In the first part of this series (click here if you missed it or want to review), we identified the four main dispense system temperatures as:
- Substrate temperature
- Fluid temperature
- Ambient temperature
- Storage temperature
We then promptly dismissed storage temperature and ambient temperature as ineffective and costly choices for controlling our dispensing process. So, now let’s examine the relationship between substrate and fluid temperature and how these two key temperatures interact in various processes.
The Variability of Substrate Temperature
Clearly, the temperature of the substrate that we are dispensing onto has a significant impact on the fluid being dispensed. If the substrate is too warm, it can cause the viscosity of the dispensed fluid to decrease, flowing into places we don’t want it, or even falling off the surface completely. Or, it can cause the fluid to cure prematurely, interfering with subsequent steps in the assembly process. If the substrate is too cold, it can cause the viscosity of the fluid to increase and also prevent it from flowing out the way we need it to.
None of these are really good for our process.
The problem is that we often have little or no control over the temperature of the substrate. Changing substrate temperature can also be challenging. It may be small and delicate, like an integrated circuit. Or it may be massive, like an automobile.
Why Bother Controlling Fluid Temperature?
Controlling the temperature of the fluid being dispensed is practical, efficient, effective, and economical. Maintaining a stable and consistent fluid temperature assures a stable and consistent viscosity – a major factor in dispensing system performance. But if it is just going to change when it hits the substrate, why bother?
To demonstrate the interaction between substrate and fluid temperature, let’s look at two completely different operations: applying hem flange adhesive to a car door and coating the inside of a beverage can.
The Hot Door Example
A car door is made in two pieces – an inner frame and an outer shell. These are then assembled by tightly rolling the edge of the shell over the edge of the inner frame. This is called a hem. Prior to hemming the two parts together, an adhesive is robotically applied around the perimeter of the inner frame to bond and seal the hem joint. This provides structural integrity and prevents rust from forming in the joint. Missing adhesive is a serious problem.
On a hot day, the parts are warm and the adhesive viscosity is low. Between the start of the dispense process and the joining of the two halves, the adhesive slumps, and worst-case, runs out of the hem area. In addition to the structural and durability issues, this displaced adhesive gums up the hem system, requiring maintenance and downtime. This reduces throughput, starves downstream operations, and costs the automaker a lot of money.
So, how do we solve it?
This process runs at up to 120 parts per hour. Bringing the large steel parts to a specific temperature in less than 30 seconds, while technically feasible, is both complex and costly. But if the inner frame is warm when the adhesive bead is being applied, the warm metal will heat the adhesive and reduce its viscosity. How can changing the temperature of the adhesive affect the outcome?
The key is time.
In Part I, we noted that these adhesives are mostly inert ingredients and have a low thermal conductivity, so it takes time for the heat to migrate through the bead. But the time between applying the bead and hemming the two parts together is very short. If that time is shorter than the time it takes for the bead to warm and migrate out of position, controlling the adhesive temperature will be successful – and it is – every time!
The Hot Can Example
Beverage cans are spray coated on the inside after they are formed. This thin layer isolates the beverage from the metal can to prevent corrosion and preserve the flavor of the beverage. This extends shelf life and preserves profitability for the beverage makers.
Because they are made of very thin metal, these cans reach ambient temperature very quickly. On a hot day, a condition appropriately called “hot can” arises where the heat from the can causes the thin coating to cure too quickly resulting in uneven film, “metal exposure” failures and high scrap rates.
These plants can process more than a thousand cans per minute, so controlling the temperature of the cans prior to spraying is both complicated and costly. So, how can changing the temperature of the coating affect the outcome?
The key is mass.
Unlike a car door, a can has a very low mass. If the coating is cooled below its optimal temperature as it is being applied, the mass of the coating absorbs the heat from the mass of the can, reaching a temperature that is optimal for the coating to properly flow out over the entire inner surface of the can prior to starting the curing process.
So as we can see, though substrate temperature has a greater impact than fluid temperature on the outcome of the dispense process, in many cases, controlling the fluid temperature can bring the overall process into control and produce the desired result effectively and economically.