In our post, Why Customer Returns are the Worst for Can Makers, we noted that the worst of all customer reject scenarios falls on manufacturers of the lowly metal can.
So we know it’s bad, but what can we do about it?
Finding a Fix
Let’s turn to our little missive, Controlling Temperature to Reduce HFI's, where we noted:
“…the manufacturer’s lab data shows it [compound] to change [viscosity] from 2800 cps at 60°F to 2400 cps at 90°F – a change of nearly 15% over that 30°F window – which equates to a rate of change of roughly 0.5%/°F. It’s even fairly linear over that temperature range.”
This direct, linear relationship between temperature and viscosity makes it easy to see that, if the temperature of the compound varies, so will its viscosity, which will cause variation in the bead applied – a common source of customer rejects. This is clearly demonstrated in the Weight vs. Temperature data plot shown here. Conversely, if you can hold the compound steady at a specific temperature, its viscosity – and therefore the weight of each bead applied – will remain consistent.
Seems easy, doesn’t it?
I Have Temperature Control…Don’t I?
By now you may be thinking, “I know all of this already, and I’ve already installed temperature control so this doesn’t apply to me.”
This is a common problem we run into all the time. Realizing the issue, manufacturers will turn to technologies they already know in an attempt to solve the problem. In this process, for instance, it is common to install in-line heaters or electrically heated hoses to “control” compound temperature.
So where’s the problem?
The problem is, that this addresses the issue at the point where these heaters are installed. Often – in fact usually – this is not the point at which the problem is occurring. They are often installed at the pump or the regulator – convenient places to get into the system.
It’s All About Location
Below is a typical plot for a can end liner system. In this case, the manufacturer has implemented an electrically heated hose to control compound temperature. From the top trace, we can see that it is holding pretty stable at 100°F – with some variation when the line pauses.
These guys are pretty smart. They understand the importance of viscosity on pressure regulation, so they’ve placed their heated hose before the regulator. But if you think about it, the place that really matters is where the compound is being applied to the end. If we look at the center traces we can see that the temperature at the inlet to the gun falls about midway between the exit temperature of the heated hose and ambient. Furthermore, a little more analysis shows that this relationship holds true for this particular line as ambient varies.
How Big Can this Problem Be, Anyway?
If you spend a few moments with the Weight vs. Temperature graph, you can see that, for this setup, a change of 1°F results in a change of 1.2mg in compound weight. Let’s say your target is 80mg/end and you have a ±10mg tolerance. Though, at ±12.5% for a total range of 25%, this seems like a huge window, it is actually a spec directly from the industry.
But do the math…At 1.2mg/°F, a change of just 8.33°F produces a change of 10mg in compound weight. This means that a swing of just 16.5°F would consume our entire tolerance – and that doesn’t account for any mechanical, pressure, or formulation based variations.
It’s a Constantly Changing World Out ThereThe problem is that a morning-to-evening swing of 20°F is not at all uncommon. In our post, Daily vs. Seasonal Temperature Changes, we compared average daily highs and 2 lows over the year for more than a half-dozen cities across the globe and found that virtually all of them experience daily temperature swings in excess of 20°F!
If we look at that from the perspective of the system we examined above, and this temperature swing is cut in half such that the compound only moves 10°F over the course of the day, we are still looking at a swing of 12mg – a full 60% of our total manufacturing tolerance – and we thought our temperature was under control.
It’s no wonder we get rejects!
What’s Really Important
Clearly, controlling temperature is essential to controlling your process and minimizing the opportunity for rejects. But the key is to control it at the point-of-application…the nozzle.
If you can hold your compound temperature at the gun inlet to within ±1.0°F, you can cut your variation by 80%!
Suddenly, your chances of getting a reject for compound weight are nearly eliminated – at least based on temperature variations.
Now you can go look at other opportunities for improvement in your process!
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