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Processes S2 > Moulding > Flashcards

Moulding Flashcards

1
Q

Mostly used with thermosets

A
  • Compression
  • Transfer
  • Reaction injection
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2
Q

Mostly used with thermoplastics

A
  • Rotational
  • Injection
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3
Q

Compression Moulding Steps

A
  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
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4
Q

Compression Moulding Advantages

(7)

A
  • 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
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5
Q

Compression Moulding Disadvantages

(4)

A
  • 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)
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6
Q

Transfer Moulding Overview

A
  • 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
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7
Q

Transfer Moulding Advantages

A
  • 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
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8
Q

Transfer Moulding Disadvantages

A
  • 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
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9
Q

Reaction Injection Moulding Overview

(RIM)

A
  • 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
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10
Q

RIM Process

A
  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
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11
Q

RIM Advantages

(4)

A
  • 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
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12
Q

RIM Disadvantages

(3)

A
  • High capital investment
  • Slow cycle times compared to injection moulding
  • Expensive raw materials
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13
Q

Rotational Moulding Process

A
  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
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14
Q

Rotational Moulding Advantages

(6)

A
  • 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
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15
Q

Rotational Moulding Disadvantages

(4)

A
  • 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)
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16
Q

Injection Moulding Process

A
  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
17
Q

Injection Moulding Advantages

(6)

A
  • 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
18
Q

Injection Moulding Disadvantages

(3)

A
  • 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
19
Q

Injection Moulding Machine

A

Divided into two components - injection unit and clamping unit

20
Q

Injection Unit

A
  • 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)
21
Q

Injection Moulding

Clamping Unit

A

(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

22
Q

Injection Mould Cycle

A
  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

23
Q

Injectiton Moulding

Clamp Force

A

Clamping force required

P inside mould
x
projected A of mould cavity

Limits the maximum area of moulding

24
Q

Injection Moulding

Main Features

A
  • 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
25
Q

Injection Moulding

Parts

A
  • 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
26
Q

Injection Moulding

Mould

A
  • 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
27
Q

Injection Moulding

Cold Two Plate Mould

A
  • 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
28
Q

Injection Moulding

Three Plate Mould

A
  • 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
29
Q

Injection Moulding

Hot Runner Mould

A
  • 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
30
Q

Injection Moulding

Mould Tool Design

A
  • 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

31
Q

Injection Moulding Variations

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

Processes S2 (8 decks)

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