Quite a few years ago someone once said, "No matter how many millions of dollars one spends on a container glass melting furnace, it cannot make products without an orifice ring that costs less than $20." The price of the orifice ring will give you an idea of how long ago this was said, but the meaning still holds true today. Indeed, expendable refractories have increased in cost over the years like everything else, but they are relatively cheap compared to other components that help make glass container products, and they are just as essential to the glass maker as any other. They must conform to certain quality criteria, as minor defects on the glass contact surface of the refractory are often copied onto the glass gob, and out of tolerance dimensions on the refractory can lead to rejections in glass production and lower plant efficiencies.
The vast array of sizes and complexity of expendable shapes poses many challenges to the refractory manufacturer. There are only a handful of processes used to manufacture expendable refractories. None of them are easy, and Pyrotek like all other suppliers, is constantly trying to improve all parts of its manufacturing process to help the end-user produce consistently good glass.
Pyrotek has been manufacturing expendable refractories for a brief time compared to many of its competitors, but from the beginning decided to use a process that allows near net shape (NNS) casting, as this has several clear advantages over other processes. The definition of NNS is a process that produces parts to the required size and tolerance, thereby minimizing or excluding subsequent finishing steps such as machining. Near net shape is usually associated with the foundry industry, whereby the cast pieces require little or no machining.
"As we are manufacturing refractories, the tolerances do not have to be as accurate as engineered metal parts, but our process allows us to cast and fire unmachined critical dimensions to tolerances that are acceptable for their intended applications," says Pyrotek's Paul Chevolleau, a sales engineer and product manager.
Each supplier of expendable refractories will claim their processes own advantage. Chevolleau and others leaders at Pyrotek believe the NNS process has the following main benefits:
- Consistency
- Repeatability
- No blistering tendency
- Structural integrity
- Longer service life
- Reduced lead times
- Reduced material usage
Consistency
Left: Shows off-Center Machining. Right: Plunger in a jig showing the result of an off-center plunger shank.
When a critical diameter in a refractory is required, it is usually machined on a lathe or a device which turns the refractory against a fixed tool. Tubes for example help control the gob weight, and any deviation in set up or machining accuracy can lead to concentricity problems where the tube will turn elliptically, or out of true, and can lead to inconsistencies in gob weight. Parts such as plungers and stirrers require shanks that are accurate to less than 1 mm. Some 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. If not, plungers and stirrers can suffer from an off-center shank. The shank is where the metal chuck holds the plunger. An off-center shank can lead to the plunger nose being off-center with the orifice, which results in differences in gob weights in a multi-gob orifice ring. In the case of a stirrer, this will cause it to revolve elliptically and can lead to a disastrous breakage near the shank. A broken stirrer can have a domino effect and lead to other stirrers breaking and many hours of removing broken pieces of refractory from the forehearth and reinstalling new stirrers.
Repeatability
Before newly manufactured moulds are put on the shop floor to begin manufacturing expendables, Pyrotek quality control personnel check each critical dimension against engineering drawings. After confirming the mould is within tolerance, each piece made from the mould will be correct. This means that after firing, they will be within tolerance. This contrasts with processes that require parts to be machined, whereby each piece not only has to be set correctly in the machine but the operator must machine it accurately and the product measured again. Diamond tipped core drills are used to machine orifice ring gobs. These core drills wear each time they are used and must be checked regularly to see if they can maintain the required machined tolerance. As machining is not required with near net shape casting, this is never an issue.
Blistering Tendency
It is a well-known fact that one of the causes of blister in glass is the result of porosity in the expendable. When orifice rings are machined, the core drill cuts into the refractory and exposes any internal porosity. This releases nitrogen and carbon dioxide into the glass. In one case study, it showed that over $100,000 a year in container rejection and loss of production was attributable to this problem. The problem could not be fixed by filling the porosity, because the refractory filler fell out soon after it was exposed to molten glass. This issue was easily solved by switching to Pyrotek expendables - NNS production does not require any machining.
Structural Integrity
The result of core drill breaking through the internal surface of a feeder tube.
Compared to metal, refractories are very brittle, and machining can weaken the structural integrity of the ceramic body. An occasional problem with some 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.
Again, diamond-tipped core drills are generally used to machine the vents. Expendable refractories are deliberately made using extremely hard minerals that can withstand the harsh environment in molten glass. One common mineral is alumina, which is remarkably close in hardness to diamond on the MOH’s scale of hardness. Even using diamond core drills, it requires a lot of pressure to drill the refractory. This often results in breakout when the core drill eventually breaks through 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. "As we do not machine our refractories, this gives us a head start on our competitor’s machined products," says Chevolleau.
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 the Pyrotek plunger.
Reduced Lead Times
Pyrotek has a reputation for having class leading ex-works lead times. It can manufacture goods in as little as three weeks if production volumes allow, but Pyrotek's normal lead times are 5-6 weeks ex-works. "We can do this because we do not have to wet machine and then dry the products before packing, meaning we can cut out this lengthy procedure," Chevolleau says.
Reduced Material Usage
Pyrotek’s commitment to manufacturing sustainable products is supported by its NNS process. With no machining wastage in the process, Pyrotek helps its customer not only save raw materials, but also eliminates additional resource needs like energy and water to perform machining.
"During the relatively brief period we have been making expendable refractories, more manufacturers are turning to us because they can see clear benefits when using our products, including consistency, longer service life, and fewer production issues," Chevolleau says.
For more information, contact Product Manager and Sales Engineer Paul Chevolleau, pauche@pyrotek.com.
Tags: Glass, container glass, Refractories, Europe, Sustainability, Expendable Refractories.