Cryogenic high-speed shot blast deflashing uses liquid nitrogen, high-speed rotation and media (shot blast) to remove flash from molded rubber parts.

In the rubber industry, cryogenic high-speed shot blast deflashing uses liquid nitrogen, high-speed rotation and media (shot blast) to remove flash - the excess plastic that squeezes out at the mating points of molds - from molded rubber parts. Cryogenic high-speed shot blast deflashing is particularly effective at removing thin flash from molded parts. It also is effective at removing the inner dimensional and complex flash that cannot be removed by any other method.

The Process

Liquid nitrogen (N₂) is injected into a highly insulated chamber in which molded rubber parts are tumbled and blasted. The flash, which should be significantly thinner than the parts themselves, is embrittled by the low temperature.

At the same time, a precision throwing wheel, turning at speeds up to 8,000 rpm, throws plastic shot at the tumbling parts. The plastic shot breaks off the brittle flash upon impact. The deflashed parts remain in the chamber and the machine separates reusable media from debris such as flash and dust.

Two basic styles of cryogenic deflashing machines are used: basket and belt. The basket style was designed to process small parts and offers complete parts containment. The belt style was designed for larger and heavier parts that require more room to tumble and a stronger rotation system. With any cryogenic deflashing system, there must be room within the blasting chamber for the parts to tumble. The tumbling action exposes the parts to both the liquid nitrogen and the media stream. The actual size of the chamber should be at least twice the size of the load.

Factors Affecting Deflashing Success

When it comes to effective deflashing, the old saying, “garbage in, garbage out,” is appropriate. To consistently get a quality finished part, it is important to start with a quality unfinished part.

Anyone who molds rubber parts may feel that the ideal flash configuration is no flash at all. While flashless molding is being developed and will have a future, products molded with existing molds will continue to drive the demand for cryogenic deflashing.

So, what is the ideal flash configuration for effective cryogenic deflashing? Molders should strive to make the flash as thin as possible with a good flash base. Where sealing surfaces are involved, try to move the flash away from critical areas.

Overflows. The location of overflow in reference to the part influences the cycle time and deflashing temperature used as well as the overall ability to deflash the part. If they are necessary for the molding process, overflows should be moved as far away from the part as possible. (For instance, assume X equals the distance between the outer edge of the part and the inner edge of overflow. Then, X should be greater than the plastic shot size. In addition, plastic media must be smaller than X in order to get inside this area and deflash properly.)

The closer the overflow gets to the part, the more difficult it is to remove. The plastic shot cannot penetrate between part and overflows to remove flash. If enough room is left between the part and the overflow, shorter deflashing cycles and better deflashing quality usually can be achieved.

Tear trim design was developed to eliminate the cryogenic deflashing operation. The overflow is placed extremely close to the part so when the overflow is removed by hand, no flash remains. (In this case, X approaches zero, so there is virtually no thin area between the part and the overflow.)

This design works well until the mold starts to wear - and this usually does not take long because of the knife edge required between the part cavity and the overflow cavity. Trying to cryogenically deflash these parts is very difficult. When the part is cooled down and becomes hard, the overflow essentially becomes a part of the piece, and the shot media cannot penetrate the miniscule area between part and overflow.

One solution to permit cryogenic deflashing of parts with tear trim design once the mold begins to wear is to fill in the overflow cavity. Filling the overflow cavity leaves only a skin, or thin flash, to remove. This improves the deflashability and part quality and can result in a mold that has superior wear properties.

Parting Lines. The parting line and flash base configuration determine the overall deflashing quality. No cryogenic deflashing unit will eliminate molding problems. If there is no difference between the thickness of the flash and the part, the deflashing unit will remove both.

In these cases, if part quality needs to be improved, mold rework is necessary. When qualifying your part, the best results can be achieved when the parting line does not exceed 0.005" (0.127 mm) thickness. Also, parts need a clear, consistent demarcation of flash.

Will Cryogenic Deflashing for Your Parts?

Ultimately, the best way to qualify parts as candidates for cryogenic deflashing is to have a sampling of the parts processed, or tested, in a cryogenic deflashing machine.

Modern cryogenic deflashing machines include options such as programmable controllers with numerous deflashing recipes for automatic operation, networking capabilities, barcoding abilities, message centers, SPC reporting, and RS 232 ports that printing and report generation.

Links

• Cryogenic Systems & Parts