Flashing

Flashing is defined as excess material that has exceeded the boundary of a mold beyond the mold cavity. Flash can be found in a number of areas. The parting line is one of the more frequent locations where flash is found. When a mold does not clamp fully, this provides a space for material to flow. Vent areas are another location of flash, especially if vents are cut too deep and wide. If not fully closed, movable mold sections such as slides can move back, due to the pressure of the melt front permitting material to flash into the space resulting from the relocation of the slide. Here are some causes and solutions below for flashing.

The cause of flash on a part is inadequate drying of the material, especially hygroscopic material that can absorb moisture. The addition of water to many hygroscopic materials, such as nylon and polycarbonate, can reduce the viscosity of the material by breaking down the molecular weight of the polymer. Moisture in the form of bubbles creates an easy-flowing polymer. Solutions: Drying the material according to recommendations needs to be followed to avoid this phenomenon.

Flow properties of the material, such as viscosity and melt flow rate, affect the amount of flashing found. Low-viscosity, high-MFR material will have a tendency to flow easier, raising the potential of flashing. Solutions: High-viscosity, low-MFR materials can resist flashing since they display stiffer flow properties. Decreasing melt temperature will also prevent flashing since the viscosity of the material increases resisting flow.

Holding pressure can cause flashing, especially when packing pressures are too high, forcing more material into cavity. Solutions: Reducing hold pressure will reduce the chance of flashing. Reducing the amount of screw feed or reducing the cushion will reduce the risk of flashing, due to less material being forced into the cavity.

When a mold is designed with not enough support to the mold surface, flexing of the mold surface may occur, which will flash the tool. Solutions: Support inside the mold to resist the high pressures applied is important in preventing flash. Sufficient support of the mold surface and cavity needs to be designed to prevent flexing of the surface.

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Undercuts Design in plastic injection parts

An undercuts is an indentation or projection on a plastic injection part usually means that the core moving direction is not parallel to the draw direction of mold, so that the molded plastic injection part makes ejection from the sample two- part mold almost impossible. Because of the movement way they are oriented, some feature is introduced to injection mold design. The cores or protrusions, which used to form these indentation, referred to as undercuts, are typically used in articles such as enclosures to provide a snap fit to assemble plastic injection parts.

Internal undercuts can be molded by using two separate core pins, as shown in Fig 1. This is a very practical method to use, but flash will sometimes occur where the two core pins meet.

Fig 1 An internal undercut can be molded by using two separate core pins

    Fig 2 shows different types of undercuts, which are frequently necessary in a molded plastic part design. It can be classified as an undercut on one side wall, internal undercut, external undercut, and circular undercut of a part.

There is a very fine distinction between undesirable and impossible undercuts. Fig 2a is an internal undercuts which can be formed using movable cores that are low efficiency and expensive, and that should be avoided. A more efficiency method to molding an internal undercut is redesign the products as shown in Fig 2b.

External circular undercuts are loaded in the outside contours of the part as shown in Fig 2c. A more efficiency method to molding an external undercut is to use two retractable ejector wedges. The other less expensive method is forming by grooving the part in a lathe with a properly designed cutting tool. Side wall undercuts as shown in Fig 2d must be molded by retractable forming core or wedge which is quite expensive.

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Fig 2 Different types of undercuts in molded plastic injection parts