The Corner Welding Machine for PVC Windows: The Heart of Modern Window Production

The corner welding machine for PVC windows is the decisive component in the modern manufacture of plastic windows and doors. Without these highly specialized industrial systems, the efficient, stable and weather-resistant production of airtight PVC frames as we know them today would simply not be possible. It is the technological core that joins precisely cut PVC profiles into a monolithic, dimensionally stable frame. In an industry driven by precision, speed, and flawless aesthetics, the performance of corner-welding technology is a direct indicator of end-product quality and a window maker’s competitiveness.

This article offers a deep and comprehensive look into the world of these fascinating machines. We analyze the physical fundamentals of the welding process, compare different machine types from single-head to eight-head machines, trace the historical development from manual corners to fully automated zero-seam solutions, and discuss the economic as well as forward-looking aspects of this indispensable technology.

---

What exactly is a corner welding machine for PVC windows?

To understand the complexity and importance of these systems, a clear definition and demarcation are essential. The term “corner welding machine” describes its function precisely: it joins corners.

Formal definition: From profile to frame

A corner welding machine for PVC windows is a system specialized in permanently joining the miter-cut ends (usually 45 degrees) of rigid PVC profiles by a hot-plate welding process (also called mirror welding).

The core function is to create a cohesive (material) bond. Unlike a form-fit (e.g., screws) or force-fit (e.g., clamping) connection, the molecular chains of the parts to be joined are remeshed (interdiffused) by melting (plasticizing) and subsequent pressing together. After cooling, a homogeneous, monolithic joint is formed that ideally has the same or even higher strength than the base material itself.

---

The core principle: Why welding instead of screwing or gluing?

The choice of joining method fundamentally depends on the frame material. The corner welding machine is the technological answer to the specific properties of polyvinyl chloride.

• Wood window frames: traditionally mechanically joined (e.g., mortise and tenon, dowels) and glued.

• Aluminum window frames: not welded. They are mechanically joined by inserting corner cleats into the hollow chambers and then bonding, pinning, or crimping.

• PVC window profiles are complex multi-chamber systems. These chambers are crucial for thermal and acoustic insulation and for receiving steel reinforcements. A mechanical corner joint (as in aluminum) would not hermetically seal these chambers—leading to water and air leakage, major thermal bridges, and insufficient corner strength.

Welding is the only method that guarantees an absolutely tight, highly stable, and automatable corner joint for these hollow-chamber profiles in a matter of seconds.

---

Terminology: Corner welding machine vs. profile welding machine

The terms are often used synonymously. “Corner welding machine” is the more precise designation for the window industry, because it describes the main function—joining 90-degree corners. “Profile welding machine” is the broader technical term that can also include machines that weld butt joints or T-joints (mullions), which modern corner welders often handle as well.

---

The fundamental technology: Hot-plate butt welding (mirror welding)

Almost all corner welding machines for PVC windows operate on the principle of hot-plate butt welding—colloquially known as mirror welding. It is the only process that can heat the large and complex cross-sections of PVC hollow-chamber profiles reliably, deeply, and uniformly.

Physical basics: Plasticizing, diffusion, cooling

• Plasticizing: PVC is heated above its glass transition (~80 °C) and melting range to a processing temperature of approx. 240–260 °C. The material becomes a viscous melt.

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

• Cooling: Upon cooling, the melt solidifies. The polymer chains are now inseparably entangled—forming a homogeneous, cohesive bond.

---

The welding cycle in detail: A four-phase precision process

A complete welding cycle—often only 1.5 to 3 minutes depending on profile and machine—is a highly precise sequence divided into four phases.

Phase 1: Profile loading and precision clamping (contour jaws)

Cut profiles (usually 45° miters) are inserted and fixed by pneumatic or hydraulic clamping devices. These are contour jaws—tools milled as the exact negative of the profile cross-section.

Why it matters: PVC hollow-chamber profiles are relatively unstable. Clamping with flat plates would cause the chambers to collapse under the high forging pressure (Phase 4). Form-fitting contour jaws support the profile inside and out so it retains its shape. Profiles are positioned to within hundredths of a millimeter.

Phase 2: Heating (plasticizing) – the hot plate (“mirror”)

The heated “mirror” (one or more hot plates) moves between the profile ends.

