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In This Issue
The
Parting Line - "Considering Expansion?"
Great Quotations
Concerning Material Storage
Teach Them To Troubleshoot
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"Ask Doc" - Controlling Fill
Reducing Cycle Times
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Dear Reader,
Welcome to another free issue of The Plastics Times newsletter. If you
are already a subscriber we wish to thank you. If you have not subscribed,
please take a moment to register at http://www.iplas.com/USA/News%20Register.htm,
or use the link at the bottom of the newsletter. And tell your friends about
us too. I would appreciate that.
The Parting Line - "Considering Expansion?"
If you are planning an expansion of your molding operation you may wish
to consider the following guidelines.
The total number of square feet required for the expansion area can be based
on a set number of square feet for each molding machine. At TPT we have
determined that number to be 1200. This is a very generic value, and must be
factored up or down based on what size and type of equipment you are looking
at, but it has proven very accurate over the years.
Some of the things to consider in factoring your number are:
1 - will "full service" (secondary operations, design services, etc.) be
offered? This can add an additional 100 square feet for each machine.
2 - will you be farming out the mold building portion of the program? This
can reduce the number by 100.
3 -will you be using robots and other labor-saving devices? This can
actually add 100 square feet to each machine.
4 - will the machines be laid out in an angled parallel design rather than
side by side? This will add 150 to the number.
5 - if you are planning manufacturing "cells" you can add another 300 to the
basic number for each machine.
As you can see, there are many things to consider when determining square
footage requirements for a molding facility expansion. While it may seem
obvious, it is wise to play around with cutouts on grid paper ahead of time.
Use cutouts for each molding machine size, secondary equipment,
transportation aisles, and storage areas. And, don't forget auxiliary
equipment such as granulators and temperature control units for molds. While
you're at it, remember to leave plenty of room for maintenance, both planned
and unexpected. Removing an injection screw for cleaning can be a nightmare
if there is too little room between machines.
There have been a few good books and/or articles written about starting
or expanding a molding operation. One I can suggest (modestly) is our own
"Manufacturing Startup and Management". This is a nuts-and-bolts book based
on decades of personal experience and the starting up of three completely
different molding facilities from scratch and expanding five others. That
way you can learn from my own mistakes and successes. You can find this book
(and others) in hardcover or ebook format in our Catalog Section at httP://www.iplas.com.
View CV of Douglas Bryce
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Great Quotations |
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"He that has no children brings them up well."
Unknown
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Concerning Material Storage |
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Proper storing and movement of raw materials are important to the
success of any molding operation. If materials are stored too long, or
improperly, additional costs are incurred through the need for excessive
storage space and potential contamination to the resins. And,
transferring the materials from storage to process equipment is more
costly if performed manually than if performed utilizing automated
equipment and methods. In addition, excessive waste can occur through
spillage if proper methods are not practiced during the transfer
process. Another area of concern is the proper mixing of raw materials.
Most molders utilize some type of compounding, either to mix color
materials (and/or other fillers) to natural resin, or for blending
regrind to virgin materials. This, too, is considered part of the
storage and handling process.
The amount of storage space required is determined by what is being
stored. There must be enough space for raw virgin plastic, accumulated
regrind, coloring materials (if you are doing your own coloring), other
fillers, auxiliary materials such as hydraulic oil, packaging, and
finished goods. There also may be requirements for work-in-process (WIP),
molds, fixtures, inspection gages, and any incoming goods used for
producing a finished product. While some of these items may be planned
for storage in specific areas (such as inspection equipment being stored
in the Quality Control Department) much of it will be located in a
central storage area or warehouse. It is a good idea to hold as little
consumable material in storage as possible. This reduces the amount of
real estate required, increases the amount of available cash, and
minimizes all costs (such as taxes, insurance, etc.) associated with
maintaining inventory. A common practice is to maintain storage equal to
the needs of 30 days worth of production. This keeps inventory moving
and allows greater flexibility in providing what your customer wants.
Also, the cost of material is in constant flux. You will want to be in a
position to purchase when the cost is low if possible. Minimal storage
provides that position. Of course, molds, tooling, fixtures, and the
like are not considered consumables and will no doubt be stored for
longer than 30 days.
It is common to use steel racks for storage. The racks are available
in many sizes and weights, but a common size provides a "storage cell"
that is approximately 4 to 6 feet wide, 4 feet tall and 4 feet deep.
