dimensional control20rev 203 given by scott

8/6/2019 Dimensional Control20rev 203 Given by Scott

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Die Casting Dies: Dimensional Control

Table of Contents

Page

1 - Introduction 1

2 - Standard, precision, and thermal control tolerances 1

3 - Dimensional capability analysis 2

4 - Shrinkage variation and shrink factor 4

5 Draft 5

6 - Linear dimensions 6

7 - Linear cross parting dimensions 8

8 - Slide to cavity dimensions 10

9 - Parting line shift variation 12

10 - Flatness variation 16

11 - Die warpage 17

12 - Parting line placement and cavity orientation 18

13 - Plan parting surface and slide fit 20

14 - Dimensional philosophy for steel safe tool development & tolerancing 21

15 - Comprehensive example 22


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1 Introduction

This text is designed to assist a die casting technical employee to work with the customer,tool maker, and production staff to specify particular product design features, tooling

design features, and process control technologies and plans to assure that a casting meets

the customers dimensional expectations. This text is written in a manner that allows thedie casting technical employee to quickly find and utilize the information effectively.

The discussion is also designed to assist a die casting company to be more effective in

dimensional accuracy and to lower costs of product startup and manufacturing. Greaterdimensional precision will open new markets for die castings. Die castings have long

been known to be dimensionally inaccurate and inconsistent. As shown in the proceeding

discussion, changes in product design, tooling design, and process control practice have

the potential to make die castings dimensionally competitive with other metal and plasticforming and machining processes.

2 Standard, precision, and thermal control tolerances

NADCA has long provided standard tolerances for die castings for use by our industryand our customers. The primary intent of these standards was to assist a product

designer. However, these tolerances were developed by surveying a large number of

parts made by many die casting companies. Therefore, since these tolerances reflect theaggregate capabilities of the North American die casting community, these tolerance

standards can be considered as the worst case dimensional capability. Furthermore, these

standards may easily be exceeded by using good thermal design and process control.Therefore, as we discuss tolerances we will describe three sets of standards.

First, the NADCA standard tolerances should be used when possible. However,these standards should not be considered as the best case. The standard tolerance

assumes that the maximum casting ejection temperature will be within +/- 150F

from shot to shot.

Secondly, precision tolerances, which are 50 to 80 percent of the standardtolerance, can be used when good thermal design and good process controls are in

place. The precision tolerance assumes that the casting ejection temperature will

be within +/- 75F from shot to shot. Thirdly, thermal control tolerances, which are roughly 20 percent of the standard

tolerance can be used when careful consideration of thermal design, process

design, and closed loop thermal and process control are in place. The thermalcontrol tolerance assumes +/- 30F from shot to shot. Thermal control tolerances

should only be used if thermocouples are used properly to monitor and control via

a closed loop mechanism. The best control mechanism is Proportional IntegralDerivative control. Good thermal control would mean that die temperature,

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measured by a thermocouple mounted in the die, should be controlled within +/-

10F of the desired set point to achieve the +/- 30F casting ejection temperature

range.

Good thermal control has been shown in many cases to exceed the standard tolerances by

a factor of five to ten times. For example, in one case a 2-inch linear dimension within asingle insert was shown to vary over time by less than +/-0.0002 inch. The process

variation component of the standard linear tolerance would be +/-0.002 inch.

Along with the NADCA standard tolerance guidelines, formulas will be provided in this

text to calculate standard, precision, and thermal control tolerances for any dimension.

These tolerances are based on the expected ejection temperature variation for each

standard. It is important to note that the precision and thermal control tolerances

are beyond the NADCA product design standard tolerances. The intent within this

text is to describe how higher levels of dimensional accuracy are attainable through

design and process control methods.

3 - Dimensional capability analysis

Although standard, precision, and thermal control tolerances are provided, the only way

to accurately determine your casting tolerance capability is by completing dimensional

capability studies on your products for each type of dimension. However, the bestmethod is an altered form of a Multi-Vari study. This study measures the results of all

types of process variation. For a full description of the Multi-Vari study, see the Die Cast

Problem Solving course and textbook on this subject.

The Multi-Vari study should be completed over two setups. During each setup two

consecutive parts should be collected for two consecutive hours for three consecutiveshifts for two consecutive days. See the example spreadsheet below. Therefore, a Multi-

Vari study for dimensional capability analysis will contain a minimum of 48 total parts.

Then different dimensions should be measured using a device with a known good GRR(Gage Repeatability and Reproducibility). Linear dimensions within a given insert, cross

parting, slide to cavity, parting shift, and flatness should all be measured if possible.

(Please refer to later sections of this text to clarify these types of dimensions.) The

data should then be evaluated statistically. The result will be an average and standarddeviation for each type of dimension. The data should be summarized and cataloged so

that it can then be used to establish tolerances for future products with your customers.

For example, assume you have studied cross parting variation on several parts. Your

customer asks you what your capability would be for a cross parting dimension on a new

product design. A similar sized casting shows that standard deviation for the crossparting dimension is 0.0004 inch. The customers quality requirements state that 10

PPMs is an acceptable quality level for dimensions. Therefore, using standard Z-curve

tables on a normal distribution, a tolerance that would be plus and minus 4.5 standard

deviations would have less than 10 PPMs. Therefore, the customer should be told in this

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Example Multi-Vari Collection Schedule for Dimensional Capability Analysis

