SHAPING PROCESSES FOR PLASTICS Chapter 13- Part 2 Injection Molding Manufacturing Processes, 1311 Dr Simin Nasseri Southern Polytechnic State University

SHAPING PROCESSES FOR PLASTICS Properties of Polymer Melts Extrusion Production of Sheet, Film, and Filaments Coating Processes Injection Molding Other Molding Processes Thermoforming Casting Polymer Foam Processing and Forming Product Design Considerations

Injection Molding 13.6

Injection Molding Polymer 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.

Injection Molded Parts Complex and intricate shapes are possible Shape limitations: Capability to fabricate a mold whose cavity is the same geometry as part Shape must allow for part removal from mold Part size from  50 g (2 oz) up to  25 kg (more than 50 lb), e.g., automobile bumpers Injection molding is economical only for large production quantities due to high cost of mold

Polymers for Injection Molding Injection molding is the most widely used molding process for thermoplastics. Some thermosets and elastomers are injection molded Modifications in equipment and operating parameters must be made to avoid premature cross‑linking of these materials before injection. Parts manufactured by injection molding

Two principal components Injection Molding Machine Figure 13.21 Diagram of an injection molding machine, reciprocating screw type (some mechanical details are simplified). - Melts and delivers polymer melt - Operates much like an extruder Two principal components - Opens and closes mold each injection cycle

Injection Molding Cycle Figure 13.22 Typical molding cycle: (1) mold is closed. (2) melt is injected into cavity. (3) screw is retracted. (4) mold opens and part is ejected.

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.

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.

Injection Molding Machines Injection molding machines differ in both injection unit and clamping unit Name of injection molding machine is based on the type of injection unit used Reciprocating-screw injection molding machine Plunger-type injection molding machine Several clamping designs Mechanical (toggle) Hydraulic

Shrinkage Reduction 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.

Shrinkage decreases if the following quantities increase: Shrinkage Factors Shrinkage decreases if the following quantities increase: Fillers in the plastic tend to reduce shrinkage Injection pressure – higher pressures force more material into mold cavity to reduce shrinkage Compaction time - similar effect – longer time forces more material into cavity to reduce shrinkage Molding temperature - higher temperatures lower polymer melt viscosity, allowing more material to be packed into mold to reduce shrinkage

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

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.

Compression Molding A 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.

Compression Molding A 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.