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The extrusion process of rubber materials
avails itself of the following components:
• Extruder
• Die
• Curing oven
• Cooling bath
• Spooler
• Cutter
• Postcuring oven
The extruder is the predominant machine
of an extrusion plant.
It allows the material to reach the
required break-down and to flow through
the die, which is firmly assembled to its
head.
Raw rubber is normally cut in stripes and
feeds the internal screw of the extruder,
which transfer energy to the rubber itself and makes it machinable from the die into
the desired profile.
The most important parameters of temperature
and internal pressure are ruled
by PLC and controlled by skilled personnel.
The production lines of Tecnoextr s.r.l.
actually include extruders both horizontal
than vertical, for the manufacturing of
a wide range of different profiles and
tubes, with diameters from a few tenth to
around 130 mm.
Extrusions dies for rubber are usually machined by electroerosion from steel discs, having diameter and thickness as a function of the rubber material and of the kind of profile and, in case of hollow shape, cores supported on the slotted disc with a steel frame.
 Even if it is required just an easy machining
to get them, the dies must be carefully
designed and, if necessary, modified
for being tuned.
First of all, extreme care and a long experience
must be dedicated to designing the
orifice.
 A rubber uncured extruded profile flowing out from the die is free to varying its own geometry, because after being piped under a growing pressure on the screw, leaving the die the pressure drops to relative zero.
Because of this rush of pressure, which
leads to swelling due to the increase of
material entropy at a high temperature
and low viscosity, the extrudate may
collapse and vary its shape under relaxation.
 This is a typical case of reverse engineering,
when one has to determine a system
to producing the wished result starting
from the result itself: the extruded profile
according a drawing.
It has to be evidenced that all the dies
must be developed and tuned according
to the material of the profile, which may
have a certain hardness or viscosity, and
to the profile geometry.
It has to be furthermore remarked that,
even if the orifice geometry one can see
on the external side of the die may be
considered as ideal and correct for a certain
profile, the internal side is usually
shaped differently and then eroded step
by step in order to get a uniform material
flow in the die land.
 For example, it is possible to accelerate the material flow reducing friction losses dulling the edges or creating converging sections. On the contrary one can slow down the flow rate locally by means of artificial friction losses inserting so called brakes.
Of course, when possible, it is better to get a uniform wall thickness of the profile so that a constant mass flow rate is reached and to avoid areas thicker than the rest or edges where material stagnation or cracks may take place. The whole wide range of extrusion dies manufactured by Tecnoextr s.r.l. have been developed inside the company thanks to a long experience in this field, so today die design has become a daily task and the results of this activities have been valued and called for by the market.
A few centimeters downstream from the die the profile is carried on the curing oven belt, where it crosslinks at controlled temperature according imposed parameters depending upon extruded material and profiles geometry.
Soon afterwards, if necessary the profile is cooled down in water, spooled, cut or punched.
Depending upon the material extruded, the profiles are postcured in hot air ovens according specific cycles to completing crosslinking and to completely releasing additives and vulcanization additives.
Tecnoextr s.r.l. supplies extruded endless profiles in bundles or spooled over plastic or paperboard spools.
A collection of profiles is showed on the last pages of the catalogue.
This list, here showed partially, is continuosly updated and integrated with new
articles.
Tecnoextr s.r.l. remains at disposal of his own customers for further information and to suggesting the best fitted solution for the application required.
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The table shown here below indicates the weight per meter of standard silicone tubings.
Values indicated on the table are related to nominal dimensions and to a standard compound, so they may be subjected to variations.
Tubes and cords are standard products and they are manufactured in a wide range of different materials and hardness.
Tecnoextr s.r.l. transform a wide range of rubber materials, both compact than cellular, into reliable and carefully manufactured extruded products.
What it may concern about material performances is mentioned on tables and graphs in the introduction area of this catalogue.
Other detailed information related to our products may be requested to our technical department, which remains always at disposal of the customers.
Extruded rubber profiles require a certain technological cycle of production, similar to what it is used for other materials, but with a distinctive difference.
What it makes the difference between rubbers and the rest of the polymers is the vulcanization process a rubber part has to withstand before its use.
This process, which implies an irreversible crosslinking of the molecules, takes place at atmospheric pressure transfering heat to the profile.
This heat transfer must be gradual and it is dependent from profile geometry, profile dimensions and polymer type.
It is evident that, even if one may try to
accelerate the process of vulcanization, it
starts and then it stops the shape variation
only after a certain period, usually
some seconds.
Anyhow it is furthermore intuitive that
during this short time the profile is under
an unstable equilibrium of forces and that
its shape may change under its own
weight because of the material relaxation.
Termoplastic extruded profiles, on the
contrary, are usually calibrated and this is
made possible because of the absence of
vulcanization and so it is possible to cool
them rapidly all over calibrators
Tecnoextr s.r.l. manufactures extruded
profiles in conformity to severe stan-dards with dimensional tolerances mostly
according UNI ISO 3302-1.
According to the above mentioned standard
norm, the suggested class of tolerances for
cross-sectional dimensions of extruded
profiles, except special cases, is the class
E2.
Concerning products cut from tubes or
cords and profiles in general, instead, we
apply the class L2.
Tecnoextr s.r.l. manufactures even extruded
profiles made in different qualities of
cellular rubbers.
Tolerances related to such this kind of
materials haven’t been included in a standard
norm.
Because of the difficult prediction of the material behaviour during manufacturing, due to an enormous swelling coefficient related to the amount of blowing agents added in the compound, Tecnoextr s.r.l. suggests for these materials the class E3 for cross-sectional
dimensions.
However the E2 class is allowed for sim- plified geometry like square or circular cross sections.
It is advisable to consult our technical department for further details and information.
Tecnoextr s.r.l. plays a distinctive role on the market because of the products quality and of the technical support to the customers.
A huge amount of different applications
have been covered by alternative series of
products:
• Inflatable seals, which are extruded hollow
profiles joined as closed circles or as
opened frames with vulcanized ends, able
to expand while inflated with compressed
air.
• Profiles modelled from uncured extrudate,
usually silicone hoses handmade or
vacuum modelled.
• Tubes and profiles with moulded ends,
usually tubes with connectors or elbows
moulded over to improve assembling features.
• Seal frames, manufactured from cut
profiles joined by corner moulding or
using specific glues.
• O-rings, joined by mouding
or glued from cords or tubes.
• Flat gaskets and hoses cut from tube.
Tecnoextr s.r.l. is certified UNI EN ISO
9002 since 1994.
Quality procedures rule the whole manufacturing
process, from design to deliveries
and produce an efficient pre and post
sales service and a reliable finished product
marked distinctively by competitive
prices and by an excellent global quality
level.
Since raw material inspection to com-pound manufacturing, due to the constant
inspection of the products and to the
measurement of efficiency parameters,
until the desired admittable quality level is
reached, the quality system of Tecnoextr s.r.l. guarantees the traceability of the
process and its own total reliability.
Some of the numerous cases studied by
our technical department and later on
being manifactured are discussed here
forward.
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