Moulding

Question 1

Mostly used with thermosets

Answer 1

• Compression
• Transfer
• Reaction injection

Question 2

Mostly used with thermoplastics

Answer 2

• Rotational
• Injection

Question 3

Compression Moulding Steps

Answer 3

1. Charging - a pre-weighted amount of charge (moulding compound) is loaded into the bottom half of a heated mould
2. Closing the press - mould halves brought together to compress charge, forcing it into shape of cavity
3. Applying H and P - charge is heated by hot mould to polymerise and cure the material into solidified part
4. Discharging - mould halves are opened and the part is removed

Question 4

Compression Moulding Advantages

(7)

Answer 4

• Low capital cost (due to simplicity of process)
• Low die wear
• Lower scrape rate when compared to injection moulding (since there are usually no sprues or runners)
• Low product shrinkage (<1%)
• Capable of high V production
• Produces repeatable precision forming
• Metal inserts, grooves, holes and threads can be moulded-in

Question 5

Compression Moulding Disadvantages

(4)

Answer 5

• Limited complexity (difficult filling of complex shapes compared to transfer or injection)
• Longer cycle times (extremely thick and heavy parts, preheating charge can shorten curing time)
• Material waste (higher compared to others due to flash
• Higher labour costs (for loading and unloading as well as trimming excess)

Question 6

Transfer Moulding Overview

Answer 6

• Material is pressurised in seperate chamber and then forced by transfer plug to flow through runners and gates into closed cavity/cavities
• Thermosets that are cross-linked with heat are used for transfer-moulding applications
• Used lots for integrated circuit packaging

Question 7

Transfer Moulding Advantages

Answer 7

• Preheating enables lower P requirements for operations
• Multiple cavities = higher yield
• Very high dimensional accuracy (due to flow being gated = used for intricate features and critically positioned inserts)
• Shorter production cycles than compression but slower than injection
• Provides more consistnecy as it allows tighter tolerances and more intricate parts

Question 8

Transfer Moulding Disadvantages

Answer 8

• Tooling can be expensive since design can be more complex
• Higher V wasted - material left in pot, sprue and runner have to be discarded as irreversibly reacted (cross-linked)
• Slower production cycles than injection

Question 9

Reaction Injection Moulding Overview

(RIM)

Answer 9

• Two types of monomers are mixed together before injection - they polymerise to synthesise the polymer
• Reinforcing agents can be added to improve mechanical properties
• Used for production of high quality, high styled parts - e.g. automotive panels, bumpers, fenders, enclosures for electrical equipment

Question 10

RIM Process

Answer 10

1. Liquid components pumped into a mixing head
2. Reacting mix of polymers is injected into the heated mould
3. Mould is kept under P whilst reaction takes places and polymer solidifies

Question 11

RIM Advantages

(4)

Answer 11

• Alternative to compression and injection moulding for large surface A
• Large, light-weight and thin-walled items can be produced (roughly 2.5mm)
• Larger design flexibility
• No moulded-on stress to cause part warping or cracking after demould

Question 12

RIM Disadvantages

(3)

Answer 12

• High capital investment
• Slow cycle times compared to injection moulding
• Expensive raw materials

Question 13

Rotational Moulding Process

Answer 13

1. Measured amount of polymer (usually powder or graule form) is added to mould
2. Mould is closed and then rotated in an oven around two axes - polymer melts and coats inside wall of mould
3. Mould is removed from oven and cooled - may be carries into chilled chamber, this stage of cycle can be quite lengthy
4. Part is removed

Question 14

Rotational Moulding Advantages

(6)

Answer 14

• Versatile technology suited to wide range of parts - e.g. trash cans, buckets, large tanks, boats and plastic footballs
• Different parts can be made at once on same machine and in any order (mould dictates process)
• Comparatively cheap moulds (don’t have to withstand P)
• Accomodates production complexities such as stiffening ribs, moulded insert and different surface texture
• Hardware made of various materials can be moulded-in for strenth and durability

Question 15

Rotational Moulding Disadvantages

(4)

Answer 15

• E intensive and uses non-renewable energy sourcces like natural gas and keroscene
• High labour costs
• High cost of raw materials due to high thermal stability requirements
• Long cycle times (e.g. 30mins to 3hrs)

Question 16

Injection Moulding Process

Answer 16

1. Thermoplastic pellets are placed in hopper and fed through extruder for melting
2. Melted polymer is injected at P through the nozzle into mould
3. Parts cooled in mould
4. When part is solid enough clamp pressure is removed, mould opens and part is ejected

Question 17

Injection Moulding Advantages

(6)

Answer 17

• Complex shapes and various sizes
• Very high production rates
• Good dimensional accuracy
• High-rate production
• Cycle times of 5-60s (slower for thermoset)
• Very versatile - complex and intricate shapes possible

