Non-Marking Press Brake Bending: How to Prevent Scratches on Brushed, Mirror-Finish & Film-Protected Sheets

ByFrancis Pan
Francis Pan

Francis Pan

Francis Pan is the Foreign Trade Manager of RAYMAX, with over 10 years of experience in sheet metal fabrication equipment and CNC machinery. He has worked closely with manufacturers worldwide on press brakes, fiber laser cutting machines, fiber laser welding machines, and practical production-oriented metal processing solutions.

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Key takeaways

Surface scratches on high-end sheet metal during bending are often not primarily caused by the tooling or protective layers, but rather result from the combined effects of friction patterns, tooling cleanliness, and equipment stability.

For high-surface-finish workpieces such as brushed sheets, mirror-finish sheets, and film-protected sheets (i.e., sheets with protective film), there are typically three core approaches to preventing scratches:

  • Barrier-based protection: Laying a urethane protective film in the lower die contact area to minimize direct metal-to-die contact;
  • Die design and tooling improvements: Using non-marking lower dies with nylon or polyurethane inserts, or roller-type non-marking lower dies;
  • Long-term batch production solutions: In batch production, these solutions must be considered in conjunction with equipment precision, compensation capabilities, and support methods.

30-Second Quick Reference Chart

Surface types of sheets

Common surface defects

Preferred protection solutions

Key precautions

Brushed stainless steel

Scratches, damaged grain pattern

Urethane protective film, non-marking lower die with nylon inserts, roller-type non-marking lower die

Avoid dragging along the grain; keep the tooling clean

Mirror-finish stainless steel

Indentations, drag marks, bright spots

PU pad, non-marking lower die with polyurethane inserts, roller-type non-marking lower die

Extremely sensitive to localized contact pressure and minute foreign particles

Film-protected sheets

Film damage, curled edges, creases

Urethane protective film or PU protective pad, a lower die with a large radius or a larger shoulder radius

The original film does not guarantee absolute safety; pay attention to localized pressure around holes and cut edges

Painted/Pre-coated sheets

Coating indentations, abrasions

PU pad, non-marking lower die with nylon inserts

Maintain as even a contact pressure as possible; avoid sharp contact

Anodized aluminum

Cracks in the oxide layer, scratches

Polyurethane underlay or insert, roller-type non-marking lower die

Avoid excessive bending, deformation, and sliding friction

Why do your high-end sheets always get “marred” during bending? What constitutes true non-marking bending?

Why are workpieces with high surface finish requirements more prone to being scrapped than ordinary sheets?

Ordinary sheets generally do not have high surface finish requirements. During bending, minor friction or slight marks often do not affect delivery. However, workpieces with high surface finish requirements—such as brushed sheets, mirror-finished sheets, film-protected sheets, and coated sheets—are extremely sensitive to minor scratches, drag marks, and localized indentations. Even a single instance of damage can result in the entire sheet being scrapped.

Why do traditional bending methods easily “damage the surface”?

In common V-die air bending, as the sheet is pressed into the lower die, it typically undergoes relative movement against the two contact edges of the lower die, resulting in sliding friction. If these contact edges are too sharp or rough, or if localized stress on the sheet is excessive, the contact surface is prone to scratches, drag marks, indentations, or damage to the coating.

Workpieces most prone to issues

  • Brushed-finish workpieces: Brushed surfaces have a unique texture, especially in stainless steel bending, are highly sensitive to scratches; once scratches disrupt this pattern, they become highly noticeable.
  • Mirror-finish workpieces: Mirror-finished parts reflect light, so even minor indentations or drag marks are clearly visible in the reflection.
  • Film-protected workpieces: For film-protected parts, it is essential to prevent indentations or scratches on the metal surface beneath the film, as well as to prevent the film from tearing or curling.
Brushed, mirror finish, coated parts
Brushed, mirror finish, coated parts

What constitutes true non-marking bending?

