Some Cautions Of Plastic Injection Molding

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Some Cautions Of Plastic Injection Molding

Posted By Jin hong     April 8, 2021    


In Injection plastic mold, the mold consists of multiple metal parts that come into contact with each other and with the molten plastic: Two halves of the mold come together at a parting line. The molten plastic resin material travels through sprues and runners and is injected through openings called gates. After cooling, ejector pins push the molded part(s), the runner, sprue, and gates out of the mold. At any meeting place between these different parts of the mold, a thin layer of plastic could flow outside of the cavity, creating flash. Flash is common along the parting line but can also occur at ejector pin locations and at other locations.

Causes of Injection Molding Flash and How To Remove Them
Flash can occur for several reasons, from variations in the process to material to tooling trouble. Flash during the injection molding of a part can occur in filling (the first injection stage) or during packing/holding (the second stage). It can occur with conventional parts and with thin-walled parts. Below are some causes of flash, and options for fixing it in each case.

  1. Parting Line Mismatch
    Flash can be caused by the parting line being mismatched in the tool. To prevent parting line mismatch:

Clean off any plastic, dirt or contaminants that could keep the mold from closing properly, especially behind slides and inside the leader pin bushings.
Ensure proper mold plate fit and mold seal:
Clean the molding press from any obstructions.
Check to see if mold plates have become deformed from molding pressure during injection and packing. Correct this with pillar supports or by thickening or reducing mold plates.
Give the mold plates adequate surface areas that seal off at the parting line, evenly distributing the clamp force during molding. Always provide a sufficient "land" of even, adequate seal-off area around the part cavity.
2. Improper Venting
Flash can be caused by inadequate vents in the mold. To address or rule out improper venting as a cause of flash:

Check the mold vent size against the supplier recommendations: Obtain the recommended vent sizes from the supplier of the plastic resin being used. If the current vents are not the recommended size, correct the vent size. If the tool is old and showing wear, re-machine the vents if needed.
Remove air from the mold as fast as possible by making vents as deep as the material viscosity will allow. The viscosity of the plastic being molded determines the depth of the vent. Stiff materials can utilize deeper vents, but fluid materials require thinner vents.
3. Clamping Pressure In the Packing / Holding Phase
In addition to occurring during the injection phase of plastic injection molding, flashing can occur during packing or holding. In packing, pressure is adjusted and additional material is injected into the mold to account for material shrinkage and backflow. In holding, the material is held in place at a pressure equilibrium until gate freeze occurs, and cooling begins. In either of these parts of the process, if there is inadequate clamping pressure, the parting line of the mold may be pushed apart, allowing plastic to flow out and cause flash.

To address or rule out clamping pressure as a cause of flash:

Do a first stage short shot test: If the product passes this test with no flashing, then the problem is occuring after the injection phase and you are right to consider clamping pressure as a possible cause.
Increase the clamp tonnage: The clamp pressure needs to be strong enough to withstand the forces generated by the plastic material flowing through the mold. Size the mold to run in the proper machine to ensure that adequate clamp pressure will be used.
4. Sprue Bushing Issues
Hardened-steel sprue bushings in the mold provide the opening for molten plastic to be transferred to the mold cavity. Sprue bushing issues can cause flash, either when there is inadequate support for the force of the injection pressure, or when the mold is held open.

Provide adequate support in the mold for the sprue bushing: When the press nozzle seals on the sprue bushing, it can create forces from 5 to 15 tons. If there is inadequate support in the mold for the sprue bushing, this pressure could transfer to the parting line, allowing plastic to flow out. Address this by adding the needed amount of support in the mold for the sprue bushing.
Adjust the sprue bushing length: If the face of the sprue bushing is sealing off on the parting line, adjust the length to ensure that nozzle pressure or thermal expansion does not hold the mold open and allow plastic to flow out.

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