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9
Plastic Pressure (Pack/Hold)
9.1 Plastic Pressure The plastic pressure is what to measure on the process. It is calculated as the maximum hydraulic pressure multiplied by the intensifying ratio of the press (Ri). This will apply to the fill pressure, pack pressure, hold pressure, and back pressure. Even though the hydraulic pressure and the intensifying ratio will change from press to press, the plastic pressure (specific pressure) generated will remain the same because it is plastic pressure in front of the screw that we are trying to control.
9.2 Dynamic versus Static Dynamic pressure is the pressure that is experienced in the first stage, or boost portion of the cycle. This is where the machine is under velocity control and has abundant pressure and flow from the pump to make sure it maintains that velocity control, with shear thinning of the plastic. Static pressure comes under the pack and hold phase of the process. This is pressure control, where only pressure is controlling this phase of the process.
9.3 Viscosity Changes Viscosity changes happen from lot to lot of material or can happen within a lot of material. With a viscosity shift, the material gets thicker or thinner than what was previously running.
94 9 Plastic Pressure (Pack/Hold)
When the viscosity changes, the chain formation of the molecules is changing to more aligned or less aligned. The machine will see a viscosity change when going into pack phase because the machine is no longer under velocity control and no longer shear thinning the material. When the viscosity goes up, the material becomes thicker and harder to push.
9.4 End of Cavity Pressure Loss The end of cavity is where the largest pressure drop will be noticed. As plastic flows further away from the gate, its tendency to freeze increases, and every step of the way there is a pressure loss (so to achieve a fully packed out part, make sure to have ample pressure in front of the screw). It is known that there will be viscosity shifts in the material. If there is a viscosity shift that could use up the remaining pressure left in front of the screw, a 10% abundance of pressure must be maintained to accommodate viscosity changes. For example, if there is an injection pressure of 2000 psi hydraulic, then our transfer pressure cannot be above 1800 psi hydraulic or there is a chance of becoming pressure limited. If the viscosity goes up (meaning stiffer material), there is a good chance of running out of injection pressure before transfer.
9.5 Part Shrinkage versus Cavity Pressure With increased pressure applied to the pack or cavity, less shrinkage will be observed. When the molecules are packed tighter together they have less of a chance to move during the shrink phase. More shrinkage will occur in a semi-crystalline material because of the crystalline structure. Remember that the crystalline structure acts like a spring and the increased pressure inhibits the spring from returning to its original shape. But it can cause problems if the part is to be used in a heated environment; when the plastic is reheated the molecules will want to relax, which could cause warp or distorted parts.
9.6 Maximum Average Pressure at Parting Line before Flashing
Figure 9.1 Pressure vs shrink, from RJG’s Master Molder classSM
9.6 Maximum Average Pressure at Parting Line before Flashing Having a good understanding of why a tool flashes is important. Remember there is all this pressure over a specific area and eventually the force of injection pressure will overcome the force of the clamping unit and that is when flash appears. Figure 9.2 shows the relationship between clamping force, projected area, and maximum average pressure.
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96 9 Plastic Pressure (Pack/Hold)
Figure 9.2 Relationship between clamp force, projected area, and maximum average pressure
Example: Consider a 300 ton press at full tonnage. Clamping force in pounds (lbs.): Clamping force = 300 × 2000 = 600,000 lbs. (2000 lbs. per ton) Projected area: Projected area is the area of the part measured at the parting line. If a part is rectangular and measures 3.5″ by 5.25″ then the projected area will be 18.375 in2. Maximum average pressure: Maximum average pressure is an average of the post gate cavity transducer pressure and the end of fill cavity transducer pressure. Let’s say there is 12,000 ppsi post gate and 3,000 end of fill: 12,000 + 3,000 = 15,000 ppsi Now take 15,000 and divide by 2 to get the average or 7,500 ppsi. (To get the average of anything, take all the set points and add them up and divide by the number of set points.) Now it is known that 7,500 ppsi is the maximum average pressure at parting line without flashing the tool.
9.6 Maximum Average Pressure at Parting Line before Flashing
So what if there is no transducer in the tool? Take the clamp force in pounds and divide by projected area to get maximum average pressure at parting line before flashing. There are two ways to look at this: take clamp tonnage down until it flashes and now the mold has exceeded the maximum average pressure, or take pack pressure up until the mold flashes, again exceeding the maximum average pressure. When starting to get flash, the first two things that are tried are going up on clamp tonnage or down on pack pressure, therefore making the changes to the process that make sure the clamp force overcomes the injection force.
