What is Blow Molding?
We go into more detail on this in Chapter 3 where we compare the various types of blow molding.
But, just to break you in gently…
Blow molding is a process used in manufacturing where hollow parts are formed and can be joined together.
It is usually used for plastic, but glass can be used too, and the most common application of this is to mold bottles.
You’ll start the plastic blow molding process by melting down the plastic and forming it into either a parison or a preform depending on which method you’re opting for.
A parison is essentially a balloon made from the blown air inside the hot plastic.
This process involves blowing heated air into the balloon while inside a mold, until the parison’s shape matches that of the mold.
A preform differs slightly in that the mold is simply injected with the plastic and sets to the shape of the preform.
The History of Blow Molding
Walk down the aisles of any grocery store, and you’ll see just how much blow molding has evolved.
The soda aisle alone would blow your mind if you didn’t have the custom blow molding technology we now have to produce the volume of bottles we currently consume!
It’s hard to imagine a time where we didn’t have such a scale of blow molding in our day to day lives, but let’s take a look at where it all started…
The process of blow molding originates from the idea of glassblowing.
Enoch Ferngren and William Kopitke were the first to produce a blow molding machine which was sold to the Hartford Empire Company in 1938.
This is where the beginning of the commercial blow molding process began.
As a slight digression… but bear with us as this really shows how introducing machinery to the commercial production impacted on the industry as a whole…
Back in 1904, there were only 155 independent glass container companies in America. These had reduced to only 100 by 1923 at the time when the glass blowing machine was sold to the Hartford Empire Company.
There were only 40 of these companies left by 1937, of which 37 were licensees of the Hartford Empire Company, but the total production of the remaining 3 companies produced less than 4% of all of the glass containers produced.
So you can clearly see just how much the commercialization of the process completely overhauled the process and that having the right tools gave the Hartford Empire Company virtual ownership of the entire industry!
Unsurprisingly, if you search the internet, you will see just how protective the Hartford Empire Company were over their process too.
There is a multitude of cases where they have sued many organizations for the use of the highly patented processes including the United States!
Looks like they wanted to keep their monopoly but luckily for us, the process managed to evolve anyway!
So, back to that blow molding…
Despite the new technology, blow molding still got off to quite a slow start, so during the 1940s the selection and number of products were still very limited.
When the variety and production rates started to build, blow molding took off and the number of products being created soon followed.
The mechanics of the process of producing a hollow bodied piece were established early on using glass.
As it was so breakable though, the introduction of low and high-density polyethylene which was so suitable for blow molding and consumer packages, soon started to see it replacing glass.
The very first plastic bottles being commercially produced were in America in 1939.
Germany followed suit and is currently one of the leading manufacturers of blow molding machinery.
Back to that soda aisle though – this next part is pretty mind-blowing – no pun intended…
The plastic containers being produced by the United States soft drink industry went from almost zero in 1977 to ten billion pieces in 1999.
It is estimated that half a trillion plastic bottles will be sold in 2020.
We know it isn’t just soda bottles it’s used for, but those are some crazy figures!
Different Types of Blow Molding
So now you have a bit of background (and scale!) on the industry, let’s look at which types of blow molding there are.
There are numerous types of blow molding, but the three methods most commonly used are:
- Extrusion blow molding
- Injection blow molding
- Injection stretch blow molding
Their differences lie mainly in the method used to form the finished product, and there are distinct advantages and disadvantages to each method.
We look at these in more detail in Chapter 7.
Extrusion Blow Molding
The extrusion blow molding process uses the basic steps found in glass blowing.
In short, with extrusion blow molding, the extrusion blow molding machine will take a plastic tube and fill it with air then heat it.
As we know, the heated air expands so this will inflate the plastic just like blowing a balloon up.
This plastic balloon you’ve just created is called a ‘Parison.’
By clamping a mold around your parison while it’s inflating, the expanding plastic fills the mold and allows you to create your desired shape.
