What to Look For in a Mold Cooling Tunnel

A ChocolateLife member asked, “Do you have a preference at the same price, [for] a Selmi Spider or an FBM Clima 50?”

Disclaimer: I actively represented FBM equipment in the US and to ChocolateLife members around the world from 2012 to the start of the pandemic. I am extremely familiar with the pros and cons of their machines and, in the course of promoting them, became familiar with the capabilities of FBM with respect to other manufacturers. I was instrumental in convincing FBM to release the Kleego conche and helped outline the original specifications for the Kleego (which is a pun on my name) as well as for FBM’s Rumbo melangers.

This is not going to be a commercial for FBM. Instead, I am going to take a look at the factors I look at when recommending any equipment to a consulting client. My cardinal rule when consulting is to keep the needs of my clients above any commission relationship I might have with a supplier.

Some Basic Physics and Maths

The configuration of any cooling tunnel is rooted in the question: How long will it take to cool the chocolate while maintaining proper temper?

There are some simple physical constraints that factor into finding an answer to that question. Perhaps the most critical is how to efficiently and effectively remove the latent heat of crystallization radiated from the chocolate in the mold. It turns out there is a lower limit to the temperature that can be effectively used – below that temperature the chocolate cools too quickly and temper is negatively affected; you can’t simply lower the temperature beyond a certain point to speed things up.

We use the term “dwell” time when starting our calculations, asking: “What is the shortest length of time a mold needs to be in the tunnel, at an optimum temperature, and assuming adequate airflow, to cool properly?” That dwell time (the total length of time the chocolate needs active cooling) is influenced by 1) the amount of chocolate in each mold cavity, 2) the total amount of chocolate in a mold, 3) the geometry and spacing of cavities in the mold, and 4) the depth of the mold cavity. Depth can be the most important factor; chocolate in a 12mm-deep cavity is going to take longer to cool than the same amount of chocolate in a 6mm-deep cavity; the difference is less critical between 6mm and 5mm unless you have really high production demands. This is where airflow becomes a critical factor. If there is no air flowing under the mold, e.g., you’re using a belted tunnel suitable for cooling enrobed items as a mold cooling tunnel, the chocolate cools unevenly as air circulating over the surface of the chocolate removes heat more efficiently because there is no airflow under the mold. With thicker mold cavities this can lead to cracking if the temperature is too low and the mold moves through the tunnel too quickly.

If it takes twelve minutes to cool a mold and the transport mechanism moves at one meter per minute, you need a linear cooling tunnel with an active cooling section that is twelve meters long – about 39.5 feet – not including the space required for the molding line, the entrance and exit sections of the tunnel, and sufficient space to access all parts of the line that need to be reached.

If each mold is 275 x 205mm and you divide 205mm into 1000mm (the distance the belt moves in one minute), you get four molds per meter and you can deposit at four molds per minute and 48 molds will fit at a time in the tunnel.

If you want to go faster – you want/need to cool more molds per minute – you need a longer tunnel. At two meters per minute with a twelve-minute dwell time you need a twenty-four meter tunnel. If you need a longer dwell time at a specific transport speed (molds/minute) you need a longer tunnel. Everything is based on the dwell time required.

Space and Form Factor

It is something of a luxury to have the space to install a 12-meter long tunnel, and for some, 4 molds per minute will not meet production demand.

Finding a solution to this conundrum requires changing the geometry of the tunnel and there are two basic approaches taken in this question: 1) folding the tunnel; or 2) using a spiral. Whichever approach is taken the goal is the same: to reduce the footprint – the number of square meters of floor space – the tunnel occupies.

Selmi offers both folded and spiral solutions. Their linear tunnels can be kitted out in two levels. The top level uses a rail transport mechanism that exposes both the top and bottom of the mold to circulating chilled air to remove heat. At the end of the upper level the mold slides down a ramp to the lower level, which is a conventional belt, and the mold is returned to the entrance end of the tunnel.