Date Time ShiftCasting

#

1 LinearDimension

1 CrossParting

Dimension

1 Slide toCavity

Dimension

1 PartingLine ShiftDimension

1 FlatnessDimension

Setup 11/1/200

7 9:00 AM 1 1

1/1/2007 9:00 AM 1 2

1/1/2007 10:00 AM 1 3

1/1/2007 10:00 AM 1 4

1/1/2007 5:00 PM 2 5

1/1/2007 5:00 PM 2 6

1/1/2007 6:00 PM 2 7

1/1/200

7 6:00 PM 2 81/2/200

7 1:00 AM 3 9

1/2/2007 1:00 AM 3 10

1/2/2007 2:00 AM 3 11

1/2/2007 2:00 AM 3 12

1/2/2007 9:00 AM 1 13

1/2/2007 9:00 AM 1 14

1/2/2007 10:00 AM 1 15

1/2/200

7 10:00 AM 1 161/2/200

7 5:00 PM 2 17

1/2/2007 5:00 PM 2 18

1/2/2007 6:00 PM 2 19

1/2/2007 6:00 PM 2 20

1/3/2007 1:00 AM 3 21

1/3/2007 1:00 AM 3 22

1/3/2007 2:00 AM 3 23

1/3/200

7 2:00 AM 3 24Setup 2

2/1/2007 9:00 AM 1 25

2/1/2007 9:00 AM 1 26

2/1/2007 10:00 AM 1 27

2/1/2007 10:00 AM 1 28

2/1/2007 5:00 PM 2 29

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2/1/2007 5:00 PM 2 30

2/1/2007 6:00 PM 2 31

2/1/2007 6:00 PM 2 32

2/2/2007 1:00 AM 3 33

2/2/2007 1:00 AM 3 34

2/2/2007 2:00 AM 3 35

2/2/2007 2:00 AM 3 36

2/2/2007 9:00 AM 1 37

2/2/2007 9:00 AM 1 38

2/2/2007 10:00 AM 1 39

2/2/2007 10:00 AM 1 40

2/2/2007 5:00 PM 2 41

2/2/2007 5:00 PM 2 42

2/2/2007 6:00 PM 2 43

2/2/2007 6:00 PM 2 44

2/3/2007 1:00 AM 3 45

2/3/2007 1:00 AM 3 46

2/3/2007 2:00 AM 3 47

2/3/2007 2:00 AM 3 48

case that the cross parting dimension tolerance to achieve 10 PPMs would be at least +/-

0.0018 inch (0.0004 inch x 4.5).

Multi-Vari studies should be done on several product sizes, machine tonnages, and for

each level of process control technology implemented within your facility. Therefore, if

you have some dies with thermal control technologies and others without, you shouldunderstand the dimensional control difference that this technology provides.

4 - Shrinkage variation and shrink factor

One aspect of dimensional control that must be accounted for in the die design phase is

the dimensional shrinkage that will occur between the time when the casting is ejecteduntil it is measured or used. Typically, die casting companies have used standards for the

dimensional shrinkage. For example, the aluminum industry has used 0.006 inch per inch

as a linear shrinkage standard. This comes from the linear shrinkage calculation:

L (length) = T (Casting Eject Temp. Ambient Temp.) x Ca (Coefficient of

Thermal Expansion for Aluminum) - T (Die Eject Temp. Ambient

Temp.) x Ca (Coefficient of Thermal Expansion for H13 Steel)

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L = (700 70) x (13 x 10-6) (400 70) x (6.3 x 10-6) = 0.006 per inch

In the above equation, 700 is the typical casting ejection temperature, 400 is the typical

die ejection temperature, and 70 is the typical ambient temperature. Therefore, if a

different ejection temperature is used, it is advisable to use different shrinkage factors.

Furthermore, the major source of dimensional variation is in the variation in the amount

of shrinkage that occurs after ejection. This is due to variation in casting ejectiontemperature. Control of thermal variation via good thermal design and closed loop

thermal control will reduce casting ejection temperature variation and thus minimize

shrinkage variation. This process of dimensional shrinkage and shrinkage variation iswell illustrated in the figure below.

The amount of shrinkage tends to be consistent for a particular feature shape. The shape

will change the amount of shrinkage. For example, a linear dimension on a long flat plate

may shrink by 0.007 inch per inch, but an inside diameter on a cylinder may only shrinkby 0.004 inch per inch.

The amount of shrinkage will vary due to the die design, cycle time consistency, process

controls, and the thermal control technology in place.

It is important to note that hot oil systems are not considered to be a thermal control

technology. Although, hot oil units may control the oil temperature within the unit, theydo not control die temperature. Hot oil simply works as a passive cooling medium.

(insert fig. 5-1 page 40 without the worded description)

Therefore, when a customer requires tighter dimensional control, the die casting company

should first look to careful die design and process control to better maintain dimensions.

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Good die design with closed loop thermal control technology using water as the cooling

medium is the best and most cost effective way to achieve tight dimensions for die

casting customers. Furthermore, utilizing these methods and technologies can frequentlymake die casting dimensional variation very competitive with precision machining

operations.

5 Draft

Draft is the amount of taper in the direction of pull to allow for ejection of a casting.

Because die castings are experiencing volumetric shrinkage from the transition from

liquid to solid and linear shrinkage due to cooling, some features such as outside angles

need very little draft, and others such as round cores and inside angles need more draft.

(insert the sketch only from the NADCA S-4A-7-03 draft tolerance specification)(on core change A to I and change B to O)

Although the draft requirement is somewhat sensitive to thermal variation, it is mostly

driven by the quality of the die release coating applied to the die, the condition of the diesurface, and the temperature differential between injection and ejection (Note: this

temperature differential is driven by dwell time). Therefore, a thermal control tolerance

does not apply to draft. The precision tolerance should be used when absolutelynecessary, with the knowledge that the appropriate die release agent and minimal dwell

time is used. The formula for draft is as follows:

D = (L)0.5 / C

In the above equation, D is the amount of draft, (L)0.5 is the square root of the length of

the dimension, and C is the constant from the table below (L = O I for cores):

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(insert table only from the NADCA S-4A-7-03 draft tolerance specification, dont enter

Copper)

The precision formula for draft is shown below:

D = (0.8 x (L)0.5)/ C

The precision draft is always 80 percent of the standard draft.

