When we last spoke, we discussed how Injection Molders sell “TIME” and how producing scrap parts drastically effects that time. Well’ here’s another major factor that negatively effects your time and profits: “blocking cavities.”
Blocking cavities has always been an easy fix to get you limping through the night or to finally complete the order when all seems lost. Seldom do we realize the implications of this action.
Yes, it can be a quick fix to get you through a difficult shift at night until your tooling department or lead processor shows up in the morning. But leaving yourself down a cavity for long periods of time or for entire production runs has some serious implications. For this example; let’s take a simple four cavity tool and imagine you block 1-cavity. First, let’s look at the processing side and what effects that will take place. If this tool is a convention runner/sprue, depending on your runner size/shape, you can lose anywhere between 10-20% shot size. If it is a hot runner tool you will lose up to 25% of your shot size. This drastic swing will cause you to have to change your shot size, transfer position and cushion position.
Now depending on what size machine you are running this tool on, these new changes may put you outside the processing window of the 20-80% rule which can reduce your shot to shot repeat-ability and part quality. Further important processing changes that will take place include changes to your injection velocity and material residence time. Even more problematic, your runner is potentially unbalanced which causes major quality issues.
Set aside all the headaches that that blocking a cavity can cause on the processing side. Now let’s take a quick look at the financial implications. Let’s say that on this same 4-cavity tool you earn a 20% profit margin. If you shut off 1-cavity then you instantly lose 5% profit from this job instantly. You will additional lose 25% production rate which will change the Lead time causing the production job to now take 25% longer than originally quoted pushing back other production jobs that are behind this one. Now that this production run is taking 25% longer than quoted, you will now also have an increase in machine running cost and payroll cost to finally complete this job.
You can see how this can quickly snowball and how many factors are negatively affected just by blocking one cavity. It doesn’t matter whether it is a 2-cavity or a 100-cavity tool. Blocking a cavity will always increase your time and decrease your profits. The more cavities you block, the worse your losses will become. Next time, before you decide to run a tool with some down cavities you may want to crunch your numbers to determine the severity of your loses. I have always found that it better to stop and repair.
Ready to reduce your production downtime to protect your profits? Learn more about how purging compounds and process efficiency work in tandem.
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