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B.3

.3

Plastic parts with integrally molded threads

COPYRIGHT: All rights reserved, in particular for reproduction and copying, and for distribution as well as for translation. No part of this publication may be reproduced or processed by means of electronic systems, reproduced or distributed (by photocopying, microfilm or any other process), without written permission by Ticona. © 2004 Ticona GmbH, Kelsterbach NOTICE TO USERS: To the best of our knowledge, the information contained in this publication is accurate, however we do not assume any liability whatsoever for the accuracy and completeness of such information. The information contained in this publication should not be construed as a promise or guarantee of specific properties of our products. Further, the analysis techniques included in this publication are often simplifications and, therefore, approximate in nature. More vigorous analysis techniques and prototype testing are strongly recommended to verify satisfactory part performance. Anyone intending to rely on any recommendation or to use any equipment, processing technique or material mentioned in this publication should satisfy themselves that they can meet all applicable safety and health standards. It is the sole responsibility of the users to investigate whether any existing patents are infringed by the use of the materials mentioned in this publication. Properties of molded parts can be influenced by a wide variety of factors including, but not limited to, material selection, additives, part design, processing conditions and environmental exposure. Any determination of the suitability of a particular material and part design for any use contemplated by the user is the sole responsibility of the user. The user must verify that the material, as subsequently processed, meets the requirements of the particular product or use. The user is encouraged to test prototypes or samples of the product under the harshest conditions to be encountered to determine the suitability of the materials. Material data and values included in this publication are either based on testing of laboratory test specimens and represent data that fall within the normal range of properties for natural material or were extracted from various published sources. All are believed to be representative. These values alone do not represent a sufficient basis for any part design and are not intended for use in establishing maximum, minimum, or ranges of values for specification purposes. Colorants or other additives may cause significant variations in data values.

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Contents

1. Introduction 3

2. Requirements for jointswith integrally molded threads 3

2.1 Fixed joints 3

2.2 Movable joints 3

3. Types of thread and key dimensions 4

3.1 Fixed joints 4

3.2 Movable joints 6

4. Design calculations for threaded joints 6

4.1 Fixed joints 6

4.1.1 Stress in the vulnerable cross-section 6

4.1.2 Flank pressure p 7

4.1.3 Deformation of the threaded section 8

4.2 Movable joints 9

5. Calculation examples 10

5.1 Reversible-flow filter made fromHostaform C 2521 10

5.2 Hose connector made fromHostalen PPN 1060 11

5.3 Vehicle jack with threaded nut

made from POM 12

6. Design notes

6.1 Self-locking6.2 Stress concentration in the

threaded section

6.2.1 Notch factors K in disengagedthread zone I

6.2.2 Stress distribution within the internal

thread engaged with the external thread

(zone II, fig. 10)6.2.3 Stress concentration in the thread runout

6.3 Static sealing of plastic componentswith integrally molded threads

7. Injection molding of componentswith integrally molded threads7.1 External thread7.2 Internal thread

8. Examples of applications8.1 Water filter housing8.2 Drainage plug for water separator

on diesel vehicles

8.3 Grain hopper for corn mill8.4 Fastening nut for spare wheel

9. Explanation of symbols

10. Literature

13

13

14

14

14

15

15

17

17

17

19

19

19

20

20

21

22

HostaformAcetal copolymer (POM)

HostacomReinforced polypropylene (PP)

Hostalen PPPolypropylene (PP)

CelanexPolybutylene terephthalate (PBT)

= registered trademark

1. Introduction

The unique freedom of design afforded by injection molding enables this process to be used for the production of

components with integrally molded threads. The additional complexity and cost involved in terms of the injectionmold (for demolding the thread) is relatively small.

Integrally molded threads are used for detachable fixed

joints in many different components. Examples include

housing parts for washing machines and dishwashers,filters, valves and submersible and circulating pumps;fittings and screwed pipe joints; locking rings, e.g. on

water meters and mincers; closures for packaging such

as tubes, bottles and drums.

Integrally molded threads are also used for movable jointsto convert rotary motion into linear motion or torque intolinear forces and vice versa. Examples include valve stems,

telescopic spindles for ventilation windows, spindle nuts

on car jacks, electric rear view mirror and seat adjustingsystems and central locking in motor vehicles, reversingspindles in vending machines etc.

2. Requirements forjointswith integrally moldedthreads

2.1 Fixedjoints

The function of the above-mentioned housing parts is

generally to keep different materials safely apart fromeach other. For this purpose, the joint must be fixed and

leaktight and should be suitably designed to prevent it

loosening by itself. Leaktightness is achieved by integralsealing elements or additional sealing elements such as

O-rings. In filter housings, e.g. for the fluff filter on

washing machines, the joint should be quick and easyto undo in order to clean or change filters.

2.2 Movablejoints

Movable threaded joints, also known as helical gears,should permit smooth, jolt-free transmission of motion.These threads are designed as single- and multi-start.

Single-start threads are generally self-locking with low

efficiency. The product of flank pressure p and speed v

may not exceed certain limit values to ensure that unac

ceptable heating up of the thread flanks does not occur.

Good low-friction and wear properties are exhibited bythreads produced, for example, from the modifiedHostaform grades C 9021 TF, C 9021 K, C 9021 G andC 9021 M.

Once-only lubrication of the thread improves the slipproperties of the mating thread and should always becarried out if possible.