This accommodates the popular "Gaylord" style cardboard container used
by most material suppliers. A Gaylord holds from 1,200 to 2,000 pounds
of material depending on the specific gravity of the resin. Each of the
cells can be stacked upon another. The maximum safe level is usually
considered 4 cells high (16 feet). Besides resin, all of the basic
consumables can be stored in these cells, regardless of the individual
container sizes, because they can be attached to skids which then fit
nicely into the cells. When using storage cells (of any style) it is
wise to assign identification numbers to each cell. Then, the material
stored in each cell can be tracked and traced for inventory control
requirements. The most common method is the use of alpha and numeric
locator combinations starting with A-1, A-2, A-3, etc., and continuing
through the alphabet. After using Z, the next alpha designator would be
AA, and so on. This provides for an unlimited number of identification
combinations.
Some industries, such as medical, usually don't allow any regrind to be
used. If you are producing products which result in the accumulation of
regrind, you will need to find proper storage for it. It should be
treated like virgin from this standpoint and be kept properly contained,
sealed, and identified. It is a good practice to keep regrind stored
separately from virgin so it will not inadvertently be used instead of
virgin. Regrind can be sold for about 20 to 50% its original cost
depending on quality and demand. Or, you may be able to salt it into a
virgin material being used for a less sensitive product. Or you may want
to develop and market a product yourself (such as an advertising piece)
that can be molded with all regrind. In any event, try to dispose of
accumulated regrind in the same time span that you dispose of virgin
materials, for the same reasons mentioned earlier.
Silos
If you are molding a large volume of a specific resin you will want to
consider use of a silo. Silos are available in sizes ranging from around
40,000 pounds to over 100,000 pounds. The advantages of using a silo are
that the cost of the material is much cheaper when buying in large
volumes, and the storage is outside the building, taking up less
valuable space. The major disadvantage is that you have a large volume
of material that somehow must be transferred to the molding press.
Usually this is accomplished using a vacuum system that pulls a
predetermined amount of material from the silo and transfers it to a
drying operation, a central handling system, and/or directly to the
press. The major disadvantage of a silo is that it is impossible to know
exactly how much material is left in storage at any one time. The silo
supplier will provide a table showing the approximate volume of material
at pre-determined and marked levels. But, because the silos are not
transparent, it is difficult to tell exactly at what level the stored
material is standing. Usually this is accomplished by tapping the side
of the silo to determine only the approximate level. Then the volume can
be determined for that level by checking the suppliers chart. But the
total weight of material can only be estimated if the specific gravity
of the plastic is known. This still becomes an issue, however, because
material towards the top of the silo is less dense than material towards
the bottom, due to gravitational pull. Therefore, a "false" reading may
take place. When utilizing silos, it is important to understand that an
exact knowledge of how much material is in the silo at any one time can
only be a guess.
Contamination
Next to moisture, contamination is the primary cause of defects in
molded parts. This contamination comes from many sources, such as dust
and dirt falling from ceilings, or trash being dumped into an open
container of raw material that was improperly marked (if at all). The
most common cause of contamination is the mixing of an improper material
with a proper material. This takes place when regrind is being mixed
with virgin, for example. But, if the regrind was not properly stored,
identified, and covered, there is a good possibility that the regrind
selected for mixing is not the one needed or intended. For every one of
the 20,000 (+) materials to choose from today, there are a number of
grades to select from. Nylon, for instance, has over 400 grades
available. If the wrong grade is used for filling the hopper of a
machine already in production, that machine may begin to produce defects
if the "wrong" nylon cannot run at the same parameters as the "right"
resin. Contamination of this sort can be minimized by making sure all
material containers (regrind and virgin) are properly identified and
kept tightly covered.
A more detailed discussion of Material Storage and Handling can be
found in Chapter 6 of "Plastic Injection Molding - Manufacturing Startup
and Management", available in hardcover or ebook versions through the
link that follows.
Visit Our Catalog of Books and Ebooks »
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Teach Them To Troubleshoot! |
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Many process technicians and engineers have not been taught the
fundamentals when it comes to basic troubleshooting. What must be
understood in the first place is what really causes defective parts to
happen. Too often the wrong thing is blamed and troubleshooting
activities start out improperly. The first place to look for defect
causes is the machine, followed by the mold, the material, and the
operator in that order. In fact, 60% of the time defects are caused by
machine problems, 20% by tooling, 10% by material, and only 10% by the
operator.
The next thing is to make only one change at a time, and allow 10 to 20
cycles between changes to allow the process to stabilize again. If a
person makes 2, 3, 4, or more changes at once, the entire process can go
out of control very shortly and this can result in complete havoc. If a
single change does not correct the problem, re-set that parameter to
where it was and wait another 10 cycles before making any other change.
While this sounds like it may take forever to find the problem, the
opposite is true. Once the cause is understood (by examining the
machine, then the mold, then the material, then the operator) the
solution will be determined within a few iterations.
