Issue 03/2016

Injection moulding
Wall thickness dependent flow
characteristics of bioplastics
For the plastics industry, the component weight is of
critical importance for the material costs. A good
way to keep it light-weight is to produce components
with low wall thickness. Reducing the component weight
means to save on material and costs. In addition, it helps
to improve the carbon footprint, especially for products
with long transportation ways. Besides, a reduced carbon
footprint fits in well with the green image of bioplastics.
As of now, thin-wall components present a technological
challenge especially for injection moulding. The lower
the wall thickness of a moulded part, the greater the requirements
regarding rheological properties of the material.
This applies to bioplastics as well as to conventional
plastics. For bioplastics, however, the specific parameters
have not yet been available, which makes it very difficult
for interested manufacturers to identify bioplastics that
are suitable for thin-wall technology or may serve as
points of comparison.
Bioplastics vs. conventional plastics
For these reasons, the absence of specific information
relevant to the manufacturing process is a major
impediment to a wider range of applications for bioplastics.
This is the background for a project entitled “Processing
of Biobased Plastics and Establishment of a Competence
Network within the FNR Biopolymer Network”, initiated by
a research alliance as part of a larger programme funded
by the German Federal Ministry of Food and Agriculture
(BMEL) and managed by the German Agency for
Renewable Resources (FNR). This collaborative endeavour
deals with the processing technologies currently in use
for plastic materials (injection moulding, extrusion, fibre
production, thermoforming, extrusion blow moulding,
welding etc. and examines a wide range of marketable
bioplastics with respect to their process-specific data,
most of which have not become available yet from the
material suppliers. The entire test results generated by
the research alliance can be accessed free of charge
and unrestricted at www.biokunststoffe-verarbeiten.de
(German language). The test outcome described here
represents partial findings only. To obtain comparable
data for biobased and conventional plastics, various
materials from both categories were tested using identical
methods. The results were evaluated according to the
wall thickness of each tested material, whereby high flow
length at simultaneously low wall thickness indicates high
flowability. The tests were conducted in cooperation with
UL TTC (Krefeld, Germany); they are based on standard
values for thermal properties of polymer melts (thermal
capacity, conductivity, and density), the Carreau-WLF
model for viscosity, the cooling-off and shear heating at
a given melt and mould temperature. An equation system
is used under the parameters of isothermal mould filling
and a filling pressure limited to 800 bar for a test plate
(without gating system). The limitation is necessary due to
the process design for high-quality moulded parts, which
requires a limitation of the filling pressure because of the
inherent residual stress.
Flow behaviour of conventional plastics
as a point of reference
The first step is to establish a basis for comparison
by charting the flow behaviour of conventional plastics.
The examined materials represent a cross-section of
commonly used plastic materials (fig. 1).
Flow behaviour of bioplastics
Biobased plastics meanwhile comprise a portfolio of
characteristics that is nearly as broad as that of their
conventional counterparts. In the case of Polylactide
(PLA), which currently seems to be most suitable for mass
markets, a number of optimized material variants are
already available. The table 1 lists those bioplastics that
have been tested in this project, along with their material
class.
The parameters chosen for the tests were identified
by means of extensive pre-tests and can be considered a
processing recommendation.
PLA-based bioplastics
The graph in figure 2 shows the test results for PLAbased
bioplastics and illustrates these in comparison
with the flow behaviour of conventional plastics. Evidently,
polyester-based PLA has a flow behaviour which settles
in the lower range compared with the tested conventional
plastics and thus corresponds to the flow behaviour
of conventional polyamide. Especially PLA filled with
60 wt% natural fibres (NF) shows surprisingly good flow
Table 1: List of examined bioplastics
Material
Nature Works Ingeo 3251D
Nature Works Ingeo 6202D
Material class
PLA Injection moulding grade
PLA Fibre spinning grade
Nature Works Ingeo 3052D PLA Injection moulding grade 2
Hisun Revode 190
Jelu WPC Bio PLA H60-500-14
Metabolix Mirel P1004
FKuR Terralene HD 3505
Evonik Vestamid Terra HS16
Showa Denko Bionolle 1020MD
Jelu WPC Bio PE H50-500-20
PLLA
PLA + 60 wt% NF
PHB
Bio PE
Bio PA
PBS
Bio PE + 50 wt% NF
22 bioplastics MAGAZINE [03/16] Vol. 11