Polymers- Injection Moulding and Thermoforming

Question 1

Is injection moulding discrete or continuous?

Answer 1

Discrete

Question 2

What is good about injection moulding?

Answer 2

Simple of intricate, complex parts of varying sizes can be made. High quality with good dimensional tolerance. Quick

Question 3

Pressure used in injection moulding

Answer 3

About 100MPa

Question 4

Costs involved in injection moulding

Answer 4

High initial equipment and mould costs. Low running and per part costs

Question 5

Sources of scrap from injection moulding

Answer 5

Sprues, runners, flash, out of spec, trimming, start up/shut down. Much can be ground and reused but not many times because the heat cycle changes the properties of the polymer

Question 6

What types of plastic can be injection moulded?

Answer 6

Most thermoplastics and increasingly thermosets too

Question 7

General process for injection moulding

Answer 7

Rapid injection of specific quantity of homogenised polymer melt into a closed two part cavity mould. Pressure held on polymer in mould until it solidifies. When temperature low enough the part is removed. Repeat cycle

Question 8

Set up of injection moulder

Answer 8

Similar to extruder but the single screw can move back and forth in the barrel so is a reciprocating screw.

Question 9

Injection phase

Answer 9

Injection: prepare molten plastic charge in metering zone of extruder, push screw forwards forcing charge into cavity mould, a check valve prevents charge returning back up screw

Question 10

Phases after injection

Answer 10

Packing: maintain pressure to ensure mould remains filled.
Cooling: allow plastic to cool and prepare next charge.
Ejection: open mould, eject part and collect.
Repeat

Question 11

PVT plot for injection moulding

Answer 11

Specific volume against temperature with diagonal lines as isobars. Starts at maximum v and T then straight down to lowest (highest P) isobar. Follows isobar diagonally towards origin. Then horizontal across to top isobar. Follow isobar down to room temperature. Kink at Tg to reduce gradient

Question 12

Explain the PVT diagram for injection moulding

Answer 12

Liquid enters mould at max temp and is pressurised at constant T so v decrease. Held at high P as T decreases so v decreases (means more pumped in). T drops such that gate freezes and no more enters mould and v constant. Further T drop causes P drop and v drop. Polymer solidifies and RT reached. Contraction in final part roughly 1%

Question 13

What is fountain flow and what does it cause?

Answer 13

Molten polymer solidifies first near the cold mould wall. Means injected polymer continues to flow through solidified polymer (fountain flow). Final part usually has outer skin of densely packed polymer with core of less dense polymer

Question 14

Injection phase problems

Answer 14

If polymer solidifies too soon it will block the mould creating a short shot. If polymer injected at too high P it can leak out between two halves of mould causing flash

Question 15

Packing phase problems

Answer 15

As polymer cools density increases so injection pressure must be maintained until it solidifies. Stopping injection early can lead to polymer falling back into mould creating sink marks or voids. Too high a T can scorch the part

Question 16

How does the mould work?

Answer 16

Part next to injection moulder fixed while other is mobile. Cooled by water or air down cooling lines. Ejection pins strike ejector plate and move to push part out of mould as it opens.

Question 17

Material used for mould

Answer 17

Almost always surface treated tool steel. Can cope with high injection pressures. Moderate material cost. High machining cost. Mould often cost more than machine.

Question 18

Describe polymer orientation in mould

Answer 18

Typically follows flow direction within the mould but this is influenced by fountain flow

Question 19

How is polymer melt guided to cavities within mould?

Answer 19

Enters through sprue. Is then fed by runners to the gates which are connected to the cavities.

Question 20

How to ensure balanced filling of cavities?

Answer 20

Need to have the same filling time to have the same heat cycle and properties. Can have complex runner paths so that all cavities reached by melt at the same time. Can restrict flow through gates and runner for the nearest cavities

Question 21

What do hot runners do?

Answer 21

Mean less waste material, reduced manual labour, more expensive

Question 22

What are weld lines?

Answer 22

The weakest part of the product cause by large cavities not filling properly. Can solve by using more than one gate or use sprue gate

Question 23

Intrinsic defects to the process of injection moulding

Answer 23

Parting line caused by small gap between two halves of mould. Gate Mark caused by where the part is snapped off from the sprue. Ejector pin marks caused by auto-aim aged ejection systems (press into soft plastic)

Question 24

Co-injection moulding

Answer 24

One polymer melt forced into part of mould (skin, more expensive). Is then displaced by second molten polymer (core, cheaper). Add a bit more of first polymer at end to cover it all.

Question 25

Foam core injection moulding

Answer 25

Similar to co-injection by the core polymer foams. Used for surfboards.

Question 26

Injection compression moulding

Answer 26

Partly fill mould with injection then use clamping system to finish off moulding process by pushing one half towards the other. Means less residual stress so reduced distortion. Used for thin items

Question 27

Spencer-Gilmore equation for thermoplastic injection moulding

Answer 27

(Pe+pi)(v-v0)=83.1T
Pe is external pressure
Pi is internal pressure 
v is specific volume
v0 is specific volume at 0K
Can have varying coefficients for each term depending on plastic

Question 28

How does thermoforming work?

Answer 28

Softened plastic sheet (from extrusion) is forced over a convex or concave single-sided mould. Is a secondary process. Some waste from trimmed edges

Question 29

What is thermoforming used for?

Answer 29

Two-sided object’s of limited complexity. Thin gauge such as lids, trays, blister packs, food packaging. Thick gauge such as auto panels, fridge liners

Question 30

How is polymer orientation affect by thermoforming?

Answer 30

Two processes (including extrusion) so there is double orientation. Not true biaxial orientation which can lead to anisotropy and weaker than intended side walls

Question 31

Vacuum/pressure thermoforming

Answer 31

Soften the polymer over the mould. Heat to just above Tg if amorphous or just below Tm if semi-crystalline. Clamp in place over mould to form a seal. Apply force using vacuum alone (suck polymer down) or pressure from above as well. Cool and eject

Question 32

Mechanical thermoforming

Answer 32

Same as vacuum thermoforming but use a plug to assist forcing the sheet into the mould. Used for thick-walled or deep draw parts

Question 33

Problems and solutions with thermoforming

Answer 33

Difficult to control wall thickness distribution. Can inflate first to do most of stretching before applying vacuum. Sharp corners difficult to fill and cause tearing. Use rounded corners

Question 34

Why use thermoforming not injection moulding?

Answer 34

For short/medium runs, is cheaper. Requires lower pressure so cheaper mould materials can be used. Zn alloys, Al alloys for medium runs. Wood, resin, hard plastics for shorter runs