What are Blow Molds?
Defining Blow Molding
There are many types of processes in the world of plastics manufacturing and often the complexity and technical nature of these processes leaves them shrouded in mystery. While blow molding is somewhat of a self-explanatory title, the exact details of what it is, how it works, and what it is used for remain unclear. Here we will provide you with a comprehensive explanation that demystifies everything from what blow molding is, different types of blow molding ,How the process works, what type of products are created from blow molding, what industries it is utilized in, and advancements within the blow molding arena.
First lets start by defining blow molding in broad terms. The process known as blow molding or blow moulding can be defined as a manufacturing process utilized for the fabrication of hollow objects such as bottles , containers, or any other type of hollow item. Materials commonly used in blow molding include plastic, glass, and in some cases metal. If you have ever drank from a bottle of any kind then odds are you have already seen and experienced a blow molded product first hand.
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As mentioned, blow molding is utilized to create hollow objects. But how exactly do blow molders produce these items? Well first its important to mention that there are different types of blow molding techniques designed to serve certain applications all with different capabilities. The the two main techniques are Extrusion Blow Molding and Injection Blow Molding. However, there is also a third type know as Stretch Blow Molding which is a essentially a combination of the two main types of blow molding. It is also important to mention that there are different methods that fall under the two most common blow molding types such as continuous extrusion blow molding which falls under the extrusion blow molding category. Most variants have to do with the difference in the handling of the parison, molds, or other features. Now that we've given a general introduction lets take a more in-depth look at the different processes.
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Extrusion Blow Molding
Extrusion Blow Molding (EBM) is the more commonly used technique of the three, in fact when people mention "blow molding" they are almost always referring to EBM. This particular process involves heating up a plastic or other material and forming it into a parison. If you are not familiar with what a parison is it is essentially a small tube that is made up of the material that will produce the end product. Usually the dimensions of the parison will determine the thickness of the hollow item. Once the parison has been formed the semi-molten item is then placed inside a mold and is held by a split die on both ends. The mold seals the parison's bottom end and the top is hooked up to an air supply that forces compressed air into the semi molten parison located inside the mold. The air essentially forces the parison to take the shape of the mold by blowing it up until it fills the entire shape of the mold. When the semi molten material makes contact with the mold the heat is transferred through the mold to the cooling lines which then cool the molded product and allow it to become rigid. Once everything has sufficiently cooled the mold is opened and the blow molded product is ejected and any excess flashing is trimmed. |
Extrusion Blow Molding
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Continuous Extrusion Blow Molding
With this extrusion blow molding variant the extrusion machine continuously extrudes the parison to a sufficient length. The Parison is then cut by the closing mold halves. The mold halves can be modified into different variations depending on the mounting and movement of the mold. One variation is the rotating wheel technique in which the molds are on the edge of a rotating wheel. Here the the molds are used constantly as they are rotated on the wheel. This type of configuration is ideal for high volume production. In this case different parts of the rotating wheel are used for different purposes in the process such as inflating, cooling, and other tasks. In addition to rotary wheel equipment other commonly used equipment includes shuttle machinery. One down side to continuous extrusion blow molding is that calibrating the wall thickness is difficult due to to the fact that the weight of the parison is what drags the parison during extrusion. Nevertheless this type of EBM is ideal for small or medium sized products and EBM also promotes faster cycle times which prevents degradation and unwanted curing of the material. This aspect makes this process a go to for the blow molding of heat sensitive plastics such as PVC.
Intermittent Extrusion Blow Molding
One difference between continuous extrusion blow molding and intermittent extrusion blow molding (IEBM) is the use of equipment. in IEBM accumulator head machinery and reciprocating screw machinery is commonly used. in this process extrusion of the parision begins only when the extruder head or accumulator is adequately filled with enough molten material mass. As mentioned, either accumulator head equipment or reciprocating screw equipment is used to extrude the parison. The reciprocating screw method works by retracting the screw as the molten material collects in the extruder head. Once there is sufficient melt in the extruder head the screw starts rotating and compressing the melt which causes the parison to be extruded. The accumulator type on the other hand works by collecting the molten material inside the accumulator head and pushing it through using a ram to extrude the parison. IEBM is mostly used to create larger objects because of its independent rates of accumulation and extrusion, which allows for faster extrusion of larger parisons.