Non-marking bending does not imply “absolutely zero contact” between the metal sheet and the die. More accurately, it is a process objective aimed at minimizing surface damage to parts with high surface finish requirements. We can minimize surface damage by modifying contact methods, adding protective barriers, reducing sliding friction, and avoiding localized hard pressure, while also ensuring more stable equipment force application and bending operations. Ultimately, this achieves the process goal of “non-marking bending.”

Why isn’t non-marking bending simply a matter of purchasing a die?

Many factories believe that using a non-marking die alone will completely resolve surface issues on workpieces. However, in actual production, surface indentations and scratches are not caused solely by the die; unstable equipment, improper handling, and non-standard operating procedures can also lead to surface damage.

The ability to consistently achieve non-marking bending for parts with high surface finish requirements is often determined by a combination of contact methods, tooling cleanliness, operating procedures, and equipment stability—not by any single component.

Next, we need to understand exactly how these scratches and indentations are formed.

What causes scratches and indentations?

Sliding friction in the lower die contact area

When the sheet metal is pressed into the lower die, sliding friction typically occurs along the contact edges of the lower die. If these edges are too sharp or rough, scratches may form on the sheet metal surface during the friction process.

Indentations caused by localized overpressure

When the bending tonnage is too high, local pressure at the lower die contact edge is excessive, the punch tip radius is too small, or the press brake applies uneven force during bending, it can cause excessive localized stress on the sheet metal. This makes the sheet surface prone to indentations, which are more common on mirror-finished, film-protected, or soft-surfaced materials. Therefore, non-marking bending addresses not only scratches caused by friction but also indentations caused by localized overpressure.

Dirty tooling and microscopic foreign objects

A single tiny metal chip, scale, or burr residue on the tooling can easily be pressed into the sheet surface under high tonnage, forming dents or scratches. Many instances of surface damage may not result from incorrect process parameter settings, but rather from a dirty tooling surface or the presence of microscopic foreign objects.

Seams in segmented tooling, chipped edges, and worn areas

When tooling joints are uneven, there are height differences between segments of a segmented tooling, or the tooling has localized chipping, wear, or indentations, these can leave regular abrasion marks on the sheet surface.

Improper handling, turning, feeding, and support methods

In addition to surface damage caused during the bending process itself, many issues arise before loading the machine or after unloading.

For example:

  • Long, thin sheets without proper support are prone to sagging, causing the edges to drag against the die or frame;
  • If an operator drags the sheet across a rough work surface, scratches may also result.

Surface defects in workpieces with high surface finish requirements are rarely caused by a single factor; they are typically the result of a combination of contact methods, localized stress, the cleanliness of the contact interface, and handling practices.

Workpiece surface scratches
Workpiece surface scratches

Core solution 1: physical isolation—how to use protective layers, padding, and isolation media

This is the solution with the lowest barrier to entry and the fastest results.

This solution does not require replacing the entire tooling setup from the outset, making it well-suited for small-batch production or prototyping scenarios.

What are common protective media?

  • Thin urethane protective film: This is the most common protective method, typically involving laying the urethane protective film over the lower-die contact area or V-opening.
  • Thicker elastic padding: More suitable for high-tonnage or thick-sheet scenarios, using thicker polyurethane padding to provide cushioning.
  • Factory-applied surface protective film: Many stainless steel or aluminum sheets come with a layer of PE protective film applied at the factory to prevent surface scratches during handling.

When to use each

  • For situations requiring light surface protection or frequent workpiece changes, thin protective layers are more suitable;
  • For high-tonnage or thick sheets, thicker elastic pads generally provide greater stability;
  • The presence of a factory-applied protective film does not mean the die requires no additional protection. When localized stress on the sheet is high, the factory-applied PE protective film may still suffer indentations, tears, or curled edges, resulting in surface damage to the sheet.

