Transformation and vulcanization of rubber

TRANSFORMATION AND
VULCANIZATION OF
ELASTOMERS
11/01/2022

TRANSFORMATION
 In the manufacture of molded rubber artifacts,
the raw compound is introduced into the
cavity of a heated metallic mold, with the
shape of the artifact to be manufactured, and,
under molding pressure, the plastic
compound flows and acquires the shape of
the cavity; the heat transmitted from the
press to the mold vulcanizes the compound,
which permanently acquires the configuration
adopted in the mold.

TRANSFORMATION
 The main molding techniques are:
compression, transfer and injection.
 In compression molding, a suitable amount of
compound is introduced into the mold cavity,
which is kept open during the loading
process, then we close the mold under a
compressive force and the compound fills the
cavity.

TRANSFORMATION
 In transfer molding, the compound is loaded into a
transfer chamber, which is connected through feed
channels with the mold cavity, which in this case will be
closed from the start of the operation. Compression force
is applied to the transfer piston, which fits into the
chamber and forces the compound contained therein to
flow through the channels and into the mold cavity. The
transfer chamber is connected to the mold cavity
throughout the molding cycle.

TRANSFORMATION
 As in transfer molding, in injection molding the
compound is introduced through a cavity under pressure
into a closed mold; however, in this case the injection
unit is independent of the mold; it is usually only
connected to this during the cavity charging time plus an
additional time; another difference is that the injection
unit performs plasticization and heating of the compound
before it is introduced into the mold.

COMPRESSION MOLDING
 We highlight the different heating systems of the
plateaus:
 Steam – the plateaus have zig-zag channels for steam
circulation; the system has a set of traps for the
elimination of condensed steam. To regulate the volume
of steam, the use of pressure switches is recommended,
which ensure the consistency and precision necessary to
maintain the vulcanization temperature.

COMPRESSION MOLDING
 Thermofluids – fluids are liquids that withstand
temperatures of up to 200ºC or more, without boiling or
suffering appreciable thermal degradation, for prolonged
periods; circulate on the plateaus in place of steam; the
condensers are eliminated and the work is done at low
pressures, which simplifies the problem of tightness. As
the thermofluid is kept circulating in a closed circuit,
usually of short length, temperature control is easier.

COMPRESSION MOLDING
 Electric – the system is clean and simple, but it usually
presents problems of temperature uniformity over the
plateau surface and fluctuations over time; with
electronic regulation of pulses, frequency and duration
between the real and the theoretical temperature, it is
possible to reduce these oscillations. Recently, a system
for heating plateaus by induction began to be marketed,
resulting in faster heating and better regulation and
uniformity of temperature.

COMPRESSION MOLDING
 As for the hydraulic drive system of the pressures, it is
differentiated into two distinct phases. The first consists
of the simple approximation of the plateaus which, for
reasons of productivity, has to be as fast as possible.
Once the approach is completed, the mold closing phase
begins, for which a minimum plateau pressure of 4-5
MPa is required, and as a consequence, high pressure
of the hydraulic fluid.

COMPRESSION MOLDING
 To operationalize this operation, it is common to use
mixed systems with a low pressure line, fed by rotating
gear pumps that provide a speed of 4m/min and a
pressure of 10-25 bars; when the circuit pressure
reaches the pump's capacity limit, the high pressure line
(250-400 bars) is automatically connected, fed by a
piston pump with an approximation of 1 mm/min.

COMPRESSION MOLDING
 A factor to consider is that the mold cavities must have
dimensions greater than what we want to obtain with the
mold; rubber compounds generally have shrinkage
ranging from 1.5-2% and this factor must be considered
when designing the mold.
 Before molding the part, we must establish the time and
temperature of its vulcanization; this is easily obtained
through a rheogram for artifacts whose thickness does
not exceed 6.25 mm; we established in this way, the
process T90. For thicker artifacts and different
temperatures, some rules must be followed:

COMPRESSION MOLDING
 1) For artifacts up to 6.25mm, a 10ºF variation, the time will change
1.5 times. This means that if a compound cures for 30 minutes at
300ºF, at 310ºF it will cure at 310/1.5 = 20 minutes; at 290ºF it will
need 30 x 1.5 = 45 minutes. The relationship between vulcanization
time and vulcanization temperature (ºF) can be defined by the
following equation: t1
 t2 = ---------------------
 1,5 (( T2 – T1)/10)
 Where: t2 = time required to vulcanize at temperature T2
 t1 = time required to vulcanize at temperature T1