3. Types of thread andkey dimensions

3.1 Fixedjoints

Threaded joints in housings can have very large threaddiameters in relation to the comparatively low housingwall thickness typical of injection molded parts. Thisresults in low inherent housing rigidity, so that the inner

part can be compressed under radial force Fr and the outer

part expanded. There is a risk that the thread flights willslide over each other and the joint will therefore fail. Thisrisk is reduced the lower the radial force Fr and the greaterthe thread depth HI of the thread profile, fig. 1.

The radial force Fr is proportional to the tangent of thehalf angle of thread ß.

Table 1: Comparison of important thread dimensions

Fr = FI tan ß (1)

A large thread depth reduces the sensitivity of the joint to

production tolerances.

Fig. 1: Force relationships on the thread flank

F,FIF

Radial forceLinear forcePerpendicular forceHalf angle of thread

.3, HI Thread depth

Table 1 compares the half angle of thread ß and thread

depth H! of different thread types.

Thread type

Metric

ISO thread

Metricbuttress thread

Metric ISO

trapezoidal thread

Whitworth

pipe thread

Threads preferablyused for plasticcontainers

buttress thread

trapezoidal thread

Round thread

DIN

13

513

103

228

6063

part 1

part 2

405

Half angleof thread

ß[]

30

3

15

27.5

10

10

15

Thread

depth

h3 = 0.613 P

H! = 0.541 P

h3 = 0.868 P

H! = 0.75 P

h3 =0.5P+ a

H, = 0.5 P

H! = 0.640 P

c = 0.5P

c = 0.5P

H4=0.50P

(P = thread pitch)

Metric ISO threads and Whitworth pipe threads have

large half thread angles /?, which lead to comparativelylarge radial force Fr. The metric ISO trapezoidal threadand the buttress and trapezoidal threads developed for

plastic containers are more favorable, fig. 2.

Fig. 2: Thread types; illustration and explanation of terms

Fig. 2 a: Metric ISO thread (DIN 13)

TÏ1.

- %%2 threadQ Q Q ///Y//////////t

P

b: Metric buttress thread (DIN 513) Fig. 2e: Thread preferably to be used for plastic con

tainers, buttress thread (DIN 6063 part 1)

D; d Outside diameterDI; d| Root diameter

c Profile depth; c =

z Profile width

P Pitch

Fig. 2c: Metric ISO trapezoidal thread (DIN 103) Fig. 2 f: Thread preferably to be used for plasticscontainers, trapezoidal thread (DIN 6063 part 2)

U-P-

D ; d Outside diameterDI; dj Root diameter

c Thread depth; c =

z Profile width

P Pitch

Fig. 2d: Whitworth pipe thread (DIN/ISO 228)

- -5 W//Ï. Male thread %///////s////////////////.

Q Q O

Fig. 2g: Round thread (DIN 405)

o -a -o

The lowest, practically negligible radial force Fr occurredwith the metric buttress thread, fig. 2b, with a half angleof thread ß = 3 (tan 3 = 0.0524). This thread also has

a large thread depth HI = 0.75 P (P = thread pitch).

The round thread specified in DIN 405, fig. 2g has a

favorable half angle of thread ß = 15 and the thread

depth H4 = 0.5 P is also in the range of other thread

profiles. As a result of the special profile design, however,the flank overlap H5 is only 0.0835 P so that locallyhigh surface pressure loadings are the result.

High radial forces with the risk of impermissible stresses

and deformation may be expected with Whitworth pipethreads with cylindrical internal and conical externalthreads (DIN 2999). This thread type is therefore unsuitable for plastic components.

3.2 Movable joints

For movable joints, the metric ISO trapezoidal thread

(DIN 103) has proved successful. In rare cases, a non-

standardized flat profile (half angle of thread ß = 0) is

used.

4. Design calculations

for threadedjoints

4.1 Fixedjoints

In designing threaded joints, the following stress parameters must be taken into account:

stress oz in the vulnerable cross-section A^ 2

surface pressure loading p on the thread flanks

change in flank diameters Zld2 and 4D2.

4.1.1 Stress in the vulnerable cross-section

Fig. 3: Vulnerable cross-section AI and A2

From the linear force F] (fig. 1), the following tensile

stress results:

QZ ="1

A,, 2

It

[N/mm2]

where A, - = (d32 - d;2)

(2)

(3)

(4)A2 = | (Da2-D2) ist.

d3 root diameter [mm]d; inside diameter [mm]Da outside diameter [mm]D nominal diameter [mm]

According to DIN 13, part 21, the (more exact) stress

cross-section AS

n ( d2 + d3 YAS=" 4\

~

is calculated, which takes into account the thread tooth

cut over half the circumference as a loadbearing surface.

While this is justified with metal screws with high materialutilization, in the case of plastic threads, the root diameterds is used to determine the loadbearing cross-section.

The linear force FI is composed of prestressing force Fyand operating force FB.

FI = FV + FB (5)

The operating force may be an external force actingon the joint or be produced by, for example, internal

pressure p;.

The prestressing force Fv results from the tighteningmoment M according to the equation

FV =2M

P +d2-fcn cos ß

+ dA fA[N] (6)

M

P

fefAd2dA

tightening moment [N mm]thread pitch [mm]thread friction coefficientfriction coefficient of the contact surfaceflank diameter [mm]mean diameter of the contact surface [mm] (fig.6)half angle of thread

The friction coefficients fG and fA depend on a number of

influencing factors such as the particular combination of

mating materials, surface roughness, sliding speed and the

presence or absence of lubricant. Guide values are shownin table 2.

