Safety precautions for injection molding

>> Saturday, 12 March 2011

Safety is everyone's responsibility in the workplace. Safety is most often related to good maintenance and good housekeeping. Safety needs to be an attitude that if always present in your daily activities. Employees should not be hesitant to voice safety concerns in the workplace. Management is just as committed to safety as the operators on the floor; the primary difference is that the operators are usually the closest to unsafe conditions; keep management advised of unsafe conditions.

The following list includes items which should be maintained to assure a safe working environment:

1. Floor and machine should be kept free of oil
2. Floor and machine should be kept free of pellets.
3. Never reach over or under machine guards.
4. Never climb between the bars when pumps are running.
5. Retract injection. unit before entering the bar space.
6. The front gate should have an electrical, hydraulic and mechanical safety device preventing clamp from closing when the front gate is open.
7. The rear gate should have an electrical interlock preventing clamp from closing when rear gate is open (there is often a hydraulic interlock here also).
8. Readjust mechanical safety each time the mold open daylight space is adjusted.
9. The purge shield should prevent injection forward if the purge shield limit switch is not made.
10. Catwalks or platforms with railing should be present if hoppers such as drying hoppers stand tall enough whereby access requires climbing onto machine.
11. Know location of portable fire extinguishers; there should be an extinguisher no farther than 75 feet.
12. All electrical outlets should be marked as to voltage.
13. Never reach into the throat of an operating granulator. Unplug granulators before working on.
14. Always wear suitable foot and eye protection; safety glasses should be worn and steel toed shoes are recommended; soft soled shoes should not be worn.
15. Doe not operate any equipment unless suitable training has been supplied.
16. All employees should be advised of any chemicals in the facility which are considered hazardous; read further about "Right To Know" laws for each particular state.
17. First aid kits should be available.
18. Advise operators that injection molding resin pressure can reach 30,000 psi and that hydraulic line pressure can reach 2500 psi. Clamp tonnage developed equals 2000 lbs of force for each ton; operators be advised.
19. Be conscious of sharp square corners on ejector pins; many cuts result from protruding ejector pins.
20. Razor knives also require extreme caution as their use results in many cuts.
21. NEVER use steel tools on the mold cores, cavities or parting line... Use brass, copper or aluminum. Brass can scratch highly polished steel, so use caution.
22. Do not stick fingers or rods into the barrel/screw feed throat area.
23. Examine air hoses and electrical cords to verify condition is proper; do not use cords with damaged insulation. Be especially observant when working near nozzle heater bands as these wires are easily
24. damaged.
25. Use only swivel type safety eyebolts; screw eyebolts far enough in such that thread engagement is 1.5 times the diameter.
26. Never stand directly below a mold suspended in air.
27. Avoid back injuries; lift properly with the back upright and straight; know your limitations and do not exceed them; use proper tools and get help when needed.

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9. Design of Experiments (DOE or DOX) - Validation procedure for injection molds

The ninth step in validating a injection mold with the overall process shown in injection mold validation flow chart is Design of Experiments. The steps before:


1. Mold certification
2. Dry cycle mold
3. Process stability test
4. Gage repeatability & reproducibility (R&R) test
5. Mold viscosity test
6. Balance of fill analysis
7. Gate Freeze Test
8. Commissioning (multi-cavity analysis)

Purpose:
The purpose of the Design of Experiments (DOX or DOE) is to identify the optimum mold process and the mold process window. A solid statistical understanding of DOX is necessary. There are many different types of software that can be used to assist you in performing design of experiments properly. Use the software that you are most comfortable with.

The DOE can require a large amount of time to perform depending on the number of variables selected to test. In some cases a properly conducted DOX can require 2-3 days to perform as well as additional time to measure part attributes. It is best to perform an extensive DOE on the pre-production tooling (Pilot Tool) which replicates the production tooling. This will give a good representation as to which key variables will effect the critical attributes of the molded part from the production tool. It is also recommended the DOX be performed immediately after the commissioning test. Especially is you are confident with the skill and precision of your mold builder to produce a mold which has minimal variation between cavities.

