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Different Options for Blow Molding Techniques

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Any manufacturer that produces plastic bottles, or any other hollow plastic container, is familiar with their common production methods.  The most likely method is a technique known as blow molding, and there are several processes by which it is accomplished.

Even if a company is not directly responsible for producing containers, but instead purchases them from a distributor, it is helpful to understand the various techniques for creating them as it will determine what’s possible in terms of packaging design, production workflow and economy of manufacturing.

The three commonly used methods of blow molding are extrusion blow molding, injection blow molding and injection stretch blow molding.

What is blow molding?

Blow molding is a common manufacturing process that involves the production of hollow plastic parts.  One of the most prevalent uses of this technique is to create plastic containers, although many other hollow shapes are also possible.

All these methods have certain steps in common.  It starts by melting plastic resin and shaping it into a parison or preform.  A parison is defined as an extruded tube of molten plastic that allows compressed air to enter, expanding and cooling it.  A preform is a plastic tube, much like a test tube in apprearance, that is heated to an elastic temperature and internally pressurized.

The parison or preform gets enclosed in the two halves of the blow mold cavity, at which point it will be filled with air.  This pressure causes the plastic to expand against the mold, forcing it to take on the mold’s shape.  After the plastic has cooled, the mold opens and ejects the container or other part.  It is then ready for the next stage of the production process.

Blow molding dates to the late 1930’s, when Enoch Ferngren and William Kopitke took the basic principles of glass blowing and applied them to the first blow molding machine. Over the next decade, blow molding struggled to take hold at an industrial level, as there was not enough variety in the early machines and the production rates were too slow.

As retailers, especially in the food and beverage industry, sought a packaging alternative to glass, plastic became a very attractive option.  Once the process was optimized it took hold rapidly.  In 1977, there was not a single soft drink sold in a plastic container.  In 1999, more than ten billion such bottles were sold.

What is extrusion blow molding?

The first of the techniques, extrusion blow molding, involves the container material being melted and extruded into a hollow tube.  This tube is known as the parison. Next, it is enclosed in a metal mold, such as aluminum or stainless steel; air is then forced into the parison, causing the plastic within to take on the shape of the mold. At this point, the plastic is cooled with water flowing through lines machined into the mold for this purpose. Finally, the mold opens and the workpiece is ejected.

The advantages of extrusion blow molding start with the fact it is a fast production method, with relatively low tooling costs. It’s possible to create a wide variety of complex parts using this method, even with handles being incorporated into the design.

On the other hand, there are some key disadvantages.  It only works for hollow parts, and the finished packages tend to be of low strength.  If the jar or container requires a wide neck, it may be necessary to perform the additional process of spin trimming.

Some of the various parts and containers that can be made using extrusion blow molding include hollow polyethylene products, milk bottles, automotive ducts, watering cans, and industrial drums.

What is injection blow molding?

The next option is known as injection blow molding.  Of the three main methods, this is the least common, despite high production capability.

The process begins with an injection mold extruder barrel and screw assembly, which melts the polymer and injects it into the preform mold consisting of a cavity and core pin.

The preform opens so that the core rod can be rotated and clamped into the chilled blow mold.  Pressurized air is pushed through the core rod into the still heated preform, inflating the material to the desired shape.  The part is then cooled, and the mold is opened and rotated, where it is ejected.  Any given mold might have anywhere from three to sixteen cavities, depending on the size of the container and the required output.

While this method offers the benefit of an injection molded neck for accuracy, a major drawback is that can only be used with small capacity bottles without handles.  This is why it is commonly used for small medical and single serve bottles.

What is injection stretch blow molding?

Injection stretch blow molding can either be a single- or double-stage process.  Both involve the application of a rod to stretch the preforms’ length during the blow molding operation.  In the former, the preform molding and the blow forming happen in the same machine.  On a double-stage machine, these steps are separated, with the first stage relying on the injection molding process; the second stage involving cool preforms being fed into an infrared preheating stage inside a stretch blow molding machine.

The one stage process is used with low volume production whereas the double stage is more appropriate for high volume runs.  While the single stage is more limited in the range of shapes that can be produced, the double stage process allows for cylindrical, rectangular, and oval containers.

Aluminum and stainless steel are two of the best options when it comes to creating blow molds thanks to these materials’ many benefits.  To learn more about the procurement of blow molding materials, contact one of our friendly and knowledgeable customer service representatives today.

 

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