Sponsored By

Managing Regrind for Maximum Quality

July 14, 1998

5 Min Read
Managing Regrind for Maximum Quality

Most moulders use regrind in a continuous loop. Virgin resin is blended with a percentage of regrind and fed back into the machine, then virgin resin is blended with a percentage of regrind and fed back into the machine, and then virgin resin is blended with a percentage of regrind and fed back into the machine. Is this familiar?

Experts argue that after each pass through the machine, the amount offirst-pass material that remains in the regrind becomes less and less,but it is never completely removed unless the entire regrind system ispurged and restarted. These experts say that polymer degradation is a time/temperaturephenomenon. In a continuous regrind loop it doesn't take very long beforethere will be a number of molecules in the mix that have been passed throughthe moulding machine for a number of heat histories (Figure 1).

Some say polymer molecules are pretty stable when it comes to heat history.They add that the number of these older, heavily processed molecules isvery small. And they can prove in the lab that key performance propertiesof the materials can be maintained in a continuous loop. But what effectdoes this continuous looping have on the real-world processing environment?

Isn't it reasonable to imagine that even a few parts per million ofan incompatible material can make a virgin material useless? What if amaterial is subject to hydrolysis? Can't processing even a very small amountof wet material and introducing it into the continuous loop have a badeffect on parts after the hydrolysis occurs? And how do you keep trackof how many of these overprocessed molecules are in the loop unless youdump each batch and clean the granulator? How do you do that if you'reusing a central regrind system that may be handling multiple lots of differentmaterials? And how do you accurately predict and evaluate worst-case scenarioswith so many variables?
 

Cascade Regrinding

An alternative regrind use method was developed by Rodney J. Groleauof RJG Assoc., Traverse City, Michigan, USA called the "cascade" approachto regrinding. The technique was developed with the assistance of SuzanneJanicki of Dow and Milton Bain, Jr. of Michigan Technological University(Houghton, Michigan, USA).

Here's the proposition: Store regrind from the first-pass virgin rununtil the virgin material is completely used up. Run the first-generation100 percent regrind while storing the second-generation regrind material.Then run the second-generation 100 percent regrind when the first-generationregrind is used up. Continue in this cascading manner until the regrindis almost entirely used (Figure 2 and table).
 
 
 
 
 
 
 
 
 
 

PERCENT MATERIAL REMAINING AFTEREACH PASS

Percent regrind

50

25

20

10

First Generation

50

25

20

10

Second Generation

25

6.25

4

1

Third Generation

12.5

1.55

.5

0

Fourth Generation

6.25

.39

0

0

Fifth Generation

3.12

0

0

0

 

You can keep track of those reprocessed molecules' heat histories witha suffix notation after the lot numbers. The notation records the numberof runs the material has passed through. Material suppliers could labellots of filled or compounded material, indicating how many processing stepsthe material was exposed to after it came out of the reactor. Also, in-mouldmarkers could tell reclaimers how many heat histories recycled materialshave been through, aiding their material separation efforts. The expertssay cascade regrinding has seven major benefits:

  • Known heat histories. The number of a material's heat historiesbecomes a known variable, and overprocessed molecules are not allowed tostay in the system long enough to degrade and cause any trouble.

  • Self-cleaning. Any contamination that enters the regrind stream(even floor sweepings) is removed from the system when the regrind is usedup.Contamination can only affect a very small number of parts, since itis always being flushed from the system. Any part has only one lot of materialin it, so lot control is simple.

  • Easier colouring. Colour is introduced with the virgin materialonly. It is not necessary to proportion colour to account for precolouredmaterial.

  • Simpler blenders. You can sell your regrind blenders or reduce yourinitial capital investment outlays.

  • Regrind empowerment. Since it can be classified and tested, yourregrind shakes the negative connotations associated with the word "regrind."It can be characterized as a material type in and of itself since it isno longer a batch of used plastic with unknown properties.

  • Easier worst-case testing. With each generational lot of 100 percentregrind, you know what you've got. You can benefit from reduced liabilityand designers get a better idea of worst-case specs.

  • Basically, it's easier. That's what the experts say they've beentold by moulders that have adopted the cascade method. It just takes adifferent kind of discipline.

An extensive series of tests comparing first-, second- and third-generationregrind against virgin resin has been conducted by these researchers. Theysay their tests prove that their approach is a viable one: moulded partproperties did not change significantly.

Contact Information

RJG Associates
Mr. Matt Groleau
Vice President
Traverse City, Michigan, USA
Tel: +1 (616) 946-3111
Fax: +1 (616) 947-6403

Sign up for the PlasticsToday NewsFeed newsletter.

You May Also Like