Question 18

Injection Moulding Disadvantages

(3)

Answer 18

• Up-front costs can be high, particularly regarding tooling
• Not ideal for producing large parts as a single piece
• Single thickness - cooling creates residual stress

Question 19

Injection Moulding Machine

Answer 19

Divided into two components - injection unit and clamping unit

Question 20

Injection Unit

Answer 20

• Consists of a barrel fed from one end by a hopper
• Inside the barrel is a reciprocating screw which
  (1. Turns for mixing and heating the polymer)
  (2. Acts as a ram to rapidly inject molten plastic into the mould)

Question 21

Injection Moulding

Clamping Unit

Answer 21

(Related to the mould operations)
1. Hold the two halves of the mould in proper alignment with each other
2. Keep the mould closed during injection by applying a clamping force sufficient to resist the injection force
3. Open and close the mould at the appropriate times in the moulding cycle

Question 22

Injection Mould Cycle

Answer 22

1. Mould closed and clamped - force applied
2. Screw moves forward, injecting a shot of polymer melt into mould at preset injection pressure
3. Screw stops and maintains preset hold pressure (to pack in melt to compensate for contraction)
4. Gate freezes
5. Screw moves back and begins rotating
6. Turning screw melts resin and builds up preset shot size of molten material ahead of screw
7. Meanwhile part cools and solidifies
8. When part is solid enough, clamp pressure is removed, mould opens and part is ejected

Cycle time can be as slow as 4s for small parts ot many mins for larger parts

Question 23

Injectiton Moulding

Clamp Force

Answer 23

Clamping force required

P inside mould
x
projected A of mould cavity

Limits the maximum area of moulding

Question 24

Injection Moulding

Main Features

Answer 24

• Special tool custom-designed and fabricated for the given part to be produced
• Typically made from hard tool steels
• Mould design critical for success - challenge is to fabricate a mould whose cavity is the same geometry as the part, that also allows for part removal
• Moulds can be very expensive
• Mould consists of:
  o One or more cavities that determine part geometry
  o Distribution channels through which the polymer melt flows to the cavities
  o An ejection system for part removal
  o A cooling system
  o Vents to permit evacuation of air from the cavities

Question 25

# Injection Moulding Parts

Answer 25

- **Core and cavity**: the two halves of the mould - **Sprue**: injection point into the mould - **Runners**: carry plastic from the sprue to the individual part cavities for multi-cavity moulds - **Gate**: entry point that constricts the flow of plastic into the cavity - **Ejector pins**: remove parts from mould after opening

Question 26

# Injection Moulding Mould

Answer 26

- Feeder system has to be removed from part, creating scrap - Different designs of runner systems affect the economics of the process o Cold runner, two plate mould o Cold runner, three plate mould o Hot runner mould

Question 27

# Injection Moulding Cold Two Plate Mould

Answer 27

- Results in *material waste* – higher material cost - *Cycle time can be longer* compared to other designs due to needing to cool runner systems along with part – can reduce overall productivity and increases manufacturing costs - Can produce *more scrap* parts - Generally *lower tooling costs* compared to other designs due to simpler mould construction

Question 28

# Injection Moulding Three Plate Mould

Answer 28

- Can **reduce material wastage** by allowing the runner and sprue to be separated from the part during ejection (results in lower material costs compared to two plate) - May have **longer cycle times** due to additional movement required for plate separation and ejection (can affect productivity and increase manufacturing costs) - **Minimise scrap rates** by separating the runner and sprue from the part, reducing the risk of defects like hot runner systems - **Typically higher tooling costs** than cold runner moulds but lower than hot runner systems due to the additional complexity of the mould design

Question 29

# Injection Moulding Hot Runner Mould

Answer 29

- Significantly **reduces material wastage** due to no sprue or runner – leads to lower material costs and increased efficiency - Can **reduce cycle times** by keeping mould at elevated T – allows faster filling and cooling of parts – improves productivity and lowers manufacturing costs - **Minimise scrap** rates by ensuring uniform filling of mould and reducing occurrence of defects - **Tooling costs are higher** than cold runner moulds due to complexity of the system and additional components required

Question 30

# Injection Moulding Mould Tool Design

Answer 30

- **Fillers** in plastic tend to reduce shrinkage and residual stress - To compensate for shrinkage, dimensions of mould cavity must be larger than specified part dimensions **Dc = Dp +DpS + DpS^2** | Dc - dimension of cavity (mm) Dp - moulded part of dimension (mm) S - sh

Question 31

Injection Moulding Variations

Answer 31

- Multi-component (mould more than one type of plastic into the same mould) - Insert moulding (moulding around a part in the mould) - Blow moulding