Application of SLIW in Precise Extrusion Molding

In plastic molding industry, extrusion molding possesses advantages as high efficiency, consistency, low costs and adaptability, but it also has low precision and low added value. To develop precise extrusion molding technology and equipment with great accuracy and high added value to meet requirement of producing precise products is currently a hot R&D issue of plastics processing technologies.

Precise extrusion molding is a molding process that obtains accurate product dimensions and average material morphology by precisely control of process elements. Main features of precise extrusion molding are: process parameters fluctuate slightly during extrusion; extruding equipment operates stably and geometric accuracy is 50% higher than that of conventional extrusion molding.

As a significant technology in precise extrusion molding, loss-in-weight measurement is widely used for producing optical fiber, medical catheter, optical films, composite films and pipes.

Loss-In-Weight Metering & Controlling System

According to automatic control theory, closed-loop control should be adopted to achieve better accuracy. That is to take control volume as feedback and compare it with the set value. Regarding to precise extruder, it is hourly extruding quantity that needs to be manipulated from which the following picture is obtained:

From Picture 1 we realize that sensor should be used to collect the actual feeding quantity and give it back to the controller to compare it with the set value. Actual value being smaller than the set value, the controller increases rotation speed of the extruder to make the actual value approximate to the set value and vice versa. What can be learned is accuracy of the sensor and sampling rate are closely related to control accuracy.

How does the sensor measure the actual feeding quantity? Usually the sensor is a load cell that survey the weight of the whole feeding device (including materials in the storage tank), and total reduction of materials equals to discharging quantity. Suppose that the reduction is ΔG during a period of time Δt, the average feeding quantity during this time is ΔG /Δt. This value can be considered as immediate feeding quantity if Δt is small enough. As the actual feeding quantity comes from reduction of the total weight, this measuring method is called loss-in-weight measurement.

Why to Use Loss-In-Wight Measurement

The extruding quantity fluctuates no matter how stably the extruder operates. Fluctuation of single-screw extruder results in undulation of axial dimensions of products. For instance, in pipes production line, diameter of the pipe varies with the extruding quantity, and in multi-layer co-extrusion line thickness of films in each layer are uneven which may influence product performance. In order to avoid the foregoing occasion, a closed loop should be formed between observed output and rotation speed or traction speed of the screw to reduce accuracy undulation of product axial dimensions. Owing to that extrusion output is difficult to measure, loss-in-weight metering device is developed and supplies solutions for real-time measuring of extrusion output.

Typical structure of loss-in-weight metering device is shown in Picture 2 which consists of material storage tank, discharging valve, loss-in-weight tank and weighing sensor etc.

The first and second line in Picture 3 is variation curve indicating extruding output of extruders that doesnt adopt the loss-in-weight metering device; the third one shows extruders employ that device. The third line that exhibits extrusion variation is nearly a straight line representing steady wall thickness and layer thickness of products when utilizing loss-in-weight metering device and it will greatly reduce wastes of materials.

Shini Newly Developed SLIW

Shini Group launched SLIW-10 (Picture 4) in ChinaPlas 2010 in April in Shanghai to comply with requirements of loss-in-weight metering device.

Shini Loss-In-Weight Meter Regulator can be used in the fields such as plastics extrusion, film blowing and blow molding. Introducing weight loss measurement, the system converts the decrement of materials in the hopper of extruder per unit time into immediate output of the extruder, and the output analog signal is given back to the extruder by control system. Closed-loop control is realized between output and traction speed or rotation speed of the screw by the feedback signal which also enhances product quality (weight per meter and thickness etc.)

Main features of SLIW are: sampling period of feeding velocity can be set freely (0.5s~60s); hourly consumption of materials can be observed at any time; output trend display is available; analog signal of hourly consumption output function is convenient for the extruder to adjust rotation speed automatically.

Controller with two RS-485 communication ports is standard configuration in which one for HMI and the other is applied for communication with the extruder. This machine is characterized with precise measurement, and output memory is shown in the historical trends display which is convenient to check. Rotation speed of the motor is automatically adjustable to reach the set output.