The 4-Head Welding Machine for PVC Windows: The Gold Standard of Industrial Window Production

The 4-head welding machine for PVC windows is the undisputed industry standard and the technological heart of modern, automated manufacturing of plastic windows and doors. These highly advanced systems are the key to efficiency, precision, and profitability. They are responsible for fusing four precisely cut PVC profiles into a complete, monolithic, permanently airtight, and structurally stable frame in a single, simultaneous operation. In a sector driven by cycle time, quality, and flawless aesthetics, the performance of four-head welding technology is the decisive factor for a window manufacturer’s competitiveness.

While single-head and two-head machines have their place in bespoke production or small shops, the four-head welder is the definition of industrial series production.

This comprehensive technical article dives deep into the technology, the operating principles, the various machine variants, and the future outlook for four-head welding machines. It serves as a thorough guide for technical managers, production planners, investors, and all professionals who want to understand the complexity and advantages of these impressive machines.

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What Is a 4-Head Welding Machine? A Technical Definition

To understand these systems’ complexity, we first need a clear definition of their function and how they differ from other joining methods.

Basic Function: Simultaneous Four-corner Welding

A four-head welding machine (often abbreviated as 4-head welder) is a stationary industrial system equipped with four separate welding units (heads). These heads are typically arranged in a square or rectangular layout.

Its core function is to weld the four 45° mitered corners of a frame (sash or outer frame) simultaneously. The operator (or a robot) places the four cut profiles (two jambs and two rails) into the machine. The machine clamps, positions, and welds all four corners in a single, uninterrupted cycle that often takes only 1.5 to 3 minutes.

Comparison with Single-head and Two-head Machines: A Quantum Leap in Productivity

• Single-head welder: Welds one corner at a time. Producing a frame requires four separate operations with manual rotation and re-positioning. This is slow (10–15 min/frame) and dimensional accuracy strongly depends on the operator. Ideal for special shapes (arches, angles).

• Two-head welder: Welds two corners at once. Often used as V-welding (one corner with two units) or parallel welding (e.g., for mullions/T-joints). A frame still needs two to three operations.

• 4-head welder: Welds all four corners in a single cycle. That is the decisive difference.

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Why Welding? The Need for a Material-Bonded Joint in PVC

The four-head welder is the technological answer to the specific properties of polyvinyl chloride (PVC).

PVC window profiles are complex multi-chamber systems. These chambers are crucial for thermal and acoustic insulation as well as for housing steel reinforcements. A mechanical joint (as used with aluminum or wood) would not hermetically seal these chambers. The result would be water and air leakage, severe thermal bridges, and inadequate corner strength.

By welding, the four-head machine creates a material-bonded joint: it melts the profile ends and fuses them inseparably, producing a monolithic, absolutely tight, and extremely strong corner—on all four corners at once.

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The Decisive Advantage: Why Four Heads Make the Difference

Choosing a four-head machine isn’t about luxury—it’s about economics and quality. Simultaneously processing all four corners offers three unbeatable benefits over sequential processes (as on single-head machines).

Unrivaled Dimensional Accuracy and Squareness

Perhaps the most important technical advantage. On a four-head machine, all four profiles are clamped simultaneously in an exact 90° position before welding begins. The machine acts as a high-precision jig.

Result: The finished frame is guaranteed square (exactly 90°) and dimensionally accurate.

Comparison: On a single-head machine, corners are joined one after another. Small cutting tolerances or minor positioning errors add up. The fourth “closing” corner often doesn’t fit perfectly, leading to stress in the frame.

A dimensionally accurate frame from a four-head machine simplifies all downstream processes (glazing, hardware installation) and guarantees the window’s tightness.

Maximum Productivity and Cycle-time Efficiency

The speed advantage is enormous. Consider a standard frame’s cycle time:

• Single-head machine: 4 welds + 4x handling (insert, rotate, remove). Realistic cycle time: 10–15 minutes.

