Polymer_3 Flashcards
Describe the Injection Moulding process. Factors?
Homogenised molten plastic is forced into a two part cavity mould using a reciprocating screw.
Discrete process for relatively intricate, 3D parts of varying sizes.
High initial cost, but low running costs.
Creates moderate amounts of scrap (which can be ground and reused)
- Sprues, runners
- Flash
Most thermoplastics.
What does an Injection Moulder compose of and look like?
(Draw)
Name and describe the Injection Moulding phases.
Injection
Prepare a molten plastic ‘charge’ in the metering zone of an extruder.
Push screw forwards which forces the charge into a cavity mould.
Packing
Maintain pressure to ensure mould remains filled.
Cooling
Allow to cool
Prepare next charge
Ejection
Open mould
Eject part and collect
(Repeat)
Describe what is happening at each of the Injection Moulding Phases.
A-B
Liquid polymer enters the mould and is pressurized at constant temperature causing the volume to decrease.
B-C
Liquid polymer is held at high pressure while the temperature decreases.
This causes the volume to decrease so more polymer is pumped into the mould to account for this.
C-D
Once the temperature drops below a certain value, the gate freezes over and no more polymer can enter the mould.
The volume is now fixed and further reductions in temperature cause the pressure to drop.
D-E
The temperature and volume drop, the polymer completely solidifies and finally room temperature is reached.
The contraction in the final part is related to the volumes at points D and E (typically approx. 1%)
What are the Injection Moulding Issues during the Injection and Packing phases?
Injection phase issues:
Injected polymer adopts ‘fountain flow’ where molten polymer continues to flow through solidified polymer.
If the polymer solidifies too soon, it will block the mould creating a ‘short shot’.
If the polymer is injected at too high a pressure, it can leak out causing ‘flash’.
Packing phase issues:
As the polymer cools it’s density increase so injection pressure must be maintained until it solidifies. Stopping injection early can lead to the polymer falling back into the mould creating distinctive ‘sink marks’ or voids.
Describe two part moulds.
Two part mould; 1 part fixed, 1 part mobile.
Cooling
Water or air.
Ejection
Automated ejection important for multiple cavity moulds.
Material
Almost always tool steel to cope with high injection pressure.
Moderate material cost.
High machining cost.
Polymer orientation
Strong areas.
Weak areas.
Describe the general mould features.
Sprue
Where the material is injected from the screw.
Runner
Where the polymer melt flows into the cavity. Feed system.
Gate
Where the polymer melt enters the cavity mould
What are the differences between single and multiple cavity moulds?
Multiple cavities are more efficient but moulds can become very complex.
Ensure balanced filling cavities by restriction of gates and runners, complex runner paths.
Hot runners
Less waste material
Reduced manual labour.
Describe the use of Gates in Injection Moulding.
Large cavities may need more than one gate which can lead to ‘weld lines’.
Gate design is critical to obtaining the desired part quality.
Design Mould to internalise defects;
Parting line.
Gate mark.
Ejector pin marks.
Describe Injection Moulding Variants.
Co-injection moulding
Two sequential extrusions
1st molten polymer molten (skin) is displaced by 2nd molten polymer (core).
Cost, combination of properties.
Foam core injection moulding
Similar to co-injection moulding but the core polymer foams.
Surf boards.
Overmoulding
1st polymer forms core then 2nd polymer forms skin.
Golf balls, comfort grips.
Gas or water assisted injection moulding
Inject a molten polymer which is then displaced using gas or water to create a skin.
Good wat to injection mould hollow parts.
