RJG molding.ppt

Appendix C:Reading&Interpreting Graphical Data,Mold Pressure Gate End of Cavity,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,A,F,D,B,Cavity Packedat Peak,Plastic ReachesSensor Location,Cooling Rate,Transfer Fill To Pack,Cavity Flow Time After Transfer at Pack Rate,Cavity Flow Time to Transfer at Fill Rate,Plastic Flow Into Mold Time Zero,Cavity Full,C,E,Pack Rate,MP Set Point,C,Mold PressureA classical cavity pressure curve for an amorphous polymer such as polystyrene,ABS or acrylic is shown The“electrocardiogram”of how the part was created This is pressure near the gate and inside of the cavity of a typical injection molded part During the injection process,plastic travels from the nozzle and until plastic reaches the location of the sensor,no pressure is observed,Mold Pressure Gate End of Cavity,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,A,F,D,B,Cavity Packedat Peak,Plastic ReachesSensor Location,Cooling Rate,Transfer Fill To Pack,Cavity Flow Time After Transfer at Pack Rate,Cavity Flow Time to Transfer at Fill Rate,Plastic Flow Into Mold Time Zero,Cavity Full,C,E,Pack Rate,MP Set Point,C,Mold PressureOnce the plastic reaches the transducer location,pressure starts to build up,Point A This pressure is called dynamic pressure because it results from the dynamic forces of viscosity during flow As the plastic flow front travels across the cavity,a pressure loss exists from the machine barrel through the flowing material to the flow front which is always at zero pressure Cavity pressure during flow builds slowly as shown by the“dog leg”,Mold Pressure Gate End of Cavity,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,A,F,D,B,Cavity Packedat Peak,Plastic ReachesSensor Location,Cooling Rate,Transfer Fill To Pack,Cavity Flow Time After Transfer at Pack Rate,Cavity Flow Time to Transfer at Fill Rate,Plastic Flow Into Mold Time Zero,Cavity Full,C,E,Pack Rate,MP Set Point,C,Mold PressureAn experienced eye will focus on the dog leg first because the size of the dog leg is indicative of where the transducer is located with respect to the end of cavityThe inflection point at C indicates the cavity is full,The pressure at Time B represents the dynamic pressure loss across the cavity at transfer from Fill to Pack The distance on the X axis,which is time,indicates the time it takes to flow plastic from the sensor location to the transfer position and on to when the cavity is full,Mold Pressure Gate End of Cavity,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,A,F,D,B,Cavity Packedat Peak,Plastic ReachesSensor Location,Cooling Rate,Transfer Fill To Pack,Cavity Flow Time After Transfer at Pack Rate,Cavity Flow Time to Transfer at Fill Rate,Plastic Flow Into Mold Time Zero,Cavity Full,C,E,Pack Rate,MP Set Point,C,Mold PressureA to C is a true cavity fill time The area under the dog leg is another measure of viscosityAt the inflection point,C,the cavity is full and static pressure starts to be developed in the cavity This is the end of filling and dynamic flow is completeFrom this inflection point until the peak at point E,packing occurs,Mold Pressure Gate End of Cavity,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,A,F,D,B,Cavity Packedat Peak,Plastic ReachesSensor Location,Cooling Rate,Transfer Fill To Pack,Cavity Flow Time After Transfer at Pack Rate,Cavity Flow Time to Transfer at Fill Rate,Plastic Flow Into Mold Time Zero,Cavity Full,C,E,Pack Rate,MP Set Point,C,Mold PressureThe time it takes to go from inflection point C to the peak at Point E,is packing time The slope of a line drawn between the two points can be considered the packing rate of the mold in psi per second Once the peak cavity pressure is reached,the pressure in thermoplastics starts to drop due to cooling,Mold Pressure Gate End of Cavity,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,A,F,D,B,Cavity Packedat Peak,Plastic ReachesSensor Location,Cooling Rate,Transfer Fill To Pack,Cavity Flow Time After Transfer at Pack Rate,Cavity Flow Time to Transfer at Fill Rate,Plastic Flow Into Mold Time Zero,Cavity Full,C,E,Pack Rate,MP Set Point,C,Mold PressureIn a classical method of molding,all of the plastic put into the mold will be held in the mold until the gate seals Gate seal can be determined experimentally if a transducer is located near the gate inside the cavityThe curve shown indicates that the gate is sealed at Point FWhen the 2nd stage pressure drops there is no consequential drop in the mold pressure,Mold Pressure Gate End of Cavity,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,A,F,D,B,Cavity Packedat Peak,Plastic ReachesSensor Location,Cooling Rate,Transfer Fill To Pack,Cavity Flow Time After Transfer at Pack Rate,Cavity Flow Time to Transfer at Fill Rate,Plastic Flow