dfm_die_casting_nttf

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1

School of Postgraduate Studies, NTTF, Bangalore Dr. N. Ramanj Design for Pressure Die Casting

DFM

For

Pressure Die-casting

By

Dr.N.Ramani

Principal

NTTF School of Postgraduate Studies

Bangalore

(2)

2 •Prize for one who has made the most number of

mistakes in the design of die cast components!

•Japanese methodology “OET

•Compilation of design rules how to design die

cast components without tears

Introduction:

•PDC Part design:

(3)

3

(4)

4 •First sentence in book on “Design for Castings”

•“Don’t design castings!”

•Make sketches & discuss with the founder

who is going to make the casting for you, on

matters of

•Position of parting plane

•Functional features,

•Achievable accuracies…...

(5)

5 In short the author preempted

“simultaneous engineering” (i.e.) he

suggested discussion of the details of the casting

by part-designer with the die-maker for maximum

effectiveness of the design of the casting.(?)

(6)

6 3. Know “Effectiveness of casting design”:

•Easy to cast without filling problems

•Easy to machine without the disadvantages of

either too less or too much material to remove by

machining

•The most economical in cost by reducing

•Cost of the die

(7)

7 First

Proto

Flexibility &

Opportunity

to improve

design

quality of a

new Product

0%

100%

Knowledge /

Expertise of our

Product

!

α- Proto β -Proto γ -Proto

Time

100%

0%

Cost of Change !

“QUALITY PARADOX”

of a new product

Fig.1

Concept

(8)

8 FMEA

Flexibility &

Opportunity

to improve

design

quality of a

new Casting

0%

100%

Knowledge /

Expertise of our

own casting

!

Die Mfg. Trial ProductionTime

100%

0%

Cost of Change !

“QUALITY PARADOX”

Fig.2

Concept

Design

For a PDC Die

Detailed

Design

(9)

9 AN ANALOGY OF A RIVER & A PRODUCT

WRT INFLUENCING ITS QUALITY

CUSTOMER USAGE

POST PRODUCTION

SERVICE WARRANTY

(UPSTREAM)

(MIDSTREAM)

(DOWNSTREAM)

SEA

PRODUCT &

PROCESS

DESIGN

MANUFACTURING

Fig.3

(10)

10 Part designer should

•Overall function

•Part Consolidation

•Design for attachment and assembly

•Typified by the chassis illustrated in Fig.4

4.Design for Function & Assembly

(11)

11 Example of multi-functional casting design:

Fig 4.

One-piece chassis for a high-speed electronic printer

replaced 82 separate components and fasteners in the

forerunner assembly.

Drilling and tapping have been eliminated by

incorporating cored holes, which receive

thread-forming

screws for fastening

.

(12)

12 •Ribs to reinforce part structurally

•Minimum increase in weight

•Replacing heavy sections that would be

otherwise necessary. Fig.5

5. Rib the part to add strength and stiffness &

Reduce weight:

(13)

13 Ribs fairly

easy to

incorporate

into an

existing

hardened

die!

Think “Ribs”:

Fig.5

When?

(14)

14 FIGURE 6.Castings for two-wheeler engines

show how good design can provide ribs where

needed for strength and cored holes for fastening

while maintaining fairly constant wall thickness.

Examples of strong light weight components

(15)

15 FIGURE 7. Aluminum components for a weighing

scale demonstrate intelligent use of ribbing to

promote strength and stiffness.

Examples of large light weight components

Moped

Frame?

(16)

16 Fig.8

Box shaped components strengthened by

incorporating internal ribs that run the full depth

of the part. Corners reinforced by radiusing to

avoid corner fractures in the casting

(17)

17 Fig.9

Where there would normally be a heavy

section adjacent to a cored area, introduce further

coring to create ribs, thereby removing mass &

obtaining uniform wall thickness

(18)

18 Can This be True?

(19)

19 •Acute-angle intersections cause the die

to overheat in the area between the ribs.

•Multiple intersections of radial ribs

should be avoided; otherwise, the

intersection will contain porosity.

Poor Design Of Rib

(20)

20 Typical case of how ribs

should not be designed

Number of ribs intersecting = 6!

(21)

21 Number of intersections = 4

4 >> 6

Which design is still better?

“Slightly” improved design of ribs:

(22)

22 Much improved rib design(Only 3 ribs

intersect at each point)

Optimum Rib design:

3 >> 4 >> 6

Fig.13

(23)

23 Procedure for design of Reinforcing Ribs

•Designer may under-design initially,

test sample castings, then add strength if necessary

by removal of die steel until optimum combination

of mechanical properties & casting-material

conservation reached.

•(Preferable to over-designing and having to

lighten the die casting later by welding the die,

which is a costly, life-limiting procedure.)

(24)

24 •Avoid

•Abrupt section changes

•Sharp corners

•Walls at an acute angle to one another

•(disturb the continuity of metal flow &

promote porosity and surface

irregularities)

•Blend differing sections into one another with

radii as generous as possible.

(25)

25 Fig.14

(26)

26 Radii

•Sharp external corners are undesirable because

they become a localized point of heat and stress

buildup in the die steel that can cause die cracking

and early failure.