• Mirror: a massive metal plate (e.g., cast aluminum), electrically heated and precisely controlled (PID) to the set temperature (e.g., 250 °C).

• Coating: anti-stick (usually PTFE/Teflon film or fabric) to prevent molten PVC from adhering.

• Process: Profiles are pressed against the mirror with a defined heating pressure. Heat penetrates for the set heating time (e.g., 20–40 s), melting approx. 2–3 mm of material depth.

Phase 3: The critical changeover time (racing against cooling)

After heating, the profiles retract a few millimeters. The mirror exits the weld zone as fast 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 “skin” forms (oxidation/cooling), polymer diffusion in the next phase is impaired—resulting in a cold weld that appears intact but fails under load.

Phase 4: Forging and cooling (joint formation)

Immediately after mirror withdrawal, the molten profile ends are pressed together with high forging pressure.

• Forging: the pressure (much higher than the heating pressure) compresses the melt zones, expels air, and drives intensive interdiffusion.

• Weld bead: excess molten material is displaced in a controlled way, forming the characteristic weld bead (weld flash).

• Cooling: Profiles remain clamped under pressure (or hold pressure) for a defined cooling time (e.g., 30–60 s) until the joint solidifies below Tg. Premature release can tear the soft joint or distort the frame due to shrinkage stresses.

---

The “holy trinity” of welding parameters: Temperature, time, pressure

Joint quality is not determined by the machine alone but by the exact interplay of these three parameters. They must be determined for every profile system (wall thicknesses, chamber count, material recipes) and stored in the PLC/CNC as a recipe.

Temperature (burn-off vs. cold weld)

Typical hot-plate temperature for rigid PVC: 240–260 °C.

• Too high: thermal degradation, HCl release, embrittlement, yellow/brown discoloration → unusable joint.

• Too low: insufficient plasticization, incomplete diffusion → cold weld with poor strength.

Time (heating, changeover, cooling)

• Heating time: long enough to melt to the required depth, but short enough to avoid burning; massive 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 and dimensional stability.

Pressure (heating vs. forging pressure)

• Heating pressure: relatively low—ensures full-surface contact with the mirror for optimal heat transfer.

• Forging pressure: high—ensures intermixing and final strength. Too high → “starved” joint (excessive squeeze-out); too low → incomplete diffusion.

---

The weld bead (flash): Quality indicator and technical necessity

In traditional welding the weld bead is an important quality indicator: a uniform, fully formed bead (typically 2–3 mm high) shows the process ran correctly (sufficient melt, adequate pressure). At the same time, it is a technical necessity that later must be removed—bringing us to the weld-and-clean line.

---

Typology of corner welding machines: From workshop to industrial line

The market is highly segmented and offers suitable technology for every operation—from one-person workshops to fully automated industrial plants. The decisive differentiator is the number of heads (welding units).

Single-head welding machine (1-head)

Basics: one welding unit.

• Operation: The operator performs four separate welds for one frame (corner 1, rotate/add, corner 2, etc.).

• Use cases: specialist for specials—its biggest strength is flexibility. Modern single-head machines can continuously weld angles from 30° to 180°, ideal for:

  • Slanted windows (acute/obtuse angles)

  • Arched windows (segment-wise welding of arch elements)

  • Gable elements

• Pros: lowest purchase cost, small footprint, maximum flexibility.

• Cons: very low productivity (10–15 min per frame), high labor cost per unit; dimensional accuracy strongly depends on cut precision and operator care.

• Target users: small shops, repair shops, special-build departments in large plants.

Two-head welding machine (2-head)

The flexible middle ground, typically in two variants:

• V-welding (corner welding): two units at 90° to form a corner (rare for PVC).

• Parallel welding (mullion welding): two units work in parallel.

Typical use: parallel welding for T-joints (welding a mullion into a frame). Alternatively, a frame is produced in two steps (two “U” halves welded, then closed).

• Pros: much faster than 1-head, more flexible (and cheaper) than 4-head.

• Cons: still at least two operations for a closed frame—dimensional accuracy can suffer.

• Target users: SMEs that need higher output than 1-head but don’t load a 4-head line, or frequently weld mullions.

Four-head welding machine (4-head) – The undisputed industry standard

By far the most widely used in industrial frame production.

