Qin Y. Micromanufacturing Engineering and Technology

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FIGURE 6-34 Micro-injection molded parts (a) (material = polystyrene) and complete three-dimensional meshing of the

model (b), including: sprue (c), runner (d), gate and part) (e) [8].

FIGURE 6-35 Accurate 3D meshing (right) of gate (a) and micro-features (b) (width = 300 mm) of micro-injection molded

part (left) [8].

CHAPTER 6 Micro-Injection-Molding 111

CONCLUSIONS

Micro-injection molding is recognized as a

manufacturing process enabling the mass fabrica-

tion of polymer micro-components. Fundamental

process parameters for the control of both the

micro-product and the process are the tempera-

ture of the melt, the temperature of the mold, the

injection speed, and the cavity injection pressure.

Process simulation can be used for polymer micro-

product and process design, and can provide at

the present time mostly qualitative input to the

designer and the process engineer. Further, pro-

cess analysis methods and optimization tools are

being used in order to provide reliable validation

of both process and simulations.

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112 CHAPTER 6 Micro-Injection -Molding

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CHAPTER 6 Micro-Injection-Molding 113

Micro-Bulk-Forming

INTRODUCTION

Forming of metals is a well-established process,

dating back more than a thousand years. During

the early colonization in Europe, the village black-

smith formed fittings and weapons using hammer,

anvil and forge.

The idea of plast ically deforming metals into a

desired shape makes good use of the material and

can even enhance the performance of the materia l

[1, 2]. In the ancient Asian cultures the forging of

the Samurai sword was considered to be a black

art. By forging steel alloys with different carbon

content into a sword blade, it was possible to

combine the ductility of low carbon steel with

the hardness of high carbon steel into a blade

which was both flexible and sharp. The subse-

quent heat treatment of the sword blade included

the addition of a mixture of water and clay to the

cutting edge of the blade in order to reduce the

cooling gradient in this area. Only by mastering

the craftsmanship of forming, and using his

knowledge of materials and empirical skills for

the hardening procedure, was the Samurai sword

maker able to make the ultimate weapon [3].

BASIC COLD FORMING PROCESSES

Today bulk forming is used extensively in a wide

range of industrial applications. Within the

production of transmission elements for the auto-

motive industry, the bulk forming process is the

standard. Huge tonnages of rack, axles and other

components are manufactured by the bulk form-

ing process, either into near-net components that

subsequently undergo light machining or net

shape components which are finished after the

forming process.

A typical forming process requires multiple

forming steps and comes in a wide range of

variants. Each of the variants can, however, be

categorized into the eight basic bulk forming

operations depicted in Fig. 7-1.

The industrial popularity of the bulk forming

process still relies on the underlying theory of

material flow and behavior, something that has

not changed since the early days of forging. Even

with the employment of numerical simulation and

modern computer aided design systems, a well-

functioning bulk forming system requires exten-

sive knowledge of the strain hardening of the

material and its flow behavior as well as of fric-

tion and lubrication conditions (tribology). These

parameters generally dictate the forming limits of

the bulk forming process and thereby also the

achievable geometry of the components.

A medium advanced bulk forming process

often consists of 3–5 consecutive forming steps,

either mounted in the same press system or as

individual operations. This means that a typical

bulk forming proces s actually is a process chain

of several forming operations, each carefully

designed and analyzed to arrive at the best possi-

ble end result for the finished component.

Micro-bulk forming is the utilization of the bulk

forming process to form micro-components. Com-

pared to more traditional micro-manufacturing

processes, such as turning and milling, this process

holds the potential of producing high quality

CHAPTER

Mogens Arentoft, Rasmus Solmer Eriksen

and Hans Nørgaard Hansen

114

components,fasterandwithnooronlylittlemate-

rial waste.

