You just established a stable process—now what? You probably record the machine setpoints on a setup sheet, right? You may even make the mistake of validating your process based on these setpoints. But these figures don’t always correlate directly with the actual output. Here’s an analogy for you: You have your cruise control set at 55 mph, but you are going down a steep slope and your speed goes up to 70 mph. A cop catches you and pulls you over—does he care that your cruise control was set to 55? Probably not. He only cares that you were going 70. In other words, the setpoint doesn’t matter, the output does.
Machines Wear, Setpoints Fade
The injection molding machine itself is a piece of machinery used to produce good-quality plastic parts in tolerance and within the quoted cycle time. The key word there is machinery. What happens to any piece of machinery, whether it is mechanical, hydraulic or servo-driven electric? They are all prone to wear over time, and if you do not have a robust preventive maintenance plan in place, they will more than likely wear even faster than they are intended to. Once these machines start to wear, do you know whether it is still hitting the setpoints you entered into the controller? Some machines have hundreds of setpoints, so let’s focus on a few that can have an impact on part quality.
Injection Speed Linearity Test
In RJG’s training courses, we teach about the Injection Speed Linearity test. This is one of the most important tests processors can run on their molding machines to see how accurate the actual machine’s injection velocity is compared with its injection-velocity setpoints. A few important factors can have an influence on whether your machine can actually hit these setpoints.
One key factor is the shot size. If it is less than 20% of the machine’s maximum shot size, there is a good chance it is not hitting the injection-velocity setpoint. Here’s another analogy: If you were offered any car of your choice for free but you had to hit 100 mph within 10 ft in order to receive it, would you be able to do it? Of course not. You wouldn’t have enough distance to obtain the necessary speed. That same rule applies to injection molding machines. If you do not have enough shot stroke, the machine does not have a chance of hitting the actual injection speed you’re trying to achieve.