Injection Parameters for PET Preforms

 The parameters operators can control are:

• Injection pressure
• Injection speed
• Transition point
• Hold time.
• Hold pressure.
• Material “cushion”

Most modern machines allow the operator to set all relevant values on the screen, whereas older machines may feature manual hydraulic and position controls. For this discussion it has no relevance how the pertinent values are controlled. It is important that readers understand the concepts.

1- Injection Pressure

Many operators are under the misconception that the injection pressure can be dialed in because there is a field on the screen with this name. However, this is not the case. Injection pressure is the result of how hard the machine pushes the resin and how hard the resin resists this pressure. The value on the screen merely determines the pressure at which the machine hydraulics will stop pushing and drain oil to the tank instead of sending it to the injection cylinder. The factors that determine injection pressure are:

• Injection speed settings: The faster the machine injects, the higher the pressure.

• Melt viscosity of the material (see the last chapter): This in turn depends on

◦ Temperature: The lower the resin temperature, the higher the required pressure.

◦ Intrinsic viscosity (IV) of the material: The higher the IV, the higher the required pressure.

Pressures over 100 bar (1500 psi) are not recommended for PET. They tend to shear the material too much and this can lead to burn marks. Very

Figure 3.8 The hydraulic force of 100 works on an area that is five to seven times the screw (or shooting pot) area. The force at the end of the screw is multiplied by the same factor as the difference in areas.

thin (<2.3 mm) preforms may require higher pressures, and this is one of the challenges in thin-wall molding. Use of higher pressures during start-up is a common practice when the mold is still colder compared to continuous production conditions.

The hydraulic pressure controlled on the screen is not the pressure the material is subjected to. This is because the injection piston that is pushing the screw or shooting pod forward has a five to seven times larger area than the screw or shooting pod. This leads to pressure intensification by the same ratio. Actual material pressure should not exceed 700 bar (10,000 psi), and this equates to about 100–120 bar on the screen of the machine.

2- Injection Speed and Time

Another point to note here is that injection time cannot be dialed in as well even though this time is displayed on the screen. As pressure and speed are interrelated, here are the factors that affect injection time:

• Speed setting

• Pressure setting

• Material viscosity

• Temperature

This gives the operator excellent but only indirect control over the injection time.

PET, like all other plastics behaves mostly like a non-Newtonian liquid, i.e., its viscosity (its resistance to flow) changes with the shear rate. Shear rate is controlled by the injection speed. At very high shear rates PET would be over sheared and turn to a yellow/brown color. Therefore, the process window for injection speed is limited, and the operator has to take into account that changes to the injection speed will lead to changes in the material viscosity.

Figure 3.9 Recommended injection fill time versus preform weight.

Manufacturers recommend a fill rate of 10–12 g/s, and the graph in Fig. 3.9 is based on this rate while taking into account the fact that the rate needs to increase for heavier preforms to prevent the melt from freezing up during injection.

There are usually three, four, or five settings for injection speed along the way the screw or shooting pot takes to push the required material into the cavities. The operator determines where the screw or shooting pot stroke starts, and the screw will end its travel when it comes to rest after injecting the material. The starting point of the screw is called shotsize, a slightly misleading name as we will see. Typical values for PET injection machines are 120–200 mm. In most cases, the actually required amount of resin is less than the maximum and can be calculated as a ballpark figure using this formula:

                                                       SS=W×4/(D2×π×density)

where

• SS = shot-size in centimeters,
• W = weight of one preform × cavitation in grams,
• D = screw diameter in centimeters, and
• Density(melt) = 1.15 g/cm3.

The operator should add 5–10 mm to the so-calculated value because it is not advisable to let the screw or shooting pot bottom out at the end of the stroke. The small distance between the end of the stroke and the end of the barrel is called the cushion and is usually kept at around 3–5 mm. Longer cushions are possible but increase residence time. Some machines offer closed-loop cushion control where a cushion value is set and the machine determines the correct shotsize to achieve it.

In order to get to the suggested injection time values, the operator has to try different speed settings. Most machines offer three to five different ones. Speed settings can be controlled in two ways:

• In an open-loop setting they are in percentage. Open loop means that there is no feedback between the settings and the actual speeds. The hydraulic valve opens at the set percentage and a certain speed is achieved.

• In a closed-loop system they are in millimeters per second. Here a feedback loop measures the actual speed and adjusts the valve to achieve the set speed.

In a closed-loop system values at the screen may look like this:

In this three-point example shotsize is 65 mm. The screw will then move from 65 to 55 mm at a speed of 15 mm/s, from 55 to 25 mm at a speed of 40 mm/s, and at 15 mm/s thereafter. These values will then result in a certain injection time, and the operator can manipulate them to get to the desired time.