This blow molding process was designed to manufacture high volume, one-piece hollow objects such as bottles.
Extrusion Blow Molding is a very cost-effective way of producing very uniformly shaped thin-walled containers.
If you were looking for more detail though…
The process of extrusion blow molding is where an extruder will use a rotating screw to force molten plastic through a die head.
This then forms the shape of a parison around a blow pin which is in the middle of the parison blowing heated air into the parison to inflate it.
There are also two types of extrusion used in this method – Continuous and Intermittent.
With continuous extrusion, the parison is continuously extruded while two halves of the mold move to and away from the parison.
This will usually be taking place vertically with two halves of the mold open while the plastic forms the parison.
Once the parison starts growing, the two mold halves close on the parison and blow pin. Pressurized air is then applied to inflate the parison to the shape of the mold – pressures are typically 25 to 150 psi.
With intermittent extrusion, a chamber is used to accumulate molten plastic which the extruder then forces through the die to form the parison.
In this process, the molds are usually stationary underneath or around the extruder.
Once sufficiently cooled, the plastic part can be removed from the mold.
In terms of which machines would be used for continuous extrusion molding, you would use Continuous Extrusion Shuttle machines and Rotary Wheel machines.
If you are using intermittent extrusion molding, you’d look for Reciprocating Screw or Accumulator Head machines.
The most common hollow parts made using extrusion blow molding include bottles, appliance components, industrial parts, automotive parts, industrial packaging, and toys.
This is because it is a relatively low-cost process which admittedly varies greatly depending upon the size of the machine used and its associated costs.
The cost and volume of the plastic shot used to produce your extrusion blow molded will also have a significant impact as your production numbers increase.
Injection Blow Molding
This process differs in that the hollow parison is formed by injecting molten plastic around a shaped core inside a mold. This is called a preform.
Injection molding can produce very high volumes of product once you’re up and running, but it is more labor intensive as it requires a great deal of initial engineering to develop the detailed tooling or molds.
This mold creation takes some skill as they are usually crafted from stainless steel or aluminum which can withstand the high temperatures of the molten polymers used under extreme pressure.
This a very front-loaded process – injection blow molding is well suited to high-volume orders and mass production on projects requiring hundreds of thousands or even millions of the same part.
The reason for the need for perfection at the start is that the injection molds must have a highly accurate match between the two mold halves.
This will allow material flow to be perfectly controlled so creating the mold is crucial to building a perfect, precision part.
It’s not until the production mold is perfected and finalized that the machine can be prepared for manufacturing to begin.
The injection blow molding process is divided into three steps:
An injection blow molding machine incorporates an extruder barrel and screw assembly which melt the plastic.
The molten plastic is then fed into a hot runner manifold where it can follow on to be injected through nozzles into the heated cavity mold and core pin.
The cavity mold dictates the external shape and is clamped around a core rod which forms the internal shape of the preform.
So unlike extrusion blow molding where the inflation itself is what creates the shape, with injection blow molding, it’s the gap between the core and the mold which creates the shape.
With extrusion blow molding you start with a parison which will be blown to shape.
With injection blow molding you start with a preform.
This consists of a fully formed bottle or jar ‘neck’ with a thick tube of polymer attached which, once melted and formed will create the body.
The preform looks very similar to a test tube with a threaded neck.
With the injection blow molding process, the preform mold opens, and the core rod is rotated and clamped in place inside the hollow, chilled blow mold.
The end of the core rod then opens and blows compressed air into the preform to inflate it to form the shape of the finished article.
Once cool enough, the blow mold opens, and the core rod rotates to the ejection position.
The finished article can then be stripped off the core rod – they are sometimes leak-tested at this point before packing.
An initial run will usually be thoroughly examined by manufacturers for any part defects of any sort as you’ll want to identify any flaws before you’ve created thousands of defective products.
If everything is perfect, then full production can begin, and random quality checks should also take place on a regular basis.
