0.30

0.32

0.22

0.26

0.16

1.56

1.36

1.14

0.94

0.86

0.66

0.68

0.48

0.54

0.34

0.36

0.26

0.28

0.18

1

A for

dimensions

no t

connected

to the

mold,

B for

dimensions connected

to the

mold

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Range of nominal dimensions more than/until)

53

70

70

90

90

120

120

160

160

200

200

250

250

315

315

400

400

500

500

630

630

800

800

1000

Common tolerances

±0.94

±0.84

±0.68

±0.58

±0.50

±0.40

±0.38

±0.28

±1.15

±1.05

±0.81

±0.71

±0.60

±0.50

±0.44

±0.34

±1.40

±1.30

±0.97

±0.87

±0.70

±0.60

±0.51

±0.41

±1.80

±1.70

±1.20

±1.10

±0.85

±0.75

±0.60

±0.50

±2.20

±2.10

±1.50

±1.40

±1.05

±0.95

±0.70

±0.60

±2.70

±2.60

±1.80

±1.70

±1.25

±1.15

±0.90

±0.80

±3.30

±3.20

±2.20

±2.10

±1.55

±1.45

±1.10

±1.00

±4.10

±4.00

±2.80

±2.70

±1.90

±1.80

±1.30

±1.20

±5.10

±5.00

±3.40

±3.30

±2.30

±2.20

±1.60

±1.50

±6.30

±6.20

±4.30

±4.20

±2.90

±2.80

±2.00

±1.90

±7.90

±7.80

±5.30

±5.20

±3.60

±3.50

±2.50

±2.40

±10.00

±9.90

±6.60

±6.50

±4.50

±4.40

±3.00

±2.90

Tolerances for dimensions with entered allowances

1.88

1.68

1.36

1.16

1.00

0.80

0.76

0.56

0.60

0.40

0.40

0.30

0.31

0.21

2.30

2.10

1.62

1.42

1.20

1.00

0.88

0.68

0.68

0.48

0.44

0.34

0.35

0.25

2.80

2.60

1.94

1.74

1.40

1.20

1.02

0.82

0.78

0.58

0.50

0.40

0.40

0.30

3.60

3.40

2.40

2.20

1.70

1.50

1.20

1.00

0.90

0.70

0.58

0.48

0.50

0.40

4.40

4.20

3.00

2.80

2.10

1.90

1.50

1.30

1.06

0.86

0.68

0.58

5.40

5.20

3.60

3.40

2.50

2.30

1.80

1.60

1.24

1.04

0.80

0.70

6.60

6.40

4.40

4.20

3.10

2.90

2.20

2.00

1.50

1.30

0.96

0.86

8.20

8.00

5.60

5.40

3.80

3.60

2.60

2.40

1.80

1.60

1.16

1.06

10.20

10.00

6.80

6.60

4.60

4.40

3.20

3.00

2.20

2.00

1.40

1.30

12.50

12.30

8.60

8.40

5.80

5.60

3.90

3.70

2.60

2.40

1.70

1.60

15.80

15.60

10.60

10.40

7.20

7.00

4.90

4.70

3.20

3.00

2.10

2.00

20.00

19.80

13.20

13.00

9.00

8.80

6.00

5.80

4.00

3.80

2.60

2.50

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difference is also made between dimensions connected to the mold and those not

connected to the mold.

9 4 C a u s e s o f S h r i n k a g e

The intrinsic cause for shrinkage of injection-molded parts is the thermodyn amic behavior

of the material (Figure 9.6). It is also called p-v-T (pressure-volume-temperature)

behavior and characterizes the compressibility and thermal expansion of plastics [9.5].

There is a basically different p-v-T behavior between two classes of materials

(amorphous and crystalline). As a melt, both classes show a linear dependency of the

specific volume on the temperature. For the solid phase, however, there are considerable

differences. On the basis of crystallization the specific volume decreases exponentially

with decreasing temperature while amorphous materials also have a linear dependency

in the solid phase. This difference is the reason for the greater shrinkage of crystalline

thermoplastics.

