With today’s upward spiral of resin prices, it has
become ever more important to optimize resin usage.
A significant part of optimizing resin usage is proper
use of regrind. Regrind is material that has undergone
at least one processing method such as molding or extrusion
and the subsequent sprue, runners, flash, rejected parts
etc are ground or chopped. Both the heat history of
processing and the grinding may lead to degraded physical,
chemical and flow properties for the thermoplastic resin
and subsequent parts made from this regrind. Normally
it is considered that the heat history is most detrimental
as stabilizers and antioxidants get consumed.
For perspective, note that even virgin resin may have
one or more heat histories as it must be compounded
or extruded into pellets when made. Virgin resins can
have additional heat history as additives such as color,
fillers (fibers, talc, mica, etc) or a host of additives
may require a second compounding step. This may be done
by the resin supplier or at a separate compounding facility.
This is not considered regrind but it is interesting
to note that virgin resin right out of the bag, box
or railcar can already have a “Heat History”.
To develop a strategy and optimize the value of regrind
it is important to know the issues involved. Appropriate
shop floor procedures and discipline must be in place
to avoid potentially catastrophic results. The list
of issues is significant and there are subtleties under
each.
Level actually used
Degraded polymer in the regrind
Broad of granule sizes in the regrind; should you
repelletize?
Possibility of contamination, both from another
plastic or metal, wood, dirt etc.
Excessive fines in the regrind
Tracking the actual level of regrind in a plastic
part.
Testing worst case situations using regrind. Remember
that if you keep using regrind some of the regrind
stays in the main stream. Some plastic molecules will
go through the system 100’s of times.
The Details:
Level Actually Used: Generally the molding community
targets 20 to 25% or less for blending regrind into
virgin. The emphasis is on “targets” for if one watches
what really happens at the press during production
one can see everything from 100% virgin to 100% regrind
going through the feed throat. The subtleties here
involve:
(A) Proper training of the resin handlers,
usually an entry level position with little attention
to training
(B) Improper calibration of the virgin and
regrind feeders. Some of these blenders are fairly
sophisticated, calibrations get fouled up, and
hopper geometry (angle of repose) often encourages
hang-up or bridging. With 2, 3, and 4 separate
hoppers all feed a single feed throat, hopper
size and geometry is limited
(C) Improper blending of the virgin and regrind.
Even when one uses weigh feeders the virgin, regrind
and color can be dumped together but not proper
mixed before reaching the feed throat.
(D) Lack of discipline on the shop floor. The
above details need attention on a regular basis.
Degraded Polymer in the Regrind: Most consider that
using up to 25% regrind does not significantly compromise
virgin resin properties. Unfortunately this statement
can get you in trouble. For example: Resins such as
nylon, polycarbonate, poly(butadieneterephthalate)
(PBT), poly(ethyleneterephthalate) (PET), etc, if
not dried properly before initial processing will
undergo a chemical reaction, called hydrolysis, in
the barrel of the molding machine which causes significant
lowering of the polymer chain length. This is not
visible to the naked eye; parts look good but have
lower physical or chemical properties. Often the problem
is not found until the parts are in service, which
can be a costly issue. Blending regrind of this quality
into virgin at 25% levels may significantly alter
subsequent part performance and function. Further
each processing history may influence the virgins
additive levels and if fiber filled the length of
the fibers. Processing grinds up the fibers to shorter
lengths. Then there is the problem with processing
the virgin at higher than recommended temperatures,
a sure way to accelerate polymer degradation. These
are only a sample of the potential problems of using
regrind and all of them can narrow the processing
window.
Temperature or heat history is commonly believed
to be the biggest issue in polymer degradation.
However a study done by Sue Janicki et al. Antec
1992 pgs 1201-1204; “The Material Cascade: An Alternative
Form of Regrind Utilization” provides data showing
excellent retention of physical properties through
five passes through an injection molding machine.
The study included High Impact Polystyrene (HIPS),
Ignition Resistant HIPS and Polycarbonate (PC).
This indicates that if treated properly in processing
that many resins can hold their physical properties
for a short number of regrind passes.