COMPRESSION MOLDING
 2) For every additional 6.35 mm (1/4 in) add an additional 5 minutes
to the curing time.
 Compression molding is not much different from making a cookie or
waffle. A given amount of material must be placed in a cavity,
ensuring its filling. Heat and pressure are applied causing the
compound to flow, filling the cavity and shaping the part; surplus
material flows out through flow channels (burrs).
 Compression molding is generally chosen for medium hardness
compounds, in high volume applications or applications that
particularly use very expensive materials.

COMPRESSION MOLDING
 The excess, or flash, created by large diameter parts is of particular
interest when using more expensive compounds. Compression
molding helps to reduce this excess. The preform can, however, be
difficult to introduce into a difficult-to-form mold and the compression
molding process itself is not recommended for high-hardness
compounds.The application range ranges from simple O-rings to
belts and complex diaphragms greater than 10,000 inches (254.0
mm) in diameter.The burr on a typical compression molded part has
a maximum of 0.004 x 0.010 (0.102 x 0.254 mm) to 0.005 x 0.032
(0.127 x 0.813 mm), depending on the deburring method.

COMPRESSION MOLDING
 Finally, we must pay attention to auxiliary operations, which are
often decisive in the economy of the process. The preform
preparation, that is, the portion of compound that will be introduced
into the mold, must have a similar shape to the cavity and a volume
that ensures loading, without wasting material. Another auxiliary
operation is deburring or separating the burr formed at the mold
joints. There are a variety of systems, from manual with knives to
cryogenic deburring, when parts are cooled below their Tg, using
solid carbonic anhydride or liquid nitrogen; in this state, plastic parts
are blasted over the parts to break the burrs, which are normally
thinner and more fragile. Finally, periodic cleaning of the molds is
important.

TRANSFER MOLDING
 The simplest installation for transfer molding is a three-piece mold.
The two lower pieces configure the cavities, as in a compression
mold; the intermediate piece, in addition to being the upper part of
the cavities, carries the transfer chamber that communicates with
the cavities through a system of channels. At the top we have a
transfer piston, which fits into the chamber. If possible, the lower and
upper parts must be fixed to the corresponding plateaus of the
press, to avoid alignment defects.

TRANSFER MOLDING
Transfer molding differs from compression molding; in the
latter, the material is placed in a receptacle, located
between the upper part of the mold and a piston. The
material slips into the cavity through one or more holes
called a “port” or “passage”. The burr on a small mold or
O-ring will typically be at most 0.005 (0.127 mm) thick,
extending to approximately 0.003 (0.076 mm) on the
surface of the part.

TRANSFER MOLDING
 For molding, with the lower assembly closed and at the
vulcanization temperature, the required amount of compound is
introduced into the transfer chamber and the piston is lowered by
means of a press. The movement of the piston forces the compound
to flow through the distribution channels until the cavities are filled;
the assembly is kept under pressure until vulcanization is complete.
At the end of the cycle, the three parts are separated and the
demoulding is carried out; breakage in the inlet channel is common
and, when cleaning the burrs and excess rubber from the mold,
extra care must be taken to avoid future filling defects.

TRANSFER MOLDING
 An important factor when designing the mold is the relationship
between the cross-sectional area of the transfer chamber and the
total area of the horizontal projection of all cavities and the feed
system. As the raw compound is a fluid capable of transmitting
pressure, the pressure exerted by the piston on the compound that
is in the transfer chamber is transmitted through the feeding system
to the compound that fills the cavities of the mold, which will
consequently exert pressure that forces the mold to open.

TRANSFER MOLDING
 If the mentioned ratio is equal to or less than 1, the total force
resulting from this pressure in the cavities will be greater than the
closing force of the press and, as soon as the cavities are filled, the
mold will open, letting part of the compound escape; the tendency to
open the mold will be proportional to the closing force of the press.
The only solution to solve the problem is to design the mold so that
the area ratio is greater than 1; in practice we have 1.3 - 1.5, even if
this implies not using a considerable part of the mold section for
cavities.

