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Purchasing Basics: Parts handling systems

May 23, 2001

8 Min Read
Purchasing Basics: Parts handling systems

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Parts handling equipment comes in many shapes and sizes, but usually must be selected to accommodate equipment and workflow that already exist in a plant or that dictate what a new installation will require.
Photo: Dynacon 

Molders are increasingly asked to do more than mold parts and put them in a box for shipment to someone else. Part of the value-adding operations many molders are offering as a service is handling that part further and performing a secondary operation on it, whether it is special packaging, decorating, finishing, assembly, or something else. 

Increasingly, that downstream work is being automated. There are three fundamental reasons for wanting to automate downstream parts handling. One is the fact that you don't have and can't find enough people to handle parts manually. The second is the need to provide uniformity and control to an assembly process—hard to do with manual labor. The third is an attempt to save people costs. Whichever your motivation, it pays to prepare a list of your needs to help your supplier focus on what will work best for you. 

If investing in labor-saving equipment is your choice, it pays to consider some preliminary ideas first. What steps should you take to get the most effective system for your needs? How much can you expect to improve your productivity? Can a new system be tied into what you already have? Can it be added to in the future? How can you get the most for your money? How can you make the whole process as easy on yourself as possible? 

Before you do anything you'll want to do some basic homework on how this investment will impact your operation. The first questions you will need to ask are these: 

1. How much money is available for automation projects? 

2. What rate of return is necessary to justify the expense? 

3. Where can production costs be reduced in order to justify these expenditures? 

In order to gauge the rate of return, you need to know what your present costs are. Gather all of the labor expenses you are incurring now for the operation you would like to automate for one month. Include basic labor rates, overtime, vacation time, and benefits. 

Separating parts and runners

There are several ways to separate your parts from the sprue and runner in a cold runner mold. Part-runner separation does not work for all products. A conveyor company should ask you for sample shots for testing prior to your purchase of any separator. They will determine the percentage of separation that can be achieved—85 to 95 percent is an average figure. If you need 100 percent separation, a robot or sprue picker will be your best bet.

  • Finger separators allow parts to fall through the fingers into a box or onto a conveyor. The sprues are then conveyed by the fingers up and over the top of the separator into a box or a conveyor.

  • Roller separators (mounted on the end of a conveyor) allow the part or runner to fall through the gap between the roller and the conveyor into a container below. The remaining piece passes over into another container.

  • Drum roller separators have a series of rollers mounted in a circular drum. Parts and runners enter the drum at one end, large pieces pass through the drum and out the other end, and small pieces fall through the rollers into a container. This type of unit can be mounted over a container or conveyor, or over top of a boxfilling conveyor.

  • Auger type separators are most effective for small parts. The part slides under the adjustable-height roller and the sprue is swept to the side. 

Next, figure approximately what you think it will cost to install an automation system. Then divide the cost to install by your monthly labor charges and you'll be able to project your payback period. But there are ways to accelerate that payback. Consider labor savings by running more than one shift. If you have two presses each molding parts, you probably have one operator at each press, earning $25,000 apiece per year. By streamlining the parts to arrive at one workstation where one operator can pack them, you have already saved $25,000. If you run two shifts you'll save $50,000; if you run three shifts you'll save $75,000/year. 

Automation increases the productivity of high-cost molding machines. If you are not running three shifts a day, you are paying those molding machines to stand around. By keeping them running 24 hours/day instead of 8 you need only one machine to do the work of three. Not only does that mean you have more money available for additional capital investments, but it also means you can offer significantly shorter delivery times. 

For involved applications or for special situations, you'll probably want the vendor to see the operation first hand. This will give him a chance to see what systems already exist and how they can be tied together with your new systems. He may also find solutions to other plant automation problems, and if you are not planning to automate everything at once, he can also help you develop a long-term plan. Surprisingly, this type of service often costs no more than buying out of a catalog, and can often produce far superior results. 

The Part 
First, your vendor will need to know as much as possible about the product you are molding or planning to mold. 

  • Part size, shape, and weight. 

  • Sprue size and shape, if any. 

  • Cycle time. 

  • Mold cavitation. 

  • Types of materials you are using. 

  • Temperature of the part, if not at room temperature. 

  • Does the part need to be cooled? If you are trying to reduce your cycle time, you may be tempted to remove your part from the mold while it is still warm. Ordinarily, this might distort the part. However, ambient or chilled air or water can be used to cool parts outside the mold before they are handled.

How Your Operation Works 
Just as you did in planning for material handling systems, in this case you need to provide comprehensive details about your operation. 

  • Where are you conveying the parts from

  • Where are you conveying them to

  • Are there any dimensional limitations along the path? 

  • Are there obstructions such as overhead cranes, resin feeding systems, or ceiling height restrictions? 

  • Are there auxiliaries around the machine—chillers, hot runner controllers, granulators—that will interfere with loading the parts? 

  • Is there enough space between the presses to allow for handling your project or do you need parts to come out under the clamp end of the machine instead? 

  • Does the part have to be oriented in a certain way? 

  • Are there secondary operations after the part leaves the mold, such as inspection, labeling, trimming, or assembly? 

  • Do the parts need to be boxed or bagged? 

  • Do the parts need to be stored? 

  • How many presses will be automated at this time? 

  • What are any plans for future automation? You want to be sure that anything you buy today will be flexible enough to fit into a larger, more comprehensive plan later.

Ways to fill a box

There are many ways to fill a box with parts. The method used is usually determined by the degree of accuracy desired, the size or shape of the part, or the type of molding machine controls available.

  • Weigh scale. Especially effective when small parts need to be accurately counted.

  • Photo eye. Excellent choice for large products.

  • Cycle count. Most common choice. An injection molding machine sends a signal to a counter on the conveyor PLC. It then counts the number of cycles and relays the count to an indexing, boxfilling conveyor. The conveyor indexes a box into position and then moves it away once the count is met.

  • Side-by-side boxfilling units can be used to conserve floor space. These conveyors incorporate a sweep arm for box transfer from the fill position to a gravity discharge conveyor.

  • An over/under stacking system uses an elevating station to transport boxes from the fill position to an accumulation conveyor located above.

  • Vertical stackers. Boxes can also be stacked vertically. In these systems, boxes are placed into vertical slots and indexed into position.

  • Tote stackers also save a great deal of space. Totes are nested together in a compact unit and then indexed into the fill position. 

Air Conveying Particulars
If you need to get parts over a long distance from one area of your plant to another, air conveyance is an option. Systems may be designed either as positive pressure (pushing parts) or negative pressure (pulling parts). There are a number of details that your vendor will want to know before designing a system, similar to those above, but with a few variations. Type of material and part dimensions are significant, along with weight of part, part temperature, cycle time, and number of parts in a cycle. It's also important to know if the parts will be fed into the conveying lines in timed, intermittent batches, or in continuous, metered feed. 

Like the conveying of raw materials, the horizontal and vertical distances to be traversed, the height of inlet and discharge ports, and the number of 45° and 90° bends are critical. It's a good idea to submit the parts for initial testing; one vendor recommends five cycles' worth. 

With our thanks . . .
. . . to conveying systems suppliers EMI and Sterling Systems for their assistance in contributing information for this article. 

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