Why does my bottle show a step at the parting line?

A step usually means the two mold halves are offset. Common causes are dirt or burrs on mounting faces, worn dowel pins or bushings, uneven bolt tightening, or a worn clamping base. Check the station that produces the step first.

Is flash at the parting line always caused by the clamping base?

No. Flash can also come from worn parting faces on the mold halves, insufficient clamping or locking force, or damage to the mold edges. If the mold halves are in good condition and flash remains, check the clamping base mounting and locating features.

Should I choose an aluminum or a steel clamping base?

Follow the machine design. Aluminum is lighter and conducts heat well; steel is stiffer and more wear resistant. LIHE machines clamping bases from high-strength aluminum alloy or steel according to machine requirements.

How often should clamping base alignment be checked?

Check it at every mold change, after any collision or blow-out, and whenever parting line or base defects appear in one station. Logging the results per station shows when wear starts.

What information is needed to order a replacement clamping base?

Send the machine make and model, the existing part or its drawing, the cooling connection layout and the bottle drawing. LIHE makes clamping bases from customers' existing parts or drawings.

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Accessories

Clamping Base

Clamping bases (mold carrier bases) that hold the blow mold shells and base mold in the blowing station. Accurate mounting faces and locating features keep the two mold halves and the base aligned under blowing pressure, giving a clean parting line and consistent bottle dimensions.

Function: Mounts and locates the blow mold shells and base mold in the mold carrier.
Accuracy: Precision-machined mounting and locating surfaces for repeatable alignment after every mold change.
Cooling: Water connections and channels matched to the mold's cooling circuit.
Machine compatibility: Made from customers' existing parts or drawings for SIDEL, KRONES, KHS and other blow molding machines.
Material: Machined from high-strength aluminum alloy or steel according to machine requirements.

Quotes and technical questions are handled directly by the manufacturer's international sales department.

Blow mold clamping base alignment: parting line quality, remounting and aluminum vs steel

Updated October 13, 2026Author: LIHE Precision Engineering Team

A clamping base is a machined base plate that connects the blow mold halves and the base mold to the mold carrier of a stretch blow molding machine and fixes their position. Because every mold change starts and ends at the clamping base, its mounting faces and locating features decide whether the two bottle halves meet exactly at the parting line. This guide explains what misalignment does to bottles, how to inspect and remount a clamping base, and how aluminum and steel bases compare in general terms.

What clamping base misalignment causes in PET blow molds

When the clamping base, shells or base mold do not sit in their intended position, the mold halves no longer close on a common plane. The result is visible on the bottle long before the mold is damaged.

Parting line mismatch: a step between the two bottle halves that can be felt with a fingernail and is visible on labels and transparent bottles.
Flash or a raised seam: material pushed into a gap between halves that do not close fully under blowing pressure.
Base mismatch: an off-center or stepped base when the base mold is not concentric with the side halves.
Dimensional variation: bottle diameter, ovality or height that differs between stations of the same machine.
Accelerated wear: uneven contact concentrates load on part of the parting face, which wears shells and inserts faster.
Cooling problems: shifted mounting can strain water connections and cause leaks or reduced flow in one mold half.
Heat-set (hot fill) molds run with heated walls for bottles filled at about 85–95°C, and standard PET deforms above about 60°C, so a clamping base for a heat-set mold must hold alignment while the mold is hot.

Where alignment comes from: mounting faces, locating features and fasteners

Mounting faces: flat, clean contact surfaces between the carrier, the clamping base and the shell or mold half. Dents, burrs or dried deposits tilt the mold.
Locating features: dowel pins, keys, bushings or locating shoulders that set the position of each part. Worn pins and enlarged holes allow movement after each mold change.
Fasteners: bolts tightened to the machine maker's specified torque in the specified order. Uneven tightening can pull a half out of position.
Base mold guidance: the base mold must be centered relative to the side halves at the closed position; its guide and lift mechanism also affect the base parting line.
Temperature: heated or cooled molds expand and contract, so alignment should be checked at operating temperature as well as cold.

Aluminum vs steel clamping bases: general comparison table

LIHE machines clamping bases from high-strength aluminum alloy or steel according to machine requirements. The table compares the two materials in general terms; the right choice depends on the machine design, not on a single property.

