Vulcanization Techniques

Rubber products are made by several curing techniques  and the method are based on shape and size of the product

Molding:

Molding is the operation of shaping band curing rubber compound by means of heat and pressure in a mold of proper form.

1. Compression moulding

2.  Transfer moulding

3.  Injection moulding.

Compression Moulding:

It consists of placing a pre cut rubber compound into a two piece heated mold kept in a press.

The pressure applied by the press forces the material to flow and fill the mold.

Transfer Moulding:

In transfer moulding the rubber compound is pre heated in one part of the mold and forced into the closed cavity of the mold where the rubber is cured.

Injection Moulding:

Injection moulding is a manufacturing process for producing parts by injecting material into a mould.

Blow Moulding:

Blow Moulding is a process commonly used when parts are hollow in design. Elastomer materials are injected into a hollow molten tube or parison. The mould encapsulated the parison while compressed air blow up the parison like a ballon, forcing it outward to conform to the inside shape of the mould, After cooling a hollow part emerges

Vacuum Moulding

Curing by other modes

Open cures:

1. Autoclave or steam cure

2. Hot air cures

3. water curing

4.lead curing

5.continuous vulcanization process

6.Hot air tunnel

7.Fluidized bed

8.Rotocure or drum cure

9.micro wave curing

10.Electron beam curing

   

OPEN CURE:

Hot air oven

Hot air ovens can be used to vulcanizate thin articles(ballons and like items which have been preshaped (by extrusion) or by a combination of precuring in mould followed by post vulcanization in an oven.The last process is used to remove peroxide decomposition products from items cured with peroxides.The systme is not too efficient due to the poor heat transfer of hot air, and longer cure time at lower temperature are necessary to prevent the formation of porosity or deformation of the unvulcanized products.

Open Steam

Open steam can be used in closed containers called autoclaves (which resemble home pressure cookers)the process invloves saturated steam under pressure,which acts as an inert gas,enabling better heat transfer, thus higher temperatures can be used and shorter cure times are possible making this process more desirable than the air oven,Hose, cables,builtup footwear and tires in hot pot-heater moulds are cured by this method.

Water cures:

Water cures can be used for articles not affected by  immersion.This method is useful for large items like large containers or rubber lined containers and is expecially  useful for hard rubber compositions.Direct contact with water produces better heat transfer than do air or open steam consequently with this system less deformation and faster cures are obtained.

Lead Sheathing:

Lead sheathing can be used to cover soft,large extruded sections with a protective cover for vulcanization in steam.The proces is used for garden and other hose, the lead sheath is usually applied immediately by hydraulic pressure or extrusion as the hose emerges from its extruder.

Continuous vulcanization processes:

Continuous vulcanization generally invloves some form of heating in a manner such that the vulcanization generally invloves some form of heating in a manner such that the vulcanization step usually occurs immediately after the rubber article is formed.This process is normally used for extruded goods, coated wiring,conveyor belts and flooring.

Liquid Curing Methods

It invloves suitable hot liquid baths in which extrusions may be vulcanized in a continuous proces.Items can be cured rapidly at temperature from 200 degree to 300 degree celsius, however the compounds must be modified to prevent porosity, always a problem with any extrudate.

Suitable materials for the curing medium include bismuth tin alloys,poly glycols and certian silicone fluids.

Fluidized beds:

Fluidized beds consisting of small particles suspended in a stram of heated air are efficient vulcanization systems.They are normally used for continuous vulcanization of extrusions.Heat transfer is approximately 50 times greater than with hot air alone.

Continuous Hot air

Continuous hot air tunnels can be used in the same manner that hot air ovens are used.Thin articles for example dipped goods can be vulcanized after dipping by passing an endless conveyor carrying the items through a series of heated tunnels at a rate that will completer vulcanization by the time articles are stripped from the forms.

Steam Tubes:

Steam tubes are used for the continuous vulcanization of sheathing,After extrusion of the rubber cover onto the wire, the cable passes into a jacketed steam tube containing steam under pressure.

Depending on the thickness of the cover, the period of time in the tube can be controlled to completer vulcanization or controversially the length of the steam tube can be extended.

Rotocure:

The rotocure process can be used to vulcanize largely conveyor belts and continuous flooring strips.The process involves the use of an endless steel band which presses the article against a large heated drum.Slow rotationn of the drum permits vulcanization to occur after approximately 10 minutes contact time.Belt curing presses are also used however this systme is not completely continuous although long lengths of belting are made by this method.

Cold vulcanization:

Thin articles may be vulcanized by treatment with sulfur monchloride S2Cl2  by dipping in a solution or exposure to its vapours.The process has been essentially replaced with ultra accelerators, which are capable of curing at room temperature.

High Energy Radiation:

Systems using either gamma radiation from cobalt 60 or electron beams have been used for vulcanization.The electron beam method has been used to cure both polyethylene and silicone rubbers,generally accomplished by passing the material through a beam on a conveyor.

Microwave Vulcanization:

Ultrahigh frequency fileds developed by alternating electromagnetic circuits can be used to warm up or vulcanize articles with large or uneven cross section.The proces require polar rubber mixtures since non polar materials will not absorb the energy produced.It is possible to warm articles up to 200 degree celsius within 30 seconds with some Ultra high frequency systmes.Continuous processes utilizing extruders and microwave heaters are available.

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