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V-Mold Clamp Design |
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Mini-Jector machines use a Mechanical Advantage to generate large clamping forces. A Mechanical Advantage occurs when a machine takes a small input force and creates a large output force.
Mechanical Advantage (MA) = S/T = 3.83
Clamping Force = MA * Applied Force = 3.83 * 6300 lbs = 12 Tons
The Mechanical Advantage (MA) generated by the "V" mold is calculated by the equation: MA = S/T
All Mini-Jector molds possess a MA = 3.83
Clamping force (C) is equal to the mechanical advantage (MA) times the effort force (E) generated by the pressure in the carriage cylinder:
C=MA*E
C = 3.83*6283 lbs = 24,063 lbs = 12 Tons
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V-Mold Design Basics |
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The "V" molds used in "Wasp" Mini-Jectors are adaptable to standard mold construction technology, but they also have some requirements of their own.
The unique feature of "Wasp" machines is the self-clamping "V" mold that is removed from the machine each cycle. Items commonly associated with injection molds, such as heating and cooling passages and part ejection systems, are not applicable to these compact tools. The unique requirements are dictated by the fact that the mold comes in contact with three surfaces every inject cycle:
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| 1) The two mold clamping shoes: |
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The mold clamping shoes are made from #80-60-03 ductile iron. This material is compatible with the wedging and ejecting action of the machine when using molds made of bronze, low- or high-carbon steel. Aluminum can also be used for mold construction, but it will abraid after prolonged use.
The standard Mini-Jector mold blanks (part no. 121230 - 2 5/8" x 6" and part no. 420170 - 3" x 7") and the cavity plates of the insert mold base (part no. 420180) are built from SAE #4150 annealed medium-carbon steel. This material is free-machining; it may be heat-treated up to a hardness of Rockwell HRC 55-60 and polished to a smooth finish. The surface contacting the mold clamping shoes (the 7.5° angle surface) does not require heat-treating to work well against the shoes; however, if the mold is to be heat-treated anyway, this process will cause the angled sides to last longer.
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| 2) The injection nozzle: |
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The injection nozzle, either straight bore or non-drool, is very hard. The bearing surface where the nozzle contacts the top of the mold is a 9/16" diameter round, which means that a force of nearly 25,000 PSI is pressing on the top mold surface at the sprue opening. This force will indent or "coin" the top of the mold.
The sprue plate can be replaced if necessary without having to regrind the rest of the mold. The 9/16" diameter is an optimum size selected because greater areas would not seal the sprue-nozzle interface.
Under no circumstances should any recess be machined into the mold to receive the nozzle. A flat surface is required.
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| 3) The mold ejector bar: |
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The dimensions of the mold ejector bar are 1" x 7", so adequate contact area is available for the ejection force. Since the bar contacts both mold halves, be sure that the bottom surface of the mold is evenly machined to ensure uniform contact on each half.
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Heating and Cooling V-Molds |
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Cartridge heaters in the shoes will heat the mold by contact. If additional mold heating is required, an optional auxiliary control package is available. The package includes two heat controllers, two 200 Watt cartridge heaters to insert into your mold, and two thermocouples. Pre-heating the mold in an oven will also allow the mold to reach a stabilized temperature if cycle times are kept consistent. If mold temperatures over 300°F are used, the shoes must be insulated from the machine frame.
Due to the small size of the "V" mold, it is impractical to run cooling lines to the mold itself. Molds run for a period of time will soon become fairly hot, but they will seldom reach temperatures above 105°F.
Some customers use a pair of molds to allow each mold a cool-down period between cycles. A fan can be used to cool the extra mold as a cycle is being run. It is also possible to use small cooling hoses to run cold tap water into the mold, but remember these must be removable every cycle along with the mold.
Cooling passages in the "V" mold clamping shoes do not effectively remove mold heat. |
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Part Removal and Mold Handling |
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The "V" molds do not provide for any type of ejection of the finished part, so other techniques must be used to get the finished part out of the mold.
When the mold is removed from the machine, the two halves must be separated to remove the part. Two diagonally-placed dowel pins are used in the mold base. These need to engage only 0.125" into the hole. Wiggling the two mold halves usually allows you to pry them apart. Adequate draft or taper on the molded part perpendicular to the parting line will assist the separation.
Note the handles at the end of the mold facing the machine operator. These, along with the short dowel at the opposite end, can be used as pry devices if mold separation is difficult. The handles can be bolts, long dowel pins or a wooden file handle. Notice they are placed adjacent to the mold-locating dowel pins. The pairs of protruding handles (or dowels) can be pried upon with hand tools or pushed onto an auxiliary fixture with two wedges that pass between the pairs of pins. The handles also aid the operator greatly in handling the mold, as the mold can become very warm after prolonged operation.
Screwdriver slots can be placed at the ends of the mold as an alternative to the protruding dowels; however, this does not provide the mold handling convenience of the other method.
When designing your mold, remember that large, flat parts are more difficult to remove from molds. The gate into the mold should be large enough to allow you to remove the entire shot, with runners and sprue, without having the gate break off. Proper mold release compounds greatly assist in removal of the parts. |
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V-Mold Technical Info