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1、Application Notes,Pressure Sensor in Plastic MachinaryBy Michael Sali,Plastic injection molding machine,Plastic Machinery,Plastic Injection molding machines are used to manufacture plastic parts.Injection molding machines typically include a reservoir of molten plastic and a nozzle through which the
2、 molten plastic is forced at high pressures into a mold.The plastic is thereafter packed and cooled to form the part.In the manufacture of such parts,it is important to precisely determine the pressure of the injected melt to maintain accuracy of the part because the part must be made at a constant
3、pressure.This is so because otherwise the resulting part may include voids,ripples,poor dimensional accuracy or the like.,Plastic Machinery Working Process Cycles,CST,3,Pressure sensor mounted on plastic machine,In a plastic injection machine,pressure sensors are mounted in the injection molding noz
4、zle,hot runner system or cool runner system and inside the cavity of the mold.it can measure plastic pressure from the injection molding nozzle to cavity plastic pressure somewhere in between plastic injection molding,mold filling,pressure maintaining and cooling process.Those data can be recorded b
5、y the monitoring system,to be used for true time control of the molding pressure,combine inspect or exclude product process appear malfunction after molding.,CST,4,Pressure sensors on plastic machine,there is a need for pressure transducer devices which can accurately measure the pressure of molten
6、plastic within injection molding machines.Additionally,such pressure transducers are faced with the following constraints:1.the pressure transducer must have low working stresses to avoid metal fatigue cracking under repetitive cyclic loading;2.the pressure transducer,for economic practicality,must
7、be physically arranged such that it can easily be integrated within injection molding machines shot chamber/nozzle area;and 3.the pressure transducer must not disrupt the plastic flow or have pockets where plastic will remain and eventually degrade and cause staining of the parts as the degraded pla
8、stic moves into the flow.,CST,5,Methods of switchover in a plastic machine,The available means to switch over in a modern Injection Molding machine,in increasing order of accuracy,are1.injection time2.screw position3.hydraulic pressure4.nozzle pressure5.cavity pressure,CST,6,Injection time,Temperatu
9、re effects the viscosity of the melt,which presents resistance to the advance of the screw.Increased resistance slows down the screw and prevents the cavity from filling in the given injection time.On the other hand,reduced resistance would lead to over packing.Injection time switchover is the only
10、means available in Injection Molding machines without screw position and pressure sensors.,CST,7,Screw position switchover,Screw position switchover is not effected by temperature nor viscosity.This is the preferred method in machines with screw position potentiometer.Like injection time switchover,
11、screw position switchover could be considered open-loop as screw position is not a direct measure of volumetric filling.A leaky nozzle misleads the machine computer into switching over before the cavity is filled.So could a worn screw valve and a worn injection cylinder.Furthermore,if the screw diam
12、eter is large relative to(the cube root of)the mold cavity volume,variation of 0.1 mm could give an over packed or under packed fill.Despite its deficiencies,this is the most widely used switchover method in a modern Injection Molding machine most probably because it is a standard(not optional)featu
13、re.,CST,8,Hydraulic pressure switchover,The packing of the melt in the mold cavity has to be balanced by the hydraulic pressure driving the screw forward.A rise in the hydraulic pressure during injection could be used to signal the switchover.Due to a roughly 10:1 ratio between the twin injection cy
14、linders and the screw cross sectional areas,the injection cylinder hydraulic pressure is less than the screw tip pressure by the same ratio.The pressure drop at the runners and sprue gate separates the cavity pressure from the screw tip pressure.The compressibility of the melt(between the cavity and
15、 the screw tip)delays the time the pressure is felt.As a result,hydraulic pressure is not an accurate detector of the volumetric filling point.However,hydraulic pressure switchover does have the advantage of the sensor working in a congenial environment(oil temperature below 50oC,oil pressure at sys
16、tem pressure,(usually 140 bars)and the sensing is independent of the mold(not attached to the mold).Hydraulic pressure sensor is usually an option in a modern Injection Molding machine.,CST,9,Nozzle pressure switchover,Nozzle pressure is also called injection pressure,which is the pressure of the me
17、lt in the nozzle or at screw tip.Nozzle pressure switchover is improved over hydraulic pressure as the compressibility of the melt cushion is avoided.The environment is harsher(melt temperature below 400oC,melt pressure at 1400 bars,the melt could be corrosive/abrasive),and the sensor face must be f
18、lush with the barrel interior wall.This switchover method is not often used except in research.,CST,10,Cavity pressure switchover,The most accurate measure of volumetric filling is via cavity pressure.Two methods are in common use:direct and indirect.In direct cavity pressure measurement,a sensor in
19、 the mold senses the melt pressure in the cavity.Direct cavity pressure measurement is the more accurate of the two,but requires one to drill a hole at the mold for the sensor.Since it is inconvenient to remove the sensor,one needs to dedicate at least one sensor per mold.In a multi-cavity mold,cavi
20、ty pressure measurement requires one sensor per cavity,increasing the sensor investment further.,CST,11,In indirect pressure measurement,a force sensor is placed behind an ejector pin the other end of which is in contact with the melt.Cavity pressure could be calculated from force/ejector pin cross
21、sectional area.The temperature at the sensor is much less than that of the melt.With indirect cavity pressure switchover,the sensor is not dedicated to the mold(mold independent),which comes in handy when mold changing is often.It also reduces the sensor investment.Due to the friction at the ejector
22、 pin,indirect cavity pressure sensing is less accurate than its direct cousin.,CST,12,The Principle of Hydraulic Servo System,The motor,gear pump,pressure sensor,inverter form a control loop,in which the measured pressure via pressure sensor compares with the set-point in inverter.If the measured pr
23、essure is higher or lower than the set-point,the inverter will drive motor to work slowly or quickly.Finally the stable hydraulic pressure is produced and the stable pressure will drive the motion of moving side of mould and the screw rotation.,A pressure transducer for an injection molding machine,
24、comprising:a housing coupled to a nozzle of said injection molding machine;an inner stress tube,attached to the housing,contacting material of the injection molding machine,the stress tube bellowing in response to pressure from the material;an outer ring having an opening;a plurality of flexures cou
25、pling the stress tube to the outer ring such that the outer ring expands,thereby widening the opening,when the stress tube bellows;anda displacement sensor determining the amount by which the opening widens.,CST,14,Competitors and Price Level,CST,15,Competitors:GemsTrafagMSIGefranWIKA(A-10)Price Level:End-Price in Customers at large volume:about:320-380 RMB with VATReference:,
