Die Lubricant Technology Trends to Address

Changing Industry Needs

Dr. Siddhartha Asthana

Chem Trend L.P

Academic Profile

Masters in Chemical Engineering•Ph. D. in Polymer Science from University of Akron•

Professional Experience

Working for Chem Trend, responsible for management of current technology• and development of new technologies

Has worked with HB Fuller as Vice President(Technology) for Automotive division

ALUCAST 2012 (18)

Die Lubricant Technology Trends to Address Changing Industry Needs

Abstract:

The global business environment continually challenges the die cast industry to improve productivity

and meet profitability targets under a strict globally competitive landscape. This requires casters to strive for

lower scrap rates and faster cycle times. Meanwhile, casters are also producing more intricate, technically

advanced, castings in complex dies. These factors lead to the need for advanced die lubricant technology

that provides excellent lubrication while also withstanding the severe thermal demands that result from faster

cycle times and hotter running dies. This paper discusses how recent trends in water-based die lubricant

technology help die casters achieve these productivity enhancements.

Author: Dr. Siddhartha Asthana

The High pressure die casting (HPDC)

industry has grown rapidly over many decades to

meet requirements of the advancing automotive

industry. This HPDC process particularly suited for

the industry as it is capable of making parts with high

dimensional accuracy in a cost effective way. The

HPDC global market has rapidly grown in recent

decades and mirrors the growth of the automotive

industry. Yet, as with most industries, HPDC must

still constantly adapt to meet ever changing

requirements and market trends. There seems to

be three dominant themes that have surfaced in

recent years:

1. Increased fuel economy

2. Improved productivity

3. Improved occupant safety

Automotive and Process Trends:

Rising fuel costs, a long wi th new

environmental regulations, have necessitated

improvements in automotive fuel efficiency.

Reduced weight is the key directive in the

transportation community as OEMs search for new

ways to design vehicles with better fuel efficiency.

The lower density of aluminum and magnesium

a l l o y s h a s m a d e a u t o m o t i v e O E M s aggressively seek replacement of steel

components with these lighter alloys. Aluminum alloys have been widely accepted for manufacture of engine, transmission and chassis components. The average North American vehicle contains almost 8% aluminum

as compared to 64% of ferrous metals. Magnesium incorporation in automotive is still small but will continue to grow, as will the overall trend of migration to aluminum and magnesium alloys from steel. Along with this migration towards aluminum and magnesium, these same parts will continually be redesigned to become thinner and lighter.

The global trend of improved productivity

has brought increased complexity to die casting

parts. Instead of many parts being assembled

together mechanically, increasingly complex

parts are being made as a single casted part to

reduce the number of assembly steps. This in

turn has led to higher shot weights being injected

in complex dies. These larger parts with more

intricate geometries are being casted on large

and complex dies, and as a consequence it has

become difficult to adequately cool all parts of the

dies uniformly. This places an even higher

demand on die lubricants (DL) contribution

towards thermal management of these complex

dies.

(19) ALUCAST 2012

The third major trend relates to combination of

weight savings while meeting higher occupant

safety standards. In pursuing the ongoing goal of

weight reduction, new alloys are being used for

structural parts to yield structures that have low

porosity and high ductility to meet crash worthiness

standards. Variations of the newer alloys used for

this purpose can be more abrasive, while also

having higher melt temperatures and low iron

content when compared to traditional aluminum-

silicon alloys. Along with these alloy changes,

casting processes have been modified as well to

yield low porosity parts, for example, by deployment

of high vacuum techniques in HPDC.

Coupled with these trends is the continued

push towards lower machine down time, improved

equipment e ff ic iency and ever s t r ic ter

environmental regulations. As a consequence, this

leads to even faster cycle times for larger and more

complex parts and resulting higher die operating

temperatures.

These trends have spurred the development

of new generation of die lubricants which are

capable of meeting the enhanced demands. High

surface temperatures have forced the DL to “wet”

the die surface at elevated temperatures, while also

providing faster and more efficient external cooling

to the dies. It is not uncommon for these complex

dies to have large sections with temperature in

excess of 400°C, while in the colder regions they

can be as low as 200°C.