For small angles, the draft angle can then be calculated from the equation below:

Draft Angle = (D / L) / 0.01746

6 - Linear dimensions

Linear dimensions within a core, insert, or cavity are the most basic dimensions toconsider for tolerances in die casting. These dimensions are only affected by ejection

temperature variation. Therefore, the amount of variation in the resulting casting is

directly linked to proper thermal design of the die and process control.

(insert NADCA linear dimension tolerance P-4A-1-03 picture only here)

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The tolerance specification for the linear dimensional variation is used partially to allow

for tool build variation and partially to allow for thermal process variation. The standardtolerance for tooling is 0.001 inch for any dimension below 1 inch and 0.0001 inch is

allowed per inch for dimensions longer than 1 inch.

The standard tolerance specification for process variation is 0.0009 inch per inch.

Adding the tool and process tolerance together causes the overall tolerance to be 0.002inch for the first 1 inch and 0.001 inch for every inch after the first inch.

For linear dimensions, the precision tolerance is one-half of the standard tolerance for

both tooling and process variation. The thermal control tolerance is one-half of thestandard tolerance for tooling and one-fifth of the standard tolerance for process

variation. The formulas for the standard, precision, and thermal control tolerances are

shown below:

Standard Linear Tolerance = Length (minimum 1) x 0.001 + 0.001

(This infers a maximum ejection temperature variation of +/- 150F)

* Precision Linear Tolerance = Length (minimum 1) x 0.0005 + 0.0005


* Thermal Control Tolerance = Length (minimum 1) x 0.0002 + 0.0005


* Note: These tolerances are guidelines based upon the level of tooling expertise and

process and thermal control technology in place. Internal capability studies should be

completed as described in Chapter 4 to verify dimensional capability. These tolerancesexceed current NADCA standards.

7 - Linear cross parting dimensions

A cross parting dimension is a dimension perpendicular to the parting line measured from

one surface cast within the cover die to another surface cast within the ejector die. Thecross parting tolerance is always added to the linear tolerance to obtain the full tolerance.

These dimensions are primarily affected by die seal off temperature variation. The seal

off area is the area around the cavity(s). When this temperature is not consistent, thehotter sections of the die seal area expand more than the cooler sections. The expansion

holds the die open and leads to an increase in the cross parting dimensions. The cross

parting dimension increase can also be caused by impact. If the impact force exceeds thelocking force, the ejector die will blow back. This can happen even if flashing is minor.

In addition, if a die is not supported properly, typically the ejector side, the cavity may

also flex back due to impact force. The cross parting dimension can be positively

affected by locking force. That is, the force of closing the die compresses the cavity seal

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area a small amount. Finally, die warpage can affect cross parting dimensions. See

Chapter 11, Die Warpage, for a complete description.

(insert NADCA parting dimension tolerance P-4A-2-03 picture only here)

The way to minimize parting blow is to control the cavity seal off area temperature and to

make sure that the impact force does not exceed the locking force. To control

temperature in the seal area, the die should be designed with cavity water lines tappedinto the cavity, well within the cavity seal area as shown below. This design practice will

avoid cold spots in the seal area. A second design consideration is to place hot oil around

the cavity in the seal area at a consistent distance from the cavity. This design practicewill not only help to keep the seal area at a constant temperature, it will also serve as an

effective method to preheat the die during setup.

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(redraw this picture more professionally if necessary)

The parting line dimension tolerance is always positive. That is, the parting linedimension will only increase with parting line thermal variation and parting line blow.

The parting line dimension tolerance is related to the projected area of the casting. This

is because seal area increases with projected area. Therefore, the potential for partingline dimension increase is greater with larger parts.

For parting line dimensions, the precision tolerance is one-half of the standard tolerance.

There is no added error needed for tooling tolerance. The thermal control tolerance isone-fifth of the standard tolerance for process variation. The formulas for the standard,

precision, and thermal control tolerances are shown below:

Aluminum and Magnesium

Standard Parting Line Tolerance= Projected Area x 0.00006 + Length (minimum 1) x 0.001 + 0.004

Precision Parting Line Tolerance= Projected Area x 0.00003 + Length (minimum 1) x 0.0005 + 0.002

Thermal Control Parting Line Tolerance

= Projected Area x 0.000012 + Length (minimum 1) x 0.0002 + 0.001Zinc

Standard Parting Line Tolerance

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= Projected Area x 0.000036 + Length (minimum 1) x 0.001 + 0.0039

Precision Parting Line Tolerance= Projected Area x 0.000018 + Length (minimum 1) x 0.0005 + 0.002

Thermal Control Parting Line Tolerance= Projected Area x 0.0000072 + Length (minimum 1) x 0.0002 + 0.0015

Note: These tolerances are guidelines based upon the level of tooling expertise andprocess and thermal control technology in place. Internal capability studies should be

completed as described in Chapter 4 to verify dimensional capability. Also, the number

of cavities should be known for the calculation of projected area which affects the parting

line tolerance variation.

8 - Slide to cavity dimensions

Slide to cavity dimensions are dimensions from features cast within a slide to features

cast within the cover or the ejector die. The slide to cavity tolerance is always added tothe linear tolerance to obtain the full tolerance. These dimensions are primarily affected

by slide temperature, slide temperature variation, and the slide locking mechanism.

When the cavity temperature exceeds ambient temperature it expands in all directions.

The expansion holds the slide back, which can cause the die to open, and then lead to anincrease in both the slide to cavity and the cross parting dimensions. The slide to cavity

dimension increase can also be caused by impact. If the impact force exceeds the slide

locking force, or if there is a gap between the slide and the lock, the slide will blow back.This can happen even if the remainder of the die does not flash. The slide to cavity

dimension can be positively affected by locking force. That is, the force of closing the

die compresses the slide a small amount.

The slide locking mechanism can also negatively affect the slide to cavity dimension.

This can happen if the thermal expansion of the cover die versus the ejector die issignificant. When this happens, the expansion changes the dimension relationship

between the cover and ejector. In addition, the thermal expansion may cause excessive

force between the slide lock and the slide. This force can cause bolts or dies to break or

may cause deformation of die features. This problem is discussed in detail in theproceeding section entitled Plan Parting Surface and Slide Fit. This section describes

how to design a die so that thermal process variation does not affect the mechanical

functions of the die.

