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Screw barrel for biopolymer processing



 After years of hype and exaggeration, market demand for bioplastics made from renewable resources like corn or potatoes is no longer just an ecologist’s dream. Most of the action is in film and sheet extrusion for disposable packaging.

Biopolymers established themselves first in film and sheet extrusion markets, but successful molded part applications are starting to prime the market for growth. The few current molders of bioresins are targeting mainly rigid packaging, disposable cutlery, medical parts, and consumer products.
Types of Biopolymers
polylactic acid (PLA) 
starch-based Polymers Like Mater-Bi
PTT (polytrimethylene terephthalate) polyester
PVOH (polyvinyl alcohol) based resins
PLA boxes
PLA plant on corn stover
PHA
Pectin
Starch
Chitosan
Xylan
Galactoglucomannan
Lignin
Cellulose nanofibrils (CNF)
PHBV biopolymer
Mirel is a trade name for a polyhydroxyalkanoate-based biodegradable bioplastic
Mirel bioplastic is certified soil and marine degradable and has applications in injection molding, extrusion coating, cast film and sheet, blown film, and thermoforming.
PLA
 
In US Some 70% to 80% of PLA consumption is for thermoformed trays and cups.Other extrusion markets for PLA are coating paper (for coffee cups) and blown and bi-oriented film.
Small amounts are also injection molded, stretch-blow molded, and used in blends to stiffen other biopolymers. From 6% to 30% PLA goes into some first family of biopolymers that uses substances obtained from vegetables, like maize starch (whilst preserving the chemical structure generated by photosynthesis) applications and into starch-based compound for blown film.
Biaxially oriented PLA sheet is commercial in applications like prepaid phone cards and gift cards and hotel key cards, where it replaces PVC. Sheet
PLA is used by extruders like to make blown film for lamination, hygienic films,packaging and PLA foam.
PLA film can be extruded with a general-purpose screw, though processors say output is better with a low-shear screw designed specifically for this resin.
Short, low-shear PVC screws reportedly are the best.
modified Fusion screw for PLA. special Turbo Screw for extruding PLA foam.
a mixing section on the screw and a static mixer before the die for good melt uniformity.
According to PLA sheet processor all temperatures in the extrusion system must be closely controlled, as excessive heat can be disastrous.
PLA has a density of 1.24 g/cc, lighter than PET (1.33), but a lot heavier than PP (0.90) or PS (1.04). That means the same size roll of PLA film weighs a lot more than one of OPP or PS.
No biax film line has ever been built specifically for PLA, but the resin is being run on modified BOPET lines because their stretch ratios are similar (about 3.5:1 in both MD and TD).
PLA can run on lines for BOPS film with typical stretch ratios of 2:1 MD and 4.5:1 TD. But PLA is too different from PP, which has a 5:1 MD and 10:1 TD ratio, to use most BOPP equipment.
PLA can replace GPPS, HIPS, and ABS:
PHA 
After PLA and other starch resins, PHA is due be the next biopolymer available in large volumes.
PHA is a family of bio-polyesters produced by fermentation, but unlike PLA, the polymer is formed and stored inside a living bacterial cell, similar to starch in a plant or fat in an animal.
Biopolymer blown film extrusion system
bio-PET, and bio-PE.
TPS, or thermoplastic starch—and two emerging technologies: PHA (polyhydroxyalkanoate) and PEF (polyethylene furanoate)
Thermoplastic starch is a category of biopolymers derived from carbohydrates such as potatoes, tapioca, corn, maize, or rice, among others. Starch is a naturally-occurring biopolymer that, once plasticized, can be thermally processed and handled on traditional extrusion, injection-molding, blow-molding, or thermoforming equipment. TPS is then blended with polymers such as polyethylene or PLA to create a biodegradable alternative made partially or entirely from renewable resources that offers the same—or better—characteristics than 100% polymer blends.
Biopolymer's recent applications
Mulch Films (Agricultural Films) made from Biopolymer based resin
BAGS
BLISTERS
BOTTLES & CANISTERS
CONTAINERS & BINS
COSMETICS
FOOD PACKAGING
HORTICULTURE, Ties and clips are required in horticultural applications for example to support plant shoots.
HYGIENIC FILM, For use as diaper backsheet or femcare, Sanitary napkin, Sanitary Pads products
bio-based polyamide, a partially bio-based PET, a bio-based PE
bio-based nettings, we recommend the use of Green HDPE SHE 
PAPER LAMINATION
SHRINK FILM For the production of shrink films, various Green PE products
TECHNICAL PARTS, Highly complex demands require corresponding polymers.
TOYS
TRAYS
WASTE BAGS 
biocompostable polymer materials.
long fiber thermoplastics (LFT),
TPU film machine Monolayer and multilayer co-extruded cast film extruder.
Food Extrusion-Biopolymer
Biopolymer Applications in Food Engineering - Extruded foods are composed mainly of biopolymers obtained from cereals and tubers, such as starch, protein, and fiber.
Twin-screw also used in extrusion for biopoolymer
Injection Molding Biopolymers: Processing Renewable Resins
“Processors have found PLA unique in its processing requirements Compared with standard resins like PS, they says, “PLA retains heat more, so longer cooling time is required. It tends not to flow well in thin walls over long distances. Adding more pressure to fill only increases shear, which can cause it to break down and become brittle.
It uses noncorrosive components to resist the acidic properties of PLA and its tendency to plate out acid residues on the walls of the molding system. It also has specially designed nozzle tips to minimize shear and provide high cooling capacity; low-pressure, low-shear channels; and a thermal profile to counteract PLA’s “temperature hypersensitivity.”
"Moisture sensitivity and lack of heat resistance appear to be the biggest issues surrounding unmodified biopolymers,”
“The rheology, shrink rates, and venting requirements are different from material to material, but the differences are subtle so you have to understand product design, tool design, processing equipment and the parameters of your process. We have been amassing considerable knowledge about biodegradable and biobased materials. We understand more now about their drying, about gate design and location, runner channels, flow rates, venting, and molding.”
“Many biopolymers seem tough to process because they have a small window between the melting point or processing temperature and the decomposition point. With biopolymers such as PHBV, a resin may melt at 310 F but degrade at 360 F, which is a fairly tight processing window. Too much heat can generate gels, black specs, or yellowing in your parts.” As a result, molders need to watch their melt temperature, screw speed, and injection speed—as well as proper drying, since these materials tend to be hygroscopic and moisture sensitive."
Biopolymer's Injection molded products: Toothbrush holder, bath cup, large and small bath boxes, a pump dispenser, soap dish, and bath bin, dishes, office trays, toys, rulers, pencil sharpeners, cartridges, and plant pots.
 
