Guide to Cutting Rubber｜What Designers Need to Know about Materials, Accuracy, and Comparison with Dies and Molds

Here."As a guide to cutting rubber, the designer should know the materials, accuracy, and comparison to molds."　I am making a note about the

As a mechanical designer, I am sometimes faced with problems in designing rubber parts, such as "Can this shape be made by cutting?" or "I want to make a prototype, but dies are costly and time-consuming.　　　　In designing equipment, I basically design "one product," but each time I have to worry about the cutting process for rubber.　　I think that many designers may be troubled about "whether we should verify with a mold from the stage of prototyping or cutting" when we expect a certain amount of mass-produced products.

In fact, there is a lot of information on rubber cutting, but it often describes materials and processing methods in a fragmented manner, and there is little information to systematically learn about the entire design flow.

For designers who were not satisfied with such information, this article provides notes that follow the actual design process, rather than just a list of knowledge.　　Starting with the strategic use of mold molding, which is important in the early stages of a project, the article proceeds through the characteristics of various materials, the selection of processing methods, and finally, the concept of dimensional tolerances and accuracy, which are essential for designers when making drawings, all in a single step.　　By reading this article to the end, you will have the overall picture of rubber cutting and the knowledge to proceed with your design with confidence.

Learning the basics of rubber cutting and how it differs from molds

Advantages and disadvantages compared to mold forming

When considering how to manufacture rubber parts, cutting and mold forming each have different characteristics, and it is essential to use both depending on the project objectives and production volume.　It is.

The greatest advantage of the cutting process is that it does not require a die.　　This completely eliminates the high initial cost of tooling and the production time of approximately one month.　　This allows for significant cost and time savings in prototyping from a single piece, in the development stage when specifications may change, or in small-lot production.

On the other hand,One disadvantage is that it takes longer to process each piece, so unit costs tend to be higher than those of die molding when mass production is involved.　　Also, due to the characteristics of processing with a blade, it is difficult to obtain a smooth surface (smooth surface) as if formed with a die, and cutting marks will remain.　　Furthermore, very thin-walled and complex shapes such as bellows may be difficult to process.

In contrast, mold forming has the advantage that once a mold is made, products of the same shape can be produced in large quantities in a short time in a stable manner, thus significantly lowering the unit cost of mass production.　　However, as mentioned above,Since high initial cost and time are required for mold fabrication, it is not suitable for small-lot production.

Thus, there are clear advantages and disadvantages to both.　　Therefore,Strategic use of different methods, such as "cutting for prototypes and small-lot production, and shifting to mold forming when mass production is decided," is a very effective approach to efficiently promote development.will be.

Cutting or Mold? The Break-even Point Concept

One of the most difficult decisions for designers is the question of "at what production volume does die molding become more advantageous?　　This quantity at which the total costs of cutting and die molding reverse is called the "break-even point.

Since the break-even point varies greatly depending on the complexity of the part shape, size, and material, there is no one-size-fits-all "how many or more molds" answer.　　However, there is a thought process for designers to make their own decisions.

First, we get a rough idea of the total planned production volume for the project.If these are a few to several dozen pieces, cutting should be selected without hesitationIt is.　　As you enter the realm of hundreds to thousands of pieces, you will need to be aware of the break-even point.

Next, consider the complexity of the design.　　For washers with simple shapes, the break-even point may be at the level of a few hundred units, since they can be handled with a relatively inexpensive die called a die-cutting die (Thomson die).　　On the other hand, for parts with complex three-dimensional shapes, the cost of the die will be high, so cutting will be more advantageous for a longer period of time.　　Cutting may be chosen even for production at the level of several thousand pieces.

Like this,The most reliable method is to consult with a manufacturer specializing in rubber processing once the designer has established some production prospects and design specificationsIt is.　　At that time, requesting estimates for both "cutting" and "mold forming" will clarify the specific break-even point and allow us to determine the optimal manufacturing method based on objective data.　*However, since quoting a mold at any cost puts a burden on the manufacturer, I believe it is also important to keep the design in the section from prototype to evaluation where specifications can be confirmed by cutting process.

Why the elasticity unique to rubber makes processing difficult

What makes rubber cutting fundamentally different from metal and resin cutting is rubber's unique elasticity.　　This property is a major challenge in achieving high-precision machining.

