Selection of transparent resin with few scratches｜Thorough comparison of the types and characteristics of machine covers

Here we discuss transparent resins used for machine covers.Selection of scratch-resistant transparent resins and peripheral information."　notes.

When designing a safety cover for a machine or a viewing window for a cleaning device, have you ever had the experience of simply stating "transparent resin" on the drawing and later receiving complaints on site that you could not see inside because it was covered with scratches? (I have...)

As a result of selection based on catalog values alone, you may face problems such as cracks caused by cutting oil or rippling of the cover due to temperature changes.　　Many information sites only compare the general characteristics of resins and provide little information on "specific part number selection," "differences in hard coating performance among manufacturers," and "hedging risks in design," which are faced in practice.

Therefore, this article starts with a comparison of major types such as acrylic, polycarbonate, and PVC, and notes the basics of hard coating technology to prevent scratches, as well as objective evaluation indicators based on JIS standards such as total light transmittance, pencil hardness, and impact resistance.

We have also organized the product characteristics of major Japanese manufacturers such as Mitsubishi Chemical, Takiron C.I., and Sumitomo Bakelite, and have systematized the practical knowledge that designers should know, including dimensional changes due to chemical cracks and thermal expansion, maintenance methods considering chemical resistance, and suitability as safety covers, We hope that this book will help you to improve the quality of your designs.

Selection point for scratch-resistant transparent resin

Resin properties as defined by JIS standards

When selecting materials in machine design, the ability to correctly read and understand the numerical values in catalog specifications is essential to prevent problems before they occur.　　Especially in transparent resins, it is not a matter of sensory expressions such as "transparent" or "hard,Objective and quantitative performance evaluation is possible by referring to test values based on Japanese Industrial Standards (JIS)will be.

In addition to mechanical strength such as tensile strength and flexural modulus, optical and thermal properties are described in detail in the properties table for resin materials.

By comparing these values side-by-side, specific judgments can be made, for example, "Material A has more impact absorption energy than material B, but has a lower surface hardness index.　　Many domestic resin manufacturers use JIS standards (e.g.JIS K 7113　), and designers should carefully scrutinize specifications based on these data to determine whether they can withstand the load of the environment in which they will be used.

Visibility affected by total light transmittance

For covers and peepholes used to check operating conditions inside machines, how clearly the object can be seen is a factor that directly affects the operator's work efficiency and the speed of abnormality detection.　　Total light transmittance" is widely used as an objective indicator of transparency, and is mainly measured based on standards such as JIS K 7361-1.

In general, the higher this value, the more light passes through, indicating that the material itself is brighter and more transparent.

The table below summarizes the optical properties of typical transparent resins.

Reference source: Makoto Plastics (Japanese only)https://makoto-plastic.co.jp/2025/07/post-208/）
Reference source: Mitsubishi Chemical Corporation Acrylite Technical Data (https://acrylite-help.m-chemical.co.jp/）

Thus, acrylic resin has overwhelming transparency.　　It is recommended that acrylic be selected with the transmittance value as the highest priority when used for processes that require precise positioning or as a cover through which optical devices such as image sensors or laser displacement meters pass.

Scratch resistance as determined by pencil hardness

Pencil hardness is the most useful indicator for designers to quantitatively specify "scratch resistance" performance in drawings and specifications.　　This is a test method specified in JIS K 5600-5-4 (coating films) and other standards, in which a pencil lead of a specific hardness (6B to 9H) is pressed against the surface of a material at a 45-degree angle and scratched, with the maximum hardness that does not produce any marks being expressed.

General untreated polycarbonate has a hardness of only about "2B" or "HB," which is so soft that even a dry wipe with a cloth can cause minor abrasions (scratches).　　In contrast, untreated acrylic is relatively hard at about "2H," but still cannot withstand contact with metal or hard dust.　You can actually feel the softness when you touch it.

Therefore, an essential choice when practical scratch resistance is required as a machine cover is a grade with "hard coat" treatment on the surface.　　By applying hard coating, even polycarbonate can achieve a hardness of "4H" or, depending on the product, "6H" or higher, which is close to that of glass.

When specifying materials in design drawings, the key to ensuring durability as intended is to specify specifications such as "hard coat specifications (pencil hardness: 4H or higher)" rather than simply "transparent resin.　(However, in the case of production facilities, since a large amount of transparent resin is used, untreated polycarbonate is likely to be used.)

Safety covers requiring impact resistance

High impact resistance is essential for machine tool doors, machining room covers, and other areas where operators need to protect themselves from unexpected impacts, such as the scattering of rotating workpieces or damage to tools.　　Polycarbonate is unrivaled in this characteristic.　　Polycarbonate is said to be 40 to 50 times stronger than acrylic and more than 200 times stronger than ordinary tempered glass, and it rarely breaks even when struck strongly with a hammer.

On the other hand, as mentioned above, polycarbonate has a weak point in that its surface is very soft and can quickly become scratched if left as it is.　　Therefore, when selecting a safety cover, the most rational solution is to use impact-resistant polycarbonate as the base material and to select a material with a scratch-resistant hard coating on its surface to achieve both safety and visibility.

