When it comes to accuracy and consistency, plastic co-extrusion is one of the best production methods for plastic parts and components.
In this guide, you will learn the advantages, co-extrusion process, practical applications and benefits, just to mention a few.
What is Plastic Co-Extrusion?
Plastic co-extrusion is simply the process of melting and uniformly feeding different plastics into a single extrusion die.
Plastic Extrusion Process
This process is similar to regular plastic extrusion, except for the die.
Difference Between Plastic Mono-Extrusion Vs. Plastic Co-Extrusion
Here is a table showing the main differences between plastic mono-extrusion and plastic co-extrusion.
It is an outdated technology
The technology is a bit newer
Only uses a single-layer plastic sheet material for extrusion
Can extrude multiple layers of material
Capital investment is lower for each profile
Comparatively costly per extrusion profile
It does not require a lot of technical expertise
Requires a higher level of expertise
High cost of material pieces
offers cheaper material pieces
In the end, the two techniques go hand in hand when it comes to:
- Available warranties
Types of Plastic Co-extrusion
There are two variations of the plastic co-extrusion process:
- Feed block co-extrusion
- Die co-extrusion
Feed Block Co-extrusion
The feed-block co-extrusion process combines plastic melt streams into a lamellar melt stream. This is done within a feed block system.
You’ll find the feed block system directly upstream of the extrusion dies.
When the plastic melts pass through an extrusion die, their high viscosity prevents them from combining.
Feed Block Co Extrusion
Valves inside the feed block will allow you better to control the flow rate of each component layer.
The advantage of this co-extrusion process is that it’s relatively inexpensive.
The investment costs are usually lower than for other co-extrusion processes, such as die co-extrusion.
If you compare the operating principle of the feed block system and die co-extrusion, the latter can be a bit more complex.
Die co-extrusion uses highly advanced equipment.
The technique itself involves setting independent melt path manifolds to connect at a point near the die exit.
Die Co Extrusion
Using this method, you can independently control the thickness and flow rate of individual layers.
Moreover, the technique allows you to handle polymers with substantially differing melt temperatures and viscosities.
Die co-extrusion does have a downside, however. It is very costly in terms of capital investment and maintenance.
Benefits of Plastic Co-Extrusion over Extrusion
Plastic Co Extrusion Parts
The advantages of plastic co-extrusion are:
- High potency because of severe strain hardening
- Dimensional accurateness
- Good quality finish
- Economy due to the smaller number of processes and low-cost material for filling
- Requires fewer machining
- Plastic co-extrusion products have longer shelf life than items made using other extrusion processes.
- The process does not alter material properties
- Since it combines two or more polymers, this technique improves the physical properties of the final profile, i.e., ductility and impact strength.
- Requires less energy to power the machine
- Co-extrusion gives you more design freedom
Process of Plastic Co-Extrusion
Let’s look at each step of the plastic co-extrusion process:
Step 1: Feed The Resin Into The Plastic Co-extrusion Machine
Before feeding the resin into the machine, mix colorants into the thermoplastic resin to increase its appeal.
A UV ray-blocking material and other additives are also available to add to the resin if you so desire.
Once you’re done, feed the mixture into the plastic co-extruder hopper.
Step 2: Heating The Resin
When you put the resin in the machine, it falls into a barrel.
Inside the barrel is a screw that can rotate at up to 120 revolutions per minute, sometimes more.
The screw pushes the resin or plastic beads through the barrel, gradually heating up.
Plastic co-extrusion machine barrel temperatures range between 200 degrees Celsius to around 275 degrees Celsius.
There’s automatic temperature control for the entire length of the barrel.
So the further the resin moves, the higher the temperature becomes.
This gradual heating causes the resin to melt more gradually and evenly as it moves along the barrel.
It also minimizes the risk of overheating, which can degrade the polymer.
Step 3: Cooling the Resin
As I said, the resin gradually heats up as it enters the plastic co-extrusion machine.
A co-extrusion machine can have up to three heating elements simultaneously, depending on its size. This reduces the risk of the material overheating, which can lead to many problems.
If the resin overheats beyond the set temperature, you can lower the heat with cooling fans.