The way to minimize slide to cavity dimension changes is to control the slide temperature

as low as possible while still making good product, and to make sure that the impactforce on the slide does not exceed the locking force. The best way to cool a slide so that

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(insert NADCA parting dimension tolerance P-4A-3-03 picture only here)

the slide temperature is held low while the cavity surface temperature is high enough forproduction is to use a large water line that is far away from the insert. See the NADCA

publication Designing Die Casting Dies: Thermal Control for water lines calculations.

The slide to cavity dimension tolerance is always positive. That is, the slide to cavitydimension will only increase with slide temperature and slide blow. The slide to cavity

dimension tolerance is related to the projected area of the casting. This is because largerslides tend to be on larger dies. Therefore, the potential for parting line dimension

increase is greater with larger parts.

For slide to cavity dimensions, the precision tolerance is one-half of the standardtolerance. There is no added error here needed for tooling tolerance. The thermal control

tolerance is one-fifth of the standard tolerance for process variation. The formulas for the

standard, precision, and thermal control tolerances are shown below:

Aluminum and Magnesium

Standard Slide to Cavity Tolerance


Precision Slide to Cavity Tolerance


Thermal Control Slide to Cavity Tolerance= Projected Area x 0.00002 + Length (minimum 1) x 0.0002 + 0.003

Zinc

Standard Slide to Cavity Tolerance


Precision Slide to Cavity Tolerance


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Thermal Control Slide to Cavity Tolerance


Note: These tolerances are guidelines based upon the level of tooling expertise and

process and thermal control technology in place. Internal capability studies should be

completed as described in Chapter 4 to verify dimensional capability.

9 - Parting line shift variation

Parting line shift variation is the amount and variation of the miss-alignment between the

cover and ejector die. The amount of shift is due to average temperature difference

between the cover and ejector dies and the design of the guide mechanism used to locatethe cover to the ejector. Parting line shift variation is affected by the piece to piece

temperature variation between the cover and the ejector. In the die design, die casting

companies have typically used guide pins. However, guide pins are poor in locating the

cover die to the ejector die. The two reasons are (1) they do not locate the die halves nearthe part center line and (2) they are pins that must be designed with compliance in all

directions to assure that the die halves do not seize together due to interference fit.

For example, assume we have a die that has a 36 inch square holder block. The guide

pins within this holder block are placed 30 inch away from one another in all four

corners. Assuming the die halves may vary +/-100F from cover to ejector, the potentialdimension variation must be calculated from pins from corner to corner as follows:

(insert figure 5-9 from the die casting die design text without description)

L = 6.33 x 10-6 (thermal expansion coefficient for steel) x L / tan 45 x T

L = 6.33 x 10-6 x 30 / .707 x 200 = 0.054

Therefore, the diameter of the guide pins must be built 0.054 inch less than the diameter

of the bushing to assure that they do not seize with +/-100F variation between the coverand ejector.

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The proper way to locate the cover to ejector is using guide blocks as follows. Three

guide blocks should be designed into the tool as shown below.

(insert figure 5-10 on page 46 of the die casting die design text)

One guide block controls die shift in the Y direction (up and down). One guide block

controls die shift in the X direction (side to side). The third guide block controls die

rotation. One side of the die may be left open for either drop through quench,reciprocative spray, or manual extraction. The side to be left open is up to the needs of

the die casting manufacturer. It is important to use three guide blocks because two will

not control rotation and may cause broken guide blocks and allow excessive shift. Fourwill cause guide block wear because the die will not expand equally in all directions.

This wear will lead to parting shift. The result is that the dimensional control is much

better because the guide blocks are located on the casting center, and each guide blockmay be designed with less than 0.001 inch difference between the block width and the

pocket.

If this arrangement is used in the previous example, assuming a part that fits within an 18

inch square, the calculation for parting shift is as follows:

L = 6.33 x 10-6 (thermal expansion coefficient) x L x T

L = 6.33 x 10-6 x 9 x 100 = 0.0057

Instead of the length being 30 / [tan] 45, the maximum length is only 9 inches from thecenterline. And since the maximum shift is only in one direction, 100F can be used for

the change in temperature. This represents reduction in parting shift of almost ten fold!

The thermal variation that occurs within the tool affects parting shift. If guide pins are

used, the temperature difference between the cover and ejector must be known to

determine the proper clearance between the guide pins and bushings. Therefore, whenguide pins are used, this parting variation should be expected to occur unless some other

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locating feature is used. However, if guide blocks are used, the parting shift will only be

as bad as the thermal variation. If the cover die and the ejector die are similar in

temperature, the parting shift will be very small.

Parting Line Shift Tolerance using Guide Pins

Expected thermal variation between cover and ejector50 75 100 125 150 175 200

Cornerdistancebetweenguidepins 7 0.0063 0.0094 0.0125 0.0157 0.0188 0.0219 0.0251

8 0.0072 0.0107 0.0143 0.0179 0.0215 0.0251 0.0287

9 0.0081 0.0121 0.0161 0.0201 0.0242 0.0282 0.0322

10 0.0090 0.0134 0.0179 0.0224 0.0269 0.0313 0.0358

11 0.0098 0.0148 0.0197 0.0246 0.0295 0.0345 0.0394

12 0.0107 0.0161 0.0215 0.0269 0.0322 0.0376 0.0430

13 0.0116 0.0175 0.0233 0.0291 0.0349 0.0407 0.0466

14 0.0125 0.0188 0.0251 0.0313 0.0376 0.0439 0.0501

15 0.0134 0.0201 0.0269 0.0336 0.0403 0.0470 0.053716 0.0143 0.0215 0.0287 0.0358 0.0430 0.0501 0.0573

