Agriculture Reference
In-Depth Information
10-15% of rice hull ash to the mix would give a better mix with improved compression
strength (load bearing), smoother surface and a tougher product that will last much
longer than the traditional bitumen only construction.
12.2 What is Compression Moulding?
Compression moulding is a simple and cost-effective way to manufacture many large
parts, especially for the industrial, automotive and air-travel sectors. It is similar to
many other forms of moulding parts, whereby a polymer resin is put into a mould,
heated, compressed and then cooled, with the end result virtually the same. This
process is very common in industrial moulding.
There are many types of compression-moulded parts and processes. These parts
distinguish themselves by the type of material used, the length of the reinforcing
fibres, their size and surface quality. Each type of material and part has its own unique
variation of a compression-moulding process. Here, we will discuss three of the widely
used processes: compression moulding of sheet-moulding compound (SMC) parts;
the injection-compression moulding of bulk-moulding compound (BMC) parts; and
the compression moulding of long fibre-reinforced thermoplastics. SMC and BMC
are fibre-reinforced thermosetting materials. The common matrix in SMC and BMC
is unsaturated polyester crosslinked with styrene. The automotive industry is the
largest user of SMC/BMC materials and parts. Phenolic polymers (probably the oldest
synthetic polymers) are used for parts that require excellent part stability at elevated
temperatures and high creep resistance. Furthermore, phenolic polymers deliver these
qualities at prices that are comparable with acrylonitrile-butadiene-styrene.
The heart of the compression-moulding process is mould filling. From hand filling,
this phase has advanced to specialised moulds in which charge weight is measured
exactly before it is introduced into a mould. There is some variation in SMC, so it is
often necessary to place small pieces of SMC on the centre of the charge to complete
the full-weight of the part. The automotive industry has moved beyond using just
compression-moulded parts to manufacture fibre-reinforced components to reduce
weight to increase fuel efficiency. The versatility of PCRH, for example, to produce
high-strength, lightweight and cosmetically superior parts has led to its use for critical
structural parts and body panels, showcased in most of the high-end automobiles,
such as sports cars and luxury cars.
Compression moulding usually functions by using a mould in two halves. The bottom
half is fixed and stationary, whereas the top half of the mould moves up and down
vertically. An electrically or hydraulically operated press is used with provision for
heating both halves of the mould. If using thermosets, these raw materials must be
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