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Single-Stage vs. Two-Stage Blow Molding Machines: A Guide

In the world of manufacturing, especially bottle manufacturing, blow molding machines are essential — but which is better for you: a single-stage process or a two-stage process?

Single Stage and Two Stage Blow Molding Machines - KB Delta

Blow molding machines employed in PET (polyethylene terephthalate) bottle manufacturing are of various types, and that is why manufacturers need to ascertain the actual injection stretch blow molding process to use in producing plastic bottles.

The ability to choose the right method comes with the perks of increasing productivity, reducing cost, and also minimizing the cycles required to produce these materials.

However, the significant differences between single-stage and two-stage molding processes require you to make a decision.


What are Blow Molding Machines?

Blow molding machines are an equipment that is used in manufacturing plastic and glass bottles.

It takes advantage of several processes to form hollow parts in plastics before these parts are joined together to form a bottle.

In a bid to make this possible, there are three major processes:

  • Injection stretch blow molding
  • Injection blow molding
  • Extrusion blow molding

While each of these will lead to the production of a bottle, the injection stretch blow molding process is what is required to create PET or plastic bottles.


What is the Single-Stage Blow Molding Process?

A single-stage process gains its name from the fact that it creates preforms, stretches, and blows them on the same machine before cooling.

Therefore, single stage PET blow molding machines can also be said to employ a direct method.

This level of functionality can be attributed to the injection molding system and blow molding station being built into one machine.

Likewise, the machine can either be a 3-station or 4-station system.

A 3-station single-stage machine has three stages which are injection, stretch blow, and ejection. It also uses latent heat which saves the cost of having to reheat, thereby reducing 25% of heat in tooling.

The 4-station, on the other hand, comprises of these three stages and an additional reheating stage.


Processes Involved in the Single-Stage Method

The transformation of the raw materials into a bottle using the single-stage follows the procedures outlined below:


  1. Dehumidifying and drying of PET materials
  2. Melting and injecting of the raw materials into the preform mold through the injection nozzle
  3. Cooling of the molten material in the preform mold to 120 ° Celsius
  4. Transfer of the hot preform mold to a bottle mold to begin the stretch molding


Advantages of Single-Stage Blow Molding Process

There are several advantages of a single-stage molding process and they include:

  • Compact and flexible
  • Transfer ring is optional
  • Creates blemish-free bottles
  • More suitable for low volumes
  • Control over preform production
  • Control over thread start to align with bottle’s shape
  • Less expensive in comparison to two-stage machines
  • Suitable for blowing rectangular and non-circular shapes


Disadvantages of Single-Stage Blow Molding Process

The disadvantages of employing this method include:

  • Long cycle times
  • Long changeover times
  • Restriction in the bottle design
  • Wall distribution may be uneven
  • Great expertise is required in mastering both processes and PET drying
  • Inefficient blow station due to injection station having more precedent over cycle time

Let’s take a look at some of these scenarios:

– Wall Distribution May be Uneven:

Generally, PET material can level by itself, but there are cases where an uneven wall distribution will still be evident in the final product.

What happens here is, the movement of the melt material along the barrel and hot-runner channels which exerts some amount of heat on it.

Despite this, the melt still retains some amount of thickness or viscosity and the warmer part tends to flow to the back of the new channel.

The result is a thicker design at the back of most bottles that employ this method, even though its manufacturer or brand may sometimes disguise it using labels.

– Inefficient Blow Station:

The issue of inefficient blow station has been combated with the creation of another category of single-stage machines which can be considered as integrated two-stage machines.

They are those featuring multiple pairs or triple injection cavities in each blow cavity. Their blow sections also complete two to three cycles for every injection cycle.

Consequently, this saves blow cavities while also reducing the tooling cost; a reason that makes the single-stage method more suitable for companies that are out to produce bottles in low or medium volumes.


PET Bottle Manufacturing- KB Delta


What is a Two-Stage Blow Molding Process?

The technique used in two-stage injection stretch blow molding machines involves two machines. These are the injection molding system and stretch blow molding machine.

Here, the plastic is molded into a fully-cooled preform in the first machine before being shipped to the second machine.

The preform will usually feature the neck of the bottle as well as its thread, and it can be sold to a third party even before it is blown using the second machine.

When received by the latter, they are then unscrambled, reheated using infrared radiation, and conditioned before molding.

The need to cool the preform before reheating and stretching also gives this process its name, the “cool preform method.


Processes Involved in the Two-Stage Method

  • Dehumidifying and drying of PET materials
  • Injecting of molten raw materials into the preform mold by the injection machine
  • Cooling of molten PET to room temperature
  • Processing of preform by a finishing machine
  • Transferring of the preform to reheating stretch blow molding machines
  • Blowing high-pressure air into bottles with the use of metal blow molds


Advantages of Two-Stage Blow Molding Process

Some advantages of the two-stage molding process include:

  • Flexibility
  • Fast cycle times
  • Fast changeovers
  • Less restriction to bottle design
  • The process can be stopped at anytime
  • Good wall distribution for round bottles
  • Preforms can be sold before being blown
  • Handles high volumes of 1000 to 72,000 bottles/hr


Disadvantages Two-Stage Blowing  Molding Process

The disadvantages of this process include:

  • High capital to maintain
  • Floor space required is high
  • Scratches may occur on bottles
  • Potential damage to the preforms


Do You Need PET Compressor Parts - KB Delta


Single-Stage vs. Two- Stage Process, Which is Better?

Having made comparisons between single-stage vs. two-stage process of blow molding machines, it is evident that where one fails, the other makes up for it.

That being the case, great care must be taken before settling for one.


Let’s take for instance the single-stage method, consideration must be given to the inability to inspect the preform before molding in the direct stage.

In this aspect, operators must have good knowledge about injection and blow molding in order to determine if there are any defects while the preform was formed.


Many think the same of the cool preform method which comes with its own limitation of making bottles susceptible to scratches. The latter is as a result of the need to store, align, and transfer the preform from one machine to the other.

An instance of this is where the preform tumbles onto the conveyor belt before entering the storage containers. Preforms may also be in the blow molding machines which can create blemishes on the bottle.

It is worthy to note that these scratches might not very noticeable given that some brands take the extra time of placing labels on their bottles in order to conceal such marks.

On the other hand, scratches may the least to worry about because if these preforms are purchased by a third party or vendor, they may not be in the right size,

As such, problems like finding the right neck and weight for the bottle may arise.



Having accessed the pros and cons of single-stage and two-stage blow molding machines, the choice of which to settle for will be dependent on what’s your priority.

Do you want a blemish-free bottle, in a precise oblong shape, a fixed thread, at a low capital cost? Then the single-thread is a better choice.

Alternatively, if you’re out to get something more economical, productive, and without any technical difficulty, then it would be the two-stage method due to its high scalability per hour.

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