Hobby injection molding

Hobby injection molding machines, also known as benchtop injectors, hold molds on a smaller scale. Benchtop injectors have become more common as inexpensive CNC milling machines have reduced the cost of producing molds in a home workshop.

In hobby injectors injection pressure is generated manually by the operator, with a lever [1] [2] [3] or gear [4] [5] 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.

History

It is not known when the first hobby injection molder 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.

Applications

Hobby injection molding has a variety of applications including the creation of low costing 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.

Materials

Polyethylene (both LDPE and HDPE), polypropylene, and polystyrene (including HIPS) have all been used successfully with lever-actuated benchtop injectors. [6]

Equipment

Benchtop injectors 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 a benchtop injector 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.

Mold

Injection mold epoxy casting 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.

Metal molds Low cost bench top cnc milling machines allows home enthusiast to machine molds into soft material. Rather than P20 tool steel, most grades of aluminum can be machined into working mold capable of 1000 plus cycles. Mic 6 cast aluminum is more stable post machining and during cycles than hot extruded grades like 6061 and is easy to machine however it has lower mechanical properties. 7000 series like 7050 and 7075 are the most preferable for the best mechanical properties in aluminum next to specializes mold grade aluminum like Fortal, they are all comparable to low to mid carbon steel mold. Copper alloys like pewter or bizmith alloys molds can be cast around a model to create strong mold with higher molding temperatures than epoxy molds. The casting around a model to create each mold part produces complex mold part quickly. The parts can also capture detailed fit face finishes.

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 cavitation 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 mold casting Due to the nature of oxygen entrapment in epoxy during the pouring and curing period it is common to have distortions and cavitation 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 molds Injection molding through the use of a single 1 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.[7]

See also

References

  1. ↑ What Can You Do with an Injection Molding Machine,pg 184. Popular Mechanics. Sep 1970.
  2. ↑ Gingery, Vince. "Plastic Injection Molding Machine". David J Gingery Publishing. Retrieved October 24, 2009.
  3. ↑ Newman, John. "Home Plastic Injection Molding Offered on Kickstarter". rapidreadytech. Retrieved June 10, 2013.
  4. ↑ "Hand-operated injection molding machine". Us Pat. #4,235,584. Nov 25, 1980.
  5. ↑ Gingery, Vince. "Plastic Injection Molding Attachment for the Drill Press". David J Gingery Publishing. Retrieved October 24, 2009.
  6. ↑ What Can You Do with an Injection Molding Machine,pg 186. Popular Mechanics. Sep 1970.
  7. ↑ Gingery 1996.

Bibliography

  • Gingery, Vincent Secret of building a plastic injection molding machine. DGE, 1996
  • Lancaster, Don (March 1996). "Injection molding & plastic prototyping" (PDF). Nuts & Volts Magazine. Retrieved October 25, 2009.
  • Maxon, Kenneth (February 13, 1997). "Injection Molding Basics for the Minimal Mold". Seattle Robotics Society. Retrieved October 24, 2009.
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