Rubber deflashing

Rubber Deflashing - What It Is and
Why It's Applied
Luis Tormento
LT Quimicos
09/10/2012

What is Flash?
 Flash is the result of the overflow of rubber
compounds out of the molding area.
 This overflow is undesirable and should be
removed

Deflashing
 In most cases deflashing is a necessary step
in the manufacture of molded rubber, whether
by:
 Injection
 Compression
 Transfer

Deflashing
 If overflow is controlled / planned or not, it is
undesirable and should be removed.
 The manual withdrawal is time consuming and
tedious.
 Machining processes, despite being automated,
are loaded manually and are time consuming.

Flashes
 In order to obtain better appearance and
technical quality, we must use appropriate
techniques to:
 Decrease flash generation
 Decrease size and thickness of flashes

Flashes
Techniques to reduce flash
generation

How to reduce flash - Process
 In molding we must control four basic
categories:
 Temperature
 Pressure
 Time
 Distance
Temperature
Pressure
Time
Distance

 Temperature
 Melt point control
 The flow of material in the mold includes its path
from the feeder to the mold cavity.
 Generally the material is heated and then flows
into the molding cavities
 Mold temperature control
 Variations in mold temperature are responsible for
greater or lesser contraction

 Pressure
 Initial pressure
 Applied to the fused compound, it is responsible for
filling the cavity and forms the artifact
 Final pressure (lift)
 Used to fill the final mold and maintain the shape until
the final solidification (vulcanization) of the artifact.
 Return pressure

 Time
 The time required to cast an artifact is the sum
total of:
 Mold closure
 Flow of the compound into the cavity
 Opening to eliminate voids and occluded air
 Vulcanization until reaching T90
 Opening
 Demoulding

 Distance
 Importance
 Distance control is critical in the production of high
quality products because long distances mean long
cycles of curing (curing)
 Mold closure distance
 The initial closing speed is very fast.
 The final closing speed (1 cm) is very slow
 Mold opening distance
 The mold opens slowly (1/4 ") to break the vacuum
formed in the molding.

How to reduce flash – Molding
 The molding step has a great influence on
the generation of flashes.
 Molds that do not follow an RMA molding pattern
tend to generate a greater amount of flassh.
 Equipment with pressure and temperature
deficiencies also tend to generate more flashes.

How to reduce flash - Molding
 Molding Processes
 Compression Molding
 Transfer Molding
 Injection Molding

 Compression Molding
 Most of the presses used in compression molding are hydraulic presses of
ascending lower plateau;
 In these, a hydraulic cylinder is housed in a bench or in the lower part of the
press;
 The piston, by means of hydraulic pressure, rises until it is compressed on
the upper plateau, fixed in the upper part of the press.
 To open the press simply close the inlet of hydraulic fluid and open the
outlet, this fact that dislodges the liquid of the cylinder for the descent by
gravity of the piston and lower plateau;
 Auxiliary systems, mechanical or hydraulic, are often used to accelerate the
opening and improve the productivity of the installation.

Presses used in compression molding

 We highlight the different heating systems of
the plateaus:
 Steam - the plates 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 it is recommended to use pressure switches, which ensure the consistency
and precision required to maintain the vulcanization temperature.
 Thermofluids - fluids are liquids that withstand temperatures of up to 200 ° C or
more, without boiling or suffering appreciable thermal degradation, for prolonged
periods; Circulate on the plateaus instead of the steam; The capacitors are
removed and work at low pressures, which simplifies the sealing problem. As the
thermofluid is kept in circulation in a closed circuit, generally of little extent,
temperature control is easier.

 We highlight the different heating systems of
the plateaus:
 Electrical - the system is clean and simple, but usually presents problems of
temperature uniformity over the surface of the plateau and oscillations over time;
with electronic pulse control systems, frequency and duration between the actual
and theoretical temperature, it is possible to reduce these oscillations. Recently a
heating system was started by heating plateaus by induction, obtaining a faster
heating and better regulation and uniformity of the temperature.

