Would you like to inspect the original subtitles? These are the user uploaded subtitles that are being translated: CAUTION: THE FOLLOWING ARE RECOMMENDED PROCEDURES. PLEASE USE EXTREME CAUTION WHEN PERFORMING THESE PROCEDURES. SAFETY OF PERSONNEL SHOULD BE OF PRIME IMPORTANCE. DO NOT TAKE A STEP THAT YOU THINK MAY DAMAGE THE MOLD, SUCH AS EXCESSIVE PLASTIC PRESSURES. FIMMTECH IS NOT RESPONSIBLE FOR ANY ACCIDENTS OR ANY MOLD/EQUIPMENT DAMAGES. SCIENTIFIC MOLDING PROCEDURES. 1. Developing the Viscosity Curve: 1. Set the melt temperatures to those recommended by the manufacturer. If there is a range, set the temperatures to the center of the range. 2. Set all the holding phase parameters to zero; Holding Pressure = 0 & Holding time = 0. This means that there will not be any holding phase and only injection. 3. Set the injection pressure to the maximum available. 4. Set the cooling time to a safe value such that the part will be cool and has reached the ejection temperature before mold opening. 5. Set the injection speed to ‘slow’ and make a part. The part should be short. If not adjust the transfer position to make the part such that it is filled only about 50%. 6. Increase the speed in steps and make sure that the parts are still short. Mold a part with close to the maximum injection speed and make sure that it is still short. If it is full, then adjust the transfer position, such that it is about 95 % full part. If it is less than 95 % full, then also adjust such that the part is 95% full. This means that at close to the maximum injection speed you have a 95% full part with no holding time or pressure. 7. Make another shot and record the fill time and the peak hydraulic pressure required to fill the part. Note: The peak hydraulic pressure will be the pressure required to move the screw at Frontier Injection Molding and Material Technologies, Inc. the set injection speed. This is taken from the available pressure from the machine. For example, the machine is set to 2200 psi but may require only 1850 psi to move the screw at the maximum speed of 5 in/sec. 8. Next, lower the speed by a small amount, for example from 5 in/sec to 4.5 in/sec or from 90% to 80%. Note the fill time and the peak injection pressure. 9. Repeat the above step all the way till you get to the lowest injection speed possible. Divide the available injection speed range into about 10 - 12 speeds so that you get as many data points. 10. Find the Intensification Ratio of the screw from the machine manufacturer. If this number is not available, pick it to be 10. It does not really matter since this is a constant used in the equation and will factor the viscosity proportionately. 11. Input these values in the software to generate the graph. A typical graph is shown below How to use this information: Looking at the above curve, one can notice that the viscosity stays fairly constant after about 60% of the injection speed. Therefore, setting the injection speed to 70% would ensure that the filling stage of the process will stay consistent. Any small natural variations will not cause large changes in viscosities resulting in shot to shot variations. Shot to shot variations should be reduced in order to achieve repeatable quality of parts. This is especially important in case of tight tolerance parts and multi cavity molds. Optimizing the injection speed through In-mold rheology is only the first step to achieving a robust process. Later, the holding phase and the cooling phase must also be optimized. 2. Procedure to determine the cavity balance: 1. Set the holding pressure to zero. 2. Set the holding time to zero. 3. Set the screw recovery delay time to about a value close to an estimated holding time. 4. Set the cooling time to a value such that you know that the part will be cool enough to eject. 5. Set your injection speed to the value obtained from the Viscosity Curve study. 6. With the rest of the settings the same as you had in the viscosity study, start molding. 7. Only by adjusting the transfer position, mold parts that are just short. If there is a visible cavity imbalance, then the ‘biggest’ part should be just short. Record the weights. 8. Continue to mold parts that are shorter and shorter by changing the cut off position. As an example, mold parts that are 75%, 50%, 25, % and 10% filled. Input the value in the worksheet. A typical graph is shown below How to Use this Information: Check the %variation between the maximum and the minimum fill for the cavities at each of the fill percentages. Record this info and perform the window study as described in the later section. 1. If the parts can be packed out and the process window is large, check to see if the parts are in spec. if they are the imbalance if any is acceptable. 2. If the process window is very small and you get flash on the cavity that fills first while the other is short or has sink, investigate the reason for the imbalance. Suhas Kulkarni Page 4 of 8 There are for main reasons why an imbalance can occur: 1. Runner sizes are not the same. 2. Gate sizes are not the same 3. Venting is not the same. 4. Cooling is not the same. However this has the least impact especially at start up. 5. There can be a rheological imbalance in the flow. Refer to www.beaumontinc.com for more info. 3. Procedure to determine Pressure Drop: Consider the plastic flows through the following sections: The nozzle of the machine, the sprue, the primary runner, the secondary runner, the gate and the end of fill. The procedure to determine the pressure drop is as follows. 1. Set the machine to the maximum available pressure. 2. Build a shot on the molding machine and take an air shot. Note down the peak pressure required to do so. 3. Next start molding but mold only the sprue and note the peak pressure. 4. Mold the primary runner only and note the peak pressure. 5. Mold the secondary runner only and note the peak pressure. 6. Mold a shot such that it just enters the gate and note the peak pressure. 