17 0.0152 0.0228 0.0304 0.0381 0.0457 0.0533 0.0609

18 0.0161 0.0242 0.0322 0.0403 0.0483 0.0564 0.0645

19 0.0170 0.0255 0.0340 0.0425 0.0510 0.0595 0.0680

20 0.0179 0.0269 0.0358 0.0448 0.0537 0.0627 0.0716

21 0.0188 0.0282 0.0376 0.0470 0.0564 0.0658 0.0752

22 0.0197 0.0295 0.0394 0.0492 0.0591 0.0689 0.0788

23 0.0206 0.0309 0.0412 0.0515 0.0618 0.0721 0.0824

24 0.0215 0.0322 0.0430 0.0537 0.0645 0.0752 0.0860

25 0.0224 0.0336 0.0448 0.0560 0.0671 0.0783 0.0895

26 0.0233 0.0349 0.0466 0.0582 0.0698 0.0815 0.0931

27 0.0242 0.0363 0.0483 0.0604 0.0725 0.0846 0.096728 0.0251 0.0376 0.0501 0.0627 0.0752 0.0877 0.1003

29 0.0260 0.0389 0.0519 0.0649 0.0779 0.0909 0.1039

30 0.0269 0.0403 0.0537 0.0671 0.0806 0.0940 0.1074

31 0.0278 0.0416 0.0555 0.0694 0.0833 0.0971 0.1110

32 0.0287 0.0430 0.0573 0.0716 0.0860 0.1003 0.1146

33 0.0295 0.0443 0.0591 0.0739 0.0886 0.1034 0.1182

34 0.0304 0.0457 0.0609 0.0761 0.0913 0.1065 0.1218

35 0.0313 0.0470 0.0627 0.0783 0.0940 0.1097 0.1253

36 0.0322 0.0483 0.0645 0.0806 0.0967 0.1128 0.1289

37 0.0331 0.0497 0.0663 0.0828 0.0994 0.1159 0.1325

38 0.0340 0.0510 0.0680 0.0851 0.1021 0.1191 0.1361

39 0.0349 0.0524 0.0698 0.0873 0.1048 0.1222 0.139740 0.0358 0.0537 0.0716 0.0895 0.1074 0.1253 0.1433

The charts describe the parting shift variation to be expected, and therefore represent

reasonable tolerances. These charts only describe parting shift for points that would bedirectly across the parting line from one another. If there is a tolerance requirement

across the parting line for features that are not directly across the parting from one

another, the linear tolerance must be added to the parting shift tolerance.

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Parting Line Shift Tolerance using Guide Blocks

Expected thermal variation between cover and ejector

50 75 100 125 150 175 200

Maximumd

istancefromcenterlinetocastingedge 1 0.0003 0.0005 0.0006 0.0008 0.0009 0.0011 0.0013

1.25 0.0004 0.0006 0.0008 0.0010 0.0012 0.0014 0.0016

1.5 0.0005 0.0007 0.0009 0.0012 0.0014 0.0017 0.0019

1.75 0.0006 0.0008 0.0011 0.0014 0.0017 0.0019 0.0022

2 0.0006 0.0009 0.0013 0.0016 0.0019 0.0022 0.0025

2.25 0.0007 0.0011 0.0014 0.0018 0.0021 0.0025 0.0028

2.5 0.0008 0.0012 0.0016 0.0020 0.0024 0.0028 0.0032

2.75 0.0009 0.0013 0.0017 0.0022 0.0026 0.0030 0.0035

3 0.0009 0.0014 0.0019 0.0024 0.0028 0.0033 0.0038

3.5 0.0011 0.0017 0.0022 0.0028 0.0033 0.0039 0.0044

4 0.0013 0.0019 0.0025 0.0032 0.0038 0.0044 0.0051

4.5 0.0014 0.0021 0.0028 0.0036 0.0043 0.0050 0.00575 0.0016 0.0024 0.0032 0.0040 0.0047 0.0055 0.0063

5.5 0.0017 0.0026 0.0035 0.0044 0.0052 0.0061 0.0070

6 0.0019 0.0028 0.0038 0.0047 0.0057 0.0066 0.0076

6.5 0.0021 0.0031 0.0041 0.0051 0.0062 0.0072 0.0082

7 0.0022 0.0033 0.0044 0.0055 0.0066 0.0078 0.0089

7.5 0.0024 0.0036 0.0047 0.0059 0.0071 0.0083 0.0095

8 0.0025 0.0038 0.0051 0.0063 0.0076 0.0089 0.0101

9 0.0028 0.0043 0.0057 0.0071 0.0085 0.0100 0.0114

10 0.0032 0.0047 0.0063 0.0079 0.0095 0.0111 0.0127

11 0.0035 0.0052 0.0070 0.0087 0.0104 0.0122 0.0139

12 0.0038 0.0057 0.0076 0.0095 0.0114 0.0133 0.0152

13 0.0041 0.0062 0.0082 0.0103 0.0123 0.0144 0.016514 0.0044 0.0066 0.0089 0.0111 0.0133 0.0155 0.0177

15 0.0047 0.0071 0.0095 0.0119 0.0142 0.0166 0.0190

16 0.0051 0.0076 0.0101 0.0127 0.0152 0.0177 0.0203

17 0.0054 0.0081 0.0108 0.0135 0.0161 0.0188 0.0215

18 0.0057 0.0085 0.0114 0.0142 0.0171 0.0199 0.0228

19 0.0060 0.0090 0.0120 0.0150 0.0180 0.0210 0.0241

20 0.0063 0.0095 0.0127 0.0158 0.0190 0.0222 0.0253

10 - Flatness variation

Flatness is defined as the maximum casting warpage relative to original die dimensions.Typically, flatness would be considered for a dimension that is parallel to the parting line.

The amount of flatness is measured by comparing the highest to the lowest point on a

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plane parallel to the parting line. Out of flatness condition is primarily caused by casting

ejection temperature variation within the casting.