Common issues during on-site use

  • Shift of the protective layer: If the protective layer is not properly secured, it can easily shift after several consecutive bends, causing scratches on subsequent workpieces.
  • Excessive wear: Under continuous high pressure, the protective layer will also wear out. If wear is too severe, it can cause scratches or indentations on the workpiece.
  • Angle deviation: The thickness and elasticity of the cushioning layer may have a slight impact on the bending angle; adjustments should be made based on actual conditions.
  • Improper securing: If the protective layer is not secured with clamps or tape during each replacement, or if it is secured incorrectly, it may lead to fluctuations in batch surface quality and precision.

Who is this solution best suited for?

This solution is best suited for scenarios involving small-batch, high-variety production; products in the prototyping phase; factories that have temporarily received orders with high surface finish requirements; or situations where one wishes to first verify the feasibility of scratch protection at a lower cost.

press brake indentation protection film
press brake indentation protection film

Core solution 2: die upgrades—when are true non-marking lower dies appropriate?

Why relying solely on protective film is often insufficient

When orders are repetitive, batch sizes are larger, and surface quality standards are stricter, relying solely on protective film is often not a very reliable solution.

  • First, protective film is a consumable item; the larger the batch size, the higher the cumulative cost of using it. Furthermore, each replacement of the protective film requires a production halt and re-calibration, which impacts production efficiency.
  • Second, applying protective film requires manual labor, and it is difficult to maintain consistent positioning and tension, leading to fluctuations in product consistency during batch production.
  • Finally, thicker cushioning pads can also affect the accuracy of bending angles, requiring operators to repeatedly adjust and compensate, further increasing the difficulty of process control.

In this situation, we need to upgrade to a non-marking solution by switching to more stable and efficient dedicated non-marking dies.

Non-marking lower dies with protective inserts

This is the most common type of non-marking lower die. The principle involves embedding a protective contact layer in the contact area of the lower die, which prevents direct, hard contact between the metal sheet and the lower die.

Advantages of this solution:

  • It is more durable than temporarily applying protective film, provides more stable surface protection, and ensures better batch consistency;
  • When producing repetitive parts, there is no need to frequently replace the protective layer, significantly reducing operation time.

Limitations of this solution:

  • The inserts are subject to wear and tear, requiring regular inspection and replacement;
  • When there are significant changes in sheet thickness or material, the suitability of the inserts must be re-evaluated.

Rolling-contact non-marking lower die

Why is this solution more representative?

This is an advanced non-marking die solution that transforms the sliding friction between the sheet metal and the lower die edge into gentler rolling contact, greatly reducing frictional resistance. It is particularly suitable for appearance parts with high surface finish requirements, such as brushed, mirror-finished, or film-protected parts.

For which orders is it best suited?

It is suitable for orders with extremely high surface quality requirements, high-volume production, repeated manufacturing of similar parts, and a desire to minimize fluctuations caused by manual operations and the replacement of protective layers.

When should you upgrade from a “protective layer solution” to a “dedicated die solution”?

  • When orders for similar appearance parts begin to recur;
  • When the scrap costs resulting from surface defects have clearly exceeded the cost of a single non-marking die;
  • When significant time is spent on replacing and adjusting urethane protective films/pads, impacting production efficiency;
  • When customers’ surface quality requirements for parts are becoming increasingly stringent;
  • When your core business focus has clearly shifted toward appearance parts.

If your workshop exhibits any of these signs, it typically means you need to seriously consider upgrading to a dedicated die solution.

Non-marking die
Non-marking die

Core solution 3: equipment and production line capabilities

Why do localized indentations still occur even when using high-quality dies?

Even the best non-marking die cannot guarantee a flawless surface finish on the workpiece if the press brake itself lacks precision and stability. Inconsistent stress distribution in the middle of long workpieces and variations in repeatability can both compromise the effectiveness of the protective solution.