•Therefore, radii and fillets

should be as generous

as possible, preferably at least 1.5 times wall

thickness for both inside and outside radii.

Why More Radii?

(27)

27 Sharp corners cause

uneven cooling,

while rounded

corners permit

uniform cooling

with much less

stress; Rounded

corners that

maintain uniform

wall thickness

provide the best

results

T

(28)

28 Fig.16

Why to avoid sharp corners?

Sharp edges on cores are difficult to maintain

because they are points of heat concentration,

with resulting premature erosion of the die

(29)

29 •The easiest die casting to make and the soundest

in terms of minimum porosity is one that has

uniform wall thickness.

•Sharp changes in sectional area or heavy sections

over 6 mm thick should be avoided if possible.

•When a heavy section seems to be indicated, its

underside should be cored out.

(30)

30 Importance of Uniformity of wall thickness

Fig.17

Less Problems

(31)

31 Why should we design a PDC with as less a

wall thickness as possible?

(32)

32 *Fine-grained dense structure, devoid of

porosity, thus making the skin the strongest part

of wall thickness (

?

)

8. Design with as thin a wall thickness as possible:

Fig 18

(33)

33 Strength

Weight

Ratio

Fig.19

(34)

34 Intersection of two walls should be at right

angles to avoid sharp corner hot spots

9.Let walls intersect at right angles:

(35)

35 FIGURE 21.

To avoid surface shrinks, relocate

the boss and connect it to the wall with a short

rib. The ribs should be no wider than the

thickness of the casting wall

(36)

36 Fig.22 When the draft angle is abnormally small,

even the slightest depression in the drafted

surface of the die will prevent ejection causing

drag marks in the surface of the casting

(37)

37 Fig.23

(38)

38 Fig.24

(39)

39 Fig.25

If possible, the part should be designed so that it can

be made in a simple two-piece die.

(40)

40 Fig.26

Through-wall cored holes for tapping should be

countersunk on both sides to avoid deburring;

Holes for tapping should be cored rather than

drilled. A drilling operation is eliminated, and the

tap will cut into dense material for a higher-quality

thread.

(41)

41 Fig.27

SC Should never be designed to intersect the

opposite die half, since imperfect die closure

(possibly the result of flash at the parting plane

not being fully removed) would result in a

damaged die.

(42)

42 Fig.28

(43)

43 Fig.28

(44)

44 It is possible to die-cast

internal threads in

zinc parts only

by using techniques that

involve either unscrewing the casting from

a threaded core or rotating the core out of

the casting during ejection.

(45)

45 Insert with knurled circumference (Positive

location within the die must be provided to

prevent insert movement during the casting

cycle)

Fig.29

Design of bush type inserts:(To resist both

axial and rotational forces).

(46)

46 Insert with recess & longitudinal grooves

Fig.30

(47)

47 Insert Machined & U/C square

(insert's threads must be kept away

from the casting face)

Fig.31

(48)

48 Insert with locally machined flat

Fig.32

(49)

49 Insert with drilled anchor holes

Fig.33

(50)

50 Decision based on Cost comparison for inserts

Fig.34

(51)

51 18.Limit machining Allowances to a minimum:

Fig.35

Deeper cuts could open up unsightly subsurface

porosity and possibly affect function.

(52)

52 If an area to be machined covers ejector-pin

locations, their impressions should be

0 to 0.4 mm depressed-so that they are

removed in machining.

(53)

53 20.Design For “Flash and Gate Removal”

With complex castings having massive core

slides, the cost of complete removal by a

combination of die trimming and hand operations

can be as much as the cost of the raw casting !

(54)

54 20.Flash and Gate Removal:

Fig.36

Avoid an angled junction of an external wall with

the parting line.It is preferable to add a

minimum-draft shoulder at the parting line, so that most of

the gate material will come away in trimming.

(55)

55 20.Simplify Flash and Gate Removal

Fig.37

Trimming tool to match intricate geometry would be

expensive and hard to maintain. Advisable to add a shoulder

between wall detail and parting line to allow for use of a

single trimming die or a lathe-turning operation to remove

gates and flash.

(56)

56 21.Think of Die-sinking costs:

(?)

Fig.38

If the casting is designed with convex features in

outside walls,it is a straightforward job to mill the

corresponding concavities into the steel.

(57)

57 There are two alternatives.

•The easy way is to specify that the

characters be raised in the casting.

•This can be accomplished by relatively

inexpensive engraving of the die.

•Characters depressed into the casting

•All the background steel on that face of the

die must be removed around the characters.

22. Leave the Letters Raised :

(58)

(59)

59 On Lettering

•If the designer wants the economy of raised

characters but does not wish them to

project above the surrounding surface, a raised

pad can be incorporated into the die to

form a depressed area in the casting. Then when

the pad is engraved, the lettering will

(60)

(61)

61 •The die-casting process can accommodate the

coring in of holes into the body of the

casting at right angles to the parting line.

•However, there are core-length limits, depending

on diameter, that should not be exceeded

•Long, slender cores may lead to core breakage

23.Know the limitations of cored out holes

(62)

62 Fig.39

24.Design for Assembly:

Zinc only!