Based on these industry trends, the die lubricant

must meet the following performance criteria:

· Fast and efficient wetting of die surface in a

wide temperature range.

· Building of an effective, uniform and lubricious film across all sections of the die.

· Lubricant film not to be excessive in cold areas so as to create porosity and carbon buildup, yet be enough in hot spots to prevent solder.

· Minimize ex-cavity buildup to reduce

machine down time for cleaning.

· Should meet all applicable HS&E regulations while having enhanced waste treatment capability.

Die Lubricant Functions and performance

Challenges:

Overcoming Leiden frost

Wide temperature ranges on the die

creates local hot spots, which are prime sites for 1

solder creation (metallurgical bonding) . To

prevent solder, there should be DL film between

the molten aluminum and die steel. However,

the high temperature around the hot spots poses

a severe challenge to DL technology due to the

Leiden frost effect. The Leiden frost effect is the

formation of a gas barrier between a hot surface

and a boiling liquid if the temperature difference

is great enough. When the surface temperature

is significantly higher than the boiling point of the

water, the part of the water drop that hits the pan

is vaporized, creating a thin layer of water vapor

on which the rest of the droplet rests. The vapor

prevents the physical contact of water droplet

with heated surface. Under these conditions,

heat transfer occurs via convection through the

vapor film. Consequently rate of heat extraction

from hot surface is small. This phenomenon is

shown in Figure 1. Although die lubricant

chemistry has one of the most significant effect

on Leiden frost temperature, many other

mechanical factors affect the Leiden frost

temperature, factors like spray angle, distance, 2

droplet size, spray pressures, etc.

Figure 1. Leiden frost Phenomenon, liquid

droplet not in contact with hot metal surface.

ALUCAST 2012 (20)

As the active materials in die lubricant are not

in physical contact with hot dies during this

phenomenon, they are unable to make the

protective lubricant film until the die surface

temperature is below the Leiden frost temperature.

Once the die temperature is below the Leiden frost

temperature, film formation starts. The maximum

temperature at which the film formation starts is

referred to as wetting temperature. Due to the

trends discussed in the introduction section, die

casters are demanding higher wetting temperatures

out of their die lubricants. Higher wetting

temperature helps the die caster to reduce the DL

spray time and consequently reduce the cycle time.

This need is being met by the suppliers through

development and incorporation of unique

chemistries in DL technology. Figure 2 shows the

wetting temperatures of some commonly used die

lubricants supplied by Chem-Trend.Requirement of

high wetting temperature should be tempered with

the required process conditions. A casting process

that is running optimally using a lower wetting

temperature DL would be thrown off-balance when

the DL is changed to a high wetting temperature.

This would result in excessive film formation. Some

of the possible consequences could be: increased

in-cavity carbon buildup, increased die cooling

leading to partial cavity fill, etc. Hence, a judicious

practical approach is always advisable when

process efficiency improvements are considered.

Figure 2: Wetting temperatures of a select few

die lubricants.

Once the surface wetting has started taking

place. The rate of heat extraction from the die starts

to increase, reaching a maximum at the Nukiyama

point (Figure 3). The heat transfer due to water

spray impinging on a superheated dry surface is 3categorized in four regimes .

1. Film boiling: Film boiling regime is the highest temperature regime. In this, water droplets are not making physical contact with the hot surface but are moving and evaporating while in contact with a vapor layer. Film boiling regime ends at Leiden frost temperature. The Leiden frost temperature is the inflection point between film boiling and transition regime. At this inflection point heat transfer coefficient is lowest.

2. Transition regime: During the transition regime, the rate of heat transfer continues to increase until it reaches a maximum at Nukiyama point. This is the point where heat transfer coefficient and heat flux maximizes. Nukiyama point indicates maximum heat flux, also referred to as critical heat flux (CHF). At this point, heat transfer curve transitions into the nucleate boiling regime.

