PLA can use ordinary plastic injection mold equipment to process various tableware, such as cups, plates, saucers, lunch boxes, bowls, knives, forks, chopsticks; various containers, such as bottles, barrels, plates, toothbrushes, clothes hangers, and safety helmets And other products. PLA for processing requires that the molecular weight of the resin should not be too large, the melt viscosity should be small, and the processing temperature should be high. The specific resin properties and product mechanical properties of the PLA processed by the injection mold are shown in the following table.
|Density/(g/cm 2 )||1.21||Elongation at break/%||2.5|
|Melt flow rate (190℃/2.16K)||10~30||Light impact (J/m)||0.16|
|Light transmittance||Transparent||Flexural modulus||3828|
The primary problem in PLA processing is the drying of granular materials. If the moisture content in PLA is too high, the PLA will degrade during processing, resulting in a decrease in the molecular weight of the polymer and affecting the mechanical strength and tensile modulus of the product. After leaving the factory, the moisture content of PLA in the packaging bag or storage tank is less than 0.025%. PLA needs to be stored in dry conditions, generally sealed and stored until before use. The moisture content of PLA during processing is less than 0.010%, so further drying is required before use. The curve of the water content in the PLA particle resin with time under a certain drying temperature shows that as the shuihanl is smaller, the time required for the PLA particles to be decomposed is longer. PLA needs to be dried for more than 3H at 100°C before it can be used. Amorphous PLA must be dried below 50°C because of its low melting point.
The processing temperature control of PLA is shown in the table below. PLA for injection mold processing usually requires the melting temperature not to be too high, otherwise the melt viscosity is too high, and the processing temperature needs to be increased. PLA at high temperature is easy to break, resulting in a decrease in molecular weight and mechanical properties. The melting point of PLA is generally controlled at 145℃~175℃. The control method is to adjust the optical purity of polylactic acid, that is, add a small amount of D-LA monomer to the L-LA monomer, so that the crystallinity of the PLA can be controlled, and the melting point of the PLA can be controlled. In addition, plasticizers can be added to lower the melting point of PLA. For example, the melting point of PLA used for injection molds of Shimadzu in Japan is only 145°C to 155°C, while the melting point of PLA from Cargil-Dow in the United States is 120°C to 170°C.
|Melting temperature/℃||195~200||Head temperature (3 sections)/℃||205~210|
|Feeding port temperature/℃||20~30||Mold temperature/℃||20~30|
|Temperature of feeding section (crystal)/℃||160~170||Screw speed/(r/min)||100~175|
|Temperature of feeding section (amorphous)/℃||145~155||Back pressure/MPa||0.35~0.70|
|Extrusion end temperature (℃)||195~205||Mold expansion||+-0.001|
|Temperature of metering section (2 sections)/℃||2.5~210|