Automated accurate rubber strip stacking.

Over the past several years, the requirement for uncured rubber strips, ranging from 1-1/2" to 6" wide and 3/16" to 5/8" thick, has increased dramatically. These strips must be packaged in sufficient weights and lengths to allow further continuous processing (downstream extrusion and/or injection molding processing) without the need for constant rethreading of the strips or other manual interface.

In order to provide these strips, manual stacking on skids or into baskets has been the accepted standard. Though manual stacking produces an acceptable (even artistic) stack, it is time consuming, tedious, labor-intensive and ergonomically questionable. The increased demand for strips and the pitfalls of manual stacking have led to the design of automated and accurate strip stacking equipment. No longer does the stacking and handling of rubber strips need to be an extensive manual operation, as automated and accurate strip stacking is available for most applications.

Rubber is often produced in very long, thin, narrow strips. It is not possible to store or transport the rubber in this form, so the strips are commonly folded into multiple layer piles and stacked on pallets for storage and transportation. This can be accomplished three ways. The rubber strips may be folded and stacked onto pallets for semiautomatic machines, but the resulting stack can cause the strip to overlap on itself, creating feed problems at the next processing step. Additionally, these machines do not allow dense stacking, and a high percentage of the stack is actually air, an expensive product to ship.

As already mentioned, rubber strips may also be stacked by hand. While stacking densities and accuracies increase from those of the semiautomatic machines, the labor is extensive and monotonous, and can even cause repetitive motion injuries.

Finally, fully automated machinery can be used to stack and fold the strips accurately and densely. Stacking and folding devices comprised of conveyors, distribution heads, folding flaps and control systems evenly stack the rubber strips neatly and densely for further processing. This machinery can operate at speeds greater than humanly possible, allowing higher productivity of entire mixing lines.

A number of different stacking methods is available for stacking narrow rubber strips. Table 1 compares the flexibility and relative costs of these methods, which are described later within this article.

Three basic stacking standards

The most common way to automatically stack uncured narrow strips of rubber has progressed from three old standards. The first, in-line with a batch-off method, is to simply wigwag stack the strips into a basket after the strips are directly removed from a batch-off machine. The second is to produce the strips off-line, where a full width strip of rubber on a pallet is slit into narrow strips by a cold slitter and then wigwagged into a wire or plastic basket. The stacking quality and density are often less than desired, but the strips are contained within the basket. When removing the strips, manual assistance is required, as the strips are often twisted and overlapped, making automatic removal difficult.

These problems can be resolved in part by wigwag stacking into divided baskets. With this type of stacking (which applies to both the in-line and off-line methods described above), the strips must be carefully guided on a conveyor to the wigwag unit. At the exit of the wigwag unit, guide rollers are provided at the top of each divided basket section so that the leading edge of the rubber strips can be manually threaded between the rollers and into the correct divided basket compartment. This separates the individual strips, but they can still overlap and twist.

A third method used in the past to stack narrow strips involves membrane slitting or notched knife slitting of wide strip rubber from a mill (thus producing narrow strips which are still attached to each other), and then wigwag stacking them like a wide rubber strip. Membrane slitting is where the stock is slit through part of the thickness to allow easy separations of the slab into strips alter stacking. Notched knife slitting is where the stock is cut completely through the thickness, but perforations are left that keep the strips in a full-width slab form through the stacking process. The stacking of either of these becomes critical based on the downstream use (or processing) of the stacked strips. If the strips are used one at a time, the stacking must allow individual strips to be removed without tangling, breaking or affecting the remaining stacked strips. This requires the strips to be stacked accurately along their width. The accuracy necessary is not only compound-dependent, but it also varies with strip width. Strips cut to a specific width must not be overlapped by strips beside or above them by an amount that inhibits their individual removal. As a general rule, the narrower the strip, the more accurate the stacking must be. Figure 1 is a photo of an accurate stack that allows individual strip removal from a pallet. Other common methods of preparing uncured rubber strips for further processing will be examined.

[FIGURE 1 OMITTED]

Off-line slitting and stacking

Off-line slitting and stacking takes place after the normal coating, cooling and stacking of rubber within a traditional batch-off machine. The major advantage of off-line slitting and stacking is that the traditional batch-off line is unencumbered by additional processing, thus guaranteeing no reduction throughput. The major disadvantage is that now a new process (requiring floor space, manpower, equipment, utilities and maintenance) has been added.

Off-line slitting and stacking takes a stack of wide, wigwagged rubber and unstacks it, cuts it into individual strips and then restacks the narrow rubber strips for further processing. Traditionally, the narrow strips are stacked into a divided basket as previously described, but with additional strip guides (rollers) shown in figure 2.