3- Transition Point

As discussed in Chapter 2.4.2 the purpose of the injection phase is to fill the cavity with molten material, at which point the machine switches over to hold (also called packing). The point at which this happens is called the transition point. During the hold phase the material cools and shrinks, thereby changing density. Density is a measure of mass per volume and is dependent on how energetic molecules of a given material are under given circumstances. For plastics (and most other materials) density changes with temperature. At higher temperatures plastic molecules take up more room and density decreases. As the plastic cools down, the opposite happens and the material shrinks. Typical melt density for PET is 1.15 g/cm3, whereas solid, amorphous density is 1.335 g/cm3 with

solid crystalline density being slightly higher. At around 100 °C (212 °F) demolding temperature density is about 1.3 g/cm3. The difference between the melt and the demolding density is about 13%. This means that 87% of the injection stroke should happen during injection and 13% during the subsequent hold phase.

The machine does not set the transition point automatically. Instead, it gives the operator three choices at which point hold should begin:

• When the pressure reaches a certain value

• After a certain injection time

• When the screw reaches a certain position

For PET, only the last option is used as it has been proven that it leads to the most stable process. Once the operator has established the injection parameters, the transition point is calculated as follows:

                                                          TP=(SS−CU)×13%+CU

where

• TP = transition point,
• SS = shotsize, and
• CU = cushion.

SS − CU is the distance the screw actually travels, 13% of it should happen after injection and during hold. The cushion has to be added at the end as the transition point is a fixed value. In our example TP should be (assuming a 5-mm cushion):

                                                                                      ( 65−5)×13%+5=12.8or13mm

This is a good starting point that may need adjustment to account for changes in temperature or viscosity. If the transition point value is set too high, the cavity is not filled when the machine goes into hold. This could result in sink marks or even short shots as there may not be enough resin in the mold to account for subsequent shrinkage during hold. If the transition point value is set too low, the cavity is completely filled during injection and the pressure will rise steeply as the resistance of the material in the cavity increases. This could lead to overpacking of the cavity and gate problems.

Very thin (<2.5 mm) and very thick (>4.5 mm) preforms deserve special considerations. Melt injected into thin mold channels cools down quicker than when it is traveling in thicker channels. Therefore, its density increases during injection and the transition point for these preforms should be set much lower, as far down as 6–8%. Very thick preforms

show the opposite behavior. They cool down very slowly and must therefore be run with a transition point set at a higher percentage, up to 18%. When in doubt it helps to study the injection graph (or watch the injection pressure gauge) and note the point at which the pressure increases. This is the point when the cavity is filled and where the transition point should be.

4- Hold Time and Pressure

During the hold or packing phase the material shrinks and density increases. If no more material was added after injection, this shrinkage would result in voids that are called sink marks. The operator has full control over the pressure, whereas the machine adjusts to a low speed setting without operator input.

Hold time is dependent on the wall thickness of the preform and temperature of the melt. The chart in Fig. 3.10 gives values for different melt temperatures.

This chart assumes certain cooling water conditions that may not be available on every machine. It should therefore be taken as a starting point only. Most machines feature three hold times, and the overall time should be equally divided among them (Fig. 3.11).

Also available are three hold pressures. The first is used to pack the material in the neck finish area. The material has had longer time to cool, and its viscosity has therefore increased. It requires the highest hold pressure. The second controls the body of the preform and is set to a lower value. The third controls the gate area and may be again lower but features the largest difference in possible values because wall thickness in the gate area, gate diameter, and gate cooling all come together to demand a particular

Figure 3.10 Recommended hold times versus performed wall thickness.

Figure 3.11 This machine uses a graphical user interface for the control of injection parameters. (Picture courtesy of Netstal-Maschinen AG).

setting for optimal gate uniformity and appearance. The maximum injection pressure can be read either on the screen or on a gauge mounted on the injection cylinder. It can be used to calculate hold pressures 1, 2, and 3 as 60%, 50%, and 40% of this pressure, respectively. These values should only be used as starting points and may need adjustments to overcome sink marks or gate problems.

5- Material “cushion”

In either injection system, whether reciprocating screw or shooting pot, the pressure that is present in the hot runner and the cavities must be relieved when hold is finished to allow easy demolding and prevent the material to drool into the cavity once the preform has been removed. This is done by moving the screw or shooting pot backward (“screw pullback” on the screen) and leaving it there for a period of time (“pullback dwell time”). A distance of 10 mm is usually enough, whereas the time depends on the last hold pressure used. Higher pressures will require longer dwell times. A good starting point is 0.5 s. After the dwell timer has timed out the shut-off nozzle that is situated between the extruder and the hot runner closes allowing transfer (shooting pot system) or recovery (reciprocating system)

Figure 3.12 Recommended cooling time versus preform wall thickness.

This was prepared and written by

     Eng/ Mohamed Bayoumi 

   Mobile +201550289138