• Four-head machine: 1 weld cycle + 1x handling (insert, remove). Realistic cycle time: 1.5–3 minutes.

Conservatively, a four-head welder is 4–6× faster than a single-head machine. It is the pacemaker that makes true line production possible (e.g., 150–200 units per shift).

Lower Labor Costs and Higher Process Reliability

Ideally, a four-head machine needs only one operator for loading and monitoring. To reach the same output with single-head machines would require 4–6 machines and 4–6 operators.

Moreover, the process is highly automated and repeatable. The machine runs the cycle identically every time. Human errors (mispositioning, wrong parameters) are minimized. This reduces scrap and raises overall quality.

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The Technological Basis: Hot-plate Butt Welding (“Mirror Welding”)

Four-head welding machines for PVC windows almost exclusively use hot-plate butt welding, commonly called mirror welding. It is the only process that reliably, deeply, and evenly heats the large and complex cross-sections of PVC multi-chamber profiles.

The Physics: Plasticizing, Diffusion, and Cooling

• Plasticizing: PVC is heated above its glass transition (~80 °C) and beyond its processing range, typically 240–260 °C, becoming a viscous melt.

• Diffusion: When two molten surfaces are pressed together, the long polymer chains interdiffuse.

• Cooling: As the melt solidifies, the previously separate chains are now inseparably entangled, forming a homogeneous, material-bonded joint.

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The Four-head Welding Cycle in Detail

A full welding cycle—often only 90 to 180 seconds—is a highly precise sequence with four phases.

Phase 1: Profile Loading and Precision Clamping (Shaped Jaws)

The operator inserts the four 45°-mitered profiles into the four machine heads. At the push of a button, pneumatic or hydraulic clamping devices fix the profiles.

These are shaped clamping jaws—tools machined as an exact negative of the profile’s cross-section. Since PVC multi-chamber profiles are relatively flexible, flat clamping would collapse the chambers under the high forging pressure (Phase 4). Shaped jaws support the profile inside and out, preserving its geometry. Profiles are positioned to within hundredths of a millimeter.

Phase 2: Heating (Plasticizing) – The Four Heating Plates

With the profiles clamped, the four heating plates (“mirrors,” one in each head) move between the profile ends.

• The plate: A massive metal plate (e.g., cast aluminum) electrically heated and PID-controlled to the exact setpoint (e.g., ~250 °C).

• Non-stick surface: The plate is covered with PTFE (Teflon) film or fabric so the molten PVC does not adhere.

• Process: The profiles are pressed against the plates with a defined heating pressure for the set heating time (e.g., 20–40 s), melting the surface to a depth of about 2–3 mm.

Phase 3: The Critical Changeover Time

After heating, the profiles retract a few millimeters; the four plates withdraw from the weld area as quickly as technically possible (often <2–3 s).

This changeover time is the most critical parameter. The 250 °C PVC melt cools extremely fast in ambient air (20 °C). If a surface “skin” forms (oxidation or cooling), diffusion in the next phase is impeded. The result is a “cold joint”—it may look fine but fails under load.

Phase 4: Forging and Cooling (Weld Formation)

Immediately after plate removal, the four heads (or clamped profiles) move together under high forge pressure to close all four corners.

• Forging: The pressure expels air and ensures intensive interdiffusion of polymer chains.

• Weld bead: Excess molten material is intentionally squeezed out, forming the characteristic weld bead (weld flash).

• Cooling: The profiles remain clamped under pressure (or reduced holding pressure) for the defined cooling time (e.g., 30–60 s) until the joint solidifies below Tg. Only then is the frame released and removed.

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The “Holy Trinity” of Welding Parameters

Weld quality is defined by the precise interplay of temperature, time, and pressure. In a four-head machine, these are stored as profile recipes.

Parameter 1: Precise Temperature Control

Mirror temperature is crucial—typically 240–260 °C for rigid PVC. Modern machines regulate each head individually within ±1–2 °C.

