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RE [GreenYes] margarine/yogurt tubs, HDPE
Dear All,
  Here's more on the topic of HDPE injection-mold grade compared to blow-mold 
grade.  Excuse the length, but keep reading.  There are some juicy bits at 
the end.  

  The melt index (MI) of plastic is a created thing, i.e., as I understand 
it, the molecular structure & molecular weight (which determine how stiff or 
runny the resin is) are caused by the type of reactor process that cracks the 
petrochemical feedstock, then drives the chemical reaction yielding the resin 
raw material.  These technologies are very complicated & change all the time. 
 I don't know the technical terms for current processes that create injection 
vs. blow-mold grades.  
  I can explain the molding behavior of each.  Injection-mold grade HDPE (or 
other resin created in injection-mold grade) has a high melt flow index (MFI 
or MI) meaning how fast it travels a given distance in a given time - hence 
how quickly it fills a mold.  Injection-mold grade is preferred for 
wide-mouth containers, also for objects with complicated shapes (e.g., toys) 
because it moves fast & can be molded cheaply.  The mold (die) for an 
injection-molded item has many orifices allowing the runny plastic to be 
forced into the mold through many holes to fill the mold quickly.  High unit 
output = low unit price.
  Blow-mold grade HDPE (or other resins created in blow-mold grades) have 
very low MIs.  They're often called fractional melt, because they can be a 
fraction of (less than) 1.  As  Peter said, they can also be 1, while 
injection grades have high numbers like 40.
  Blow mold HDPE flows slowly by comparison.  But the key things are its 
stiffness and strength.  Blow mold can literally be blown up like a balloon 
and will resist the pull of gravity.  This allows the "balloon" to be blown 
up inside the mold, thru a single orifice, and because the resin holds its 
shape, it will expand out to fill the dimensions/form of the mold.  Also, the 
finished product is stronger (better crack resistance, less chance of pinhole 
failure, etc) than an injection-molded item.
  Blow-molding is suited for making bottles due to their narrow-neck shape, 
and specialized molding systems make it efficient.  Blow-molding is not the 
choice for yogurt tubs, complex or irregular shapes, etc.  That's why the 
best HDPE markets are for bottles to bottles.  Another reason is that HDPE 
bottles can be used for many HICs (household industrial chemicals, AKA 
laundry or dish detergent, many other products).  This avoids the issue of 
using post-consumer HDPE bottle plastic in food-contact applications.  HDPE 
bottles are also strong for heavy loads, such as a gallon of some liquid.  
  Many HDPE injection-molded containers are for lightweight products (butter, 
CoolWhip, etc).  Toys & so on tend to be weak & break easily, tho moreso with 
PS (polystyrene) than with HDPE.  Markets for injection-molded containers are 
not so hot because (1) many applications are food-contact (a problem with 
post-consumer HDPE); and other applications such as toys are wanted in 
brilliant pure colors.  When you mix together HDPE of many different 
pigments, plus the inks on packaging, you generally get a dull army green 
colored blend.  It's very difficult to convert this color to something 
pretty; mainly secondary molders add carbon black & make black products (of 
which there are many, e.g., car part insides that aren't visible).

  Now to the juicy bits.

  It isn't true that blow-mold grade HDPE can only & forever be blow-mold 
grade, or injection grade only that.  There are many companies, called custom 
blenders (or something like that), that specialize in making many different 
blends of HDPE and other resins.  Also, there are many different products 
needing a range of MIs.  
  Take recycling set-out containers for instance.  Say the famous blue box or 
stackable containers still used in places that haven't gone to single stream 
cart systems.  And say they're being used in Canada or MN.  Many of these 
containers are molded from medium density PE (MDPE), yes, a blend of 
blow-mold & injection-mold PE that stands up better to cold temps.  
  This may be an obscure example, but my point is that custom blenders can 
formulate just about any MI.  The nature of plastics is their plasticity, & 
the nature of plastic molders is that they can & do adapt plastic MIs to tons 
of applications (products) made thru a dizzying range of molding systems.
  In other words, the plastics industry could use all available post-consumer 
injection grade HDPE in recycled products, if they put their minds to it.  
And if we consumers were not so particular about colors...

  Another option we should seriously revisit is mixed plastic lumber (and 
other multi-dimensional products now possible).  
  Yes, there are wonderful products available from LDPE & wood that are 
terrific for uses like decks.  But I'm talking about really mixed 
plastics--blow-mold & injection HDPE, LDPE, PP, plus measured amounts of PVC, 
PS, PET, & other.  Not to mention tramp materials (paper & plastic labels, 
bits of dirt, grit, glass, food, whatever).
  As long as you have 60% polyolefins (HDPE, LDPE, & PP) and half & half film 
and rigid--both of which closely match the percentage of these resins in the 
discard stream--then existing, proven technologies can produce landscape 
timbers, posts, fences, compression molded (all-of-a-piece) pallets, 
interlocking blocks for retaining walls, more.  Not to mention lumber for 
park benches, picnic tables, and other things we should not be making from 
  Mixed plastic recycling gives a solution for what to do with dirty plastics 
from single stream collections; sidesteps the issue of food contact; provides 
an end-use for #3-7 resins PLUS margarine, yogurt, & most other wide-mouth 
HDPE containers; gives products that encapsulate (hence reduce) a percentage 
of MRF outthrows, more.
  Folks have asked "how many picnic tables & park benches can we use?"  I 
look at parks, beaches, bus stops, other potential gathering places, & think 
lots more than we have now.  The technology is relatively cheap compared to 
other plastics recycling systems. And it's possible to do recipes that allow 
a range of colors besides brown, green or black. 

  It's worth some thought...

Gretchen Brewer
Earth Circle
San Diego, CA

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