Into Mold Time Zero,Cavity Full,C,E,Pack Rate,MP Set Point,C,Mold PressureThis indicates that all of the plastic put into the mold has been left in the partOnce the mold is filled and packed and cooling has progressed past the gate seal the mold pressure decay curve is directly related to cooling This curve then tends to decay at an ever decreasing rate until the part surface temperature reaches mold temperature where no heat transfer can take place,TIME(SECONDS),15,0,Pack,Fill,Cavity Pressure Gate End Showing Discharge,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Flow,Hold,Start MoldFill Time,Gate End Mold Pressure,A,B,C,D,E,F,A chart with two seconds less 2nd stage time is shown where plastic is allowed to run back out of the cavity,at point FThis is called discharge or backflowDischarge is not always undesirable in molding Many center gated parts,especially ones where flatness is desirable,must be allowed to discharge for correct part characteristics,TIME(SECONDS),15,0,Pack,Fill,Cavity Pressure Gate End Showing Discharge,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Flow,Hold,Start MoldFill Time,Gate End Mold Pressure,A,B,C,D,E,F,Coffee can lid molds are classically this type.It is important in all molding setups to know whether or not the gate is sealing It is also important to know the gate seal time,even if the gate sealing is not being used This information is an important troubleshooting tool because a change in gate seal time is generally indicative of changes in either mold temperature or material temperature which could help pinpoint the cause of the problems,On many molds,cavity pressure decays to zero long before the mold is open.There are also many molds where a residual cavity pressure exists when the mold is open.Generally,high residual pressures exist on molds which are hard to fill and pack and have relatively thin sections such as on connector molds.An actual curve of such a mold is shown where small residual pressure exists at mold open.If a moving ejector pin is being used to sense pressure,as is recommended due to its self-cleaning characteristics,then the ejector pin force will be shown on the graph.,TIME(SECONDS),15,0,Pack,Fill,Mold Pressure Near the End of Cavity,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,End of Cavity Mold Pressure,A,B,C,D,E,F,When the dog leg is small,as shown,the sensor location is very near the end of the cavity If there is no dog leg,the transducer is located precisely at the end of the cavity If a large dog leg exists,it means there is a significant amount of flow downstream of the cavity sensing location,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,End of Cavity Mold Pressure,PackRate,CavityFull,Measuring Cavity Pressure at End of Cavity(EOC),The cavity pressure curve at the end of fill is similar to the one at the gate end,with two important exceptions:The first exception is that there is no dog leg or dynamic portion of the cavity pressure curveNo cavity pressure exists during injection until the cavity is full and then a rapid static buildup is the first indication shown It is only possible to determine mold fill time from an end of fill cavity pressure sensor if an indication of the start of injection exists,CoolingRate,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,End of Cavity Mold Pressure,PackRate,CoolingRate,CavityFull,Measuring Cavity Pressure at End of Cavity,The absence of the dog leg on the cavity pressure curve always indicates that the transducer is located at the end of fill An experienced eye will focus on this when looking at a cavity pressure curve because it is a good indicator as to where the transducer is located in the flowPacking at the end of the cavity is much the same as the gate end In the same mold,the peak static packing pressure will be significantly lower at the end of the cavity than it is at the gate end of the part,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,End of Cavity Mold Pressure,PackRate,CoolingRate,CavityFull,Measuring Cavity Pressure at End of Cavity,Lower EOF is due to the nature of long polymer molecules and their inability to transmit pressure The longer the molecules,the higher the viscosity,the thinner the part,the larger the static pressure loss Monitoring one point in a cavity most often does not indicate what is going on in another area of the cavity,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,End of Cavity Mold Pressure,PackRate,CoolingRate,CavityFull,Measuring Cavity Pressure at End of Cavity,In some cases where viscosity changes are happening due to mold temperature,material temperature fluctuations,or fill time fluctuations,opposite ends of the cavity will do opposite things This is always the case in a mold where a short shot and a