97
Plastic Pressure (Pack/Hold)
9.1 Plastic Pressure The plastic pressure is what to measure on the process. It is calculated as the maximum hydraulic pressure multiplied by the intensifying ratio of the press (Ri). This will apply to the fill pressure, pack pressure, hold pressure, and back pressure. Even though the hydraulic pressure and the intensifying ratio will change from press to press, the plastic pressure (specific pressure) generated will remain the same because it is plastic pressure in front of the screw that we are trying to control.
9.2 Dynamic versus Static Dynamic pressure is the pressure that is experienced in the first stage, or boost portion of the cycle. This is where the machine is under velocity control and has abundant pressure and flow from the pump to make sure it maintains that velocity control, with shear thinning of the plastic. Static pressure comes under the pack and hold phase of the process. This is pressure control, where only pressure is controlling this phase of the process.
9.3 Viscosity Changes Viscosity changes happen from lot to lot of material or can happen within a lot of material. With a viscosity shift, the material gets thicker or thinner than what was previously running.
94 9 Plastic Pressure (Pack/Hold)
When the viscosity changes, the chain formation of the molecules is changing to more aligned or less aligned. The machine will see a viscosity change when going into pack phase because the machine is no longer under velocity control and no longer shear thinning the material. When the viscosity goes up, the material becomes thicker and harder to push.
9.4 End of Cavity Pressure Loss The end of cavity is where the largest pressure drop will be noticed. As plastic flows further away from the gate, its tendency to freeze increases, and every step of the way there is a pressure loss (so to achieve a fully packed out part, make sure to have ample pressure in front of the screw). It is known that there will be viscosity shifts in the material. If there is a viscosity shift that could use up the remaining pressure left in front of the screw, a 10% abundance of pressure must be maintained to accommodate viscosity changes. For example, if there is an injection pressure of 2000 psi hydraulic, then our transfer pressure cannot be above 1800 psi hydraulic or there is a chance of becoming pressure limited. If the viscosity goes up (meaning stiffer material), there is a good chance of running out of injection pressure before transfer.
9.5 Part Shrinkage versus Cavity Pressure With increased pressure applied to the pack or cavity, less shrinkage will be observed. When the molecules are packed tighter together they have less of a chance to move during the shrink phase. More shrinkage will occur in a semi-crystalline material because of the crystalline structure. Remember that the crystalline structure acts like a spring and the increased pressure inhibits the spring from returning to its original shape. But it can cause problems if the part is to be used in a heated environment; when the plastic is reheated the molecules will want to relax, which could cause warp or distorted parts.
9.6 Maximum Average Pressure at Parting Line before Flashing
Figure 9.1 Pressure vs shrink, from RJG’s Master Molder classSM
9.6 Maximum Average Pressure at Parting Line before Flashing Having a good understanding of why a tool flashes is important. Remember there is all this pressure over a specific area and eventually the force of injection pressure will overcome the force of the clamping unit and that is when flash appears. Figure 9.2 shows the relationship between clamping force, projected area, and maximum average pressure.
95
96 9 Plastic Pressure (Pack/Hold)
Figure 9.2 Relationship between clamp force, projected area, and maximum average pressure
Example: Consider a 300 ton press at full tonnage. Clamping force in pounds (lbs.): Clamping force = 300 × 2000 = 600,000 lbs. (2000 lbs. per ton) Projected area: Projected area is the area of the part measured at the parting line. If a part is rectangular and measures 3.5″ by 5.25″ then the projected area will be 18.375 in2. Maximum average pressure: Maximum average pressure is an average of the post gate cavity transducer pressure and the end of fill cavity transducer pressure. Let’s say there is 12,000 ppsi post gate and 3,000 end of fill: 12,000 + 3,000 = 15,000 ppsi Now take 15,000 and divide by 2 to get the average or 7,500 ppsi. (To get the average of anything, take all the set points and add them up and divide by the number of set points.) Now it is known that 7,500 ppsi is the maximum average pressure at parting line without flashing the tool.
9.6 Maximum Average Pressure at Parting Line before Flashing
So what if there is no transducer in the tool? Take the clamp force in pounds and divide by projected area to get maximum average pressure at parting line before flashing. There are two ways to look at this: take clamp tonnage down until it flashes and now the mold has exceeded the maximum average pressure, or take pack pressure up until the mold flashes, again exceeding the maximum average pressure. When starting to get flash, the first two things that are tried are going up on clamp tonnage or down on pack pressure, therefore making the changes to the process that make sure the clamp force overcomes the injection force.
97