To assess the process w ith respect to shrinkag e, the change in state in a p-v-T diagram

is very helpful. Pressure and temperature during the process are recorded isochronously

in a p-v-T diagram (Figure 9.7).

Following the volume filling of the cavity (0

— »

1) the material is com pacted in the

compression phase without substantial change in temperature (1 — » 2). The magnitude of

the locally attainable pressure in the molding depends on the magnitude of the holding

pressure exerted by the machine and on the resistance to flow in the cavity.

Subsequently, the molding steadily cools down (2 — » 3). Related to this is a volume

contraction, which can be partly compensated by the holding pressure, which supplies

additional melt to the cavity through the liquid core of the solidifying molding. If no

more melt can be fed into the cavity, e.g., by a solidified gate, the change in state is

isochoric (3

— »

4).

S

p

e

c

i

c

v

o

l

u

m

e

S

p

e

c

i

c

v

o

l

u

m

e

amorphous

crystalline

cm

3

/g

Temperature

0

C

cmfyg

Temperature

0

C

Figure 9.6 p-v-T diagram of an

amorphous (top) and a crystalline

(bottom) thermoplastic material

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The point wh ere the 100-kPa line is met (point 4) establishes the local volum e sh rinkage .

A higher volume shrinkage occurs if this point is in the range of larger volumes. Since

the volume shrinkage is equivalent to the shrinkage potential, a larger volume shrinkage

also results in a higher longitudinal shrinkage.

After the 100-kPa line has been reached , any further chang e of state is isoba ric. At the

time the molding is ejected (point 5) and constraints from the surrounding cavity cease.

9 5 C a u s e s o f A n i s o t r o p i c S h r i n k a g e

Dimensional changes of a molding in the mold are restricted or prevented by a force-

locking clamping of the mold halves [9.6] and shrinkage is non-uniform (anisotropic). A

distinction must be drawn between internal and external constraints of contraction.

External restriction of shrinkage is a mechanical restriction against a change of shape

by the surrounding mold. The restriction of shrinkage and the related stress relaxation

result in a lower level of shrinkage. The shrinkage of a restricted part is less than that

of a restricted one and there is, moreover, less dependence on the process parameters

(Figure 9.8).

The mechanical restriction, of course, is effective only as long as the molding is still

in the mold. After ejection, restricted dimensions can also shrink freely. Therefore the

temperature of demolding is a characteristic for the change in mechanical boundary

conditions and for the shrinkage and distortion behavior.

Internal restriction of shrinkage is due to both cooling-related internal stresses and to

orientation.

M olecule orientations affect shrinkage in two ways. On the one hand, the coefficients

of linear expansion dependent on orientation cause a difference in shrinkage. On the

other, re-orientation (contraction) in the direction of orientation contributes to an

increase in shrinkage. The molecular orientation is determined by the process

parameters, and primarily by the type and location of gating, but exerts much less of an

influence than does, e.g., fiber orientation in fiber-filled molding compounds.

Figure 9.7 Change of state in the p-v-T

diagram [9.3]

0 — > 1 Volumetric filling,

1 — > 2 Com pression,

2

— >

3 Effect of holdin g press ure,

3 — > 4 Isochoric pressure drop down to

temperature T

100 kPa

4

—

5 Cooling to demolding temperature T

E

5 — > 6 Cooling to room temperature T

R

4 - * 6 Volume shrinkage.

S

p

e

c

i

c

v

o

l

u

m

e

v

P

r

e

s

s

u

r

e

p

T

e

m

p

e

r

a

t

u

r

e

T

Timet

Timet

Pressure

Temperature

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Oriented by the flow processes, the fibers hinder shrinkage primarily in the direction of

orientation because they hav e a lower thermal exp ansion and greater stiffness relative to

the matrix material (Figure 9.9) [9.8]. Through the use of fibers, shrinkage may be

reduced by up to 80%. However, no further reductions in shrinkage behavior are

observed at additions of more than 20% fiberglass.

Incorporation of fillers such as glass beads and mineral powder leads to isotropic

shrinkage. The reduction in overall shrinkage that occurs is due to the lower

compressibility of the material as a whole.

