The Rubber/ Elastomer manufacturing process is a two stage process

The first stage is the mixing / compounding stage where the elastomer is mixed with various additives such as re-enforcing agents, softeners, pigments, curing agents, antioxidants and processing aids. The mixing occurs in a dispersion kneader which mixes the ingredients under high pressure, controlled high temperature and preset mixing times. All the ingredients are combined to form compound lumps which are dumped out of the dispersion kneader. The lumps are cooled and aged for a few days in order to improve final properties of the component.

These compound lumps are then mixed on a two-roll mixing mill. Curing agents such as sulphur or peroxides are added on the two roll mill and compound is milled till it is homogeneous. The compound is then taken out as sheets in the required thickness. The thickness of the compound sheet can be adjusted in the two-roll mixing mill. This compound sheet is now sent for moulding / extrusion.

 

Elastomer products may be shaped into their final form by either an extrusion or a moulding process.

 

Extruded products have a constant cross-section along the entire length of the product. Typical extruded rubber products include tubing, seals and gasket profiles and weather stripping.

In the extrusion process, the rubber compound sheet is fed into an extruder. The extruder consists of a rotating screw inside a close-fitting heated barrel. The purpose of the extruder is to soften, mix and pressurize the rubber as it is continuously conveyed to a shaping die at the extruder exit.

The die is a metal disk with a machined opening in the desired shape of the extruded part. The pressure built up by the rotating screw forces the rubber, which has been softened by the heating, through the die opening—shaping it into the profile cut in the die.

 

When the rubber shape (extrudate) exits from the die, the material swells so that the part cross-section is larger than the die cross-section. This phenomenon is called "die swell"; depending on the rubber formulation and the extrusion conditions, the dimensions of the extrudate may be increased by several percent to several hundred percent beyond those of the die. Furthermore, except for circular cross-sections (i.e., circular cords), the rubber does not swell uniformly in all directions. For example, in order to extrude a square cross-section, the die must be cut so that the sides of the square are curved inward.

After leaving the die, the rubber extrudate must be cured or "vulcanized" to maintain its shape and acquire the physical properties necessary for the application. This is accomplished by a chemical reaction (the cross-linking of molecules) from peroxides or sulphur curing agents. Vulcanization is initiated when the extrudate temperature is raised by passing it through a curing media, such as a microwave and/or a hot air process.

Part dimensions change somewhat during the curing process, so this must also be accounted for in the die design.

Curing is the final step in the rubber extrusion process, although the extrudate frequently undergoes secondary processing such as slitting, splicing, cutting to specified lengths etc.

 

A three-dimensional object, such as a ball, is processed by moulding in which the rubber is forced to conform to the surfaces of a mould cavity. Typical moulded rubber products include Washers, O Rings, Bushes, Pads, Tops etc.

In the moulding process, the compound sheets are cut and fed into heated moulds and baked for the compound to vulcanize and lock in the dimensions in accordance to the cavities cut in the mould.

Moulds are made out of metals such as mild steel, EN31 or stainless steel as per requirements. The moulds are pre heated to the required temperatures, and pre-cut compound sheets are fed into the moulds. The moulds are then clamped shut using hydraulic pressure. The compound flows like a liquid under this pressure and takes the shape of the cavities in the mould. The moulds are left to bake for pre determined times which raises the temperature of the compound and allows it to cure or "vulcanize". This allows the component to maintain its shape and acquire the physical properties necessary for the application. This is accomplished by a chemical reaction (the cross-linking of molecules) from peroxides or sulfur curing agents.

Once the vulcanization time is over, the moulds are opened, and the components are removed from the mould along with any flash. The components are then cooled and de-burred, before final dimensional checks and despatch.
For metal, plastic bonded rubber components, the part to be bonded is cleaned and then coated with a suitable bonding agent. The coated part is loaded into the mould along with the pre-cut rubber compound sheets and they are vulcanized together. The rubber is moulded onto the metal/plastic, and they are chemically bonded together. Various jute mill components, pads, tops are manufactured in this way.