Tech Briefs

Knowing how machines respond produces the most repeatable parts in the fastest time.

Hold speed or hold velocity? How many machines today have it, and what does it do to the process?

The machine controller input of hold velocity started to become more and more prevalent with the onset of electrically driven molding machines. The setting tells the machine how fast to move the screw during second stage. But, and this is a big but, second stage is a pressure-limited portion of the process. You can't have velocity control on a pressure-limited portion of a process. So how effective is the speed setting then?

The molding machine will use the speed that is input into the machine controller only until the set second stage pressure has been reached. Once the second stage pressure is reached, the screw slows down and may even eventually stop. This velocity input should be viewed as a matter of response time. The setting is telling the machine controller how fast it will get to the second stage pressure setting.

Fig 1. Hold flow vs. hold time.

So, the question becomes, “How does this setting impact my process?” The answer to that question is simple: Yes. Figures 1 and 2 represent first the injection pressure profile, and second, the part weight. These are correlated to seven different second stage velocities starting at 5 ccm/s up to 35 ccm/s moving in 5 ccm/s increments.

In Figure 1, the slowest speed is represented in orange. It shows a large droop in the injection pressure profile after transfer has been reached. The reason this occurs is because the cavity is short at transfer, and it takes a period of time before the cavity completely fills and starts to pressurize. The resistance in the cavity is now pushing back on the screw, and the second stage pressure setting is reached and stabilized. As the second stage velocity is increased, the droop gets smaller and smaller, and the time it takes to reach the second stage pressure setpoint gets shorter and shorter.

Fig 2. Shot weight vs. hold flow.

In Figure 2, part weight is plotted against the second stage speed setting. The part weight is continuing to rise until 30 ccm/s is reached. At this time, any additional second stage speed has no influence on the part weight. This is the sweet spot for where the setting should be set — at a point where the final part weight is no longer influenced by the second stage speed.

Remember to think about what is happening to the plastic and to parts during this time. This is one of the most influential periods of the process for setting dimensions on the part. The slower the speed and response are, the slower it is packing out the parts and allowing the material to cool and solidify.

What about Other Machines?

For most molders scheduling a mold in several different molding machines, it is important to understand how each of those machines reacts to the second stage speed setting. Just because 30 ccm/s works on machine 4 doesn't mean that machine 12 will provide same response. This could be due to different machine makes and models, ages of the machines, hydraulic vs. electric machines, etc. There are many factors that could contribute to the difference.

In addition, machines may not have an input for second stage velocity. There are many machines on the market that don't. In this case, the molding machine will typically use the last fill speed input on the machine controller. If that setting is slow, then the second stage velocity will also be slow.

Other Influences

Besides the second stage speed setting, the size of the fill-only part can also contribute to the droop or slow response. If the fill-only part is 60 percent full at transfer instead of 98 percent full at transfer, then it will take more time for the cavity to fill out and pressurize. When conducting this study, make sure that parts are 98 percent full at transfer to eliminate that as a potential cause in the response. This ensures that only the influence of the second stage speed velocity on the process is being measured.

The goal is to produce the most repeatable parts in the fastest, most repeatable time, and the second stage speed setting is an often-overlooked influence on the process. Knowing how the machines respond to this input is one more step toward being better than the next molder.

This article was written by Shane Vandekerkhof, Global Training & Education Integrator, RJG, Traverse City, MI. For more information, Click Here.