Composite Processing

Question 1

What is a PMC

Answer 1

a composite material consisting of a polymer embedded with a reinforcing phase such as fibers or powders

Question 2

Why are techniques for shaping composites generally less efficient than other materials.

Answer 2

(1) composite materials are more complex than other materials, consisting as they do of two or more phases and the need to orient the reinforcing phase in the case of fiber reinforced plastics
(2) processing technologies for composites have not been the object of improvement and refinement over as many years as processes for other materials.

Question 3

What are the five categories of fibre-reinforced polymer composite shaping processes?

Answer 3

(1) open mold processes
(2) closed mold processes
(3) filament winding
(4) pultrusion processes
(5) other

Question 4

Simply, what are open mold processes?

Answer 4

Open mold processes include some of the original manual procedures for laying resins and fibers onto forms.

Question 5

Simply, what are closed mold processes?

Answer 5

Closed mold processes are much the same as those used in plastic molding.

Question 6

Simply, what is filament winding?

Answer 6

In filament winding, continuous filaments that have been dipped in liquid resin are wrapped around a rotating mandrel; when the resin cures, a rigid, hollow, generally cylindrical shape is created.

Question 7

Simply, what is pultrusion?

Answer 7

Pultrusion is a shaping process for producing long, straight sections of constant cross section; it is similar to extrusion, but adapted to include continuous fiber reinforcement.

Question 8

What are the different types of thermoset polymer matrix materials and when are they used?

Answer 8

Thermosetting (TS) polymers are the most common matrix materials. The principal TS polymers are phenolics, unsaturated polyesters, and epoxies. Phenolics are associated with the use of particulate reinforcing phases, whereas polyesters and epoxies are more closely associated with FRPs.

Question 9

What are common fibre materials?

Answer 9

Common fiber materials in FRPs are glass, carbon, and the polymer Kevlar.

Question 10

How is Kevlar produced?

Answer 10

Kevlar fibers are produced by extrusion combined with drawing through small orifices in a spinneret.

Question 11

Carbon fibres are formed by the heat treatment of a precursor. What are typical precursors?

Answer 11

The precursor can be any of several substances, including polyacrylonitrile (PAN), pitch (a black carbon resin formed in the distillation of coal tar, wood tar, petroleum, etc.), or rayon (cellulose).

Question 12

What is a roving?

Answer 12

A roving is a collection of untwisted (parallel) continuous strands; this is a convenient form for handling and processing. Rovings typically contain from 12 to 120 individual strands. Continuous rovings are used in several PMC processes, including filament winding and pultrusion.

Question 13

What is a yarn?

Answer 13

A twisted collection of continuous strands.

Question 14

What is a cloth?

Answer 14

The most familiar form of continuous fiber is a cloth - a fabric of woven yarns.

Question 15

What is the difference between a cloth and woven roving?

Answer 15

A woven roving is a fabric consisting of untwisted filaments rather than yarns. Woven rovings can be produced with unequal numbers of strands in the two directions so that they possess greater strength in one direction than the other. Such unidirectional woven rovings are often preferred in laminated FRP composites.

Question 16

What is a mat?

Answer 16

Fibers can also be prepared in the form of a mat - a felt consisting of randomly oriented short fibers held loosely together with a binder, sometimes in a carrier fabric. Mats are commercially available as blankets of various weights, thicknesses, and widths. Mats can be cut and shaped for use as preforms in some of the closed mold processes.

Question 17

What does a sheet molding compound (SMC) consist of?

Answer 17

a combination of TS polymer resin, fillers and other additives, and chopped glass fibers (randomly oriented) all rolled into a sheet of typical thickness is about 6.5 mm

Question 18

In a SMC what is typically the resin and fillers?

Answer 18

The most common resin is unsaturated polyester; fillers are usually mineral powders such as talc, silica, limestone; and the glass fibers are typically 12 to 75 mm long and account for about 30% of the SMC by volume.

Question 19

In general what does an open mold process consist of?

Answer 19

The starting materials (resins, fibers, mats, and woven rovings) are applied to the mold in layers, building up to the desired thickness. This is followed by curing and part removal.

Question 20

What are the five steps in the hand lay-up process?

Answer 20

(1) mold is cleaned and treated with a mold release agent
(2) a thin gel coat (resin, possibly pigmented to color) is applied, which will become the outside surface of the molding
(3) when the gel coat has partially set, successive layers of resin and fiber are applied, the fiber being in the form of mat or cloth; each layer is rolled to fully impregnate the fiber with resin and remove air bubbles
(4) the part is cured
(5) the fully hardened part is removed from the mold

Question 21

What is involved in the spray up process?

Answer 21

It functions with a similar mechanism to hand lay up, however the the resin and chopped fibres are sprayed onto the mold with a spray gun.

Question 22

What are limitations of the spray up process?

Answer 22

.The mixing action results in random orientation of the fibers in the layer-unlike hand lay-up, in which the filaments can be oriented if desired. Another difference is that the fiber content in spray-up is limited to about 35% (compared with a maximum of around 65% in hand lay-up).

Question 23

What are advantages of the spray up process?

Answer 23

Spraying can be accomplished manually using a portable spray gun or by an automated machine in which the path of the spray gun is pre-programmed and computer controlled. The automated procedure is advantageous for labor efficiency and environmental protection. Some of the volatile emissions from the liquid resins are hazardous, and the path-controlled machines can operate in sealed-off areas without humans present.

Question 24

What are the three principal process parameters in curing?

Answer 24

Time, temperature and pressure.

Question 25

What does the term, autoclave molding refer to?

Answer 25

The curing of a prepreg laminate in an autoclave. This procedure is used extensively in the aerospace industry to produce advanced composite components of very high quality.

Question 26

What is filament winding?

Answer 26

Filament winding is a process in which resin-impregnated continuous fibers are wrapped around a rotating mandrel that has the internal shape of the desired FRP product. The resin is subsequently cured and the mandrel removed. Hollow axisymmetric components (usually circular in cross section) are produced, as well as some irregular shapes

Question 27

What are the three methods wound fibres can be impregnated with resin?

Answer 27

Wet winding
Prepeg winding
Post impregnation

Question 28

What is wet winding?

Answer 28

the filament is pulled through the liquid resin just before winding

Question 29

What is prepeg winding?

Answer 29

filaments preimpregnated with partially cured resin are wrapped around a heated mandrel

Question 30

What is post impregnation?

Answer 30

filaments are wound onto a mandrel and then impregnated with resin by brushing or other technique

Question 31

What are the two basic winding patterns?

Answer 31

Helical and Polar winding.