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Frequently Asked Questions


1. Why do we use two-stage method to product bottle?

Before PET bottles become popular, plastic bottles are extrusion blow mold.

In extrusion blow molding, the bottleneck is not smooth, and has to be scraped to remove the flashing. This avoids leakage after a cap is put on. A lot of waste plastic is produced in extrusion blow molding which has to be removed and recycled. The advantages include low investment cost, high production speed, easy to add a handle, the bottle could take many styles and shapes, bottle weight could be adjusted over a big range, extrusion blow moulds are inexpensive and have short delivery time. Hollow bottle handles could be made in the extrusion blow molding process.

In the injection stretch blow molding process, injection molding machine is used to make the preform. Using this process, the neck finish is almost perfect. Before the preform cools or by reheating, the preform could be stretched by a rod and blown (bi-directional stretching). Other than strengthening the bottle, the wall thickness is reduced so resin usage is more economical than extrusion blow molding.

2. Why do we need Hopper Drying and Dehumidified Drying?

PET, PC, nylon and ABS are examples of hygroscopic resin. They absorb water moisture in the air. If the moisture is not got rid of before injection molding, the resin will hydrolize so the strength becomes substandard and cosmetic appearance suffers, causing rejects.

Under a microscope, one could find a hygroscopic resin pellet is filled with capillaries. Once moisture gets into the capillaries, only heating could vapourize the water and get it out. During heating, temperature, duration and the source of incoming air are critical in determining the residual moisture content. Measured by weight, the residual moisture in PET resin should be below 50 ppm. When this happens, the absolute humidity of the drying air has a dew point of -40?C.

The simplest drying method is to use the hopper dryer. Indoor (moist) air is sucked in, heated to 160?C and blown upward through the hopper bottom. After drying for 5 hours, injection molding could begin. As the source of air is the atmosphere, the drying effect is dependent on the air humidity. The drying is unstable and in any case could not reach the 50 ppm requirement.

An oven works on the same principle as the hopper dryer. The difference is resin pellets are spread out in flat pans which increase their contact area with the air and improves the drying efficiency. After drying at 160?C for 5 hours, the pans are emptied into the hopper dryers for use.

Dehumidifying dryer recycles the air for reuse. Before reuse, the moisture in the recycled air is removed by desiccant. The desiccant is in turn regenerated by an independent hot air stream which enables it to continue removing moisture from the recycled air.

Dehumidifying dryer could attain the drying requirement of PET resin.

Dew point is a temperature, but it measures the absolute humidity of air.

The lower is the dew point, the dryer is the air. The dehumidifying dryer for PET is required to produce dry air with dew point at or below -40?C. Top

3. Why do we use Chiller and mould sweat protector

The efficiency of making preforms, which belong to the thick-wall domain, is highly dependent on cooling time.

Cooling time is proportional to the square of wall thickness, but inversely proportional to mould temperature difference. Mould temperature difference = melt temperature - mould temperature.

If the melt temperature is 260?C, water tower water temperature is 30?C and chilled water temperature is 6?C, cooling the preform mould by chiller instead of water tower could reduce the cycle time by 1- (260 - 6)/(260 - 30) = 10%.

To chill the mould to 6 ?C, under the usual weather conditions, the mould will sweat. Condensate will leave marks on the preform surface. But if the surrounding air has a dew point of 3?C, the mould will not sweat.

Refrigerator and air conditioner are simple air dryers. The air in a refrigerator is lowered to dew point so dew is formed. When the air temperature is below 0?C, the dew turns to ice. With the moisture so removed, the moisture content of the air is reduced. By the same token, when moisture condenses in an air conditioner, the room air is (absolutely) dried. The domestic air conditioner could not dry air down to 3?C dew point. A special purpose machine is needed.

Whether an air conditioner or a mould sweat protector is used, the clamping unit of an injection molding machine must be fully enclosed to reduce the loss of dry air.

4. How to calculate cycle time of preform molding

Cooling time dominates cycle time in thick-wall molding like preform molding. The next most critical time is plasticizing time.

When chilled water is used to cool the mould, cycle time is related to wall thickness as follows.

tc = 1.2 t2

where t = preform wall thickness in mm

tc = cooling time in s

Here, cooling time is defined as holding time. The wall thickness of a 1-litre bottle preform is 3mm. The cooling time is 10.8s. The wall thickness of a 5-gallon bottle preform is 8mm. The cooling time is 76.8s.

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