Increase Injection Velocity

>> Monday, 28 February 2011

Raising the injection velocity will reduce the time taken to fill the cavity and it is therefore possible to achieve faster cooling of the preform. However, it will also increase shear in the material. Shear is a major factor affecting overheating of the material, A.A. Generation and I.V. reduction, therefore increasing velocity will damage the PET resin.
When working with hot preform method, the injection velocity will also make a significant difference to the material distribution in the finished container. Filling faster means that the preform will be colder when the mold opens and its temperature balance will also have changed. Typically, the shoulder area will become relatively cooler than the base area giving less stretch at the top of the preform.

When working with warm / cool / cold preform method the temperature related effects of increasing the velocity are either greatly reduced or non-existent

Oil flow into the injection cylinder must be increased.

For machines fitted with electronic injection control, increase the velocity percentage value on the injection screen of the electronic injection control system. Maximum allowable setting is 99%.
For machines without electronic injection control, increase the setting of the valve found on the operator side of the injection unit.

In most cases, the five steps of injection control can be set to the same value. Different values may be advantageous in cases of complicated preform design or technically difficult bottles.
Increasing the injection velocity too far may cause other preform defects such as lowered I.V., increased A.A., silver streaks and internal sink marks.

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Reduce Injection Velocity

Keeping the injection velocity low will reduce the shear that occurs in the material. Shear is a major factor affecting overheating of the material and I.V. reduction, therefore reducing velocity will protect the PET resin from excessive damage.
When working with hot preform method, the injection velocity will also make a significant difference to the material distribution in the finished container. Filling slower means that the preform will be hotter when the mold opens and its temperature balance will also have changed. Typically, the shoulder area will become relatively hotter than the base area giving more stretch at the top of the preform.

Excessive injection velocity can also disturb the alignment of the injection core, especially if the design is long and thin.

Reducing the injection velocity will also have the effect of making the holding time shorter since the V/P time will increase.

When working with warm / cool / cold preform method the temperature related effects are either greatly reduced or non-existent.

Oil flow into the injection cylinder must be reduced.

For machines fitted with electronic injection control, reduce the velocity percentage value on the injection screen of the electronic injection control system. Minimum usable value is typically around 15~17% but beware of making a short shot at very low settings.
For machines without electronic injection control, reduce the setting of the valve found on the operator side of the injection unit. Beware of making a short shot at very low settings.

In most cases, the five steps of injection control can be set to the same value. Different values may be advantageous in cases of complicated preform design or technically difficult bottles.
Reducing the injection velocity too far may cause other preform defects such as specks of crystal in the gate area.

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Ensure Correct Preform Temperature Balance

Hot PET stretches more easily, cooler PET is more difficult to stretch.
Therefore the primary method of adjusting the positioning of material in the finished container is to use relative temperature in various parts of the preform.

If the temperature is balanced, the wall thickness of the container can be optimized and the overall strength of the container can be improved.

If the balance is incorrect, some areas may become thick leading to a mottled or grainy appearance while the thin, overstretched areas such as the corners may show pearlescence or crystallization.

Balance the temperature of the material within the preform to give the most equal strength in the finished product.

There are two major methods of doing this.
In the first method, injection velocity is used to control the temperature balance of the preform. Since most of the retained heat from the injection process is used in the blowing of the container, this method can have dramatic effects on the finished container.

Filling the injection cavity faster will have the effect of making the shoulder portion of the preform relatively cooler resulting in a container having a thicker shoulder area and a thinner heel.
Filling the injection cavity more slowly will allow less cooling time for the shoulder area of the preform (relative to the base) leading to thinner shoulders and a thicker heel.
This method is more critical where the preform is relatively long. Shorter preforms show less response.

The second method uses temperature adjustment at the conditioning station. The method of adjusting preform temperature depends on the type of conditioning system fitted.

Conditioning Systems
Oil / Water Conditioning Core With Electric Heating Pot
Electric Heating Core With Electric Heating Pot
Electric Heating Core With Oil / Water Conditioning Pot
Electric Heating Core With Split Type Oil / Water Conditioning Pot


Note that strength does not only come from wall thickness, it also comes from good bi-axial orientation of the material and physical design of the container. Therefore a container having equal wall thickness may not always have the best overall strength.
Thicker, unstretched areas of the container may have a mottled body or grainy shoulder appearance.

The conditioning station is not intended to re-heat the preform so the power and control is limited (in most container designs). The main purpose of this part of the machine is to provide a balancing function across multi-cavity molds and in some cases it actually provides additional cooling.

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6. Balance of fill analysis - Validation procedure for injection molds

>> Sunday, 27 February 2011

The sixth step in validating a injection mold with the overall process shown in injection mold validation flow chart is balance of fill 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

Purpose:

The purpose of the balance of fill analysis is to evaluate the thermal and flow balance of the plastic distribution system in the mold. The plastic distribution system encompasses the hot or cold runner system, as well as the core and cavity. Only a naturally or symmetrically balanced manifold should be specified.

