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 Stimsonite
Corp. makes numerous types of thermoplastic roadway marking material using
multiple major and minor ingredients. Major ingredients are handled two
ways. The primary ingredient, calcium carbonate, is pneumatically conveyed
from storage silos to a mixer. Other major ingredients in 50 pound bags
are manually added to the conveyor hopper and transferred to the mixer.
Each product recipe
calls for five minor ingredients on average. In the past, preparing minor
ingredients (or "odds") for batching involved carrying a 50 pound ingredient
bag to a stationary scale and slitting it open. If the recipe called for
13 pounds of the ingredient, that amount was poured into a bag sitting
on the scale. Then the partially used 50 pound bag was set aside, and
the next ingredient was weighed out into the bag on the scale.
The batching worker
followed a recipe sheet that listed each ingredient amount needed for
the batch. When all minor ingredients were weighed out in bags, they were
dumped to the same pneumatic conveyor transferring the major ingredients
for mixer loading.
Manually adding
minor ingredients creates clutter and spills
 Because
recipe preparation involved manually scooping several minor ingredients
from a 50 pound bag, various problems occurred. "There was a lot of dusting
and spillage and the problems associated with that," says Process Engineer
Bill Floor. "The batching workers would change uniforms probably two or
three times a day. They had to wear special masks, caps and gowns. And
we had material loss from spillage. At cleanup, we could fill a couple
garbage cans with various spilled materials."
The many opened 50 pound minor ingredient
bags were also a problem for the producer. And, unlabelled bags with small
quantities of weighed out ingredient lay all around the batching area.
"It was a mess; we had lots of open bags and many were identifiable only
to the worker who assembled the minor ingredients. " Floor says, "Somebody
new coming into the area wouldn't know what was what."
Manually adding minor ingredients
also created a potential for operator error. Floor says, "When adding
so many scoops of several different ingredients, it's possible to forget
where you are."
When this happened, it wasn't known
until the batch had gone through all production steps. "Sampling shows
if something isn't right. To fix a batch, we have to reprocess the whole
thing," says Floor. "If the batch was short of a material, the easy
remedy was to simply add more of the ingredient. But if the batch had
too much of a material, the remedy was more difficult."
The ingredient-adding operations
were very labor intensive as well. Bag handling was excessive because
each 50 pound bag had to be picked up and moved several times. Also, Floor
says a typical batch has "about 75 pounds of minor ingredients, and a
scoop size is one pound. So for every batch, a worker had to scoop once
for each ingredient pound needed. It was a very slow process."
The producer had considered ways
to make their minor ingredient operations more efficient and to minimize
dusting so worker exposure would stay at acceptable levels. "We did some
industrial hygiene tests on worker exposure to dust. " Floor says, "The
OSHA 8-hour exposure limit is .050 mg/m3. We found we were above that
at times, so we researched ways to improve operations and stay in compliance."
They decided to upgrade the batching operations.
Producer looks
at ways to streamline ingredient handling.
Unlabeled
bags with small quantities of weighed out ingredients lay all around
the batching area. "Somebody new coming into the area wouldn't know
what was what."
To find a solution, Floor and a coworker
visited the Powder & Bulk Solids Exhibition in Chicago. "We were searching
for anything that could help streamline weighing and loading minor ingredients,
" Floor says. "We considered dispensing odds from a standard steel container
[IBC] with a slide gate at the bottom. We considered an [IBC] with a loss-in-weight
feeder, and we looked at a fully automated loss-in-weight [bulk bag dispenser]."
"The OSHA
8-hour exposure limit (for airborne dust) is .050 mg/m3. We researched
ways to improve operations and stay in compliance."
They found a manufacturer
that built a semi-bulk dispensing station with a polyethylene bin and
a slide-gate valve. "It was exactly what we were looking for," Floor says.
After discussions with the dispenser manufacturer about the application,
the manufacturer proposed a design. "They supplied excellent drawings
that allowed us to make several revisions to fit our needs," said
Floor.
"One of
the nice things in considering the equipment was that the manufacturer
offered an upgrade program. They said they would buy back the equipment
and upgrade us to an automated system if we wanted," Floor said.
Dispensing bins
provide accurate ingredient discharge
In April 1997, the producer installed
two Ingredient Masters dispensing stations that hold eight bins each,
for a total of 16 dispensers. The bins' rectangular upper section slopes
down to a cone-shaped bottom where material discharges through a slide-gate
valve are made of Type 304 stainless steel.
The bins are available in various
capacities, and the producer selected 28 Cu. Ft. units. The inner surface
is liquid smooth and has no right angles that can trap material; each
inside corner has a 2-inch radius. These features allow a first-in, first-out
material rotation and eliminate the need for bin cleanout.
The dispensing stations have a loading
platform and a pallet cart that can move a pallet of bags to the bin being
loaded. The bins are filled through a top screw-on lid. Although workers
lift 50 pound bags to fill each bin, each bag is lifted only once and
quickly emptied into the bin.
A battery-operated portable scale
on a rolling cart is used to weigh dispensed ingredients from each bin.
"The batching worker puts a bucket on the scale, wheels it over to an
ingredient dispenser, and opens that slide-gate valve," Floor says.
As
the target weight nears, the worker can adjust the flow to a trickle.
"We probably
get 10 times the productivity from that worker now, because he or she
can accomplish a lot more with the new method."
Exact weighments are possible because
the valve is accurate, Floor says. "When we shut it off, it's an immediate
[flow] shut off; there's no spilling." After each minor ingredient is
added to the bucket, the worker tares the weight and adds the next ingredient.
When the bucket contains all the required minor ingredients, it's manually
dumped in the conveyor hopper and transferred to the mixer.
New dispensing stations
allow batch consistency, reduce cleanup
Installing the dispensing stations
significantly improved minor ingredient batching operations. Opened and
unlabelled bags no longer clutter the batching are floor. "We did a major
cleanup of the area," Floor says. "Dusting is significantly reduced, and
we've cut our cleanup time to almost nothing, because there's essentially
no spillage. We have less material loss as a result."
With the dispensing stations, the
producer keeps its operations in compliance with OSHA regulations.
Workers
dispensing material to buckets don't need to wear masks. "We have a safe
and clean work environment," Floor says.
Minor ingredient production costs
have been reduced. Only one batching worker is now needed. And the worker's
productivity has gone up. "We probably get 10 times the productivity from
that worker now because he or she can accomplish a lot more with the new
method," Floor says. "Now a worker simply opens the valve at the bin's
bottom and weighs out the material in probably one-tenth the time it used
to take."
Another productivity benefit is that
the worker has more time to perform other functions. "We de-bottlenecked
the area. Our throughput is up about 150 percent in the odds [minor ingredients]
batching area," says Floor.
Besides saving labor expenditures,
the producer saves space in the batching area, according to Floor.
"We've
reduced the floor space needed for the entire odds area. I'd say we're
using roughly 40 percent less space than before."
Floor is satisfied with the dispensing
station manufacturer and the production results. "Batch accuracy and consistency
have improved greatly. Reworking batches due to improper minor additions
no longer occurs. I've been totally satisfied with the service I received,
from initial contact to system installation."
The producer is now considering installing
similar equipment in other plants, according to Floor. "It took only six
months to get a return on our investment," he says.
Published in March, 1998 Powder & Bulk Engineering magazine.
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