For example, if one section of a casting is 700F at ejection and another is 400F, the

section at 700F will incur much more thermal contraction than the 400F section as the

casting is cooled to ambient temperature. This is irrespective of the cooling method.This will result in internal stresses and warpage within the casting. The direction of this

warpage will be affected by casting feature shape and location. Typically the casting will

shrink in the direction of casting features.

To minimize a flatness problem, dies and processes should be designed and controlled to

assure that casting ejection temperature is as constant as possible. Castings with

asymmetric geometry, inconsistent wall stock, and odd shaped features will make flatnessworse. To gain the most consistency, the die temperature should be as consistent as

possible and the cooling systems heat removal rate must be proportionate to the casting

thickness in each die section. This will cause consistent casting ejection temperature. To

properly design the die thermally, see the Die Casting Die Design: Thermal Controlbooklet. Thermal simulation should then be used before the die is built to make sure that

the casting temperature is consistent within the casting at ejection.

The second source of flatness problems is dragging during ejection. Dragging occurs

when the casting has low draft, soldering problems, or undercuts placed in the die. When

these problems occur, the casting is twisted and bent during ejection. The amount oftwisting or bending is based upon the severity of the dragging problem. If there is large

casting to casting variation in flatness, the problem is likely due to an ejection problem.

If the casting flatness problem is consistent from piece to piece, the problem is likelythermal.

For the purposes of tolerancing for flatness, the largest dimension on the surface shouldbe measured. The standard, precision, and thermal control tolerances for flatness are

shown below, based upon this dimension.

Flatness Standard Tolerance = Largest Dimension x 0.003 0.001

Flatness Precision Tolerance = Largest Dimension x 0.0015 0.0005

Flatness Thermal Control Tolerance = Largest Dimension x 0.0006 0.0002

The minimum dimension that should be used as the Largest Dimension in theseequations is 3 inches.

Note: These tolerances are guidelines based upon the level of tooling expertise andprocess and thermal control technologies in place. Internal capability studies should be

completed as described in Chapter 4 to verify dimensional capability.

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11 - Die warpage

Cavity inserts are hotter on the parting surface than on the back surface. If unrestrained,

they will expand on the parting surface and warp toward the parting line.

(insert figure 5-14 on page 49 in designing die castings die text)

(insert figure 5-15 on page 50 in design die casting die text)

To assure that die warpage does not become extreme, the die must be restrained bybolting the cavity insert to the holder near the center of the insert. An ample number of

appropriately size bolts must be used to assure that the cavity insert is held flat and that

the bolts do not break. The appropriate bolt size and number of bolts required should becalculated.

12 - Parting line placement and cavity orientation

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Several factors dictate parting line placement. The most important of these is typically

the customers product needs and requirements. These requirements, especiallyporosity and finish requirements that are not clearly shown in the product print, should be

considered in the parting line placement decision. The parting should be placed so that

the parting line seams do not cause any problems with product fit, function, orappearance.

Another important factor is metal flow direction and gate location. Enough partinglength must be available to allow for an in-gate. The in-gate should be located near

critical porosity areas or near critical surface finish areas.

Secondary operations may also affect parting line placement. Some casting featuresmust not have secondary operations to preserve their appearance or function. For

example, pressure tight castings work best when the cast surface or skin is not

compromised by secondary operations. In addition, highly decorative or chromed casting

surfaces may require that no parting seams occur on the surface. Other casting featuresmust have secondary operations, such as machining. Cast tolerances are not frequently as

accurate as machined tolerances. Therefore, the machining operation may affect, or evenallow more freedom, in the location of parting lines.

Another factor is tooling cost. Die casting companies must be competitive in the tools

overall cost. To minimize tooling cost, the amount and difficulty of tool manufacturingoperations and tool steel costs must be considered. Parting line placement decisions can

have a large effect on the tooling cost.

Slides or cores add to the cost of the tool and add parting line seams that may affect all

of the considerations above. The cost of slides and cores, and the related quality issues

must be weighed against the cost and quality of machining operations.

Ejection requirements can complicate parting line placement and casting orientation.

The best situation for ejection is to orient the part so that the most complicated features,features deepest from the parting, and/or lowest draft features are in the ejector die.

However, other factors described above can make ejection difficult. Utilizing strip

ejection in the cover die and in the slides can be used to promote good ejection.

All of the parting line placement and casting orientation issues described above lead to

variation in dimensional accuracy for many reasons. The puck example shown below

illustrates these decisions.

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(insert fig. 7-1 page 73 in Designing Die Casting Dies Book)

Assuming in all cases that the ejector die is on the right, parting decision (a) would be

very good for ejection and tool cost and may be good for secondary operations.However, metal flow may be poor. Parting decision (b) would be best for metal flow and

likely best for secondary operations, but tool cost may be higher and ejection may be

difficult. Parting decision (c) would be poor for ejection and metal flow but good for

tooling cost. Parting decision (d) would be excellent if the customer could not allow aparting on the perimeter of the casting. However, this approach would be very poor for

metal flow, tooling cost (because of die thickness requirements), and ejection (unlesssignificant draft were allowed).

A second example to illustrate the decisions is the housing shown below.

(insert picture of housing fig. 7-2 page 74 in Designing Die Casting Dies Book)

The most difficult issue for this casting would be metal flow. This casting would need to

have an in-gate perpendicular to the large diameter or would need to be gated into oneend with a ring gate. The gating/metal flow decision would be made based upon the

customers requirements.

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If the customer demanded excellent fill out and surface finish, or if the customer required

tight control over the inside diameter dimension, a one-half ring gate should be used.

The ring gate would be directed at the most critical surface(s). This ring gate would bearound the bottom core slide and could enter from either side of the casting depending

upon secondary operations and customer requirements for trim. The second decision

would be the boss. It can either be machined or cored. If the ring gate is used, the bosscan be placed in the ejector or cover die.

