The Role of Accumulators in Material Transfer
It is part of the CRG Automation mission to find ways to increase productivity and minimize material waste and errors. A significant part of figuring out automation solutions gets down to the logistics of the flow of product. Speaking in general terms an automated (or even semi-automated) production process consists of the raw elements that need to be brought together at one end of the process and the finished goods, packaged, labeled and boxed for shipment at the other end. In between the start and finish of the automated process are a number of tightly controlled steps depending on what the finished product is. This can include processes such as die cutting, gluing, filling, inspecting, sorting, coating, drying and so forth. Some of the processes are best done one at a time, or in small batches – such as inspection, whereas other processes are best done in bulk, like die cutting. Because the optimum process size and time can vary widely accommodation must be made for variations in the process flow. This is where product accumulators come into play. When talking about accumulation, often the term “transfer station” is used to indicate moving product from one part of a process to another. Accumulation is a specific process which involves bulk transfer or processing of material or finished goods.
ACCUMULATORS USED WITHIN THE PROCESS
Product accumulators are simply ways to accumulate product, so that it is more easily processed in bulk. One example is a palletizing station at the end of a manufacturing line. Let’s suppose that the manufacturing line is producing headlight retrofit kits for a certain model car. They are being packaged in groups of 12 to be sent to auto parts wholesalers. As the boxed up kits come off the production line they are gravity fed into a large apron. A robot picks up kits, orients them to the large carton, fills that carton with 12 kits and then it goes to a taping and labeling station. From there it is palletized and plastic wrapped for transport. This particular example is a transfer from continuous flow to intermittent flow. The use of a robot gives some flexibility. In case there is some upstream problem, it may be able to continue working while that problem is sorted out.
Sometimes it is necessary to accumulate material at the start of a process. This is often the case when a sortation or an inspection process is needed before the product can be moved to the next step. One example might be at a recycling station where a hopper is periodically loaded with material to be recycled. From that hopper, the material goes through several filtering steps to separate it into material categories that can be handled by that particular recycling center; for example, scrap steel, aluminum and glass. This is a transfer from intermittent flow to continuous flow.
Accumulator stations can also be used in the middle of a manufacturing process. This often happens if there are several grades of product that are being handled on the same line. For instance eggs are graded by weight. In addition to individual weights the combined weight of all of the eggs in a carton must meet a minimum weight requirement. Most common sizes of eggs are medium , large and extra-large (though the US Department of Agriculture recognizes seven weight ranges of eggs). An accumulator for this process would need to be able to sort individual eggs by weight and also to accumulate a minimum weigh requirement before a dozen eggs can be put in a carton. Eggs are accumulated by weight category, but an action to load up a carton only happens when the minimum weight requirement (or more) for a dozen is attained. This is considered an intermittent transfer.
These examples are simplified, but it is easy to see how having operations that are continuous married to operations that don’t have to be continuous can be accommodated by some form of an accumulation station.
ACCUMULATION TRANSFERS INTO THE PROCESS
In addition to the types of accumulation stations discussed above, there are others which are not so much used to manage the flow as they are to support the manufacturing process.
Probably the most prevalent of accumulators for manufacturing assembly processes is the vibratory bowl feeder. This is commonly used with small parts that need to be oriented so that they are easily incorporated in the next step of an assembly process. Let’s take the example of a small cylindrical spacer with a hole through the middle. Assume there are four holes in the body of the assembly that need the spacer. For fastest assembly, an operator would need to grab a spacer, orient it properly, insert it into a hole, repeat that process three more times and then let the assembly move further along the line. The beauty of bowl feeders is they can be designed so that the spacers can be bulk loaded into the hopper of the bowl feeder and through specifically designed manipulations, it can present four spacers oriented correctly so they can just be picked up and inserted. In fact at that point a small robotic arm may be substituted for a manual operation to insert the spacers. Other examples of these types of “helper” processes could include, orienting and assembling bolts or rivets for assemblies, or even distributing a liquid gasket material from a large drum on to a part, followed by placement of the gasket. In each case bulk (accumulated) material is distributed as needed to each part in proportion to the need.
One other type of accumulator process involves quality control. With some manufacturing processes, in order to maintain quality, it is necessary to periodically measure some physical aspect of the product, often a critical dimension. The sample size and sample interval would be dictated by the acceptable performance level. But in these cases it may be necessary to extract a sample from the production flow, say 250 parts for example. Out of this sample it may be necessary to measure 10 pieces, which can be done fairly quickly using a Coordinate Measuring Machine (CMM). If everything checks out, then the retained samples can be released to production. It may also be the case that there is a deviation from the desired dimensions. Not enough to stop the production line, but enough to either make an adjustment, or possibly change the sample interval to keep the process within control. By taking periodic samples, problems are caught early and scrap is minimized.
As we have seen within the manufacturing process there are some natural places where accumulations of product can make sense. Depending on the area, this can result in increased accuracy, provide for product buffering for some processes and minimize bottle necking. Buffer storage zones may be incorporated at the start, at the end or even in the middle of the manufacturing process.
Accumulating bulk components of the manufacturing process external to, but timed with the manufacturing process can result in an improved production rate, reduced labor, reduced work-in-process, and improved integration of operations.
The examples presented would be considered linear accumulators but the flow pattern of operations may be either in-line or rotary depending on the application. The rotary type is usually suited to circular product like labeling stations on bottles, but they should be considered when factory floor space is limited as they are very space efficient processes.