Cryogenic shot blast deflashing - what it can and cannot do.

Cryogenic high speed shot blast deflashing is a process used by the rubber industry to remove flash from molded rubber parts. The process uses liquid nitrogen, high speed rotation and media (shot blast) in varying combinations to remove the flash in a highly precise, economical and expedient manner (figure 1).

[Figure 1 ILLUSTRATION OMITTED]

Parts that have thin flash can be quickly and thoroughly cryogenically deflashed. This process is exceptionally good at removing the inner dimensional and complex flash that cannot be removed by any other method.

There are two basic styles of cryogenic deflashing machines - basket and belt. The basket style was designed to process small parts and offers 100% parts containment. The belt style was designed for larger/heavier parts which require more room to tumble and a stronger rotation system.

Molding for the best results

The old saying "garbage in, garbage out" is very appropriate. To consistently get a quality finished part, you have to consistently put in a quality unfinished part.

Ideal flash configuration

Everyone who molds rubber parts feels that the ideal flash configuration is no flash at all. Flashless molding is being developed and will have a future, but given the vast quantities of existing molds, cryogenic deflashing will be around for quite a while. So what is the ideal flash configuration? Make the flash as thin as possible with as good a flash base as possible, or, in the case where sealing surfaces are involved, try to move the flash away from the critical areas.

Overflows

The location of overflows in reference to the part has an impact on the cycle time and deflashing temperature, as well as general deflashability of the part. Overflows should, if they are necessary for the molding process, be moved as far away from the part as possible (x [is greater than] plastic shot size)(*).

(*) (x = distance between outer edge of part and inner edge of overflow. 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 the flash. If enough room is left between the part and the overflow, the following advantages are usually achieved:

* shorter deflashing cycle; and

* better deflashing quality.

Tear trim design

Tear trim design was developed to eliminate the cryogenic deflashing operation. The overflow is placed extremely close to the part (x = o)(**) so that, when this overflow is removed by hand, no flash remains.

(**) (actually, x approaches zero - there is virtually no thin area between part and overflow.)

This design usually works well until the mold starts to wear, and this usually does not take very 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 part of the part, and the shot media cannot penetrate the minuscule area between part and overflow. A solution to permit cryogenic deflashing of this part design is to fill in the overflow cavity, thereby leaving only a skin (thin flash) to remove. This improves the deflashability and quality of the part and also results in a mold that has far superior wear properties.

Parting lines

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

In these cases, if part quality needs to be improved, mold rework is necessary.

When qualifying your part:

* Best results can be achieved when the parting line does not exceed 0.005", or 0.127 mm, thickness.

* Parts need a clear, consistent demarcation of flash.

The process

Liquid nitrogen ([N.sub.2]) 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 high speeds (up to 8,000 rpm), throws plastic shot at the tumbling parts, and the plastic shot breaks off the brittle flash on impact (figure 2). The deflashed parts remain in the chamber, and the machine separates reusable media from debris (flash and dust).

[Figure 2 ILLUSTRATION OMITTED]

Load size

There must be room within the blasting chamber for the parts to tumble. The tumbling action exposes the parts to both the [N.sub.2] and the media stream. The actual size of the chamber should be at least twice the size of your load. Average load sizes are from 3.4 cu. ft. of parts to 4 cu. ft. of parts.

Cycle times

Average cycle time in most cases is three to five minutes.

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

The new generation of cryogenic deflashing machines uses the latest in technology offering options such as programmable controllers with numerous deflashing "recipes" for automatic operation, networking capabilities, bar coding abilities, message centers, SPC reporting, telephone modem hook-ups and RS 232 ports which allow the use of printers and report generation.

These improvements have allowed the equipment to function better and to insure the customer an easy transition into the world of ISO 9000.