TRANSFER MOLDING
 To overcome this limitation of transfer molding, special presses were
built with a double set of hydraulic pistons. They are generally top-
down piston presses, with a transfer cavity housed in the center of
the fixed bottom plate, in which the transfer piston acts. This system
makes it possible to independently regulate the mold closing
pressure and the transfer or molding pressure, making it possible to
make greater use of the surface of the mold cross section.

TRANSFER MOLDING
 Transfer molding has several advantages over compression
molding: it allows not using preforms or simplifies it.
 Frictional efforts in the flow of the compound through the channels,
greatly raise the temperature of the compound, which allows to
reduce the vulcanization time by 40-50% of what would be
necessary in compression molding.
 It also allows the use of higher vulcanization temperatures, as the
filling of the mold cavity and the start of vulcanization are almost
coincident. It also allows the filling of cavities, without the entry of air,
eliminating the need to degas the mold.

TRANSFER MOLDING
 In the artifacts made with the rubber-metal combination, they
simplify the fixing of the metallic inserts in the mold and guarantee
better adhesion values.
 As an inconvenience, the higher cost of the mold should be
mentioned, and the price of a transfer press is higher than that of a
press for compression molding.

INJECTION MOLDING
 The difference is that, in injection molding, the compound is
previously plasticized and introduced at low pressure into the closed
mold. In most injection molding machines, plasticization is carried
out by a spindle that rotates inside a cylindrical chamber; both the
spindle and the chamber are kept at a controlled temperature. The
spindle transports the compound to the feed inlet, to the nozzle
located at the opposite end. The compression ratio is low, at most
1.2 : 1; its basic function is to plasticize and eliminate air bubbles.

INJECTION MOLDING
Injection molding is the most automated of the molding processes.
The material is heated to a state of easy flow; It is injected under
pressure from the heated chamber through a series of holes or
“ports” in the mold.
Injection molding is ideal for high volume production of relatively
simple configuration rubber parts.

INJECTION MOLDING
 There are two types of injection presses:
 Plasticizing and spindle injection, and
 Plasticizing by spindle and injection by piston.
 In the first type, in addition to rotating, the spindle can also advance
and retract concentrically in the cylinder. During the plasticization
phase, the spindle spin accumulates plasticized and hot material in
the region of the injection mouth; the spindle is retracted to leave the
necessary space. When the necessary quantity has been
accumulated, the rotation stops, the injection unit advances until its
injection nozzle fits into the appropriate opening of the mold, which,
through distribution channels and by hydraulic means, transfers the
plasticized compound to the mold. During vulcanization, more
material will be plasticized, and after unloading the mold, the
process starts again. Generally, the injection unit is independent of
the mold,

INJECTION MOLDING
 which is kept closed in a normal compression press. The injection is
performed with two different pressures, an initial, slower one, to
accelerate the filling of the cavities; this pressure is reduced to a
value sufficient to prevent the expulsion of the material injected into
the mold and to prevent the formation of bubbles.
 In order to avoid these limitations, injection units were developed,
where a plasticizing system by spindle feeds an injection chamber.
This feed produces the return of a hydraulic piston. When filling the
chamber, the piston advances and, through an injection nozzle, fills
the mold. A valve prevents the compound from returning from the
chamber to the plasticizing cylinder at the time of injection.

INJECTION MOLDING
 An additional improvement is the use of molds with controlled
temperatures in the distribution channels, known as cold channels,
which prevent the vulcanization of the compound in the
supply/distribution channels.
 Due to the several successive heatings, the compound arrives at the
mold at temperatures of 140-150ºC, and, as the vulcanization
temperatures are in the order of 180-200ºC, the total molding time is
normally 1/10 to 1/20 of that necessary for compression molding.

