It seems that some new machines have packaging speed issues because machine manufacturers confuse processors with controllers that are more complex and less user-friendly than they need. #Best Practices

There are hundreds of variables that will affect your process, and the last thing you need is the quirks of the machine controller to cause trouble for your day. The machine controller plays an important role in the production of acceptable parts. There are subtle but significant differences between the controllers; in order to manufacture the same parts on different molding machines, it is important for processors to understand these changes. So let's take a closer look at how machine manufacturers differ in the second stage of packaging or maintaining functionality. How different are they? Even Krusty Sr. may be surprised.

All machines follow the general injection sequence. The screw starts from the "injection size" and injects the molten plastic to the preset transfer position at one or more speeds. At the moment the screw reaches the transfer or cut-off position, the machine switches from the first (injection) stage to the second (holding pressure) stage. Different machine manufacturers have different options for what happens during packing and holding. Some provide pressure time, others provide pressure and time phases. Other stages provide pressure, time, ramp time, and speed. Unfortunately, this complicates the injection molding process, and in my opinion, certain options are very detrimental to consistent parts. Want to know why?

Machine manufacturers are eager to propose more complex processing modes, but they are rarely tested by cavity pressure monitoring in production.

Since there are multiple options for packaging and holding, depending on the machine and manufacturer, we will set constant parameters and review seven variations or options. To describe these possible options, we will use the following set (constant) conditions:

 1. First stage or injection: All machines are set to injection (first stage) and transferred to the second stage at a certain position or volume. In this example, the first stage injection of 1 second ± 0.04 seconds is used. Using the same mold on each machine, we found that the pressure at the time of transfer was a "plastic" pressure of 16,000 psi. For ease of discussion, all pressures use "plastic" (non-hydraulic) numbers. This makes it easier to compare electric motors and hydraulic machines. Also, if you want the same parts, when you take the mold from one machine to another, you have to copy the plastic conditions—not the machine set point. Although hydraulics is popular among processors, it does not switch between machines due to different boost ratios.

 2. The second stage (hold pressure/hold): here we will set two holding pressures: 10,000 psi for 3 seconds, and then 8000 psi for 5 seconds. Likewise, all pressures are "plastic" rather than hydraulic. The following hypothesis attempts to prove possible changes in the second stage between different machines:

Machine type A: This machine only allows the processor to set the time and pressure for the second stage of packaging or holding pressure once. For example, hold for 8 seconds at a pressure of 10,000 psi. The pressure was increased from the transfer pressure of 16,000 psi to 10,000 psi, and the pressure was maintained for 8 seconds. See Figure 1.

This machine only allows the processor to set the time and pressure once for the second stage of packaging or holding pressure.

Machine type B: The machine allows the processor to set the holding pressure and related time for two or more stages. For example, 3 seconds of 10,000 psi plus 5 seconds of 8000 psi, the total hold time is 8 seconds. Depending on the machine manufacturer, there are many possible holding pressure responses:

 • Machine manufacturer 1: The pressure drops from the first stage transmission pressure of 16,000 psi to 10,000 psi as quickly as possible. At the end of 3 seconds, the pressure immediately drops to 8000 psi for 5 seconds. Please refer to the graph of the relationship between plastic pressure and time in Figure 2.

The machine allows the processor to set the holding pressure and relative time for two or more stages. Here, the pressure drops as quickly as possible from the first stage transmission pressure of 16,000 psi to 10,000 psi. At the end of the programmed 3 seconds, the pressure immediately drops to 8000 psi for 5 seconds.

 • Machine manufacturer 2: It takes 3 seconds for the machine to drop from the transmission pressure of 16,000 psi to 10,000 psi, then quickly rise to 8000 psi and hold for 5 seconds. In this case, the first time is actually the ramp time to reach the set pressure, not the time to reach the set pressure. Refer to Figure 3 for the relationship between plastic pressure and time.

Here, it takes 3 seconds for the machine to drop from a transmission pressure of 16,000 psi to 10,000 psi, then quickly rise to 8000 psi and hold for 5 seconds. The first time is actually the ramp time to reach the set pressure, not the time to reach the set pressure.