Bioresins are hygroscopic and must be dried or they will suffer a drop in molecular weight and melt viscosity, as well as increased potential for flashing and brittle parts.
PLA and PHA are polyesters, and drying requirements are in the range of those for PET and PBT—i.e., more strict than for ABS, nylon, or PC.
“Unlike traditional resins, melt degradation of these materials isn’t likely to clog up the molding equipment.”
Bioresin suppliers say their materials process like traditional thermoplastics such as PC or ABS and can be run on conventional machines using general-purpose screws. They don’t recommend high-shear screws, such as a nylon screw, that can generate a lot of shear heating. Suppliers also warn that you cannot have hot spots in the machine. “A general rule of thumb is for shot volume to be 30% to 80% of barrel volume, much like a standard thermoplastic,” 
THE SCREW FOR BIOPOLYMER
For screws of any plasticating process, the best material of construction is 17-4 ph stainless steel. This grade of stainless is readily available and can easily be machined. It can also have a nickel-based flight hard facing welded on to it, and is a superior product compared to a 4140 HT based screw that has been chrome plated. The 17-4 ph stainless has approximately 16% chromium in its chemical matrix, whereas 4140 steel only is 0.8 to 1.1% chromium. It is that extra chromium that provides the excellent corrosion protection.
 
The cost of the 17-4 ph stainless screw will be at a premium. But consider that on chrome-plated screws, once the chrome has worn away, the PLA will corrode the 4140 base material. Also, the 17-4 ph stainless screw can be cleaned over and over without the worry of polishing through the 0.001-in. thick chrome that normally is applied to the screw surface.
 
The other big advantage for using 17-4 ph stainless steel for the base material of the PLA screw, especially for small screws—2-in (50mm) and smaller—is that you will be less likely to overtorque the screw and break it. Again, in comparing 17-4 ph stainless to 4140, 4140 HT Steel has about 95,000 psi yield strength, whereas 17-4 has a yield strength of approximately 175,000 psi, almost twice as much. Again, a great reason to use 17-4 ph stainless for any small screw application.
THE BARREL FOR BIOPOLYMER 
With the barrel, things aren’t quite as complicated. Basically there are two kinds of barrel liners: iron-based or nickel-based matrixes. The barrels that most OEMs supply with new machines for injection molding, blow molding and extrusion, have iron-based lined barrels.
 
For corrosion protection of the barrel is it best to use the nickel-based barrel bimetallic liners.
 
When molding PLA, not only do the screw and barrel need to be protected, but the components in front of the screw and barrel also need to be made of corrosion resistance materials. The endcap, nozzle adapter and tip should be made of 17-4 ph stainless steel or chrome plated internals. The 17-4 ph stainless is the preferred way to go for the best overall corrosion protection.
 
For extrusion and blow molding applications, all of the adapters, screen changers, melt pumps, and flow tubes should also be made of 17-4 ph stainless steel or some other corrosion-resistant type of stainless, or all of the polymer flow paths should be chrome plated a minimum of 0.002-in. thick, just is done for processing rigid vinyl.
 
 

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