Co-Extrusion Blow Molding
When looking to create a more complex product with layers of different materials coextrusion blow molding (CEBM) is the go to process. In this case a coextruison die head is used in place of an accumulator head and instead of using only one type of plastic it is often the case that the the coextrusion process uses multiple plastics with different properties and all layered in a specific manner. When the multi layered molten material is ready it is pushed through the coextrusion head using multiple rams forcing the material through a die with several die cores. From here on, the parision is then processed through the typical blow molding process including attaching it to a compressed air supply, molding it, Etc.
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Injection Blow Molding
Injection Blow Molding (IBM) is the second most commonly used blow molding process. it utilizes a combination of injection molding and blow molding for the forming of the preform. Usually a plastifier and a reciprocating screw extruder are used to make the preform. Here preform molds with metal rods enclosed in the center are used on a rotating table with different sections. Usually the metal rods are mounted to the rotating table with different sections corresponding to the various stages of the blow molding cycle. Typically, the blow molding cycle consists of three steps and thus there are usually three sections on the rotating table corresponding to each step. The first step consists of the injection of the molten material into the preform. Then in the second stage the preform is transferred to a different set of molds that are of the profile that will constitute the final product and then they are blown and molded into the appropriate shape. Then the last stage involves cooling the product until it is rigid and then ejecting it form the mold. some injection molding machines have additional phases for coating, preform conditioning, and other cooling and heating cycles. |
Injection Blow Molding
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Stretch Blow Molding/Injection Stretch Blow Molding
Stretch Blow Molding(SBM) also known as injection Stretch Blow Molding (ISBM) is a essentially a modified version of the injection blow molding process. This method consists of all three stages in the injection blow molding (IBM) process with an additional step in the blow molding phase. The differing characteristic between injection blow molding and stretch blow molding is that in SBM the preform is stretched to a specific length using a stretch rod. This extra step takes place during the blow molding phase and usually it takes place immediately after the preform is transferred to the final mold but before the product is blown up to fit the mold. Of course once the material has been appropriately stretched and molded it is then cooled and ejected as usual. The main purpose of stretch blow molding is to create plastics that are stretched along two dimensions longitudinally and horizontally. This type of stretched plastic is known as biaxially oriented plastic and it is praised for its optimized tensile strength, improved barrier properties, excellent transparency, and various other attributes. However only a select few plastics can be successfully biaxially oriented these include PP, PET, and PVC. |
Stretch Blow Molding
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Where is Blow Molding Used?
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Plastics are versatile materials and as a result they are used in several applications throughout various industries. Consequently, blow molding being one of the most commonly used plastic processing methods is also employed widely in almost every industry. Industries where blow molding is utilized include:
- Medical and Healthcare
- Automotive
- Electronics
- Consumer Packaged Goods
- Food and beverage
- Sporting goods
- Outdoor equipment
- Office furniture and equipment
- Home
- Plastic Piping
- Plumbing
- Cosmetics
Products Made Using Blow Molding
Given that blow molding is widely used throughout almost every industry. It stands to reason that blow molding would be used to create various different products. Products produced using blow molding include but are not limited to:
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Technological Advancements in Blow Molding
The blow molding arena has evolved over the years and today new emerging technologies are advancing the sector at a rapid pace. New technologies such as Automation, Internet of things (IoT), and Artificial Intelligence (AI) are making their way into the blow molding arena. Automation is perhaps the most commonly used technology in the plastics processing sector having already been employed by the majority of firms for the past several years. IoT and A.I. have not yet reached widespread adoption and the technology is still in its primitive stages, however, these two technologies are expected to make a massive impact in the sector in the future.
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