In recent years, glass bottles have been replaced by plastic for such products as milk, soft drinks, bottled water and most chemical products. These plastic bottles are produced on machines called blow molders. Blow molding essentially uses glass blowing principles and applies them to plastic resin. A small tube of molten plastic called a preform or “parison” is created and placed inside a mold. The interior of the mold is the size and shape of the bottle to be produced. Air is then blown into the hollow core of the preform causing it to expand out to fill the void in the mold, thereby creating a bottle.
The first plastic blow molding machine was developed in 1938 although there was not much use for the process until many years later. The development of high-density polyethylene (HDPE) in the 1950’s created a material suitable for many uses but the development of polyethylene terephthalate (PET) in the 1970’s led to the real explosion of blow molded products. In 1977, the first blow molder that could produce PET bottles in one step was developed. Since 1977, the number PET soft drink bottles has grown into the billions.
Some of the leading producers of blow molding machines are Bekum, Uniloy, Cincinnati Milacron, Hayssen an Rocheleau
Polyethylene bags and film are produced by a method called blown film extrusion. It is like blow molding except there is no mold. Polyethylene pellets are melted and extruded through a circular die into a tube shape. Air pressure is applied in the middle of the tube causing the plastic to expand and stretch until the walls are a thin film, somewhat like blowing up a baloon. To produce bags, they simply seal it crosswise at whatever length of bag they are making. To produce film, they slit it lenghtwise along the edges so that the tube becomes two sheets.
As with blow molding, the blown film industry has become huge. There are approximately 2500 blown film extrusion lines in the United States and, together, they produce about 4 billion lbs of blown film annually.
Some of the leading producers of blown film extrusion equipment are Hosokawa Alpine, Macro Extrusion, Davis Standard, Gloucester Engineering Co and Reifenhäuser
Injection molding is used to create solid plastic parts. These parts can be complex in shape and can be very small or very large. Injection molding machines consist of an injection system and a clamping system. Plastic pellets are fed into a heated injection barrel with an internal screw and they melt as they are pushed forward. When the melted plastic gets to the end of the barrel, a piston pushes it into the mold. The injection pressure can be as high as 30,000 psi and the temperature of the molten plastic is between 300 and 600 F. The plastic part solidifies as it cools in the mold and is then unclamped and removed. Many varieties of polymers can be used in injection molders.
Some of the leading producers of injection molding machines are Cincinnati Milacron, Van Dorn, Engel and Sumitomo