S ome in the glass industry will be unaware that Pyrotek has been a big name in the aluminum industry for many decades. Its more recent success and growth in the glass industry has been the result of recruiting the best academics and investing heavily in fully equipped laboratories to develop leading-edge technologies. About 15 years ago, the company decided to use its metals industry refractory know-how to manufacture expendable refractories for the glass industry. Although it's still relatively new to the glass industry compared to others, it has proven that choosing the right manufacturing process can make a newcomer a leading supplier.
From the outset, Pyrotek decided to use a process that allows near net shape (NNS) casting, as this has several clear advantages over other processes. The definition of "near net shape" is a process that produces parts to a required size and tolerance, which minimizes or eliminates subsequent finishing steps, such as machining. NNS casting is usually associated with metal foundries, in which the cast metal pieces require little or no machining. When Pyrotek manufactures refractories, the tolerances do not have to be as exact as engineered metal parts, but its process allows it to cast and fire unmachined critical dimensions to tolerances that are acceptable for the intended applications in the industry. The shrinkage in its refractory products is so minimal that its products do not have to be made oversized to be machined to the correct critical dimensions. This alleviates a few problems, namely, excessive porosity, shrinkage cracks, structural integrity issues, lower density, and extended manufacturing times.
The primary advantages of the NNS process over some more common processes:
- Consistency
- Repeatability
- No blistering tendency
- Structural integrity
- Longer service life
- Reduced lead times
- Sustainable manufacturing
Consistency
Left: Shows off-center machining. Right: Plunger in a jig showing the result of an off-center plunger shank.
Parts such as plungers and stirrers need shanks that are accurate to less than 1 millimeter. Most manufacturers do this on a lathe and some use a pillar drill with a diamond tipped core drill. Both these methods of machining require each expendable piece to be set correctly in the machine each time. If not, plungers and stirrers can suffer from an off-center shank. When the shank is off-center, the chuck cannot correct this irregularity and the nose of the plunger will not be in line with the orifice, resulting in deviations in gob weights in multi-gob orifice rings.
An off-center shank on a stirrer will cause it to revolve elliptically, and those which deviate too much might lead to a disastrous breakage near the shank. A broken stirrer often has a domino effect, leading to other stirrers breaking and many hours of retrieving broken pieces of refractory from the forehearth and reinstalling new stirrers. A full shift for plant workers can be lost in the worst case.
These scenarios are never an issue with Pyrotek’s NNS process, as the shanks do not require any machining and are cast to very close tolerances.
Repeatability
Before newly manufactured molds are put on the shop floor to begin manufacturing expendables, Pyrotek's quality control personnel check each critical dimension against engineering drawings. Once it's confirmed that the mold is within tolerance, each piece made from the mold will be correct. This means that after firing, they will be within tolerance. This contrasts with processes that require every part to be machined independently, where each piece not only has to be set correctly in the machine, but the operator must machine it accurately and the product checked for accuracy. Diamond tipped core drills are used to machine orifice ring gobs and these drills wear by a small amount each time they are used. As such, the hole diameters will change over time and they must be monitored to ensure they don’t fall out of tolerance. As machining is not needed with Pyrotek’s NNS process, this is never an issue.
No Blistering Tendency
Most of Pyrotek's competitor’s orifice rings require the orifices to be machined to size, and the machining by core drills exposes internal porosity. It is well known that one of the causes of blister in glass is from porosity in expendable refractories. When in contact with hot glass, the porosity will release nitrogen and carbon dioxide into the glass. In one case study, it showed that more than $100,000 a year in container rejections and loss of production was attributable to this problem. The problem could not be fixed by patching the porosity with refractory filler, because the refractory filler fell out soon after it was exposed to molten glass. This made the problem worse by having refractory stones in the gobs as well as blister. This issue was easily solved by switching to Pyrotek expendables, as its NNS production does not require any machining.
Structural Integrity
The result of core drill breaking through the internal surface of a feeder tube.
Refractories are very brittle by their nature compared to metal, and machining can weaken the structural integrity of the ceramic body. An occasional problem with feeder tubes is cracking between the vents. This is usually a sign of thermal shock, but the susceptibility to thermal shock is heightened when vents are machined around the circumference of the tube.
Expendable refractories are deliberately made using extremely hard minerals that can withstand the harsh environment in molten glass. Again, diamond tipped core drills are needed to machine the vents.
One common chemical compound is alumina, which is remarkably close to diamond on the MOH’s scale of hardness. Even using diamond core drills, it requires constant pressure to drill through the hard refractory. This often results in breakout when the core drill eventually breaks through to the inside of the tube.
Breakout forces large refractory grains to break away from the main refractory body, leaving a rough, friable, and somewhat weakened edge around the outlet of the vent.
Longer Service Life
When the outer layer of a refractory is compromised by machining, glass corrosion speeds up, especially when there is internal porosity. Since Pyrotek doesn't machine its refractories, this gives it a head start against competitors' machined products. Pyrotek refractory formulations have also been designed to have low internal porosity as the picture below illustrates.
This picture shows two plungers that were removed after use in green glass. The top plunger is a competitor's plunger, and the bottom plunger is a Pyrotek plunger. There is a clear lack of internal porosity in the Pyrotek plunger, and this helps reduce glass corrosion, leading to longer service life.
Reduced Lead Times
Pyrotek has a reputation for having class-leading ex-works lead times. It can manufacture goods in as little as two weeks if production volumes allow, but Pyrotek's normal lead times are 5-6 weeks ex-works. This is possible because Pyrotek doesn't wet the machine, re-measure, and then dry the products before packing, cutting these lengthy procedures.
Reduced Material Usage
Pyrotek’s commitment to manufacturing sustainable products is supported by its NNS process. With no machining wastage in the process, Pyrotek not only saves on raw materials needed, it also eliminates the water and energy that would be needed for machining.
Summary
In a relatively short period of time Pyrotek has grown very quickly to become one of the leading global suppliers of expendables to the container glass industry. This phenomenal growth is testament to how its customers value its products as they can clearly see the advantages outlined above, compared to other suppliers’ products. More glass manufacturers are turning to Pyrotek because they experience benefits when using its products. These include consistency, longer service life, fewer production issues, and better glass quality.
The vast array of shapes and sizes of feeder expendables pose many challenges to every refractory manufacturer. There are only a handful of processes used to manufacture expendable refractories and none of them are simple, but Pyrotek passionately believes it has the best technology to produce the most consistent refractories in the market.
For more information, contact Product Manager and Sales Engineer Paul Chevolleau, pauche@pyrotek.com.
Tags: Glass, container glass, Refractories, Europe, Sustainability, Expendable Refractories.