The most significant problems are "escape" and "deformation" during machining.　　Hard materials such as metal can be cut without deformation when a blade is applied.　　However, rubber is soft, so when the blade makes contact, the material is first crushed or stretched and then deformed.　　It may be easier to understand this phenomenon if you imagine the phenomenon that when you try to cut rubber bands with scissors, the rubber band is stretched before it is finally cut.　　After the blade passes through, the rubber tries to return to its original shape, but this series of deformation and restoration makes it very difficult to achieve the desired dimensions.

Second, there is the issue of frictional heat.　　Rubber has low thermal conductivity and tends to store heat.　　Frictional heat generated between the tool and material during cutting tends to accumulate at the machining point, softening or expanding the rubber.Cause of error in final dimensions due to dimensional changes during processing and shrinkage due to cooling after processingwill be.

To overcome these challenges, cutting rubber requires various innovations to control the elasticity of the material, including the use of very sharp specialized blades, adjusting the processing speed, and in some cases using special techniques such as freeze cutting, which will be discussed later.

Why cutting is chosen for prototyping and small lot production

There is a clear reason why cutting is frequently employed in the development process of rubber parts during the prototype and small lot production stages.　　This is because it minimizes time and economic risks in development.

The biggest reason is that molds are not required.　　In general molding, it is necessary to first manufacture a mold engraved with the product shape, which usually costs several hundred thousand to several million yen and takes about one month to complete.　　If design changes are necessary during the prototype evaluation stage, additional costs and time are incurred to modify and remanufacture the mold.

Cutting, on the other hand, can be started immediately with materials such as rubber blocks or round bars and 3D CAD data.　　Since there is no mold fabrication process at all, development lead time can be dramatically reduced.　　For example, when design changes occur, we can quickly respond by simply modifying the CAD data and immediately fabricate a prototype with the modified shape.

Like this,Cutting is an extremely flexible method that can meet the needs of the early development stage, such as "I want to check the shape of just one piece" or "I want to make multiple prototypes of a pattern for comparison and study.The result is a more cost-effective and successful project.　　This allows us to verify the functionality and shape of the product without the large initial investment of a mold, which is similar to the actual product, resulting in lower overall development costs and a higher success rate for the project.

Processing basics to know at the design stage

When designing rubber parts to be fabricated by cutting, an approach that takes into account the unique properties of rubber is required, which is different from the design of metal or plastic parts.　　Understanding this is key to avoiding problems in back-end processes and producing cost-effective products.

First, while rubber's elasticity directly affects machining accuracy, it can also be a design advantage.　　For example, in sealing parts, the elasticity of rubber itself can absorb slight dimensional deviations and ensure sealing.　　It is important to take advantage of this characteristic and set appropriate dimensional tolerances within a range that does not cause functional problems.Excessively tight tolerances, in the same sense as metalworking, can cause manufacturing costs to soar unnecessarily.

Also,Care must also be taken in shape designIt is.　　For example, walls or ribs with walls that are too thin-walled will cause the material to "chatter" or "deflect" under pressure during cutting, significantly reducing dimensional accuracy.Wall thickness should be as thick as possible to ensure rigidityis the radius of the tool to be used.　　Similarly, inside corners, such as pocket machining, will always leave a radius of R equal to the radius of the tool used. Since the indication of sharp corners (R) requires additional construction methods and increases costs, it is recommended that designs allow for R if it is not a functional problem.

Understanding these and other manufacturing constraints in rubber cutting from the earliest stages of design and incorporating them into the drawings is essential for smooth manufacturing and cost control.

Materials suitable for cutting rubber and main processing methods

What is the hardness of rubber that determines machinability?

The most important physical properties that determine the success or failure of the rubber cutting process areHardness."　It is. The hardness of rubber is generally measured with a measuring instrument called a "Durometer Type A" and is expressed in units of "Shore A".　　The higher the value, the harder the rubber, and the lower the value, the softer.

There is a close relationship between hardness and machinability (machinability).　　Rubber with high hardness (e.g., Shore A 80-95) is less deformed during machining and can be machined as if it were resin.　　Since "clearance" when the cutter is applied is suppressed, dimensional accuracy is easier to achieve and cutting surfaces tend to be more beautifully finished.

Conversely, rubbers with lower hardness (e.g., Shore A 30-50) are very soft and deform more during cutting, making machining significantly more difficult.Most processors can handle a range of hardnesses from 30A to 95A, but special techniques such as freeze cutting, described below, may be required, especially when high precision is required with low-hardness materials.

