Presentation on theme: "SHAPING PROCESSES FOR PLASTICS Chapter 13- Part 2 Injection Molding"— Presentation transcript:
1 SHAPING PROCESSES FOR PLASTICS Chapter 13- Part 2 Injection Molding Manufacturing Processes, 1311Dr Simin NasseriSouthern Polytechnic State University
2 SHAPING PROCESSES FOR PLASTICS Properties of Polymer MeltsExtrusionProduction of Sheet, Film, and FilamentsCoating ProcessesInjection MoldingOther Molding ProcessesThermoformingCastingPolymer Foam Processing and FormingProduct Design Considerations
4 Injection MoldingPolymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity where it solidifies and the molding is then removed from cavity.Produces discrete components almost always to net shape.Typical cycle time 10 to 30 sec, but cycles of one minute or more are not uncommon.Mold may contain multiple cavities, so multiple moldings are produced each cycle.Similar to hot-chamber die casting.Pellets fed into heated chamber.Melt forced into split die chamber.10,000 to 50,000 psi.Mold is water cooled.Part is ejected.
5 Injection Molded Parts Complex and intricate shapes are possibleShape limitations:Capability to fabricate a mold whose cavity is the same geometry as partShape must allow for part removal from moldPart size from 50 g (2 oz) up to 25 kg (more than 50 lb), e.g., automobile bumpersInjection molding is economical only for large production quantities due to high cost of mold
6 Polymers for Injection Molding Injection molding is the most widely used molding process for thermoplastics.Some thermosets and elastomers are injection moldedModifications in equipment and operating parameters must be made to avoid premature cross‑linking of these materials before injection.Parts manufactured by injection molding
7 Two principal components Injection Molding MachineFigure Diagram of an injection molding machine, reciprocating screw type (some mechanical details are simplified).- Melts and delivers polymer melt- Operates much like an extruderTwo principal components- Opens and closes mold each injection cycle
8 Injection Molding Cycle Figure Typical molding cycle: (1) mold is closed. (2) melt is injected into cavity. (3) screw is retracted. (4) mold opens and part is ejected.
9 Injection Unit of Molding Machine Consists of barrel fed from one end by a hopper containing supply of plastic pellets.Inside the barrel is a screw which:Rotates for mixing and heating polymer.Acts as a ram (i.e., plunger) to inject molten plastic into mold.Non‑return valve near tip of screw prevents melt flowing backward along screw threads.Later in molding cycle ram retracts to its former position.
10 Clamping Unit of Molding Machine Functions:Holds two halves of mold in proper alignment with each other.Keeps mold closed during injection by applying a clamping force sufficient to resist injection force.Opens and closes mold at the appropriate times in molding cycle.
11 Injection Molding Machines Injection molding machines differ in both injection unit and clamping unitName of injection molding machine is based on the type of injection unit usedReciprocating-screw injection molding machinePlunger-type injection molding machineSeveral clamping designsMechanical (toggle)Hydraulic
12 ShrinkageReduction in linear size during cooling from molding to room temperature.Polymers have high thermal expansion coefficients, so significant shrinkage occurs during solidification and cooling in mold.
13 Shrinkage decreases if the following quantities increase: Shrinkage FactorsShrinkage decreases if the following quantities increase:Fillers in the plastic tend to reduce shrinkageInjection pressure – higher pressures force more material into mold cavity to reduce shrinkageCompaction time - similar effect – longer time forces more material into cavity to reduce shrinkageMolding temperature - higher temperatures lower polymer melt viscosity, allowing more material to be packed into mold to reduce shrinkage
14 Reaction Injection Molding (RIM) A processing technique for the formation of polymer parts by direct polymerization in the mold through a mixing activated reaction.Two highly reactive liquid ingredients are mixed and immediately injected into a mold cavity where chemical reactions leading to solidification occur
15 Reaction Injection Molding (RIM) RIM was developed with polyurethane to produce large automotive parts such as bumpers and fenders.RIM polyurethane parts possess a foam internal structure surrounded by a dense outer skin.
16 Compression MoldingA method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity.The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, and heat and pressure are maintained until the molding material has cured.
17 Compression MoldingA widely used molding process for thermosetting plastics.Also used for rubber tires and polymer matrix composite parts.Molding compound available in several forms: powders or pellets, or liquid.