If the ring was not needed for the reasons described above, the casting should be gatedwith the in-gate perpendicular to the large diameter to minimize tooling cost as shown

below.

(insert fig. 7-3 a page 74 only and do not show verbiage)

In this design the only slide required is for the cored boss. If this part were at a low

production volume, the slide may not be used. This would likely be the least expensive

tooling approach.

13 - Plan parting surface and slide fit

To assure that the thermal variation within the process does not lead to a mechanical

malfunction within the die or dimension problems, special care should given to the die

design. The die designer should work to separate thermal expansion and variation fromthe mechanical operation of the die by first understanding the amount of thermal

expansion of all inserts and slides as the die heats from ambient to normal operating

temperature. If this expansion causes parting line gaps to exceed 0.005 inch, flashingwill occur. If the variation in size exceeds the elastic compression strain limits of the

steel, the slide or the lock will either break or deform.

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To address this issue, the die should be built with compliant features to accept these

forces without flashing, breaking, or deforming. This entails the use of very strong

springs, such as tie bars or high force compression washers. We use very large tie bars inmachines to allow for the machine to generate locking force without breaking. The same

technology works for holding slides when thermal variation conditions would either

cause flashing or breaking. An example of how a tie bar could be placed within the coverdie to hold slide locks is shown below.

To calculate the diameter and steel type to use, the minimum and maximum thermal

conditions must be considered to determine the amount of tie bar stretch required. Thenthe minimum and maximum locking forces must be determined or calculated to

determine the modulus of elasticity, the strength requirement of the steel, and the lock

preload required.

14 - Dimensional philosophy for steel safe tool development & tolerancing

To assure that tooling does not require welding during the initial tool sampling phase, it is

best to design dimensions to be steel safe. That is, we should always design dimensions

so that we are removing material if corrections must be made. Therefore within cavitydimensions should always be on the small side, cores and pins should always be on the

large side, and centerlines should be nominal. It is not productive to design with

excessive steel safe conditions because this will cost money and time in the product

development process. The amount of steel safety factor is based upon the quality of thetool design methods, process control methods, and thermal control methods.

15 - Comprehensive example

The following example is designed to illustrate how the dimensional control guidelines

and tolerances can be applied. The example sketch is shown below.

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(indicate that the plane in plan view must be flat within 0.006 and that the OD to ID on

the three holes must be within 0.003 true position)

Insert figure 5-2 page 40 from die casting design text also see desex5.bmp

Flatness

The first step is to identify any flatness dimensions. The plane in the plan view has a

flatness tolerance of 0.006 inch. Since the largest dimension is 5 inches, then the

standard tolerance for flatness would be 0.014 inch, the precision tolerance would be0.007 inch, and the thermal control tolerance would be 0.0028 inch. Since the flatness

tolerance is 0.006 inch, thermal control must be used to achieve this dimension, and the

cooling system should be setup so that the within casting temperature is consistent at allpoints on the casting. Draft and ejection should be designed and checked to assure that

dragging and bending do not result during ejection.

Parting Shift

The second step is to identify any dimensions related to parting line shift. The only

dimension that requires control of parting shift is the boss that extends below the plane in

the side view. This is because the pin used to make the center hole must be located in the

opposite die half as the outer diameter of the boss to maintain the hole location toleranceof 0.900 inch +/- 0.005 inch. In other words, the three core pins have a tight tolerance

that is also a critical characteristic. To maintain this critical dimension, the pins must all

be located in the same insert. Therefore, the boss on the underside of the casting in theside view must be made in the opposite die half as the pin in the center of the boss.

If guide pins are used and we were to assume a two cavity tool, 50F variation between thecover and ejector die, and 18 inches between guide pins, the variation would be 0.0161

inch. This is far greater than the tolerance of 0.003 inch. Therefore, we must use guide

blocks. Given that guide blocks are used and placed on the center line, the maximum

distance from the center of the tool to the each cavity is about 9.5 inches. Therefore,

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when the two cavity layout of the tool is designed, both cavities should be well within 9.5

inches from center. Since the gating system will require that the cavities be placed

approximately 5 inches from the center line of the die, the parting shift for this boss willbe 0.0016 inch maximum.

Slide to Cavity

The third step is to consider any slide to cavity dimensions. There are four potential slideto cavity dimensions. Two of these will be slide to cavity dimensions depending upon

how the parting is determined. There is also a print note 0.187 inch diameter holes

marked X to be within 0.005 inch of centerline A. These dimensions are highlighted

below.

Insert fig 5-2 with 0.750 +/- 0.003, 2.500 +/- 0.015, 0.500 +/- 0.002, 5.00 +/-

0.020 dimensions and the note 0.187 diameter holes marked X to be within 0.005 of

centerline A shown only. Eliminate the other dimensions for this sketch also see filedesex2.bmp

Assuming that this casting is made in a two cavity tool, that the projected area for each

cavity is less than or equal to 6.25 square inches, and that the runner and biscuit will beless than the casting projected area, then the overall projected area will be less than 25

square inches. To consider the tolerances for these dimensions, it is best to prepare a

table showing the tolerance calculations for each of the different tolerance standards.