These checks should be for strength, color correctness, and any commonly occurring defects which can occur like flash or warping.
An advantage of injection blow molding is that the preform and blow mold can have many cavities to create additional shapes to the product.
Between three and sixteen cavities can be used depending on the article size and the required volume.
As there are three sets of core rods, this also allows concurrent preform injection, blow molding and ejection to take place.
Due to the processes within this method, injection blow molding is capable of producing more complicated parts with much more accuracy than with extrusion blow molding.
It isn’t as widely used though as it has a lower production rate.
Injection blow molding tends to be used for smaller more complex bottles such as those in the pharmaceutical industry, pharmaceutical bottles, medical parts, and cosmetic product packages.
Injection Stretch Blow Molding
njection stretch blow molding is very similar to injection blow molding except for an additional step to stretch the warm plastic into shape.
Just to complicate things further, Injection Stretch Blow Molding has two main methods…
Single-Stage Injection Stretch Blow Molding
Two-Stage Injection Stretch Blow Molding
With the single-stage process, both the preform manufacturing and the actual bottle blowing are performed in the same machine.
Single-Stage Injection Stretch Blow Molding
The single-stage process is broken down into 3-station and 4-station machines.
The older 4-station method of injection of reheat, stretch, blow, and ejection is more costly than the 3-station machine.
This is because the 3-station machine uses the latent heat in the preform to eliminate the reheat stage.
The energy saved here plus reduced tooling needs can shave around 25% off your production costs.
This process and the equipment used can handle small to high volumes of various shape and size bottles.
Two-Stage Injection Stretch Blow Molding
In the two-stage injection stretch blow molding process, the plastic is first molded into a preform using the injection molding process.
These preforms are produced including the necks of the bottles with the threads at one end.
The preforms are then positioned inside a reheat stretch blow molding machine.
This is where they have heat applied, usually with infrared heaters, to heat them above their glass transition temperature.
They will then have high-pressure air blown into them using metal blow molds.
Using the combination of the heat and the blown air, the core rod can then stretch the preform to the desired shape.
The Two Step process is well suited to very high volumes of containers of 1 liter and under.
The very conservative use of resin in this process also provides great strength and gas barrier.
Blow Molding vs. Injection Molding
The type of process you opt for will be determined by various factors such as cost, production volumes, materials being used and ultimately, what your final product is for.
We’ve looked at the processes and skimmed over their strengths, but let’s take a look in more detail at the merits and pitfalls of injection molding vs. blow molding …
Advantages of Extrusion Blow Molding
- Blow molding offers several distinct advantages including
- The costs in blow molding are lower compared to injection molding
- Machinery costs are typically lower
- Faster production rates due to the one-piece construction so there’s no need to connect part halves
- Can achieve shapes which injection molding cannot produce such as handles
- Able to mold complex products
The disadvantages of blow molding are:
- Only able to mold hollow parts
- Low final product strength
- Barrier properties are increased via multilayer parisons of different materials which stops the product being recyclable
- To make wide neck jars, spin trimming is necessary.
Blow molds tend to have more design freedom because each mold half forms its own wall shape.
Creating the mold is important, but other variables need to be considered too such as wall thinning, air leaks, flash, and streaks which must be monitored.
For instance, the variation in the wall thickness of your product is often an essential factor to be considered by product designers.
Quality control is a fundamental part of the process which adds to costs considerably but cannot be bypassed.
This is a highly competitive industry, and suppliers of products need to qualified and trained to measure, monitor, and improve each aspect of the product for their clients.
We’ve looked at the ins and outs of extrusion blow molding, so let’s look at injection blow molding now…
Advantages of Injection Blow Molding
- Parts come out with very accurate finished dimensions
- Extremely tight tolerances can be held
- No extra material is needed in the formation, so it is highly efficient.
- It produces an injection molded neck for accuracy
- Efficient material use and low scrap rates
- Flexibility in being able to change the type of material or color that is being used.
