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Friday, September 10, 2010

Fibre Forms and Architecture

a. strand - a compactly associated bundle of filaments. Strands are rarely
seen commercially and are usually twisted together to give yarns.

b. yarns - a closely associated bundle of twisted filaments or strands with
the same filament diameter each between 4-13 mm.
Yarns have varying weights described by their ‘tex’ (weight in grammes
of 1000 linear metres) or denier (weight in lbs of 10,000 yards).
Typical tex range usually being between 5 and 400.

c. rovings - a loosely associated bundle of untwisted filaments or strands.
The same each filament diameter in a roving, usually between 13-24mm.
Have varying weights, i.e. tex range is usually between 300 and 4800.
Two basic types of roving:
Direct roving - filaments are gathered together directly after melting.
Assembled roving - several strands be brought together separately after
manufacture, and usually have smaller filament diameters for better
wet-out and mechanical properties.

d. Chopped strands – continous strands which are chopped into into
various lengths, i.e. 3, 6, 9, 12, 25, 50 mm.
Commonly used in moulding compounds (BMC/DMC) or reinforcing
sharp radii, etc.

e. Chopped strand mat (CSM) - a principal form of glass fibres for fibreglass.
Chopped filament strands of 50 mm length, bonded by suitable binder
which are then pressed to form a mat and wound into rolls.
Typical CSM densities between 225 to 900 gram/m2.
Due to random orientation, common glass to resin ratio range between
1:2 to 1:2.5 by weight.

f. Continous filament mat (CFM) - a continuous glass strand of swirl mat.
Designed for press moulding (cold/hot) and RTM.
Random orientation of continuous multi-filament with low binder content
Allows exceptional wetting compared to CSM, considerable stretch,
and minimises tailoring.

g. Woven roving (WR) – glass roving which are woven to form cloth type
with densities range between 150 to 900 gram/m2.
Glass to resin ratio for WR is usually taken as 1:1, i.e. higher fibre content.
Disadvantages: difficult to cut, difficult to handle (when cut), and more
difficult to mould than CSM.
Typical construction would be WR and CSM laid-up alternately.

h. Woven fabric (cloth) – a woven product of textile yarns with various
weaving styles (plain, twill, harness satin, etc.) and densities range
between 90 to 300 gram/m2.
More expensive and use for high performance applications such as
aircrafts and electrical.
i. Surfacing mat (tissue) – use for backing gelcoat and final decorative layer.
Designed for press moulding (cold/hot) and RTM.

j. Special products/hybrids – combination of various types and forms of
glass to meet individual needs, processes and end-product performance.

Friday, September 3, 2010

Sheet Moulding Compounds (SMC)

SMC is a combination of chopped glass strands and filled polyester resin, in the form of a sheet. Processing of SMC by compression or injection moulding enables the production of bodywork or structural automotive components, and electrical or electronic machine housings in large industrial volumes. The process also penetrates sectors such as sanitary ware (baths) and urban furniture (stadium and cinema seating), etc.

The prepreg contains all the components needed for moulding the final part (resin, reinforcement, filler, catalyst, low profile additives, etc.) in a malleable and non-tacky sheet. Its characteristics allow it to fill a mould under the conditions of moulding temperature and pressure employed.

SMC prepreg is made from glass strands chopped to lengths of 25 or 50mm, sandwiched between two layers of film, onto which the resin paste has already been applied. The prepreg passes through a compaction system that ensures complete strand impregnation before being wound into rolls. These are stored for a few days before moulding, to allow the prepreg to thicken to a mouldable viscosity.

The main process for moulding SMC material is Compression moulding. The film is stripped off and the material is cut into suitable pieces. These are collated into piles of material, which are called the charge. This is positioning in the mould tool. Heated moulds are used and a compression pressure is applied. The base resin being a thermosetting material cures and
hardens. The part is ejected. Any flash is trimmed by the operator.

SMC Manufacturing