3. Nucleate boiling regime: During the nucleate boiling regimethe rate of heat extraction gradually decreases throughout this regime.

4. Film evaporation regime: Finally the curve

transitions into the film evaporation

reg ime. Dur ing th is reg ime heat

transfer rate continues to decrease as the

mode of water removal is not boiling but

evaporation.

As discussed earlier, die lubricant film

formation starts at Leiden frost temperature and

continues on through the transition and nucleate

boiling regimes, film formation is most efficient in

these regions.

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Dai Lubricant A Dai Lubricant B Dai Lubricant C Dai Lubricant D

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Die

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Std. product Generation 1 Generation 2 Generation 4

Figure 3: Change of heat flux with temperature in

forced convection cooling due to spray

impingement.

This curve gives (Figure 3)illustrates some of the ways to reduce cycle time in a DPDC process:

1. Use of die lubricants with high Leiden frost temperature

2. Use of die lubricants with short transition regime, so that CHF is attained fast

3. Use of die lubricants with high CHF point, so that maximum heat is extracted

Constant Film Deposition

The next important function that DL performs is

part release. For a casted part to be released from

the die, an optimum amount of lubricant film has to

be formed on all sections of the die cavity. Field

experience at many different die casters, in different

regions of the world, has shown us that a wide

temperature range exists between different areas of

the same die- sometimes in excess of 200°C. Die

designers are constrained regarding where internal

cooling lines can be placed, and hence it is

unavoidable for hot and cold spots to form in the

casting cavity.

The DL film will form rapidly in the cooler

sections of the die and will form at a much slower

pace in the hot regions of the die. This places

another high performance requirement on die

lubricants, i.e., sufficient film formation at all

temperatures on the die. Die Adhesion Index test is

a good way to measure different die lubricants to

meet this criterion. A comparison of select generations of technologies from Chem-Trend is shown in Figure 4.

As discussed earlier, die lubricant film

formation starts at Leiden frost temperature and

continues on through the transition and nucleate

boiling regimes, film formation is most efficient in

these regions.

1. Transition regime: During the transition regime, the rate of heat transfer continues to increase until it reaches a maximum at Nukiyama point. This is the point where heat transfer coefficient and heat flux maximizes. Nukiyama point indicates maximum heat flux, also referred to as critical heat flux (CHF). At this point, heat transfer curve transitions into the nucleate boiling regime.

2. Nucleate boiling regime: During the nucleate boiling regime the rate

of heat extraction gradually decreases throughout this regime.

3. Film evaporation regime: Finally the curve

transitions into the film evaporation

reg ime. Dur ing th is reg ime heat

transfer rate continues to decrease as the

mode of water removal is not boiling but

evaporation.

As discussed earlier, die lubricant film

formation starts at Leiden frost temperature and

continues on through the transition and nucleate

boiling regimes, film formation is most efficient in

these regions.

Figure 4: Die Adhesion Index of key die lubricant

generations.

NukiyamaTemperature

Lindenfrost Temperature

Film Boiling. Non.Wetting Zone

Transition

Transition NL Cleate Boiling.Wetting Zone

FilmExaporation

FilmExaporation

He

at

Flu

x

Surface Temperature

ALUCAST 2012 (22)

The Y-axis of this graph plots the ratio (in

percent) of the weight of the film formed at 350°C to

that formed at 250°C. This test is performed under

controlled laboratory conditions and it correlates

well with the actual on-die performance. These two

temperatures were chosen as majority of the die

areas in aluminum casting are usually around

250°C, while 350°C temperature represents the

temperature of the hot-spots. A ratio close to zero

indicates very little film formation in hot regions of

the die, and ratio of one would represent exactly an

equal amount of film formation in hot regions of die

as in the cold regions of die. The challenge for DL

suppliers is to increase this ratio close to one.

However, this is an impossible task as die lubricants

are based on a combination of organic and inorganic

materials all of which decompose (degrade) with

increasing temperatures. Hence, the higher the

ratio the better it is.