Why surface protection depends on equipment stability

  • Consistent force application: This ensures uniform force distribution along the entire length of the workpiece;
  • Repeatability: Ensures each workpiece is formed under nearly identical conditions, resulting in high batch consistency;
  • Consistent angle along the entire length: Ensures that the angles and surface conditions at the center and both ends of long workpieces are consistent;
  • Stable clamping and tool change: Ensures the upper punch is securely installed and does not shift during bending, which could cause uneven force distribution on the sheet metal surface;
  • Front support or material support capability: Ensures stable support for long workpieces, preventing edges from dragging against the die or frame due to sagging.

Which equipment capabilities are more critical for high-appearance parts

  • Stable CNC control: helps maintain consistent speed, force application, and angle control during bending;
  • Appropriate crowning compensation system: Compensates for table deflection caused by bending forces, ensuring uniform stress distribution along the entire length of long workpieces;
  • High repeatability: Improves positional consistency during batch bending, reducing angle deviations, localized over-pressing, and surface inconsistencies caused by positioning variations;
  • Fast and stable clamping method: Enhances tool change efficiency and ensures consistent tool positioning and clamping force with every setup;
  • CNC sheet followers or follow-up support capability: Provides excellent support for long workpieces, preventing damage caused by sagging or dragging of the sheet metal.

For factories that regularly process elevator door panels, kitchen cabinet panels, medical equipment housings, and high-end home appliance appearance parts, stable, non-marking bending requires a combination of high-precision press brake, suitable non-marking die, compensation systems, and standardized operating procedures.

In such applications, Raymax high-precision press brakes feature high repeatability, ensuring full-length consistency for long parts. They are compatible with a wide range of non-marking die, offering fast and stable tool changes that significantly boost production efficiency. If your production involves long-term sheet metal bending with high aesthetic requirements, it is essential to evaluate not only the non-marking die solution but also the press brake’s repeatability, compensation capabilities, support for long parts, and tool change stability. For such applications, the Raymax high-precision press brake can be considered as a viable option for evaluation.

Ready To Upgrade Your Metal Fabrication Line? ​

Email Us For A Free Consultation.​

What are the specific challenges associated with workpieces of different surface types?

Brushed-finish workpieces

  • Key challenge: The surface features a unique texture; any scratches that disrupt this texture will be highly noticeable.
  • Key focus: Pay special attention to tooling cleanliness, the condition of the contact interface, and the motion of flipping the workpiece.

Mirror-finished workpieces

  • Core challenge: Mirror-finished parts reflect light, so even tiny pinpoint indentations or drag marks will be clearly visible in the reflection.
  • Key focus: Relying on operational experience alone is generally not recommended; a more stable, non-marking die solution is required.

Film-protected sheets

  • Key challenges: It is essential to prevent indentations or scratches on the metal surface beneath the film, as well as to prevent the film from tearing or curling.
  • Key focus: Pay special attention to localized pressure around hole edges, notched edges, short flanges, and small-sized workpieces.

Coated sheets, pre-coated sheets, and anodized workpieces

  • Key challenges: Surface coatings are relatively soft and prone to wear or indentation.
  • Key focus: Pay close attention to pressure control and the condition of the contact interface.

Summary: Different surface types present different bending risks. Brushed parts are more susceptible to scratches, mirror-finished parts are more prone to indentations, and film-protected parts require simultaneous control of both metal surface damage and film damage. Therefore, when selecting a non-marking solution, we must not only consider whether the workpiece has a factory-applied protective film but also take into account the workpiece type and production environment.

The invisible killer: overlooked details on the shop floor

Tooling Cleanliness Is Not Maintenance — It Is Process Control

  • Before tool change: Check that the tooling’s contact surfaces are clean and free of foreign objects;
  • During production: Use an air gun to periodically blow away dust and particles from the tooling surface;
  • Before a new part is introduced: Check the tooling surface for burrs, residues, or indentations.

