The extrusion process can require the use of different ancillary machines. Among them, dosing systems and dryers/dehumidifiers play a relevant role in the treatment of granules, as they directly affect the quality of the cable insulation and performance.
DRYERS AND DEHUMIDIFIERS
In the extrusion process, it is essential that the polymer does not carry water inside the extruder.
Otherwise, steam bubbles would occur inside the extruder and would remain in the melt. At this point both the mechanical features and the surface quality of the electrical insulation would be compromised.
The main difference among polymers for the extrusion process lies in their ability to interact with water molecules.
Hygroscopic materials are those polymers that are capable of absorbing water.
The hygroscopic materials include the polyamides PA (trade name Nylon), Polymethyl methacrylate PMMA (trade name Plexiglass), Polyethylene terephthalate PET, etc.
Non-hygroscopic materials, on the other hand, are those which are not able to absorb water, but the water is deposited outside the granule.
The most common non-hygroscopic materials are PVC, PE, PP.
Dehumidifiers
In order to remove water from hygroscopic materials, cable manufacturers use dehumidifiers.
Electric resistances heat the air, which is then passed through silica gel granules, which have the function of trapping the water contained in the air.
The hot and dry air circulates inside a hopper containing the granules to be treated.
Dryers
To remove the water deposited outside the non-hygroscopic materials, it is enough to blow hot air inside the hopper containing the granules.
The drying or dehumidification time and temperature changes according to the physical-chemical nature of the polymer and the specific values are provided by the cable manufacturer.
In both machines, the drying time is adjusted by sizing the hopper in relation to the flow output of the extruder.
DOSING UNITS
The dosing of granules is a very important topic in the production of electric cables, as they require the use of a blend of plastic materials.
An example of mixture is the addition of color to the base material or the addition of catalyst in a cross-linkable material.
Dosing units are machines capable of supplying an exact percentage for each component of the recipe. The accuracy depends on the application and the type of machine.
There are two main types of dosing units: volumetric and gravimetric dosing.
Volumetric dosage
The dosing unit, which supplies the percentage of masterbatch, is placed on the neckpiece that connects the hopper to the feeding body.
The machine consists of a hopper mounted on a cylinder with a screw inside. The auger is rotated by an electric motor.
By controlling the rotation speed, it is possible to control the volumetric flow of material and therefore its percentage with respect to the recipe.
By setting the rotation speed of the screw as a fixed value, weight of actual capacity of the screw can vary according to the specific weight of the material to be dosed or the extruder capacity.
Volumetric dosage is an effective dosing system in case of material coloring, while it is not recommended when the material to be dosed changes the viscosity of the base material.
Benefits
- The cheapest solution
- Easy to clean
- Excellent dispersion of components in the mixture
Disadvantages
- As the specific weight or bulk density of the granule varies, the machine must be calibrated
- When the extruder output changes (often the cause is the presence of catalysts), you have an error on the dosing percentage
Gravimetric dosage
Usually, gravimetric dosage units are placed directly above the feeding body of the extruder.
Unlike volumetric dosing units, gravimetric ones are not influenced by the difference in specific weight of the additive or by the change in viscosity of the base material melt.
There are two technologies for gravimetric dosage.
1. Batch dosage
The machine is made up of a number of hoppers equal to the number of components to be dosed, which converge on a single scale assembled on a Mixer.
One at a time, the components are released into the container and weighed by percentages of the desired recipe.
At the end of the dosing, the scale releases the mixture into the mixer placed at the inlet of the extruder feeding zone.
Benefits
- Very precise dosage
Disadvantages
- Different layers for each material because materials are dosed in sequence and not simultaneously
- Difficult to clean due to the presence of a mixer
- Risk of contamination in case of color change in continuous production
Weight loss dosage
Weight loss dosing unit is a volumetric dosing unit placed on a load cell.
To keep a constant dosage percentage, the number of revolutions of the dispenser varies according to the weight loss during operation.
Since the individual components are weighed individually and continuously, there is no stratification of materials, so it is not necessary to use an additional mixer.
With weight loss dosing units, to change color it is enough to turn off a component and turn on the new one, without stopping production.
Benefits
- High Accuracy
- Automatic adjustment to keep a constant capacity
- Easy to clean
- Possibility to change color without stopping the line
Disadvantages
- Bulkier than a batch dosing unit
While dryers and dehumidifiers avoid steam bubbles inside the extruder and consequently a negative effect on the electrical insulation, dosing units guarantee the exact dosing during plastic extrusion and are essential machines when you need to work with a blend of plastic materials.
MFL Group extrusion lines are designed and built under the brand Frigeco. The Frigeco brand belongs to the Wire & Cable Machinery Division and is one of the two brands born and raised within MFL Group.
Over the last 20 years, it has consolidated its position as a top brand for Rod Breakdown Lines, Intermediate Drawing Lines, Multi-wire Drawing Lines, all kinds of Rotating Machinery, Insulation Lines, Sheathing Lines and many key Auxiliary Machines.
Thanks to decades of experience in extrusion technology, Frigeco Extruders guarantee the highest quality level of your cables and are constantly evolving to optimize the production process in terms of energy efficiency, production volumes and to work at the best new extrusion materials.
About materials used for the cable insulation, in recent years legislation has identified basic and unified characteristics in electrical cables for the whole European community to promote the free movement of products and minimize the risks to people in the event of a fire.
With the advent of new fire-retardant materials, like HFFR (Halogen Free Flame Retardant), it was necessary to create a new generation of extruders.
Our team of engineers, in collaboration with the world’s largest plastics manufacturers, have developed a full range of machines capable of working innovative materials ensuring a high temperature accuracy of the melt with outputs that were considered impossible.
The combination of innovative solutions adopted by MFL group led to an international patent on flame-resistant material processing technology.
If you need more information about our extrusion lines, take a look here or contact our expert team.