Manufacturing composites

Question 1

Resin Transfer Molding (RTM)

Answer 1

• Thermoset + fiber
• Most commonly E-glass fibers together with unsaturated polyester (UP)

Question 2

RTM - types and usage

Answer 2

1. HP RTM - High pressure whithin mold
2. Light RTM - flexible molds used

• Automobiles
• Aircraft propellers
• Components to boats

Question 3

RTM - process and properties

Answer 3

1. A dry pre-formed fiber is placed in a mold
2. Resin (=plastic) is injected under pressure and vacuum
3. Materials cure and then taken out of mold

+Good health and safety
+Both sides of component have molded surface

-Expensive equipment
-Limited to produce smaller components

Question 4

Hand Lay-up

Answer 4

• Wet hand lay-up = simplest and most versatile of all manufacturing techniques
• Heavy hand-work: spray-up can be used instead = spraying chopped reinforcement and resin into mold while hand-rolling

1. One-sided mold treated with mold-release agent is covered by a gelcoat (neat resin layer)
2. Resin is applied on top of gelcoat
3. Dry reinforcement applied
4. Hand-roller works resin upwards into the reinforcement

Question 5

Prepreg Lay-up

Answer 5

• Prepreg = pre-impregnated fibers + partially cured matrix
• Prepreg stored in freezer to maximize life-span
• High viscosity - cannot work by hand
• Prepreg materials like: HTM, MTM, LTM = High/medium/low curing temp.

Question 6

Prepreg Lay-up - usage and properties

Answer 6

• High-performance racing vehicles
• Spacecraft

+High preformance parts
+Reduces health concerns

Question 7

Prepreg Lay-up - Process

Answer 7

1. Prepreg taken out of freezer to slowly reach room temp. and is cut
2. Prepregs are stacked a one-sided mold (make sure no air or contaminants are in between piles)
3. Release film placed on stack of the prepreg - allows air and resin to escape
4. Bleeder on top of that - absorbs excess resin
5. Barrier - keeps excess resin beneath it
6. Breather to allow air to escape and keep pressure even
7. Vacuum bag

Question 8

Vacuum Injection Molding

Answer 8

• Vacuum to draw resin from container into vacuum bag/mold
• Used for boats, parts to vehicles
• Main pros: fast, cheap, less health concerns than hand lay-up, less material needed, better quality

Question 9

Compression molding (SMC and BMC) - usage

Answer 9

• Automobiles
• Containers
• Electrical components

Question 10

Filament winding

Answer 10

• Fibers threads are wetted in resin and wound around rotating mold
• Efficient
• Automated
• Cost-effective
• Limited to certain geomatries - ex. tubes

Question 11

Pultrusion - usage

Answer 11

• Window and door frame
• Fishing rods

Question 12

Manufacturing of thermoplastic composites

Answer 12

• Thermoplastic prepreg lay-up: ex. for aircraft
• Thermoplastic pultrusion: rare and few applications
• Compression molding: GFT = glass fibers; LFT = long fibers; automotive industry
• Thermoplastic filament winding: good mechanical properties (pressure vessels) but expensive

Question 13

Manufacturing defects

Answer 13

• Fiber waviness - reduced stregnth and stiffness
• Dry spots of fibers - weak
• Resin-rich regions - brittle
• Debonding
• Residual stresses
• Micro cracks
• Voids

Question 14

Voids - causes/parameters

Answer 14

• Lay-up procedure
• Debulking (prevent by using vacuum to remove air/voids)
• Pressure
• Heating rate

Question 15

Voids - consequences

Answer 15

• Reduces compressive and interlaminar shear strength
• Higher moisture uptake
• Reduces service life

Question 16

Voids - determination/testing

Answer 16

• Density measurements
• Optical microscopy - cut and polish to see
• Ultrasonic C-scan
• X-ray
• X-ray microtomography