Aside from fiber orientation, molecular orientation in the direction of flow leads to

anisotropic shrinkage.

Hodng pressure

P

H

MPa

Pa

Hodng pressure

P

H

Figure 9.8 Shrinkage of a free (left) and a confined (right) circular plate

S

h

r

n

k

a

g

e

S

|

S

h

r

n

k

a

g

e

S

i

near gate

in

center

fa r trom gate

near gate

in center

fa r

from gate

L

o

n

g

i

u

d

i

n

a

l

s

h

r

n

k

a

g

e

S

|

Temperature

o f

cavity wall

0

C

Figure 9.9 Effect of

glass fibers and spheres on

shrinkage

Part: bushing,

Material: PBPT unreinforced

( X ) ,

3 0 glass spheres (O),

3 0 glass fibers (D),

T

M

= 251°C,

P

w

= 33 MPa.

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9 6 C a u s e s o f D i s t o r t i o n

Distortion is one result of anisotropic shrinkage. Frequently, it is caused by asymmetric

cooling relative to the part thickness. A higher wall temperature on the top side as shown

in Figure 9.10 leads, for example, to higher temperatures in the upper layers and, via

greater volume contraction, to deflection towards the warmer side. This differential

cooling may also be caused by inserts, such as decorative m aterial in in-mold decoration.

Corner distortion (Figure 9.11; see also Figure 8.46) is due to poorer heat dissipation

towards the inside of a corner. This has the effect of reducing the corner angles.

Similarly, differences in the thickness of ribs will displace the temperature profile from

its symmetrical center position and result in distortion of the moldings (Figure 9.12).

This cooling-induced distortion can be prevented by altering the mold temperature,

where necessary, by relocating the cooling channels or using mold inserts of different

material. Inner corners and thick ribs need to be cooled better than other part sections

(see also Section 8.6).

To avoid distortion caused by orientation, the gate should be repositioned or the flow

path modified by changing the wall thickness. Distortion in flat parts can be coun teracted

by applying thin bracing ribs.

The low thermal conductivity of plastic results in the temperature profile shown in

Figure 9.10. Different cooling conditions exist for the different layers and so volume

contraction also varies. Due to mechanical coupling between the layers, thermal

contraction in the longitudinal and transverse directions is restricted. This restriction

does not exist in the direction of thickness, with the result that most of the volume

shrinkage takes the form of shrinkage of the cross-section. Restrictions of shrinkage of

the same kind in longitudinal and transverse direction result in the same shrinkage,

provided no warpage orientation of molecules or fibers occurs.

o)

Moded part

thckness s

b) No mechanca coupng

of layers

c) Rea change

mechancay couped)

Therma contracton

igure 9.10 Mod el of stress buildup [9.12]

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9 7 E f f e c t o f P r o c e s s i n g o n S h r i n k a g e

Other than by modifications to the mold and a change of material, the molder can only

influence shrinkage and distortion by making changes to the process. From the p-v-T

diagram, it can be seen that pressu re and temp erature are the main factors affecting

shrinkage. Design changes will affect these parameters and thus also the shrinkage. A

survey of the influences exerted by various parameters is shown in Figure 9.13 [9.11].

With amorphous as well as with crystalline thermoplastics, the holding pressure exerts

the greatest effect on shrinkage (Figure 9.8).

Under holding pressure, the material in the cavity is compressed and the volume

contraction from cooling is compensated by additional melt supply. The influence of the

holding pressure is shown in the p-v-T diagram charting the progress of the process

(see Figure 9.7). If the holding pressure is increased, the process is shifted to lower

specific volumes, reaching at lower specific volumes the 100-kPa line at which the part

undergoes lower shrinkage. The influence of the holding pressure is degressive,

however; in other words, the reduction in shrinkage decreases with increase in holding

pressure.