EXTRUSION
 Equipment and extrusion process
 Essentially, an extruder is formed by a spindle rotating inside a
coaxial chamber, which has at one end a feed mouth for introducing
the compound; by turning the spindle, the compound is transported
and accumulated on the head, which is an extension of the cylinder.
In this region there is a small passage to the outside, where we
place the dies that will give shape and dimension to the extrudate.
 Until the 60's, extruders were fed with previously heated and
plasticized compounds; the sole function of the spindle was to
transport the material to the head and generate the pressure
necessary to push the compound through the die. Therefore, the
volume of the channel formed by the cylinder wall, the spindle core
and the spirals gradually decreases from the loading mouth to the
head. The channel volume ratio between the first

EXTRUSION
 spirals and the last one is the compression ratio, which in rubber
extruders has a value of 1.2:1 and 1.4:1.
 To progressively reduce the volume of the channel, the spindle pitch
is reduced or the channel depth is reduced by increasing the
diameter of the core. Hot feed extruders use short spindles, using
the spindle diameter, D, as the unit of length measurement; the
spindles of these extruders have a length between 4D and 6D.
 The ratio between the depth and width of the channel varies
between 0.2 and 0.5, ie, deeper channels are used than those of
extruders for thermoplastics, where this ratio is less than 0.2. The
fillet width varies between 0.08D and 0.12D. The clearance between
the fillet and the cylinder wall is between 0.003D and 0.004D. To
improve flow regularity, dual-entry screws are used, as single-entry
screws produce wave-shaped flow that appears in the dimensions of
the extruded article.

EXTRUSION
1. head, 2. Extruder body, 3. Feeding mouth,4. Locomotive system, 5.
Electric motor.
Figure 5. Schematic of an extruder.

EXTRUSION
 The matrix is what gives the final shape to the extrudate, but it does
not match the final shape of the artifact. Due to the visco-elastic
nature of unvulcanized rubber, the compound undergoes some
expansion at the exit of the matrix and, due to the tensions
generated in the flow, forced by the matrix, the extrudate then
experiences a longitudinal contraction; neither swelling nor
shrinkage is the same in all sections of the artifact, as they depend
on the thickness of the section. Therefore, building a matrix requires
considerable experience and often a lot of retouching.

EXTRUSION
 For approximately 30 years, traditional extruders have been
replaced by cold-feed extruders with considerably larger spindles,
from 12D to 16D. The first zone has a very low compression ratio,
and is intended for heating and plasticizing the compound which, in
the final zone of the spindle, is compressed as in a hot-feed extruder
on the die. Cold fed extruders consume 30-40% less energy per kilo
of compound, but they need higher power motors and their hourly
efficiency is lower.

VULCANIZATION OF EXTRUDED
ARTIFACTS
 The traditional method of vulcanization of extruded artifacts is
vulcanization in an autoclave, which can be saturated steam, hot air
or superheated steam. Saturated steam autoclaves are the most
used, and in them the steam is introduced directly into a closed
autoclave, where the artifacts to be vulcanized are located; the
control is done by pressure, which sets the temperature and time.]
 Steam vulcanization is very simple and does not need any forced
circulation system. It is slower than in-mold vulcanization, due to
worse heat transmission, and vulcanizates have inferior mechanical
properties.

ARTIFACTS
 The first continuous vulcanization method was developed by DuPont
de Nemours and was called LCM (Liquid Curing Medium). In it, at
the exit of the extruder, the extruded artifact passes through a bath
formed by a eutectic mixture of molten salts; the original eutectic
mixture was composed of KNO3, NaNO2 and NaNO3, in the
proportion of 53:40:7, with a melting point of 141ºC; was sold under
the name HYTEC®. Currently, to avoid the problem of nitrite toxicity
in wastewater, a product called SABALITH® is sold, which melts at
130ºC and does not contain nitrites. The bath temperature during
vulcanization is 200-260°C.

ARTIFACTS
 At the end of the 60's, the continuous vulcanization system by
microwaves or ultra-high frequency waves, UHF (Ultra-High
Frequency) was introduced in the market, where the heat is
generated by the movement of molecular dipoles when trying to
follow the orientation movements of the oscillating magnetic field. In
the case of vulcanization ovens, the alternation frequency of the field
is 2450 MHz, that is, the field changes its orientation 2.45 x 1012
times per second.
 The microwave heating system differs from all others: in others, heat
propagates from the outside to the inside, as the heating devices are
external. In the case of microwaves, heat is generated internally,
thus avoiding problems of poor thermal conductivity.

