Polymers- Injection Moulding and Thermoforming Flashcards
Is injection moulding discrete or continuous?
Discrete
What is good about injection moulding?
Simple of intricate, complex parts of varying sizes can be made. High quality with good dimensional tolerance. Quick
Pressure used in injection moulding
About 100MPa
Costs involved in injection moulding
High initial equipment and mould costs. Low running and per part costs
Sources of scrap from injection moulding
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
What types of plastic can be injection moulded?
Most thermoplastics and increasingly thermosets too
General process for injection moulding
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
Set up of injection moulder
Similar to extruder but the single screw can move back and forth in the barrel so is a reciprocating screw.
Injection phase
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
Phases after injection
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
PVT plot for injection moulding
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
Explain the PVT diagram for injection moulding
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%
What is fountain flow and what does it cause?
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
Injection phase problems
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
Packing phase problems
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
How does the mould work?
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.
Material used for mould
Almost always surface treated tool steel. Can cope with high injection pressures. Moderate material cost. High machining cost. Mould often cost more than machine.
Describe polymer orientation in mould
Typically follows flow direction within the mould but this is influenced by fountain flow
How is polymer melt guided to cavities within mould?
Enters through sprue. Is then fed by runners to the gates which are connected to the cavities.
How to ensure balanced filling of cavities?
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
What do hot runners do?
Mean less waste material, reduced manual labour, more expensive
What are weld lines?
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
Intrinsic defects to the process of injection moulding
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)
Co-injection moulding
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.
Foam core injection moulding
Similar to co-injection by the core polymer foams. Used for surfboards.
Injection compression moulding
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
Spencer-Gilmore equation for thermoplastic injection moulding
(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
How does thermoforming work?
Softened plastic sheet (from extrusion) is forced over a convex or concave single-sided mould. Is a secondary process. Some waste from trimmed edges
What is thermoforming used for?
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
How is polymer orientation affect by thermoforming?
Two processes (including extrusion) so there is double orientation. Not true biaxial orientation which can lead to anisotropy and weaker than intended side walls
Vacuum/pressure thermoforming
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
Mechanical thermoforming
Same as vacuum thermoforming but use a plug to assist forcing the sheet into the mould. Used for thick-walled or deep draw parts
Problems and solutions with thermoforming
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
Why use thermoforming not injection moulding?
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
