No1 Welding guy in the universe Flashcards
What are polymer?
MAcromolecules made by joining together thousands of small molecular units know as monomers
What is the degree of polymerisation?
number of units of polymers in a chain
Advantage of Synthetic polymers?
They have good flexibility, strength, resistivity, chemical internees, and properties. Due to its fine quality of high strength and elasticity, it is used in making ropes for climbing, nets, parachutes, fishing nets, etc.
What do reinforcing fibres do?
Increase tensile strength
Increase flexural modulus
What do conductive fillers do?
improves electrical and thermal conductivity
Advantage of thermoplastics over thermosets?
-if you raise the temperature of a thermoplastic over the melting point you can form them and if they solidify you can process them without degrading the material.
doesn’t apply to thermosets –> can’t be reprocessed.
/What are the 2 divisions in thermoplastics
Crystalline (ordered) and Amorphous (random) structure.
What are some important considerations for thermoplastic polymers?
- solid materials at room temperature but viscous liquids when heated
-they can be easily and economically shaped into products.
-the can be subjected to heating and cooling cycles without significant degradation.
What advantage of branched structure over linear?
-increases entanglement among molecules which makes them stronger in solid state and more viscous at a given temperature.
what does crystallinity refer to?
the degree of structural order of a solid.
what is glass transition temperature?
the temperature at which an amorphous solid become soft when you heat them up and become brittle when you cool it down
Why is glass transition temperature?
-because not all thermoplastic materials have a melting temperature, they only have (Tg). They will only become less viscous which will allow you to shape them.
What do bending polymers do?
the brittle behaviour of amorphous polymer will be reduced.
What are some characteristics of amorphous polymers?
-usually transparent
-low shrinkage
-low chemical resistance
-poor fatigue and wear
What are some important characteristics of crystalline polymers?
-sharp melting point
-usually opaque
-high shrinkage
-good fatigue and wear resistance
What are thermoset polymers?
-they are made from polymeric resins that are capable of forming chemical cross-links.
-thermosets are shaped and then chemically cross-linked. This is called curing
Differences between Thermoplastic and thermoset?
Thermoplastics
-a weak, straight chain that can be broken and hence reprocessed.
-they dissolve organic solvents
-on heating, they will soften and then melt
-the melting point is lower than the degradation point
-in solid state, their structure consists of hard crystalline regions together with elastic amorphous regions.
Thermoset
-strong chemical bonds which do not separate on heating
-Do not dissolve in organic solvents
-on heating they will char and not melt
-degradation point is lower than their melting point
-when solid their structure consists of thermosetting resin interspersed with reinforcing fibre.
How does the curing process work?
-provide energy, initiate chemically cross-linking. energy increase
What events occur during curing/cross-linking?
Gelation - liquids-to-rubber transition (increase in viscosity)
vitrification- liquid or rubber-to-glass transition.(still not 100% cured)
Advantage of polymer over metals/ceramics?
-easy processing and no post-processing required
-cost competitive
-less processing energy
-transparency
-low-density materials with good mechanical, chemical and electrical properties.
Why is injection moulding used?
it can use a wide range of thermoplastics and thermosets at a low scale.
plus good flexibility and good resolution.
High manufacturing rate
What are the 3 units of injection melding equipment?
Unit 1 - injection unit consists of the injection cylinder allows us to melt and inject the material into the mould cavity.
Unit 2 - 2 halfs of the mould
Unit 3 - clamping unit
What does the injection unit do?
-it heats the thermoplastic material to its appropriate viscosity and then forcefully injects it into the mould.
How does reciprocating screw/injection moulding work?
Raw material and colour are fed into the barrel of the machine via the hopper. In the barrel, the raw materials are subjected to pressure and temperature until they are melted and malleable. The reciprocating screw inside the barrel will turn, injecting a precise amount of the now molten plastic into the injection mould at a set and controlled temperature, pressure and speed. Once the mould is filled with the molten plastic the injection process will shut off.
What are the 4 processes in the injection moulding cycle?
-Plastification
-injection
-packing/cooling - compensate for shrinkage due to solidification process
-demold/ejection
Why is the clamping unit important?
-A clamping unit for an IMM is used to keep the mould shut against the forces developed when injection pressure pushes plastic into the closed mould. It includes a drive mechanism for moving a moving platen of the injection molding machine in at least one direction
-if the clamping force is too small then leakage of material happens
-if too high then part might burn
Clamping force formula?
F=PxAxKxSf
A-total cavity projected area
K-viscosity
Sf- safety factor
What does viscosity depend on?
Molecular weight
what does the cavity pressure depend on?
wall thickness and the ratio of flow path/wall thickness
Effect of high crystallinity on the moulding process?
-Shrinkage and warping
-changes in part dimensions
What are the different process parameters in injection moulding?
Melting temperature
-the melt temp is generally below the melting point of the plastic
-the melt temp depends on the polymer morphology
The Temperature of the mould (factors that affect it)
-shot size
-injection rate
-size of the runner system
-part thickness
Injection pressure and holding pressure
-injection pressure = pressure applied to the mould
Injection rate
-the time to force the molten material into the cavity.
Dwell time
-the time that force or pressure is applied to the cavity after the cavity is filled. depends on the solidification
Cooling time
-interval after the pressure is relieved and before mould is opened
Ejection time
-The ejection time counts the time to open the mould, eject the part and close the mould.
what are sink marks?
depression on the surface - caused by excessive shrinkage during solidification.
