the mini-jector


injection molding process
At the beginning of the machine operating cycle, the "V" mold is placed on the mold ejector bar and inserted against the mold stop. At this time, the clearance between the "V" mold and the mold clamping shoes is rather loose at about 0.010". As the injection plunger begins its stroke, the mold ejector rods are released, lowering the mold ejector bar about 0.188". The "V" mold is then allowed to fall further down between the mold clamping shoes until its sides contact the side surfaces of the shoes.

In plunger ejectors like the Lab's, the plastic granules in the heating cylinder resist the downward motion of the the injection plunger, causing the heating cylinder assembly to press its nozzle down against the top of the mold. The force exerted against the mold by the machine's nozzle is equal to the hydraulic pressure generated by the plunger cylinder. This force pushes the "V" mold into the slot formed by the two mold clamping shoes. The wedging of the mold against the 15 degree included angle of the system is amplified by the inverse of the tangent of the angle which is a factor of 3.73 : 1. This forces the mold shoes apart slightly, which stretches the two tie bars that hold the shoes. This stretching creates a tension, much like a spring, in the tie bars which presses back against the mold , holding it together during injection. When the carriage cylinder sees a pressure of 1950 PSI, a pressure switch is tripped, starting injection. This insures the mold being fully clamped.

When the hot plastic is injected into the mold, the force in pounds per square inch (PSI) created by the hot plastic is amplified by what is termed the "projected area" or shadow area of the cavities and runners on the same plane as the mold parting line. This force is proportional to the pressure of the molten plastic times the projected area. It attempts to push the mold halves apart during injection, which can cause the plastic to "flash" out between the mold halves.

The tension force stored in the tie rods resists the injection force, keeping the mold tightly closed until injection is completed. This clamping pressure is measured in tons, which is just under 12 tons. Due to varying viscosities of the various plastic resins, the actual melt pressure within the mold cavity is impossible to calculate, but a good rule of the thumb is to have 2 tons of clamping pressure for every square inch of mold projected area. For low viscosity materials such as nylon or high melt index polyethylene, at least 3 tons is recommended. The practical limit of these machines is therefore 6 square inches.

When the injection timer times out, the plunger retracts. The plunger will trip a limit switch about .188" short of retracting fully. The operator then turns the selector switch to "retract-eject " which pulls the ejector bars up fully ejecting the mold from the "V" slot by the ejector bar. The mold can then be removed. The limit switch may be set to permit full retraction and ejection without need to turn the switch.

insert and loose-core molding
Insert and loose-core molding can be performed economically on Mini-Jectors like the one at the Lab. Insert molding involves molding plastic around a metal or wire inserted object or onto a previously molded plastic part (sometimes called "overmolding") to form a finished assembly. Complete encapsulation of miniature electrical or medical devices can be performed on these machines. Loose cores are metal forms inserted into the mold prior to molding to form hollow portions in the finished part. The core is then removed from the mold and finished piece after the mold is taken apart. An example of loose core molding would be forming the hollow portion of a fishing lure.

Insert molding of electrical parts such as sockets, cord sets, and slip ring and brush assemblies would be typical insert applications, as would molding handles on tools or knife blades. The beauty of insert molding using "V" molds is that the insert or loose core can be placed in the mold; the two halves assembled and the entire assembly held in one's hand prior to placing it in the machine. This permits molding around very fragile inserts that could not be safely molded in high tonnage machines.

economics
Because of the low cost of Mini-Jector machines and tooling, even very short production runs of 100 pieces or less are economically possible. A typical case in point would be a model shop, where miniature scale models of industrial equipment and hardware are assem- bled in precision scale models of factories and manufacturing facilities. A large variety of parts in small individual quantities are required in such an undertaking. Prototyping of new or experimental parts, color testing, ASTM tensile test specimens, and educational programs in schools and universities are other typical applications for Mini-Jectors.

specifications

	Injection volume-in. 3 @ (plunger diameter)
	1/2	1.178
	5/8	1.841
	3/4	2.651
	7/8	3.608
	Injection pressure-PSI @ 2000 PSI @ (plunger diameter)
	1/2	32,000
	5/8	20,480
	3/4	14,223
	7/8	10,449
	Max. shot size-ounces Polystyrene @ (plunger diameter)
	1/2	.33
	5/8	.50
	3/4	.75
	7/8	1.00
	Cylinder Heater Wattage	1,050
	Nozzle Heater Wattage	150
	Plasticizing Capacity-Lbs. Per Hour	10
	Injection Cylinder Bore-inches	2.0
	Injection Cylinder Stroke-inches	6.0
	Approx. Maximum Clamp Force-Tons	11.75
	Maximum Casting Area-in. 2	6.0
	Mold Ejection Force-Lbs	3313
	Hopper Capacity-Lbs. Polystyrene	13
	Max. Mold Size-WxLxH-inches	7.00x3.00x2.125
	Power Consumption-AMPS (230 Single Phase)	19.6
	Machine Shipping Weight-Lbs	550
	Overall Dimensions-WxLxH	33.25x25.63x43

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