THE SIZE EFFECT

By definition, a micro-component is a component

with one or more geometrical dimensions or func-

tional featu res of less than one millimeter in size

[5]. By decreasing the size of the components in

small mechanical systems, the weight and volume

of the device can be reduced without sacrificing

functionality, an d often the micro-component

technology is the enabler of a new miniaturized

product. Micro-components are often a central

part of the mechanical systems in consumer pro-

ducts such as hearing aids, computer drives or

mobile phones. The medical sector is another

important application for micro-components.

Here the components are used as parts of dental

implants, in spine fracture repair kits or as ele-

ments in drug delivery systems. These applica-

tions often require low failure rate in combination

with high functionality; giving rise to strict quality

and tolerance requirements on the components

making up the system. It is not uncommon to

see 100% inspection rate requirements for these

types of components, something that is incompat-

ible with mass production at low cost.

Size effect is a term for the deviation from

the linear continuum theory, when the scale is

reduced to micro-size. That is, when the scale is

reduced to the micro- level, some of the ru les

change and a new set of theories have to be

applied. Or putting it in another way, when the

size on the component is within orders of magni-

tude of the physical elements of the process, e.g.

material grain size, the linear theory of the macro-

domain breaks down [6].

The contributors to size effects can be put into

three groups: density, shape and structural effects.

Density effects relate to the inhomogeneity of

materials at small scale. If the average grain size

of a material is considered to be of the order of

40 mm, a macro-size component would contain

millions of grains, meaning that the material

could be modeled as homogeneous. Going to the

micro-domain, a 500 mm feature would only con-

tain about 12 grains over the cross-section, result-

ing in the properties of the individual grains influ-

encing the forming process.

Shape effects are closely related to the surface-

to-volume ratio of the component. Consider a

dice with a scalable side length of a. The volume

then scales with the third power a

, whereas the

surface area scales to the second power a

, mean-

ing that a small component will have a higher

surface-to-volume ratio, by a, when compared

to its larger equivalent. The surface-to-volume

ratio is an important parameter when considering

an ejection or handling situation, where the fric-

tion is a factor of the component surface area and

the component strength is related to the material

volume. Designing an ejector system to remove a

micro-component from the forming die is chal-

lenging, due to the risk of collapse or reverse form-

ing during ejection. Further, small components

are prone to the sticking effect, where adhesive

forces between the gripper and the component

outweigh gravitational forces. These adhesive

forces primarily consist of surface tensions, van

der Waals, and electrostatic forces, and can be the

limiting factor of a handling system for micro-

components.

The third effect contributing to the size effect is

the group of micro-structural effects. This group

of effects is made of physical elements which

either experience a physical length limitation,

FIGURE 7-1 Basic cold forging processes [4].

CHAPTER 7 Micro-Bulk-Forming 115

where it is not practical to scale the micro-

geometry or where secondary scaling effects come

into play. Surface roughness is an example of a

quantity that, in practice, is not fully scalable. The

roughness topography of a tool is often an inher it

property of the manufacturing process and is only

scalable within a certain window, resulting in

higher relative roughness at smaller size.

Manufacturing tolerance is another important

property prone to the size effect. High precision

tooling machines are costly with an approxi-

mately exponential correlation between achiev-

able precision an d machin e cost. However, the

machine precision is independent of workpiece

size and is strictly bound to the precision of

the tooling machine itself. However, when the

overall size of the component is reduced to the

millimeter scale, it is seldom acceptable to inherit

the tolerance band of the macro-scale, which is

normally within a few orders of magnitude of a

millimeter.

Figure 7-2 depicts prominent scaling effects

affecting the micro-forming process; the bound-

ary roughness characteristics, the fixed tolerances

inherited from the tool manuf acturing process,

the grain size determined by the physics of

metallurgy and the geometrical non-linearity of

the surface/volume ratio.