Table 2: Guide values for the friction coefficients fc andfA of unlubricated and lubricated surfaces

Material combination

PCR/PCR (same)PCR/PCR (different)PCR/metalPCR/AP

Friction coe

dry

0.3 -0.4

0.15-0.25

0.1 -0.3

0.15-0.25

ficient fG, fA

lubricated

0.04-0.1

PCR partially crystalline plasticsAP amorphous plastics

For the operating force FB resulting from the internal

pressure p;, the following applies:

The tensile stress <7z calculated with linear force

FI = FV + FB must be equal to or less than the permissiblestress.

Oz > perm. (8)

Table 3 gives guide values for long-term permissibletensile stresses as a function of temperature (safety factorS has already been taken into account).

Table 3 : Permissible tensile stresses 0perm. in N/mm2 as

a function of temperature

Material

Hostaform C 9021

Hostaform C 9021 GV 1/30

Hostalen PPN 1060

Hostacom M2 N02

Hostacom M4 N01

Hostacom G2 N02Hostacom G3 N01

Celanex 2500

Celanex 2300 GV 1/20Celanex 2300 GV 1/30

Temperature [C]

20

10

40

6

8

10

12

15

8

20

25

60

6

20

3

4

5

9

10

5

16

20

80

3

15

2

2

3

6

7

2

14

18

100

_

10

-

-

4

5

_

12

15

120

_

-

_

-

-

-

-

_

10

12

If the calculated stress GZ is greater than the permissiblestress, the loadbearing cross-section AI or A must be

enlarged by increasing the outside diameter Da and/or

reducing the inside diameter d;.

4.1.2 Flank pressure p

Although the linear force FI is not uniformly distributedover the intermeshing thread flights (see section 6.2.2),this is assumed for the purpose of calculating the flank

pressure p. Thus:

P =F,

z jt d2 H![N/mm2] (9)

pidp

FB = P, J dp2 [N] (7)

internal pressure [N/mm2]diameter of the pressure-stressed surface [mm]

L

P

F} linear force [N], see section 4.1.1

d2 flank diameterHI thread depth [mm]z number of loadbearing thread flights

Lz

thread reach [mm]thread pitch [mm]

(10)

Guide values for permissible flank pressure pperm. are

quoted in table 4 (safety factor S is already taken into

account).

Table 4: Guide values for permissible flank pressure pperm.in N/mm2 as a function of temperature

Material

Hostaform C 9021

Hostaform C 9021 GV 1/30

Hostalen PPN 1060

Hostacom M2 N02

Hostacom M4 N01

Hostacom G2 N02

Hostacom G3 N01

Celanex 2500

Celanex 2300 GV 3/30

Celanex 2300 GV 1/20

Celanex 2300 GV 1/30

Temperature [C]

20

20-25

45

6

10

12

15

18

20

30

40

45

60

15

35

4

4

5

7

9

7

16

32

38

80

12

30

2.5

2.5

3

6

8

3

10

24

32

100

_

20

1

1

2

5

6

2

8

20

25

120

_

-

_

-

-

3.5

4.5

-

-

15

20

If the calculated flank pressure p is greater than the permissible, the thread reach L must be increased.

4.1.3 Deformation of the threaded section

It is necessary to check radial deformation of thethreaded section if

- the thread has a large half angle of thread ß (metricISO thread, Whithworth pipe thread)

- a fine pitch is chosen which results in a low thread

depth HI- continuous loads have to be transmitted at elevated

temperatures.

Fig. 4: Explanatory diagram

The radial force Fr resulting from the half angle of thread

ß (see fig. 1 and equation 1) leads to expansion of the outer

part and compression of the inner part. This causes theflank overlap to be reduced. To calculate this deformation,it is assumed that the radial force Fr is distributed uni

formly over the thread overlap area C:

C = JT-d2-L = ^-d2-z-P [mm2] (11)

da flank diameter [mm], see fig. 2

L thread reachz number of loadbearing thread flightsP thread pitch [mm]

This gives pressure pD, which is comparable with theinternal pressure

PD =TC d, L

[N/mm2] (12)

With wall thicknesses si and s2 of the tubular segments

Da-D1 =

S2 =

d3-dj

[mm]

[mm]

(13)

(14)

which are generally small in comparison with the meandiameters Dm and dm, the stress a and the deformation

e can be calculated by the following formula:

ffi = y_ J [N/mm2] for the outer part (15)jLXg JL*

a2 =PD' m [N/mm2] for the inner part (16)d3 - dj

Hence compression of the mean diameter on the outer

part is

PD-Pm"

Et(Da-D)

and on the inner part

pD-dm

(17)

(18)E2 (d3 - di)

1,2 flexural creep modulus [N/mm2], see table 5

Expansion of the mean diameter on the outer part is

JDm = 8l - Dm = /^r^r [mm] (19)LI (L>a - L>)

On the inner part, it is

Adm = e2 dra = Pp-dnE2 (d3 - d;) [mm] (20)

The decrease in the flank overlap /iHi should not exceed20 to 30 % of the initial value.

JHi S (0.2 to 0.3) Hi (24)

The deformation of a plastic component is not only time-and temperature-dependent but is also a function ofstress level. Strictly speaking, separate characteristicvalues should be determined for each type of stress.

However for deformation ^ 2%, the variation betweenthe characteristic values is negligible so that, for example,the deformation of a component under compressivestress may be calculated with sufficient accuracy usingthe flexural creep modulus.

For E therefore, the flexural creep modulus is used as thecharacteristic value, table 5.

