Qin Y. Micromanufacturing Engineering and Technology

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the positions of weld lines permits study of the

influence of different processing conditions on

the filling capability of the micro-molding process.

In fact, the different paths of weld lines

obtained with the different setting s of process

parameters can be employed to study the effect

of different process parameters and determine the

conditions for better cavity filling conditions [18].

In particular, a variation of mold temperature and

of injection speed produces modification of the

path of the weld lines, pushing towards the end

of micro-features as the polymer melt flows. For

example, in the case of polystyrene, the effect of

melt temperature and packing pressure was

extremely limited.

The flow front depth of filling (i.e. the ease of

the melt flow to fill micro-structure s) increases

with the width of the structures to be filled. The

meeting points of weld lines out of channels

200 mm and 300 mm wide, as well as the horizon-

tal weld line in the channel 150 mm, show such

behavior (see Table 6-1).

Complete filling of micro-injection molded fea-

tures (i.e. a good filling performance) can be

FIGURE 6-17 Optical image of weld lines due to melt

flow front separation [8].

FIGURE 6-18 Effect of mold temperature (a), injection speed (b), packing pressure (c), and melt temperature (d) on the

positions of weld lines shown in Figure 6-17 [8].

CHAPTER 6 Micro-Injection-Molding 101

therefore obtained by using a high temperature of

the mold (which decreases the viscosity of the melt

and prevents premature solidification) and high

injection speed (which also decreases the viscosity

of the melt due to viscous thinning and viscous

heating, as well as decreasing the injection time,

thus avoiding premature short shots and incom-

plete filling). On the other hand, it is not conve-

nient to increase the temperature of the melt, first

due to the limited benefit on the filling perfor-

mance, and second to avoid material degradation

due to material overheating. An elevated packing

pressure is also not advantageous because it can

produce high internal tension on the polymer

matrix as well as induce high stress on the mold

itself.

Micro-injection Molding Process

Control and Anal ysis

The reliable manufacturing of polymer-based

micro-components on a mass production perspec-

tive is directly connected to the capability of con-

trolling the micro-injection molding process. The

influence of process parameters on m-injection

molding (mIM) can be investigated with a mold

with a sensor applied at injection location. It can

be used to monitor actual injection pressure and

to determine the cavity filling time.

At the injection location where the melt is

pushed into the cavity (see Figure 6-21, detail

A) by the injection piston, a piezoelectric pressure

sensor is usually placed (see Figure 6-21, detail B).

FIGURE 6-19 Injection molding of 300 mm wide micro-features: optical microscope images of the flow front position shift

due to an increase of injection speed (injection direction from left to right, T

melt

= 220



C, T

mold

=55



C) [8].

FIGURE 6-20 Simulated flow front pattern to be compared with weld lines used as flow markers [8].

102 CHAPTER 6 Micro-Injection -Molding

The recording of the in-cavity pressure at injection

location over time is one of the methods to mon-

itor conventional and micro-injection molding

processes. It allows comparative studies to be per-

formed on different process conditions and eval-

uation of the process repeatabi lity when molding

under the same process conditions. Moreover,

especially in the case of micro-molding, it is a

powerful method to calculate the cavity filling

time which would not be possible by other means,

since filling of micro-cavities takes place in times

in the order of tens of milliseconds.

The cavi ty pressure profile, in mIM as well as in

precision injection molding, is a factor directly

correlated to the quality of the part [20]. The

cavity pressure control, expressed in terms of both

absolute value and repeatability (i.e. standard

deviation), is fundamental for an optimized part

and process realization and it is the critical process

parameter for the precision molding of high accu-

racy thermoplastic parts [21] as micro-molded

components. For example, an excessive value of

the cavity pressure will lead to defects such as

flashes (see Figure 6-22); whereas a large value

of the standard deviation of the pressure indicates

a poor cycle-to-cycle process repeatability (i.e.

different filling conditions) and therefore different

properties of the molded part.

Analysis of the cavity injection pressure shows

that an increase of the temperature of the melt

causes an increase of the cavity injection pressure.

This is due to the fact that higher temperature

TABLE 6-1

Experimental Depth of Filling Depending on the Channel Width

Channel No. Width (mm) Depth of Filling (mm) End of Cavity

1 400 900

2 450 900

3 200 301 –

4 600 900

5 150 22 –

6 300 710 –

7 600 900

CHAPTER 6 Micro-Injection-Molding 103

reduces the melt viscosity, which has as a conse-

quence the reduction of the pressure drop through

the noz zle and runners, resulting in higher cavity

injection pressure. To attain higher injection

speed, a higher injection pressure must be

applied, which in turn increase s the cavity injec-

tion pressure. The influence of the temperature is

also of importance. At higher mol d temperature,

close to the glass transition temperature of the

polymer, the melt viscosity is decreased, which

in turn reduces the pressure drop, resulting in a

higher cavity injection pressure.

