What do you do after you have established a stable injection molding process? You might document parameter settings on machine on a process parameter setting sheet, right? You might even make mistake of validating process based on these set points. But these numbers are not always directly related to actual output.

An injection molding machine is a complex machine used to produce high-quality plastic parts to tolerances within a defined cycle time. Key word here is machinery. What happens to any mechanical component, whether mechanical, hydraulic or servo-driven electrical? They're all prone to wear and tear over time, and if your facility doesn't have an effective preventive maintenance program in place, it's likely that these critical components will wear out faster than expected. Once these machines start to wear out, there will be a noticeable difference between actual output value of machine and set value. Injection molding machines have hundreds of set points, and we need to focus on a few that can affect part quality.

In RJG's training course, we teach injection speed linearity test experiment. This is one of the most important tests in scientific injection molding system, main purpose is to understand difference between injection speed of actual machine and its injection speed, the smaller difference, the stronger control ability of machine, the higher consistency between settings and output. A few important factors can affect whether your machine actually hits these set points.

If shot volume is less than 20% of machine's maximum barrel size, it is very likely that machine cannot reach the set value of injection speed. Here's another analogy: If you can get any car of your choice for free, but you have to accelerate car to 150 km/h within 2 meters to get it, can you do it? Of course not. You don't have enough distance to accelerate to get necessary speed. Same rules apply to injection molding machines. If you don't have enough injection distance, there is no way for machine to reach speed set by machine.

 

Figure 1 Use injection speed linear test to determine actual injection speed of machine. (Actual: actual speed, Setpoint: machine set speed)
Figure 1 shows an example of linearity test results for injection speed using only 6% of machine's maximum barrel capacity. Machine was set at 5 inches/sec but it only hit 2.5 inches/sec, a 50% difference. This was not because machine was broken, but because it was running 20% less plastic shot than our recommended barrel size (or machine's maximum shot size).

Surprisingly, we still find that many injection molding plants still use hydraulic pressure as injection, packing and holding pressure in process. These pressure settings do not always produce identical parts between machines. This is mainly due to difference in enhancement ratio of injection cylinder area to screw area between different machines. Plastic pressure converted by different strengthening ratios is different.
You also lose ability to monitor changes in plastic viscosity, which you would see if you monitored plastic pressure during filling stage. To help people understand this better, I like to use analogy of playing darts. How hard is it to hit bullseye on every pitch if you're standing on pitching line? For most of us, this can be quite difficult.
This is similar to measuring hydraulic pressure on the back of injection unit. These settings do not guarantee that machine will produce same part from one die to another, every day or every year. However, if we place cavity pressure sensor inside mold, observe post-gate pressure and end-of-fill pressure, it's like throwing a dart directly in front of bullseye, and things become very simple.

By placing sensors at appropriate locations in mold, injection molding machine can capture data such as short shots, dimensional changes, feed rates, cooling rates, and gate freeze times. These are considered outputs that can be monitored, and alarm intervals can be set for these important parameters to ensure that each model is within parameters.

 

This also makes it easier to match process when transferring molds from one machine to another. Two machines may not perform exactly same, even if they are same brand and have exactly same specs. This means that if you try to match a process based on machine settings, you may end up with two completely different products. However, if you match curve of cavity pressure sensor, you can better replicate process and make same part regardless of whether machine is worn, viscosity of material changes, or which machine is running on.
Another example: when you just look at temperature of an injection molding machine, they are rarely able to run exactly at set point. If all barrel temperature zones are set to 240℃, how many of them will be exactly 240℃? If you look at temperature controller during production, you may find that actual temperature may be below or above set point. Temperature is high or low depending on machine's temperature controller, PID loop, thermocouple position, and heater itself.
All of these variables can be different if one set of molds and processes are transferred to another machine. Even if you use same temperature setpoint of 240℃, you may see completely different temperature readings on controller. If you take into account different barrel sizes, dwell times, and screw designs between two machines, you'll see that even at same process set point, actual melt temperature can be quite different. Figure 2 shows difference between barrel temperature setpoint on machine controller and actual barrel temperature. Set value is 460F, actual value is 474F and 468F.

 

Figure 2 Just because you set barrel at a specific temperature doesn't mean it's actually at that temperature.
When we took a melt temperature measurement using a thin wire probe and a thermometer, we found that actual melt temperature was 463℃ (see Figure 3). While in this case melt temperature is very close to machine setpoint, this is not always case. Or temperature control ability of this injection molding machine is excellent.

 

Figure 3 Using a melt meter to measure actual melt temperature of plastics.
In this regard, I have an actual case for your reference. A few years ago I was in a consulting job helping an injection molding plant investigate yield differences between two "identical" injection molding machines, both purchased from same manufacturer and built to same specifications. They ran two identical sets of molds using same material and same process set point. Customer did not understand why scrap rate for one of machines was around 3%, while scrap rate for the other was over 10%. Actual melt temperature of the two machines differed by nearly 40℃ from a rise measurement. In this case, if mass production and comparison are carried out according to set temperature of machine, it will be difficult for products to be consistent.

Some machine controllers will read setpoint for certain processes as a percentage. If you only record percent setpoint and not actual value, you may run into trouble in the future. Machines that read percentages are fine by themselves, but when running these machines, two important variables must be considered: maximum value for that set point and actual output.
Figure 4 shows a machine controller that reads injection speed as a percentage. If you don't know what max injection speed of machine is, then you won't know what 60% is actual speed. Actual value of this 60% for an injection molding machine with a max speed of 10 inches/sec and another with a max speed of 8 inches/sec is definitely not same.

 

Figure 4 If your machine controls injection speed in percentages, you will need to determine maximum speed to understand what these percentages mean.
This doesn't mean either of the two machines can't produce good parts, it just means there's a big difference in 60% speed setting between two machines. In this case, it is important to record volume of material you injected and actual fill time of process. If you inject same volume of material at the same time, you will match volume flow rate of this process, which will make it easier to replicate every time you run process validation.
In actual production, if set point of process is no longer able to consistently produce parts of acceptable quality, it is often necessary to readjust and validate process. In today's highly competitive market, an injection molding plant must be cost-effective to be competitive, and if you continually revalidate your process, your profits will be lost (waste material, wasted labor, wasted machine time, etc.).
At the end of day, you will be more successful if you are monitoring and recording process output values rather than machine setpoints. This gives you a process that is perfectly matched to any machine wear, plastic viscosity changes or machine changes.