2 + 2-cavity (Disposable, Standard), 5-gallon preforms
4 + 4-cavity, Standard 5-gallon preforms
Dual-mould technology doubles productivity.
5-gallon preforms are intermediate products before they are made into 5-gallon bottles. The preforms are made in an injection moulding process. A subsequent process reheats, stretches and blows mould the preforms into bottles. The two processes together are known as the 2-stage process. The material is PET. 5 gallons = 18.9 litres.
5-gallon bottles are used in bulk water storage commonly found in offices. A smaller version at 3-gallon is used at home.
5-gallon preform weighs 680 – 750 g and has a wall thickness as high as 9.6 mm.
3-gallon preform weighs about 300 – 380 g and has a wall thickness about 6.4 mm. Disposable 5-gallon bottle is made from 3-gallon or lighter preform.
The issue with making preforms so thick is a very long holding pressure time and cooling time. The two together is more than half the cycle time.
Long holding pressure and cooling time
Injection fills the mould cavities with PET material which is then in full contact with the cooled mould surfaces. As the preforms cool, they shrink. The stage from the end of injection to the start of cooling is called holding pressure, which fills the cavities with more material to eliminate the shrinkage. The designed preform weight is also reached.
To further cool the preforms to 80 °C when the mold opens, a stage called cooling is used.
PET material is not a good conductor of heat. The 120 s cycle time of 5-gallon preforms has more than 70 s of holding pressure and cooling time which have to do with the cooling process (wall thickness, water channel design, chilled water temperature, flow rate and pressure), rather than the injection moulding machine.
A common solution is after an initial cooling in the mould, the mould is opened and preforms removed by a robot arm where the remaining cooling is done. The mould can then be closed for the next cycle.
However, this solution does not reduce the holding pressure time which has to be done inside the mould.
Two clamping units
The 5Gal series of injection moulding machine has two clamping units to hold two moulds. It also has two holding pressure devices so the holding and cooling time of one mould could overlap with the stages of the other mould. The processes in the two moulds are roughly staggered half a cycle time apart. Doubling productivity has been proven.
The patent for this design has been approved.
One screw and barrel
Each machine has only one screw and barrel the shot weight of which is good for one mould. It takes care of plasticizing and injecting the material for the two moulds, but one at a time. The residence time of the material in the barrel is halved. The heat loss is reduced from that of a screw and barrel at double the shot weight.
For 380 g 3-gallon preforms on the 5Gal2-1000, the energy consumption was measured at 0.33 kWh/kg of preforms.
There are 3 models in the series, for a maximum of 2 + 2 cavities and 4 + 4 cavities of 3-gallon or 5-gallon preforms. 2 + 2 means there are two cavities on one mould, plus two cavities on the other mould.
The models cover three sizes of screw and barrel at PET shot weights of 1000 g, 2100 g and 4200 g.
The following table will help you in the selection of a suitable model.
|Production / min.
|5-gallon, 750 g
|3-gallon, 380 g
Selection guide according to preform weight and number of cavities or productivity.
For example, to make 3-gallon preforms using a mould at 2 + 2 cavities, the production rate is 2.67 per minute or 160 an hour. The machine model is 5Gal2-1000.
To make 5-gallon preforms at 4 per minute or 240 per hour, use a mould at 4 + 4 cavities and the machine model 5Gal4-4200.
Robot arm and cooling conveyor
A robot takeout arm removes the preforms alternatively from the two moulds. Preforms are deposited into the entrance of a cooling conveyor at the exit of which the preforms are cool enough for packing. Cooling can be done at room temperature or using chilled air depending on wall thickness.
Moulds and peripheral equipment
Tat Ming also provides the moulds and peripheral equipment from our partners to deliver a turnkey system.
Peripheral equipment includes 3-in-1 hopper dryer, dehumidifying dryer and 2 autoloaders. There is also a mould sweat protector to avoid condensation on the chilled mould surfaces. A chiller cools the mould. If the preforms are coloured, a master batch dosing unit is needed.
To crush 5-gallon preforms, a powerful crusher at 40 hp is used.
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