Since 10.2005, Dr. Lei Xie has been working as scientific staff in institute of polymer materials and plastics engineering, Clausthal University of Technology, Germany.In the same institute, He achieved his doctor engineering title in July 2010.
He is active in broad fields of polymer materials and plastics engineering, especially expertised in micro injection molding technology, as well as polymeric composites.
Abstract: A simple and novel combination of ultra-precision diamond ball-end milling and micro injection molding technique is described to produce precise microlens arrays out of polycarbonate (PC), polymethylmethacrylate (PMMA) as well as polystyrene (PS). The microlens arrays consist of 100 lenses in a 10 à 10 array with a lens radius of 273 μm, a lens diameter of 300 μm and a lens depth of 45 μm. Pitch between the lenses is fixed at 800 μm. The injection molding parameters were optimized to get precise microlens geometries with low surface roughness. The results show a precise diamond milled mold insert and injection molded microlens arrays with minor deviations in radius and surface roughness of the microlenses, particularly for microlens arrays out of PMMA.
Abstract: Flax fibers were chemically modified on their surface with aldehyde-based glutaraldehyde, glyoxal or butryaldehyde to enhance their adhesive interconnections with polypropylene (PP) matrix. Mechanical (tensile and elastic) properties were most improved with glutaradehyde modification compared with glyoxal or butryaldehyde. Using glutaraldehyde modified flax fibers, the optimal modification parameters that produce the most improvement to the mechanical properties and tensile strength were then evaluated by Fourier Transform Infrared Spectroscope (FTIR) and scanning electron microscopy (SEM). Optimal modification of flax resulted from an acetalation reaction involving glutaradehyde and anhydro-D-glucopyranose (AGU) flax at a concentration of 3 mol/AGU, performed at 60°C for 5 hours (pH = 2.0). Composites were then created by mixing the modified flax fibers with PP matrix at a 20:80 wt% ratio at 180°C for 40 minutes using a co-rotational twin screw inner melt mixing kneader. FTIR and SEM analysis demonstrated that reinforced PP composites had a 31.63% increase in tensile strength and a 19.61% increase in the elastic modulus when compared with the original flax fibers. Compared with other reported surface modification methods, this method exhibited better performance in enhancing the mechanical properties of flax/PP composites.
Abstract: Micro injection molded polymeric parts coated with functional thin films/layers show off the promising applications in microsystems area. But the unfavorable and unavoidable defect of weld line in micro injection molding part leads to detrimental mechanical and surface properties. The possibility of the functional thin film for enhancing micro injection molded weld lines was investigated. Two typical coating materials (aluminum and titanium) with various film thicknesses (400, 600, 800 nm) were deposited on one side of the micro injection molded weld line tensile sample via physical vapor deposition (PVD) method. The coated micro weld line samples were characterized by tensile tests. The results show that PVD films of aluminum and titanium can reinforce the strength and stiffness of micro injection molded weld line, even at thin thickness levels. But when the film thickness is increasing, the weaker adhesion between metallic films and polymers decreased the PVD filmsâ enhancing performance for micro weld line mechanical properties due to the degradation of polymers related to longer time exposure under high temperature.
Abstract: Nano fillers reinforced polymer composites have been widely applied in microsystem technologies. As one popular micro fabrication technology, micro injection molding is playing the key role in manufacturing micro scale parts made of nano thermoplastics composites. As a defect in micro injection molding parts hard to avoid, the weld line causes detrimental mechanical and physical properties, particularly in case of nano filled composites. In this study, the hybrid carbon nanofibers (CNFs)/TiO2 nano particles (1:1) filled polypropylene (PP) was prepared by inner melts mixing process with various weight contents(10, 20, 30 and 35 wt%). Micro tensile samples with weld lines for all composites were formed by micro injection molding process at constant processing conditions. Mechanical properties of micro weld lines were tested by tensile test characterizing method. The results show that comparison with neat PP, the samples with weld lines made of hybrid nano PP composites are showing lower tensile strength and elongation percent, but higher E modulus. As increasing nano filler contents, the tensile strength and elongation of micro weld line samples are decreasing, while the E modulus is improved considerably.
Abstract: As a defect of micro injection molding parts, weld
line is unfavorable since it will influence the surface quality and
mechanical properties of micro parts. Therefore, the investigation
on the developing process of weld line would be a
significant issue for improving the quality of micro injection
molded parts. In this study, one injection mold with four micro
tensile sample cavities was designed and constructed. Every
cavity responses to various gate dimensions, which is marked
as Gate Nr:1ð1:5 0:1 0:5mm; width depth lengthÃ,
Nr:2ð1:00:10:5mm; widthdepth lengthÃ, Nr:3ð1:0 0:05 0:5mm; width depth lengthÃ, and Nr:4ð0:5 0:1 0:5mm; width depth lengthÃ. The effects of gate
dimension of the mold on mechanical properties of weld line
have been studied by experiments in different processing
parameters. The tensile test was used to characterize the micro
injection molded weld line strength. The results for polypropylene
show that with the changing of injection pressure and
mold temperature, Gate Nr.3 is corresponding to the strongest
weld strength; the next is Gate Nr.2; Gates Nr.4 and Nr.1 are
in the end. The difference between them is not obvious. For
high-density polyethylene, Gate Nr.1 is not able to be
completely filled, which is due to the blocking of stick
materials and dirt based on the simulation analysis. The
investigation was only carried out for the other three gate
sizes; results present that Gate Nr.3 always gives the best weld
line strength whatever the processing parameters are, Gate
Nr.4 is next and then Gate Nr.2. There are always middle
optimal values for processing parameters leading to strongest
weld line strength, when injection pressure is 80 MPa,
injection speed is 90 cm3/s, melt temperature is 200°C, and
mold temperature is 130°C. Higher and lower processing
parameters result in reduced weld line strength.
