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,