As a rough guide for designers to get a sense of hardness, a typical eraser is 30A-40A, and an automobile tire is about 60A-70A.　　It is very important in design to select materials by considering the balance between the function required for the part (e.g., sealing performance) and the hardness needed for processing.

Characteristics of easy-to-cut urethane rubber

Among the many rubber materials, urethane rubber (U) is particularly well suited for cutting.　In particular, those with a hardness of Shore A 90 have such excellent machinability that they can be processed as if they were plastic resin.

The reason urethane rubber is easy to cut is its high hardness and mechanical strength.　　This allows stable cutting with little deformation or "runaway" of the material when the cutter is applied during machining.　　As a result, it is easier to achieve higher dimensional accuracy and smoother finished surfaces than other rubber materials.

Urethane rubber also has the functional feature of being extremely abrasion resistant.　　This property makes it ideal for parts that are repeatedly subjected to friction and impact, such as rollers, sealing materials, cushioning materials (bumpers), and pads.

Thus, urethane rubber has both "ease of processing" and "excellent functionality," making it a material for rubber parts manufactured by cutting,It is considered one of the most promising options to consider first.　　This is especially true for prototypes and small-lot production, where precision is required.

Processing point of NBR with excellent oil resistance

Nitrile rubber (NBR) is a synthetic rubber that is very widely used in the industrial field due to its excellent oil resistance.　It is the standard choice of material for oil seals, packings, O-rings, and other parts used in environments exposed to mineral oils and lubricants.

From a cutting perspective, NBR is classified as a relatively easy material to machine.　　In particular, grades with hardness of 65A to 80A, which are commonly distributed, have moderate hardness, enabling stable cutting.　　Although not as hard as urethane rubber, good dimensional accuracy and surface finish can be expected.

One of the key points in processing NBR is to consider its intended use.　　Since NBR is often used for sealing parts that require oil resistance, the surface roughness of the sealing surface may be important.　　However, as mentioned above,Since it is difficult to obtain a smooth surface like that of a mold-formed product by cutting, it is important to clarify at the design stage what level of surface roughness is required and to indicate this in the drawings.

On the other hand, NBR has the weakness of poor weather resistance.　　It is not suitable for outdoor use because it is prone to cracking and other deterioration when exposed to direct sunlight (ultraviolet rays) and ozone for long periods of time.　　Thus, it is necessary to understand both the advantage of excellent oil resistance and the disadvantage of low weather resistance before applying NBR to parts in appropriate operating environments.

Cautions for silicon rubber, a difficult-to-process material

Silicon rubber (Si)is a material widely used from the food and medical fields to industrial products because of its wide temperature range (excellent heat and cold resistance) and high safety.　　However, it is also used in a wide range of applications, from food and medical fields to industrial products,One of the "difficult-to-machine materials" that are difficult to handle from the cutting point of viewIt is known as the

The main reason for this is that most silicone rubber has a low hardness of Shore A 30-50.　　Because it is so soft and highly elastic, it is greatly deformed when a knife is applied to it, making it extremely difficult to cut it to exact dimensions.　　Forcible attempts at machining can cause rough cutting surfaces and frequent burrs.

For this reason, cutting silicone rubber may require special techniques and ingenuity.　　For example, approaches such as the use of very sharp specialized tools and precise control of processing conditions are taken.　　In addition, when high precision is required or when processing particularly soft grade materials, a special method called "freezing cutting" (described below) is sometimes used, in which the material is temporarily hardened with liquid nitrogen or the like before cutting.

Designers should be aware that when silicone rubber is fabricated by cutting, it requires wider dimensional tolerances than other rubber materials.It is. In addition, taking advantage of the cutting process's advantage of not requiring a die, it is a realistic option to be applied to the production of prototypes and small-lot production.

Use of turning and milling

The most typical methods of rubber cutting are "lathe turning" and "milling.　　These two machining methods differ fundamentally in terms of whether the workpiece (material) or the cutter rotates, and each has its own specialty.