• Too high: PVC thermally degrades (“burns”), becomes brittle, and discolors (brown/yellow).

• Too low: Insufficient plasticization; diffusion is incomplete → cold joint.

Parameter 2: Time Management (Heat, Changeover, Cool)

• Heating time: Long enough to melt 2–3 mm deep, yet short enough to avoid degradation. Heavier 7-chamber profiles need longer than slim 3-chamber profiles.

• Changeover time: As short as technically possible.

• Cooling time: Long enough for full solidification under pressure to ensure geometric stability.

Parameter 3: Controlled Pressure (Heating vs. Forge Pressure)

• Heating pressure: Low, ensuring full-surface contact with the plate for optimal heat transfer.

• Forge pressure: High, ensuring thorough intermixing and final strength. Too high → “starved” joint (excess material expelled); too low → incomplete diffusion.

Each profile system (different suppliers, geometries, wall thicknesses, and compounds) requires its own validated recipe stored in the PLC/CNC.

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The Aesthetic Revolution: Four-head Welding and Zero-seam Technology

The biggest innovation of the last 15 years addressed an aesthetic challenge: colored and laminated profiles.

The Traditional Problem: Weld Bead on Colored Profiles

As demand shifts from white to colors (e.g., anthracite) and woodgrains, traditional welding faced a major issue:

• Weld bead (e.g., ~2 mm high) forms and is then milled off by a corner cleaner.

• Dilemma: The cutter removes not only the bead but also the surface color/foil layer.

• Result: An unsightly cleaning groove at the miter exposing a lighter core—ruining the premium look.

• Old “solution”: Manual touch-up with paint pens—costly and error-prone.

Zero-seam Principles (V-Perfect / Seamless Welding)

Modern four-head welders can be equipped with zero-seam (V-Perfect, seamless, contour-following) technology. It prevents the uncontrolled formation of exterior beads on visible surfaces.

Technical Implementations in Four-head Machines

• Mechanical limitation (e.g., 0.2 mm): Blades/stops on the plate or jaws limit melt flow at the surface, leaving only a tiny, barely visible line.

• Reforming/displacement: Moving tools (slides, blades) actively displace melt inward (into chambers) or into hidden areas (e.g., gasket grooves) during forging.

• Thermal forming: Precisely shaped, sometimes heated tools “iron” the miter during cooling so foil layers meet perfectly at the edge.

Benefits: A pristine corner like a single piece—or like a perfect wood miter.
For manufacturers: No manual touch-up, major labor savings, higher process security, premium product.
For end users: Superior aesthetics, higher perceived value, easier cleaning (no cleaning groove).
Companies like Evomatec have driven the development of such precise and robust solutions to open access to this market-leading technology.

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The Four-head Machine as the Heart of the Weld-and-Clean Line

A four-head welder never works alone in industrial production. It is the pacemaker and core of an integrated weld-and-clean line.

Pacemaker of the Line

The four-head machine’s cycle time (e.g., 2–3 minutes per frame) defines the speed of the entire line. Upstream and downstream processes must match this takt.

Downstream: Buffering, Turning, and Corner Cleaning

Immediately after welding, the warm frame is transferred automatically:

• Cooling buffer: Short rest for the corners to fully set.

• Turning station: The frame is rotated or turned as needed.

• Corner cleaning machine (CNC): The most important downstream step.

Why a Corner Cleaner Is (Almost) Always Needed

• Traditional welding: The cleaner must process all four corners—externally (aesthetic) and internally (functional)—milling/knifing away beads.

• Zero-seam welding: External milling is eliminated (or greatly reduced). Internal beads (in glazing and hardware grooves, gasket channels) still exist because melt was displaced there; functional internal cleaning remains essential.

Thanks to extensive project experience, we ensure that acceptance of such integrated lines meets the highest standards of quality and CE-compliant safety with meticulous attention to detail.