It is recommended that a fractional factorial be performed to study the main effects. Variables that do not have a significant effect on critical part dimensions can be eliminated by performing the fractional factorial first. Significant variables for a full factorial experiment are determined from the fractional factorial, eliminating guesswork. The information from the full factorial as well as the fractional factorial analysis can then be used to perform Statistical Process Control (SPC).
Fractional Factorial Description:
This step should be performed when it is not clear which variables will effect key dimensions and attributes of the part. In some cases, it may be necessary to perform the fractional factorial DOE to determine the main effects for the aesthetics of the part. This will ensure that a full factorial DOX for the critical dimensions can be run while achieving aesthetically pleasing parts. The fractional factorial is to be used as a screening devise for identifying significant processing variables and not for process optimization.

Caution: The fractional factorial will not determine interaction effects.

All the steps during the procedure that involve intimate contact with the injection molding machine are to be done by a qualified injection molding machine operator.

Fractional Factorial Procedure:

1. The multi-cavity analysis has been successfully completed, i.e., there is no difference between cavities on the critical part dimensions.
2. Set up fractional factorial experiment to include all significant molding variables.
3. Typical mold process variables to include in DOX are:
Melt Temperature, Hold Pressure, Hold Time, Cooling Time, Mold Coolant Temperature, Cavity Pressure at Injection Cutoff, Peak Cavity Pressure (Packing), Screw Speed, Back Pressure.
4. Test extreme conditions (corners) of the DOE to verify you are within the molds processing window.
5. If a test "Fractures" then stop the DOX and rerun the experiments with acceptable degrees of variation for each factor.
6. For each change in processing conditions, allow time for system to equilibrate for changes in machine set up. For example, changes in mold temperature may require the mold running for 1 hour before it reaches steady state conditions.
7. Collect 5 shots for each process conditions. Attach a process set-up sheet and test code to samples.
· For each destructive test method, collect one additional shot.
8. Condition parts for 24 hours at 23C (73.4F) and a relative humidity of 50% with a standard tolerance of 2.0C (3.6F) and 5% relative humidity, respectively, for 2 days.
9. Measure and record all key attributes for one cavity, representative of the mold (see multi cavity analysis).
10. Analyze the data using any statistical software with a DOX option. Use coded units for the analysis.
11. Identify significant processing variables to be evaluated in the full factorial DOX.

Full Factorial DOX Description:
The full factorial is to be used to determine the optimum processing conditions based on key part dimensions. When it is unclear which variables are significant, the fractional factorial should be performed first.

All the steps during the procedure that involve intimate contact with the injection molding machine are to be done by a qualified injection molding machine operator.

Full Factorial Procedure:

1. The multi-cavity analysis has been successfully completed, i.e., there is no difference between cavities on the critical part dimensions.
2. Set up full factorial experiment using significant variables determined from the fractional factorial (if applicable).
3. Typical mold process variables to include in DOX are:
Melt Temperature, Hold Pressure, Hold time, Cooling Time, Mold Coolant Temperature, Material Lot (different lot of same resin), Colorant, Cavity Pressure at Injection Cutoff, Peak Cavity Pressure (Packing), Screw Speed.
4. Test extreme conditions of the DOE to verify you are within the molds processing window.
5. If a test "Fractures" then stop the DOX and rerun the experiments with acceptable degrees of variation for each factor.
6. For each change in processing conditions, allow time for system to equilibrate for changes in machine set up. For example, changes in mold temperature may require the mold running for 1 hour before it reaches steady state conditions.
7. Collect 5 shots for each process conditions. Attach a process set-up sheet and test code to samples.
· For each destructive test method, collect one additional shot.
8. Condition parts for 24 hours at 23C (73.4F) and a relative humidity of 50% with a standard tolerance of 2.0C (3.6F) and 5% relative humidity, respectively, for 2 days.
9. Measure and record all key attributes for one cavity, representative of the mold (see multi cavity analysis).
10. Analyze the data using any statistical software with a DOX option. Use coded units for the analysis.
11. Perform the optimization of the process based on the various critical dimensions and most efficient cycle time.
12. Identify upper and lower control settings for each processing variable evaluated (create a mold processing window).

