Polymer

Question 1

Properties compared to metals

Answer 1

Lower strength, hardness, stiffness, density and T resistance

Question 2

Polymer Properties (6)

Answer 2

• High strength or modulus to weight
• Low electrical and thermal conductivity
• R to corrosion and chemicals
• Relatively low cost
• Variety of colours
• Easy processing

Question 3

Amorphous Thermoplastics

Architecture and structure

Answer 3

• Random molecular structure
• Easy to thermoform (soften over range of T)
• e.g. polycarbonate, acrylic, PETG, PVS, ABS

Question 4

Semicrystalline Thermoplastics

Architecture and structure

Answer 4

• Separated domains - amorphous and crystalline or highly ordered molecular structure
• Have two transition Ts
• e.g. polyolefins, PEEK, PET, POM

Question 5

Thermoset

Architecture and structure

Answer 5

• 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

Question 6

Tg of Amorphous Thermoplastics

Properties

Answer 6

• Can be formed at T>Tg
• At Tg - leathery (can be pressed into shape)
• > Tg - rubbery
• <Tg - relatively little mobility

Question 7

What do processing techniques rely on?

Answer 7

• Heat, pressure and time
• Type of polymer and its response to heat, pressure and length of processing time

Question 8

What form of plastic get’s processed?

(5)

Answer 8

• Powders
• Doughs
• Granules
• Sheets
• Liquid resins

Question 9

Parameter’s of Processing

Considerations

Answer 9

• Tool cost
• Cycle time
• Material cost
• Shape compatability
• Scrap

Question 10

Phases

Answer 10

1. Heating - to soften or melt plastic
2. Shaping/forming - under constraints of some kind
3. Cooling - to retain shape

Question 11

Extrusion

Answer 11

• 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

Question 12

Screw Extruder

Steps

Answer 12

1. Raw material (granules) are fed into hopper
2. Hopper delivers material into barrel
3. The screw drives material along barrel through various sections - feed, melt and pumping/metering section
4. Melt passes through filter screen and breaker plate to remove contaminants and removing the materials rotational memory
5. Filtered melt is forced thorugh the die to produce shaped extrudate
6. Extrudate is pulled and cooled

Question 13

Screw Sections

Extruding Machine

Answer 13

1. Feed - polymer conveyed to melt section, little friction or compression
2. Melt - polymer compressed by screw and forced against wall, melt film forms due to shearing along wall and melt pool collected againt screw shelf
3. Pumping/metering - remaining solids disperse in melt. Pressure builds up in die

1. Highly efficient melting process. 2. Much less efficient

Question 14

End of Extruder

Parts and what happens

Answer 14

• 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

Question 15

Extruder Screw Design

Answer 15

• 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

Question 16

Extrusion Die

Two types

Answer 16

• Annular - enables cable coating, film blowing
• Shaped - complex cross-section

Question 17

Die Swell

(Barus Effect)

Answer 17

• 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)

Question 18

Cooling Extrudate

Answer 18

• 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

Question 19

Extrusion of Tubes and Pipes

Answer 19

• 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

Question 20

Extruded Wire

Answer 20

• 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

Question 21

Sheet and Film

Answer 21

• 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

Question 22

Thin Film and Bags

Answer 22

• 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)