This is an attractive solution on the surface because it means the tunnel can be used for both enrobed and molded items and it roughly halves the length of the tunnel; a four-meter tunnel is the equivalent of an eight-meter tunnel and an eight-meter tunnel has a capacity roughly equivalent to a sixteen-meter tunnel. In my experience, if you have enough production demand to require both a belt tunnel for enrobed items and a tunnel for cooling molded items it makes sense to have dedicated lines if you have the space and budget. The multi-level tunnel is also less than ideal if the backs of the bars are going to be sprinkled with anything as loose pieces will slide off when the mold slides down the ramp and this could require nearly constant operator attention (I have been told it has, in one factory I visited) to ensure uninterrupted production.

The spiral solution uses a gently-curved ramp circling around within an enclosed box. The spiral changes the footprint of the linear tunnel into roughly a square and it might be possible to squeeze the equivalent cooling capacity of a twelve-meter tunnel into a cube that is two- to three-meters on a side within a reasonable ceiling height.

FBM offers dedicated mold cooling tunnels built on the folded approach using a “tower” geometry. Molds enter the cooling chamber and are lifted up on a vertical conveyor. When a mold reaches the top of the up tower it is moved to a down tower using pneumatic pushers that coordinate movement and timing. This approach makes it possible to build a cooling tunnel capable of holding up to 50 molds at a time in cube that is roughly one meter deep, two meters long, and two meters tall.

Both the folded and spiral approaches offer ways to expand cooling capacity in spaces where using a linear tunnel is not an option. And, depending on the mold capacity and orientation, a folded or spiral tunnel is almost certainly going to be less expensive than a linear tunnel of the same capacity.

Breaking the fourth wall ...

... linear tunnels can offer a benefit that folded and spiral tunnels generally do not, and that is the option to have multiple temperature zones. A longer linear tunnel could be divided into multiple zones where the entrance and exit sections would step down from ambient room temperature and a longer central section might be at a colder temperature. The theory is that this reduces temperature shock, but it is a really finicky point to ponder and is most applicable when the chocolate is very thin.

Comparing FBM’s Folded Towers and Selmi’s Spirals

Neither of these approaches is mechanically simple. FBM’s machines require not only a compressed air supply, but if there is more than one machine using the same source of compressed air there must be a pressure regulator on each line to ensure that the volume and pressure to each is constant, and the compressor must be sized to supply the necessary volume and pressure of air when all the machines attached to the supply are running at full speed. This is because the actuation time of pneumatic devices is governed by air pressure. If the pressure goes down the devices slow down. This can mess up the timing of everything and the molds may jam. Fortunately, this is an easy problem to solve – and avoid in the first place through proper planning.

Although it is not possible to expand an FBM Clima 50 to a Clima 100 after it is manufactured, the tower design means that the Clima 100 can handle 100 molds in a space in the same depth (about a meter), the same height (about two meters), and about twice the length of a Clima 50 (about four meters). That’s a lot of cooling capacity in a very small amount of floor space. Running at the recommended maximum continuous depositing speed of an FBM Unica, Maestria, or Jumbo – six molds/minute – that gives a maximum dwell time of sixteen-and-a-tad-minutes.

Other Considerations

I am assuming that the cooling capacity of the tunnels is sized appropriately by the manufacturer. There will be differences in airflow patterns but that is a topic for another time.

FBM’s folded tower design means there is a vertical spacing limit on the towers. This is more than 25mm, which is greater than the height of most polycarbonate molds but it is something to keep in mind.

The folded tower design also means, if you’re using a flood/scrape mold filling method the sides and tops of the molds need to be clear of chocolate or there is the risk of jamming.

Both spiral and folded tunnels should be able to accommodate 275 x 175mm and 275 x 205mm molds (but ask). When an FBM Clima is ordered you do need to let them know you want to run both sizes as last I heard, changing is accomplished in software, which changes timing parameters, it is not accomplished using photo-detectors or some other mechanism on the fly. Dynamically mixing mold sizes in the same production run should not an issue because most depositing situations require changing out the depositing plate assembly first  to change the mold size – in this case telling the cooling tunnel is something to remember to do. It could be an issue using a flood/scrape molding line, but I would not recommend using flood/scrape and any mold cooling tunnel because excess chocolate on the sides/top of the mold could gum up the works.

