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Purchasing Basics: Material handling systems

May 23, 2001

6 Min Read
Purchasing Basics: Material handling systems

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With all the material handling equipment well planned and installed on the mezzanine, every machine on this molding floor can get any material that is run in the entire plant. 

From the moment the resin you use is ordered from the distributor, events are put into motion that affect how that resin is handled all the way to the throat of the molding machine. Molders setting up new material handling systems, or simply adding to equipment already in place, have countless options and roads to be explored. As one supplier told us, it's better if the customer does as much research as possible before contacting a supplier. Otherwise, it's possible that the customer will get what the salesman wants to sell, rather than what will best meet the molder's needs. 

What follows is a series of questions designed to stimulate thought on how to prepare for shopping for materials handling equipment. Put your best logistics experts on this task, and put as much as you possibly can down in writing. 

The Material 
What exactly are the materials to be stored, conveyed, and loaded? What is the bulk density, particle size, shape, melt point, and dusting character for each different component? Is your material virgin or regrind or a mix? Is some filled with an abrasive filler that will mean stainless steel elbows need to be installed? Will it need to be dried centrally? 

If the material is in powder form, your supplier will probably want an analysis of particle size by percent in various micron ranges, or at the very least a sample for his own analysis. Regrind and various square-cut pellets don't flow as well as new round pellets; they also require a different cone angle. 

Packaging guidelines

Here are all the different ways your resin can be packaged for delivery.

Package

Capacity

Bag

50 lb

Box

55 lb

Drum (carboy)

300 lb

Gaylord

1000 lb

Bulk bins

5000 lb

Truck

40,000 lb

Railcar

180,000 lb

In what kind of container(s) will material arrive at the plant? The options stretch from railcar down to a 50-lb bag, with quite a few options in between (see box at left). How much material will pass through your process in an hour, a day, a week, a year? How many different grades? In addition to the resin itself, what kinds of additives or colorants need to feed into the process? At what stage and in what ratio? 

Material Logistics 
Where will material be stored? Outdoors? Inside? In conditioned space? If it arrives by railcar, to be unloaded into a silo, what is the horizontal and vertical travel distance from one to the other? Are there bends in the travel path? Is there a specific transfer rate from the railcar required? 

Inside the plant, is there a central material handling area that does central blending, central drying, or distribution? What are the conveying distances inside, both horizontal and vertical, and how many bends would there be? How many molding machines need to be fed by a central system? Is there a central reclaim area that requires a flow of materials from the presses and/or back to the presses? 

Do you need to keep different lots of the same material separate? If material traceability is an issue, your vendor will likely specify several small silos rather than one large one. If you are able to specify which molding machine will be using which resin and provide an exact location, you could save considerably on the amount of piping to be installed. 

Are present machine hoppers adequate for efficient automated loading? What is the estimated feedrate of each molding machine? Does the material need to travel in a controlled (e.g., dry) environment? 

Rules of Thumb

  • It takes 40,000 lb/month of a single resin to justify installing a silo.

  • Vacuum/pressure conveying provides double the conveying rates of vacuum alone.

  • Nowhere is this more true than in material handling: The shortest distance between two points is a straight line.

  • The maximum conveying distance should be 800 equivalent ft.

  • A properly designed system generates resin velocities of 5000 ft/min. At speeds more than that you create angel hair.

  • A gradual upward slope is never better than a vertical lift.

  • When resin hits a 45° or 60° elbow, it ricochets back and forth, creating turbulence that destroys momentum.

  • PVC pipe is bad for conveying lines; static electricity will be generated.

  • Try this test: Take a handful of the resin to be conveyed and squeeze it firmly. If upon opening your hand the lines in your palm are filled with fines, it will be difficult to convey. 

Blending Needs 
If blending is part of your equation, what are the needs? Is batch blending suitable, where materials are dispensed sequentially one at a time? Or is the blending likely to be continuous, where additive materials are dispensed simultaneously? What is the volume of blended materials in ratio to the main plant materials conveying requirements? 

Is in-house addition of color part of the process? What are typical letdown ratios? Is there a need for liquid color feeding devices? 

The Plant Environment 
Many of these issues relate only to installation, but they are crucial to proper planning. A layout of the whole property is essential; if it is not to scale, crucial distances should be marked. Items that need to be shown: roads, driveways (concrete, asphalt or dirt), trenches, power lines, and other utility feeds. Be sure that the area designated for a silo isn't on soggy soil, for instance. 

It's especially critical to note power lines at the silo location, to plan for crane maneuverability. What are the plant's exterior and interior walls made of—steel, concrete, brick, and so forth? If railcar delivery is the chosen method, is the railcar siding so close to the plant that it might hinder pneumatic unloading of the plastic? 

What's the load on the equipment? Is the operation 24/7, or is there downtime for regular cleaning and maintenance? How much plant traffic will have to take place near the material handling system? If forklifts have to pass by often, for example, reinforcement or protection of vulnerable areas should be planned. 

Civil Authorities 
If this is a first-time installation, you should contact the building department in your locality to determine height restrictions or other building code issues. 

With our thanks . . .
. . . to experts in material handling from Comet Automation, Conair, L-R Systems, and Novatec for their contributions to this article. 

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