KMT uses a variety of different materials and techniques to produce the composite components we deliver. We try to balance the objectives of a project to choose the proper materials and methods to achieve the goals required, be they structural, finish, or as is often the case economy. We believe that in balancing these requirements and analysing the needs of the component we can produce the right product with the appropriate technology.

Matrixes and their reinforcement (thus composites) are fundamentally different from isotropic structural materials, such as metals, in that they can first achieve their maximum mechanical potential when the reinforcement is oriented in the direction of load.  Without a proper matrix these, fibres are like a coiled rope – it has to be straightened out in the direction of load before it can begin to work.  It is the fibres in a laminate that should be doing all of the work, as they are considerably “stronger” than the resin they are locked into.

Fibre

Density gr/cm3

Tenacity Mpa

Modulus Mpa

Elongation at break %

Water absorbtion %

E-Glass

2,58

2000

73

3,5

0,5

S-Glass

2,53

3500

86

4

0,3

Carbon HT

1,78

3400

134

1,5

0,1

Carbon HM

1,8

2250

392

1,1

0,1

Aramid HT

1,44

2800

80

3,3

4,5

Twaron

1,45

2800

125

2

2,2

Technora

1,39

3470

81

4,5

3,2

Dyneema

0,97

2700

90

3,5

0

Certran

0,96

1300

55

4

0,2

Polyester HT

1,38

1160

14

13

0,5

Vectran

1,41

2860

65

3,3

0,1

The matrix’s job is to hold the fibres in place, and in line, while transferring off axis and inter-laminar loads to the reinforcement. Additionally the matrix converts the fibre-stack into a solid (and thus makes it waterproof). It needs to be able to have good fatigue and thermal properties, while being able to retain a hold on the reinforcement. Matrix resins come in many different forms, and have a wide variety of both working and mechanical characteristics:

Epoxies:

Epoxies are one of the most advanced matrixes used in a composite structure today. They are very resistant to water, can absorb allot of energy (thus making them though), and they are very resistant to cyclic fatigue and chemical degradation. Epoxies also perform exceptionally as an adhesive. As epoxy is not formulated with the quantities of solvents present in some of the other available matrixes used today, it exhibits very little shrinkage during cure. Epoxies are, however, more costly than some of the more popular resins mentioned bellow.

Vinyl esters:

Vinyl esters are closely related to the more well-known, polyester resins so popular in boat building. They offer reasonably high mechanical properties, while offering a higher resistance to attack by water than is available from Polyester resins.

Polyesters:

Unarguably the most common resin matrix still used in boat building today.  Polyester offers allot of boat pound for pound. This catalysed resin system is not as tough, nor as resistant to hydrolysis as either epoxy or vinyl ester. Polyesters are not good as glues, and often require good surface preparation to assure a reliable bond.

From the above we can see that a good laminate has a very high fibre content, just like a good breakfast, and has a minimum of voids (nothing is weaker than air). The reinforcement/ resin combination we choose, and the architecture of the fibre stack is dependent on the needs of the end product.

At KMT we use hand lay-up techniques in situations where we are either trying to build up bulk quickly in a laminate, and the laminate is not expected to be subjected to unacceptable loads, or in situations where we need to build up a resin rich surface for example in coating a piece of mahogany with clear resin to make it waterproof. Hand lay-up gives the lowest fibre content of all the laminating techniques we use, but it can be a very cost-effective, and quick way to achieve an objective. With skilled labour a satisfactory laminate can be achieved saving the customer money on materials and sometimes labour.

Except in special cases where we are trying to create an epoxy vapor barrier, or build a deep lustre finish, hand lay-up is almost always done with polyester resin.

All these vacuume techniques have two things in common, the first is that they are limited by the amount of pressure which can be applied (unless we alter atmospheric pressure by laminating on some really heavy planet or in an autoclave – neither solution of which is inexpensive); the second is that in reducing the local pressure inside the bag they encourage any asses trapped in the matrix to come out of solution. This second restriction means that you really have to keep your eye on resins with high solvent levels like Polyester for example – otherwise you will “boil-off” the solvent).

Bladder moulding involves clamping the part against the mould using positive air pressure. We build a closable mould lay an impregnated fibre stack up in the tool, close the tool, and inflate a high pressure balloon inside the tool. The balloon pushes the laminate out against the inside face of the tooling with a force equal to the air pressure applied to it. Limiting factors to the applicable pressure is mould strength.