[FIGURE 2 OMITTED]

Another method of off-line slitting and stacking is to start with a wide strip of rubber stock from the mill and slit it into narrow strips with a notched knife slitter system and stack it as described above. Then those stacks are removed to an off-line system where the strips are individually pulled from the stack and slit again into two narrower strips. In figure 3, the input is a wigwagged stack of notched slit rubber stock with 3" wide strips. The skid of stock is held in a stock tipper, threaded through a conveyor with a cold slitter within it to slit the 3" strips into two 1-1/2" wide strips. Then the 1-1/2" strips are threaded into a dual head RSC WigWag for stacking into a divided basket. The strips may be individually removed at the next process.

[FIGURE 3 OMITTED]

Spooling

Additional methods for storage and handling of narrow strips include spooling. Spooling can be used at the end of the batch-off machine as an in-line process or off-line at the exit of the cold slitter. Figure 4 shows a cold slitter slitting 28" wide sheet stock into 1-1/2" strips. The strips are then wound around a shaft. The spool of rubber is then removed for unwinding at the next operation.

[FIGURE 4 OMITTED]

Spooling onto reels is yet another option. Again, this can be done in-line or off-line. Shown in figure 5 is a reeling system for four individual strips. Here, multiple shafts are used so that the full reels can be removed while empty reels are threaded and the winding started again.

[FIGURE 5 OMITTED]

Spooling has the advantage of keeping the individual strips easily separated from each other for further processing, but there are two serious disadvantages. First, not much weight (or length of strips) can be placed on a reel or shaft without extra equipment required for handling (i.e., removing from the shaft, storage and transportation). Secondly, let-off devices of some type are required at the downstream processing equipment. But when there are concerns about stock deformation and stickiness, spooling is sometimes the best solution.

Some types of rubber stock cannot be dipped or sprayed with an anti-tack coating. For these stocks, an additional method of spooling is available; spooling with liner material. This spooling can be from a standard festooning system or from a cooling conveyor. The rubber stock can be partially slit or slit through, but some uncoated stocks will not stay in the slit form and cold flow back together. This spooling system also requires let-off equipment for the downstream processing. And the liner material, whether fabric or poly, must also be spooled, stored and handled. But when anti-tack cannot be used, this type of spooling is a useful solution for preparing and handling rubber strips.

Rotary strip cooler (RSC) and WigWag

The RotoStrip Cooler (RSC) and RSC WigWag are a combination of two pieces of equipment to coat, cool, dry and stack a single narrow strip. The RSC can be placed at the exit of a small mill or extruder which produces the rubber strip. The RSC WigWag is used to stack the strip into baskets or onto pallets. The RSC WigWag has two axes of controlled motion (back and forth/left and right), and is therefore able to fill a basket or pallet more effectively than a standard wigwag. These systems are simple solutions for preparing strip rubber for further processing within a plant or for shipping to another plant, but the stack is not dense and can tangle when shipped long distances.

Figure 6 shows a RotoStrip Cooler and RotoStrip WigWag. Note that the RotoStrip WigWag can also be supplied as a stand-alone machine with two or more stacking heads to allow stacking of multiple strips from a conveyor' or batch-off into divided baskets.

[FIGURE 6 OMITTED]

(Part 2 of this article will appear in the April 2004 issue.)

Table 1--stacking equipment flexibility and cost comparison

                                                                 Spool-
                               WigWag     Into                    ing
                      Hand      into     divided     WigWag       on a
                      stack    basket    basket     slit slab    shaft
Strips
Narrow (<1.5")          X        X          X           X          X
Medium (1.5 to 6")      X        X          X           X          X
Wide (>6")              X        X          X           X          X
Cost
Equipment               $        $         $$          $$         $$$
Labor                 $$$$$      $          $           $         $$$
Throughput            $$$$       $$       $$$$          $         $$$$
Transportation          $      $$$$$      $$$$          $        $$$$$
Use @ next op.          $       $$$        $$          $$$        $$$$

                      Spooling                 RotoStrip     Wind-up
                        onto      RotoStrip      WigWag       with
                       spools      WigWag      multi-head    liners
Strips
Narrow (<1.5")           X            X            X
Medium (1.5 to 6")       X            X            X            X
Wide (>6")               X            X                         X
Cost
Equipment               $$$           $            $$         $$$$
Labor                   $$$           $            $           $$
Throughput              $$$$         $$            $            $
Transportation         $$$$$        $$$$          $$$         $$$$
Use @ next op.          $$$$         $$$           $$          $$$

                      Multi high     Accu-
                       density       Strip
                        WigWag      stacker
Strips
Narrow (<1.5")
Medium (1.5 to 6")        X            X
Wide (>6")                X            X
Cost
Equipment                $$$$        $$$$
Labor                     $            $
Throughput                $            $
Transportation            $            $
Use @ next op.            $$           $