.Information .
  About PET resin  

PET is short for PolyEthylene Terephthalate. It is a type of thermoplastic PolyEsTer.

Since the 1973 registered patent to make bottles for carbonated drinks using PET, PET starts to become popular. Of the 140 million tons of plastic resin produced worldwide in 2000, PET takes up 25 million tons, although not all PET resin is used to make bottles. Currently, bottle-grade PET resin is suppliedby Voridian, KoSa and other suppliers.

Resin could be classified into crystalline and non-crystalline types. PET, PP, PE, nylon and PBT are crystalline. PC, ABS, PS, SAN, PPO and PVC are non-crystalline.

When crystalline film is stretched, crystals will align themselves in the stretch direction, increasing the film strength. On the other hand, non-crystalline resins do not increase their strength when stretched.

If uni-directional stretching increases the PET film strength in one direction, bi-directional stretching will increase its strength in both directions. The injection stretch blow molding process could stretch the PET film in two directions in the stretch blow process.

Another function of bi-directional stretching is to reduce wall thickness, which reduces resin usage.
PET material is different than other crystalline materials in that it is crystal clear with light transmission similar to that of glass.

The advantages of PET bottles are they are less prone to breaking and are light. A 0.7-litre glass bottle weighs more than its content. However, the weight of a 1-litre PET bottle is less than 20% of gross weight. During transportation, spoilage and transportation fee could be reduced.

PET bottles allow oxygen to migrate in and carbon dioxide to migrate out and therefore could not be used alone to contain beer. Oxygen will oxidize the beer composition, changing the taste of the beer. In general, beer manufacturers request the shelf life of beer to be at least 6 months.

There are two classes of methods to overcome this shortcoming: resin barrier and coating barrier.

PEN (PolyEthylene Naphthalate), nylon and EVOH have a much higher barrier to oxygen than PET. They could be sandwiched between two PET layers.

Using barrier coating, coating like carbon film is sprayed onto the inner or outer surface of the bottle.

In the hot processing of PET, e.g. injection molding, when the shearing is too high, PET degrades to AcetAldehyde, which is abbreviated to AA. AA is not odourless. Especially when the bottle contains mineral water which tastes bland, the AA smell becomes unacceptable.

PET bottles will soften when containing water at 80oC. Many drinks like tea, juice and diary products are hot filled or hot pasteurized afterwards. In such cases, PET is not the right choice.

The most common solution is to mix for example 25% PEN material into PET to increase its hot fill temperature. Alternatively, the bottle neck is crystallized (and turns white). Crystallized PET bottle neck could stand up to a high hot fill temperature. In the hot fill arena, clarified PP is becoming popular with a hot fill temperature of 95oC, but its clarity is not comparable to that of PET. Top
  Two molding ways   

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

After heating and friction heating by the rotating screw, the resin turns to a melt and is extruded through a die in the form of a cylindrical tube (parison). The moud closes on the parison and air is injected through the neck. The cylinder expands and touches the mould surface which cools it down.

After opening the mould, the bottle is removed.

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.

The disadvantages of injection stretch blow molding is difficult to add handle and expensive preform mould, which has a long delivery time. Bottle weight, which is the same as preform weight, is essentially non-adjustable. The way a handle is added to PET bottle is to insert the injection mold handle into the blow mould. The handle has undercuts and is wrapped around by the bottle wall during blow molding. Top
One-stage versus two-stage

If the process of injection stretch and blow molding are performed within one machine, the process is called one-stage. In the one-stage process, when the preforms cool to about 110oC, the stretch and blow molding process begins.

    The advantages are
    1) energy efficiency as reheating is not necessary,
    2) high productivity as cooling to room temperature and reheating are not necessary.
    3) no contamination as the human hand is out of reach. Top
   Drying PET resin  

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 -40oC.

The simplest drying method is to use the hopper dryer. Indoor (moist) air is sucked in, heated to 160oC 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 160oC 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. Top

   The moisture content of air  

The most familiar measure of the moisture content of air is relative humidity. April in Hong Kong is foggy with relative humidity approaching the maximum of 100%. During winter when the monsoon from the North is blowing, relative humidity could be as low as 30%.

Relative humidity is defined as the ratio of moisture content of air to the saturation (maximum) moisture content at the same temperature expressed as a percentage. In the diagram, the dotted line is the 60% relative humidity curve. As it turns out, the higher the air temperature, the more saturation moisture it can hold. On the other hand, the lower the air temperature, the less saturation moisture it can hold. When saturated air is cooled, its moisture is released in the form of fog. By the same token, spectacle wearers coming out of an air conditioned room has dew on their glasses which is when the outdoor air is cooled to the glass temperature and becomes saturated, releasing the moisture on the glass surface.

In the diagram, the 60% relative humidity point at outdoor temperature corresponds to a fogged spectacle temperature, which is called the dew point.

The dew point is the highest temperature to which the air cools to saturation moisture content.

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 -40oC. Top
   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 260oC, water tower water temperature is 30oC and chilled water temperature is 6oC, 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 6oC, 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 3oC, 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 0oC, 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 3oC 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. Top
  Preform moulds  

Multi-cavity preform moulds do not use the cold runner design. An often-used design for the runners is hot plate design.