Abstract: Two kinds of wood particles with different sizes and properties were compounded with polypropylene (PP) in highly concentrated levels (by 50% and 60% weight concentration). Their flow abilities were estimated by viscosityâshear rheological test and spiral flow mold respectively; the results show that higher concentrations result in poor flow ability. However, to estimate the relation between particle size and flow ability, spiral flow-mold testing results are not consistent with those of the rheological test. The thermal melting, crystallization, and stability of the highly filled wood/PP composites were measured by differential scanning calorimetry (DSC). The results clearly showed that the melting temperature, thermal stability, and crystalline degree of composites decrease when filler contents and size increase. The mechanical properties of these materials were assessed by tensile test machine and testing results also show that the filler concentration and size dramatically affect the mechanical properties, such as E module, tensile strength, and breaking strength of wood-particle-filled PP composites. The weld-line strength of all wood composites with the contents was also tested in the same way. By testing, one found that the usual defect of weld line in injection-molding process had a very significant negative impact on the mechanical properties of wood composites and the relation of wood-particle-filling fraction and size to weld-line strength was also analyzed and clarified.
Abstract: Weld lines are the unfavorable defect not only
in normal injection molding process but in micro injection
molding process. In this study, polypropylene (PP) was
chosen as the processing material and a micro dog-bone
tensile test sample was selected as the objective part. The
micro tensile part was prepared by the double gate injection
mold. An ultrasonic generator was integrated in this mold
in order to investigate the effect of ultrasonic oscillation on
the micro injection molded weld line strength. The experiments
were carried out for studying how the ultrasonic
output power and the oscillation inducing time affect the
weld line strength. Three output power levels (400, 600 and
800 W) and two inducing mode (Mode 1. the oscillation is
induced from injecting moment to ejection moment; Mode
2. the oscillation is induced from injecting moment to
packing procedure finishing) were set. The results show
that ultrasonic oscillation has obvious influence on the
weld line strength; Mode 2 always has better performance
than Mode 1 for reinforcing the weld line strength; and
when output power is 400 W the weld line strength is the
highest. The mechanism of ultrasonic oscillation affecting
the micro injection molded weld line was also analyzed by
AFM (atomic force microscope) and polarized microscope.
Abstract: In order to reduce the âtrial-moldâ risk and cost, numerical simulation method was applied to micro injection molding
weld line development investigation. The micro tensile specimen which has the size of 0.1 mm (depth)Ã0.4 mm (width)Ã12
mm(length) in test area was selected as the objective part, and polypropylene (PP) as the experimental material. Respectively with
specific commercial software (Mold Flow®) and general computational fluid dynamic (CFD) software (Comsol® Multiphysics), the
simulation experiments for development of weld line in micro injection molding process were executed and the real comparison
experiments were also carried out. The results show that during micro injection molding process, the specific commercial software
for normal injection molding process is not valid to describe the micro flow process, the shape of flow front in micro cavity flowing
which is important in weld line developing study and the contact angle due to surface tension are not able to be simulated. In order to
improve the simulation results for micro weld line development, the general CFD software, which is more flexible in user defining
function, is applied. The results show better effects in describing micro fluid flow behavior. As a conclusion, as for weld line forming
process, the numerical simulation method can give a characteristic analysis results for processing parameters optimizing in micro
injection molding process; but for both kinds of softwares quantitative analysis cannot be obtained unless the boundary condition and
micro fluid mathematic model are improved in the future.
Abstract: In micro injection molding, the melt flow behavior is important for the final product quality. However, the current process monitoring and measurement technology are not adequate enough to provide a direct analysis access. In the presented study, a glass insert mold designed for performing the direct visual analysis for melt flow phenomena in micro injection molding is introduced. The micro tensile specimen with 0.1 à 0.4 mm2 (depth à width) cross section dimension is chosen as the objective part. The correlation between processing parameters (injection pressure, injection speed, mold temperature) and flow behavior was investigated and analyzed. The results show that the injection pressure put an obvious effect on the filling speed through micro cavity. Injection speed can influence the filling time dramatically also. Higher mold temperature brings positive influence with the flowing speed, due to the lower viscosity of polymers in higher mold temperature.
