Why Do I Have to Wait 5 Weeks for Custom Elements?
One of the more common questions we receive is, “why does it take so long to get MoSi2 and SiC heating elements?” We understand that the wait can be frustrating, especially since often times, the lead times can stretch from 4-6 weeks depending on how busy the factory is at the time of order. As a customer, this may come across as needless waiting at best, and at worst, a lack of care for your situation – maybe even poor planning.
The fact is, neither are true, and the hope is that this post can shed some light on the situation by providing some insight into the element manufacturing process.
Let’s Start at the Beginning
This may sound strange, but there is no such thing as an “off the shelf” heating element. This is true for both MoSi2 Super and SiC Silicon Carbide heating elements. The furnace OEM – the company that made your sintering oven – decides the size of the element, and this size is based on what they need a furnace that fits within a specific physical footprint to do.
Essentially, they start with a design spec. For example, “this furnace needs to sinter 5 kg of zirconia crowns in 8 hours, run from line voltage (120V or 220V), and sit on a standard sized dental lab desk.” From there, they start backing out the specifications of the furnace:
- How big does the box need to be to fit 5kg of zirconia crowns on appropriately sized trays?
- Based on the size of the furnace box, how much power does the furnace need to have in order to complete a sintering cycle in 8 hours?
- Now that we have a power number (usually measured in kW, or kilowatts), how many elements are required to get that amount of power?
- Elements have a maximum power loading that can be applied to them before the burn out/ fail. This is typically called surface loading, and it is expressed as power related to the surface area of the element (i.e. Watts/cm^2).
- Generally, a good rule of thumb is to design the furnace such that the elements are being run at a surface loading less than or equal to 80% of the maximum element loading.
- How does this all relate to to the power supply? Does the furnace need to run on a 120V/20A circuit? Is the customer limited when it comes to current (as in, is the furnace connected to a standard wall outlet)? Are there other control considerations?
- Once the furnace OEM has a figure on the number of elements, then they try and fit them into the furnace, keeping in mind recommended clearances and other design parameters. The furnace OEM can change the element dimensions as required to make the system fit.
That last part about changing the element dimensions is key, and this is where the Le, Lu, and a measurements come from – not to mention the terminal and hot zone outer diameters.
- The outer diameter of the terminal is set by finding an element that can handle the surface loading. There are 4 terminal ODs – 3mm, 4mm, 9mm, and 12mm – and each one of these has a set hot zone diameter, 6mm, 9mm, 18mm, and 24mm, respectively.
- The Le is the hot zone length, usually calculated to be close to the overall furnace height (taking into account clearances).
- The Lu is the Le plus the furnace insulation depth and the standard terminal length.
For SiC, the measurements are similar, but the Lu is the overall length, and the Le is referred to as the spiral length. The outer diameters specifications are less rigid as there are no standard ODs like MoSi2, but it is still a critical measurement that needs to be supplied for a quote.
The key takeaway from the last 200 words or so is that the furnace manufacturers create new element sizes to fit their designs. Since they don’t share those measurements with us (hmm, wonder why?), every element ends up being a custom element.
Time to Make Some Elements
Once we have the measurements for the elements, we send those measurements to our partner manufacturer in Sweden for MoSi2 and the UK for SiC elements. The manufacturer who we have partnered with is a major global manufacturer of both, and once they receive our order, they place it into their global manufacturing queue and send us an order confirmation with a target ship date.
Time to Ship Some Elements
Weeks later, when the elements are finished, they are shipped to New York City for entry into the US, and once they clear customs (Europe to clearing customs in the US takes approximately 1 week), they ship to the Dental Furnace Elements world HQ (another 2-3 days), and we try and turn them around to you in 24 hours or less.
We Don’t Like Waiting Either
Our ultimate goal is to stock elements for popular furnace types. We keep statistics on popular furnace types and the sizes of elements ordered by our customers. Once we start to see an uptick in run rates for specific furnace models, we consider bringing in stock to better serve our customers. If you are wondering if we have a particular element in stock, just drop us a line.