Table 5: Flexural creep modulus values based on

DIN EN 20 899-2 (DIN 54 852)

Material

Hostaform C 9021

Hostaform C 9021 GV 1/30

Hostalen PPN 1060

Hostacom M2 N02

Hostacom M4 N01Hostacom G2 N02Hostacom G3 N01

Celanex 2500

Celanex 2300 GV 3/30Celanex 2300 GV 1/20Celanex 2300 GV 1/30

Flexural creep modulus

[N/mm2],23C,ob = 10 N/mm2

1-min value

2800

9000

1300 1}

16002)3300

4000

5500

2800

4100

7100

9500

6-day value

1500

7000

_

500 2)

1600

2400

3800

2000

2900

5900

8600

4.2 Movable joints

In the conversion of rotary into linear motion and vice

versa with helical gears, the friction PR and resultant tem

perature increase on the thread flanks is a crucial deter

mining factor in transmittable power. The followingapplies for the frictional energy:

PR = fG-Fi-v[W]

fc thread friction coefficient, see table 2

FI linear force [N], see section 4.1.1

v sliding speed [m/s]

Fig. 5: Helical thread (schematic)

(25)

T

:> <Tb = 5 N/mm2 2> ob = 6 N/mm2

The changes in flank diameter AD2 and Ad2 also approx- With flank pressure p from equation (9)imately correspond to the diameter changes ZlDm and^dm,

B

[N/mm2]

<4D24Dm (21)

Jd2~Zldm (22)

so that the change in flank overlap is calculated as

and

P =

v =

z n d2 H

n d, n

60[m/s] (26)

a p v value can be determined (similar to that for plainbearings) and used as a design criterion for the helical

arli ~ ^Dra +Zldmgear Guide values for permissible p v values for a

Hostaform/steel combination are given in table 6.

Table 6: Permissible p v values (guide values) for a

Hostaform/steel combination5. Calculation examples

Sliding speed v

[m/s]

^0.5

1.0

1.5

Permissible p v

[W/mm2]

0.15

0.10

0.06

value

The quoted values apply to unlubricated sliding partners.If once-only lubrication is carried out before assembly,up to 50 % higher p v values can be permitted.

With plastic/plastic combinations, e.g. Hostaform/

Celanex, the permissible p v values should be halvedbecause of the poorer thermal conductivity.

5.1 Reversible-flow filter made from Hostaform C 2521

The two housing halves of the reversible-flow filter shownin fig. 6 are screwed together with a cap nut with S 128 x

6 metric buttress thread. The filter is designed for nominal

pressure PN 10 (= 1 N/mm2) at room temperature.

Fig. 6: Reversible-flow filter (schematic diagram)

Stress in the vulnerable cross-section

Because of the lower wall thickness, the vulnerable cross-

section is in the inner part, fig. 7.

Fig. 7: Vulnerable cross-section in the inner part

10

The loadbearing area A] is calculated according to equation (3)

nA, = j (d32 - d,2) [mm2]

A, = f (1212 - HO2)

= 1994.7 mm2

In this filter, which is sealed with a radially deformed

O-ring and thus requires a negligible prestressing force

FV, the linear force is essentially determined by theinternal pressure

Fi =FB

DIN 19632 specifies that mechanically operating filters

must be tested with three times the nominal pressure

rating, in this case with 30 bar = 3 N/mm2.The pressure-

stressed area is thus

A = | d,2 [mm2]

^ HO2 mm2

= 9498.5 mm2

Thus

FB = 3 PN A [N]

= 3 IN/mm2 9498.5mm2

= 28 495.5 N

The maximum tensile stress ffm,x

omax. = - [N/mm2]

28 495. 5 N

1994.7mm2

= 14.28 N/mm2

The long-term-acting tensile stress is

PN AOz = ^r~

1 N/mm2 -9 498. 5 mm2

1994.7mm2

= 4.76 N/mm2

and is thus smaller than aperm. = 10 N/mm2, see table 3.

Flank pressure

As explained above, FB = FI. Thus the flank pressure p iscalculated from equation (9)

P =

FBit d, H, [N/mm2]

d2 flank diameter [mm]

d2 = d-0.75P= 128 - 0.75 6

= 123. 5 mm

H, =0.75- P= 4.5 mm

z number of loadbearing thread flightsL

2 =P22.5

6

= 3.75

Thus the maximum flank pressure is

28 495.5 NPmax.

JT 123.5 mm 4.5 mm 3.75

= 4.35 N/mm2

In the long term, a flank pressure of p = 1.45 N/mm2 at

PN = 10 bar can be accepted.

5.2 Hose connector made from Hostalen PPN 1060

The cap nut made from polypropylene shown in fig. 8 is

part of a hose connector for irrigation systems. The aimin this case is to check how great the expansion of the nut

is when tightened with a torque of M = 1000 N mm.

Dimensions of the RVs thread:

Outside diameter of threadFlank diameterPitchThread depthOutside diameter of nut

Thread reachHalf angle of thread

Mean diameter ofcontact surface

D = 16.7 mm

D2 = 15.8 mm

P = 1.337 mm

Hi = 0.856 mm

Da = 19.5 mm= 10 mm= 27.5

(cos 27.5 = 0.887,tan 27.5 = 0.5206)

L

ß

dA = 15 mm

For the present calculation example, the operating forceis ignored: FI = Fv.