Range of granule sizes: When rejected parts, sprues,
runners etc go through a grinder there is the potential
to get a wide range of granule sizes. Everything from
fines, small dust like particles, to ¼ inch
or larger chunks. Preventive maintenance on grinders
is often rarely done in many shops. During plasticizing
or screw rotation the screw does not melt these different
size granules the same. Some start to melt earlier
than others and this leads to non-uniformly melted
plastic forming the part, again with potentially compromised
part properties. Further, processing is less robust
due to this inconsistency. Repelletizing will eliminate
this problem and it allows for the regrind to be melt
filtered to remove non-plastic contamination. Unfortunately
this will add $0.12 to $0.20 per lb. For process stability
grinders should be well maintained, blades sharp,
cleaned properly, and screen working properly.
Possibility of contamination: It has been my experience
that the biggest problem with regrind is not excessive
heat history but contamination, both foreign plastic
and foreign materials. How many of us have seen production
stop due to a plugged hot tip. This has forced many
to use nozzle filters to prevent downtime and the
significant cost of cleaning out the hot runner system.
A nozzle filter creates a pressure drop and further
reduces process robustness. This is the reason why
you can save money by running only virgin in hot runner
tools and use the regrind for cold runner tools. Not
many have this luxury but when possible it is a winning
strategy.
Further if there is contamination how do you purge
the resin system? The only sure way is to sell all
of your regrind and start over. There is no way
to purge the resin stream of contamination with
20% regrind.
Excessive fines in the regrind: Excessive fines
are often a special problem for they melt differently
than larger granules. If you have a clear application,
fines are the source of both black and white speck
development. There are numerous applications that
require fines removal before processing. Fines separators
can be bought. Fines are generated in the both grinding
and resin conveying, again reasons to do preventive
maintenance on grinders.
Tracking the actual level of regrind in a plastic
part: While this seems possible on paper when using
a 25% regrind stream, practically out on the shop
floor it is impossible. First there is confusion in
use, is it 80% virgin and 20% regrind or do you add
20 lbs. of regrind to 100 lb. of virgin. While the
first blending may be 20% regrind all subsequent passes
always have some of the previous regrind blend. Resin
from the first pass never leaves the resin stock.
In fact it does not take much to calculate that after
a few months of processing some molecules have gone
through the injection molding cycle hundreds of time.
Ask your resin supplier how many times his plastic
can go through a molding process before properties
begin to decrease by more than 10%. Also which properties
are the first to show signs of degradation.
Testing worst case situations using regrind: If
the application has any liability to it, how can you
test a worst case situation? To prove you are OK in
using regrind. It is impossible, and many application
use only virgin for safety’s sake.
An Alternative Approach:
Instead of blending regrind and encountering all the
above problems, one may want to evaluate using 100%
regrind. That is there will be no blending. The regrind
is held until all the virgin resin is used up and then
the regrind is fed into the machines at 100%. The benefits
of this approach are several:
There is no question about amount of regrind or
if it is properly blended.
If there is polymer degradation the machine will
tell you if you are using velocity control and watching
actual pressure at transfer.
Range of granule size may affect drying time but this can be addressed with a good dryer and appropriate drying conditions.
Possibility of contamination still exists but this
strategy is self-purging, it will be removed from
the resin stream.
Excessive fines are still a problem, no change
here.
Tracking the regrind becomes simple. Parts are
labeled with the resin lot number and the number of
passes regrind is used. Even if bad parts are made,
if coded correctly only these have to be pulled from
inventory or discarded. There is far less chance of
contaminating virgin resin.
Testing worst case situation becomes possible.
Test parts made from virgin and 5 pass 100% regrind.
See how they compare. If all properties are good stay
use on a maximum of 4- passes.
Capital costs are lower because you do not need
to buy blending equipment.
There are issues with using 100% regrind. As with
most strategies there are compromises to be made. One
has to watch fiber length in fiber reinforced resins
and there is the issue of color matching.
Your Expert
About
John Bozzelli Competent in resin characterization and analysis, John's
specialty is practical, hands-on injection molding training with both small and
large machines. Learn
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About
John Bozzelli