• Operation: four welding units arranged in a square (one per corner). The operator (or automation) loads all four cut profiles at once. The machine clamps, positions, and welds all four corners simultaneously in one cycle.

• Key advantage: accuracy and productivity—the frame is clamped as a whole, yielding unmatched dimensional and angular precision (true 90°). Cycle time per complete frame drops to 1.5–3 minutes.

• Pros: extremely high throughput, superb precision, low labor per unit, high process reliability.

• Cons: high investment, larger footprint, reduced flexibility for special angles (though modern machines can handle them with more setup effort).

• Target users: industrial manufacturers with medium to high volume (≈ 30–50+ units/day).

High-performance class: Six- and eight-head machines (6-/8-head)

For absolute mass production.

• Operation: e.g., a 6-head machine can produce a frame including a welded-in mullion in a single cycle (4 corners + 2 T-joints). An 8-head can weld two small sashes at once or complex door frames (e.g., with two mullions).

• Pros: maximum output per time, maximum automation.

• Cons: extremely high investment, very low flexibility; economical only for large, standardized series.

• Target users: large-scale industry, project manufacturers in highly standardized markets.

Special forms: Vertical vs. horizontal welding systems

• Horizontal (standard): profiles lie flat—ergonomic loading, ideal for flat production lines.

• Vertical: profiles stand upright—often more space-saving, integrates well with automated logistics, buffers, and carts.

---

The evolution of the corner: From “cleaning groove” to perfect zero-seam

The biggest innovation of the past 15 years has been the answer to an aesthetic challenge: colored and foil-laminated profiles.

The traditional challenge: Weld bead on colored/foiled profiles

With the boom in trend colors (e.g., anthracite) and wood-grain foils, a major problem emerged.

• Issue: Traditional welding forms a weld bead (e.g., 2 mm high).

• Follow-up: The bead is milled off by a corner-cleaner.

• Dilemma: The cutter removes not only the bead, but also the foil or color layer underneath.

• Result: An unsightly, exposed (often white or brown) cleaning groove at the miter that spoils the high-end look.

• Old “fix”: costly, manual, error-prone touch-up with paint pens.

The technological revolution: Zero-seam technology (V-Perfect / seamless welding)

“Zero-seam” (also V-Perfect, seamless or contour-following welding) solves this by preventing uncontrolled bead formation on visible outer surfaces.

How does seamless welding work? (technical approaches)

Often used in combination:

• Mechanical limitation (e.g., 0.2 mm): knives or limiters at the mirror or clamps cap the melt squeeze-out to a minimum; a tiny, barely visible line remains—no broad external cleaning required.

• Forming/displacement: movable tools (sliders, blades) actively displace the melt inward (into chambers) or into defined, non-visible areas (e.g., gasket groove) during forging.

• Thermal forming: the V-miter is perfectly brought together; specially shaped (often heated) tools “iron” the corner during cooling. The foil is slightly formed at the edge so both sides meet perfectly.

Benefits for manufacturers and end users

The result is a visually flawless corner—like a one-piece frame or a perfect wooden miter.

• Manufacturer: no manual touch-up, major labor savings, higher process security, premium product.

• End user: superior aesthetics, higher perceived value, easier cleaning (no groove that traps dirt).

Companies like Evomatec have advanced such precise and robust machine solutions to give window manufacturers access to this market-leading technology.

---

The corner welder as part of a system: The weld-and-clean line

A corner welding machine never works alone in industrial practice. It is the pacemaker and core of an integrated weld-and-clean line (a complete system).

Why the welder rarely works solo

As noted, the weld bead must be removed. Even with zero-seam machines that perfect the outer edge, internal beads still form (glass rebate, hardware rebate, gasket grooves).

The indispensable partner: Corner cleaning machine (CNC corner cleaner)

Directly after the welder (often via cooling/palletizing/turning systems) follows the corner cleaning machine.

• Functional cleaning (always required): special internal-corner knives cut the bead out of profile grooves so glass, gaskets, and hardware can be installed.

• Aesthetic cleaning (traditional): without zero-seam, a contour cutter mills off the outer bead, creating the (undesired) cleaning groove. With zero-seam, this milling step is eliminated.

