Processing of TPU in more advanced injection molding such as micromolding and water
assisted injection molding are generally aimed at improving the TPU product quality,
production speed, and efficiency by implementing advanced technologies and innovations.
These processes achieve complex geometries and high precision, and in some cases more
than one material, beyond what is possible with the standard injection molding process.
Advancement in injection molding technology can be attributed to advancement in
automation, robotics and tooling as well as advancement in materials science. This in turn
leads to higher productivity, highly repeatable processes that help bring down the cost of
manufacturing while increasing quality and design flexibility.
It’s been shown that the microstructure“ and properties of a TPU part is significantly affected
by the method of processing. Such advanced processing technologies aim to tune the
properties of TPUto meet specific performance requirements.
Microcellular Injection Molding
This process involves injecting CO2 or Nitrogen into the mold. The gas, which at supercritical state, then
expands as it fills the mold cavity alongside the TPU melt. The mold is then cooled. This
results in micron scale pores or microcells being formed within the part.
This process achieves a lightweight part that is more dimensionally stable. Formation of
microcells allow faster solidification of the part, hence shorter cycle times. It also means the
injection pressure can be lower since the expansion of the gas exerts the pressure needed
in the mold. Less material is also used. This is particularly important because TPU is a
relatively expensive plastic.
Microcellular molding creates micron-scale porous structures within the TPU part that are not
visible to the naked eye yet can make significant changes in the material properties. Such
parts have applications in gas filtration, separation systems for cells, water filtration and
more,
The most widely used process for microcell processing is the MuCell process. The IQ foam
process is another variation of microcell processing.
Injection Compression Molding
A combination of compression and injection molding as the name implies, in this process,
TPU gets injected into a partially open mold which then compresses the TPU upon closing.
The closure of the mold exerts pressure in the mold cavity as the material cools.
This results in even stress distribution and dimensional stability which addresses problems
such as flow lines, airtraps or warping. The injection pressure can also be lower since the
mold pressure is achieved during compression.
Expanded Bead Foam process
In this process the polymer is already foamed before being fed into the injection unit. High
temperature steam is fed into the injection unit to help sinter and bind the beads together.
This process results in foamed parts with excellent elasticity, resilience and hardness. They
are often used in applications such as shock absorption, insulation, impact resistance, and
heat absorption.
Micromolding and Precisnjection ion IMolding
Micromolding is aimed at manufacturing small parts with high precision. The parts are
typically with 10 to 100 micrometres dimensions. Precision injection molding is not limited to
small parts. The process focuses on achieving exceptionally higher accuracy than what can
be achieved in standard injection molding. This is usually around +/- 0.025mm.
These processes require materials with superior mechanical properties such as TPU. They
also require machining and tooling to achieve complex parts with high precision.
Variable Temperature Processing
This technology allows the temperature of the mold to be strategically varied according to
the phase of the injection molding stage. During filling, the temperature is elevated and
during cooling the temperature is reduced.
Typically the mold is designed with a cooling channel to take away heat from the mold to aid
part cooling. Having to supply heat at specific stages in the cycle requires more complex
mold design and advanced technologies such as induction heating and pulsed cooling.
Variable temperature processing can prevent flow and adhesion issues common in TPU
processing while also achieving shorter cycle times and better control over process
parameters. It is often applied to TPU parts with very thin walls or microcellular parts.
Multicomponent Injection Molding
This involves molding multiple materials simultaneously using multiple injection units. This
process is employed when there is a need to have multiple materials with stringent
processing requirements that need to be simultaneously filled into one molded part. This is
different from two-shot, overmolding or insert molding and requires more advanced
machinery.
Water Assisted injection Molding
This process involves passing water under pressure through a mold, exerting pressure on
the walls of the material while hollowing the center. This creates a part with high density
walls. It also results in faster cooling as the water removes heat from the part. Less material
is used and the injection pressure is lower because the even pressure distribution needed to
achieve dimensional stability is provided by the water. Defects like airtraps, flow lines and
burns are avoided.
Materials Preparation
Drying
The most important material preparation for TPU in advanced processing is drying. It
becomes more important that the moisture is eliminated from TPU in advanced processes
due to the tighter tolerance and accuracy these processes target.
For example in advanced processes where the parts are within micron scale, the parts
are much smaller and even microscopic imperfections like tiny bubbles which may be
invisible to the naked eye, gets noticed in quality checks as they affect the performance and
quality of the part.
Metering into feed and channels
In advanced processes where more than one material gets injected within a cycle, it is
important to ensure the right material is fed into the right channel and that the materials do
not come into contact prematurely. While in standard injection mold the TPU resins or pellets
are simply charged or metered into the hopper, in some advanced processes, there could be multiple injection units such that metering of TPU into the machine requires special
preparations. The material preparation needs to be such that the TPU is fed into the injection
unit at precisely the right time during the cycle.
Mechanical properties Testing
This will give values such as the tensile strength, elongation at break, yield stress and
compression force. These values give a quantitative measure of how strong the material is,
how much load it can bear and more.
Visual Inspection
This could be by trained human eyes where the inspector is trained on how to spot defects.
It could also be done with technology aids such as a magnifying glass, optical microscope or
scanning electron microscope (SEM). In advanced processes like micromolding or
microcellular molding SEM is used to observe the morphology of the part. This is wear
defects such as micro cracks which may not be visible to the human eyes can be identified.
Resin testing and characterization before use
While you would typically source your raw materials from a trusted supplier who would
already have provided you with the properties of the materials. It is important to take
samples and confirm what you’re working with before a production run. This could save you
a lot in cost.
This is even more important in cases where the raw materials have been sitting in storage
for a long time. Properties might change over time and due to storage conditions and deviate
from the manufacturer’s specifications. Especially in advanced processing where the
tolerance is tight and slight variation in properties will require significant modification to the
processing conditions.
Thermogravimetric analysis (TGA), Differential scanning calorimetry (DSC), Gel permeation
chromatography (GPC) and fourier transform infrared spectrometry (FTIR) are some of the
tests carried out to determine melt temperatures, degradation temperature, molecular weight
distribution/polydispersity, and molecular structure of the polymer sample.
Calibration of instruments
Where there is a need to keep tight control of parameters, then you have to make sure the
instruments doing the measurements are well calibrated and in good condition. Pressure
gauges, temperature sensors, flow meters and other necessary instruments need to be regularly calibrated against reliable standards.
Conclusion
Over the years a wide range of advanced injection molding processes have been developed.
Many of these are widely applied to TPU thanks to its excellent properties. This advanced
processing further expands the applicability of TPU. Material preparation requirements vary
with the different processes. Quality control can be as simple as a visual inspection or may
employ more complex technologies such as CMM. In advanced processing of TPU, mold
design implements more complex technologies to meet the process requirements.