Lathe machining

Lathe turning is a method of machining by rotating a round rubber rod or other material to be machined at high speed and applying a fixed bit (blade) to it.　　Since the material rotates, this process is used to produce cylindrical or concentric rotating parts such as O-rings, rollers, shafts, and sleeves.　　It is possible to shave the outer and inner diameters and flatten the end faces.

milling

Milling is the opposite of lathe turning, in which a cutting tool (blade), such as an end mill, is rotated at high speed and applied to a fixed rubber material to cut it.　　By moving the tool in the X-, Y-, and Z-axis directions, it is possible to cut a flat surface, dig a groove or pocket (recess), or make a hole.　　It is suitable for creating three-dimensional shapes from square objects or plates.

Thus, if the part to be designed is cylindrical in shape, it should be lathed; if it is square or a complex three-dimensional shape, it should be milled.　　Some parts may be manufactured by a combination of both of these processes.Cutting BasicsIt would be good to be able to design parts while at the same time understanding the

Water-jet processing with no thermal effects

In preparing this article, I was reminded of this innovation myself.Waterjetting."It is a　　This is a very unique processing method that uses the power of water to cut materials. An ultra-high pressure pump compresses water to several thousand times atmospheric pressure and shoots it through a microscopic nozzle at approximately three times the speed of sound.　　This powerful water energy is used to cut the material as if it were being chipped away.

The greatest advantage of this processing method is that it is a "non-thermal processing" that does not generate any heat during processing.The heat is not a problem for heat-sensitive materials such as rubber.　　Even with heat-sensitive materials such as rubber, there is absolutely no concern about deformation or deterioration due to heat. Also, since this is a non-contact process that does not use a blade, no pressure is applied to the material, and even soft rubber can be cut into the designed shape without deforming it.

It is especially effective in the production of gaskets and packing with complex contours from rubber sheets.　　Because the machine can cut the gasket or packing into any shape based on CAD data, there is no need to make a mold (die), and prototypes and repair parts can be produced quickly and at low cost, even from a single sheet.

For actual processing examples,Websites of specialized manufacturers like Yamakata Corporationand examples of processing of various rubber materials are published and are very informative.

What is effective freeze cutting for soft rubber?

As mentioned above,Very soft rubbers such as silicone rubber are extremely difficult to cut normally　The "key card" to solving these problems is "cryogenic machining.　　A special technology that can be called a "trump card," so to speak, to solve these problems is "cryogenic cutting (cryogenic machining).　　In the course of researching this article, I was reminded that this technology greatly expands the approach to difficult-to-machine materials.

This is,Technology to temporarily freeze and harden rubber materials to be processed by cooling them with liquid nitrogen (approx. -196°C), etc.It is.　　Rubber in its glassy hardened state is cut using ordinary machine tools such as lathes and milling machines.

Freezing dramatically suppresses material "escape" and deformation during processing, making it possible to cut even soft materials that would be impossible to process in their normal state with high dimensional accuracy.　　Once processing is completed and the material returns to room temperature, the original softness and elasticity of rubber are restored.

This method is employed to fabricate prototypes of low-hardness rubber parts that cannot be handled by other processing methods, or materials that are so sticky that blades can become entangled.　　However, costs tend to be relatively high due to the need for special equipment, such as the use of liquid nitrogen.　　Specialized companies are using this technology to solve problems with difficult-to-process materials.

A must-see for designers! Accuracy in Rubber Cutting

Realistic approach to dimensional tolerancing

Dimensional tolerancing is a very important aspect of drawing rubber parts.　　It is especially easy for designers with extensive experience in metal fabrication to fall into the trap of setting tight tolerances in the same sense as for metal.

First,As a basic premise, general tolerances such as JIS B 0405, which assumes removal processing of metals, should not be applied directly to rubber cut products.It is.Rubber has a dimensional change (coefficient of linear expansion) with temperature that is 10 times greater than that of metal.and because of its large size and elastic deformation, it is not realistic to control it with the same precision as metal.

The dimensional tolerances achievable in cutting rubber depend primarily on the "hardness" of the material.The higher the hardness, the less deformation, so tighter tolerances can be targeted.　　Designers should establish realistic tolerances that can be manufactured, depending on the hardness of the rubber used.　　Below are some guidelines for tolerances by hardness.

Note: The above is only a general guideline and will vary depending on the shape and size of the part.

The key to designing parts that are cost-effective and easy to manufacture is to specify tight tolerances only where functionality demands tight tolerances, and to set tolerances as loose as possible in other areas.

Surface roughness to be noted in drawing instructions

Surface roughness　is a measure of how smooth or rough the surface of a part is.　　In drawings, the arithmetic mean roughness "Ra" is commonly used to indicate this.