The cavity injection pressure cycle-to-cycle

repeatability can provide a valuable parameter to

determine the process stability. In micro-molding,

a standard deviation in the order of 10–50 bar,

which corresponded to a coefficient of variation

between 1% and 5%, can be obtained.

The injection pressure rise at the injection loca-

tion and the subsequent reaching of the maximum

cavity pressure can be employed to determine the

cavity injection time (see Figure 6-24) under dif-

ferent process conditions. Due to the very short

filling time in micro-molding (of the order of a few

tens of milliseconds), sampling rates of the order

of 5–25 kHz are recommended to be employed.

Experimental results show that an increase of

both the temperature of the mold and of the injec-

tion speed lead to a shorter cavity injection time,

with injection speed having the greatest influence.

The cavity injection time can be used also to esti-

mate the cycle-to-cycle repeatability of the micro-

molding process. Standard deviation valu es in the

range between 1 and 3 ms are usually encoun-

tered during micro-molding (which correspond

to a coefficient of variation of below 1%).

FIGURE 6-22 Effect of cavity injection pressure: micro-flashes do not occur at lower pressure (553 bar, (a)) and appear at

higher pressure (778 bar, (b)). Melt temperature, mold temperature and injection speed were: (a) 260



C, 70



C, 100 mm/s

and (b) 260



C, 70



C, 900 mm/s [8].

FIGURE 6-21 Two-micro-cavity mold (A) equipped with

in-cavity pressure sensor at injection location (B) [8].

104 CHAPTER 6 Micro-Injection -Molding

DEFECTS OF MICRO-INJECTION

MOLDED PARTS: WELD LINES

Weld lines a re a reality of the injection molding

of complex parts. Multiple gating, splitting of the

melt flow due to inse rts in the cavity or through-

holes, as well as changes of thickness give rise to

points within the structure where the flowing

fronts will recombine and wel d. An imperfection

is observed as a line on the surface of the molded

part (see Figure 6-25). In the molding of very

FIGURE 6-23 Cavity injection time determination from the cavity pressure vs. time plot [8].

FIGURE 6-24 Pressure vs. time curves from the in-cavity sensor. Moldings at two different injection speeds (100 mm/s and

900 mm/s) are shown. Temperatures of the melt and of the mold were 220



C and 55



C, respectively. In the chart, two curves

sampled from the same molding, carried out under the same processing conditions, are shown. The increase of the injection

speed produced a decrease of the average cavity injection time from 67 ms to 30 ms and an increase of the average

maximum cavity injection pressure from 612 bar to 672 bar [8].

CHAPTER 6 Micro-Injection-Molding 105

complex components a multiplicity of weld

lines is g enerated. The weld lines are formed as

the mol d is being filled. Weld lines reduce the

mechanical strength of components in the

macro- [22] as well as in the micro- [15] dimen-

sional range. In particular, an area where the

properties are different from the bulk is crea ted.

Weld-line factors (defined as the ratio between

the strength of workpieces containing a weld line

and workpieces with the same geometry but

without weld lines) as lo w as 20% were found

on micro-injection molded tensile strength speci-

mens. The main causes ar e incomplete molecular

entanglement or diffusion, the formation of V-

notches at the weld surface, the pres ence of con-

tamination of micro-voids at the weld-line inter-

face, and unfavorable molecular or fiber orien-

tation at the weld [15].

It is therefore of great importanc e to optimize

the injection molding process, and especially the

filling phase, in order to decre ase such defects. In

particular, injection speed and mold temperature

set at a convenient level can be beneficial in order

to decrease the depth and width of weld lines

(see Figs. 6-29 to 6-31). A higher temperature of

the mold allows a higher molecular mobility

(i.e. lower viscosity) which permits obtaining of

smaller weld lines. Higher injection speed causes a

decrease of the injection time, which has the

consequence of avoiding premature freezing of

the polymer melt, allowing higher mobility of

the polymer at the interface melt front/mold sur-

face. As a conclusion, higher temperature of the

mold and higher injection speed are preferabl e

when molding micro-components with poly-

styrene polymer grade in order to decrease the

importance of weld lines.

Furthermore, the position of the gate with

respect to a considered area of the part with weld

lines is also important. In particular, the longer

the flow length, the larger the weld lines that

will form. Increase of width and depth of 30%

were observed when measuring weld lines far

from the gate compared with weld lines near to

the gate. To this respect, multi-gating solutions

can be employed to shorten the flow length

along the part of the polymer melt during the

filling of the cavity (see Figure 6-32).

PROCESS SIMULATION

Simulation programs in polymer replication

micro-technology are a pplied with the same

purposes as in conventional injection molding.

