Polymer processing techniques

Question 1

name the main high volume techniques used to manufacture polymer products (7)

Answer 1

Extrusion, injection moulding, blow moulding, compression moulding, thermoforming, coating, rotational moulding

Question 2

what is the difference between batch and continuous manufacturing?

Answer 2

Batch processes are completed in defined cycles, producing a number of components per cycles
Continuous involves processes that generate product continuously, requiring cutting to discrete lengths

Question 3

What are the basic steps/necessities involved in any polymer manufacture process

Answer 3

=Polymer preparation-mixing and heating
=polymer flow forming-processed as a viscous liquid or softened solid
=part shape definition
=solidification-cure of a TS or solidification of a TP

Question 4

Describe the 3 sections in an extruder

Answer 4

Feeder: used to convey solid pellets away from hopper towards compression
Compression: compresses material, heat conduction and shearing causes melting, air and volatiles are forced out of melt
Metering section: provides strong mixing and ensures homogeneity of melt. shearing provides pressure required at die/ mould

Question 5

What are the three main things an extruder does

Answer 5

• melting of the TP (from pellet form)
• Removal of air and volatiles from the melt
• provides driving force

Question 6

Why would you use twin screws in an extruder?

Answer 6

for materials that require thorough mixing or high volumes

Question 7

wat is the difference between corotating and counterrotating screws in an extruder?

Answer 7

counterrotating-majority of material gathers at intersections and is pushed along quickly (suitable for high flowrates)
-corotating:Material takes along path resulting in more thorough mixing

Question 8

Describe the extrusion process (production rate,continuous/batch, capital expense, typical materials)

Answer 8

rate: medium to high (continuous)
capital expense: extruder and supporting equipment is expensive
typical material: TP’s with lw to medium viscosity

Question 9

Describe the injection moulding process (production rate,continuous/batch, capital expense, typical materials)

Answer 9

Batch, medium to high production
Capital expense: moulds cost (increases with part quality, size, and production rate)
typical material: TP with low to medium viscosity

Question 10

Describe the blow moulding process (production rate,continuous/batch, capital expense, typical materials)

Answer 10

rate: very high
capital: machinery is high cost
typical materials: commodity TP

Question 11

Descirbe the compression moulding process (production rate,continuous/batch, capital expense, typical materials)

Answer 11

Rate: low to medium
Capital expense: prodominantly through the press, moulds are relatively cheap
Material: typically TS in powdered form

Question 12

Describe the thermoforming process (production rate,continuous/batch, capital expense, typical materials)

Answer 12

rate: low to high
expense: reasonably economical, can be performed using modest through to complex machinery
Material: TP sheets + films

Question 13

Describe the rotational moulding process (production rate,continuous/batch, capital expense, typical materials)

Answer 13

Rate: low to medium
expense: relatively low due to low pressure low cost moulds
Material: majority powdered polyethlene, other TP and liquid TS possible

Question 14

Describe the coating process (production rate,continuous/batch, capital expense, typical materials)

Answer 14

Rate: high
Expense: extruder based for TP, cheaper for TS (dip)
material: Low viscosity extrudable TP, and variety of TS liquid resins

Question 15

Name and briefly describe four different types of Extrusion processes?

Answer 15

Profile: A die is used to extrude complex shapes
Sheet: Produces flat sheets and films, utilises a coat hanger die
Blown film: Die produces a continuous film of melt, air is blown through the centre
Filament: produces synthetic fibres for cloth or rope

Question 16

What are the four basic steps in an injection moulding cycle?

Answer 16

Mould closes, melt injection, part cooling, part ejection

Question 17

What are the three main parts to an injection moulding unit?

Answer 17

Injection unit, mould, clamping unit

Question 18

What are the three main steps in blow moulding?

Answer 18

melting the polymer, creating a preform, blowing the preform tube into the desired shape

Question 19

How would you improve the standard steps in a blow mouliding cycle to increase production rate?

Answer 19

larger production rates achieved by working multiple moulds through cycles. a single extruder is used to feed the moulds. ( picture is circular with moulds at different steps i.e. blow, cooling, eject, clean etc

Question 20

List some main advantages between extrusion and injection blow moulding

Answer 20

ext: tooling cost lower, shorter cycle time, wider choice of polymers, more flexibility in part design
Inj: better accuracy, uniform wall thickness, no seam lines or pinch marks, more transparent part as better control of crystallisation

Question 21

List the main disadvantages between extrusion and injection moulding

Answer 21

ext: requires significant part trimming, requires additional equipment to grind scrap and reintroduce extruder
inj: higher tooling costs, limited choice of polymers

Question 22

Name the 5 main steps in compression moulding

Answer 22

placement of charge
mould closure
squeeze to dimensions
polymer solidifies
demoulding

Question 23

What material charge forms are moulded in compression moulding?

Answer 23

powder and sheet TP and TS

Question 24

Describe the 5 basic steps in thermoforming

Answer 24

load blank into diaphragm
heat blank
clamp into mould
draw down into mould
de-mould and trim

Question 25

What are the four different ways a thermoforming process can be undertaken (with regards to the different moulds)

Answer 25

free forming (no moulds, positive air pressure forms shapes)
straight vaccuum forming (blank is drawn into relatively low cost mould
Matched mould forming: good control of part thickness is obtained through having two moulds that are pressed together
Plug assist vacuum forming: used for deep drawing parts, blank is pressed into mould and then through a vaccum is drawn against the mould surface

Question 26

what are the four main steps in rotational moulding?

Answer 26

charging the mould
heating the mould (start spinning)
cooling the mould (continue spinning)
demoulding

Question 27

What are two different types of coating processes?

Answer 27

Extrusion coating and dip coating