- Detailed, highly engineered tooling with multi-cavity mold options
Disadvantages of Injection Blow Molding.
- As it is difficult to control the base center during blowing, it’s only suitable for small capacity bottles
- The material is not biaxially stretched so no increase in barrier strength
- Handles cannot be incorporated.
That’s quite a list of pros vs. the cons!
So last, but most definitely not least, let’s delve into injection stretch blow molding…
Advantages of Injection Stretch Blow Molding
- Stronger final product due to using PET and PP polymers
- Lighter products due to increased strengths
- Very high volumes are produced
- Very little restriction on bottle design
- Is suitable for cylindrical, rectangular or oval bottles
- Preforms can be sold as a completed item for a third party to blow.
The polymers most commonly used for injection stretch blow molding are PET and PP. These have physical characteristics which are enhanced when the stretching part of the process takes place.
Where this excels is that the stretching gives the product additional strength so a bottle of the same size and wall thickness under the other processes would be far weaker by comparison.
This process and the equipment used within it can handle small to high volumes of bottles of various shapes and sizes.
Disadvantages of Injection Stretch Blow Molding
- High capital cost for machinery and set up
Floor space required is high, although more compact systems are being introduced,.
- Only a champagne base can be made for carbonated bottles.
Materials Suitable for Blow Molding
Many polymers are suitable for the blow molding process, either in a single layer or multiple layer structures or combinations of both.
These include materials such as:
- High-Density Polyethylene – HDPE
- Low-density Polyethylene – LDPE
- Polypropylene – PP
- Polyethylene Terephthalate – PET
- Polyvinyl Chloride – PVC
- Ethylene vinyl alcohol – EVOH
- Ethylene Vinyl Acetate – EVA
- Thermoplastic elastomers – TPE
- Cyclo Olefin Polymers COP and COC
- Acrylonitrile Butadiene Styrene – ABS
Obviously, this isn’t an exhaustive list, but you can see it’s a useful process which lends itself to many modern-day polymers.
It’s also still suitable for glass of course, and some metals are making their way into the mix too, particularly amorphous metals.
Amorphous metals are also known as metallic glass or glassy metal. This is because despite being a solid metallic material (usually an alloy) they have a disordered atomic-scale structure.
Put more simply, most metals in their solid state are crystalline meaning they have a highly ordered arrangement of atoms.
Amorphous metals are non-crystalline having a glass-like structure so they are true glasses and will they soften and flow upon heating.
This makes them extremely easy to process in the same way as polymers and glass, such as in injection molding.
This versatility has resulted in amorphous alloys being commercialized for use in sports equipment, medical devices, and within electronic equipment too.
Some containers such as jars create an excess of material during the molding process – usually with extrusion blow molding.
This is usually trimmed off by spinning a knife around the container to cut the excess material away.
Spin Trimmers are used on several materials, such as PVC, HDPE, and PE+LDPE, but wherever possible, this plastic surplus is recycled to create new moldings.
Some materials are easier to trim than others due to their physical characteristics impacting the trimming process.
For instance, moldings produced from amorphous materials are far more difficult to trim than crystalline materials, so various materials need to be used to manage this.
Titanium coated blades are often used rather than standard steel now as this can increase the blade life by a factor of 30 times.
The good news is that some modern blow spinning machines are now managing to eliminate the trimming process which will bring down the overall production costs over a period of time.
So – the big question…
Did that help?
Hopefully, we have answered any questions you may have had on the differences between the blow molding methods.
As you should now know, there are clear advantages and disadvantages to all three types depending upon what you’re aiming to achieve.
If we’ve done our job correctly, you’ll have a much better idea of where to start – as well as know the history of where the blow molding itself started.
We’d love to hear if it helped or if there’s anything we can cover to help you out. Make sure you drop a comment below, and we’ll check out your musings when we’re not mining for more info…