The further steps are required in validating a injection mold according to injection mold validation flow chart is dry cycle mold:

10. Qualification (process capability study)
11. Mold metal Adjustments - centering process
12. Verification (30-day run)

Read more...

8. Commissioning (multi-cavity analysis) - Validation procedure for injection molds

The eight step in validating a injection mold with the overall process shown in injection mold validation flow chart is Commissioning (multi-cavity analysis). The steps before:


1. Mold certification
2. Dry cycle mold
3. Process stability test
4. Gage repeatability & reproducibility (R&R) test
5. Mold viscosity test
6. Balance of fill analysis
7. Gate Freeze Test

Purpose:
The purpose of commissioning is to ensure that all cavities in the mold deliver the same quality, i.e., there is no difference between cavities in the mold on the critical dimensions. The time required to perform this analysis is a function of the number of cavities and number of critical dimensions. The time on the injection molding machine is minimal compared to the time required to measure the parts. However, by performing this analysis it will significantly reduce the number of parts required to test and measure for future experiments on the multi-cavity tool. A solid understanding of creating and interpreting statistical control charts is necessary to perform the multi-cavity analysis.

All the steps during the procedure that involve intimate contact with the injection molding machine are to be done by a qualified injection molding machine operator.

Procedure:
1. Set melt temperature at resin manufacturer's recommended mid-range.
2. Set mold temperature at resin manufacturer's recommended mid-range.
3. Set fill rate and transfer method (position, time or pressure) based on result of the 5. Mold viscosity test.
4. Set hold time based on result of the 7. Gate Freeze Test
5. Set cooling time long enough so that parts eject without being distorted.
6. Allow the mold to run at least 1 hour, which should be long enough for the system to reach thermal equilibrium and for the process to stabilize. Longer stabilization time may be necessary for higher cavitation tooling like 128 cavity stack molds.
7. Ensure the measurement technique is stable and measurement sigma is known. The measurement technique must be accurate and precise enough to capture small amounts of variation in part dimensions and attributes.
8. Take one all-cavity shot every 15 minutes for an hour (n=5) with each cavity identified. (Base your choice of time on capturing most of the variation that is present.)
Caution: For destructive test methods, additional all-cavity shots will be required - to sample every 15 minutes. For every destructive test method, collect one additional shot.
9. Group all the parts from a particular cavity together to get samples of 5 observations each. The number of samples corresponds to the number of cavities.
10. Condition parts for 24 hours at 23C (73.4F) and a relative humidity of 50% with a standard tolerance of 2.0C (3.6F) and 5% relative humidity.
11. Measure and record all critical dimensions, attributes and variables for all parts.
12. Construct an and R Chart where the Range Chart will capture the variation within a cavity over an hour and the Chart will capture the variation between cavities. Calculate the Cp, CpK and target Z. This information will be used as a flag if the mold is considered off specification. For constructing an and R Chart use a spreadsheet Commissioning (multi-cavity analysis) which should be modified for your specific requiment.

