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Polymer-process and Machining Fundamentals, Machining, . (Temperature) -…
Polymer-process and Machining Fundamentals
Types
Injection molding
Permanent mould (like the die casting for metals)
(Movable) Cores of metal
Ejector pins
(shrinkage is larger than in metals) to make sure the material can be removed
Cooling system for organized solidification
Injection pressure: 20 - 200 MPa Depends on size, shape +material
Clamping force: 200 - 50 000 kN
Variants
Multi-component
1st: first component injected
Rotates
2nd component is injected
Gas-assisted
1: inject component into mold
2: Gas is blown into mold, and makes component hollow
Less material = needed
Foam
While liquid lnjected, gas is also injected
1: apply pressure
2: loosen mold --> gas will spread
Reaction
2 diff components
1: cure diff components
2: inject into mold
Design rules:
Avoid undercut
use draft angles: instead of 90
*
make 100
*
because shrinkage will occur
Scars will be visible
Uniform wall thickness
Prevent: internal stress, sink marks, cavities, sharp corners
Extrusion of plastics
Working:
Heating material with screw
Continuous process
no leftovers
Die design comparable to metals
Extruded plastic will swell after leaving die
Extrusion Blow molding
Plastic is fed through tube
is clamped together on bottom
Blown into shape
Scar
= visible, line on bottom of object
Injection Blow molding
air is blown in pre made part to give shape
plastic bottles: small
scar
on bottom
Coca cola bottle
Ribs are designed to make plastic structure stronger
Thermoforming
Starts with sheet plastic
During rubbery state of plastics (easy to deform)
Corners of products = thinner
Mechanical thermoforming
Plug (sort of shape) pressed down on preheated sheet of plastic into mold
Pressure Thermoforming
Pre Heat sheet, apply outside pressure (by blowing on sheet)
Or Apply pressure by making a vacuum that pulls sheet into shape
Rotational Molding
Starts with shapeless material (Powder)
Placed in mold
Heated to melt and make material Viscous
Rotated around 2 diff axis (for even division inside mold)
Solidifies and becomes hollow product
Used for Large/Complex hollow products (circles, square,...)
small wall thickness: 0.4 -15 mm
Not always uniform (depends on forces from rotational axis)
Chip forming
Principle
Cutting Wedge with:
alfa + beta + gamma = 90*
Gamma: Rake angle
Alfa: Clearance angle
Beta: Wedge angle
Shearing Process
Hear Angle Fi
Smaller angle = higher forces needed to remove chip
Friction between chip + surface
Chip type
Continuous chip
Problem: chip becomes longer and longer --> can intertwine with tool
This type depends on material (aluminium has more chance)
Discontinuous chip
Breaks away easier
Serrated chip
Creates small particles
Problem: break off makes small shocks that damage tool
Process limitations
Turning process
Vf = translation speed of tool
f x n x z
f = feed per tooth, n = number of revolutions, z =
Vc = Cutting speed of tool
speed of tool at tip (circonfrence)
High speed = rough
pi x diameter x n
Fc = Main Cutting Force
Depends on:
Workpiece Material (soft = less F)
Rake angle
Width of cut (big width = more F)
Thickness of cut
F = cutting force
Ff = Feed Force
Fp = Radial force
Cutting power: Pc = Fc * Vc
Heat is generated in shear zone
Friction Heat generated
Wear
(caused by)
(Thermal) hardness of tools
Friction Forces
Types
Flank wear (Vb(a))
Between flank piece + work piece
Crater wear on rake face (Kt(b))
Between chip and rake face
Makes material thinner
When wear is too much --> replace tool
Tool life (T) = Max operational life
requirements for Cutting (tool) material
Thermal hardness
No melt, break or having thermal cracks from heating up
Tough
Withstand shocks from serrated chip
Thermal shock resistance
Withstand differences in temp
Resistance to oxidation and adhesion
Particles no glue onto tool
Creates rough surface finish
Prevention using cutting fluids
Good for heat discharge
Better accuracy
Less friction (Lubrication)
Smoother surface
Disposal of chips
For Lubrication
Mineral oils
Vegetable oils
For cooling
Emulsions
Aqueous solutions
Plastics
What are they?
Polymers (Many monomers) CH2
Types
Thermoplastics
Linear CH2's in a row
Branched CH2's in a row, with extra branches
Held by Weak VDW forces
Can Deform
easy to work with
Thermoset
Rigid (not flexible) 3D network covalent bonds
Rubber / Elastomer
Flexible 3D network with small crosslinks (Holds layers together)
Structure & Properties (Thermoplastics)
Easy to work with
Mobility between molecules
Plus low VDW-Forces that keep it together
How it works:
Heat = Liquification
Cool = Solidification
Chain Type
Linear chain:
Low Viscosity (like water)
Crystallization (structured)
Can withstand higher deformation
Branched chain:
High Viscosity (Syrop)
More force needed for deformation
Amorphous (Chaotic)
Ductile, soft and less heat resistant
Properties
higher temperature, less deformation resistance
Less crystalized = more rubbery (once it starts heating up)
After Tg
(Templerature in solid state)
Rubber state only in plastics
After Tm
(Melt temp), no matter the crystalization: they all melt and become Viscous liquid
Processing
Elastomers
Cross link structure, 3D network
From Vulcanization
Don't soften with increase temp
Thermosets
Mix 2 components, then they cure
Thermoplastics
Heated and Shaped, over and over again
Reusable
Vacuum Thermoforming
Cold deformation
Blow molding
Extrusion/Injection molding
Characteristics:
High viscosity
Bad heat conductor
More Shrinkage in solid state than metals
Process selection
Thermoplastics
Tooling costs
Cycle time
Cool down time
Machine costs
Batch size
Classification Machining processes
Rotating workpiece
Turning
Drilling
Linear moving workpiece
Planing
Rotating tools parallel to axis of rotation
Drilling
Boring
Reaming
Tapping
Honing
Rotating tools Perpendicular to axis of rotation
Milling
Grinding
Lapping
Linear moving tools
Planing
Sawing
Slotting
Pull type broaching
Lapping
Tool life (T)
Regular wear criteria
Crater wear > 0.1mm
Tool is worn out
Flank wear > 0.3mm
Calculation (Taylors Formula)
Vc * T^n = cte
n (= Exponent of Taylor): A measure for Temperature sensitivity
n(Cemented Carbide) = 0.15 <--> 0.30
n(Ceramics) = 0.25 <--> 1.00
n(HSS) = 0.08 <--> 0.20
Found In table
log(Vc) + n*log(T) = cte
Tool with Large chip cross section (A= b*h)
Recommended: Lower cutting speed
Else: Shorter tool life
At optimal cutting speed --> machine operation costs are minimized
Reduction of tool life K = Tool life 1/ Tool life 0
Cutting materials
Tool steel
Vc < 0.2 m/s
HSS (High speed steel)
Vc < 0.3 to 0.5 m/s
Cemented Carbide
Vc < 1.5 to 3.0 m/s
Coated carbides
Vc < 0.6 m/s
Cermets
Vc < 0.9 m/s
Ceramic cutting material
Vc < 20 m/s
CBN (Cubic boron nitride)
Diamond
Indication of cutting materials
Normalized (ISO 513)
(Ex) HC-P 15
15 = measure for mechanical stress resistance
HC =
main group
, coated cemented carbide
Workpiecematerial
, P = steel
K = Cast iron, non-ferrous
M = Stainless steel
Machining
Tool generates desired shape (unlike replication)
Complex shape = possible
Good size and surface quality = possible
No startup time
.
Temperature
.