Blow Molding vs. Injection Molding: Differences and ...

Injection molding machines work by injecting molten thermoplastic into a two-part mold. The first injection molding machine was developed in by John Wesley Hyatt. This machine was used to mass produce items like buttons. The first screw injection molding machine, which is what modern machines are based on, was built in by James Watson Hendry. 

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The process works by melting plastic resin pellets inside a barrel. The barrel contains a screw with a shaft whose diameter increases along its length. As the screw rotates, the plastic is forced into an incrementally smaller area. This compression is the primary mechanism responsible for melting the plastic. The barrel is also heated to further aid the melting process. Once enough plastic to fill the mold has melted, the screw retracts. The screw is then pushed forward to force the plastic into the mold at very high pressures. 

Injection molds are made from at least two parts. They are held together with hydraulic rams so that the high-pressure plastic injected into the mold cannot escape from the parting line. The mold has cooling channels that cool the part enough so that it can be ejected from the mold without damaging it. 

Injection molding can achieve high production volumes unmatched by any other plastic processing technique. It can easily produce millions of parts per machine annually. Injection molding can create complex parts. It is used for everything from gears to garden chairs. Injection molding is better than blow molding for the high-volume production of complex plastic parts. To learn more, see our guide on the Molding Manufacturing Process.

A typical injection molding setup is shown in Figure 2:

Advantages of blow moulding

• Low tool and die cost
• Fast production rates
• Ability to mould complex parts
• Handles can be incorporated in the design

Disadvantages of blow moulding

• Limited to hollow parts
• Low strength
• To increase barrier properties, multilayer parisons of different materials are used (thus not recyclable)
• Trimming is necessary to make wide neck jars spin
• Limited to thermoplastics (rotational moulding can be used with thermosets)

Advantages of rotational moulding

• Though used principally for thermoplastic polymers, applications for thermosets and elastomers are becoming more common
• Parts range in size from small plastic bottles of only 5ml to large storage drums of 38,000 litres capacity
• Rotomoulding tends to favour more complex external geometries, larger parts, and lower production quantities than blow moulding
• Compared to injection or blow moulding, moulds are simple and inexpensive, as rotational moulding is often performed on a multi-cavity indexing machine

Disadvantages of rotational moulding

• Blow moulding is more suited to the mass production of small disposable containers
• Despite cheaper moulds, the cost of rotational-moulded parts is generally higher than both blow-moulded and injection-moulded parts because volumes are lower and cycle times higher
• Production cycle is much longer, lasting perhaps 10 minutes or more

Blow moulding basics

The process begins with melting down the plastic and forming it into a parison or, in the case of injection and injection stretch blow moulding (ISB), a preform. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass.

The parison is then clamped into a mould and air is blown into it. The air pressure then pushes the plastic out to match the mould. Once the plastic has cooled and hardened, the mould opens up and the part is ejected.

Blow moulding is most commonly used to make containers, packaging (such as soda bottles), or industrial applications (like fuel tanks). There are multiple variants, detailed below.

Extrusion blow moulding

Extrusion blow moulding can be used to process many different polymers including polyethylene, polyvinyl chloride, polypropylene and more. The process begins with the conventional downward extrusion of a tube. When the tube reaches the desired length, the mould is closed, catching and holding the neck end open and pinching the bottom end closed. Then a blow-pin is inserted into the neck end of the hot tube to form the threaded opening and inflate the tube inside the mould cavity. When the mould is completely cooled, it is opened to eject the bottle and the excess plastic is trimmed from the neck and bottom areas.

Furthermore, there are (at least) two variants of extrusion blow moulding.

• In continuous extrusion blow moulding, the parison is extruded continuously and the individual parts are cut off by a suitable knife. With continuous extrusion, the weight of the parison drags the parison and makes calibrating the wall thickness difficult.
• In intermittent blow moulding, there are two processes:
  • Straight intermittent is similar to injection moulding, whereby the screw turns, then stops and pushes the melt out.
  • With the accumulator method, an accumulator gathers melted plastic and when the previous mould has cooled and enough plastic has accumulated, a rod pushes the melted plastic and forms the parison. The accumulator head uses hydraulics to push the parison out, quickly reducing the effect of the weight and allowing precise control over the wall thickness by adjusting the die gap with a parison programming device.