7. Mold a shot such that it just reaches the end of fill and note the peak pressure. 8. Generate a graph as shown and look for the sections that have a high pressure drop. Note: Set a short screw recovery delay time. In doing so you will prevent immediate back flow into the cavity. If the gate is not frozen, the back pressure will influence the cavity fill. Suhas Kulkarni Page 5 of 8 How to use this information: The maximum pressure used in the process should never be equal to the maximum available pressure at the machine. For example, if the maximum available hydraulic pressure is 2200 psi, then the end of fill pressure should not be equal to 2200. If this is the case, it means that the screw needs more pressure to move at the set injection speed and cannot do so because of the limited pressure. Such a condition is called ‘Pressure Limited’. Typically, you should a maximum of about 90% of the maximum available pressure. Therefore in this case, where the maximum is 2200, the end of fill pressure should not be more than 1980 psi. In the generated graph, if you are pressure limited or more than 90% of the maximum, look for steep increases in the pressure and try to reduce these. For example, If the secondary runner section shows a steep increase, then this means it takes a lot of force to move the plastic through this section. Increasing the diameter of the runner will help in reducing this pressure. 3. Procedure for determining the Process Window. A process window is the most important study one must do. Typically the window must be developed with hold pressure versus melt temperature for amorphous materials and holding pressure versus mold temperature for crystalline materials. The procedure below is given for Hold pressure versus melt temperature. For hold pressure versus mold temperature, replace melt temperature by mold temperature. 1. Set the barrel temperatures to attain the lower value of the recommended melt temperature. 2. Set the injection speed to the value obtained from the viscosity curve experiment. 3. Set all holding times and pressures to zero Suhas Kulkarni Page 6 of 8 4. Set the cooling time to a value more than what would be typically necessary. E.g. If the estimated cooling time is 10 seconds, set the cooling time to 20 seconds. 5. Start molding and adjust the transfer position to make a part 95 t o98% full. 6. Let the process and the melt stabilize by molding the parts for about 5 to 8 shots. 7. Now set the hold time to a value such that you are sure the gate is frozen. (In the next section we will learn how to optimize this time.) This would be based on some previous experience. For example, for a 30% glass filled PBT or Nylon with a gate size of 0.070” this time ends up between 6 and 8 seconds. In this case, for this part of the experiment, we would set the hold time to 10 to 12 seconds. 8. Increase the holding pressure in small increments and note the pressure where an acceptable part is made (no shots, flash, etc. …). 9. Note down this pressure as the ‘Low Temperature - Low Pressure’ corner. 10.Increase the pressure further in similar increments and note down the pressure where there is an evidence of an unacceptable situation such as part sticking in the mold or flash, warpage, etc. ….. Note down this pressure as the ‘Low Temperature - High Pressure’ corner. 11. Repeat steps (9) and (10), but at the high end of the recommended melt temperature. This time the two extreme pressures would be the ‘High Temperature - Low Pressure’ and ‘High Temperature - High Pressure’ corners. 12. Joining these four corners would now generate the process window or the molding area diagram. 13.Set the process to the center of this window. Suhas Kulkarni Page 7 of 8 How To use this information: The process window is an indicator of how much can you vary your process and still make an acceptable part. An ideal situation is you have a wide process window. If the process window is very narrow, then there is always a danger of molding parts with defects. For example in the above graph, if the process window is very small, then one could get occasional short shots or occasional flash due to the natural variation in the process. A robust process is one that has a large process window and accommodates the natural variation. 5. Procedure for determining the Hold Time. 1. Set the injection speed to the value obtained from the viscosity curve experiment. 2. Set the process at the center of the process window from the process window study. 3. Set the cooling time to a value to ensure that the part is cooled before ejection. 4. Drop the holding time to zero and start molding. Mold approximately 5 to 8 shots. 5. Increase the holding time to one second and collect a shot. 6. Increase the holding time to two seconds and collect a shot. Similarly collect shots at increments of one second. 7. Weigh the shots and plot a graph of part weight versus time similar to the graph in the picture. 8. Determine the gate seal time. How to use this information: Once the graph is generated, choose your holding time, such that you are just beyond the time where the part weight is constant. In the above graph, the part weight is constant after 8 seconds. Set the holding time to 9 seconds. Suhas Kulkarni Page 8 of 8 6. Procedure for determining the Cooling Time. 1. Mold three shots at various cooing times. 2. Measure the critical dimensions. 3. Plot a graph of dimension versus the cooling time. 4. Analyze the data to see how the critical dimensions are influenced with the cooling time. 5. Decide on the cooling time that best fits the data. 6. Run 30 shots at this cooling time and perform a statistical analysis to determine the process capability at this cooling time. Cycle time is the most important factor since that is what makes the bottom line profit. In most case, if the process is capable at lower cooling times, one can make a change in the mold steel and achieve the same dimensions at lower cycle times. 12543