1
Q
Plastics
A
- made up of small molecules bonded together into long chains called polymers
- group or large molecules organic compounds, primarily produced as a chemical product and susceptible to shaping under combination of pressure and heat
- Both thermoplastics and thermosetting plastics may be processed by molding, casting and extrusion
2
Q
Plastic structure
A
- Plastics are all polymers
- smallest unit structure or molecule that IDS the chemical involved is called a monomer
- may be made to join and grow into a much larger molecules by the process of polymerization
3
Q
Thermoplastics
A
- have a degree of polymerization control in the initial manufacture of the plastic raw material or resin
- Soften with increasing temp and regain rigidity as the temp is decreased
4
Q
Thermosetting plastics
A
- may be resoftened by reheating, but the thermosetting reaction is chemical in nature and irreversible so that once it has taken place, further heating results only in gradual charring and deterioration
5
Q
matched die molding
A
- Most important area of plastic processing
- Compression molding and cold molding are like forging and powder metallurgy in that the material is introduced to the open die and the forming pressure is applied by the closing of the dies
- transfer molding is cold chamber die casting
- Injection molding is like hot chamber die casting
6
Q
Compression molding
A
- oldest and simplest of plastic molding processes is compression molding
- used primarily for thermosetting plastics
- not really size restricted, largest molded articles made by this method
- no material waste and little erosion of the dies, distortion and internal stresses minimized
- filling clearances in the order of .025mm (.001”)
7
Q
Transfer molding- thermosetting plastics
A
- like cold chamber die casting
- predetermined quantity of molding compound always included in some excess
- Excess material in the transfer chamber, as well as the sprue and runner system, also set, resulting in a cull that must be removed. Cull is scrap since the thermosetting reaction may not be reversed
8
Q
Injection molding- thermoplastics
A
- prolonged heating is not necessary or desirable
- temps are controlled so that the sprue separates at the nozzle where the parts are removed
- some injection molding of thermosetting materials is done, but have to be precise temp and time to prevent premature setting when used for these materials known as jet, Flow and offset molding
9
Q
Casting of plastics
A
- casting of plastics is primarily a low tooling cast procedure restricted to thermosetting resins and used for low production of jewelry, novelty items, lab specimens and similar parts
10
Q
Extrusions
A
- Most plastics are finished as sheets, tubes, rods, filaments, films other shapes of uniform cross sections produced by extrusion
- two methods used for producing film
- 1) film is extruded though a slit of appropriate size
- 2) extruded as a tube and then expanded or passed and slit between heated rollers where it is then welded
11
Q
Fibrous fillers- thermosetting resins
A
- Glass fiber and paper are the most common filler material used
- limited to thermosetting materials
- filler and resin are brought together in the process itself and the thickness of the molded part is eastablished
12
Q
Contact layup- filler, resin
A
- Simpilest procedure is contact layup
- successive layers of manually placed filler material are brushed or sprayed with resin as they are applied to the mold
- with proper winding techniques, tensile strengths up to 1000MPA ( 150,000 PSI) have been reported
13
Q
Contact layup variations
A
- common variations of the contact layup method are vacuum bag, expanded bag, autoclave molding are all methods for developing some pressure on the surface of the molding to permit a lower- resin to filler ration
- Vaccum bag identical to contact layup except that a sheet of vinyl plastic film is placed over mold
14
Q
Compression process for sheet materials
A
- matched metal dies are used to form reinforced products
- used most for flat sheets manufactured for table and countertops
- only way to produce good finish on both sides of the part
15
Q
Post forming
A
- widest use of post forming operations is made on thermoplastics in sheet form that are heated and made to conform to a single surface mold or pattern
- draping: only granity is used
Drawing and stretch forming: identical
- blow dieting- combined and air bulging operations
- vaccum forming: no external film is used
16
Q
Composite materials
A
- Engineered material consisting of one or more reinforcing agents and a matrix binder acting together as a physical unit while retaining their identities
- Interface between reinforcing material and the matrix binder is physically identifiable
- Mixtures are a type of composite with random orientation and shape of constituents
16
Q
Adhesives
A
- Material with some tackiness or stickiness
- modern adhesives have a wider range of
16
Q
Cements
A
*Concrete: mix of gravel, sand, and Portland cement when combined with enough water used to support compressive loads
- cement: Portland cement is 80 percent carbonate of lime and 20 percent clay, 100 percent strength is required in 28 days after 1 year may reach 150 percent or more.
