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Mold:
The mold or die refers to the tooling used to produce plastic parts in molding. Traditionally injection molds have been expensive to manufacture and were only used in high-volume production applications where thousands of parts were produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminum, and/or beryllium-copper alloy. The choice of material to build a mold from is primarily one of economics. Steel molds generally cost more to construct but offer a longer lifespan that will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel molds are less wear resistant and are primarilly used for lower volume requirements or larger components. The hardness of the pre-hardened steel measures typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining, making them superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC).
Aluminum molds cost substantially less than steel molds, and when higher grade aluminum such as QC-7 and QC-10 aircraft aluminum is used and machined with modern computerized equipment, they can be economical for molding hundreds of thousands of parts. Aluminum molds also offer quick turnaround and faster cycles because of better heat dissipation. They can also be coated for wear resistance to fiberglass reinforced materials. Beryllium copper is used in areas of the mold which require fast heat removal or areas that see the most shear heat generated.
Injection Molding Process
With injection molding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly pushed forward by a screw-type plunger, the plastic is forced into a heated chamber called the barrel where it is melted. As the plunger advances, the melted plastic is forced through a nozzle that seats against the mold sprue bushing, allowing it to enter the mold cavity through a gate and runner system. The mold remains at a set temperature so the plastic can solidify almost as soon as the mold is filled.
Injection Molding Cycle
The sequence of events during the injection molding of a plastic part is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected.
Different Types of Injection Molding Processes
Although most injection molding processes are covered by the conventional process description above, there are several important molding variations including:
• Co-injection(sandwich)molding
• Fusible(lost, soluble)core injection molding
• Gas-assisted injection molding
• In-mold decoration and in mold lamination
• Injection-compression molding
• Injection molding of liquid silicone rubber
• Insert and outsert molding
• Lamellar (microlayer) injection molding
• Low-pressure injection molding
• Microinjection molding
• Microcellular molding
• Multicomponent injection molding(overmolding)
• Multiple live-feed injection molding
• Powder injection molding
• Push-Pull injection molding
• Reaction injection molding
• Resin transfer molding
• Rheomolding
• Structural foam injection molding