Don't be too quick to blame the material. Most often, what appears to
be a material problem is actually a machine problem in disguise. For
instance loose heater band on the nozzle will cause an overheating of
that zone which could result in splay or burning. This may appear to be
from material that is not dry or contaminated. But a change in material
will only prolong the problem and result in running a great deal of
scrap or purgings. If you understand that the machine is probably the
culprit 6 out of 10 times, it will be easier to spot the loose heater
band because you will be looking for machine problems, not material
problems.
And finally, at least in this shortened version of training, the
process person must understand the basic structure of the plastic
material being molded. This includes information as to whether it is
amorphous or crystalline; the intended shrinkage factor, recommended
melt temperature, and recommended mold temperature. Proper machine size,
water turbulence, and residence time are also major items that need to
be considered.
We hope these thoughts help you. For additional help, you can order
our Troubleshooting For Injection Molders at www.iplas.com in the
Catalog section. It addresses the 24 most common defects, what causes
them, and how to correct them.
View our Catalog of Books and Ebooks »
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"Ask Doc" - Controlling Fill |
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Carl James, from Pacific Coast Plastics, writes:
"Do you prefer monitoring injection "time" or "distance" as the most
effective means of controlling fill of cavities?"
This is a controversial subject. If you ask 5 different molders,
you'll get 6 different answers. At TPT we don't prefer either time or
distance, but rather, pressure.
Controlling the fill by monitoring pressure allows you to ensure that
all the cavities FINISH filling at the same time. This is what balanced
systems are all about. This monitoring can be accomplished with pressure
transducers placed in each (or at least one) cavity, or by readouts on
the control panel. Either way is effective. Proper readings indicate
that the cavity is being filled at the highest pressure NECESSARY. In
addition, this should usually occur rapidly, so the fastest fill
possible is also desirable.
Hope this helps you, Carl. Good luck! And keep those questions coming
in folks. I love it.
Ask Doc a Question! »
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Reducing Cycle Times |
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What else can I do to minimize cycle times?
I'm sure you've tried all the obvious things like using chillers on the
mold and machine, minimizing wall thickness, speeding up the material
flow, etc. But there are a few other items you may want to consider. The
following suggestions are provided by various process engineers within
Texas Plastic Technologies who have successfully implemented them for a
variety of clients. They may seem obvious, but it is amazing how even
seasoned veterans overlook the obvious sometimes.
1 - Clean out mold water lines. It has been found that even a
thin scale of 1/64" ( 0.015 inch ) can reduce the efficiency of a 1/4"
pipe water line by 40%! There is not a single area of the country that
has "good" water. Every location lays claim to the "worst water around".
It is apparent that water contains such items as lime, sulfur, iron, and
a host of other chemicals that cause scale to build up in water lines.
This scale is evident even in manufacturing plants that have "treated"
water systems and should be removed on a periodic basis by running an
acid- based cleaning solution through the lines.This can be done without
interrupting production cycles by attaching a portable circulator to the
mold . A few minutes per machine, per week, will keep water lines clean
and wide open.If yours is a shop with frequent mold changes the cleaning
process can take place after the mold is removed from the press, and
before it goes into storage.
2 - Clean out heat exchanger. The machines heat exchanger has
copper tubes running through it that circulate water for cooling the
machines hydraulic system oil. If the oil temperature is not properly
controlled, injection and clamp pressures will fluctuate and cycle times
may have to be increased to compensate for these temperature
fluctuations. The same principle is at work here as with the mold water
lines. The copper tubes of the heat exchanger tend to form scale easily
and a 1/64" scale will cause a 30 to 40% reduction in cooling
efficiency. The same portable acid circulator that is used for mold
water lines can be connected periodically to the heat exchanger tubes,
again without interfering with production. Or, the unit can be brought
in during normal maintenance.
3 - Minimize temperature swings. Consistent, controlled melt and
mold temperatures will ensure efficiency and reduce cycles by focusing
temperature control where it is needed. This can be accomplished by
utilizing such items as insulation blankets on the injection barrel and
insulation sheets on the mold. Use of these items will result in a
reduction is energy costs by up to 50% and cycle times by up to 10%.
4 - Use aluminum molds. While aluminum molds have been used
extensively for prototype parts for decades, they have been side-stepped
for use in molding production (volume) parts. But, recent case studies
(available from Texas Plastic Technologies) show that aluminum molds can
be designed and built to support annual production volumes of 250,000
parts and more, even when molding glass reinforced materials. The
biggest advantages of using aluminum are reduced lead times, lower mold
costs, and faster cycles. The cycles are faster because of the ability
of aluminum to dissipate heat faster and more equally, thus minimizing
warp and allowing quicker ejection of solidified parts from the mold.
While this listing of suggestions for reducing cycle times only
scratches the surface of possibilities, it does show that there are many
ways to improve cycles without investing huge amounts of additional
money, time, or other resources. There are many other similar areas to
be investigated. Common sense and attention to the little things will
help you discover the answers.
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