Polymer Flashcards
Properties compared to metals
Lower strength, hardness, stiffness, density and T resistance
Polymer Properties (6)
- High strength or modulus to weight
- Low electrical and thermal conductivity
- R to corrosion and chemicals
- Relatively low cost
- Variety of colours
- Easy processing
Amorphous Thermoplastics
Architecture and structure
- Random molecular structure
- Easy to thermoform (soften over range of T)
- e.g. polycarbonate, acrylic, PETG, PVS, ABS
Semicrystalline Thermoplastics
Architecture and structure
- Separated domains - amorphous and crystalline or highly ordered molecular structure
- Have two transition Ts
- e.g. polyolefins, PEEK, PET, POM
Thermoset
Architecture and structure
- Network is amorphous but the cahins are cross-linked
- Strengthens when heated - cannot be remoulded or heated after initial forming
- e.g. epoxy, silicones, polyurethanes and phenolics
Tg of Amorphous Thermoplastics
Properties
- Can be formed at T>Tg
- At Tg - leathery (can be pressed into shape)
- > Tg - rubbery
- <Tg - relatively little mobility
What do processing techniques rely on?
- Heat, pressure and time
- Type of polymer and its response to heat, pressure and length of processing time
What form of plastic get’s processed?
(5)
- Powders
- Doughs
- Granules
- Sheets
- Liquid resins
Parameter’s of Processing
Considerations
- Tool cost
- Cycle time
- Material cost
- Shape compatability
- Scrap
Phases
- Heating - to soften or melt plastic
- Shaping/forming - under constraints of some kind
- Cooling - to retain shape
Extrusion
- High volume - raw thermoplastic is melted and formed into a continuous profile
- Produces constant cross section - e.g. pipe, UPVC window frames
- Constitues largest production technique for plastic products by volume of material
Screw Extruder
Steps
- Raw material (granules) are fed into hopper
- Hopper delivers material into barrel
- The screw drives material along barrel through various sections - feed, melt and pumping/metering section
- Melt passes through filter screen and breaker plate to remove contaminants and removing the materials rotational memory
- Filtered melt is forced thorugh the die to produce shaped extrudate
- Extrudate is pulled and cooled
Screw Sections
Extruding Machine
- Feed - polymer conveyed to melt section, little friction or compression
- Melt - polymer compressed by screw and forced against wall, melt film forms due to shearing along wall and melt pool collected againt screw shelf
- Pumping/metering - remaining solids disperse in melt. Pressure builds up in die
- Highly efficient melting process. 2. Much less efficient
End of Extruder
Parts and what happens
- Filter screen - captures un-melted polymer
- Breaker plate - changed helical movement into longitudinal translation movements (also improves final mixing)
- Sensors (T+P) - enable process monitoring and optimisation
Extruder Screw Design
- Aim to achieve: full melting of plastic, avoid overheating, ensure melt homogeneity, max throughout rate
- L of each section and detailed geometry can be changed to accomodate different polymers
- Pins can be added to improve mixing
- Vents along screw allow water vapour to escape
- Some machines have twin or tripled screw designs
Extrusion Die
Two types
- Annular - enables cable coating, film blowing
- Shaped - complex cross-section
Die Swell
(Barus Effect)
- Extrusion forces polymer chains into alignment
- Soft extrudate relaxes on exit of die (changing cross-section)
- Die shape is designed to compensate for relaxation
- For complex shapes, relaxation is hard to predict (iteration and design experience is needed)
Cooling Extrudate
- Cooling rate and uniformity is important to minimise shrinkage and distortion
- Cooling achieved by blowing air, in water, in cooled vacuums (to prevent collapsing)
- When cooling amorphous polymers, the T needs to be <Tg to freeze the final part dimensions
Extrusion of Tubes and Pipes
- The die contains a mandrel in the shape of the hole
- This mandrel is attached to the die by one or more ‘bridges’
- As the material encounters bridges it is forced to seperate, but it flows around the bridges and joins up again
- Air may be pumped into the centre of the die to stop tube collapsing
Extruded Wire
- Electric cable is extruded and coated in plastic insulation
- A constant feed rate of wire and extrusion ensures a uniform thickness
- The wire is checked continusously for total coverage with plastic and marked automatically with a roller
Sheet and Film
- Sheet >0.5mm thickness
- Film <0.5mm thickness
- Produced using a special flat die called coat hanger die
- Distributes polymer evenly
- Can be adjusted to create different thicknesses
- Extrudate first taken up on cooled rolls and then rubber coated pull-off rolls
Thin Film and Bags
- Thin film (<0.254mm) products - e.g. plastic bags, cling film - are made from blown films
- A thin walled tube is extruded vertically and expanded by compressed air
- Thickness is controlled by pressure of air, speed of tube and a cooling ring that surrounds the “balloons”
- Slitting knives turn the tube film into a continuous sheet or it is pinched to create bags
Blow ratio = ratio of blown : to extruder d (typically 3:1)