flashed part exist at the same time during start up In this molding situation,the gate end of the part experiences an excessive cavity pressure while the end of fill exhibits none,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,End of Cavity Mold Pressure,PackRate,CoolingRate,CavityFull,Measuring Cavity Pressure at End of Cavity,Remember a short shot is zero pressure at the end of the cavity,Because less packing occurs at the end of the cavity than at the gate end,this is the area where sinks are likely to occur With less packing there is less plastic in this area after unpacking or cooling When plastic cools,it shrinks,reducing the pressure in the mold,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,End of Cavity Mold Pressure,PackRate,CoolingRate,CavityFull,Measuring Cavity Pressure at End of Cavity,To make a properly packed part which does not have high tensile stresses in the center,it is important to properly pack it so that upon cooling there is enough plastic so that voids or sinks will not occur Cavity pressures generally drop to zero at the end of the cavity before the mold opens.When cavity pressures go to zero,cooling rate is reduced,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,End of Cavity Mold Pressure,PackRate,CoolingRate,CavityFull,Measuring Cavity Pressure at End of Cavity,This is most important in crystalline plastics where cooling rate determines the degree of crystallinity Crystallinity determines shrink rate Different cooling rates at different ends of the cavity can many times cause warp This is sometimes confused with packing induced warp,TIME(SECONDS),15,0,Pack,Fill,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,End of Cavity Mold Pressure,PackRate,CoolingRate,CavityFull,Measuring Cavity Pressure at End of Cavity,All process variations get amplified at the end of cavityFor monitoring purposes,the end of cavity mold pressure is the process variable which detects all of the variations in the process Cavity pressure at end of the cavity is never the best location for control using mold pressure techniques,TIME(SECONDS),15,0,Pack,Fill,Pre-Gate Mold Pressure,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,Pre-Gate Mold Pressure,Mold pressure curves in the runner before the gate are shown During the filling and packing of the mold,this curve is identical to one shown inside the gate except in proportion The dog leg indicates the dynamic pressure loss from the sensor location to the end of cavity,TIME(SECONDS),15,0,Pack,Fill,Pre-Gate Mold Pressure,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,Pre-Gate Mold Pressure,The area under the dog leg is a measure of the viscosity of the plastic during flow.Once the cavity is full,packing occurs and is measured in identical fashion to other transducer locations Packing takes place from the end of the dog leg at Point B until peak cavity pressure is reached,TIME(SECONDS),15,0,Pack,Fill,Pre-Gate Mold Pressure,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,Pre-Gate Mold Pressure,Pressures in the runner system are always higher than pressures in the cavity Runner pressures provide the least information about part quality of all cavity transducer locations Runner pressures are many times the only effective alternative for process monitoring.This is especially true in small multiple cavity molds,TIME(SECONDS),15,0,Pack,Fill,Pre-Gate Mold Pressure,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,Pre-Gate Mold Pressure,Runner pressure may be an excellent control point on these molds Runner pressure allows the actual control of conditions just outside the cavity,which is an effective compromise on molds where blocked cavities could cause problems with sensor locations inside the moldThe major difference between the monitoring of runner pressures and those inside the cavity is that in the runner,gate seal and cooling rate cannot be seen,TIME(SECONDS),15,0,Pack,Fill,Nozzle Pressure,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,NozzlePressure,HydraulicPressure,Monitoring the pressures in the nozzle of the injection unit offers a promise of simplicity and ease of application to all jobs and is an enticement which is hard to resist Nozzle pressure readings correlate much better to hydraulic pressure than to the pressures in the mold Here we show nozzle pressure along with a hydraulic injection pressure and cavity pressure,TIME(SECONDS),15,0,Pack,Fill,Nozzle Pressure,FLOW(Cu IN/SEC)VOLUME(Cu IN)PRESSURE(PSI),Hold,Start MoldFill Time,NozzlePressure,HydraulicPressure,A look at the comparison of this nozzle pressure to the pressures in the mold shows that there is very little correlation between the two This is why nozzle pressure is not suitable for monitoring or control It appears to be physically