Rubber Goods 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

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

echniques to reduce flash generation

How to reduce flash - Process

In molding we must control four basic categories: Temperature Pressure Time Distance

TemperatureTemperature

PressureTime

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

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.


Compression Molding

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.


Compression Molding


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.

How to reduce flash - Molding 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.


Injection Molding There are two types of injection presses:

Plastification and injection by spindle, and Plastification by spindle and injection by piston.

How to reduce flash - Molding Injection Molding


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.

How to reduce flash - 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

How to remove flashes 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 ºCFaixa de

Temperatura de Rebarbação ºC

Etileno Propileno (EPDM, EPT, EPR)‑ -68 -70 a -96Borracha de Cloropreno (Neoprene) -46 -60 a -70Butadieno Acrilonitrila (Buna NBR)‑ -46 -50 a -70Epicloridrina (Hydrin, ECO) ‑34 ‑43Poliuretano (Millathane) -29 -45Fluopolímero (FPM) -37 -51Poliacrilato (Vamac) ‑51 ‑60 a 80‑Polietileno Clorosulfonado (Hypalon) ‑51 ‑62 a 70‑Silicone ‑85 ‑87 a 118‑Fluorsilicone ‑68 ‑68Fluorelastômero -29 -29Borracha 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 -90Borracha de Estireno Butadieno (Buna S, SBR)‑ ‑59 ‑80 a 100‑

Thank you

Luis A. Tormento

LT Químicos

[email protected]

Tel: (11) 55810708

www.ltquimicos.com.br

mailto:[email protected]

Rubber Goods Deflashing

Engineering

Transcript of Rubber Goods Deflashing