Home Injection Molding
Home injection molding has become more common as inexpensive CNC milling machines have reduced the cost of producing injection molds in a home workshop.
In home injection molding, pressure is generated manually by the operator, with a lever or gear, translating the operator’s effort to the required pressure. The most common hobby injection machine uses a handle to press down with. This enables the user to generate roughly 250 pounds-force (1.1 kN) of downward force, through the use of leverage.
It is not known when the first home injection molding was constructed. Before the development of inexpensive CNC milling machines, producing a metal mold was prohibitively expensive for most hobbyists. With a small CNC mill and personal CAD tools, though, even complex shapes can be cut easily and accurately.
Home injection molding has a variety of applications including the creation of low-cost prototypes, new inventions, replication of lost or broken parts, and provides homeowners the opportunity to build anything. Hobby injection molding is a low cost method of repeatable production.
Home injection molds are smaller and simpler than their larger commercial counterparts because they rely on the operator to manually inject melted polymer into the mold and remove the finished part from the mold. Production injectors automatically inject melted polymer at a prescribed rate into the mold, cool the mold to rapidly solidify the polymer, then eject the part from the mold once it’s cool. The two halves of the mold must be pressed together with great force to prevent a defect in the part where the two halves meet, and the nozzle of the injector must be pressed tightly against the inlet port of the mold to prevent the escape of melted polymer and, again, a defect in the finished part. In home injection molding, this is done manually by clamping or bolting the mold together and clamping the complete mold into the injector. In a production injector this is accomplished with hydraulic or pneumatic actuators, which increase the cost of the machine but dramatically reduce the labor required to produce a finished part.
Epoxy Casting: Epoxy casting is the use of a liquid epoxy that has been mixed with a metal alloy. Typically, the most common form of epoxy based molds are combined with atomized aluminum. The atomized aluminum allows for the distribution of heat from the mold surface outward toward the edges. This preserves the surface quality of the mold for typically 50 – 100 cycles on a single epoxy mold.
Pressurized Mold Casting: Due to the nature of oxygen entrapment in epoxy during the pouring and curing period, it is common to have distortions and cavitations in the final injection mold. Pressurizing epoxy during the curing period is a form of surface quality retention. External pressures can be created with the use of a pressure pot connected to an air compressor. The external pressure causes air to be trapped inside the epoxy mold and crushed during curing. As time passes over a 24 hour period, the oxygen bubbles will not be able to escape and will cure directly inside the mold. With sufficient pressure these small cavitation holes will be invisible to the naked eye.
Degassing Injection Mold Casting: Due to the nature of oxygen entrapment in epoxy during the pouring and curing period, it is common to have distortions and cavitations in the final injection mold. Degassing the epoxy during the curing period will require the use of a vacuum chamber and will require a pressure of 100 kPa (29 inHg) in order to create near vacuum conditions. This can be achieved with the use of a 2 stage vacuum pump that is capable of 2 Pa (15 μmHg).
Single Use Injection Molds: Injection molding through the use of a single use injection can be achieved through the use of “plaster of paris.” The mold will produce one solid part. However, the mold will break down after the first shot and will rarely allow for the injection of a second shot.