Shaping Operations

RUBBER MOULDING

Moulded rubber parts can be produced by different manufacturing methods. Major techniques are:

Compression moulding

Compression moulding is a process in which a compound is squeezed into a preheated mould taking a shape of the mould cavity and performing curing due to heat and pressure applied to the material. The method uses a split mould mounted in a hydraulic press Compression moulding process involves the following steps:

1. A pre-weighed amount of the compound is placed into the lower half of the mould. The compound may be in form of putty-like masses or pre-formed blanks.

2. The upper half of the mould moves downwards, pressing on the compound and forcing it to fill the mould cavity. The mould, equipped with a heating system, provides curing (cross-linking) of the

Compound

3. The mould is opened and the part is removed for necessary secondary operations

Injection moulding

Injection moulding is a process in which the compound is forced under high pressure into a mould cavity through an opening (sprue).The rubber material in form of strips is fed into an injection moulding machine. The material is then conveyed forward by a feeding screw and forced into a split mould, filling its cavity through a feeding system with sprue gate and runners. An injection moulding machine is similar to an extruder. The main difference between the two machines is in screw operation. In the extruder type the screw rotates continuously providing output of continuous long product (pipe, rod, and sheet).The screw of the injection moulding machine is called a reciprocating screw since it not only rotates but also moves forward and backward according to the steps of the moulding cycle. It acts as a ram in the filling step when the compound is injected into the mould and then it retracts backward in the moulding step. The mould is equipped with a heating system providing controlled heating and vulcanization of the material. The compound is held in the mould until the vulcanization has completed and then the mould opens and the part is removed from the mould. Injection moulding is a highly productive method providing high accuracy and control of shape of the manufactured parts. The method is profitable in mass production of large number of identical parts. A principal scheme of an injection moulding machine is shown here.

Transfer moulding

Transfer moulding is a process in which a pre-weighed amount of a compound is preheated in a separate chamber (transfer pot) and then forced into a preheated mould through a sprue, taking a shape of the mould cavity and performing curing due to heat and pressure applied to the material. The picture below illustrates the transfer moulding process. The method uses a split mould and a third plate equipped with a plunger mounted in a hydraulic press.

The method combines features of both compression moulding (hydraulic pressing) and injection moulding (ram-plunger and filling the mould through a sprue).The scrap left on the pot bottom (cull), in the sprue and in the channels is removed. Scrap of vulcanized rubber is not recyclable.

The transfer moulding cycle time is shorter than compression moulding cycle but longer than the injection moulding cycle. The method is capable to produce more complicated shapes than compression moulding but not as complicated as injection moulding.

RUBBER EXTRUSION

In the extrusion process of rubber, the compound including polymers, various types of additives and fills like curing agents, antioxidants, pigments are fed into the extruder. The extruder typically consists of a rotating screw inside a closely fitted heated barrel. The primary purpose of the extruder is to do three things, a) soften b) mix c) pressurize the rubber as it is fed continuously to the die at the extruder exit.

The die is a sort of metal disk that has a machined opening in the desired shape of the part that needs to be extruded. The rubber already softened by heating is then forced by the rotating screw through the die opening into the shape of the profile cut in the die. A typical phenomenon called die swell takes place as the rubber shape leaves the die. Because of this the part cross-section becomes larger than the die cross-section. The part cross-section depending on the material may rise up to several folds over the die.

Subsequently the processes of vulcanization or curing takes place as the last step in the extrusion process. This aids the rubber extruded profiles to maintain its shape and acquire necessary physical properties. Typical examples of extruded rubber parts are profiles, hoses, strips and cords.

CALENDERING

                The calendar is an important processing machine in polymer processing industry including both rubber and thermoplastic materials. Calendering is used to produce (1) sheet and film from thick polymer sections and (2) to embed polymeric materials into textile structures. A calendar comprises a large number of rolls or bowls held in a framework. The rolls rotate to produce sheeting and, by adjusting the distance apart of the rolls. The rubber industry normally uses three or four rolls and occasionally for rough gauge sheeting, a two roll calender. While two rolls are normally vertical, modern three roll calenders may have an offset top roll instead of vertical configuration. The offset top roll assists the feeding of the calender from a feed strip. Four roll calenders can have a vertical, an inverted L, or on the most modern calenders, a Z configuration on the rolls. Different Calender configurations are shown below.