WORKPIECE MATERIALS

Material knowledge is central for the bulk form-

ing process, which fundamentally relies on differ-

ences in flow stress between two materials. These

two materials are often termed ‘workpiece mate-

rial’ and ‘tooling material’, relating to their

respective functional purpose in the bulk forming

process. Properties such as price, flow stress, duc-

tility and strain hardening are important for the

forming material, whereas the yield strength,

machinability and ductility are performance para-

meters for the tool material.

In order to accurately analyze and simulate

the f orming process, workpiece material data

are needed. These data can be acquired by per-

forming an upsetting test, where the m echanical

strain/stress c urve of the material can be

acquired using length and force transducers

connected to a data acquisition device. The

resulting data can be fed into a numerical simu-

lation program and the forming process can be

simulated.

FIGURE 7-2 Overview of dominant size effects in micro-bulk forming, the influence of roughness scaling, grain size scaling,

tolerance scaling the non-linear scaling for surface-to-volume ratio.

116 CHAPTER 7 Micro-Bulk-Forming

The selection of workpiece material is nor-

mally a trade-off between requirements set by

the component applicat ion and formability per-

formance. Materials with a high strength are gen-

erally desirable, leading to lower mass and a more

compact design; however, these materials are

often difficult to form in a bulk forming process.

On the other hand, a soft material is easier to

extrude or form but the strength of the finished

component is often not adequate for high perfor-

mance applications. The choice of workpiece

material further influences tool life expectancy,

forming temperature, the choice of lubricant,

coating and the number of forming operations

required.

Amorphous Metals

Amorphous metals are a new range of non-

crystalline materials with interesting properties

for manufacturing of micro-metallic components.

The amorphous structure of the materials is

generally realized by combining alloying metallic

elements of considerably different atom sizes

with rapid cooling when transitioning from the

liquid to the solid phase. By rapid cooling the

alloy is frozen in the amorphous state, thereby

not allowing the metallic atoms to combine into

the well-known lattice structure characterizing

conventional metals. The required criti cal cooling

rate generally sets limits on the achievabl e forms

in which amorphous metals can be realized, typi-

cally thin ribbons, foils and wires. However,

recent development has enabled the casting of

amorphous metals with dimensions exceeding

one millimeter: these are known as bulk metallic

glasses.

Bulk metallic glasses are interesting for the

forming of micro-components because the mate-

rial is not subject to the grain size forming limita-

tions, dislocations and slid ing planes of normal

crystalline materials and can be formed at the

micrometer scale with good results. The forming

of bulk metallic glasses takes place between the

glass transition temperature T

and the crystalli-

zation temperature T

.AtT

the BMG becomes a

super-cooled visc ous liquid exhibiti ng decreasing

flow stress with increasing temperature. A second

dependent parameter of the forming temperature

is the crystallization time t

cryst

, at which the amor-

phous structure is lost and the material turns crys-

talline. There is a time/flow stress trade-off when

selecting the forming temperature; a higher tem-

perature equals low flow stress and low viscosity

but limited forming time due to the decreasing

crystallization time. On the other hand, a lower

forming temperature results in increased flow

stress and viscosity of the BMG, resulting in lon-

ger process time and increased tool loads, whereas

the window of amorphous processing time

increases.

Further, BMG materials are generally very

strain rate dependent and fast processing results

in high flow stresses. It has been claimed that

BMG above the glass transition temperature

behaves like asphalt on a summer ’s day, since

the viscosity of both BMG and asphalt is strongly

dependent on temperature and strain rate.

Figure 7-3 depicts the flow stress dependency

of Mg60Cu30Y10 bulk amorphous alloy in com-

parison to a conventional crystalline magne-

siumaluminum alloy.

Figure 7-4 depicts a micro-component

formed by bulk forming in BMG and a soft

aluminum alloy. By comparison it is observed

that there is better form filling in the case of

the BMG material (a) even though the forming

process is not fully completed. As marked by

the arrows, the outer rim o f the component is

subject to rounding in the case of the aluminum

material whereas the BMG case exhibits sharp

edges.

