Plastics Molding & Manufacturing/Position

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Control of position is critical to producing high-quality products at reasonable cost. This is because that excessive distance requires excessive time for moving parts to move, and in manufacturing line,time is money. Due to this, the various functions involving distance are basically the same as those related to time.

Mold-close distance

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There are two phases to the mold-close distance: the initial close, covering the major portion of the closing process, and the final close, covering the very last portion of the closing process.

Initial Close
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For initial mold closing, the mold halves position should have approximately 5 to 15 mm gaps. The initial mold closing normally traveled at high speed and is done to get the mold closed as soon as possible so that overall cycle time can be minimized. But if the mold halves are allowed to slam together under high speed, they will eventually crack and collapse. The presence of an foreign objects such as broken plastic will cause damage to the mold if mold slammed together at high speed.

Final Close
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For final mold closing, the mold is slowed to a crawl, but placed under high closing pressure. This occurs for the entire 5 to 15mm distances, until the mold halves are closed tightly against each other. This slow final closing is done to minimize closing damage. If any foreign object is caught in the mold, the closing action will stop at this point and the object can be removed without causing any further damages to the mold. The mold is fully closed under full clamp tonnage and the injection phase is allowed to begin.

Injection distance

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There are two phases to the injection distance: the initial injection and the injection hold.

Initial injection distance
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Initial injection distance must be set to ensure that approximately 95% of the intended material is injected into mold cavity. This distance varies depending on how big the mold cavity is and how much of the barrel capacity is being injected for one shot. The limit switch governing the shot size would be set half-way back (50% of the barrel size) on the measurement scale. In any case, the scale is incremental and can be adjusted anywhere between 0 and 100% of barrel capacity.

Holding distance
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With the initial injection setting allowing 95% (approximately) of the material to be injected, the machine switches to holding pressure. This finishes filling the mold and holds pressure against the injected material. The position at which the hold pressure takes over should be set almost at the very end of the stroke of the injection screw.

Mold-open distance

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To break the vacuum created in the cavity image during the injection process, the mold must be opened slowly. After the mold has opened for about 5 mm, the vacuum on the stationary side is relieved and the mold can be opened fully at a faster rate so that the cycle can be completed quickly and the next cycle begun. The total distance a mold is to open should not be more than what is absolutely necessary because it takes time to open the mold, and time is money. The mold should open a total distance equal to twice the depth of the molded part [For molding machine with auto ejector]. For example, if the part being molded is a square box with a depth of 150mm., the mold should be allowed to open no more than 300 mm. If possible, this dimension should be made smaller. All that is needed is enough open space to allow the finished part to fall clear of the mold after ejection or for the operator to reach in and remove the ejected part.

If the mold is running with an operator who must manually remove the part from the mold, the mold-open distance should be adjusted to whatever level is necessary from the standpoint of safety and comfort, in addition to simply allowing enough room for manipulating the part. In most cases, it rarely exceed 2.5 times the depth of the part.