Injection blow moulding (IBM)

Injection blow moulding is the least-used of the three blow moulding processes. IBM is used for producing hollow plastic (or glass) objects in large quantities, typically small medical and single-serve bottles. The process is divided into three steps: injection, blowing and ejection.

The process starts with the injection moulding of a polymer onto a core pin which is then rotated to a blow moulding station to be inflated and cooled.

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The injection blow moulding machine is based on an extruder barrel-and-screw assembly which melts the polymer. The molten polymer is fed into a hot runner manifold where it is injected through nozzles into a heated cavity and core pin. The cavity mould forms the external shape and is clamped around a core rod which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a thick tube of polymer attached, which will form the body, similar in appearance to a test tube with a threaded neck.

The preform mould opens and the core rod rotates and clamps into the hollow, chilled blow mould. The end of the core rod opens and allows compressed air into the preform, which inflates it to the finished article shape.

After a cooling period, the blow mould opens and the core rod rotates to the ejection position. The finished article is stripped off the core rod and as an option can be leak-tested prior to packing. The preform and blow mould can have many cavities, typically three to 16, depending on the article size and the required output. There are three sets of core rods, which allow concurrent preform injection, blow moulding and ejection.

Advantages of injection blow moulding

• It produces an injection-moulded neck for accuracy

Disadvantages of injection blow moulding

• Compared to extrusion-based blow moulding, the injection blow-moulding process has a lower production rate (why it is less widely used)
• Only suits small capacity bottles, as it is difficult to control the base centre during blowing
• No increase in barrier strength as the material is not biaxially stretched
• Handles cannot be incorporated

Stretch blow moulding

The process first requires the plastic to be injection moulded into a &#;preform&#; with the finished necks (threads) of the bottles on one end.

The preform is then heated above its glass transition temperature and blown, using high pressure air, into bottles using metal blow moulds. At the same time, a core rod stretches the preform to fill inside of the mould.

The main applications of stretch blow moulding includes jars, bottles, and similar containers because it produces items of excellent visual and dimensional quality compared to extrusion blow moulding. Strain hardening occurs as part of the stretching process of some polymers (such as Polyethylene Terephthalate) which allows the bottles to resist deforming under the pressures resulting from carbonated beverages (typically around 60 psi).

Injection stretch blow moulding

Injection stretch blow moulding is the common method for producing soda bottles. The process begins with an injection moulded perform. The perform is typically pre-heated then stretched in the axial direction and blown into its final shape by a stretch blow moulding machine.

Single-stage process is again broken down into three-station and four-station machines. In the two-stage injection stretch blow moulding process, the plastic is first moulded into a "preform" using the injection moulding process. These preforms are produced with the necks of the bottles, including threads (the "finish") on one end. These preforms are packaged, and fed later (after cooling) into a reheat stretch blow moulding machine. In the ISB process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high-pressure air into bottles using metal blow moulds. The preform is always stretched with a core rod as part of the process.

Advantages of stretch blow moulding

• Very high volumes can be produced, though also highly suitable for low volumes and short runs
• The strength is ideal for filling with carbonated drinks
• Preforms can be sold as a completed item for a third party to blow
• Is suitable for cylindrical, rectangular or oval bottles
• As the preform is not released during the entire process the preform wall thickness can be shaped to allow even wall thickness when blowing rectangular and non-round shapes

Disadvantages of stretch blow moulding

• Restrictions on bottle design &#; for example, carbonated bottles can use only a champagne baseHigh capital cost
• Requires high floor space, although compact systems have become available

In the single-stage process, both preform manufacture and bottle blowing are performed in the same machine. The older four-station method of injection, reheat, stretch blow and ejection is more costly than the three-station machine which eliminates the reheat stage and uses latent heat in the preform, thus saving costs of energy to reheat and 25 percent reduction in tooling.

Barrier blow moulding

Coextrusion allows blow moulders to improve the barrier properties of their products by keeping either oxygen or moisture out, which is critical in food packaging applications. In industrial applications such as gas tanks, the barriers layers are designed to regulate hydrocarbon emissions.

Spin trimming

Products often have an excess of material due to the moulding process, which must be trimmed off by spinning a knife around the container which cuts the material away. This excess plastic is then recycled to create new mouldings.

Spin trimmers are used on a number of materials, such as PVC, HDPE and PE+LDPE. The physical characteristics of different types of the materials affect trimming. Mouldings produced from amorphous materials are much more difficult to trim than crystalline materials. Using titanium-coated blades rather than standard steel can increase life.

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