Property	Aluminum alloy base	Steel base
Density	About 2.7 g/cm³, so parts are light and easier to handle during mold changes	About 7.8 g/cm³, so parts are heavier and may need lifting aids
Thermal conductivity	High; heat moves quickly between mold and cooling circuit	Lower; temperature changes more slowly
Wear and dent resistance	Softer surfaces; locating features may need hardened inserts or bushings	Harder surfaces; locating features hold their size longer
Machinability	Fast to machine to tight tolerances	Slower to machine; may need heat treatment and grinding
Corrosion in water channels	Depends on alloy and surface treatment; water quality matters	Carbon steel can rust unless protected or made from corrosion-resistant grade
Typical use	Machines that require light mold carrier parts and fast changes	Machines or positions that require high stiffness and wear resistance

Inspection steps before remounting a clamping base

Step 1: Stop the machine, follow the lockout procedure, release blowing air and drain the cooling circuit of the mold station.
Step 2: Remove the mold halves and base mold and clean all mounting faces on the carrier and clamping base with a non-abrasive method.
Step 3: Inspect mounting faces for dents, burrs, corrosion and deposits; remove raised burrs carefully without changing the face geometry.
Step 4: Check dowel pins, keys and locating bores for wear, looseness and enlarged diameters, and replace worn locating parts.
Step 5: Check flatness of the clamping base on a surface plate or with a straightedge and feeler gauges, and compare with the drawing.
Step 6: Inspect water connections and seals, and replace seals that are flattened or cracked.
Step 7: Record the findings per station so that recurring problems on one position can be traced.

Remounting steps for clean parting lines

Step 1: Mount the clamping base on its locating features first, then tighten the fasteners to the specified torque in a cross pattern.
Step 2: Mount the mold halves on the clamping base, engaging the locating pins before any bolt is tightened.
Step 3: Close the mold slowly in setup mode and check that both halves meet evenly; a contact check with marking compound shows gaps on the parting faces.
Step 4: Check centering and height of the base mold in the closed position.
Step 5: Connect cooling water, pressure the circuit and check every connection for leaks before heating or blowing.
Step 6: Run a short trial, then inspect the parting line, base center and key dimensions of bottles from each station.
Step 7: Keep the first bottles from each station as reference samples for the next mold change.

Clamping base replacement and dimensional verification

Replace a clamping base when its locating bores or mounting faces are worn beyond what new pins or bushings can correct.
LIHE makes clamping bases from customers' existing parts or drawings for SIDEL, KRONES, KHS and other blow molding machines.
Water connections and channels are matched to the mold's cooling circuit, so send the existing cooling layout with the order.
LIHE has an in-house Zeiss CMM lab, and inspection reports are supplied on request.
LIHE's stated lead time for replacement parts is 4 weeks; plan replacements together with scheduled mold changes.
For hot fill bottles, where the mold runs at a different temperature than for cold fill (filling at about 85–95°C), state the fill type so the base and its cooling connections suit the application.

Frequently asked questions

Why does my bottle show a step at the parting line?: A step usually means the two mold halves are offset. Common causes are dirt or burrs on mounting faces, worn dowel pins or bushings, uneven bolt tightening, or a worn clamping base. Check the station that produces the step first.
Is flash at the parting line always caused by the clamping base?: No. Flash can also come from worn parting faces on the mold halves, insufficient clamping or locking force, or damage to the mold edges. If the mold halves are in good condition and flash remains, check the clamping base mounting and locating features.
Should I choose an aluminum or a steel clamping base?: Follow the machine design. Aluminum is lighter and conducts heat well; steel is stiffer and more wear resistant. LIHE machines clamping bases from high-strength aluminum alloy or steel according to machine requirements.
How often should clamping base alignment be checked?: Check it at every mold change, after any collision or blow-out, and whenever parting line or base defects appear in one station. Logging the results per station shows when wear starts.
What information is needed to order a replacement clamping base?: Send the machine make and model, the existing part or its drawing, the cooling connection layout and the bottle drawing. LIHE makes clamping bases from customers' existing parts or drawings.

Clamping Base

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