Published - 18th Oct 2019
Part 3- Processing into finished articles- Moulding
Each moulding process has its own requirements for the preparation of the uncured material.
Compression moulding will require a blank that can be prepared from a sheet of uncured rubber sometimes cut out using a ribbon knife cutter. Alternatively an extrusion or a performed blank prepared by cutting an extrusion into accurately controlled blanks as it comes out of the die.
Transfer moulding will require pre-weighed blanks usually cut from sheet or extrusion.
Injection moulding machines are usually fed directly by uncured strip rubber.
Industrial Rubber uses all these methods of preparing blanks or performs. We have a Barwell performer that extrudes the uncured rubber that can be cut to weight using a flying knife. We also have a conventional extruder and a two roll mill that can make sheets along with clicker presses that can punch shapes out using ribbon knife cutters.
There are three common methods of moulding and which one used will depend upon many things such as how many parts are required? Is cost an important consideration? How critical is the finished part for dimensions? Is the part to include a bonded insert?
This is a simple form of moulding when a piece of the uncured rubber of the correct weight and shape is placed directly into a hot mould cavity. A blank or perform is placed into the mould cavity which is then closed and compressed usually in a hydraulic press with heated platens. The blank volume is more than the cavity volume and the excess is forced out in the form of flash. The heat from the platens is transferred to the mould which in turn transfer it to the rubber compound and this causes it to vulcanise (or set in shape). After some time (referred to as the Cure Time)- which is dependent on the compound, the temperature, and the thickness of the product being moulded the mould can be opened and the moulding removed.
There are some advantages and disadvantages of this type of moulding.
Advantages- Tooling is generally cheaper to manufacture. It is good for low volumes of production. The moulding equipment can be very simple.
Disadvantages- A blank or perform needs to be made often by hand and cut from sheet or possibly using a performing extruder that will extrude and cut a blank to shape and weight. The cycle times are longer due to poor heat transfer. The flash is usually thicker than other forms of moulding. The process is less easy to automate so will generally require an operator.
TRANSFER INJECTION MOULDING
The heated mould is closed in a press and the rubber is injected through a feed hole in the cavity. The injection can be either by a hydraulic cylinder that is part of the press or it can be part of the mould called a transfer pot.
Advantages- The mould is fully closed before the compound is transferred into the cavity,this will help reduce flash levels. The compound is heated when it is transferred through the small feed hole which helps to reduce the cure time. This process is good when producing rubber to metal bonded parts as it allows the insert to be fully trapped in position by the mould before the rubber is transferred into the mould cavity. It also presents a fresh rubber surface to the bond area which is essential if a good bond is to be achieved.
Disadvantages- The tooling is a little more expensive. It is not generally suitable for large volume production unless multi-cavity tooling is used. It an be wasteful on material as there is always material left in the transfer pot. There is still a requirement to make a pre weighed blank.
Industrial Rubber has compression moulding presses from 25 tons to 450 tons capacity.
Our largest press is 2.25 metres x 1.25 metres. It only has a locking pressure of 150 tons. But it is very useful for sponge moulding as much lower pressures are required for this.
This process lends itself to larger volume production. Multi-cavity moulds are often used with extended run times of the same material. This method requires special moulding equipment. The mould will be closed before the material is injected. The screw injection unit is usually fed by a continuous strip of uncured rubber. It consists of an archimedean screw that will carry out the preparation of the material for moulding. It will warm it and plasticise it making it plastic enough to be able to inject into the mould. It will also prepare a specific shot weight. So there is no blank or preform preparation required. The material in injected through a spur (hole) either directly onto the part or through a runner system and feed gate. For multi-cavity tools there will be a runner system that will feed the material to the individual cavity ( see examples of runners)
Advantages- There is no requirement to make a perform eliminating this part of the process. The cycle times are faster as the uncured rubber is preheated in the barrel and it is then forced through small feed gates which generates heat in the material reducing the cure times.
Disadvantages- Special moulding equipment is required & tooling is more expensive. Multi cavity tools often require complicated runner systems to ensure even filling of each cavity. These runner systems are cured during the moulding process so they are waste. This can be overcome by the use of a cold runner block system.
Industrial Rubber has injection moulding capacity up to 500 tons of locking pressure and 2 litres shot weight. They are CNC controlled allowing innjection volumes, pressures, times to be set and automatically controlled.