The most important point that designers should be aware of regarding the surface finish of machined rubber parts is that they will not have the same smooth surface as a mold-formed part.　In mold forming, the polished mold surface is transferred directly to the product, resulting in a very smooth surface.　　On the other hand, in the cutting process, a certain degree of roughness is produced because marks (tool marks) are always left behind where the cutting tool has passed through.

It is recommended that surface roughness be indicated only where functional smoothness is required, such as sliding areas like sealing surfaces or areas that affect fluid flow.　　In many applications, the elasticity of rubber will absorb slight irregularities in the contact surface, so overly severe surface roughness requirements are not necessary.

Although it is only a guess, the following is a roughness guideline for surface roughness that can be achieved in general cutting operations by hardness.

Note: The above values are reference values assuming general cutting finish and are not guaranteed. Smoother surfaces can be obtained by polishing or other finishing processes.

It is also worth keeping in mind that very soft rubber may be difficult to measure accurately with a contact-type roughness meter because the measuring instrument's stylus may dig into the surface.

Summary: Rubber Cutting Essentials

This article has provided a comprehensive overview of what a machine designer needs to know to design rubber parts by cutting.　　Finally, we note the essentials for making the best choices.

• Cutting process requires no tooling and greatly reduces initial cost and delivery time
• It is the best method for prototype, small lot, and high-mix low-volume production
• Inferior to die molding in terms of cost in mass production
• Runaway" or deformation due to rubber elasticity makes processing difficult
• Frictional heat during machining also affects dimensional accuracy.
• The key to machinability is "hardness," and the harder it is, the easier it is to machine.
• Urethane rubber (especially hardness 90A) has extremely high machinability
• NBR is a general-purpose material with excellent oil resistance and good machinability
• Silicon rubber is soft and difficult to process.
• Turning" for cylindrical shapes and "milling" for three-dimensional shapes are basic
• Waterjet processing has no thermal effects and is ideal for cutting out soft materials
• Freeze cutting is a special technology for processing ultra-soft materials
• Dimensional tolerances are not based on metal standards, but on the hardness of the rubber
• Indicate surface roughness only where functionally necessary to avoid excessive quality
• It is a sensible flow to use cutting in the early stages of development and consider molds when transitioning to mass production.

Summary: Rubber cutting related links

The following five websites provide useful information on the "rubber cutting process.

• Fuji Rubber & Chemicals Co.
  • URL:. https://www.fujigom.co.jp/manufacturing/molding-method/54/
  • Description: A specialist manufacturer with in-depth knowledge of all aspects of rubber processing. In particular, this page provides a detailed comparison of the advantages and disadvantages of cutting and mold forming, and compiles very useful information to help designers make decisions on how to use both from prototype production to mass production. Special machining processes such as freeze cutting are also explained.
• Ono Rubber Industry Co.
  • URL:. https://www.onogomu.co.jp/knowledge/
  • Description: This page covers a wide range of rubber-related topics, from basic knowledge to specialized processing techniques. The characteristics of each type of rubber and the differences in cost structures between mold forming and cutting are explained in an easy-to-understand manner, providing authoritative reference information for material selection and cost calculation.
• Taiyo Rubber Industry Co.
  • URL:. https://jushi-gomu-sessaku.com/qanda/1131/
  • Description: This is a technical information site dedicated to the cutting of resins and rubbers. The pages provide very professional and practical information that is directly useful to machine designers in preparing drawings, with specific numerical values for the dimensional tolerances achievable for different hardnesses of rubber.
• Yamakata Corporation
  • URL:. https://www.yamakata.co.jp/waterjet/works_rubber.html
  • Description: Specializing in waterjet processing. On this page, you will find many examples of waterjet processing of various rubber materials, such as natural rubber and silicone rubber, with photos. This is a reliable source of information, especially when considering waterjet processing, because you can see the actual results of non-thermal and non-contact processing.
• Cletus Corporation
  • URL:. https://cretas.co.jp/rubber_processing_detail?actual_object_id=554
  • Description: A trading company specializing in industrial rubber and plastic products. This page systematically organizes the different types of rubber processing, and in particular, clearly explains the basic differences between "molding processing" and "cutting processing. The structure is easy to understand for designers who are learning rubber processing methods for the first time, and is highly reliable for obtaining basic knowledge.

That's it.