Takt-time optimization: The production bottleneck

Overall efficiency depends on synchronization between welder and cleaner. The welder’s cycle (e.g., 2–3 min per frame) sets the takt. The corner cleaner must finish all four corners within that time before the next frame arrives.

The safety and reliability of such integrated lines are top priorities. Thanks to extensive project experience, we can ensure during acceptance that inspections meet the highest standards for quality and CE-compliant safety with utmost rigor.

---

Quality assurance, maintenance, and operational safety (CE compliance)

A corner welding machine is a precision system. It delivers consistent high quality only when optimally maintained and calibrated.

Typical welding defects and root causes (troubleshooting)

• Cold weld (insufficient strength): breaks under low load; fracture surface looks brittle/crystalline, not tough-fibrous.

  • Cause: temperature too low, heating time too short, or (very often) changeover too long (melt cooled in air).

• Burnt joint (visual defect): PVC at the joint discolors (yellow/brown) and turns brittle.

  • Cause: temperature too high or heating time too long → thermal degradation.

• Angle error/warpage (dimensional defect): frame not exactly 90° or dimensions off.

  • Cause: mechanical misalignment (poor calibration), improper clamping (e.g., dirty contour jaws), cooling time too short (frame warps on removal).

The importance of “profile recipes” (parameter management)

Each profile system (different system houses) has distinct geometry, wall thickness, and formulation. A 7-chamber system welds differently from a 3-chamber system. A modern system must store and recall hundreds of recipes (temperature, time, pressure) to ensure consistent quality.

Maintenance of critical wear parts (PTFE film, clamping tools)

Most common causes are wear and contamination.

• PTFE (Teflon): the anti-stick mirror coating (usually a film) is the key wear part—inspect and clean daily. Burnt PVC deposits impair heat transfer and appearance. Replace regularly.

• Contour jaws: PVC dust/chips accumulate and prevent exact seating → dimensional errors.

• Guides & pneumatics/hydraulics: all moving parts must run smoothly and precisely; air pressure must be stable to keep heating/forging pressures exact.

Corner strength testing: Validating weld quality

Professional QA includes regular corner strength tests (destructive). Welded corners are loaded to failure; results must meet system-house specs and standards (e.g., DIN EN 514). This validates parameter settings.

CE conformity and occupational safety: More than a sticker

Industrial corner welders pose significant risks: > 250 °C temperatures, high forces (often several tons of forging force), and fast-moving heavy assemblies. Compliance with the European Machinery Directive (CE) is non-negotiable.

This includes guarding, light curtains, two-hand controls (during loading), and redundant emergency-stop systems. With years of experience across many customer projects, we ensure inspections are carried out with the utmost care for quality and CE-compliant safety—protecting operators and the legal operation of the system.

---

Economics: Cost, payback (ROI), and efficiency

Purchasing a corner welding machine for PVC windows is one of the largest single investments for a window manufacturer.

CAPEX: From single-head to fully automated line

Costs vary widely with head count, automation level (manual transfer vs. automated line), and technology (zero-seam or not):

• New, high-quality single-head (angle-adjustable): approx. €15,000–30,000

• New two-head: approx. €35,000–70,000

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

• Integrated weld-and-clean system (4-head, traditional): approx. €180,000–250,000

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

OPEX: Energy, labor, maintenance

• Energy: heating massive hot plates (four or more) is the biggest energy consumer—even with optimized heating cycles.

• Labor: the biggest savings lever. An automated 4-head line typically needs just one operator for loading/monitoring, whereas single-head plus manual cleaning ties up multiple people.

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

ROI example (detailed)

Upgrade from old single-head + manual cleaning to modern 4-head weld-and-clean line (traditional):

• Old system (1-head + 2 people cleaning):

  • Welding takt: ~12 min/frame (1 operator)

  • Cleaning takt: ~10 min/frame (2 operators)

  • Personnel: 3 operators

  • Shift output (450 min): ~35–40 frames

  • Labor/frame: (3 FTE * wage) / 40

• New system (4-head line):

  • Line takt: ~3 min/frame (1 operator)

  • Personnel: 1 operator

  • Shift output (450 min): 150 frames

  • Labor/frame: (1 FTE * wage) / 150

Result: Labor cost per unit drops dramatically (often > 80%), while potential output quadruples. Even with a €200k investment, payback is often < 2–3 years through labor savings (2 FTE saved) and higher margin (more units sold).