The forming of BMG material is not trivial and

remains a topic of research [7, 8].Someofthe

issues yet to be solved relate to fracture strength

and high temperature tool development. Low tem-

perature BMG, with a glass transition temperature

below 200



C, is brittle with facture strength as

low as 500 MPa and may easily facture during

ejection or use. Zirconium and iron-based BMGs

are formed at between 360 and 600



C and have a

higher yield and fracture strength, but here the

design of the tooling set-up becomes challenging

due to the high temperature that gives rise to

CHAPTER 7 Micro-Bulk-Forming 117

thermal deflection, oxidation and lubrication

breakdown.

TOOL MATERIALS

The most dominant tool steels for bulk forming

tools are state-of-the-art powder metallurgical steels

with high toughness, hardness and metallurgical

homogeneity. Due to the high surface pressures

involved in bulk forming, a tool material hardness

of greater than 50 HRC is normally required. The

tool steels used to be cut in their soft state and

subsequently heat treated to their maximum hard-

ness and ultimately polished or surface treated.

However, modern tooling machines can directly

cut hardened steel, allowing for better tolerances

of the finished tool by avoiding the inevitable geo-

metrical deflections arisingduringheattreatment.

FIGURE 7-4 Side-by-side comparison of a potentiometer axle for a hearing aid bulk formed in: BMG Mg60Cu30Y10 (a);

and in an annealed aluminum EN AW-6060 (b).

FIGURE 7-3 Flow stress for Mg60Cu30Y10 bulk amorphous alloy.

118 CHAPTER 7 Micro-Bulk-Forming

In micro-bulk forming the geometrical toler-

ances of the tool are very narrow and deflections

of the tool during heat treatment often exceed the

total allowable tolerance deviation of the tool.

Thus too ling materials for micro-bulk forming

need to be hardened already from bulk and will

be machined in their hard state using grinding,

hard micro-milling or electro discharge machin-

ing. Owing to the small size of the tools it can be

time and cost effective to use known prefa b ele-

ments such as standard industrial punch needles

or bushings as a starting point for a bulk forming

tool, as these ISO standardized machine elements

are cost effective, easy to source and have the

mechanical properties required.

Hard metal, ceramics or solid tungsten carbide

materials are other options for tooling materials.

The cost of these materials is often much higher

than traditional tool steel but since the amount of

material used for a micro-bulk forming die is

small, the solid tungsten carbide can be an eco-

nomically viable tool material. The benefits of

using tungsten carbide as tool material are the

increased hardn ess of 62–70 HRC and a very

low elasticity. The drawbacks of using tungsten

carbide are the limited available machining pro-

cesses, normally only electrical discharge machin-

ing, and the low tensile fracture strength of

the material (as low as 400 MPa). Especially, the

presence of tensile hub stress is unwanted for

hard metal dies because of the crack initiation

risk. With a correctly dimensioned pre-stress sys-

tem, in the form of conical stress rings, it is pos-

sible to superimpose compressive stresses onto

the hard metal forming die. As a result, the stress

rings counteract the tensile stresses arising from

the forming process, thereby eliminating any

effective tensile stresses on the hard metal die.

This is also touched upon in the next section deal-

ing with machine and tool design.

MACHINE AND TOOL DESIGN

The design and manufacturing of machine and

tooling elements play an important role in the over-

all performance of a micro-bulk forming process.

Ideally the machine should scale accordingly,

meaning that the micro-bulk forming could take

place on small mobile table-top machines. This

vision has been proposed by several researchers as

the factory lines of the future. In orderto realize this

vision, several technologies have to work together

to form a fully working micro-bulk forming plant,

from raw material to the final component.

Billet Preparation

Workpiece material is delivered in bulk, typically

as rods. These have to be split into billets with the

right volume for the subsequent bulk forming

operation. A straightforward way to split the bulk

material is by machining. This is, however, not

very appealing for the reasons of cost and time.

