Extrusion blow molding is a process used to make hollow thermoplastic products.
Plastic resin is melted inside the extruder and pushed through a die head to form a hollow tube called a parison.
The mold closes around the parison. Compressed air is then introduced into the inside of the parison, forcing the hot plastic against the mold cavity.
After the plastic cools and becomes rigid enough to hold its shape, the mold opens and the product is removed.
This process is widely used for products such as:
- Detergent bottles
- Lubricant bottles
- Chemical bottles
- Agrochemical containers
- Laboratory reagent bottles
- Jerry cans
- Wide mouth containers
- Plastic toys
- Floats
- Ocean balls
- Automotive air ducts
- Other hollow industrial parts
How Does an Extrusion Blow Molding Machine Work?
Plastic Feeding → Plasticizing & Extrusion → Parison Forming → Mold Closing → Blow Molding → Cooling → Mold Opening & Product Removal
This is the basic production cycle of an extrusion blow molding machine.
1. Plastic Feeding and Plasticizing
Plastic pellets are fed into the hopper and enter the heated barrel.
The screw rotates inside the barrel, moving the material forward while it is heated, compressed and mixed. By the time the plastic reaches the die head, it should have formed a stable and uniform melt.
Temperature control is important here.
If the melt temperature is too low, extrusion may be unstable and the surface of the parison may not be smooth.
If the temperature is too high, the melt becomes too soft. A long parison can stretch under its own weight before the mold closes.
The temperature therefore needs to suit both the material and the product.
2. Parison Forming
Molten plastic passes through the die head and forms a hollow tube known as the parison.
The final product is formed from this parison, so its diameter, length and thickness have a direct effect on product quality.
For a simple bottle, relatively uniform parison thickness may be sufficient.
For a Jerry can with a handle, shoulder and large bottom area, material distribution becomes more important. Different parts of the product may need different amounts of material.
In this case, a parison control system can adjust the die gap during extrusion and change the thickness of different sections of the parison.
The purpose is not simply to make the container thicker. It is to place material where it is needed.
3. Mold Closing
When the parison reaches the required length, the mold closes around it.
The two mold halves pinch the plastic and form the basic shape required for blowing.
The clamping unit must provide enough space for the mold and keep it closed during the blowing process.
This is why machine selection cannot be based only on bottle volume.
Two products may both be 5L, but one may be tall and narrow while the other is short, wide and fitted with a handle. Their mold dimensions can be very different.
Product dimensions and expected mold size should always be checked before confirming the machine.
4. Blow Molding
After the mold closes, compressed air enters the parison.
The hot plastic expands until it contacts the inner surface of the mold cavity.
The cavity determines the external shape of the product, including the body, shoulder, handle and other molded details.
Blowing pressure, blowing position and mold venting all affect forming quality.
If air cannot escape properly from the mold cavity, some areas of the product may not form clearly even when enough blowing pressure is available.
5. Cooling
The plastic must cool inside the mold before the product can be removed.
Cooling water circulates through channels in the mold and removes heat from the plastic.
Cooling is a major part of the blow molding cycle, especially for large or heavy containers.
A lightweight 500ml bottle normally needs much less cooling than a thick wall 20L Jerry can.
This is also why machine output is not determined only by automation level. For the same product, extrusion system and mold configuration, semi automatic and automatic machines can have similar molding output.
Product weight, extrusion rate, cooling efficiency, mold design and forming cycle usually have a greater influence on output.
5. Cooling
The plastic must cool inside the mold before the product can be removed.
Cooling water circulates through channels in the mold and removes heat from the plastic.
Cooling is a major part of the blow molding cycle, especially for large or heavy containers.
A lightweight 500ml bottle normally needs much less cooling than a thick wall 20L Jerry can.
This is also why machine output is not determined only by automation level. For the same product, extrusion system and mold configuration, semi automatic and automatic machines can have similar molding output.
Product weight, extrusion rate, cooling efficiency, mold design and forming cycle usually have a greater influence on output.
6. Mold Opening and Product Removal
After sufficient cooling, the mold opens and the product is removed.
Extrusion blow molded products usually have flash around areas such as the neck, bottom or handle.
With a semi automatic machine, product removal and flash treatment are generally completed manually.
A fully automatic machine can integrate product removal, deflashing, conveying and scrap handling into the production process.
The main difference is therefore in post molding handling and labor requirement rather than a large difference in basic molding output.
Main Parts of an Extrusion Blow Molding Machine
Although machine designs vary, most extrusion blow molding machines contain several main systems.
Extrusion System
Its job is to plasticize the material and deliver enough molten plastic for every molding cycle.
The extrusion system normally includes:
- Hopper
- Screw
- Barrel
- Heating zones
- Drive motor
- Gearbox
When choosing an extruder, screw diameter is only one reference.
Actual extrusion capacity is more important.
For example, a 20L container weighing 1kg requires much more material per cycle than a lightweight 5L bottle.
If the extruder cannot prepare enough material within the required cycle, extrusion becomes the production bottleneck.
Die Head
The die head forms the molten plastic into the parison.
Depending on the product, a machine may use:
- Single head
- Double head
- Multiple heads
- Continuous extrusion head
- Accumulator head
Small and medium bottles are commonly produced with continuous extrusion.
Large and heavy products may require an accumulator head, which stores a larger quantity of molten material and extrudes it within a shorter period.
The die head should be selected according to product weight, size, number of cavities and required parison dimensions.
Clamping Unit
The die head forms the molten plastic into the parison.
Depending on the product, a machine may use:
- Single head
- Double head
- Multiple heads
- Continuous extrusion head
- Accumulator head
Small and medium bottles are commonly produced with continuous extrusion.
Large and heavy products may require an accumulator head, which stores a larger quantity of molten material and extrudes it within a shorter period.
The die head should be selected according to product weight, size, number of cavities and required parison dimensions.
Blowing System
The blowing system supplies compressed air to form the product inside the mold.
The blowing position depends on product and mold design.
Bottle neck structure, handle position and opening design can all affect the blowing arrangement.
Hydraulic and Control System
The hydraulic system controls movements such as mold opening and closing.
Servo hydraulic systems can adjust hydraulic output according to actual machine demand and reduce unnecessary power consumption.
The electrical control system coordinates extrusion, heating, mold movement, blowing, cooling and cycle timing.
PLC and touchscreen control allow production parameters to be adjusted and stored for different molds and products.
What Products Can an Extrusion Blow Molding Machine Make?
Extrusion blow molding can be used for many hollow plastic products.
Common examples include:
- Detergent bottles
- Shampoo and household chemical bottles
- Engine oil bottles
- Lubricant bottles
- Agrochemical bottles
- Laboratory reagent bottles
- Jerry cans
- Chemical containers
- Wide mouth containers
- Plastic toys
- Ocean balls
- Floats
- Automotive air ducts
- Other hollow industrial plastic parts
The suitable machine depends on more than product volume.
For example, two 5L products can require different machine configurations if one weighs 150g and the other weighs 400g.
Product dimensions, weight, shape and mold structure all matter.
Planning a new blow molding project? Send I-TOP Machinery the product photo, dimensions, volume, weight, material and required output. We can check the product first and recommend a suitable machine and mold configuration based on the actual production requirement.