Although acrylic is superior in terms of transparency and hardness, it has the risk of sharply breaking and shattering when subjected to a strong impact, so careful measures such as significantly increasing the thickness of the board or using shatterproof film together are necessary for use as a safety barrier.It is.

Dimensional design considering thermal expansion

One of the physical properties that machine designers must pay most attention to when fastening plastic materials to metal frames or enclosures is the "coefficient of linear expansionThe difference is that resin expands and contracts to a much greater degree than metal.　　Resins expand and contract much more greatly than metals with changes in temperature.　　For example, the coefficient of linear expansion of polycarbonate is approximately five to six times that of steel or stainless steel.

As a specific example of calculation, let us assume a polycarbonate cover of 1,000 mm in length. Suppose the ambient temperature rises 30°C from 10°C in winter to 40°C in summer.

Expansion and contraction ΔL = 1000 × (7 × 10^-5) × 30 = 2.1 mm

This means that the cover will extend approximately 2 mm.　　If,If this cover is firmly fixed with bolts and the temperature rises with no escape, the force of expansion can cause the cover to ripple and deform, and stress concentrated around the bolt holes can cause cracks (cracks).

As a design technique to prevent this, it is important to take sufficient "clearance holes" to make the bolt holes larger than the screw diameter, or to process long holes to create a structure that can absorb expansion and contraction.　　It is also important to consider dispersing the tightening pressure by placing a resin washer or rubber packing between the bolt and the resin to press the bolt with a surface rather than a point.

Reference source: AGC Polycarbonate Technical Data (https://www.agc.com/products/electoric/polycarbonate/pdf/12.pdf）
Reference source: D-Engineer Resin Property Data (https://d-engineer.com/plastic/pc.html）

Types and products of scratch-resistant transparent resins

Mitsubishi Chemical's high hardness acrylic

Mitsubishi Chemical Corporation (MCHC), a major Japanese resin manufacturer, has developed a new acrylic resin, "MMA", which is used in the manufacture of a wide range of products.Acrylite™.In the "Acrylite MR (SHINKOLITE MR)" series, we are developing value-added grades with high surface hardness.　　The representative product, ACRYLITE MR (SYNCOLITE MR), dramatically improves scratch resistance by forming a special hardened film on the surface while maintaining the outstanding transparency inherent to acrylic (total light transmission rate of 93%).

The surface hardness of this product may reach "6H" to "7H" on a pencil hardness scale, which is the level of hardness at which light rubbing with steel wool will not cause scratches.　　Therefore, this product is best suited for applications where maintaining aesthetics and visibility is a top priority, such as protective panels for LCD displays, touch panel covers for operation panels that are frequently touched by human hands, and instrument windows.

However, since the base material is acrylic, its impact resistance is inferior to polycarbonate. It is recommended for use in locations where strong impact is unlikely to occur, or after taking measures to prevent shattering.　　It is the first choice when both extremely clear visibility and scratch resistance are required.

Polycarbonate from Takiron C.I.

Takiron C.I. is a manufacturer with a high market share and technical capabilities in the field of polycarbonate plates.　　The company's "PCMRHard coat grades (surface hardening grades), such as polycarbonate, are widely used in industry as products that overcome polycarbonate's greatest drawback, its susceptibility to scratching.

This greatly reduces wiping scratches during daily cleaning and minor abrasions caused by flying chips during machining.　　Because of its high dimensional balance between "resistance to cracking (high impact resistance)," which is essential for safety covers, and "scratch resistance (abrasion resistance)," which can withstand long-term use, it is one of the most versatile and reliable options for protective covers for machine tool doors and automatic machines.　　The lineup also includes grades with enhanced weather resistance, which can be applied to outdoor facilities.

Sumitomo Bakelite's Multifunctional Board

Sumitomo Bakelite also has a "Polycase® (Polycarbonate®)The company offers a wide range of high-performance polycarbonate sheets under the brand name of "Polycarbonate".　　The company's lineup is extremely diverse, and in addition to the general "weather-resistant grade (both sides)," there are a number of products with enhanced specific functions.

Also attracting attention in recent years is a special grade that can be bent, which has been difficult with conventional hard-coated sheets.　　Normally, the hard coat layer is glassy and brittle, causing minute cracks on the surface when bent, but the company has developed a "weather-resistant molding hard coat" technology that takes moldability into consideration and offers products that can be thermally bent at a certain radius of curvature.　　This has made it possible to realize scratch-resistant hard coat surfaces even for covers with a well-designed R shape.

To digress a little from the topic of transparent resin bending, some machine manufacturers use transparent resin bending covers of unspecified material to guard CVs that transport products over long distances, because "aluminum frame + flat panel" guards are too difficult to guard.

Advantages and Disadvantages of PVC Covers

As a material for machine covers, vinyl chloride (PVC), so-called polyvinyl chloride, is frequently compared along with acrylic and polycarbonate.　　The greatest advantage of selecting this material is its overwhelming "cost performance" and excellent "chemical resistance.

