"The Science of Socks": A Visit to Naigai's Quality Control Department — A Look Behind the Scenes of Quality Testing at a Specialized Manufacturer

Our editorial team visited the research facility of Naigai Co., Ltd., a company with a long history as a sock manufacturer.Naigai Quality Control DepartmentI visited [...].
This is not merely a place where socks are manufactured; it is a research and testing hub designed to scientifically validate the “quality” and “comfort” of each and every product. It is particularly noteworthy that this testing laboratory is located right next to the Naigai Customer Service Center, allowing us to immediately review customer feedback.　Furthermore, having obtained registration under the JNLA (Industrial Standardization Act Testing Operator Registration System), it also functions as a testing laboratory recognized by public authorities.

As soon as I stepped into the lab, I saw testing machines for checking color transfer caused by sweat or laundry, devices for quantifying resistance to abrasion and tearing, and even mannequins that replicate actual foot shapes to verify comfort—it was an environment that could almost be described as a “comprehensive hospital for socks.”

I’m truly impressed by the dedication of a specialized manufacturer that goes to such lengths to establish such comprehensive testing and standards for something as everyday as socks. On this visit, I toured their research facilities firsthand and uncovered the secret behind why Nagai has continued to earn people’s trust for so many years.

Will the color fade from sweat or washing? How much force does it take to tear the fabric? Will it keep its shape even after wearing it for a long time?—

At first glance, socks may seem simple, but behind the scenes, they undergo far more testing than you might imagine. In this section, we’ll introduce some of the experiments actually conducted at the Naigai Research Institute.

1. Sweat Hardness and Water Hardness

It’s inevitable that socks will get wet from sweat or rain. So, we tested them in a controlled environment to verify whether they truly resist color fading and color transfer.

• Overview: We used two types of artificial sweat—alkaline and acidic—to simulate actual wear conditions. We bonded the materials at 37°C, which is close to body temperature, to check for color transfer.

• Purpose: We tested whether the color fades or bleeds when exposed to sweat.

• Method: Soak in artificial sweat for 30 minutes → Press at 37°C for 4 hours → Evaluate after drying. Grade using a grayscale scale.

Place product scraps and a pure white cloth on the plate of this device. Since human sweat can be either alkaline or acidic, we prepare two types of artificial sweat solutions—one alkaline and one acidic—to conduct the test. We also prepare two types of white cloth—cotton and nylon—for verification.

Use weights to secure the sides while the fabric is under load. (*For demonstration purposes, fewer boards than usual are being used.)

With the test specimen sandwiched between them, maintain a temperature of 37°C (±2°C) to simulate the conditions of the fabric being pressed against the skin at body temperature. Leave it undisturbed for 4 hours and observe any changes. Remove the product sample and the white cloth, then allow them to dry. This allows you to determine the extent of color transfer to the white cloth.

2. Wash Fastness Test

It’s the daily cycle of washing that’s the biggest enemy of socks. These are washing tests conducted under conditions that are even harsher than real-life use.

• Overview: Testing for color bleeding during washing. Since the test is conducted at a high temperature of 50°C, it simulates conditions that are more severe than those encountered in real life.

• Purpose: Check whether the quality can be maintained after repeated washings.

• Method: Wash the test fabric in a detergent solution for 30 minutes → Check for fading or color transfer. Evaluate using a grayscale scale.

As shown here, the product samples and綿とNylona piece of fabric sewn onto it,

The test is conducted by placing the sample in a dedicated stainless steel container along with a detergent solution of a specified concentration heated to 50°C.

Seal it tightly and place it in a machine capable of simulating the washing process.

Fill this machine with water at 50°C, place the stainless steel cylinder mentioned earlier inside, and let it spin for 30 minutes. This test is conducted under conditions far more rigorous than those of a typical household washing machine. After the test, the item is rinsed with water, hung to dry, and then checked for any color transfer onto the fabric.

3. Burst Strength Test

How much force can sock fabric withstand? This is a dramatic test in which the fabric is inflated from below, just like a balloon, to measure the exact moment it bursts.

• Overview: The fabric is inflated from the bottom, and the pressure at which it ruptures is measured.

• Purpose: Quantifying fabric durability.

• Method: Inflate the rubber membrane hydraulically → Measure the pressure at which the fabric ruptures.

Attach the fabric you want to measure, and

A rubber membrane is inflated from below by hydraulic pressure, pushing the dough upward.

The fabric tore. Apparently, it’s considered safe as long as the pressure is 400 kPa.

This fabric ruptured at 660 kPa.

4. Wear Test

"Friction" is something that inevitably occurs when you walk. We replicated its effects thousands of times using metal plates to thoroughly test whether the socks were resistant to abrasion.

• Overview: To simulate the friction that occurs when the shoes are actually worn and to test their resistance to wear.

• Purpose: Verify whether holes form after repeated use.

• Method: Repeatedly rub against a metal plate (more than 500 times) → Check for changes in the surface.

