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7 Tips To Master PET BLOW MOLDING? Without Breaking A Sweat

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7 Tips To Master PET BLOW MOLDING.Without Breaking A Sweat

Are you looking to manufacture drinks bottles but not sure what type of plastic best meets your needs?

Perhaps you’ve been asked to supply Polyethylene terephthalate (PET) drinks bottles instead of Polypropylene (PP), and you want to find out how to adapt your business?

This article will provide everything you need to make an informed decision about PET bottle blow molding machines.

Firstly, we must acknowledge what the options are for machinery, with three main types on the market:

  • Single Stage Injection Blow Molding
  • Two-Stage Injection Blow Molding
  • Extrusion Blow Molding

The exact type of machinery that your bottle manufacturing requires is dependent on output quantity, available space, the contents of the bottle and desired shape/size.


What exactly is blow molding?

It is a manufacturing process typically used for creating plastic bottles; however, it can also be used to produce oil cans and larger plastic containers.

Blow molding is also used in the mass production of glass bottles and jars.

For this article, we will focus on PET plastic blow molding.

PET starts by being inserted into the machines via a preform or parison; these can be made within the same factory or purchased separately.

The PET plastic will need to be heated up and formed into the desired size, shape, and weight if manufacturing them in-house.

This is calculated in advance, so you end up with the exact bottle you need.

Your preforms or parisons then get heated up to make them workable before being blown into the desired size and shape.

The hot pressurised air is blown into the plastic preform or parison to produce the bottles.

If you have a stretch blow molding machine, there will be one more step here to shape the bottle, after which the finished bottles are allowed to cool and then ejected so they can move to the next step of your setup.

According to Habits Of Waste, “481.6 BILLION plastic bottles were used worldwide in a single year. That’s 40 billion per month and 1.3 billion per year.”

With numbers like this, the plastic bottle industry is only poised to continue growing in the years ahead as the uptake increases in countries that still rely on glass over plastic.

Since most beverage bottles are manufactured from PET plastic, the demand for reliable, efficient machinery will also grow.


What Are The Processes Involved In Blow Molding?

As with all manufacturing processes, some steps need to be followed systematically, ensuring a consistent product and quality for your customers.

Whether you are using a single-stage or two-stage blow molding machine for producing bottles, the following steps need to happen:

  • Thorough drying of PET materials and dehumidification
  • Heating of the PET material to correct temperature where it is workable
  • Injection of liquid PET into a preform mold specific to the bottle shape and size
  • Cooling of the preform, so it holds shape
  • Transfer of the preform
  • Shaping of the bottle
  • Cooling of the finished product
  • Remove the cured and shaped, cooled bottle from the equipment

There are, of course, some variations of the exact steps depending on whether you’re operating a single-stage or two-stage production line.

Key differences here are:

  • If you need a parison or preform, for PET bottle blow molding, you will require preforms.
  • Stretch molding of the bottle shape vs blow molding
  • Whether the preform is allowed to cool after being injected, then reheated to temperature once ready to form or if the preform is kept sufficiently hot so that the reheating aspect is not needed.
  • If the preform is run through a finishing machine before being turned into a bottle

To discover more about the specific details & science of PET plastics, this website can prove useful.

Preforms vs Parisons

At face value, these may sound very similar and can be easily mistaken for one another; of course, we’ll explain some key differences below.

PET plastic can be used from either form; the choice of which suits you best will depend on the machinery used and desired bottle products.

Preforms for bottle blow molding

These are produced using plastic injection molding; the raw PET plastic pellets and heating them to adequate temperature can flow like a viscous liquid.

Preform molds are contained within the machine where the plastic is injected under pressure, incorporating the bottleneck and thread as specified.

Once cooled sufficiently, these PET preforms can be stored until needed, when they’ll be reheated and stretch blow molded to their final shape and size.

Working in this manner means you can produce the preforms yourself in-house and then store them until needed. It may be more cost-effective to purchase the preforms from another manufacturer.

Modern advances in the production methods of preforms have improved accuracy, and their shipping is relatively efficient as they’re compact and lightweight.

Filling sites tend to favour doing things in this way since they don’t have to acquire the injection molding apparatus themselves or have skilled operators run it and check tolerances.

If your bottling plant relies on injection blow molding or injection stretch blow molding, you’ll need preforms of PET.

Parisons for bottle blow molding

This style of plastic molding relies on creating a hollow tube mold of the plastic, similar to the preforms but is done so for extrusion blow molding.

Output for this method would commonly be plastic containers, drums, jars and complex shapes, usually higher tensile strength, thicker walls and solid colours instead of transparent finishes.

Materials that are used most often would be HDPE, LDPE or PVC.

Due to the desired properties of PET plastic and its use for beverage bottles, injection blow molding and stretch injection blow molding methods are favoured.

For more information about blow molding, check [this] out or visual references and an alternative site.


Single Stage Blow Molding

This type of injection blow molding refers to the machine being a single-stage; of course, several processes need to occur to produce bottles within this.

