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.
