Hobby injection molding
Encyclopedia
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 machine
s 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 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 pound-forces (1.1 kN) of downward force, through the use of leverage
.
tools, though, even complex shapes can be cut easily and accurately.
(both LDPE and HDPE), polypropylene
, and polystyrene
(including HIPS) have all been used successfully with lever-actuated benchtop injectors.
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 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 will break down after the first shot and will not allow for the injection of a second shot in almost 100% of the time.
Milling machine
A milling machine is a machine tool used to machine solid materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. Both types range in size from small, bench-mounted devices to room-sized machines...
s 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 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 pound-forces (1.1 kN) of downward force, through the use of leverage
Mechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force...
.
History
It isn't 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 CADComputer-aided design
Computer-aided design , also known as computer-aided design and drafting , is the use of computer technology for the process of design and design-documentation. Computer Aided Drafting describes the process of drafting with a computer...
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
PolyethylenePolyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
(both LDPE and HDPE), polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
, and polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
(including HIPS) have all been used successfully with lever-actuated benchtop injectors.
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 castingEpoxy Casting. Epoxy Casting is the use of a liquid epoxy that has been mixed with a metal alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
. 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 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
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
. 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
Vacuum chamber
A vacuum chamber is a rigid enclosure from which air and other gases are removed by a vacuum pump. The resulting low pressure, commonly referred to as a vacuum, allows researchers to conduct physical experiments or to test mechanical devices which must operate in outer space...
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
Vacuum pump
A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...
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 will break down after the first shot and will not allow for the injection of a second shot in almost 100% of the time.