17
Q
Stresses in aircraft structures
A
- Tension, compression, torsion, shear, bending
17
Q
Engineering ceramics
A
- majority of ceramics are constructed from clays ( silica and alumina) mixed with water, shaped to proper form
- Ceramics are poor conductors of heat and electricity
*. Ones used for cutting tool inserts, made almost of pure alumina ( aluminum oxide)
18
Q
Variety of composite structure
A
- sandwich panels utilizing either a foam or honeycomb between two skins
- level of complications is seen when all these elements are combined in hybrid reinforcements
19
Q
Metal Bonding
A
- Property of adherence to a surface
- surface to which an adhesive adheres or are of the bodies held to another by an adhesives is known as adherend
- four mechanisms responsible for
- electrostatic/ covalent bonds: sharing of electrons by different atoms account for the formation of most common chemical compounds
- van der waals forces provide additional bonding between the atoms
- mechanical interlocking- secondary to true adhesion
20
Q
Adhesive contact
A
- Proper performance of an adhesive requires that intimate contact in addition to adherends cleanliness be established between the adhesives and the surfaces to be joined
- Important adhesives for the bonding of metals are thermosetting compounds
21
Q
reinforcement
A
- Composites are generally made up of a matrix and reinforcement
22
Continuos Fiber
* Yarn or strand in which each of the filaments is the same unbroken length as the strand
23
Discontinuous fiber
* Random assortment of fixed length or chopped random length fibers are bound together on a flat mat
24
Glass fibers
* used in over 90% of composites
* high tensile strength< resistance to fire, moisture and chemical don’t conduct electricity and small coefficient of thermal expansion, inexpensive
* EGLASS: excellent electrical insulation
SGLASS: Magnesium aluminum silicate fiberglass named because of its stiffness
Low modulus of elasticity
25
Aramid Fibers
* Extreme tensile strength impact resistance and vibration dampening
* highly moisture sensitive and very sensitive to UV light
26
carbon fibers
* very high tensile strength and make very stiff, lightweight structures
* high compressive strength and a negative coefficient of thermal expansion
* graphite different have a higher carbon and tensile modulus of elasticity
27
Ceramic Fibers
* used for many high temp applications ex- jet engine shields, exhaust deflectors, electrical high temp heat shielding
28
Core material
* Normally low strength material, but higher thickness provides the sandwich composite with high quality bending stiffness and overall low density
* used for the same function: to lighten, stiffen and strengthen utilizing the sandwich principle
29
Fabrication
* divided into primary and secondary
* process ny which the reinforcing agent and the matrix agent are integrated to form a composite material and proceed to form a useful structure
30
Primary fabrication
* Reinforcing agent and the matrix agent are integrated to form a cured composite material
* construction and stacking or continuous fiber pre- impregnated piles in specific patterns
* ply is a single layer of unidirectional or woven fibers, basic steps are layup and curing
31
Layup
* Positioning of the uncurled composite material in or on the mold, mandrel, fixture or tool that provides shape to the part
32
Pre-preg materials
* are a filler material unfirectional or woven that has been pre impregnated or wetted out with a resin system
* it is expensive and not the best choice for every application
* tack: sticky to touch, quality of a fresh matrix
* out time: Cumulative time that a preregistration been exposed to temps above the storage temp
33
Coefficients of thermal expansion (CTE)
*if differing CTE can result in residual stresses that can warp a part of component
34
Curing
* Once layup has been completed, unconsolidated laminate is prepared for the curing process and then cured
* most continuous fibers are consolidated and densified or debunked by threebag molding method: vaccum, pressure and autoclave
* consolidated; Bonding of prepreg piles and other adherends into a unified mass