New vs. used: Opportunities and risks

• Wear: guides/ball screws can be worn → dimensional inaccuracy.

• Obsolete controls: spare parts for old PLC generations may be unavailable.

• Technology: used systems rarely offer zero-seam.

• Safety: older systems often don’t meet current CE standards.

Expertise is crucial, especially for used equipment. With deep project experience, we ensure that inspections of legacy systems are carried out with utmost care for quality and CE-compliant safety to avoid bad investments.

---

The future of corner welders: Industry 4.0 and new materials

The development of PVC corner welding machines is ongoing. “Smart factory” trends shape the next generation.

Networking & smart factory: The machine in a digital ecosystem

The weld-and-clean line is no island. It is fully integrated with digital production planning (ERP/PPS). A barcode scanner reads the profile label; the system (welder and cleaner) automatically loads the correct recipe (parameters and cleaning contours) and adjusts sizes.

Predictive maintenance & remote service

Modern machines monitor themselves—counting PTFE film cycles and notifying replacement before quality suffers. Via online access, service technicians (e.g., from Evomatec) can diagnose and often fix issues remotely, avoiding costly travel.

Robotics & full automation: The “manless” welding cell

Next step: full automation. Robots load profiles from the saw into the welder, unload finished frames, pass them to the cleaner, and stack them.

Energy efficiency & sustainability (welding recycled cores)

With rising energy costs, hot-plate efficiency is optimized (faster heat-up, better insulation). Another trend: robust welding of profiles with recycled-PVC cores (co-extruded)—which behave differently and demand tighter temperature control.

AI-assisted process optimization & QA

The future is self-optimizing. Vision systems could monitor bead formation or the finished zero-seam in real time. AI could detect deviations (e.g., material batch variance) and dynamically readjust parameters to guarantee perfect results.

Beyond mirror welding?

Alternatives are being researched. Laser welding of plastics promises ultra-fine seams but remains extremely costly and technically challenging for complex geometries and PVC (poor laser absorption). Infrared welding (non-contact) is another niche technology.

---

Choosing the right machine: A strategic decision

Investing in a corner welding machine for PVC windows defines a plant’s competitiveness for a decade or more.

Needs analysis: Throughput, flexibility, aesthetics

• Throughput (productivity): units per shift → defines head count (1, 2, or 4) and automation (stand-alone vs. line).

• Flexibility: many specials (angles, arches) or mostly rectangles?

• Aesthetics (market positioning): processing colored/foiled profiles? Then zero-seam is virtually a must today.

The importance of an experienced system partner

Selecting the right system and integrating it with existing processes (sawing, hardware assembly, logistics) requires deep process know-how. An experienced partner like Evomatec analyzes not only the machines but the entire workflow to avoid bottlenecks.

Our long-standing experience from numerous customer projects allows us to guarantee during planning and acceptance of new systems that all inspections are carried out with the utmost care regarding quality and CE-compliant safety. This ensures a smooth start as well as longevity and security for your investment.

---

FAQ – Frequently asked questions

What’s the difference between a 4-head and a 1-head corner welder?

A single-head welder joins one corner at a time. The operator must position the frame four times. It’s slow but flexible (ideal for special angles) and inexpensive. A four-head welder joins all four corners of a window frame simultaneously in one operation. It is extremely fast (takt < 3 min), highly precise, and the standard for industrial series production.

What does “zero-seam” mean, and do I need it?

Zero-seam (also V-Perfect) is a modern welding technology that creates a visually seamless corner without the usual visible weld bead (flash). If you process only white profiles, it’s “nice to have.” If you make colored or foil-laminated profiles (e.g., woodgrain, anthracite), zero-seam is a decisive competitive advantage today. It eliminates time-consuming, costly manual touch-up with paint pens and delivers superior aesthetics.

How hot is PVC welded?

The welding temperature (hot-plate/mirror temperature) for rigid PVC used in window profiles is typically in a tight range of 240–260 °C. Too low leads to a cold weld (breaks). Too high burns the material, embrittles it, and releases harmful gases.

Request a free consultation: Click here