11

Fig. 8: Cap nut

/;

-19.5-

-RÏ/8-

-14.95-

S

--18.1-

E (from table 5) 1300 N/mm2

^ Da + D 19.5 + 16.7...Dm= - - = - - = 18.1 mm

61 =

2 2

0.3 N/mm2 18.1 mm

1300 N/mm2 (19.5 mm - 16.7 mm)

= 1.49 ID'3

The flank diameter D2 expands by

ZlD2 = ! D2 [mm]= 1.49 10-3 15.8 mm

= 0.023 mm

This expansion is not critical in relation to the thread

depth H! = 0.856 mm.

The tightening torque M leads according to equation (6)to a linear force

Fi =2M

*+

D2-fGcos ß

[N]+ dA fA

The friction coefficients fA and fG are assumed to be 0.2.

2 1000 N mmF,=

1.337mm 15.8 mm 0.2+ - + 15 mm 0.2

it 0.887

= 286.2 N

The linear force FI leads to a radial force (equation 1)

Fr = FI tan ß= 286.2 N 0.5206

= 149 N

This force corresponds to a pressure (equation 12)

[N/mm2]PD = W^-L149 N

Jt 15.8 mm 10 mm

= 0.3 N/mm2

This pressure leads to an expansion (equation 17)

PD DmS, =

5.3 Vehicle jack with threaded nut made from POM

On the jack shown in fig. 9, a threaded feed nut injectionmolded from POM engages with a steel spindle. Theserviceability of the jack is tested in a long-term trial ona special rig in which the jack is lowered from its highestposition by a specified distance and then raised again50 times under the nominal load F = 4500 N. A liftingspeed of v = 150 mm/min is maintained through the

speed of the drive motor. The aim here is to determinewhether the permissible p v value is exceeded in the trial.

Fig. 9: Jack with threaded feed nut (arrow)

E (Da - D)

12

Dimensions of the M 12 x 2 thread in the threaded feed

nut:

Flank diameter D2 = 10.9 mm

Thread depth H! = 1.0826 mm

Number of loadbearingthread flights z =21

For the flank pressure, equation (9) applies

FP =

Jt D2 H, z

4500 N

[N/mm2]

n 10.9 mm 1.0826 mm 21

= 5.78 N/mm2

The lifting speed of 150 mm/min corresponds to a

spindle speed of

n = 75 min"1

Thus the sliding speed

n D2 nv = [mm/s]60

n 10.9 mm 75

60s

= 42.8 mm/s ^ 0.0428 m/s

The p v value is calculated as

p-v = 5.78 N/mm2 0.0428 m/s

= 0.25 W/mm2

In table 6, for a sliding speed of v = 0.5 m/s, a permissiblep v value of p vperm. = 0.15 W/mm2 is quoted for unlu-bricated operation. With lubricated operation - as is possible in the present case - this value can be increased byup to 50%.

Thus a permissible p v value of

p-v.perm. = 0.15-1.5

= 0.225 W/mm2

is obtained.

The calculated p v value is about 10 /o higher than the

permissible. Hence in the long-term trial, pauses are provided between the individual cycles to allow the threaded

nut to cool to room temperature again.

6. Design notes

6.1 Self-locking

Equation (6) in section 4.1.1 describes the relationshipbetween linear force FI and tightening torque M at the

point when the threaded parts are turned as far as theywill go, i.e. an additional frictional force becomes effective in the contact area. Before this position is reached, the

following equation applies

M = FB --y- tan(y')

tan (a y') ** tan tan y'

P

(27)

tan =

tan y

d, JT

fr2 \d2-n ' cos/? (28)

The plus sign in brackets denotes tightening under load

FB, the minus sign denotes withdrawal (reversal of direc

tion). In this case the linear force FB is converted into a

torque M if the expression in brackets is positive, i.e.

d, -JT>

cos ,

No self-locking takes place. Self-locking occurs when the

expression in brackets is negative, i.e.

frcos ß d2 n

In the case of single-start threads, the quotient

P

d2 -n

is generally between 0.016 (Whitworth pipe thread) and0.060 (metric buttress thread) so that with unlubricatedsurfaces self-locking occurs (see friction coefficients IG in

table 2). With lubricated surfaces (fG = 0.04 to 0.1), it is

necessary to check in each individual case whether self-

locking still takes place, especially since external vibrations

can further limit the range of self-locking. In some situa

tions, a special thread with a smaller pitch P should be

used.

13

6.2 Stress concentration in the threaded section

6.2.1 Notch factors aK in disengaged thread zone I

Fig. 10: Characteristic values

For the components discussed here, the tensile stress

resulting from linear force FI has the greatest importance.

In components under a constant tensile stress, the stress-

related notch factor

KO =

ON

The grooves of a thread correspond in their effect to a

series of notches. These notches lead to increased stress

as compared with a smooth molding. The ratio of the

maximum stress in the notch root = thread root <Tmax. to

the nominal stress 0N of the notch cross-section

A,.:-f (d,2-di2)

and A2 = (Da2 - D2), see section 4.1.1,

is described as the notch factor or stress concentrationfactor

K0 =

On

ON

Since linear-elastic material behavior is assumed, an ana

logous factor for strain concentration can be defined

Ke =

eN

This notch factor depends on

- design and dimensions of the molded part- notch geometry: radiusing R at the transition between

the thread flank and thread root,thread depth HI, inside diameter d;and outside diameter Da

- type of stress (tensile, flexural, torsional).

In [10] an extended method of calculation is presented,which enables stress concentration to be taken into

account for non-linear-elastic and plastic material behavior

as well. This method is applied in [11] to viscoelasticmaterial behavior of plastics.

decreases over time as a result of creep processes at the

point of highest stress, while the strain-related notchfactor

Kf :

N

increases with time. These strains can produce flow

zones and microcracks which, in the presence of certain

media, can lead to failure of the component.