In hot plate design, every two cavities share a heating zone the temperature of which is measured by a thermocouple. There will be a sprue at the bottom of the preform of no more than 5 mm. After stopping the injection molding process for a while, there is a chance the runners are blocked and must be cleared by a time consuming process. Hot plate design is economical and is popular in China.

The most sophisticated runner design is the valve gated hot runner design. Each cavity has its own heating zone and is temperature measured by a thermocouple. There is no sprue at the bottom of the preform. When the injection molding stops, the valve gate closes the runner so the runners will not be blocked. The cost of valve gated hot runner is US$2,500 per cavity. An 8-cavity preform mould costs more than the injection molding machine that holds it. Top
  The number of mould cavities  

The king of preform is that of 5-gallon (19-litre) bottle used in drinking water machines. It weighs between 650 and 750g. Even the disposable version weighs 400g. So the more popular moulds are single cavity based which allows the cold runner design.

At home, 3-gallon (12-litre) bottles are getting popular. Their preforms weigh between 300g and 520g. Their preform moulds could be 2-cavity based and use a hot plate or hot runner design.

The other number of cavities include 4, 6, 8, 12, 16, 24, 32, 48, 60, 72, 96 and 144. To increase productivity, maximize on the number of cavities by using the clearance between tiebars, the platen dimensions and the injection weight of the injection molding machine to the most.

Most preform moulds are rectangular and are longer than they are wide. There are even injection molding machines dedicated to making preforms having a rectangular tiebar clearance to fit the needs of making preforms. The diagram shows the cavity layout for 32 cavities or fewer. Top
  Cycle time  

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.

The few methods to increase plasticizing time without lengthening cycle time all depend on simultaneous action. Pre-plasticizer allows plasticizing during almost the whole cycle. The only function of the pre-plasticizer is plasticizing, injection is left to another unit called the plunger. The plasticized melt accumulates in front of the plunger. Only during injection (including holding time) is plasticizing stopped.

Pre-plasticizer and injection plunger
A holding pressure unit allows the pre-plasticizer or a reciprocating screw to plasticize during the long holding time. Plastic shrinks during cooling.

Holding packs more material in to take up the shrunk space so the preforms are without sink marks.

Hydraulic shut-off nozzle allows plasticizing during mould motion and ejection.

Barrier screw (also called double flight screw) is a design to increase plasticizing capacity.

Everything else unchanged, the bigger the screw diameter, the bigger is the depth of the screw flight in the metering section, which increases plasticizing capacity. Often times, especially when simultaneous action is not used, plasticizing capacity is the bottle neck that limits cycle time from getting shorter. Selecting a screw diameter bigger than the shot weight need would help. Top
 Injection molding machines dedicated to making preforms 

Manufacturers of injection molding machines dedicated to making preforms include well-known names like Husky (Canada), Netstal (Switzerland) and Krauss-Maffei (Germany). Recently, Tat Ming (Hong Kong) has joined in.
Such injection molding machines have a high shot weight, their screws have a high length to diameter ratio, low rotational speed, use the barrier design or pre-plasticizer design.

The clamping unit has a large clearance between tiebars, big mould opening stroke, big maximum mould height and big ejection stroke. The clamping unit is totally enclosed to contain dry air.

Avariable displacement pump could save energy during the long holding time.

There are five models in the Tat Ming PET series, covering a shot weight from 356g to 1880g. The series could handle 1-litre mineral water bottle preforms from 16 to 48 cavities. Top

   Injection molding of bottle caps  
Injection molding of bottle caps is distinctly different from that of preforms in that it is thin-wall molding.

Bottle caps are made of PP, LDPE or HDPE which are all non-hygroscopic. Even a hopper dryer is not necessary which simplifies the auxiliary equipment a lot

Manufacturers of bottle caps materials include Basell (formed by the merger of BASF and Shell), Borealis and ExxonMobil Chemical companies.
Basell 6331 is widely used in making mineral water bottle caps. Basell SB912 is used in carbonated drink bottle caps.

Borealis BD4560MO has good impact strength at low temperature and is therefore suited to carbonated drink bottle caps. Their HG313MO is suited to making caps with integrated hinge. All the above are PP material.
ExxonMobil Chemical has HD6801 YN and HMA 016 which are HDPE material for bottle caps.

Thin wall molding needs a high plasticizing capacity but a small shot weight, which could only be satisfied with a screw of large diameter. Barrier screw and pre-plasticizing design helps to reduce cycle time.

As bottle caps are low cost items, multiple cavities is used to increase efficiency which in turn requires high clearance between tiebars. The above two points are common to PET preform molding.
Tat Ming's CAP series is dedicated to making bottle caps. It has high mould closing and opening speeds, high ejection speed, large clearance between tiebars, large screw diameter and semi-closed loop proportional valve design.

The models in the series are CAP75, CAP110, CAP150 and CAP250 covering 16-, 24-, 32- and 48-cavity 28mm diameter bottle cap moulds.
Using a hot plate design and chilled water cooling, cycle time could reach 7s. The productivity of a 24-cavity mould is close to 0.3 million 28mm diameter caps per day (24-hour). Top