 Compression molding is not very different from the
manufacture of a cookie or waffle. A given quantity of
material must be placed in a cavity, ensuring that it is
filled. Heat and pressure are applied by taking the
compound to flow, filling the cavity and shaping the part;
The 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.

 The excess, or flash, created by pieces of large
diameter, is of particular interest when using more
expensive compounds. Compression molding helps to
reduce this excess.
 The range of application ranges from simple O-rings to
belts and complex diaphragms with a diameter greater
than 254.0 mm.
 The flash in a typical compression molded part has a
maximum of 0.102 x 0.254 mm at 0.127 x 0.813 mm,
depending on the deflashing method.

 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 slides into the cavity
through one or more holes called "door" or "passageway".
 The flash in a small mold or O-ring will be at most 0.127 mm
thick, extending to approximately 0.076 mm on the surface of the
part.

 Transfer Molding

 Injection Molding
 Injection molding is the most automated of the molding
processes. The material is heated to an easy flow
state; is injected under pressure from the heated
chamber through a series of holes or "ports" in the
mold.
 Injection molding is ideal for high production volumes
of relatively simple configuration rubber parts.

 There are two types of injection presses:
 Plastification and injection by spindle, and
 Plastification by spindle and injection by piston.

 Degassing
 The removal of gases (air entrapped) generates
flashes that are removed by various methods:
manual extraction, cryogenic grinding or sanding.
It is advisable to maintain a good tolerance in
mold closure to reduce your losses to a minimum.

How to reduce flash - Mold
 Desgassing
 As tolerance for these degassers it is suggested:
Material hardness Depression or typical projection
from the surface
Less than 50 0.381mm
50 or more 0.178mm

 Feeding
 The number, size and location of feeder holes
vary greatly, depending on the molding process,
material hardness, dimensional tolerances,
cosmetic considerations and other customer
requirements.
 The correct material entry design is a decisive
factor in the reduction of scrap in the process

 Feeding
 Here are the five most common mold feeding
processes:

 Corners
 Two key points should be considered when
designing corners:
 The corner should be rounded to facilitate removal of the
tooling
 Whenever possible, the mold should open both
horizontally and vertically.
 Thus, when the operator removes the part from
the mold, it will separate the central part and the
piece will slip away, thus avoiding tearing losses.

 Corners
 The figure below shows an example of this type of
mold.

 Holes
 Always try to use the basic rule of 2: 1, ie: the
height of the hole should not be more than twice
the diameter, thus reducing the pressure required
to remove the material from the mold.

 Durability of molds / dies
 As far as possible, we must always use "clean" or
low dirt materials on the mold; this is relevant
because in the injection molding processes the
cleaning of the molds is very complicated and
may require hours for its completion.
 As far as possible, use polymers of controlled
viscosity, avoiding the use of process aids or even
release agents (a mold surface finish is
necessary).

 Durability of molds / dies
 Additional care should be taken with the use of peroxides, as
they release products that cause oxidation of the molds; for these
applications it is recommended to use chromed or stainless steel
finish.
 Take care of the cleaning of the molds: very abrasive cleaners
will wear out the mold and deform the dimensions generating
more flashes.
 Sandblasting was banned in 1999, as it causes silicosis.
Currently plastic media are used for this process. Ideally, the
mold will last for 100,000 times.

How to remove flashes
 After we have taken all the care in process
control, compounds and molds to reduce
flashes, we will now explain the processes of
removal of them:

Deflashing

Deflashing
 Several processes and methods are used in
the deflashing of rubber artifacts:
 Manually
 Drilling
 Cutting
 Milling
 Cryogenics

 Manual Deflashing - Deflashing was initially a
manual operation. Dozens of workers, seated
in tiny workstations, picked up the pieces one
by one and removed the excess rubber (burr)
with knives, scissors and special devices.
Even today, some artifacts need to be
deflashed in this way, due to their complexity
and size.

