Injection Moulding Flashcards
What are the advantages of injection moulding?
Wide range of component sizes
Complex shapes
Range of materials
Repetitive accuracy and consistency
Automation for mass production
What’s the difference between screwback and injection?
Screwback - rotation and axial/backward motion
Injection - axial/forward motion (no rotation)
What is Max Shot/Swept volume?
Max amount that can be injected into a mould
Vmax = Volume of barrel (xA) + Vnozzle
What is max clamp force?
Max clamp force needed to hold the mould faces closed together
What are the timed stages within injection moulding?
Injection
Packing
Screwback
Decompression
Mould Opening
Ejection
Mould Closing
What is cycle time for injection moulding?
Sum of times for each stage
How can flow be balanced with a feed system to ensure injection moulded parts are of equal quality?
Runners could have different diameters like the ones near the nozzle could be thinner
What is a gate in injection moulding?
Entrance to the mould cavity, fed by runners - aim to prevent back flow and provide easy de-gating at ejection
How are injection moulds kept at temperature?
Cooling channels drilled into mould plates
Electrical resistance cartridge heaters eg. In heated runner systems
Why might you need heated moulds in injection moulding?
Feasibility of moulding - affects pressure and flow
Structural development - affects microstructure, shrinkage, distortion etc.
Promotion of crystallisation
Product quality - surface and certain features
What are the disadvantages of heated moulds?
Increased cooling times and therefore cycle time
How do you calculate pressure needed through a injection moulder?
Sum of pressure drops at nozzle, sprue, runner, gate and cavity
What happens if pressure in an injection moulder is above or below the required pressure difference?
Above = successful mould filling
Below = short shots
How does frozen layer affect effective cavity thickness?
Heffective = Htotal - 2xthickness of frozen layer H
What’s the source of stresses in injection moulding?
Close to the mould wall - high shear stress and rapid cooling - high orientation
Towards the centre - low shear stress and slower cooling - orientation is lower