The cavity-to-cavity weight difference is an indicator of the quality of the hot runner system. It is critical to have the flows balanced to each cavity or the part-to-part variation may be large and process capability may not be achievable. Recall, the purpose of the mold validation procedure is to reduce variation throughout the injection mold. A typical mold is generally within +/- 10%. If the mold you are qualifying exceeds these values it should not cause you to cease the qualification process. Do investigate for possible causes such as, blocked gates, hot tip controller malfunction, hot tip failure, and non-uniform cooling. If the issue is not identified, document the data and continue. This data will be of use while analyzing the data collected during commissioning (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.

Caution: This test is part geometry dependent. The part may stick in mold and need to be removed manually.

Procedure:

1. Set melt temperature to resin manufacturer's recommended mid-range.
2. Set manifold and probe tip temperatures to resin manufacturer's recommended mid-range.
3. Set mold temperature to resin manufacturer's recommended mid-range.
4. Set hold time and hold pressure to zero.
5. If the machine is equipped, set pack time and pressure to zero.
6. Set cooling time long enough so that resin has cooled and parts eject consistently.
7. Set fill rate using the results from the mold viscosity test.
8. Transfer from injection to hold phase by screw position.
9. Have sufficient cushion to prevent the screw from bottoming out against the barrel during injection.
10. Adjust feed stroke so that the heaviest part is approximately 90% filled by weight.
Note: To achieve accurate results it is imperative none of the parts be completely filled.
11. Add adequate hold time and pressure, as well as pack time and pressure (if the machine is equipped with pack time and pressure) so no sink marks appear and cycle for five shots.
12. Remove hold time and pressure, as well as pack time and pressure.
13. Collect one all cavity shot.
Note: With certain mold designs it is difficult to perform a manifold balance test because of ejection issues with short shots. For example, some parts are designed with a slight undercut to ensure the part will stay on the side of the mold with the ejection action. If this is the situation, the undercut might not be filled during a short shot and the part will stick on the wrong side of the mold. In this case, you should evaluate the number of samples to be collected. In addition, study the parts to verify the balance of fill is not a result of poor venting. Lack of venting can artificially make the mold appear in balance.
14. Repeat steps 11-13 until 3 full shots have been collected.
Note: It is necessary to cycle five shots before each collection of shots to ensure each collection is subjected to the same thermal conditions.
15. Weigh all the parts and average the data per cavity.
16. Chart the weight by cavity.
17. For each cavity, use the following formula to calculate % imbalance:
%Im balance = (Wf-Wn)*100/Wf, where Wf = Weight of heaviest cavity, Wn = Weight of cavity n, where n = cavity number
18. Graph and interpret results.
Figure Balance of Fill Analysis depicts typical results for a mold which is not properly balanced, i.e., cavities 9, 10, and 12 are not within 5% of the weight of the full cavity #7.
Balance of fill analysis

Plotting the data relative to a mean weight is also a good method of identifying problem cavities. The 0.00% level of imbalance is based on the mean weight of all cavities in the mold. Cavity to cavity imbalance calculated against the mean part weight. Showing the data in this manner helps to highlight cavities to both extremes - higher than normal, or lower than normal. The formula below shows the calculation for calculating the imbalance using this method. %Im balance = (Wa-Wn)*100/Wa, where Wa = Average Weight of all the cavities, Wn = Weight of cavity n, where n = cavity number
It is also recommended the data be plotted in other ways. Doing so will make it easier to trouble shoot molds when necessary. Figure Balance of Fill Analysis shows the cavities in “clusters” according to their position in the mold. This is a good method to identify problems in a runner system, cooling system, one face of a stack mold, or inadequate clamp tonnage across the entire face of the mold. The “cluster” method of plotting the data according to mold construction is probably the best method. This method is very useful when plotted using average part weights for calculated imbalance.
On a hot runner system if you plot the imbalance and find that the majority of the cavities are reasonably well balanced but you have one or two obvious deviations (i.e. on a 32 cavity mold 30 cavities are 4 % imbalanced and 2 are 20% imbalanced) it is evident there is an issue with those two cavities. Steps you could take are:
1.Ensure correct hot tip operation, read power usage and temperature setting/variation on display unit.
2.Check for a foreign particles which may be blocking the gates of the suspect cavities
3.Measure gate size/shape (i.e. circularity) in cavity. Measure hot tip height in relation to the gate.
If one or two cavities have substantially (over 10%) higher imbalance than others the above analysis should highlight the reasons.

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

7. Gate freeze test
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)

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Optimize Secondary Blow Delay (Start)

The purpose of the Secondary Blow air is to make the final shape of the container with all details. The Secondary Blow air should NOT be controlling or influencing the inflation of the preform.
As with the Primary Blow Delay timer, there is no "correct" setting for Secondary Blow Delay. The optimum setting will depend on many other factors upstream of the stretch blow process and will include the setting of the Primary Blow Delay. Therefore, adjusting the Secondary Blow Delay time should be the last processing adjustment to be made when setting up a machine.