Plastic co-extrusion machines usually have built-in fans that work automatically.
As soon as they’re switched on, they switch off the heaters, and the temperature drops.
Without heat and with cooling, the molten plastic stays at a constant temperature.
Step 4: De-contamination
After the heating process, the plastic leaves the screw at the front end of the barrel.
The molten plastic passes through a screen pack to remove any impurities or defects.
Your plastic products will look better, be more durable, and last longer as a result.
Step 5: Extrusion
A part of the machine called a die receives the melt after passing through the screens.
The die gives the product its final shape or profile.
Thus, the design should allow the molten plastic to flow evenly from a cylindrical profile to the final product.
Note that the plastic co-extrusion process requires multiple extrusion lines.
The extruders force two or more different materials through the same die.
The procedure consequently forms parts with layers.
The properties of the co-extruded part may differ from those of a part made from a single plastic.
This is because of the combined plastic materials.
Sometimes you can use five or more materials in a single cycle.
Nevertheless, each plastic retains its original properties when you combine them into one part.
Step 6: Cooling The Finished Products
After the die does its job, the machine’s work is done.
Plastic co-extrusion involves cooling the finished extrusions (profiles), which is a delicate operation.
A failure to cool the plastic will make the whole process ineffective.
We usually use sealed-water baths to ensure that our products cool evenly and with as little risk as possible.
For more delicate products, we use plastic sheeting cooling rolls. This technique prevents the products from collapsing.
As soon as the products cool down, you can cut them into manageable lengths or spool them for later use.
Plastic Co Extrusion Process
Types of Plastic Bases Used in Plastic Co-Extrusion Process
There are quite a few different materials you can use for plastic co-extrusions.
Some of these may include:
- Polyethylene terephthalate (PET) – this environmentally friendly plastic is popular for beverages, food, and packaging because of its durability.
- High-impact polystyrene (HIPS) – this is a rigid plastic that can withstand a lot of force before breaking.
It is ideal for products that require high-impact strength.
- Polyethylene terephthalate glycol (PETG) – this is a modified PET material suitable for packaging.
- High-density polyethylene (HDPE) – this petroleum-based plastic is perfect for hard hats and 3D printing.
- Acrylonitrile butadiene styrene (ABS) – You will commonly use this plastic in the medical industry.
It makes wires and tubing for most medical and pharmaceutical equipment.
- Polyvinyl chloride (PVC) – this plastic is versatile but is most common in structural applications.
Keep in mind that despite the variety, not all plastic base materials are suitable for plastic co-extrusion. This is because some polymers simply don’t adhere to others.
In addition, the melting temperature between two or more plastics can be very different.
The large differences in melting temperatures can degrade the lower melting material.
This can happen if you heat the material with the lower melting point to the temperature of the higher melting material.
Disadvantages of Plastic Co-Extrusion
Despite its many advantages, plastic co-extrusion also has some disadvantages
- Slight variations in physical properties are necessary to achieve a desirable combination in plastic co-extrusion.
Unfortunately, these differences can also make two materials incompatible.
- Since this process mainly uses polymers, it can negatively affect the environment.
- The polymers you use in this process must have the same melt viscosity to guarantee laminar flow. This is not easy to ensure.
- Co-extrusion requires lots of planning and design.
The entire process can be quite time-consuming and may take up more resources than necessary.
- This technique requires high-tech equipment that can be expensive to purchase.
Applications of Plastic Co-extrusion
Applications of co-extrusion are thin film and sheet extrusions.
You can use these in sealing, packaging, electronics, medical equipment, automobiles, aircraft, and other applications.
Some examples of common products we make using plastic co-extrusion include:
- Window frames
- Storm shutters
- Stripped tubing, i.e., earth wires and drinking straws
- Weather stripping
- Fencing posts
- Boat docks
- Fencing posts
- Water tanks
- Insulation for wires
Using plastic co-extrusion, we can also make minuscule items like toy figures.
I bet you didn’t look at or play with that small action figure and thought, “this was co-extruded”!
For successful plastic co-extrusion, a thorough and accurate knowledge of the process is essential.
To that end, I hope this guide has been helpful.