Horizontal vs. Vertical Line Concepts

Classic four-head machines run horizontally (profiles lying flat). Vertical weld-and-clean lines are increasingly common—space-saving and ideal for automated logistics with buffers and carts.

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Quality Assurance, Maintenance, and Safety (CE Conformity)

A four-head welder is a precision system. Consistent high quality depends on optimal maintenance and calibration.

Typical Welding Defects and Root Causes

• Cold joint (insufficient strength): Breaks at low load; fracture surface appears brittle/granular.
  Cause: Temperature too low, heating time too short, or (very often) changeover too long.

• Burnt joint (visual defect): Yellow/brown discoloration, brittleness.
  Cause: Temperature too high or heating time too long.

• Angular/size errors (distortion): Frame not exactly 90° or wrong dimensions.
  Cause: Mechanical misalignment (heads not calibrated), poor clamping (dirty jaws), cooling time too short.

Corner Strength Testing (Break Test)

Professional QA includes regular corner strength tests. Welded corners (or coupons) are loaded to failure in a test rig. Achieved values must meet system supplier specs and standards (e.g., DIN EN 514), validating correct parameters.

Critical Maintenance Points

• PTFE (Teflon) films: The most critical wear part on the four heating plates. Inspect and clean daily. Burnt PVC residues impair heat transfer and aesthetics; replace regularly.

• Shaped clamping jaws: PVC dust/chips clog contours; profiles no longer seat perfectly → dimensional errors.

• Guides and pneumatics/hydraulics: Heavy heads must run smoothly and precisely; stable air pressure is vital for consistent forging forces.

CE Conformity and Occupational Safety

Four-head welders present significant risks: >250 °C temperatures, high forces (often several tons), and fast, heavy assemblies. Compliance with the EU Machinery Directive (CE) is non-negotiable: guarding, light curtains, two-hand controls (during loading), and redundant E-stops. Our long-standing project expertise ensures inspections are performed with maximum care for both quality and CE-compliant safety—protecting operators and ensuring legal operation.

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Economics: ROI of a Four-head Welding Machine

A four-head welder is one of a window shop’s largest single investments.

CAPEX: From Single-head to Fully Automated Lines

• Used four-head (traditional): ~€30,000–70,000, depending on age/condition

• New four-head (standard, traditional): ~€90,000–160,000

• New four-head (with zero seam): ~€140,000–220,000

• Integrated weld-and-clean line (4-head, zero seam, automation): ~€250,000–500,000+

OPEX: Energy, Labor, Maintenance

• Energy: Heating four massive plates is the main consumer. Modern machines optimize heating, but demand remains significant.

• Labor: The biggest saving. An automated four-head line typically needs one operator.

• Wear parts: Regular replacement of PTFE films, knives, and cutters on the corner cleaner.

ROI Example

Upgrade from old single-head + manual cleaning to modern four-head weld-and-clean line (traditional):

• Old system (1-head + 2 people cleaning):
  Welding: ~12 min/frame (1 operator)
  Cleaning: ~10 min/frame (2 operators)
  Staff: 3 operators
  Output per shift (450 min): ~35–40 frames

• New system (4-head line):
  Line takt: ~3 min/frame (1 operator)
  Staff: 1 operator
  Output per shift (450 min): 150 frames

Result: Labor cost per window unit drops dramatically (often >80% reduction). Output quadruples. Even at an investment of ~€200k, payback through labor savings (eliminating two FTEs) and higher margin (more units sold) is often <2–3 years.

New Purchase vs. Used: Opportunities and Risks

• Wear: Guides/lead screws in heavy heads may be worn → dimensional errors.

• Controls: Obsolete PLCs can be hard to service.

• Technology: Used systems rarely offer zero-seam.

• Safety: Older machines may not meet current CE standards.