As the dies have become more complex in

recent years, there has been an increased demand

placed on die lubricant film to spread on the die

surface. As is well known, not all areas of the die

cavity receive direct spray from spray nozzles. Yet

the die lubricant film is expected to spread to these

areas and form a film. This is measured through a

test referred to as Spreading Ability test. Figure 5

shows a representative graph using some of the

commercially important die lubricant chemistries.

The challenge from die lubricant technology point of

view is to develop chemistries which will aid in

spreading of the DL film at high temperatures

encountered on die surface.

Figure 5: Spreading ability of some commercially

important die lubricants.

Ex-Cavity Buildup

Another focus area for die casters globally

is to reduce machine down time so as to increase

productivity. To meet this goal, newer generation

of DL chemistries have evolved to incorporate

higher solder protection, better lubrication of

moving parts (like core and ejector pins), less

carbon buildup, etc. These trends have been

discussed in detail in previous sections. Now a

new area of focus that has become apparent in

recent years is to have DL chemistries that

exhibit lower ex-cavity buildup properties. There

is no good way to protect ex-cavity areas, such

as vents, etc. from die lubricant over spray.

These areas of the die are significantly cooler

than in-cavity areas, and consequently die

lubricants have a tendency to accumulate in

these regions. This process happens through

evaporation of water from DL, thus leaving

behind active components of DL chemistry on

these colder sections of the die. This

accumulation can greatly affect casting quality if

it occurs in vent areas as it inhibits evacuation of

air/gasses, thus potentially leading to gas

porosity and incomplete fill. If this accumulation

happens in the retainer area of the die it can lead

to improper closing of dies, and subsequent

safety problems.

As a consequence of this, die casters are

forced to frequently clean this accumulation by

mechanically scraping the die, which leads to

loss of productivity. An area of focus in recent

years has been to develop DL chemistries which

reduce this accumulation/buildup. Figure 6

shows comparison of over spray accumulation of

a standard technology product with that of latest

technologies. As is evident from the graph,

these newer technologies can reduce this

buildup by a very significant amount. Field

results have matched with our expectations from

lab results.

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Figure 6: Comparison of ex-cavity buildup

properties of standard technology with those of new

generation of die lubricant technologies.

To test this in the field, a trial was performed

with new high temperature die lubricant

incorporating low ex-cavity buildup technology.

Customer was casting a truck oil pan (A380 alloy)

with a 9 Kg shot weight using a conventional die

lubricant at 1:80 dilution. While running 73 sec.

cycle time, the die temperatures ranged from 230°C

to 400°C before spray. According to the customer,

in-cavity buildup, release and solder protection were

acceptable, but ex-cavity buildup was poor with

conventional die lubricant. The trial run was set at

two hundred parts. Customer designed a metal

template that outlined a major ex-cavity portion of

the die.Using this template, mold scrapings were

collected after 200 piece runs of the conventional

and new technologies on both the cover and

ejection sides of the die. Figure 7 shows the region

of interest where die lubricant buildup was collected

from. Care was taken to collect DL buildup material

only from the area enclosed by the template, and no

process variable was changed.

Figure 7: Pictures of dies from trials for low ex-cavity technology.

As can be seen from the Figure 7, the

technological advancement yielded a cleaner

shut-off area and cleaner vents as compared to

the conventional technology DL. The actual

amount of buildup collected after the trial with

new technology die lubricant was almost 50%

less than that with conventional technology. In

addition to these benefits, the new technology

provided improved release and solder

protection, with classification moving from

“acceptable” to “excellent” rating. In some

cases, this new technology has been able to

reduce the ex-cavity cleaning frequency from 2-3

times a shift to once in 8 shifts.

Die Lubricant Dilution Trends

Along with changes in demands on die

lubricants, die casters are beginning to reverse

the trend in dilution levels. Standard die

lubricant dilution rates in North America and

Europe are around 1:80. Meanwhile it is not

uncommon for die casters in developing regions

to try to extend dilutions to 1:200. The rationale

behind extended dilutions is perceived cost

savings. However, laboratory studies and new

industry trends show that this is not the most

efficient manner to utilize die lubricants.