Because it is available at a lower cost than acrylic or polycarbonate, it is a strong choice for extensive coverings that cover entire large pieces of equipment or for projects with tight budget constraints.

Also,It has very strong chemical resistance to acids and alkalis, and can be used safely as a cover for plating tanks and chemical cleaning lines.　　Furthermore, its self-extinguishing properties and its resistance to spreading even if a spark should come into contact with it are also advantageous as a safety measure for factory equipment.

However, disadvantages include low surface hardness, easy scratching, and a low heat resistance temperature of approximately 60°C.　　Care must be taken because it is easily softened or deformed in high-temperature environments.　　Even transparent grades have a distinctive bluish or purplish tint, and do not provide as clear a view as acrylics.It is suitable for use in places where it is possible to say, "Even if it gets scratched, just replace it periodically," or where there is a high risk of chemical spattering.

Troubleshooting for scratch-resistant transparent resin

Causes and countermeasures for chemical cracks

One of the most alarming problems for machine designers when using transparent resin covers is "chemical cracking.　　This is,A phenomenon in which the adhesion of certain chemicals or oil to a resin material under stress (residual stress from molding, distortion from bending, bolt tightening force, etc.) reduces the bonding strength of the resin's molecular chains, causing internal micro-cracks (crazing) that eventually lead to rupture.It is.

Polycarbonate and acrylics, in particular, are amorphous resins and very delicate to organic solvents.　　If thinner, acetone, benzene, etc. adhere to the surface, it will whiten or crack in a short time.

A frequent failure at design sites is the use of "threadlocker (anaerobic adhesive)" to prevent bolts from loosening, which invades the resin and causes it to crack radially from the mounting holes.　　To prevent this, it is necessary to prohibit the use of general threadlockers around resin parts in the illustration or select a special adhesive compatible with the resin.

Also,Since some types of cutting oils (coolants) attack resins, it is essential to be sure to check the resin compatibility chart published by the oil manufacturer.

Chemical resistance of each resin material

As mentioned earlier, the types and levels of chemicals that can be withstood vary greatly depending on the type of resin.　　In order to ensure appropriate material selection, we have organized a list of general chemical resistance trends for each material.

Reference source: Mitsubishi Chemical Corporation Acrylite Technical Data (https://acrylite-help.m-chemical.co.jp/）

Thus, acrylics and polycarbonates have a weakness against chemicals.　　Although some hard coat grades have temporarily improved chemical resistance with the coating layer acting as a barrier,Cutting surfaces and the inner surfaces of drill holes expose the substrate, from which chemicals can penetrate and lead to cracking.　　In environments where chemicals are splashed, it is important to choose PVC or take protective measures to prevent chemicals from getting on the resin structurally.

Proper maintenance extends service life.

Even if a high-performance, "scratch-resistant" hard-coat resin is selected, it will not maintain its performance for long if maintenance methods are not followed during the operational phase.　　Designers are responsible for ensuring that on-site operators are familiarized with the correct cleaning methods through instruction manuals and maintenance manuals after delivery.

The most important thing to avoid is the use of strong cleaners containing alcohol or highly concentrated solvents in an attempt to remove stains.　　These are a direct cause of the chemical cracks mentioned above.

Also,Scrubbing hard with a dry cloth may drag fine sand or metal powder on the surface, which may scratch even the hardcoat layer, no matter how hard it is.　　The recommended cleaning method is to use lukewarm water with diluted neutral detergent and gently rinse with a soft cloth or sponge.

Even with persistent grease stains, the transparency and safety of the cover can be maintained over time by specifying in the instructions or on the nameplate that a special cleaner for plastics should be used.

Summary of scratch-resistant transparent resin selection

• Scratch resistance of machine covers is judged using the JIS standard "pencil hardness" as an indicator.
• Untreated polycarbonate is very soft, with a pencil hardness of about 2B, and scratches even when wiped with a cloth.
• The best means to prevent scratches is to select a resin board with a "hard coat" specification
• Hard coated products are reinforced to a pencil hardness of 4H to 6H and have resistance to steel wool.
• When visibility is the highest priority, acrylic (PMMA) with a total light transmittance of 931 TP3T is optimal.
• Polycarbonate (PC) is essential when impact resistance is a priority as a safety cover
• Hard coating on acrylic can achieve hardness close to that of glass (6H-7H)
• Polycarbonate hard-coated products are a practical "unbreakable & unscratchable" solution.
• Mitsubishi Chemical, Takiron C.I., and Sumitomo Bakelite to supply major hard coating products
• Vinyl chloride is inexpensive and chemical resistant, but has low heat resistance and is easily scratched, so use it in the right places.
• Resin has greater thermal expansion than metal, so bolt holes should be made to provide clearance by making stupid holes or long holes.
• Threadlockers and alkaline detergents are the main cause of chemical cracks.
• In environments where cutting oil is applied, the compatibility with resin materials should be checked and selected in advance.
• Establish a rule to avoid alcohol and solvents when cleaning and use neutral detergent and soft cloths.
• Safety and aesthetics of machine covers can be maintained over the long term with appropriate material selection and design considerations.

That's it.