Overall Length vs Heated Length in PET Infrared Lamps

One of the most frequent procurement errors in plastic packaging plants is ordering an infrared replacement lamp specified only by a single “length” figure—for example, “350 mm lamp.” When the shipment arrives, technicians often find that while the glass tube fits into the oven cassette, the heated filament inside is 50 mm too short, resulting in cold preform shoulders and severe container wall thickness defects. A quartz infrared heating emitter has at least four interdependent axial dimensions. Understanding how these dimensions are defined, measured, and communicated is critical for seamless machine maintenance.

1. Defining the Core Dimensional Reference Points

A standard linear quartz infrared emitter consists of an active tungsten coil enclosed within a fused silica quartz tube, terminated at both ends by hermetic pinch seals and ceramic bases.

|<-------------------------------- Overall Length (OAL) -------------------------------->|
+ ---------+=========+=============================================+=========+---------+
| Ceramic |  Pinch  |                 Active Coiled               |  Pinch  | Ceramic |
|  Base   |  Seal   |               Tungsten Filament             |  Seal   |  Base   |
+ ---------+=========+=============================================+=========+---------+
          |<------->|                                             |<------->|
           Cold End                                                Cold End
          |<------------------------ Heated Length (HL) ------------------->|

Overall Length (OAL)

The total mechanical length of the lamp assembly from tip to tip. Depending on the base style, OAL is measured:

• For**SK15 ceramic bases:**From the outer ceramic face of one end cap to the outer ceramic face of the opposite end cap.
• For**R7s ceramic bases:**From the tip of the spring-loaded metal contact button to the opposing button.
• For**Clear bare-end tubes:**From the glass tip-off seal to the opposing glass end.
• Critical Fitment Constraint:* If the OAL is off by as little as $\pm 2.0\ \text{mm}$, the lamp will either fail to compress into its retention clips or rattle loosely, causing electrical arcing at the terminals.

Heated Length (HL) / Filament Length

The active span of the coiled tungsten filament that actually glows incandescents and radiates infrared energy when energized.

• Critical Process Constraint:* In a blow moulding oven, the heated length must match the exposure width of the preform travel path. If the heated length is narrower than the OEM design:

• Preforms entering or leaving the cassette face an unheated “shadow zone.”
• Preform shoulders or bases receive insufficient radiant flux, requiring the machine to run at slower speeds or higher voltages.

Conversely, if the heated length is too long, radiant energy spills directly into the oven frame, overheating transport chains and bearing lubricants.

Cold End Length (CL)

The distance from the end of the active coiled filament to the outer mechanical tip of the lamp. This zone includes:

1. The uncoiled straight tungsten lead wire.
2. The hermetic molybdenum foil ribbon embedded within the compressed quartz pinch seal.
3. The ceramic mounting collar and cement.

During normal operation, the temperature of the pinch seal must remain below**$350^\circ\text{C}$**. If the cold end is too short or lacks proper ventilation, excessive heat conducts from the filament into the molybdenum foil, causing thermal oxidation, foil embrittlement, and catastrophic vacuum failure (gas leakage).

2. Common Pitfalls on Supplier Technical Drawings

When comparing specifications from different lamp manufacturers, watch out for inconsistent terminology:

3. Physical Measurement Protocol: How to Measure an Existing Lamp

To capture accurate dimensional data from a broken or sample lamp in the maintenance shop:

1. **Use a Rigid Steel Rule on a Flat Surface:**Never measure quartz lamps with a flexible fabric tape measure. Place the lamp on a granite surface plate or flat steel workbench alongside a rigid metal ruler.
2. **Align the Zero Datum at the Ceramic Face:**Align the $0\ \text{mm}$ mark precisely with the outer ceramic edge. Record the overall tip-to-tip dimension (OAL).
3. **Locate the Filament Transition Points:**Look closely through the quartz tube under strong backlighting. Identify the exact point where the tungsten wire transitions from a tight, uniform coil into a straight lead wire attached to the molybdenum foil ribbon.
4. **Calculate Symmetrical Cold Ends:**Measure Cold End A and Cold End B. In standard linear emitters, cold ends are symmetrical (e.g., $35\ \text{mm}$ on each side). However, custom blow moulding ovens occasionally utilize asymmetrical cold ends to clear internal conveyor obstructions. Verify whether $\text{HL} = \text{OAL} - (\text{Cold End A} + \text{Cold End B})$.
5. **Measure Tube Outer Diameter (OD):**Use a digital vernier caliper to measure the quartz tube diameter at three points along the body. Standard diameters are $10\ \text{mm}$, $11\ \text{mm}$, $12\ \text{mm}$, and $19\ \text{mm}$. Ensure the caliper jaws do not pinch or scratch the quartz.

4. Photographic Evidence to Send with Inquiries

When requesting replacement lamps from manufacturers like Goldisgood, provide three clear photographs alongside your measurements:

+ ----------------------------------------------------------------------------+
| PHOTOGRAPHIC VERIFICATION PROTOCOL                                         |
|                                                                            |
| [Photo 1: Wide Shot]   --> Full lamp placed directly against a steel rule. |
| [Photo 2: End Detail]  --> Close-up of ceramic base, markings, lead wire.  |
| [Photo 3: Transition]  --> Close-up showing filament coil to foil joint.   |
+ ----------------------------------------------------------------------------+

Including these visual reference points eliminates dimensional ambiguity and guarantees that the replacement lamp installs smoothly on the first attempt.

Related Guides

• How to Specify a Replacement PET Preform Heating Lamp
• Clear vs Reflector Lamps in PET Reheating: What to Compare
• PET Preform Heating Zones Explained

Related Goldisgood Heating Solutions & Dimensional Verification

Eliminate dimensional ambiguity by working directly with an emitter manufacturer that provides fully dimensioned engineering drawings:

• **Engineered Heated Lengths (GIG-010 / GIG-011 Series):**Custom-configured overall length (OAL), heated length (HL), and cold end margins engineered to your exact oven cassette specifications. View Specifications on Goldisgood
• **Direct Terminal Integration:**Available in SK15 slotted ceramic bases, R7s caps, and custom high-temperature lead wire configurations.
• **Drawing Verification Service:**Send your existing lamp sample photos or CAD prints to info@goldisgood.com for formal dimensional cross-checking before order placement.

Sources & References

• Deutsches Institut für Normung / European Standard (DIN EN 62798): Industrial Infrared Emitters - Measurement Methods for Dimensional and Radiative Performance.
• International Electrotechnical Commission (IEC) 60061: Lamp Caps and Holders Together with Gauges for the Control of Interchangeability and Safety.
• American National Standards Institute (ANSI) C78 Standards for Electric Lamps: Dimensional and Operating Tolerances for Quartz-Tungsten Halogen Lamps.
• Goldisgood Engineering Specification: Dimensional Reference Standards for Single-Tube Quartz Infrared Emitters.