Significant development efforts have been

put forth by die lubricant companies to develop

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Std. Technology New Technology-1 New Technology-2

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ALUCAST 2012 (24)

products with advanced features. As noted previously in this paper, die lubricant chemistry performance has a major impact on die cooling, film formation, wetting, spreading and release characteristics. All of these performance attributes are dependent upon lubricant chemistry-not dilution water. Thus, as lubricant dilution is extended, the lubricant functionality is diminished. This can be counterproductive, and can lead to longer cycle times and lost productivity. Figure 8 outlines wetting time increase with increased dilution. Due to increased wetting times, cycle time would also go up thus resulting in loss of productivity. Similar trend of loss of productivity with increased dilution exists for other die lubricant characteristics, like spreading, wetting temperature, cooling characteristics.

Figure 8. Effect of increased dilution on wetting

temperature for select die lubricants

The diminished performance leads to a ripple

effect in lost productivity and quality. Die lubricants

with a lower Leiden frost point and wetting

characteristics will obviously equate to longer cycle

time and lost productivity. However, they also lead

to an inferior film build on the die. In contrast, a

properly diluted die lubricant will quickly deposit a

lubricant film, at a higher temperature, allowing die

caster to run an even faster, hotter running process

(greater productivity). The following points outline

the various costs associated with excessive

dilutions:

· Reduced Leiden frost temperature and film characteristics

Longer spray cycles°§ Increased cycle time (loss of productivity)§ Increased waste-water (eff luent)

generation and treatment cost

Inferior film build°§ Quality and rework (appearance and

dimensional issues)

§ Increased down time (polish dies)§ Broken cores and pins (replacement

cost and down time)

· Required lower die temperatures (for film build)

More heat extraction / additional stress on °die

Increased heat check°§ Lower die life (premature replacement

cost)§ Lower part quality & higher rejection

rates towards the end of die life

When looking for cost saving opportunities,

die casters should be cautious to be sure that

they are focusing on entire process optimization,

which takes into account hard and soft costs

associated with each of the process steps. Many

die casters globally are moving towards reduced

dilution levels to optimize and match die lubricant

characteristics with their process. Along with

this, significant effort is being placed on tooling

design to optimize the internal cooling. This

allows die lubricant functionality to focus on

release and die protection characteristics,

instead of external cooling properties. If properly

designed, this leads to greater productivity and

improved tooling life. In fact, many advanced

casters globally are adapting High Efficiency

Release Agent (HERA™) die lubricant

technology. These specialized, concentrate

sprays not only improve part quality with vastly

improved cycle times but they also greatly

improve die life and eliminate plant effluent.

Summary

HPDC technology is rapidly evolving to

meet these new market trends. The die lubricant

suppliers will have to continue developing new

technologies to be able to meet market

demands. As the leading global supplier to the

HPDC industry, we continue to develop new

technologies to improve quality and productivity

in die casting plants. There are a number of DL

innovations which have been successfully

launched by Chem-Trend in last few years to

meet these new market challenges.

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References

1. S. Shankar, D. Apelian, “Die Soldering:

Mechanism of the Interface Reaction

between Molten Aluminum Alloy and Tool

Steel”, Metallurgy Transactions, 33B, pages

465-476 (2002).

2. J. L. Graff, L. H. Kallien, The Effect of Die

Lubricant Spray on the Thermal Balance of

Dies, Paper T93-083, Presented at North

America Die Casting Associating Meeting,

Cleveland, Oct 18-21 (1993).

3. J.D. Bernadin, C. J. Stebbins, I. Mudawar,

“Mapping of Impact and Heat Transfer

regimes of Water Drops Impinging on a

Polished Surface”, International Journal of

Heat Mass Transfer, 40, pages 247-267

(1997).

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