In [12], the notch factor for metric thread M 8 undertensile stress was determined - using an enlarged,geometrically similar thread model - as

K = 2.3 to 2.4

The radiusing R associated with pitch P has an importantinfluence on the notch factor. Since the ratio P/D becomes

smaller with increasing nominal thread diameter D, the

notch factor increases with greater thread size.

Notch factors for the metric ISO trapezoidal thread are

given in [13]. Depending on the nominal thread diameter,the notch factor is

K = 2 to 3

(high notch factors for large nominal thread diameters

and vice versa).

6.2.2 Stress distribution within the internal thread

engaged with the external thread (zone II, fig. 10)

In sections 4.1.2 and 4.1.3, it was assumed that the individual thread flights in the engaged thread zone play an

equal part in force transmission. Studies on model threads

[12], however, show that about 30 to 40% of the totallinear force FI is absorbed by the first loadbearing thread

flight when the inner and outer parts consist of the same

material. The proportion of force absorbed by the first

loadbearing thread flight depends on the level of linearforce FI; with increasing linear force, the proportiondiminishes.

14

To describe the stress concentration in the engaged thread

zone, therefore, a (load-dependent) notch factor a* is

defined, which additionally takes into account load application and deflection on the thread flank. The measurednotch factors in the thread root are

* =5 to 8.7

These high notch factors can be reduced if force trans

mission is distributed evenly over the individual thread

flights. A possible design to achieve this is shown in

fig. 11.

Fig. 11: Design measure for improved force trans

mission in the individual thread flights

Fig. 12: V-notch on the internal thread produced bythread runout

!:iÎ5G3i"T3

OSu< "5 *

_x

>X

/f ^5

v/,\%\&->%a

40 30 20 10 0%

Proportion of loadtaken by the force-transmitting thread

flights

40 30 20 10 0%

Proportion of loadtaken by the force-transmitting threadflights

By introducing a recess at the end of the nut, this regionis designed to be more yielding and hence the load is

distributed more evenly over the thread flights. A similareffect can be achieved if the internal and external partsare made from materials with different rigidity. Example:screw fitting with internal part made from glass-fiber-reinforced polypropylene and cap nut from unreinforced PP.

6.2.3 Stress concentration in the thread runout

The internal thread of a plastics molding is formed in an

injection mold by a core with external thread. Because

the cores are generally machined, a runout is producedon the external thread which forms a V-notch with verysmall radius of curvature on the internal thread (fig. 12).

Thread runout

V-notch

The large notch factor K which results can lead to fail

ure of the component at this point when under internal

pressure stress. Thread runouts such as described in

DIN 76 part 1 and part 3 must be avoided with integrallymolded threads; the thread flight on the mold core must

end with the complete thread profile.

6.3 Static sealing ofplastics components with integrallymolded threads

Sealing should basically take place outside the thread, i.e.

no seals (hemp, sealing tape) should be used in the thread.

In the case of flexible, deformable plastics (e.g. polyethylene, elastomer-modified plastics), the components can be

sealed with molded-on sealing lips or other readily deform

able zones (fig. 13).

Fig. 13: Screw closure with integral seal

In the case of hard, rigid plastics, additional seals mustbe used.

15

A joint is sealed when the contact pressure in the seal is

greater than the pressure difference between the two

sides of the seal.

Fig, 14: Flat gasket

Fig. 15: Automatic sealing effect with elastic seals

Seal locatedin sealing groove

Flat gaskets (fig. 14), because of the relatively large contact

area, require high prestressing forces Fy to achieve a satis

factorily high contact pressure. Owing to bedding downprocesses in the thread and stress relaxation in the tensile-stressed thread zones, these prestressing forces are not

constant but decrease with time. As a result, the joint can

start to leak.

O-rings placed in appropriately dimensioned grooveswhich cause them to be deformed in the closed joint are

a more suitable solution. The force required to deformthe rings and thus to create contact pressure is much lessthan is the case with flat gaskets. In addition, O-rings havethe advantage that the contact pressure necessary forsealing is increased by the pressure of the medium beingsealed, fig. 15.

O-rings can be installed in rectangular and triangulargrooves, fig. 16. The groove dimensions and tolerancesspecified by the manufacturer should be observed.

In the examples shown in fig. 16, the O-rings are deformedin the longitudinal direction by prestressing force Fv. The

arrangement in fig. 17 is more favorable, in which deformation of the O-ring is achieved by suitable dimensioningof the internal and external parts and is not dependent on

a prestressing force.

Seal compressedbut not

pressure-loaded

Squeezing processas a resultof initial compression Pv

Seal,pressure-loaded

Squeezing processas a resultof initial compression poand sealing _

pressure p

PB = p + pv

Fig. 16: Triangular and rectangular grooves, axialdeformation of O-rings

Fig. 17: Rectangular groove, radial deformationof the O-ring

16

7. Injection molding ofcomponents with integrallymolded threads

7.1 External thread

The external thread forms external undercuts in slides.These undercuts are released by the action of angle pinsthat allow movement of the slides at right angles to themold opening direction, fig. 18.

Fig. 18: Slide mold for external thread

In the contact area of the slides, flash can be formed as a

result of the particular molding conditions or wear. Thisflash makes it difficult or even impossible to screw thethreaded parts. The problem can be avoided by flatteningthe thread in the area of the contact surfaces. Any flashwill then lie out of the way of the opposing thread and

not be an obstruction, fig. 19.