These processes are carried out by the same machine; it is essentially a production line housed within one piece of equipment, taking care of three or four separate steps.

Each of these stations is automated so the equipment can run continuously without human input; this can help increase your overall productivity within the factory and then dispense the finished bottles straight into the next step of your production line.

By reducing the number of machines, you can save valuable floor space and run a more compact operation overall.

This style of blow molding machine tends to be the most cost-effective solution, mainly when producing quantities of containers and bottles for widespread use. Still, it does come with higher initial capital investment to acquire the equipment.

On the plus side, you’ll be saving money on your electricity bill as the preforms are not allowed to cool, removing the step of having to reheat them in between.


Two-Stage Blow Molding

The two-stage blow molding process will require the use of preforms; these are PET plastic casts that have the neck and thread of the bottle already made.

Your machine will take the PET preform and reheat it to a temperature when it becomes workable before being blown to size and shape with pressurised air, giving you the bottle that you need.

Once cool enough, these finished products can move to the next stage of your bottling operation, to be quality checked, filled with liquid, and then labelled accordingly.

This method allows bottles of up to 1 litre to produce a sound production output with consistently high quality.


Two-Stage Stretch Blow Molding

Also known as ISBM (Injection Stretch Blow Molding), this method is favoured by those wishing to produce very high volumes of bottles, popular for PET usage.

After the preforms are heated up to temperature, they are blown to size with hot, food-grade air, as with the standard two-stage blow molding; however, this method has one more critical step.

Using a metal core rod, the soft plastic is stretched into the desired shape before cooling.

Stretching the PET plastic in this way results in optimum strength; this is how the heated polymers respond, creating a more tightly woven molecular structure.

Also, improving gas permeability in the process is two of the main reasons carbonated beverages are commonly filled into this type of PET bottle. They do a fantastic job of keeping the pressurised contents intact and retaining the fizz when end users come to enjoy their drink.

Bottle sizes up to 2 litres can be produced using this method with high accuracy.


Why Use PET For Bottle Blow Molding?

Polyethylene Terephthalate (PET) is a commonly used thermosetting plastic for drinks and beverage bottles for the following reasons:

  • Strong.
  • Lightweight.
  • Shatterproof.
  • Glossy and smooth finish.
  • Doesn’t leach chemicals such as BPA or phthalates into the liquids contained.
  • Transparency of the material.
  • Won’t degrade or break down due to the beverage contents contained inside.
  • The cost to manufacture is relatively low.
  • Can be transported efficiently.
  • Recyclable.
  • Inert.
  • Impressively low gas permeability properties so contain carbonated drinks well compared to other plastics.
  • It is safe and has been approved for use worldwide by health and safety organisations such as the FDA in the US, Food Standards Agency in the UK or TGA in Australia.
  • Preforms can be ordered to meet the exact specification or manufactured in bulk then stored until needed without issues.
  • You can create an identifiable, brand-specific bottle shape that stands out against your competitors.

As you can see, these are very compelling reasons to use PET over other compounds for beverage packaging.

When wanting to select the correct type of PET for your bottle manufacturing, it is helpful to know there are two types, which are:

  • APET (Amorphous Polyethylene Terephthalate)
  • CPET (Crystalline Polyethylene Tetraphthaltate)

Since you’re interested in bottling production, the type of PET used will be APET; this has the most suitable properties for the application.


What Are The Drawbacks For PET Use?

As much as we may rely on the widespread use of PET for beverage applications, like any material, it does come with some downsides or disadvantages; when deciding which route to explore for your bottling plant, it is advisable to be aware of.

Here is our list of perceived downsides for PET use in bottling:

  • Cannot pasteurise within the bottles as the excess heat will cause them to warp and disfigure, a specially designed PET can tolerate a temperature of around 75°C
  • The PET bottles can absorb a small amount of flavouring, resulting in slightly more needed.
  • Acetaldehyde produced during bottle production can impair the taste of the contents, especially noticeable when water is being used to fill the containers; it may be only slight but can be detectable.
  • Inefficient to recycle
  • Low melt temperature can cause problems with bottles or preforms if subjected to high temperature in storage, transit or otherwise.
  • Complex shapes are not quickly produced if possible vs extrusion blow molding and other plastics.
  • It needs to be manufactured using the injection stretch blow molding technique for the best bottles to be produced
  • Low tolerance and a high level of accuracy are needed, which is not necessarily a bad thing but makes the manufacturing process less forgiving and smaller margin for errors.

Here at bottling, we supply iBottling equipment to meet your bottling plant needs; our two-stage stretch blow molding machine can help ensure consistent and reliable output of up to 15,000 bottles per hour.

These high-quality bottles produced are up to 2 litres in size, and the blow molding machine is capable of working up to 8 cavities around the clock, making your operation more productive.

To find out more information about this, or any other machinery we produce, simply submit your request by below form, or send an email to [email protected] or head to this page to discover what we can help you with.

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