To avoid the risks of costly re-engineering,

the functions of the final products as well as

FIGURE 6-25 Simulation of the formation of a weld line due to the presence of two micro-features and the meeting of

three melt flow fronts (left) and large-range AFM scanning of the actual meeting area on a polystyrene micro-molded part

(right). The large range scanning of 700 mm  200 mm was obtained using a software tool for stitching three-dimensional

surface topography data sets [20]: 18 different scannings 50 mm  100 mm were employed for the reconstruction [8].

106 CHAPTER 6 Micro-Injection -Molding

FIGURE 6-27 Atomic force microscope measurement of wel d lines produced at the meeting area of t wo flow

fronts [8].

FIGURE 6-26 Scanning electron micro scope (SEM) images of weld lines on the surface of the 300 mmwidemicro-

features [8].

CHAPTER 6 Micro-Injection-Molding 107

the manufacturing steps are sim ulated ex-

tensively before starting the actual manu-

facturing process: important economic factors

are the optimization of the molding process

and of the tool , using differ ent simulation

techniques.

In polymer micro-manufacturing technology,

software simulation tools adapted from conven-

tional injection molding can provide useful assis-

tance for the optimization of molding tools, mold

inserts, micro-component designs, and process

parameters.

FIGURE 6-30 Effect of the injection speed on the weld line profile [8].

FIGURE 6-29 Effect of the temperature of the mold on the weld line profile [8].

FIGURE 6-28 Atomic force microscope measurement of weld lines produced at the meeting area of three flow fronts [8].

108 CHAPTER 6 Micro-Injection -Molding

At present, commercially available simulation

tools can work adequately from a qualitative

point of view but numerical values cannot be cal-

culated as precisely as necessary [25] and there-

fore simulation is not integrated in the product

development process as extensively as for

macro-products. In addition to this, most pro-

grams have difficulties in simulating exactly the

filling of micro-structures with high aspect ratio.

The reason is that commercial software tools

developed for macroscopic applications do not

consider microscopic aspects properly.

FIGURE 6-31 Three-dimensional visualization of the effect of temperature of the mold and of injection speed on weld line

topography [8].

FIGURE 6-32 Multi-gating design of a polymer mi cro-product produced by injection mo lding (hole diameter is

300 mm) [8].

CHAPTER 6 Micro-Injection-Molding 109

The main limitations encountered are related

to the fact that the rheological data used in current

packages are obtained from macroscopic experi-

ments and that a no-slip boundary condition is

employed, with the consequence that wall slip

cannot be predicted [26]. Moreover, surface ten-

sion is not taken into account, but plays a role on

the filling of micro-structures [27]. Usually a con-

stant heat transfer coefficient (HTC) is assumed,

but it cannot describe the flow through micro-

channels [26, 28] and its standard value suitable

for the simulation of macro-parts differs sub stan-

tially from values indicated for mIM [28–29].

Moreover, rheology data provided by the soft-

ware’s database are obtained at shear rates and

pressures typical of capillary rheometers (i.e. over

significantly lower ranges if compared with those

of micro-molding), and there fore are not directly

applicable and not suitable for micro-scale poly-

mer flow applications.

However, a proper implementation strategy

employed during the set-up of the simulation

can improve the quality (i.e. the accuracy) of the

simulated results. There are a number of aspects

to be considered in order to improve existing soft-

ware packages’ results:

At the machine/software interface boundary:

the implementation of the actual injection

speed profile during the filling stage of the cav-

ity, the implementation of the actual cavity

injection pressure profile developed during

the filling stage, the use of the actual cavity

injection time [27].

Concerning part modeling and meshing: three-

dimensional modeling of the whole molded

component including sprue, runner, gate, part

and micro-features, to consider the meshing

tolerance compared to the actual dimensions

of the micro-features and therefore using an

element size down to at least a few tens of

micrometers, to optimize meshing accuracy

for high precision modeling of part and

micro-features with surface and edge definition

within at least 5 to 10 mm [19].

Regarding the material characterization: the

use of experimental micro-rheological data of

the polymer material inst ead of the default rhe-

ology available in the software database, to use

experimental data obtained with micro-sized

cavities (see Figure 6-33) and high speed rheo-

metry experiments (at higher shear rates of

1/s and higher) [8] [28].

FIGURE 6-33 Polystyrene 143 E micro-rheology curves for a channel 200 mm wide at melt temperatures of 200



C, 225



and 250



C and 143 E macro-rheology at 200



C (i.e. obtained with conventional capillary rheometer). Viscosity of polymer

melt flowing in micro-channels is lower than in the macro-dimensional range due to the wall-slip effect [8].

110 CHAPTER 6 Micro-Injection -Molding