13. Interpret the Range Chart.
• If the Range Chart is in control (pass C, P, and R), all the cavities deliver the same variability which can be calculated by , where d2=2.326 for n=5 (See Factors for constructing variables control chart on page 45), and is the average of the range measured within each cavity.
• If the Range Chart fails, and #7 was done, there is a particular cavity that is different from the others relative to the variability it produces. When the upper control limit (UCL) and the lower control limit (LCL) are not statistically meaningful, in comparison to the upper specification limit (USL) and lower specification limit (LSL), continue with the validation process.
14. Interpret the Chart (if the Range chart is in control ONLY)
• If the Chart is in control (pass C, P, and R), all the cavity averages are statistically not different. All the variability in the mold occurs within one cavity and all cavities are statistically not different.
• If the Chart fails, the special cause cavity must be investigated and corrected, i.e., is it because of mold, material, process, or people? When the upper control limit (UCL) and the lower control limit (LCL) are not statistically meaningful, in comparison to the upper specification limit (USL) and lower specification limit (LSL), continue with the validation process.
15. Upon completion of the molding run for the multi-cavity analysis, explore the molds processing window to determine key parameters (factors) to be varied during a design of experiments and the amount of variation (levels) of each.

A multi-cavity analysis curve shown in figure.

1. The range chart is in control (pass C, P, R): The variation (range) within any cavity is not significantly different than the mold average-range of 0.0027" (.0068 cm).
2. The average chart fails C (cavities 2, 9, 13): Cavity 2 is consistently bigger than the rest of the mold while cavities 9 and 13 are consistently smaller than the rest of the mold. All other cavities are not significantly different than the average inside diameter of 2.1728" (5.52 cm).

The cause for these out-of-control cavities should be investigated and identified. Root causes may be measurement errors, different steel dimensions, imbalanced runner system (small/large gates, different probe tip temperatures, etc.), different cooling conditions, etc. Use your investigative skills to identify the special cause. When the upper control limit (UCL) and the lower control limit (LCL) are not statistically meaningful, in comparison to the upper specification limit (USL) and lower specification limit (LSL), continue with the validation process.

A multi-cavity analysis curve shown in figure.

1. The range chart is in control (pass C, P, R): The variation (range) in weight within any cavity is not significantly different than the mold average-range of 0.121 grams.
2. The average chart is also in control (pass C, P, R): All weights are not significantly different than the average preform weight of 29.024 grams, i.e., any one cavity is representative of the quality (weight) of the 16 cavity tool.
Since there is no significant weight difference between cavities, on-going monitoring of the weight (as an overall process stability indicator) can be achieved by sampling ONLY one cavity randomly from the 16 cavities in the mold. Remember to spread the observations within samples to capture the "true" process variation.

The further steps are required in validating a injection mold according to injection mold validation flow chart is dry cycle mold:

9. Design of experiments
10. Qualification (process capability study)
11. Mold metal Adjustments - centering process
12. Verification (30-day run)

Read more...

7. Gate Freeze Test - Validation procedure for injection molds

The seventh step in validating a injection mold with the overall process shown in injection mold validation flow chart is Gate Freeze Test. The steps before:


1. Mold certification
2. Dry cycle mold
3. Process stability test
4. Gage repeatability & reproducibility (R&R) test
5. Mold viscosity test
6. Balance of fill analysis


Purpose:

The purpose of the gate freeze test is to identify the hold conditions necessary to freeze the gate. The extent and duration of hold pressure has a large effect on the dimensional stability and outer appearance of the molded part. If the hold time is too short, the gate will not have had enough time to freeze off and sink marks could appear on the part. This is especially true of larger parts and when higher hold pressures are employed. After the mold gates "freeze", hold pressure has no effect and should be terminated at that point. It is typically better to be a little high on the hold pressure timer setting. This will cause a slight wear increase on the hydraulics of the injection press but the molder will be able to ensure higher dimensional accuracy.

Hold pressure is set at a pressure which allows no plastic melt to enter or leave the gate as the part solidifies. A high hold pressure setting could pack the part excessively, beyond dimensional tolerances. A low hold pressure setting will allow the melt to exit through the gate causing sink marks and voids in the part.

