Barrier tec There are various technologies for increasing the barrier
performance of PET containers. One of the concepts - overmoulding -
has, until now, failed to be successfully applied. However, Netstal-Maschinen
AG, in close collaboration with other specialist partners, has
succeeded in developing this technology to produce high-quality
preforms.hnology for UV light protection

Cross-section of a preform. The grey layer on the inside and
the white mark of the support on the base are visible

Holes created by the core support
One of the proven barrier solutions is the multi-layer preform,
produced by co-injection. The manufacturing process starts with the
injection of the PET base material into the preform mould. A layer of
barrier material is then injected into the still hot and soft core of
the PET to create a middle barrier layer. The “conventional”
alternatives to multi-layer are (a) the use of PET pre-blended with
some secondary barrier material, or (b) coating the bottle after it
has been blown. However another possible solution - overmoulding -
has, in recent years, been neglected. The overmoulding process starts
with the injection of an “inner” preform which is then over-moulded
with an additional layer or layers during further process steps.
In the industry the overmoulding process for manufacturing preforms is
generally regarded as being both difficult and complex. The reasons
for this are as follows: for efficient manufacture with this
technology each individual layer must have a very thin wall thickness.
However, as a result of pressure differences, the core can be
displaced from the central axis. Consequently, the distribution of the
wall thickness is no longer guaranteed. Furthermore, after the first
cycle, the preform body has to be transferred to another cavity for
the injection of the next layer. Until now this was achieved via
rotation. The delamination of the layers poses a further challenge.
This problem is familiar from multi-layer methods but it can also
occur as a result of temperature differences during overmoulding and
the metal contact with the mould.
Development with partners
After intensive development work in close collaboration with Spanish
mould-maker Molmasa and raw materials manufacturer Novapet, also based
in Spain, Netstal-Maschinen AG has succeeded in making the
overmoulding process economically viable, reliable and reproducible.
By making some ground-breaking modifications to both the mould and the
machine, preforms can now be produced with a wall thickness of just
1.2 mm after the first cycle. In addition, by including the raw
material as part of the development programme the risk of delamination
has been eliminated. The new concept includes the parallel movement of
the rows of cores when changing cavities, which means that complex
rotations are no longer necessary. These advances pave the way for a
variety of barrier and other applications. In order to create
different colour effects other materials can be used that adhere to
the inner layer. The outer layer can now also be made of recycled
material which does not come into contact with the contents of the
package.

The result of the overmoulding process is a pure white
bottle...

...with perfect light barrier
Prelactia
A particular application of overmoulding is the manufacture of
preforms for dairy products with the necessary light barriers. The
product is known as Prelactia (preforms for light absorption by
carbon-titanium addition) and was first introduced at K 2007 in
Düsseldorf.
With PET, both white colour and titanium dioxide (TiO2) have to be
added in order to reflect UV light. This is generally added at a
concentration of about 12%. Adding grey or black colour would greatly
enhance UV light absorption and would also mean a reduction in the
amount of TiO2 used. However, it has been proven that neither grey nor
black milk bottles sell well on the dairy products market. The
overmoulding concept is an ideal way of circumventing this
disadvantage and is thus a particularly important technology.
To develop the solution an injection-moulding machine from the Netstal
SynErgy 2C 2400-2150/900 range with two injection units was used. Raw
materials manufacturer Novapet quickly recognised the technical and
economic potential of this method and made a significant contribution
to its development by producing a suitable material that, on the one
hand, has flow properties that are ideal for the thin preform walls,
and on the other hand was modified to prevent delamination. The raw
material, known as “Dairy2 complex”, was produced specially for the
manufacture of Prelactia preforms and is a ready-to-use material to
which only 7.2% TiO2 is added during the polymerisation process. In
addition, a carbon-grey liquid colour was developed for the inner
layer and the white outer layer was retained. Thanks to this
carbon-grey liquid colour, after the stretch blow-moulding process, a
metallic grey surface is created inside the bottle and is the key to
perfect light absorption.
The mould was manufactured by Molmasa specially for the
overmoulding process. The preform body, that in the first cycle is
manufactured without a neck and with a wall thickness of 1.2 mm, has
three small holes in the base area. These are a result of the core
support, which prevents the cores from moving when filling the cavity.
A patent application has been filed for this solution that guarantees
the uniform wall thickness of the first layer. In the second phase,
the preform bodies, which are still grey, are coated with a bright
layer of white 2 mm thick. This produces a neck and thread and the
three holes in the base are filled.
Customers for an outstanding development
In order to realise such innovative ideas it is essential to have
interested customers. The application was thus presented to the
company PDG Plastiques in France. This company is located next to the
dairy filler LSDH and so is already familiar with the packaging of
dairy products. PDG Plastiques was able to quickly blow the Prelactia
preforms into bottles and subsequently test them. After six months
1000 bottles filled with UHT milk were assessed according to extremely
strict criteria and demonstrated their superiority over mono-layer
bottles. Not a single bottle revealed any tainting of the milk. This
formed the basis for a successful collaboration with PDG on PRElactia
and, in autumn 2007, PDG successfully commissioned its first Netstal
system for the manufacture of PRElactia preforms.
The bottle’s pure white colour is immediately apparent when holding
the final product in the hand, despite the grey inner layer. In
contrast to the multi-layer method the dark inner layer does not show
through. If the bottle is cut open, it is possible to see how well the
materials have been matched. They bind firmly together making
delamination impossible. The light measurements are particularly
impressive and reveal a 99.99% absorption and reflection of light of
up to a wavelength of 700 nm.