CALENDERING
 Calendering equipment and processes
 In the rubber industry, calendering is used in two
processes:Continuous rubber blade, and Rubberized
fabrics.Basically, the process consists of forming a sheet of
compound by one or more successive passes between pairs of
cylinders that rotate in opposite directions; however, in the first case,
the blade is the final product, and in the second, it is applied on a
textile support.
 The calenders can have two, three or four cylinders.
 In those with three cylinders, they can be arranged vertically, with
their axes in the same vertical plane or with the axes in two planes.
The four-cylinder calenders can be arranged in I, in L, in inverted
L or in Z.

CALENDERING
 To drive the calenders, variable speed direct current motors are
used, with transmission through a Unidrive system. In some modern
ironers, each cylinder has its own speed-controlled drive motor.For
lamination, all cylinders rotate at the same speed. In the
rubberization of fabrics, there are two processes: frictionless and
friction; by the first process, the rubber passes between the
cylinders, compressing it on the fabric; for adhesion to occur we
must resort to the use of adhesives.In the second process, the
cylinders have different speeds: the cylinder on which the rubber is
placed rotates faster, generating shear forces; this causes a certain
mechanical anchorage, which, in some tissues, is sufficient to
ensure adequate adhesion.

CALENDERING
 Often, calendered artifacts are semi-finished articles used in the
manufacture of other more complex artifacts, such as tires, conveyor
belts, etc.; in others, it is the end product.
 As with extruded artifacts, vulcanization methods can be
discontinuous or continuous. In the batch process, we place a fabric
between the layers of calendered material to avoid gluing between
them; vulcanize in autoclave.

CALENDERING - VULCANIZATION
1. vulcanizing drum; 2. primary pressure cylinder; 3. high
pressure cylinder; 4. band tensioning cylinder; 5. guide
cylinder; 6. after heating; 7. steel band; 8. previous heating;
9. calendered laminate

REDUCED BURS
 Degassing
 The removal of gases (occluded air) generates burrs that are
removed by several processes: manual extraction, cryogenic
grinding or sanding. It is recommended to maintain a good tolerance
when closing the molds to reduce their losses to a minimum.

REDUCED BURS
 FEEDING
 the number, size and location of feed holes vary greatly depending
on the molding process, material hardness, dimensional tolerances,
cosmetic considerations and other customer requirements.
 The correct design of the material inlet is a decisive factor in the
reduction of scraps in the process:
 Below are the five most common mold feeding processes:

REDUCED BURS
 Corners
 Two key points must be considered when drawing corners:- The
corner must be rounded, to facilitate the removal of the tooling
 Whenever possible, the mold should open both horizontally and
vertically.
 Thus, when the operator removes the part from the mold, he will
separate the central part and the part will slide out, thus avoiding
losses due to tearing.
 The following figure shows an example of this type of mold.

REDUCED BURS
 Holes
 Always try to use the basic rule of 2:1, that is, the height of the
hole should not be greater than twice the diameter, thus
reducing the pressure necessary to remove the material from
the mold.

REDUCED BURS
 Durability of molds / dies
 As far as possible, we should always use “clean” or low-dirt
materials for the mold, because in injection molding processes,
cleaning the molds is very complicated and can take hours for total
cleaning.
 As far as possible, use polymers of controlled viscosity, avoiding the
use of process aids or even release agents (a surface finish of the
mold is required).
 Additional care must be taken with the use of peroxides, as they
release products that cause mold oxidation; for these applications it
is recommended to use chrome finish or stainless steel.

REDUCED BURS
 In extrusion dies, always choose the steel that has the best abrasion
resistance, because, even when it does not have mineral fillers,
rubber causes considerable wear on the extrusion dies.
 write your specs
 When starting any new design or formulation study, write down your
specifications and demand the same from your customer: a very
detailed specification is a good starting point for designing a
compound/product suitable for your process to meet customer
requirements. your client.
 Take as much information as possible before starting to study a new
formulation.

Transformation and vulcanization of rubber

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Transformation and vulcanization of rubber

Similar to Transformation and vulcanization of rubber (20)

More from Luis Tormento

More from Luis Tormento (17)

Recently uploaded

Recently uploaded (20)

Transformation and vulcanization of rubber