Operator technique directly determines surface quality

  • Gloves must be clean: Dirty gloves may be contaminated with metal debris or oil, which can contaminate or scratch the workpiece surface; therefore, operators must wear clean gloves.
  • Do not drag sheet metal across the bed: Dragging sheet metal across the bed may cause scratches. Operators should pay attention to how they handle the sheet metal.
  • Support long sheets during loading and unloading: Long workpieces are prone to sagging without support, which can cause dragging against the lower die or frame. A CNC sheet follower system or sheet-following device must be used.
  • Flip the workpiece carefully and in a controlled manner: Improper flipping of the workpiece may also cause collisions between the workpiece, the tooling, and the bed. Therefore, operators must perform flipping motions gently and steadily.

Why long workpieces are more prone to surface defects

  • More pronounced deflection: When bending long workpieces, the bed and ram may experience a certain degree of deflection. In general, the longer the workpiece, the more pronounced the deflection tends to be. This results in a greater disparity in force distribution between the center and the ends of the workpiece, leading to a larger difference in angles between the center and the ends.
  • Support is more difficult: Handling, positioning, and supporting long workpieces are more challenging than with short ones, making them more prone to dragging and scraping.
  • Ensuring uniform force distribution is more difficult: Due to their length, it is more difficult to maintain consistent force distribution along the entire length of long workpieces.

On-site scratch prevention checklist

Before bending parts with high surface finish requirements, we must check the following items:

  • Are the die shoulders clean?
  • Are there any foreign objects in the contact area?
  • Are the seams smooth?
  • Are the operator’s gloves clean?
  • Is the support in place?
  • Is the protective layer worn?
  • Has the first piece been inspected for the reflective surface rather than just the angle?

Basic pitfalls to avoid: don’t let incorrect parameters undermine your efforts to prevent scratches

When performing non-marking bending, we often encounter a common problem: why do indentations still appear even when the protective measures are correct?

This usually happens because the bending load exceeds what the protective layer can tolerate. The main reasons are:

  • V-die opening is too small: A V-die opening that is too small causes the required bending tonnage to rise sharply, increasing the pressure on the edges of the lower die and potentially causing the protective layer to be “pressed through.”
  • Excessive localized pressure: If the contact area between the sheet and the tooling is too small, localized pressure on the sheet’s surface may increase, leading to indentations.
  • Incorrect parameter compensation: To correct angular errors, blindly adjusting the Y-axis downward depth causes a sharp increase in localized compression on the sheet from the edges of the V-die. This can instantly tear the sheet’s factory-applied protective film and may leave deep indentations on the sheet’s surface.

Achieving non-marking bending requires proper die opening, contact conditions, and force control.

How to choose the right solution? A single table clearly outlines workpiece types and production scenarios

When selecting a solution, we must make a comprehensive assessment based on order type, batch consistency, and production objectives.

Workpiece types

Production status

Most common issues

Recommended solutions

Why it’s suitable

Production considerations

Brushed stainless steel

Small-batch prototyping

Scratches and minor marks on the textured surface

Urethane protective film / PU pad

Low investment, flexible changeovers

Carefully inspect the tooling cleanliness, the bending action, and the contact areas for metal shavings

Brushed stainless steel

Repeat orders

Regular scratches appearing in batches, with inconsistent quality

Non-marking lower dies with protective inserts

More stable than temporary protective film

Pay attention to the wear of the insert and the condition of the contact areas

Brushed stainless steel

Long-term bulk production

High scrap costs

Roller-type non-marking die solution + high-precision press brake

Better suited for high-standard, long-term operations

Evaluate the equipment’s repeatability, compensation, and support capabilities

Mirror-finished stainless steel

Small to medium-batch production

Spot indentations, scratches, and localized marks are significantly magnified

non-marking lower die with polyurethane or nylon inserts

Higher requirements for surface uniformity

For the first piece, focus on the reflective surface and localized indentations

Long-term bulk production

High batch consistency requirements; localized overpressure can easily result in the entire batch being scrapped

Roller-type non-marking lower dies

Better suited for long-term batch production and scenarios with high aesthetic requirements

Pay close attention to compensation consistency, repeatability, and support for long parts

Pay close attention to compensation consistency, repeatability, and support for long parts