What are the causes of sink marks and their solutions?
An insufficient polymer in the mould
-increase packing pressure,injection hold time,injection speed and gate size
Polymer ‘flowing back’ out of the mould into the runner system and barrel
-Increase injection hold time
-decrease mould temperature and increase cooling time
Polymer temperature too high
-Reduce back pressure and barrel temperature
-Improve mould temperature controls
Ejected part too hot
-increase cooling time
How are weld/knit lines caused?
due to multiple gates in your mould
what are burn marks and how do they happen?
Burn marks and/or discolouration are common causes of part rejection. These types of defects are
generally caused by trapped gasses in the cavity or thermal degradation of the polymer
What are the 4 main causes and corrective action for weld lines?
Slow injection speed
-increase injection speed
Under packing of the part
-increase hold pressure
Polymer too cold
-increase melt temp,mould temp and injection speed
Multiple Gates
-decrease no of gates
-increase gate size
-change gate locations
Causes and corrections for burn marks?
Trapped gases in the cavity
-clean vents, provide more vents and decrease injection speed
Thermal degradation
-decrease injection speed and melt temp
What are short shots?
A short shot occurs when a plastic injection moulded component is incomplete because the molten plastic has not filled the entire mould cavity. In other words, there is a portion of the part where there is no plastic.
Prevent it by increasing injection speed and adding vents and controlling mould temperature
What is metal forming?
A metal forming process achieves a pre-determined component shape by re-disturbing raw material under the action of applied forces.
What are some raw materials used in metal forming?
Billet,sheet,plate and bar
Advantages of metal forming?
-not much waste
-the forming process can refine and re-distribute the grain structure, improving the mechanical properties of the finished part
what are the 3 classifications of forming process?
-Bulk vs sheet
-Hot vs Cold Processes
-Steady-state vs non-steady state
What is open die forging?
-in open die forging the part is compressed between two flat surfaces or platens
-it is a hot process – it is soft – high ductility
-the part is manipulated to achieve its final shape
what is closed die forging?
Here the shape of the product is defined by the shape of a punch and a die.
Closed-die forging produces a flash - flash forms around the forging at the punch/die interface during the last step.
The high deformation resistance of the flash results in material completely filling the cavity rather than being extruded sideways out of the die. The flash is important in ensuring you fill the cavity in the die.
Frictional resistance at the flash allow closed die forging to work.
However the flash has to be trimmed away from the forging.
What is rolling?
The metal elongates plastically as it passes through the rolls.
how does direct extrusion work?
-you put a billet in a chamber and push it forward out through the die so that the ram and the die are moving together
what has reversed extrusion?
-where the ram is hollow and is pushing on a moving die and the bilet is stationary and extruded backwards
What is deep drawing?
The deep drawing process is a forming process which occurs under a combination of tensile and compressive conditions. A flat sheet metal blank is formed into a hollow body open on one side or a hollow body is formed into a hollow body with a smaller cross-section.
what is redundant work?
-refers to the extra work needed to change the size and shape of the metal as it proceeds through the
plastic region of the billet and into the die.
What is piping?
Oxide skin at the surface of the billet can sometimes
become entrained into the body of the extrude as an
impurity defect.
Solution is to use a punch of smaller diameter than
the billet and chamber. This collects the skin as a “skull”.
What is fir tree cracking?
A combination of large reductions, fast extrusion rates and low
melting point materials can result in this type of cracking. One
solution is to reduce the speed of extrusion.
What are rear cavity defects?
-Near to the die, in the plastically
deforming region of the billet, the billet
material is moving faster at the centre
than nearer to the container walls. (ie. there is a non-uniform velocity distribution across the billet near to the
die due to friction at the walls).
-Since the punch is rigid and cannot
participate in the flow, the billet material
near the centre moves ahead of the
punch, creating a void (cavity).
-The cavity becomes wider and deeper
as extrusion proceeds further and can
extend through the die into the
extruded product.
What is deep drawing?
-In deep drawing, sheet metal is stretched into the final part shape.
-The raw sheet material is first cut into predetermined shapes called blanks.
-The drawing process uses specially designed tools to form the blank into a 3-D shape.
-Deep drawing generates mostly tensile stresses in the blank during forming.
-Deep drawing is most effective with ductile metals
Deep drawing defects?
Ears
wrinkling
tearing
How does wrinkling occur?#
Wrinkling can occur in the
the flange of a partly drawn cup of thinner sheet due to the
development of compressive hoop stresses
These stresses develop because the material occupies
a decreasing circumference as it is drawn radially inwards.
Wrinkling is a buckling phenomenon
How to prevent the flange?
A blank holder must be used to provide restraining pressure normal to the flange.
What is welding?
A union between 2 pieces of metal rendered plastic or liquid by heat or pressure or both. A filler metal with a melting temperature of the same order of that of the parent metal may or may not be used.
a metallic bond forms across the interface between parent material and well metal - the material microstructure is continous.
What is welding?
A union between 2 pieces of metal rendered plastic or liquid by heat or pressure or both. A filler metal with a melting temperature of the same order of that of the parent metal may or may not be used.
a metallic bond forms across the interface between the parent material and well metal - the material microstructure is continuous.
