Screen Options

The fractional and millimeter dimensions are nominal hole diameters.

In general, the maximum thickness a sheet of stainless steel in which a hole is punched is limited to one gauge thickness less than the diameter of the hole. Thus the small perforation material is very thin, typically 24 to 28 gauge steel. In contrast, 14 gauge sheets are used for the 3/32" perf.

The 3/32" perforation is the standard used in orange peel presses and most other applications. It is preferred because its thickness makes it resistant both to tramp metal damage and to wear when screw-to-screen interference occurs.

The smaller perforation sizes are used where the material being pressed tends to extrude through the holes. This typically occurs with producers of fruit juices (apples, grapes, pineapple, cranberries, pear, tropicals, etc).

The fibrous solids that get through the holes are screened from the press liquor, and then added back to the flow entering the press. To improve the bursting strength of the 1/32" and half millimeter materials, a 1/8" thick back-up reinforcing screen is employed. Back-up screens are made of 3/8" or 1/2" perforated plate.

Perforated screens have the advantage of being an inexpensive material that can be changed out with a minimum of labor.

Perforated screens generally have 20% to 25% open area. Surprisingly, in most applications the smaller holes have significantly more press throughput capacity than larger holes. Of course, the smaller hole screens are thinner and more susceptible to damage.

A completely different type of screen is made of slotted wedgewire. Wedgewire screens are made by resistance welding wedge shaped bars of stainless steel parallel to each other. The slot width between these bars is specified. These bars have a truncated triangle cross-section. They are 1/4", 3/8" or 1/2" in height.

Wedgewire screens are produced in flat panels that are subsequently rolled to the nominal screw diameter. The wide base of the triangle cross section faces the material being pressed, thus providing relief on the liquid discharge side.

Typically, Vincent presses use nominal slot widths of 0.010" and 0.016". These have a tolerance of plus or minus 0.003". Manure presses use 0.015", 0.030" and 0.040" slots. For cooking oil, slots in the range of 0.008" to 0.012" are specified.

An important advantage of wedgewire is that it can be self-cleaning. In some applications there is less chance of a wedgewire screen blinding than there is with perforated metal.

Another advantage lies in the greater thickness of the bars as compared to perforated sheet. It means that the useful life is extended when pressing abrasive materials (paper, plastic, sand, glass) or materials containing tramp metal. This thickness also results in greater burst strength.

Wedgewire screens typically have 12% open area. Surprisingly, in general they have the same screw press throughput capacity as perforated screens with double the open area.

CROSS SECTION OF WEDGEWIRE SCREEN	PERFORATED SCREEN WITH BACK-UP PLATE