Making a Decision

Space, how much and the shape and height, is the first consideration. You want to allow at least one meter around all four sides of the tunnel and the tempering/molding/loader/transport – don’t forget a melter/feeder if you have one attached, and more than one meter at the exit end. Make a drawing of the whole thing to scale on your floor plan – or better yet, actually tape it off on the floor to help you visualize which will fit more comfortably in the space.

Imagine yourself taking molds out of the tunnel for hours at a stretch. Where do the molds go? On to what? If on trolleys how do you get the trolleys in and out of that space? Where does the demolding get done? By whom? What else is getting done in the space? Will people have to transit through the space to get from one part of the workshop to another in the middle of production? Can everything needed to keep the molding/cooling line working at full speed be kept close at hand?

When I purchased my first 35mm SLR back in the late 1970s I narrowed down the candidates to the ones that had the features I wanted. Then I went to a camera store and picked up every single one of them. I considered the placement of the controls – was a dial on the top plate to change the shutter speed more or less comfortable than one on the lens mount? – and how the camera felt in my hands, evaluating not only the position of all the controls, but the weight, balance, and more. In the end I purchased an Olympus OM1 because it fit my hands and felt right. It was not the cheapest nor was it the most expensive option and it did not have the cachet of the Nikon brand, but I never fought the camera itself to trip the shutter release.

Similarly, assuming I had the space and budget for every possible type of tunnel, I would most closely consider the workflow when I need to put molds in the loader (or have them near at hand if filling manually), and what has to happen after the molds exit the tunnel. If a square footprint restricts production workflow more than a rectangular on (in any serious fashion), that option may not be the best choice.

There are also ambient environment issues to take into consideration. One is how loud is the machine? There are refrigeration compressors inside. The speed and location of the fans, and other factors (blade shape) will determine how loud they are. Also, because these tunnels operate by refrigeration, all the heat from the chocolate has to go someplace, and that's back into the room. So – when considering these two factors some way to duct the heat out of the room might be necessary, and if there are highly sound-reflective surfaces that will bounce the sound around some form of sound-dampening should be on the BOM.

Other factors to consider are 1) the load limit of the floor, 2) the width and height of any doorways, and 3) whether or not there are any elevators or bends in any corridors that need to be navigated, and the width and height of the corridors.

I’ve had to remove equipment from crates on the street, remove pieces, then remove doors – frames and all – to get a piece of equipment to a desired location in a production facility. The moral of that story is to measure everything at least twice, and, if you can, build a model (on paper) and think hard about the mechanics of the move, especially if you need a pallet jack or a group of friends to load in. One fun question to ask is if the equipment can be tilted from the vertical to get it through a doorway.

If no option restricts production in any serious way, and I already owned a tempering/depositing solution from a company, then I would lean towards purchasing the cooling capacity from that company. However, not all versions of machines from the same manufacturer will “mate” cleanly, and that is definitely something to take into account when using machines from two different companies. One clear advantage FBM has in this regard is that because every machine is made to order, every machine can be customized. I helped sell an FBM tunnel to a company with a [name brand] enrober when they needed the tunnel sooner than [name brand] could deliver. FBM got the sale because they could customize the entrance section of the tunnel to fit the exact dimensions of the exit section of the [name brand] enrober.

Closing Thought

If I was looking to purchase a tempering/depositing/cooling solution I would imagine myself tending the machine for hours at a time and think about what I would come to hate the most about it and choose the one that offers the fewest obstacles to using the machine.

Listing image credit – Original by Марьян Блан | @marjanblan on Unsplash.

Your thoughts? Agree or disagree? Do you have specific experiences with cooling solutions you want to share? Leave a comment!