Projected Area 25 square inches

PrintDimension

PrintTolerance

StandardTolerance

PrecisionTolerance

ThermalControl

Tolerance

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0.750 0.003 0.0125 0.00675 0.0037

2.500 0.015 0.014 0.0075 0.004

0.500 0.002 0.0125 0.00675 0.0037

5.000 0.020 0.0165 0.00875 0.0045

Insert figure 5-4 with description here

Because the standard, precision, and thermal control tolerances all exceed the print

tolerance for the 0.750 inch dimension and the 0.500 inch dimension, both end surfaceson the print must be made by the slides. Therefore, the 0.750 inch and 0.500 inch

dimensions require linear tolerances and the 2.500 inch and 5.000 inch dimensions are

slide to cavity tolerances. However, there is a problem. The 5.000 inch dimension isnow made from one slide to the other. Therefore, the only way to tolerance this

dimension is to break into two slide to cavity dimensions of 2.500 inch each from the

cavity centerline. Because this is a tolerance based upon statistical variation, one should

take the square root of the sum of squares of the two 2.500 inch slide to cavity tolerances.This changes the table as follows:


PrintDimension

PrintTolerance

StandardTolerance

PrecisionTolerance

ThermalControl

Tolerance

2.500 0.015 0.0140 0.0075 0.0040

5.000 0.020 0.0198 0.0106 0.0057

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The print note 0.187 inch diameter holes marked X to be within 0.005 of centerlineA is a slide to cavity dimension that is on the same centerline. This is a slide to cavity

dimension due to the basic dimension 0.300 inch NET noted in the side view. To

calculate tolerances for this dimension, zero will be used as the linear dimension.Therefore the standard, precision, and thermal control tolerances will be +/- 0.0115 inch,

+/- 0.0063 inch, and +/- 0.0035 inch respectively. Therefore, thermal control must be

used to achieve this tolerance.

Cross Parting

The fourth step is to identify any cross parting dimensions. Because the cross parting

centerline dimensions are based upon the cored hole on the left side of the side view, and

this slide will be positively locked in the cover die, the center line will be defined as a

dimension formed in the cover die. However, for a dimensional safety factor, anydimension that is measured from the centerline will be considered a cross parting

dimension. There are four cross parting dimensions that are highlighted below.

Insert Fig 5-2 with 0.050 +/- 0.010, 0.550 +/- 0.010, 0.400 +/- 0.010, and 2.00 +/-

0.010 Diameter shown. All other dimensions should not be shown. Also, seedesex3.bmp

These dimensions should then be analyzed in a chart as shown below.


PrintDimension

PrintTolerance

StandardTolerance

PrecisionTolerance

ThermalControl

Tolerance

0.050 0.010 0.0065 0.0033 0.0015

0.550 0.010 0.0065 0.0033 0.0015

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0.400 0.010 0.0065 0.0033 0.0015

2.000 0.010 0.0075 0.0038 0.0017

Each cross parting dimension should work within standard tolerances.

Linear

The final step is to identify all dimensions on the casting sketch that are linear

dimensions within one insert. These dimensions are noted on the sketch below.

Insert figure 5-2 again with 0.650 +/- 0.005, 0.187 +/- 0.002 x 4, 0.900 +/- 0.005 x

2, 0.750 +/- 0.003, 0.500 +/- 0.002, 1.800 +/- 0.005 diameter, and 0.400 +/- 0.010

shown. Eliminate all other dimensions. Also see desex4.bmp

The dimensions should then be placed into a chart to analyze them individually as shown

below.

PrintDimension

PrintTolerance

StandardTolerance

PrecisionTolerance

ThermalControl

Tolerance

0.650 0.005 0.002 0.001 0.0004

0.187 0.002 0.002 0.001 0.0004

0.900 0.005 0.002 0.001 0.0004

1.800 0.005 0.0028 0.0014 0.0006

0.500 0.002 0.002 0.001 0.0004

0.750 0.003 0.002 0.001 0.00040.400 0.010 0.002 0.001 0.0004

To this point we have only discussed customer print tolerances versus standard, precision,and thermal control tolerances. With the advent of automotive quality systems in the last

20 years such as QS-9000, customers require suppliers to not only achieve tolerances but

exceed them to statistically assure that the tolerances are not exceeded. Currently,

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automotive customers require Cpk values of 1.33 up to 2.00. Below are added notes

indicating special print requirements indicating Cpk requirements.

Insert sketch similar to desex5.bmp here

The final step in verifying tolerances for a casting is to summarize all tolerances and Cpk

requirements to make sure that the process will be capable of the print requirements andto identify the process technology requirements. The summary of all dimensions is

shown below.

PrintDimension

PrintTolerance

Type ofDimension

StandardTolerance

PrecisionTolerance

ThermalControl

ToleranceCpk

RequiredEffective

Tolerance

ProcessTechnology

Required

Flat Surface 0.006 Flatness 0.014 0.007 0.0028 none 0.006 Thermal Control

Boss to Pin 0.003 Parting Shift 0.0161 0.0016 NA none 0.003 Precision

2.500 0.015 Slide to Cavity 0.014 0.0075 0.004 1.33 0.0113 Precision

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5.000 0.020 Slide to Cavity 0.0198 0.0106 0.0057 1.33 0.0150 Precision

X holes 0.005 Slide to Cavity 0.0115 0.0063 0.0035 none 0.005 Thermal Control

0.050 0.010 Cross Parting 0.0065 0.0033 0.0015 1.33 0.0075 Standard

0.550 0.010 Cross Parting 0.0065 0.0033 0.0015 none 0.010 Standard

0.400 0.010 Cross Parting 0.0065 0.0033 0.0015 none 0.010 Standard

2.000 0.010 Cross Parting 0.0075 0.0038 0.0017 1.67 0.0060 Precision

0.650 0.005 Linear 0.002 0.001 0.0004 1.33 0.0038 Standard

0.187 0.002 Linear 0.002 0.001 0.0004 2.00 0.0010 Thermal Control

0.900 0.005 Linear 0.002 0.001 0.0004 2.00 0.0025 Standard

1.800 0.005 Linear 0.0028 0.0014 0.0006 1.33 0.0038 Standard

0.500 0.002 Linear 0.002 0.001 0.0004 1.67 0.0012 Precision

0.750 0.003 Linear 0.002 0.001 0.0004 1.33 0.0023 Standard

0.400 0.010 Linear 0.002 0.001 0.0004 none 0.010 Standard

Since three of the dimensions require thermal control to achieve the tolerances,thermocouples should be placed in the tool with closed loop cooling to maintain careful

control over die temperature. Otherwise, these tolerances and Cpk requirements would

need to be relaxed by the customer to allow for precision tolerances.

dimensional control20rev 203 given by scott

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