 We can say that manual deflashing has
several drawbacks:
 The quality of manual finishing is unstable
 Manual deflashing is a slow and time-consuming
process,
 Due to the extensive use of labor it is gradually
becoming a costly process.

 Other manual deburring processes include:
 Drilling
 Machining
 Sanding
 Cleaning

 Manual Process uses a series of techniques
and instruments:

 Vibrators and Rotating Chambers
 The first cryogenic machines were vibrators and
rotating chambers. (Model 300)
 In these machines, the artifact is frozen by means of
liquid nitrogen until reaching its glass transition
temperature. The machine has the internal hexagonal
shape and the hardened flash is removed by impact
between the frozen parts or by means of some type of
media.
 Plastic or ceramic media are generally used to assist in
impact and break flash.

 Vibrators and Rotating Chambers
 They are limited: they are efficient when the artifact must
be deflashed externally; are of low efficiency for internal
deflashing.
 Currently the vibrators and rotating chambers are
efficient in the deflashing of several artifacts; in other
artifacts, serve as a pre-treatment for finishing the most
efficient cryogenic equipment - one of these equipment
is the rotating blasting basket (Polyblaster 2.0).

Polyblast modelo 300

 Shotblasting Equipment (Polyblaster 2.0)
 A large increase in the productivity of the
deflashing came with the development of basket-
type equipment by blasting media.
 In this equipment the artifacts are frozen in a
basket with a volume of 28 to 112 liters. This
basket is positioned in a thermally insulated
chamber after reaching the desired temperature
(according to the type of elastomer).

 Shotblasting Equipment (Polyblaster 2.0)
 The basket rotates and on the artifact is injected
plastic media to break the flash; This flash falls
along with the media to a set of sieves that makes
removal of the flash and broken media.
 The media returns to a container and, via
compressed air, returns to the deflashing
chamber.
 After the preset time the artifact is free of flashes
and ready for use.

Polyblaster 2.0

 There are only four basic controls on these
machines:
 Temperature
 Injection Wheel Speed
 Basket speed
 Cycle time

 These media are polycarbonate, especially
composite to withstand impact at -184°C.
(For example, silicone deflashing)
 These polycarbonate media are cylindrical in
shape.
 It has already been proven that the shape of
the media has little effect on the deflashing
process, but the cylindrical shape is the
easiest to handle.

 The temperature needed to crystallize the
flash varies from compound to compound.
 Important factor in the productivity of this
process is to determine the ideal temperature
(Tg) for each compound; With this it is
possible to better deflashing and saving of
nitrogen.

 Reference Temperatures:
Denominação Temperatura do Ponto
de Rigidez ºC
Faixa de
Temperatura de
Rebarbação ºC
Etileno-Propileno (EPDM, EPT, EPR) -68 -70 a -96
Borracha de Cloropreno (Neoprene) -46 -60 a -70
Butadieno Acrilonitrila (Buna - NBR) -46 -50 a -70
Epicloridrina (Hydrin, ECO) -34 -43
Poliuretano (Millathane) -29 -45
Fluopolímero (FPM) -37 -51
Poliacrilato (Vamac) -51 -60 a -80
Polietileno Clorosulfonado (Hypalon) -51 -62 a -70
Silicone -85 -87 a -118
Fluorsilicone -68 -68
Fluorelastômero -29 -29
Borracha de polibutadieno -73 -80 a -100
Borracha Natural -60 -70 a -100
Borracha de Isopreno (Poliisopreno) -59 -70 a -90
Borracha butílica (IIR) -59 -70 a -90
Borracha de Estireno Butadieno (Buna-S, SBR) -59 -80 a -100

Thank you
Luis A. Tormento
LT Químicos
Ltormento@gmail.com
Tel: (11) 988990267

Rubber deflashing

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