Expert evaluation is crucial. With deep experience across many customer projects, we ensure every legacy machine inspection is conducted with the utmost care for quality and CE-compliant safety, avoiding costly mistakes.

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Four-head Welders in Industry 4.0: Connected Production

Modern four-head welders are fully integrated into the smart factory.

Integration with ERP/PPS

Work orders (dimensions, profile type, color, quantity) are created in the office (ERP) and sent digitally to the machine. With CNC-driven heads, the machine auto-positions to the exact frame size.

Automatic Profile Identification and Recipe Management

Cut profiles are often labeled with barcodes. A scanner reads the code, identifies the profile (e.g., “Frame XY, 7-chamber, anthracite”), and automatically loads the correct welding recipe (temperature, time, pressure).

Predictive Maintenance and Remote Service

Machines monitor themselves—counting PTFE cycles and prompting replacement before quality suffers. With online access, service engineers (e.g., from Evomatec) can diagnose and often fix issues remotely, avoiding travel time.

Robotics and the “Lights-out” Cell

The next step is full automation: robots load profiles from the saw into the welder, remove the finished frame, feed it to the corner cleaner, and stack it.

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Outlook and Trends

Energy Efficiency & Sustainability (Recyclate Cores)

With rising energy costs, heating elements are optimized (faster warm-up, better insulation). Another trend is reliable welding of profiles with recyclate cores (new material outside, recycled PVC inside). Different melt behavior requires tighter temperature control.

AI-assisted Process Optimization & Quality Control

The future is self-optimizing equipment. Cameras can monitor bead formation or zero seams in real time. AI can detect deviations (e.g., due to bad material batches) and dynamically retune parameters to guarantee perfect results.

Beyond Mirror Welding?

Alternatives are being explored. Laser welding of plastics can produce extremely fine seams but remains very expensive and technically challenging for complex geometries and PVC (poor laser absorption). Infrared welding (non-contact) is a niche not yet widely adopted.

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Choosing the Right Four-head Machine: A Strategic Decision

This investment shapes a plant’s competitiveness for a decade or more.

Needs Analysis: Volume, Flexibility, Aesthetics

• Volume (productivity): Units per shift (e.g., 50, 100, 200+) define the automation level.

• Flexibility: Many special shapes (angles, arches)? A 4-head machine may need a flexible 1-head companion.

• Aesthetics (market positioning): Working with colored/laminated profiles? Zero-seam is practically a must.

The Importance of an Experienced System Partner

Selecting the right machine and integrating it into existing processes (sawing, hardware assembly, logistics) demands deep process knowledge. An experienced partner like Evomatec analyzes not only the machines but the entire workflow to avoid bottlenecks.

Our long-standing project experience guarantees that all commissioning and service operations are carried out with maximum diligence, ensuring adherence to quality standards and CE-compliant safety. This not only secures a smooth start but also protects the longevity and safety of your investment.

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FAQ – Frequently Asked Questions

What is the main advantage of a 4-head over a 1-head welding machine?

Twofold: productivity and precision. A four-head machine is 4–6× faster because it welds all four corners simultaneously (cycle ~2–3 minutes vs. 10–15 minutes). It is also far more precise, as the entire frame is fixed in a single clamping at exactly 90°, yielding perfectly dimensionally accurate frames.

What does “zero seam” mean—and can every 4-head machine do it?

“Zero seam” (also V-Perfect) is a modern welding technology that creates a visually seamless corner without the typical visible outer bead. No, not every four-head machine can do this—it is a special, often optional configuration with extra tools in the welding heads to shape or displace the melt. It is especially important when processing colored or laminated profiles to avoid manual touch-up with paint pens.

How long is a complete welding cycle on a four-head machine?

The full cycle (loading, clamping, heating, forging, cooling, release, unloading) for a standard frame typically takes 1.5–3 minutes on a modern four-head machine. Exact time depends heavily on the profile system (mass, number of chambers) and color, as these factors determine heating and cooling requirements.

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