Place the fabric with the heel section cut out in position, and rotate the top and bottom in the same direction while rubbing the steel blades together.

Rub the fabric 2,000 times to see if it tears during that process. The standard is whether it can withstand at least 500 rubs.

As you can see, the system is designed so that you can see how many times you've swiped the meter.

5. Friction Hardness Test (Dry and Wet)

We check whether the color transfers to other fabrics when the fabric is dry or wet. This is a meticulous test designed to prevent common clothing problems, which is why it is conducted in a dedicated room.

• Overview: Rub with a dry cloth and a wet cloth to check for color transfer.

• Purpose: Check to ensure the color does not transfer to other clothing or shoes.

• Method: After friction, evaluate in grayscale.

Constant Temperature and Humidity ChamberThis is a test conducted in a room—known as a —where the temperature and humidity are kept constant.
This room is also equipped with the burst tester and abrasion tester mentioned earlier, allowing the same tests to be conducted under constant temperature and humidity conditions.

Prepare a dry white fabric and a damp white fabric, attach each to the machine, and rub them 100 times. This simulates wet and dry friction conditions to check how much color transfers onto the white fabric.

You can see a slight color transfer.

6. Color Assessment Under a Light Source

We verify color transfer onto a white test cloth under standard lighting conditions through various tests.

• Overview: Accurately evaluate color fading and color bleeding by comparing the fabric to a grayscale under D65 lighting (standard light source).

• Purpose: To accurately evaluate color changes and determine whether a product meets quality standards

• Additional Information: Once a year, we also coordinate with an external organization to conduct our own verification of certification qualifications.

White test specimens that have undergone color transfer during various tests are evaluated using a grayscale scale.
There are two types of grayscale, andThe whitish one in the foreground shows "how much color has adhered."This is for the purpose of verifying [...].
On the other hand,The darker one in the back shows "how much color has faded."It is used to verify [...].

We objectively evaluate all types of color transfer—including yellow and red—using this grayscale. In the past, there were separate color evaluation scales for each color, but it was ultimately determined that judging based on “shades of gray” was more accurate, so the system has now been standardized to the grayscale.

Grades such as “Grade 1,” which are commonly seen, are indicated on the back of the grayscale chart. Grading is based on half-grade increments and is performed visually.

However, since there are inevitably individual differences, they reportedly submit the same fabric to other public institutions to verify that there are no discrepancies in the results.

Apparently, covering the test specimen in this way makes it even easier to see.

7. Fit Test Using Foot Impressions

They put socks on lifelike mannequins modeled after employees' feet to check for changes in elasticity and fit after washing. This allows for testing that closely mimics actual wear.

• Reason: Check for any changes in stretch or fit after washing.

• Method: Realistic mannequin feet modeled after employees' actual footprints are used.

• Overview: Evaluated under conditions that closely resemble actual wear.

Unlike typical mannequins, we create our own foot molds that closely resemble standard foot shapes. We then put actual socks on these molds and have them worn again after washing to verify how they stretch and fit under real-world conditions.

8. Observation of Fibers Under a Microscope

When you examine the fabric under a microscope, you can even identify the causes of wear and tear or fractures. It’s a detective-like investigative test that’s also useful for handling customer complaints.

• Overview: We examine thread breaks and fuzziness under a microscope to identify the cause of the defect.

• Utilization: Also used for handling customer complaints and investigating defective products.

By examining the fabric under a microscope, you can verify the actual condition of the holes. Furthermore, magnification allows you to determine the condition of the fibers—for example, whether the holes were caused by snagging or by being cut with scissors. (The photo shows an example observed at 100x magnification.)

This is how it looks on colored fabric.

The elastic yarn used in socks (such as NDX) cannot be observed directly under a microscope because it is woven together with surrounding fibers. Therefore, when there are complaints regarding the elasticity of the yarn, it is said that the elastic yarn is removed strand by strand for inspection.

Socks are something we use without a second thought in our daily lives, but I was surprised to learn that such multifaceted and meticulous testing is conducted to ensure their quality.

From color transfer caused by sweat or water, to changes resulting from repeated washing, durability against tears and wear, and even the subtle assessment of color under various light sources—I was reminded once again that behind every pair of socks we feel “safe to wear” lies a wealth of scientific experimentation and verification.

What impressed me most was how every test was strictly focused on “replicating real-world usage conditions as faithfully as possible.” From crimping at temperatures close to body temperature to abrasion tests exceeding 500 cycles, their commitment to verifying quality under conditions even more rigorous than those encountered in daily use truly reflects their pride as a specialized manufacturer.

After completing the tour, I was thoroughly convinced that it is precisely because of these rigorous tests that Naigai’s socks have continued to earn the public’s trust for so many years.

Next up is Naigai Lab.

When people hear “sock manufacturer’s research lab,” many might imagine a place where the strength of fabrics and fibers is tested.
However, what Naigai Lab is truly focused on is not “socks” themselves, but rather what needs to be protected through socks—A person's footThat's it.

