Loss-in-weight Measurement: A New Technology of Masterbatch Dosing

Today with that injection molding technology develops rapidly, people has more demands to live a high quality life. As a result, masterbatch materials are widely used in daily life. In order to produce high quality products, accurate proportioning of masterbatch is particularly important. At present, the injection manufacturers often use volumetric or gravimetric ways to dose masterbatch and for manufacturers with low accuracy requirement or low labor cost, adopt manually weighing and mixing. This way has been gradually replaced by the high precision and high efficiency automation equipment.

Volumetric dosers adopt screws driven by motors. By the uniform rotation of the screws, masterbatch evenly falls into the hopper to be mixed with the main materials. However, the dosing accuracy of the volumetric dosers is determined by the machining accuracy of the measuring screw and the bulk density of the masterbatch. Therefore, volumetric dosers are not fit for injection technology with high demands of masterbatch proportioning precision.

In order to meet the high precision requirement, the manufacturer developed a new model named gravimetric doser. Compared with volumetric doser, the measuring method of gravimetric doser is much more precise. Gravimetric dosers adopt formula batch measuring, electronic gravimetric sensor can accurately calculate the discharge weight of each batch and the error compensation function makes material proportioning more accurate. Because of the high precision effect, high level configurations such as PLC and gravimetric sensor, as well the high processing technology request, the price is much higher. To meet the high requirements of masterbatch proportioning, manufacturers have to bear the high cost.

Working Principle of Gravimetric Doser

Driven by brushless DC motor, the screw rotates to take masterbatch in its grooves to the main material. The dosage of masterbatch varies with the rotation speed of the motor. Gravimetric sensors are used, by the principle of loss-in-weight, to accurate control the output of the masterbatch and realize the proportional dosing. The gravimetric doser effectively solve the problem that volumetric doser has. The actual dosage is readable. Customers can get to know the masterbatch dosage from the PLC. The dosage equals to the weight decreased in the masterbatch hopper. Figure 2 and 3 show the relationship between the variation of masterbatch weight in the hopper and the actual weight discharged by the screw.

The newly developed Shini gravimetric doser SGD:

•  Compared with volumetric doser, the new model added gravimetric sensor and adopt loss-in-weight weighing technology to control the mixing process, it can achieve higher production precision requirements.
• Standard network interface (Ethernet network).
• Automatic compensation functions of material level.
• Aberration compensation functions of regrinds.
• Automatic adjustment of calculation when bulk density changes.

 Operating with the current injection molding technology; gravimetric doser can be used in the injection machine mode and extrusion machine mode.

•        Injection Molding Machine Mode

Input the weight of each mould, masterbatch ratio and melting cycle, the system will automatically calculate masterbatch dosage. When gravimetric doser receives melting signal from the injection molding machine, it will automatically calculate the needed amount and through loss-in-weight principle, the masterbatch is accurately added to the main materials by the screw. This series doser can realize two kinds of granular additives or masterbatch dosing at the same time.

•        Extruding Machine Mode

Input hourly output of the extruder and masterbatch ratio and the system will automatically calculate the materbatch or additive dosage per minute. According to the operation signal of the extruder, the screw of gravimetric doser rotates with the extruder screw and automatically adjusts the dosage according to the actual amount of last minute.