How will you design a spherical shaped

handle for a door?

(63)

63 Fig.39

24.Design for Assembly:

(64)

64 •The as cast dimensional variations of a die

casting depend on part size.

•This dependency is largely due to thermal

expansion and contraction of both the die and the

casting. The die expands at operating

temperatures, and the casting shrinks after it

leaves the die.

•Variations in die operating temperatures and the

temperature of the molten metal entering the die

add to the need for a design tolerance on

die-casting dimensions.

(65)

65 •Tolerances across the parting line and between

core slides and main die blocks must be greater

because of the clearances that are incorporated

into these features in the die to enable them to

function at elevated temperatures.

•Recommended tolerances also allow for

gradual wear in die components over the life of

tooling. This is a significant factor.

•Another is warpage.

(66)

66 Some case-studies of Problems encountered

during die casting of engineering components

(67)

67

School of Postgraduate Studies, NTTF, Bangalore Dr. N. Ramanj

Problem: Non-filling of window portions?

Design for Pressure Die Casting

10 Outside view of Motor Body Housing Casting

~ 180 dia; ~100mm deep; 800gm

Support

pin

locations

(68)

68 Problem: Non-filling of window portions

Nominal

Wall

thickness

~ 2 mm

CI Insert

for bearing

Inside view of Motor Body Housing Casting

(69)

69 Original

runner

location near

heavy sections

Motor Body Housing Casting

Fig. 42

(70)

70 Fully filled

windows

after the

addition of

extra runner

Motor Body Housing Casting

(71)

71 Housing For Hand-grinder

Typically asymmetric flow of metal required

200 long

Semi- Cyl.

Portion

60 dia Cyl.

250gm; Wall thickness 3mm

Fig. 44

(72)

72 Runner

Housing For Hand-grinder

Problem: Non-filling of semi-cylindrical

portion;Part Sticking to cavity (?)

(73)

73 Runner

Housing For Hand-grinder

Subrunner

(74)

74 Design For Die-casting

NTTF

How did they solve the problem of

‘Part sticking to the cavity’?

(You guessed it right)

(75)

75 Motor Housing Non-drive End

(Outside view)

Dia 180mm; 100mm high;750gm

(76)

76 Motor Housing Non-drive End

(Inside view)

2.5mm Wall thickness

(77)

77 Problem: Fin area not being filled at all

Cracks

near the

opening

Motor Housing Non-drive End

(78)

78 Problem: Fins not at all filling

Location of

runners near

heavy

sections

Motor Housing Non-drive End

Cracks

below

opening

(79)

79 Extra

sub-runner just

below

rectangular

opening

added

Motor Housing Non-drive End

(80)

80 Fan Casting : Dia ~ 150mm; ~75mm long; gm

Problem: Fins Breaking; Ejection sticky

Fig. 52

(81)

81 Fan Casting

Fig. 53

(82)

82 Fan Casting

• Draft angle of blades increased from 2

0 _{to 5}

0 • Extra ejection pins 6 added ( )

(83)

83 Family die for Flange & Bearing Cover

50dia

40dia

Fig. 55

5mm

thick;

8gm

20mm thick;

30 gm

Anything Wrong?

(84)

84 Family die for Flange & Bearing Cover

Problem:

Only one set of

good parts obtained

out of each shot

•Severe Blow-holes

Fig. 56

Analysis: Severe

unbalanced Mass-flow;

C.f. : 46gm Vs 68gm

(85)

85 Family die for Flange & Bearing Cover

(86)

86 Solution: Progressively reducing runner width

Increase radii for improved flow;

(87)

87 Design For Die-casting

NTTF

Lesson learnt:

•For optimum results, the degree of imbalance

between the two sides of a family die should

not exceed + / -

10%

(88)

88 Gear Cover Casting

Fig. 59

(89)

89 Gear Cover Casting

Problem:

Non-filled area

(90)

90 Gear Cover Casting

Fig. 61

Sub-runner

added

(91)

91 Base Casting for Electric Cutter

~300mm X ~ 150mm; 400gm

Not Filling

(92)

92 Base Casting for Electric Cutter

Not Filling

Fig. 63

(93)

93 Base Casting for Electric Cutter

Fully filled with redefined gatings

(94)

94 Cover for Cutter

Dia 240mm;120 mm; 2mm nominal thickness

Non- Filling

(95)

95 Wafer-thin wall

Cover for Cutter

(96)

96 Solution:

Maintain uniform wall thickness

(97)

97 Cause

Effect

20 %

80 %

20 %

80 %

Conclusion:

Pareto Principle

Fig. 67

(98)

98 Design For Die-casting

NTTF

“ Have always the simple mind of a student,

because on the simplest of foundations are built

the most wonderful works of nature.”

(99)

99 Design for Die Casting

NTTF

References:

1.Die casting book by Dr.Arthur Street

2.Metals Hand book Volumes 5A,5B,15 SME

3.Principles of Metal Casting By Richard W.Heine

4.Design for Manufacturability By James Bralla

(100)

100 Thank You