Molybdenum Electrodes for All-Electric Pharmaceutical Glass Furnaces: Engineering Checks for the Borosilicate Upgrade

Pharmaceutical glass is going through a material transition: consistency evaluation for injectables is pushing low borosilicate toward medium borosilicate, whose B₂O₃ content is roughly 6%–11% and whose water resistance must reach Class 1. Capacity expansions for molded vials, tubing vials and prefilled syringes land mostly on all-electric furnaces, the glass category with the highest all-electric penetration. The melting zone is heated by rod-shaped molybdenum electrodes in the bottom or sidewall, and electrode specifications are tied to refining practice and the batch blanket structure. Small-batch demand for prefilled syringes and biopharmaceutical packaging is also driving more small-capacity electric furnaces into service.

Why the Batch Blanket Is Tied to Medium Borosilicate

All-electric melting brings three direct benefits for medium borosilicate. The batch blanket holds the surface temperature down, confining boron and alkali volatilization to a narrow range and stabilizing glass composition, which is critical to delamination performance and alkali leaching in pharmaceutical glass; there are no combustion products in the melt, so fewer bubble and cord sources arise, and with sulfate refining the process can dispense with arsenic and antimony refining agents, matching pharmacopoeia residual limits; furnace capacity is typically 10–60 tons/day, allowing flexible scheduling and low changeover cost across formats. Medium borosilicate melt has relatively high conductivity, so the power supply design is in fact comparatively economical.

The blanket thickness must be adjusted to the magnitude of volatile components; uneven surface coverage lets boron volatilization rise again. Melting-zone electrodes are mostly inserted vertically through the bottom, with sidewall layouts used mainly for supplementary power. If a local area of the blanket collapses during operation, the current density there rises instantaneously and end wear accelerates, so melt level and surface shape should be part of the shift inspection rather than only the temperature curve.

A Formulation Change Means a New Set of Power Parameters

Converting low borosilicate to medium borosilicate is not a one-line recipe change. As boron content rises, melt viscosity and conductivity change together, and the supply voltage, electrode spacing and forehearth temperature drop of the old furnace all need rechecking; some furnaces that have run for many years are simply rebuilt, which explains why new furnaces and electrode orders often appear together. The tubing vial route adds another layer: medium borosilicate must first be melted into highly homogeneous gobs and then remelted for tube drawing, and the electric heating and molybdenum components of the second melting line form a separate consumption stream. Checking the two stages separately keeps forehearth and orifice parts from being missed.

A common mistake in retrofit projects is converting power only from the original furnace tonnage. The melting temperature range of medium borosilicate is close to that of low borosilicate, but refining temperature and homogenization demands are higher, so the same daily melt often needs a higher specific power density, with electrode count and diameter changing accordingly. If the old electrode holes are retained, power can only be made up by raising current density, loading the ends more heavily and shortening life. The sound approach is to define power zoning first, then work back to electrode count and layout, then check brick openings and installation depth.

Electrode Layout and Power Zoning

Pharmaceutical electric melting furnaces generally divide the melting zone into several power zones corresponding to the charging zone, the hot spot and the refining zone. The charging zone takes the raw batch heating and sees wide load swings; the hot spot determines the convection loop and homogenization; the refining zone holds high temperature so bubbles have time to rise. The electrodes of the three zones may share one power supply or be regulated independently by zone; the latter is friendlier to formulation changes but has more complex control and protection logic.

The table below gives configuration ranges for common capacities, for scheme discussion only; actual count and diameter must be determined from the furnace heat balance.

On standards, electrode products are controlled to YS/T 1312-2019 Molybdenum Electrodes; molybdenum rods per ASTM B387 / GB/T 17792; molybdenum plates and discs per ASTM B386 / GB/T 3876, with acceptance based on the quality certificate and measured records.

The insertion depth of bottom electrodes directly affects end temperature and glass flow erosion. Too shallow, and the end sits near the bottom brick joints, where it is easily enveloped by deposits; too deep, and the end enters the high-temperature convection zone, accelerating wear. During installation, record the measured brick opening size, the electrode length allowance and the permitted deviation, so the site does not fit parts by experience and damage the dense surface layer.

Sulfate Refining and Bubble Control

Medium borosilicate pharmaceutical glass mostly uses sulfate refining, and the refining temperature window is tied to the redox state of the glass melt. Uneven electrode power allocation can leave local temperatures below the range needed for refining, so bubbles are carried into the forming section before they can rise, showing up as bubbles or cords in the vial body. When investigating such defects, first check whether refining-zone power is being squeezed by the hot spot, then whether the blanket surface is locally overcooled, and only then suspect raw materials and refractories.

Another easily overlooked factor is the electrode surface condition. If processing oil or an oxide layer remains on a new electrode when it enters the furnace, it releases gas initially and briefly increases the bubble count. Standard practice is to clean and preheat before installation and to run a staged heating curve rather than applying full power at once. When bubbles persist, use the electrode replacement record to determine whether a particular electrode is involved instead of adjusting the formulation across the board.

Consumption Rhythm and Replacement Windows

Electrode consumption in pharmaceutical furnaces usually shows as slow end thinning and local pits; the former relates to current density and running hours, the latter mostly to glass flow erosion and melt-surface fluctuation. Unlike flame furnaces, all-electric furnaces have no maintenance season, so electrode changes are often scheduled into planned shutdowns or load-reduction windows; electrode life data should therefore be kept per furnace, recording each electrode's installation position, accumulated running hours and reason for replacement.

For spares strategy, we recommend stocking common specifications equal to 20% of the electrodes in service, with separate stock for the cold-repair plan. Small furnaces have varied specifications and frequent changeovers, so electrodes of similar diameter can be made to a universal length and cut on site; large furnaces should lock the drawing once and order accordingly to reduce site machining. Purchase contracts should state batch traceability, test reports with the shipment, and time limits for handling breakage and transport damage, terms that matter especially when the shutdown window is tight.

Quality System and Traceability Requirements

Pharmaceutical glass is a pharmaceutical packaging material, and associated review requires a stable source of raw and auxiliary materials with controlled changes. Although electrodes do not directly contact the drug, their impurity release affects the water resistance of the glass, so supplier batch management, test items and change notification should be written into the quality agreement. Incoming acceptance should at least check the material certificate, density and dimensions, with end-structure sampling where necessary.

For new lines, review the electrode technical agreement together with the furnace thermal calculation, specifying power zoning, permitted current density and cooling conditions. For retrofit projects, recheck the old furnace's brick openings and power supply capacity before fixing electrode specifications, so electrodes do not arrive and then prove impossible to install. Product parameters can be referenced in the selection points for this duty and on the molybdenum rod page.

Capacity, power and consumption figures in this article are common industry ranges for scheme discussion and budget estimates; actual selection should be based on the furnace heat balance, operating conditions and the technical agreement between the parties.