As a general rule, the Secondary Blow Delay timer should be set as short as possible to get the best definition in the container, but without upsetting the blow-up of the preform. Having the Secondary Blow Delay set too short is likely to cause off center gates and neck rings.

If there is too much delay between the introduction of the primary air and the secondary air, it may cause pearlescence in the corners of the container.

Before setting up the Secondary Blow Delay time, the Primary Blow Delay setting should be finalized. Then, adjust the Secondary Blow Delay to the earliest possible setting without upsetting the blow-up of the preform.

After the Primary Blow Delay time has been set to give the best blown bottle using Primary Air only, the machine should be re-started.
Reduce the Secondary Blow Delay setting from its long setting to a setting of about 0.5~0.8 seconds.
While blowing bottles, decrease the Secondary Blow Delay setting in steps , this should be about 0.1 second per step. Collect a sample container at each step, mark it and place it on a table. At first, there will appear to be no or very little difference in the containers but on close inspection you should be able to see improvements in the definition of ribs, logos and corners. At some point, the container quality will go bad, typically this will be an off center gate or a neck ring. There is no point in going any further once this happens and it is possible that a preform will split leaving fragments of PET in the mold if you continue.
Once the quality has turned bad, slightly increase the Secondary Blow Delay setting a little until the container quality comes good again. This is the optimum setting for the Secondary Blow Delay timer.

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Optimize Primary Blow Pressure

The purpose of the primary blow air is to inflate the preform as much as possible to the final shape of the container. The setting of the primary blow pressure can be extremely critical with containers having flat panels although it is not normally so important for round containers.
If there is insufficient inflation in the primary blow, the extreme high pressure of the secondary blow may cause damage to the inflating preform such as pearlescence in the corners or splitting in the base.

Too much primary air pressure can cause uncontrolled inflation of the preform leading to splitting preforms or buckling of flat panels.

Turn off the Secondary Air and test the inflation of the preform at varying Primary Pressures.

Referring to Optimize Primary Blow Time, increase the time delay of Secondary Air to the same as the blow time. This will prevent the secondary air from starting so that the Primary Air inflation can be more easily observed.
Reduce the Primary Air Pressure to around 0.3~0.4 MPa (3~4 Kg/cm²), then start the machine.
Wait for the preform temperature to stabilize, then turn on the blow mold.
Collect a sample bottle, make a note of the pressure used, then increase the pressure by 0.1 MPa (1kg/cm²) and repeat the sample collection.
Continue this until the pressure has reached around 2.5 MPa (2.5 kg/cm²) or the inflation of the preform becomes uncontrolled.

If any other parameters such as injection or conditioning settings are changed, it may be necessary to re-optimize the pressure.

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Optimize Primary Blow Delay (Start) for ISBM

The purpose of the Primary Blow Air is to inflate the preform as much as possible to the final shape of the container. The setting of the Primary Blow Delay time is one of the most critical processing adjustments that can be made in the molding process. Very small changes can have a significant effect on the container quality. There is no "correct" setting for Primary Blow Delay. The optimum setting will depend on many other factors upstream of the stretch blow process and it is also important to understand that the optimum setting will change if one of those upstream conditions changes. Therefore, adjusting the Primary Blow Delay time should be one of the last processing adjustment to be made when setting up a machine.
If Primary Blow Delay is too early, the container may suffer from neck rings or off center gates. If it is too late, the shoulder may show a grainy appearance, there may be a constriction or the preform may split in the body.
If there is insufficient inflation in the primary blow, the extreme high pressure of the secondary blow may cause damage to the inflating preform such as pearlescence in the corners, splitting in the base.
It is nearly impossible to judge the effect of the Primary Blow air on the inflation of the preform if the Secondary Blow air is also operating normally. Therefore, the first step is to disable the Secondary Blow air, after which the primary air time setting can be adjusted to various settings to find the optimum.

Optimize Primary Blow Delay (Start) for Injection Stretch Blow Molding
1. Increase the setting of the Secondary Blow Delay timer to the same or more than the Blow timer. This will prevent the Secondary Blow from taking place.
2. Reduce the Primary Blow Delay time to 0.0 seconds.
3. Start the machine, allow the preform temperature to stabilize, then turn on the blow mold. Take a sample then increase the Primary Blow Delay setting, this should be about 0.05~0.1 seconds per step. Collect a sample container at each step, mark it and place it on a table. At some point, the container quality will go bad, typically this will be a constriction mark around the shoulder area. There is no point in going any further once this happens and it is possible that a stretch rod tip may be broken if you continue.
4. Stop the machine and study the sample bottles produced, it is likely that the quality will have gone through a range as shown by the red curved line, somewhere around the middle setting, the best formed bottle will be seen. This is the optimum setting for the Primary Blow Delay timer.
5. After the Primary Blow Delay has been optimized, you should then optimize the Secondary Blow Delay setting.

The technique used here can also be applied to the Primary Blow pressure and Primary Blow flow settings.
Some container designs, notably square and flat-oval shapes are very sensitive to these adjustments, whereas round simple shapes may not be.

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