Figure 9.12

Distortion caused

by differences in wall thickness

Figure 9.11

Corner distortion through poorer heat

dissipation in internal corners

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Wall thickness

Fow restriction

Runner profile

cross-section

Runner length

Holding

pressure

time

Holding

pressure

Fow path/wall thickness ratio

Gate size

Mold

temperature

Melt

temperature

Injection

speed

Figure 9.13 Relationships between shrinkage and characteristic parameters [9.11]

In

direction

o f

flow

Perpendcuar to

direction o f flow

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With an increase in holding pressure, a reduction in shrinkage of up to 0.5% can be

obtained in crystalline materials. With amorphous materials, a reduction of just 0.2%

max. is feasible because of their overall lower level of shrinkage.

The second major influence on shrinkage is the temperature of the material.

Theoretically, a higher injection temperature has two opposing effects on shrinkage:

on the one hand, a higher temperature results in a higher thermal contraction potential of

the material (see also Figure 9.7 and [9.7]) and, on the other, the decrease in melt

viscosity causes a better transfer of pressure and with this a reduction in shrinkage.

Given sufficiently long holding-pressure stages, the effect of improved cavity pressure

predominates in the case of crystalline materials (Figure 9.14).

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h

r

n

k

a

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e

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|

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Met temperature

TM

Figure 9.14

Effect of melt temperature on shrinkage (top: Crystalline, bottom : Amorphou s

material)

Melt temperature TM

near gate

:'

in center :

fa r from gate

near gate

in center

fa r from

gate~

0

C

0

C

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With crystalline materials, a reduction in shrinkage can be obtained of up to 0.5%; with

amorphous plastics, the figure is up to 0.15%. All other parameters determine the

shrinkage behavior via the pressure and temperature. While a greater wall thickness

leads to better pressure transfer, the poor thermal conductivity of plastics makes volume

contraction more noticeable at high temperatures and increases the shrinkage.

Restrictions to flow impa ir pressure transfer and so increase shrinkage. In contrast, larger

runner profile cross-sections and thicker gates make for better pressure transfer. A large

runner length leads, just as does a large flow path to wall thickness ratio, to a drop in

pressure and thus to greater shrinkage. H ot runners, however, reduce shrinkage.

The influences of the holding pressure time can be used to again illustrate the most

important criterion concerning shrinkage. As the holding pressure time increases, the

forcing of additional material into the cavity reduces shrinkage. This can only happen,

however, as long as the melt, particularly the gate and sprue, has not frozen. Prolonging

the holding pressure time beyond that has no further effect. For this reason, a part for

homogeneous molding materials should always be gated at the thickest point and the

wall thicknesses should be such that holding pressure can take effect even in those areas

furthest away from the sprue.

With glass-reinforced materials, there are some particularities (Figure 9.15). In the

direction of fiber orientation, it is not possible to affect shrinkage by modifying pro-

cessing parameters, as the rigidity of the fibers exerts an extremely strong influence. The

effect perpendicular to the direction of fibers is approximately the same as it is with the

matrix material only.

9 8 S u p p l e m e n t a r y M e a n s f o r P r e d i c t i n g S h r i n k a g e

The s implest

w a y t o

est imate shr inkage

fo r

d imens ioning

a

m o l d

is to

cons ult tables

(Table 9.3). They are provided b y t h e raw-mater ia l suppl iers in the dat a shee ts fo r their

respect ive mater ia ls .

However , the par t ly wide range o f listed data is pro ble ma tic b ec aus e i t doe s not al low

a sufficiently accurate prediction

o f

shr inkage ;

n o r a r e

per t inent process param eters

k n o w n o r conf igurat ions o f mol ding s f rom whi ch t h e shr inkag e was obtained. Transfer

to othe r con figu rati ons is , ther efor e, difficult.

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i

u

d

i

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l

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e

Cavity pressure

p

c

max

Figure 9.15 Effect on shrinkage of glass fiber reinforced materials

10OkPa

Molding: plate

Material: Nyon

6,

30% gass

reinforced

Thickness:

3 mm

Reaton to direction

of

flow:

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A more accurate prediction can be made on the basis of a collection of data gained

through experience. This is the most reliable method for predicting linear shrinkage so

far. In such a collection the shrinkage data of all parts are listed which have been

produced in the past as well as their processing conditions. Because of different

restrictions to shrinkage and geometry elements, families of dimensions are formed.