  • Machine manufacturer 3: The machine takes 3 seconds to drop from the transmission pressure of 16,000 psi to the holding pressure of 10,000 psi, and then it takes 5 seconds to drop from 10,000 psi to 8000 psi. Both holding times are "ramp time", not the time under the set pressure. Refer to Figure 4 for the relationship between plastic pressure and time.

It takes 3 seconds for the machine to drop from the transmission pressure of 16,000 psi to the holding pressure of 10,000 psi, and then it takes 5 seconds to drop from 10,000 psi to 8000 psi. Both holding times are "ramp time", not the time under the set pressure.

 Machine Type C: These machines allow the processor to set two or more stages of holding pressure, time, and speed. Using the same pressure and time as above, we will now set the speed of the first holding stage to 35 mm/s and the second holding stage to 15 mm/s.

 • Machine builder 4: Only the first stage of maintenance has a speed setting, and pressure dominates in both stages. The pressure drops from a transfer pressure of 16,000 psi to 10,000 psi at a speed of 35 mm/sec until it reaches a holding pressure of 10,000 psi. At this time the speed control is disabled (pressure is limited) and the machine maintains a constant 10,000 psi at any time within 3 seconds. At the end of 3 seconds, the pressure quickly dropped to 8000 psi and held for 5 seconds. The relationship between pressure and time is shown in Figure 5.

Here, pressure dominates in both stages. The pressure drops from a transfer pressure of 16,000 psi to 10,000 psi at a speed of 35 mm/sec until it reaches a holding pressure of 10,000 psi. At this time the speed control is disabled (pressure is limited) and the machine maintains a constant 10,000 psi at any time within 3 seconds.

 • Machine manufacturer 5: The pressure covers the set speed of the two holding phases. The pressure drops from a transmission pressure of 16,000 psi to 10,000 psi at a speed of 35 mm/sec, until the forward pressure of the drive screw reaches 10,000 psi. At 10,000 psi speed control is lost (pressure limited) and the machine remains constant at 10,000 psi until the remaining time of 3 seconds. At the end of 3 seconds, the pressure rises to 8000 psi at a rate of 15 mm/s until the pressure reaches 8000 psi and maintains 8000 psi for the remaining time of the preset 5 seconds. Again, this step is pressure limited, and I suspect that there will be any speed control when the holding pressure is from 10,000 psi to 8000 psi. The relationship between pressure and time is shown in Figure 6.

 • Machine manufacturer 6: Speed ​​takes precedence over set pressure. The pressure gradually drops from the transfer pressure of 16,000 psi and will be driven by the speed control to any pressure required to reach a speed of 35 mm/sec for 3 seconds. Speed ​​takes precedence over pressure setting, pressure may not be 10,000 psi. At the end of 3 seconds, the machine will run for 5 seconds at a speed of 15 mm/s. Likewise, the speed control overrides the pressure setting. Figure 7 shows several injections under these conditions (red is plastic pressure, green is cavity pressure; (the ratio is not the same). It can be seen that after several hours of experimentation, I tried to obtain process consistency, but failed.

Speed ​​takes precedence over set pressure. The chart shows several shots under these conditions (red is plastic pressure, green is cavity pressure; the ratios are not the same). After several hours of experimentation, attempts to achieve consistency in the process were unsuccessful.

Confused? Me too. This is more complicated than it should be. The processor has enough processing. Machine manufacturers are eager to propose more complex processing modes, but they are rarely tested by cavity pressure monitoring in production. As new and faster computers become available, programmers with good intentions are adding features that hinder product consistency—such as packaging speed.

Bottom line: Many machine manufacturers have made controllers more complex and less user-friendly than they need by adding options with questionable value. Speed ​​control on packaging without pressure limitation or pressure cutoff is a prime example. In order to evaluate any machine controller with a second stage (packing or packing) speed control, it is best to have cavity pressure sensing.

Cavity pressure data is essential for evaluating machine performance and consistency. Figure 6 provides an example. Please note that the machines are fairly consistent (red curve), while the cavity pressure (green curve) is different. Perform performance evaluation based on data rather than hype.

About the author: John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Michigan, which provides training and consulting services for injection molding companies (including LIMS) and other professions. Contact john@scientificmolding.com; sciencemolding.com.

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