Film-protected sheets

Occasional processing

Film damage, curled edges, and localized indentations

Urethane protective film / PU pad

Low cost, suitable for initial trial production

The fact that the sheet metal comes with a factory-applied protective film does not mean it will not be damaged during bending

Film-protected sheets

Consistently unstable yield rates

Simultaneous damage to the metal surface and factory protective film, resulting in significant batch-to-batch variation

Insert-type or roller-type non-marking die solution

More stable than temporary protective layers

Focus on inspecting hole edges, notches, short flanges, and areas subject to localized high pressure

Painted sheets / Pre-coated sheets

Routine production

Surface indentations and edge abrasions

Protective layer solution or non-marking lower die with polyurethane or nylon inserts

Reduces surface damage caused by hard contact

Pay attention to edge scuffing and protection of the backgauge contact points

Anodized aluminum parts

Routine production

Surface prone to scratching, with visible indentations

Protective layer solution

Sensitive surface requiring gentler handling

Ensure surface cleanliness, stable support, and proper workpiece handling methods

Anodized aluminum parts

Frequent prototyping, unstable order volume

Difficulty standardizing tooling and protective solutions

Urethane protective film / PU pad

High flexibility, suitable for switching between multiple product varieties

Consistency is typically inferior to dedicated die solutions

Anodized aluminum parts

Repeated production of small to medium-batch appearance parts

Minor issues with individual pieces, but scratches appear in batches

non-marking lower die with polyurethane or nylon inserts

Balances stability and cost

Establish a system for mandatory first-piece inspections and regular replacement of consumables

Long appearance parts

Prominent surface defects

Localized indentations in the center, drag marks along the long edges, and poor consistency along the entire length

Non-marking dies + CNC sheet followers + compensation system

Issues with long parts are often not just a tooling problem

Pay close attention to support for long parts, full-length stress distribution, and the operating path

For a practical application example involving long appearance parts, see our guide to non-marking bending for elevator panels.

Summary:

  • For one-off, ad-hoc orders, we recommend first using a protective layer and standardizing on-site operations;
  • For recurring orders, prioritize the use of non-marking lower dies;
  • For long-term, high-quality batch production, evaluate the use of non-marking lower dies in conjunction with equipment capabilities.

Conclusion

To achieve consistent, non-marking bending, several factors must be taken into account, including tooling selection, protective media, equipment precision, on-site management, and operating procedures.

If you are struggling with bending parts that have high surface finish requirements, please send us your part drawings, material specifications, thickness, and surface finish requirements. Raymax’s engineering team can provide customized CNC press brake solutions for appearance parts and a tailored quotation, covering everything from tooling selection to machine configuration.

Ready To Upgrade Your Metal Fabrication Line? ​

Email Us For A Free Consultation.​

Frequently Asked Questions (FAQs)

In most cases, yes. The PE protective film applied to the sheets at the factory is primarily intended to prevent scratches during transportation and handling; it is not designed to minimize damage during bending. The factory film may still be crushed or torn in areas such as holes, notches, cuts, and localized high-pressure zones on the workpiece.

There will be some impact, but it is usually manageable. Protective pads have a certain thickness and elasticity; when subjected to pressure, they may compress, resulting in a slightly larger actual bending angle. Therefore, in CNC systems, first-piece calibration should be performed in conjunction with the control system being used, and necessary program compensation should be made based on the actual bending results.

There are three most common causes. First, the urethane protective film/pad or die insert wears out after continuous use, losing its protective capability; second, metal shavings or particles gradually accumulate on the die surface during continuous bending; third, the operator increases the operating speed and handles or positions the workpiece improperly.

The service life of die-side urethane protective films or PU protective pads depends on material thickness, V-die opening, bending angle, contact radius, and specific operating conditions; there is no fixed number of cycles. If the protective layer becomes thinner, turns white, becomes damaged, covered with debris, or if surface marks after bending become noticeably more severe, it should be replaced promptly. The actual service life is subject to the supplier’s specifications and on-site trial production data.

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