Fig. 19: Slides with flattened thread

If even parting marks from the slides are not allowed to

be visible, the thread must be formed in a single threadedsleeve which is unscrewed after injection molding.

7.2 Internal thread

Depending on the intended length of the production run

and thread requirements, various demolding options maybe considered.

For short runs, so-called lost cores are inserted into the

injection mold and ejected with the part after injectionmolding. Outside the mold, the core is then unscrewed

manually or with a special device. For longer runs, thethreaded cores are unscrewed inside the mold. The

necessary rotary movement is produced either with theaid of a coarsely threaded spindle via the mold openingmovement or via a hydraulically operated rack and

pinion system or drive motor.

Fig. 20: Unscrewing mold for internal thread

Another way to demold internal threads is to use a collapsible core. This type of core is divided into segmentswhich collapse inwards so permitting release of the

thread, fig. 21.

17

Fig. 21: Collapsible core, left: injection moldingposition, right: demolding positionManufacturer/sales :

DME-Zentrale, 74196 Neuenstadt, GermanyRudolf Riedel GmbH, 58579 Schalksmühle, Germany

Fig. 22: Segmented internal thread

demoldabieundercut

Internal threads can be demolded much more simply, i.e.

via angle slides, if a continuous thread is not required butIn general it should be noted that a molding with internal

thread shrinks onto the threaded core and that with in-

instead individual threaded segments are sufficient to take creasing cooling time the shrinkage forces also increase.

the linear force FI, fig. 22. With unscrewing cores in particular, this can lead to

demolding difficulties if the prescribed cooling time is

exceeded, e.g. in the event of interruptions to production.

18

8. Examples ofapplications

8.1 Water filter housing 8.2 Drain plugfor water separator on diesel vehicles

Photo 1 shows a water filter housing made from Hosta-form C 9021. The two housing halves are screwed

together with an S 80 x 4 buttress thread; the numberof loadbearing thread flights z = 2. This housing, whichis constantly under mains water pressure, is sealed withan axially deformed O-ring. The wall thickness of theinternal part si = 6.1 mm; the wall thickness of external

pans s2 = 4.4 mm.

The external thread is formed in splits; the core formingthe internal thread is unscrewed in the mold.

The drain plug shown in photo 2 and fig. 23 is screwedinto the base of the water separator with an M 10 x 1.5

thread.

The plug is sealed with an O-ring. The plug is held captivein the separator housing by two snapfit hooks. To achievethe required deformability for snapfitting, the screw boltis centrally bored and laterally recessed.

Although the transition from the screw bolt to the flangeis well rounded (R = 0.8 mm), fig. 23 left, stress crackingoccurred in this region owing to the sharp-edged threadrunout. This problem was remedied by shortening thethread and ending it with a complete profile, fig. 23 right.

Fig. 23: Drain plug

19

8.3 Grain hopperfor corn mill 8.4 Fastening nutfor spare wheel

This hopper (photo 3) made from Hostalen PPT 1070 has

an M 90 x 2 metric external thread and a non-standard80 x 20 P 4 five-start, rectangular internal thread, thread

depth HI = 1 mm.

The external thread is formed in two slides. The internalthread is divided into two threaded segments, each cover

ing an angle at the circumference of 120. The thread is

demolded via two slides running on the tapered inner

core.

The nut shown in photo 4 made from Hostacom G3 N01

(PP + 30% w/w glass fibres, chemically coupled) is used

to fasten the spare wheel of a car onto a threaded bolt.The M 8 x 1.25 nut thread is however not complete but

formed only over half the circumference in each half of

the thread reach, fig. 24.

The threaded areas are offset by 180. This makes it easyto push the slightly tilted nut quickly over the threadedbolt.

Only through contact with the spare wheel is the axis of

the nut aligned with the axis of the threaded bolt so that

the thread flights of the nut and bolt can become engagedand the nut tightened up.

Fig. 24: Interrupted M 8 x 1.25 thread

20

9. Explanation ofsymbols

Symbol

A,, 2

B, L

C

d, D

d2,D2

d3

dA

di

dp

dm, Dm

Da

E

f

fA

fc

FB

Fi

F

Fr

Fv

H,,h3,H4, c

H5

Unit

mm2

mm

mm2

mm

mm

mm

mm

mm

mm

mm

mm

N/mm2

_

_

N

N

N

N

N

mm

mm

Explanation

vulnerable cross-section,notch cross-section

engaged thread zone, thread reach

thread overlap area

nominal thread diameter

thread flank diameter

thread core diameter

mean diameter of the

contact surface

inside diamter of a joint

diameter of the pressure-stressedÇ11 t"T3 PPdU.1 letL/C

mean ^\ Da + Ddiameter 2

outside diameter of a jointflexural creep modulus

friction coefficient

friction coefficient of the

contact surface

thread friction coefficient

operating force

linear force

perpendicular force

radial force

prestressing force

thread depthflank overlap

Symbol

M

n

P

PD

p;

Pperm.

p v value

p

PR

PN

R

s

v

z

a

K, Kö,

KS

a*

ß

r

e

Oz

"perm.

Unit

N-m

min"1

N/mm2

N/mm2

N/mm2

N/mm2

W/mm2

mm

W

bar

mm

mm

m/s

-

o

-

-

o

o

N/mm2

N/mm2

Explanation

tightening torque

spindle speedflank pressure

pressure

internal pressure

permissible flank pressure

material-dependent designdimension for helical gears andslide bearingsthread pitchfrictional energy

nominal pressure

radiusingwall thickness

sliding speednumber of loadbearingthread flightshelix angle

notch factors

load-dependent notch factor

half angle of thread

angle of friction

strain

tensile stress

permissible tensile stress

21

10. Literature

[1] DIN 2244

Gewinde, Begriffe[2] DIN 202

Gewinde, Übersicht[3] DIN 13, Bl. l Metrisches ISO-Gewinde.