If the mold for a similar part has to be designed, these data can be used for

dimensioning.

Another and increasingly more accurate estimate of shrinkage behavior is provided by

FEA simulation [9.12] see also Chapter 14). Exact dimensioning of parts with the aid of

this method is not yet possible. However, the process and the part can be optimized in

respect to shrinkage and distortion behavior. The ability to predict part dimensions and

volume shrinkage, but more so the temperature and pressure behavior, are important

tools for accomplishing this. Since several physical parameters such as crystallization

behavior cannot as yet be determined, some material data cannot be determined with

sufficient accuracy batch fluctuations cannot be allowed for, and it is still not possible to

make an exact predictive simulation of part dimensions.

R e f e r e n c e s

[9.1] Hov en-Nievelstein, W. B.:

Die

Verarbeitungsschwindung thermoplastischer Formmassen.

Dissertation, Tech. University, A achen, 1984.

[9.2] German Standard: DIN

16901:

Kunststoff-Formteile Toleranzen und Abnahmebedingun-

gen fiir LangenmaBe.

[9.3] Schmidt, Th. W.: Zur Abschatzung der Schwindung. Dissertation, Tech. University,

Aachen,

1986.

[9.4] German Standard: DIN 53464: Prufung von Kunststoffen. Bestimm ung der Schwindungs-

eigenschaften von PreBstoffen aus warmhartbaren PreBmassen.

[9.5] Geisbiisch, P.: An satze zur Schw indungsberechn ung ungefiillter und m ineralisch gefullter

Therm oplaste. D issertation, Tech. University, Aach en, 1980.

[9.6] Zipp, Th.: Erfahrungsanalyse

zur

Erm ittlung

des

notwen digen WerkzeugiibermaBes beim

SpritzgieBen. Unp ublished report, IKV, Aache n, 1985.

[9.7] Stitz,

S.:

Analyse

der

Forrnteilbildung beim SpritzgieBen

von

Plastomeren

als

G rundlage

fiir die ProzeBsteuerung. Dissertation, Tech. University, Aachen, 1973.

[9.8] Menges, G.; Hoven-Nievelstein, W. B.; Zipp, Th.: E rfahrungskatalog zur V erarbeitungs-

schwindung thermoplastischer Formmassen beim SpritzgieBen. Unpublished report, IKV,

Aachen, 1984/85.

Table 9 3 Shrinkage

of

some thermoplastics [9.10]

Material

Nylon 6

Nylon 6-GR

Nylon 6/6

Nylon 6/6-GR

Low-density polyethylene

High-density polyethylene

Polystyrene

Styrene-acrylonitrile

Polymethyl methacrylate (Acrylic)

Shrinkage

%

1-1.5

0.5

1-2

0.5

1.5-3

2-3

0.5-0.7

0.4-0.6

0.3-0.6

Material

Polycarbonate

Polyoxymethylene (Acetal)

Polyvinyl chloride, rigid

Polyvinyl chloride, soft

Acrylonitrile-butadiene-styrene

Polypropylene

Cellulose acetate

Cellulose acetate butyrate

Cellulose propionate

Shrinkage

%

0.8

2

0.5-0.7

1-3

0.4-0.6

1.2-2

0.5

0.5

0.5

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[9.9] Baur, E.; Schleede, K.; Lessenich , V.; Ort, St.; FiIz, P.; Potsch, G.; Gro th, S.;

Greif,

H.:

Formteil- und Werkzeugkonstruktion aus einer Hand - Die modernen Hilfsmittel fur den

Konstrukteur. Contribution to 14th Technical Conference on Plastics, Aachen, 1988.

[9.10] Strack Norm alien fur Form werk zeuge. Hand book, Strack-Norm a Gm bH , W uppertal.

[9.11] Frados, J.: Plastics Engineering H andbook. Van Nostrand R einhold, New York, 1976.

[9.12] Potsch, M. G.: Prozessimulation zur Abschatzung von Sch windung und Verzug thermo-

plastischer Spritzgussteile. Dissertation, RWTM, Aachen, 1991.

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