Rightfully, a claim could be made that if the billets

have to be machined, the whole component should

be finished when the part is in the cutting machine.

By cropping billets from rods the machining

operation is avoided. Cropping is a process where

the bulk material rod is divided into smaller pieces

by a static and a movi ng cu tting edge. Cropping is

not a high precision process and some volume

deviation is to be expected between the final bil-

lets. An investigation of the cropping quality of an

Ø1.9 mm aluminum rod has been made and the

results are depicted in Fig. 7-5 [9]. It is clear that

the cut in Fig. 7-5(b) exhibits only little scatter in

volume among the cut billets due to the high qual-

ity cut. It was further found that the billet was

geometrically deformed in the high speed cutting

scenario, where the faces of the cutting dies move

with speeds exceeding 10 m/s, Fig. 7-5(c).

It was further found that cropping is a fast and

inexpensive way of manufacturing billets for

micro-bulk forming. The best results are achieved

using a cropping process where the bulk material

is kept under axial stress, close to the yield

strength of the material, while the billet is sheared

from the bulk.

Press System

Conventional macro-size hydraulic or excenter

presses are not suitable for micro-bulk forming

processes due to their size, and their low load

CHAPTER 7 Micro-Bulk-Forming 119

control capability. Bulk forming of high quality

components requires high stiffness of the press

frame, high speed and good load and position

control of the press. The load requirements often

range between 5 and 30 kN, depending on the

component and the material. With these require-

ments in mind a number of micro-bulk forming

press systems have been built [9–11]. By using a

servomotor as the actuator for the press move-

ment, the required speed and force can be deliv-

ered while maintaining good control over the

position of the press axis. The actuator itself can

be a direct drive linear motor, where a linear rod is

moved by the magnetic field of surrounding coils.

This solution offers very high speeds and accelera-

tions of up to 40G while having a peak force

output at about 0.5 kN. Another commonly used

set-up is a configuration with a nut and roller

screw driven by a servomotor, through a gearbox.

The roller screw solution is capable of positioning

to within a few micrometers while delivering high

loads at moderate speed. The press frame should

be designed for high stiffness in order to minimize

elastic deflections of the frame and the consequent

positional inacc uracy and lateral defections. The

press frame system is normally not prestressed, as

deflections can be kept within tolerable limits.

The Tool Die System

Due to the required precision on tooling ele-

ments and the machine in general, manufacturing

process chain desi gn decisions will have to be

made in order to keep tolerances within accept-

able limits. Examples of such process chain design

decisions include machining several tooling ele-

ments in the same run, thereby eliminating inac-

curacies arising from fixation and long-term

deflections of the tooling machine.

Precision and Alignment of Tooling

Elements

In general the precision requirements of a tooling

system for micro-bulk form ing can be divided into

precision issues relating to the precision of the

individual tooling element, such as the die and

the punch, and the precision of the relative align-

ment of the individual parts, relating to the frame-

work of the tooling system.

Achieving the required tolerances on the indi-

vidual tooling parts is relatively easily done by an

appropriate choice of tooling machine and process

chain. Here high speed milling and turning, and

electro discharge machining (EDM), are processes

of choice. These processes can work directly in

hardened material and can achieve good precision

down to about 5 mm. With subsequent surface

treatment such as polishing or micro-spraying,

high quality tooling components can be achieved

with a short lead time and at low cost.

When considering the alignment of the indi-

vidual tooling elements, there are a number of

approaches to achieving the alignment of the

tooling parts. The straightforward way of ensur-

ing the alignment of tooling elements is by a

FIGURE 7-5 Cropping quality comparison of an Ø1.9 mm aluminum rod with three different cropping scenarios.

(a) conventional cropping, (b) cropping under hydrostatic pressure and (c) high speed cropping.

120 CHAPTER 7 Micro-Bulk-Forming