Regelgewinde von l bis 68 mm

Gewindedurchmesser.DIN 13, Bl. 9 Feingewinde mit Steigung 4 mm

von 40 bis 300 mm Gewindedurchmesser.DIN 13, Bl. 12 Regel- und Feingewinde von l bis

300 mm Gewindedurchmesser. Auswahlfür Durchmesser und Steigungen.

[4] DIN 513

Metrisches Sägengewinde(Gewindedurchmesser 10 bis 620 mm)

[5] DIN 103

Metrisches ISO-Trapezgewinde(Gewindedurchmesser 8 bis 300 mm)

[6] DIN/ISO 228

Rohrgewinde für nicht im Gewinde dichtende

Verbindungen[7] DIN 6063

Gewinde vorzugsweise für KunststoffbehältnisseTl. l SägengewindeTl. 2 Trapezgewinde

[8] DIN 405

Rundgewinde(Gewindedurchmesser 8 bis 200 mm)

[9] DIN 19632

Mechanisch wirkende Filter in der Trinkwasser

installation, Anforderungen, Prüfungen, techn.

Regeln des DVGW

[10] DIN EN 200

Sanitärarmaturen

[11] H. Neuber:Über die Berücksichtigung der Spannungskonzentration bei FestigkeitsberechnungenKonstruktion 20 (1968) 7, p. 245

[12] E.Weiß:Zur Berücksichtigung der Kerbwirkung beiviskoelastischem MaterialverhaltenPlaste und Kautschuk 35 (1988) 2, p. 67

[13] K.H. Kloos,W. Thomala:

Spannungsverteilung im SchraubengewindeFirmenschrift der Fa. Richard Bergner, Schwabach

[14] G. Pahl, K. Bordas, A. Oedekoven:

Berechnung von TrapezgewindenKonstruktion 37 (1985) l, p. 25

[15] DIN 76, Tl. l + 3

Gewindeausläufe, Gewindefreistiche

[16] H. Gastrow:Der Spritzgießwerkzeugbau in 100 BeispielenCarl Hanser Verlag, Munich Vienna 1990

22

Engineering plasticsDesign Calculations Applications

Publications so far in this series:

A. Engineering plasticsA. 1

.1 Grades and properties - Hostaform

A. 1.2 Grades and properties - HostacomA. 1.4 Grades and properties - Hostalen GURA. 1.5 Grades and properties - Celanex,

Vandar, ImpetA.2.1 Calculation principlesA.2.2 Hostaform - Characteristic values and

calculation examplesA.2.3 Hostacom - Characteristic values and

calculation examples

B. Design of technical mouldingsB. 1.1 Spur gears with gearwheels made from

Hostaform, Celanex and Hostalen GURB.2.2 Worm gears with worm wheels made from

HostaformB.3.1 Design calculations for snapfit joints in

plastic partsB.3.2 Fastening with metal screws

B.3.3 Plastic parts with integrally moulded threadsB.3.4 Design calculations for press-fit jointsB.3.5 Integral hinges in engineering plasticsB.3.7 Ultrasonic welding and assembly of

emgineering plastics

C. Production of technical mouldingsC.2.1 Hot runner system - Indirectly heated,

thermally conductive torpedoC.2.2 Hot runner system - Indirectly heated,

thermally conductive torpedoDesign principles and examples of moldsfor processing Hostaform

C.3.1 Machining HostaformC.3.3 Design of moldings made from

engineering plasticsC.3.4 Guidelines for the design of moldings

in engineering plasticsC.3.5 Outsert molding with Hostaform

In this technical information brochure, Hoechst aims to

provide useful information for designers who want to

exploit the properties of engineering polymers such as

Hostaform. Our technical service team will be pleasedto advise you on materials, design and processing.

This information is based on our present state of knowl

edge and is intended to provide general notes on our

products and their uses. It should not therefore be con

strued as guaranteeing specific properties of the productsdescribed or their suitability for a particular application.Any existing industrial property rights must be observed.The quality of our products is guaranteed under our

General Conditions of Sale.

Applications involving the use of the Hoechst materialsHostaform, Hostacom, Hostalen PP and Celanexare developments or products of the plastics processingindustry. Hoechst as manufacturers of the starting material will be pleased to give the names of other processorsof plastics for engineering applications.

© Copyright by Hoechst Aktiengesellschaft

Issued in August 1996/1 st edition

23

Hostaform®, Celcon®

polyoxymethylene copolymer (POM)

Celanex®

thermoplastic polyester (PBT)

Impet®

thermoplastic polyester (PET)

Vandar® thermoplastic polyester alloys

Riteflex®

thermoplastic polyester elastomer (TPE-E)

Vectra®

liquid crystal polymer (LCP)

Fortron®

polyphenylene sulfide (PPS)

Celstran®, Compel® long fiber reinforced thermoplastics (LFRT)

GUR®

ultra-high molecular weight polyethylene (PE-UHMW)

EuropeTicona GmbHInformation ServiceTel.: +49 (0) 180-5 84 26 62 (Germany) +49 (0) 69-30 51 62 99 (Europe)Fax: +49 (0) 180-2 02 12 02eMail: infoservice@ticona.deInternet: www.ticona.com

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