From 3D foot measurements and analysis of pressure on the soles of the feet during walking to the Locomo Test, which assesses the health of older adults, Naigai Lab, as a “comprehensive foot research institute,” is engaged in initiatives that span from everyday comfort to healthy life expectancy.

Studying socks means, in other words,Researching the Future of the Human Foot。
In the following section, we’ll introduce some of the unique measurements and tests actually being conducted at that site.

9. Foot Measurement Using a 3D Scanner

By accurately digitizing foot shapes, they were able to identify tendencies such as bunions and flat feet. They went beyond just socks and ventured into the field of “foot research.”

• Overview: The shape of the foot is measured in three dimensions using a 3D scanner.

• Purpose: We digitize foot shapes and measurements and use the data to develop socks.

• Additional Information: The tendency toward bunions and flat feet is also quantified.

This machine allows you to scan feet in three dimensions.

This is an actual scan of a foot.

We can digitize all foot measurements—such as foot length, width, and heel width—using a scanner. The results revealed a significant difference in size between my left and right feet.

10. Measurement of Pressure on the Soles of the Feet While Walking

Where does your center of gravity shift when you walk? By visualizing the pressure on the soles of your feet using a color-coded map, you can uncover walking habits you might not normally notice.

• Overview: Walk on a special mat to visualize the weight distribution on the soles of your feet as you walk. It displays numerical values showing where the force is concentrated.

• Purpose: Identify walking habits and imbalances in body weight distribution, and use this information to provide advice on healthy walking and to inform sock design.

• Additional Information: It can also help identify issues such as “floating toes” and “weight shifting to one foot,” which in turn helps prevent ingrown toenails and joint problems.

It can measure which parts of the soles of your feet bear the most weight when you walk.

The areas highlighted in red are where particularly strong pressure is being applied. The measurement results showed that your center of gravity is shifted forward on your left foot, while you walk with your weight on the heel of your right foot. Additionally, since no pressure is being applied to the toes other than the big toe, it would be a good idea to make a conscious effort to use all your toes when walking from now on.

11. Center of Gravity Variation Meter and Balance Age Assessment

It quantifies body sway while standing upright, leaning forward, or leaning backward. The results are displayed as your “balance age,” providing an easy-to-understand indication of your core strength and stability.

• Overview: It measures body sway while standing upright, leaning forward, leaning backward, and tilting to the left or right, and calculates stability as a "balance age."

• Purpose: Objectively assess core strength and balance to help prevent falls and improve exercise habits.

• Additional Information: The system is designed so that the smaller the variation, the lower the result; the difference between this result and your actual age serves as an indicator of your health.

This is a machine that measures the range of motion and stability of the center of gravity and calculates a person’s “balance age.” The subject stands upright on the machine and then leans forward, backward, to the right, and to the left, holding each position for 10 seconds while the machine takes measurements.

The dark blue lines indicate the trajectory of the center of gravity in each posture, while the light blue lines indicate the range of motion. The "balance age" comprehensively reflects factors such as core strength and ankle flexibility; the lower the number, the better the balance is considered to be.

12. Locomotive Syndrome (Locomo) Test

A test to determine whether older adults can walk on their own. There was also equipment that measured health based on daily activities, such as standing up from a seated position on one leg.

• Overview: A test to assess the physical health of the legs and lower body in older adults. The assessment is based on multiple evaluation criteria, such as whether the person can stand up from a certain height.

• Purpose: To assess whether individuals are maintaining their ability to stand and walk on their own—that is, their mobility—and to use this information to help prevent the need for long-term care and extend healthy life expectancy.

• Additional Information: Participants undergo a test to assess their ability to stand up on one leg or both legs, as well as a two-step test to measure their stride length; if their results fall below the standard, they are classified as “at risk for locomotive syndrome” and are advised to take steps to improve their condition.

Naigai Lab brings its measuring equipment to senior care facilities in Minato Ward and other locations to conduct foot measurement sessions. Based on the results of 3D foot scans, locomotion tests, and physical fitness assessments, they create a “foot report card” to determine whether older adults are experiencing difficulty walking due to weakened leg and lower-body muscle strength.

In addition, the organization actively engages in initiatives that connect its research activities with the local community—such as holding workshops for children that utilize scrap materials from socks—in addition to the health screenings for seniors.

During my recent tour of Naigai’s Quality Control Department and Naigai Lab, I was strongly struck by the fact that sock manufacturing goes far beyond simply “producing textile products.” In addition to quality tests—such as checking for color fading and bleeding caused by sweat and washing, as well as durability against wear and tear—the company scientifically collects data on foot shape and gait characteristics, and even engages in health monitoring for the elderly and community activities.

I was surprised to learn that such thorough research and testing goes into something as everyday as socks. Naigai is not only a “sock manufacturer” but also a “research institute dedicated to protecting the comfort and health of people’s feet.”

Through this site visit, I truly came to realize that it is this cumulative effort that has built the trust that has made this sock brand a beloved one for many years.