Optimizing the Injection Settings for PET Preforms

 Preforms can be divided into four sections. 

Figure 1 To better tailor the injection profile to the preform it is cut into sections.

• Gate area covering the hemispherical portion, also called end cap.
• Body area
• Transition area where the thicker body merges into the thinner neck.
• Neck area

To keep shear constant and achieve a uniform preform temperature, the melt should move at a constant speed inside the cavity. However, the amount of material that fills each cavity section is not uniform because of the differences in wall thickness and diameter. Therefore, the fill speed should be adjusted to account for these differences. Here is how this is done.

The first step is to cut the preform at the three places indicated in the drawing and weigh each piece, also noting its wall thickness. For the end cap and the transition zone, the wall thickness in the middle of each part should be noted. If only three speed and position settings are available, only two cuts are made, cap and body are weighed together, and the body wall thickness is used for the calculations.

Next, each weight is calculated as a percentage of the total weight. The weight percentages are then transferred into position settings from shotsize to transition point and subtracted from the shotsize to yield the numbers to enter on the screen. Let us look at an example:

• Shotsize: 120 mm
• Transition point: 23 mm
• Distance between them: 97 mm

Here are the weights and calculated results:

Next the speeds have to be calculated by using the wall thickness of the different parts. Let us assume that the preform in our example has the following wall thicknesses:

• End cap: 2.8 mm
• Body: 3.4 mm
• Transition: 2.5 mm
• Neck finish: 1.5 mm

Taking the body as 100%, i.e., the highest fill speed, it is now easy to calculate the other wall thicknesses as percentage of the maximum:

• End cap: 2.8/3.4 = 82.3%
• Transition: 2.5/3.4 = 73.5%
• Neck finish: 1.5/3.4 = 44.1%

The maximum speed has to be determined by trial and error to get the desired injection time as outlined earlier. If it is 50 mm/s, the relevant values would be:

• End cap: 50 × 82.3% = 41
• Transition: 50 × 73.5% = 37
• Neck finish: 50 × 44.1% = 22

Optimizing the Injection Settings Equations

1-    1- Fill Time Equation:

                                                t = Vcavity / Q

Where:

• t = fill time
• Vcavity = cavity volume
• Q = volumetric flow rate

2- Volumetric Flow Rate Equation:

                                                            Q = mdot / ρ

Where:

• mdot = mass flow rate
• ρ = melt density

3- Melt Density Equation:

                                                      ρ = ρ0 * (1 - α(T-T0))

Where:

• ρ0 = density at reference temperature T0
• α = thermal expansion coefficient
• T = melt temperature

4- Viscosity Equation:

                                               η = η0 * e^(E/RT)

Where:

• η0 = viscosity constant
• E = activation energy
• R = gas constant
• T = temperature

By optimizing injection speed, pressure, screw retraction rate and melt temperature profile using these equations, volumetric flow rate Q and melt density ρ can be controlled to achieve minimum fill time t for best part quality. Understanding temperature effects on ρ and η also helps fine tune the process.

Combined with modeling, these mathematical relationships provide guidance for methodically optimizing settings across the multi-variable injection process.

This was prepared and written by

     Eng/ Mohamed Bayoumi 

   Mobile +201550289138