Abstract: As a suitable mass and cost efficiency fabrication
method, micro injection molding is doing a very good performance
in micro plastic parts production. The mold design
is an important part affecting micro parts properties. In this
study, a micro injection mold with multi cavities of micro
tensile bar is used. These micro cavities are fabricated by a
micro milling process in different cross section shapes
(semicircle R = 0.5 mm, equilateral triangle D = 0.3 mm,
and trapezoid D = 0.336 mm t = 0.2 mm bottom angle =
95). With an Arburg 320C injection molding machine,
micro tensile test sample are prepared in different processing
parameters so that a correlation between the cross section
shapes with micro weld line strength in different conditions
could be investigated by tensile test. Final results show that
when the cross section shape is different, their corresponding
weld line strength is also different. Equilateral triangle cross
section is leading to strongest weld line, and then followed by
trapezoid, semi-circle is the last. By analysis of these tensile
test results, the quantitative factor a is defined as the ratio of
perimeter to area of cross section shape, and higher a value is
corresponding stronger weld line. After weld line strength
comparison in different processing conditions, the results
show that higher injection pressure induced to lower weld
line strength whatever the cross section shape is. By higher
mold and melt temperature, equilateral triangle cross section
gives improved weld line strength. But mold and melt temperature
affect weld line strength negative for other cross
section shapes.
Abstract: As a hot fabrication technology for micro scale parts, micro injection molding is receiving increasing market attention. Improving mechanical properties of micro parts should be an important issue in the micro injection molding process. The relation between weld line strength in micro injection molding parts and processing parameters is investigated. A visual mold with variotherm unit is designed and constructed, in which the micro tensile specimen with weld line are prepared. Polypropylene (PP) is used as the research material in this study, and six processing parameters were chosen as investigating factors, which were melt temperature, mold temperature, injection pressure, packing pressure, ejection temperature and injection speed. In order to achieve optimized processing parameters and their order of significance, Taguchi experiment method was applied in this presented study. The prediction formulation of the strength of micro weld line was built up by multiple regression analysis based on Chebyshev orthogonal polynomial. The results showed the influencing significance order of parameters from strong to week separately are mold temperature, melt temperature, injection speed, ejection temperature, packing pressure and injection pressure. And the tolerance of micro weld line prediction formulation was found to be lower than 21% through confirmation experiments.
Abstract: In order to study the weld line developing
process and its influence on mechanical properties in micro
injection molding, a visual mold with variotherm system
mold was designed and fabricated. In this mold, a visualization
design and a rapid heating/cooling system were
integrated, and specimens with different cross section
shape and micro dimensions could be molded for weld line
study. The building process for the visual and variotherm
mold was presented and the experiments were executed.
The specimens for weld line study of micro injection
molding were produced applying different processing
parameters. A problem of flash in molded specimens needs
to be solved.
Abstract: Understanding the weld line forming process is
important for micro parts molded by injection molding
technology. In this paper, the flow fronts advancing during
weld line forming process of micro injection molding is
investigated by both experimental and numerical methods.
A glass insert flow visualization mold is devised to record
the whole process of the melts flowing and weld line
forming. Arburg® 220S is chosen as the micro injection
molding machine and PP is the objective material. The
molding part is a micro dumbbell tensile sample, which is
totally 24 mm long and has 12 mm long micro scale test
area with rectangular cross section (0.4 mm widthÃ0.1
mm depth). The numerical simulation is realized by
software Comsol® Multiphysics 3.4 taking surface tension
account into. In order to find out how the processing
parameters affect weld line forming in micro scale, the
experiments were carried out in different processing
conditions, injection pressure (25, 30, 35, 40 MPa),
injection speed (15, 20cm3/s), mold temperature (120,
135 ºC). Flow visualization experimental analyses indicate
that injection pressure, injection speed and mold
temperature all have effects on the flowing speed of the
melts. Injection pressure and mold temperature also
influence with the shape of flow fronts. Numerical
simulation results give a good confirming description for
the flow fronts shape which is consistent with
experimental observation. In addition the characteristic
prediction for the v notch size is carried out.
Abstract: As a hot fabrication technology for micro scale parts, micro injection molding is getting
more and more market attention. Improving mechanical properties of micro parts should be an
important issue in micro injection molding process. The relation between the strength of weld line in
micro injection molding parts and processing parameters is investigated. A visual mould with
variotherm unit is designed and constructed, in which the micro tensile specimen with weld line are
prepared. Polypropylene(PP) is used as the research material in this study, and 6 processing parameters
are choose as investigating factors, which are melt temperature, tool temperature, injection pressure,
packing pressure, demold temperature and injection speed. In order to get optimizing processing
parameters and their significant order, Taguchi experiment method is applied in this presented study.
And the prediction formulation of the strength of micro weld line is built up by multiple regression
analysis based on Chebyshev orthogonal polynomial. The results show the influencing significant order
of parameters from strong to week separately are tool temperature, melt temperature, injection speed,
demold temperature, packing pressure and injection pressure. And the tolerance of micro weld line
prediction formulation is lower than 20%, proved by confirmation experiment.