* densified / debollering: when voids and excessive resins are removed from the stacked piles
35
Secondary fabrication
* Involves the machining and joining of structural members to provide useful structures
* mating of composite substances to other sub structures including but not limited to composite material
36
Joining
* fiber reinforced composites are normally joined by two methods mechanical and adhesive, sometimes a combination of both
* Mechanical includes: bolting, riveting, pinning
Noble fasteners ( titanium, stainless steel) used to join graphite or carbon reinforced composites to reduce corrosion galvanic
37
Adhesive bonding
* Process which the adherends are bonded by an adhesive material
* bond strength related to two factors : cohesion and adhesion
* Cohesion refers ro the ability of the adhesive to hold itself together
* adhesion: refers to the ability of the adhesive layer to attach itself to the adherend
* adhesion forces always exceed cohesion forces in bonds
Adhesives is the weak link
38
Forms of adhesive bonding
* Secondary and co-cure bonding
* Secondary : process of bonding pre-cured composite structures together and/or other structures composites to form useful component
* co- curing: simultaneous layup and curing of a structures several components
* process attractive because it eliminates the cost and fit up problems associated with secondary bonding
39
Low velocity impact damage
* There is an energy threshold below which no damage is done to a given composite material or structure
* above this threshold is an energy range in which external damage occurs without visual indications at the surface
40
Damage to a honey comb panel
* impact to a honeycomb panel crushes the core
- crushed core
- water entrapment
- sheared core
Core splice
Disbonds
Delamination
41
Matrix cracking
* first type of failure, usually caused by low velocity impact
* shear cracking ( 45 degree indication)
* bending cracking( vertical Indination
* Impact response of the structure is not affected by matrix cracking
42
Delamination
* Forms between the layers in the laminate
* in a low velocity impact, the most critical damage mechanism in composites is Delamination
43
Fiber damage
* fiber pullout and fiber breakage are the most common failures under low velocity impact testing
* Occurs because of the high stress fields ad identification effects
44
Disbond
* Can be two kinds, adhesive ( interfacial) when disbanding occurs between the adhesive and the adherend
* cohesion fracture obtained if a crack propagates in the bulk polymer that constitutes the adhesive
45
Moisture
* Moisture in all of items physical states has the potential to degrade composite materials and structures
46
Damage assessment
* Damage assessment compromises 3 distinct steps
1. Discontinuity detection/ location
2. Discontinuity assessment
3. Defect removal assessment
46
Repair material
* Repair joints are basicallly bolted, bonded or a combination of these
* Bolted joints usually use metal sheets and plate for the patch material, titanium preferred, but aluminum and stainless steel are also used
* aluminum patches must be isolated from graphite or carbon to prevent galvanic corrosion
47
Coatings
* Paint and other coatings prevent good bonding of fraying surfaces
* reduce friction in mechanically fastened joints
* frequently accomplished by carefully controlled mechanical abrasion
48
Drying
* Resin matrix composites especially epoxies are normally dried before making repairs involving structural bonding especially important if the laminate temp will exceed 93 Degrees
49
Scarfing
* tapering the edges of the remaining material in order to establish high strength joints and return the original external contour
* straight scarfing: exposes each play over a gradual slope do not always overlap on the original laminate piles
* Step scarf stronger than straight scarf because it provides overlap of repair piles over the remaining original. Piles
50
Patches
* used to make bonded join repairs on laminate skins are built from pre preg composite tape
* common practice to make a repair using a pre preg patch to which co cure adhesive film has been applied
Asnt Studycards Basic Material And Processes
flashcards
Decks in class (10)
# Cards
