Preforms/PET-bottles
Typical products: Bottles for mineral water, beer, soft drinks, parfumes, soap, liquids ...
Typical materials: PET, PLA, additives
Virgin |
Agglo- |
Bottle |
Preform |
Foil |
Typical material throughputs: Total range 50-1500 kg/h, main range 500-1000 kg/h
Process description
In principle, the manufacturing process for PET or PLA bottles is divided into two steps. First of all, so-called preforms are injected into moulds with up to 144 cavities where the thread or the mouthpiece for the cap of the finished bottle is included. In the second step, the preform is brought to its final shape in a stretch blow moulder.
Used materials.
For the manufacture of preforms, PET is mainly used which is available as virgin in granular form. For special applications, PLA is also processed, which is gained from renewable raw materials, such as corn.
Today, more and more PET and PLA is recycled in the production process. Genuine regrind from PET or PLA bottles is preferred in order to guarantee the quality of the bottles. If amorphous regrind is used, it must be crystallised before drying.
Material colouring:
For the production of coloured bottles, either coloured masterbatch is dosed volumetrically or gravimetrically via a dosing unit on the inlet of the machine, or colorant is brought into the material inlet via a liquid dosing system. This is how transparent green, blue or brown bottles are usually manufactured. For the bottling of milk, PET bottles are coloured white and opaque.
Material preparation:
Prior to the manufacture of preforms, PET or PLA must be dried, otherwise streaks and degradation through hydrolysis may occur in the preform. In virgin material, PET frequently holds up to approx. 0.1% initial moisture and is therefore dried for approx. 6 hours at 160 °C to 175 °C in order to achieve a residual moisture content of maximum 50 ppm. Regarding PLA with an initial moisture of up to 0,25%, a residual moisture of below 0.01% is achieved with drying temperatures from 60 °C to 100 °C and drying times from 4 to 8 hours.
1=Silo, 2=Crystallisation, 3=Drying unit, 4=Blending with subsequent drying, 5=Blower, 6=Colour dosing with MINICOLOR, 7=Mould air conditioning
Preferably, the drying system is installed above the injection moulding machine so that after drying, the material is directly transported into the processing machine via a gravity pipe - without additional transport devices. This is the only way to lead the material into the plastification unit without significant temperature losses and additional energy expenses. Drying is equipped with the ETA plus energy saving system in order to assure optimum drying with minimum energy consumption. This package includes automatic air flow control adjusting the air flow to material inlet conditions and thus adapting continuously and optimally the energy consumption to processing. In addition, the drying process is equipped with a heat exchanger in the regeneration heating process as well as a closed regeneration cooling circuit. In order to keep heat losses low, the heater for drying is located directly in front of the drying bin.
Gas-fired drying:
If the production plant is connected to the gas supply, it makes sense to install a gas-fired drying installation. Since gas prices are often lower than electricity, gas-fired drying gives a fast return on investment. Applications with both electric and gas-fired heaters have been installed which allows the customer the choice of his medium.
1=Exhaust air regeneration, 2=Dry air dryer LUXOR, 3=Optional: Super fine dust filter, 4=Tubular heat exchanger, 5=Material/return air temperature, 6=Heat exchanger regeneration air, 7=Frequency controlled blower, 8=Burner, 9=Combustion chamber.
Mould air conditioning or mould dehumidification
The moulds in an injection moulding shop always must be treated with dehumidified and cold air from an air conditioning system to keep away humid ambient air from the cooled mould and to cool rapidly down the preforms after injection moulding.
Without mould dehumidification, the cooled mould surface areas would immediately be covered with condensate when opening the moulds in humid ambient air. Small amounts of humidity could lead to streaks on the preforms. motan's product range includes a series of mould dehumidification units LUXOR MD A 300 to 3000, which provide for cooled and dehumidified air with capacities of 300 to 3000 m³/h.
Blending and crystallisation of material
Nowadays, there is increased demand to recycle bottle regrind or agglomerate into the production process in order to assure material recirculation. To do this, a certain percentage of the recycling material should be added to the virgin material. Usually, the regrind percentage is 10 to 50%.
For these applications, gravimetric dosing units have stood the test. These units operate in batch processing and are installed directly onto the drying bins. The benefits:
- ”Conveying” into the drying bin through gravity
- low requirement of conveying capacity (throughput)
- low investment costs
- small footprint
Colouring of material
The units of the MINICOLOR PET series are ideal for colouring with masterbatch. They work with cooled dosing screw housings, because in many cases, masterbatch materials cannot cope with high temperatures. The dosing houses have extra insulation to keep heat losses low. These dosing units operate only volumetrically.
Crystallisation of PET and PLA
- For processing, PET must be dried at 180 °C to achieve a residual moisture below 40 ppm.
- However, during the recycling process, PET is in an amorphous condition since it is cooled down rapidly when coming out of the melt. It is amorphous and therefore transparent.
- When heating PET, there is a transition phase from amorphous to crystalline structure when reaching 120 °C to 140 °C. In the transition phase, PET becomes sticky and must be moved in an agitator bin to avoid formation of lumps.
- After crystallisation, the material turns white and can now be dried at 180 °C without sticking together.
- motan offers a complete series of crystallisers with reactor volumes of 125 to 2500 liters for material throughputs of 30 kg/h to 1000 kg/h.
motan-colortronic’s benefits:
motan-colortronic has specific skills and particular knowledge of the injection moulding process and proposes an increasingly wide range of products specifically developed for this sector. The solutions that motan-colortronic offer are ideal above all for production processes of PET and PLA preforms in any size. With a high engineering capacity, motan-colortronic is able to develop and implement customised systems with the customer, OEM and preform injection moulding machine manufacturers.