What Is the Blow Molding Process in the Plastic Industry?
Blow molding is a type of manufacturing and production process that allows for the forming of hollow plastic parts. Air pressure is traditionally used to inflate a soft plastic into the mold cavity. While this technology originally derived from the glass industry, the blow molding process in the plastic sector competes in the recyclable and disposable market.
There are three main types of blow molding techniques: extrusion, injection stretch, and injection blow molding. The blow molding process can construct jars and containers such as PETE and PET containers and jars for packaging.
Blow molding is a critical industrial process for manufacturing any type of one-piece hollow plastic parts such as plastic piping, water cans, containers and bottles.
What is PET and PETE?
PET are plastics that you come in contact with throughout your everyday life. PET or PETE, known as Polyethylene Terephthalate, is best known as the clear plastic used for soda bottle and water containers. As a raw material it is globally recognized as a flexible, lightweight, strong, non-toxic, and safe material that is 100% recyclable. In fact, PET is the most widely accepted recycled plastic available in the entire world. Almost all US municipal, recycling programs accept PET type packaging. Checking the backs or bottom of containers for the resin identification code can easily identify PET type plastics. This symbol can be recognized by the #1 surrounded with “chasing arrows”.
Recycled PET, along with virgin PET, can be used for soda and water bottles, cake packaging, cosmetic clam shell packaging for toys, razors, cosmetics and other retail items. It can also be found in containers for salad dressing, shampoo, peanut butter, sleeping bags, clothing, carpeting, and so much more. PET is so prevalent in our modern day society that it’s important to recycle it for reuse when and wherever possible. Under the recycle code 1, PET recycling rates are growing faster than ever.
PET is polyester product that is generally crafted with the injection blow molding process into clear containers. While it is possible to use the blow molding extrusion method in PET plastics, it is less common because the resin requires a drying period that is extensive.
How does the blow molding process work?
The plastic blow molding manufacturing process is made up of two parts. The first part of this process begins with the creation of Parisons, or starting tubes of molten plastic. These Parisons are the base for extrusion blowing, regardless of the type of container or plastic part that is being manufactured. When the Parisons have been fabricated they are ready for the second part of the process – blow molding into the desired shape.
Note: If we were talking about injection or injection stretch blow molding, the first step of the blow molding process would be called Preform instead of Parison.
1. Extrusion Blow Molding (EBM)
- Parison Extrusion
As explained above, the extrusion blow molding manufacturing process begins with a parison. This process begins with a hollow tube (the parison) which hot air is blown into. This step inflates the tub into the hollow part, taking the shape of the mold cavity. Parts in the extrusion blow-molding sector can contain jars, containers, and plastic bottles. When the plastic has cooled, the mold is opened and the blow-molded piece is ejected. It is then replaced by a new parison for the process to be repeated.
The injection blow molding process begins with the molding of a polymer onto a core pin in a heated cavity. The cavity mold then forms the outer shape based off of the core rod, which shaped the inside of the preform. This preform is generally shaped as a bottle or jar neck with larger amounts of polymer attached to it. This polymer is what will eventually form into the jar body. When the preform mold is opened, the core rod brings the piece to the blow station. The core rod then opens up and compresses air into the preform. At the final equipment station, the part is blown, cooled, and ejected.
This manufacturing technique is generally used for smaller containers and hollow objects in larger quantities. However, this technique is the least commonly used by manufactures of the three techniques discussed here today.
3. Injection Stretch Blow Molding
This final blow molding process is widely used by PET material throughout the manufacturing process of hollowed plastic containers.
Similar to the blow molding injection technique, molten polymer flows on a hot runner block into the injection cavity to make the Preform. As explained in the injection process, this core pin is what produces the inner diameter, while the injection cavity works to shape the outside of the preform. Following the injection molding process, the piece is held by its neck and rotated 90 degrees. When the preform reaches the right temperature, it is air infused (blown), and stretched into its final shape.
In the blow-mold area, the molds close and the stretch-rod works stretches the part using two different levels of air pressure. When the part is cool enough it gets discharged. This method produces containers for all different types of consumer related industries.
IBM and EMB: What’s the Difference?
The primary difference between injection blow molding and extrusion blow molding is that extrusion involves the squeezing of plastic through the mold. This mold is actually called a die (in blow molding terms). This can be thought of like pasta dough extruding from a machine. During this process, you can adjust the shape, length, and thickness of the pasta. In other words, extrusion molding involves squeezing plastic into a die in order to make the molded part, whereas injection blow molding pushes the plastic into a mold, then ejects the part and inflates it with air once it has cooled.
Why use PET Plastics
As a whole, plastics make up 13% of municipal waste. However, it is estimated by the EPA that only 1% of all municipal waste can be attributed to PET containers. This 1% may seem small, but it is in fact a lost opportunity, to recycle and reuse those PET plastics for new plastic packaging and clothes (that can further be recycled and reused again!). US households, which use approximately 45 lbs of PET containers and plastic bottle per year could make a serious impact by choosing to recycle over other waste methods.
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