The gate freeze test is designed to achieve minimal dimensional variation by ensuring no plastic leaves the cavity before the gate is frozen. Hold times in the sharply rising part of the weight curve introduce additional variation. The gate freeze can be accurately determined via flow analysis programs which utilize the cooling circuitry layout, mold geometry, tool steel and hot/cold runner configuration. A predominant amount of the flow analysis programs assume isothermal conditions in the mold. This would produce best case results and typically the gate will never freeze off in this short of time. Properly interpreted, the flow analysis results serve as a good starting point.

Notes: 1) When using a valve gated system, no hold time is required. Once the valve is closed, no material will enter or leave the cavity. The valve is held open long enough to properly pack the part, and then closed. At this point, no pressure needs to be applied. 2) Having poor shot size control on your molding machine and an imbalanced mold will lead to less than desired results (lack of precision).

All the steps during the procedure that involve intimate contact with the injection molding machine are to be done by a qualified injection molding machine operator.

Procedure:

1. Set melt temperature to resin manufacturer's recommended mid-range.
2. Set mold temperature to resin manufacturer's recommended mid-range.
3. Set cooling time long enough so that parts eject without being distorted.
4. Set fill rate from results of mold viscosity test and if desired, profile the injection stroke to have velocity controlled pack. At this moment, record the dosing stroke and change over position. For the remainder of the validation process this will remain constant.
5. Set hold time based on the machine operators experience, take their estimate and multiply by 1.5. Use this as the starting point for hold time. If after running the test you do not identify a point at which the gate freezes off, increase the hold time incrementally. Caution: On some molds, high hold pressure and hold times can cause ejection issues. Pay heed to the advice of the mold builder and do not increase the hold pressure and time to the point the parts are difficult to eject off the cores. If you see this issue on your pilot mold, you may want to modify the mold or part design to make your part ejection more robust.
6. Set hold pressure so that there are no visual sink marks.
7. Collect and weigh 3 consecutive shots to 0.01 grams or better.
8. Subtract one second from hold time.
9. Add one second of time to cooling in order to maintain a consistent molding cycle.
10. Begin a table similar to Table: Gate Freeze Test.
11. Graph the shot weight versus the hold time Figure 1: Gate Freeze Test.
12. Repeat steps 7 - 9 until the part weight begins to decrease.
13. Repeat the test with a high mold temperature and high melt temperature to document the worst case scenario for hold time.

A typical "gate freeze test" curve is shown in Figure 1: Gate Freeze Test. There is a region on the left side of the graph where small changes in hold time result in large changes in part weight. These large changes in part weight may result in part quality variation with regards to dimensions or mechanical performance. The region on the right side levels out at 3.5 seconds indicating that the part weight is more stable and that the gate has frozen off, i.e., no polymer melt enters or leaves the cavity.

Table: Gate Freeze TestTable: Gate Freeze Test
Figure 1: Gate Freeze TestFigure 1: Gate Freeze Test

Figure displays a gate freeze test when the gate never froze. This is typical for hot tips directly gated onto the part. The thicker the part, the more likely this will occur. When such a curve is graphed, identify the region on the curve where a change in slope is evident. In figure 2, this area is at 1.0 seconds. A hold time of at least 1.5 seconds is recommended.
Figure 2:  Gate Freeze TestFigure 2: Gate Freeze Test

It is possible the part will become over-packed and either flash or stick in the core, causing ejection problems, before obtaining a level curve. The above test had to be stopped at 4.0 seconds due to the part sticking in the core at longer hold times. Therefore, the hold time could be set anywhere between 1.5 & 4.0 seconds. If cycle time is not limited by hold time, a design of experiments could be performed using hold time as a variable. This will help to select the optimum hold time.

The further steps are required in validating a injection mold according to injection mold validation flow chart is dry cycle mold:

8. Commissioning (multi-cavity analysis)
9. Design of experiments
10. Qualification (process capability study)
11. Mold metal Adjustments - centering process
12. Verification (30-day run
)

Read more...

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