JP2012032285A - Foam inspection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inspection method for nondestructively detecting an internal defect of a foam so as to solve the problem in which there has been no effective inspection method since an ultrasonic wave, which is generally used for a nondestructive inspection method, is reflected against a boundary surface between a film layer and an air layer, which constitute a foam, and is not transmitted through a foam.SOLUTION: The problem is solved by using a burst wave ultrasonic wave or a chirp wave ultrasonic wave as the ultrasonic wave for a nondestructive inspection since they are transmitted through the foam.

Description

  The present invention relates to an inspection method for nondestructively detecting internal defects of a foam.

  Inside the foam product, voids may occur during the manufacturing process, or in the foam molded by the in-mold foaming method using the bead method, defects called poor fusion may occur, where the beads are not fused together. is there. The defects occurring inside these products are difficult to discover unless the product is cracked and inspected, and will be discovered for the first time when the foam is used at the customer site.

In order to deal with this problem, the foam manufacturer's factory conducts a sampling inspection, that is, an inspection is performed in which products are extracted at appropriate time intervals, and these products are divided to check for defects inside the products. If there is, the conditions of the manufacturing process are adjusted so that defects do not occur.
The product used for the inspection is discarded because it cannot be used as a product, and its cost is a problem.
In addition, since the frequency of product inspection cannot be increased, detailed process management is difficult. Therefore, the manufacturing process conditions are set on the safe side with a margin. As a result, production is performed with excessive heating, excessive cooling, and excessive molding time, resulting in a decrease in productivity and a decrease in energy efficiency.
Therefore, in a factory that produces foam, an apparatus for inspecting internal defects without destroying the product has been required.

  As a method for inspecting the internal defect of the foam, an apparatus using soft X-rays can be considered. However, since it is expensive and measures against radiation exposure are required, it is difficult to introduce the foam into a manufacturing factory. In addition, poor fusion that occurs in the in-mold foam molding method using the bead method cannot be discriminated whether the beads are fused or in contact with each other even if an image inside the molded product is seen. X-rays are not effective.

Therefore, application of ultrasonic flaw detectors has also been attempted, but the ultrasonic waves that have been used in conventional ultrasonic flaw detectors are pulse waves, and most of the pulse wave ultrasonic waves are formed between the foam substrate and the voids. Since it is reflected at the interface and does not pass through the inside of the molded body, it is difficult to detect internal defects.
On the other hand, as described in Patent Document 1, under special conditions, ultrasonic waves may pass through the molded body. However, the measurement object is limited to a foam having an epidermis, and is limited to special measurement conditions such as close contact with a probe that transmits and receives ultrasonic waves, and has low practicality.

  Accordingly, there is no appropriate inspection method for detecting the internal defects of the foam in a nondestructive manner, and development of the method has been awaited.

JP2000-187024

  The objective of this invention is providing the inspection method which can detect the internal defect of a foam comparatively cheaply and safely.

  As a result of intensive studies on the above problems, the present inventor has detected the transmittance of ultrasonic waves transmitted through the foam sample using burst wave ultrasonic waves and chirped wave ultrasonic waves having a relatively low frequency, and thereby the inside of the foam. It has been found that defects can be detected, and the present invention has been completed.

That is, the present invention
[1] An inspection method for nondestructively detecting internal defects of a foam,
Transmits burst wave or chirp wave ultrasonic waves toward the foam, receives the ultrasonic waves transmitted through the foam, and detects internal defects of the foam from the attenuation rate of the intensity of the transmitted ultrasonic waves A non-destructive inspection method for foam,
[2] The foam non-destructive inspection method according to [1], wherein the ultrasonic wave transmission and reception probe is non-contact with the foam to be inspected,
[3] The nondestructive inspection method for foam according to [1] or [2], wherein the frequency of burst wave ultrasonic waves or chirp wave ultrasonic waves is any of 16 kHz to 200 kHz,
[4] The nondestructive inspection method for foam according to [1] or [2], wherein the frequency of the burst wave ultrasonic wave or the chirp wave ultrasonic wave is any of 30 kHz to 150 kHz,
[5] The foam according to any one of [1] to [4], wherein the foam is a foam molded by an in-mold foam molding method using a thermoplastic resin as a base resin. The foam non-destructive inspection method and [6] The foam non-destructive inspection method according to [5], wherein the internal defect of the foam is poor fusion of the raw material foam beads.

  As in the non-destructive inspection method of the present invention, the intensity of transmitted ultrasonic waves obtained by transmitting burst wave ultrasonic waves or chirped wave ultrasonic waves toward the foam and receiving the ultrasonic waves transmitted through the foam is obtained. The internal defect of the foam can be detected from the attenuation rate of the foam.

  By the nondestructive inspection of the present invention, it is possible to perform a nondestructive inspection that can detect an internal defect of a foam, which has been difficult in the past, relatively inexpensively and safely.

It is a figure which shows an example of a structure of the test | inspection apparatus which concerns on the embodiment of this invention. It is a figure which shows arrangement | positioning of a probe based on the embodiment of this invention. It is a figure which shows arrangement | positioning of the probe based on another embodiment of this invention. It is a figure which shows the relationship between the fusion rate which concerns on Example 1 of this invention, and the echo height of a transmitted wave.

  The nondestructive inspection method for foam according to the present invention transmits burst wave ultrasonic waves or chirp wave ultrasonic waves toward the foam, and receives the ultrasonic waves transmitted through the foam. This is a nondestructive inspection method for a foam, wherein an internal defect of the foam is detected from an attenuation rate of strength.

  Burst wave ultrasonic waves and chirp wave ultrasonic waves are a kind of ultrasonic waves. A burst wave is a pulse wave with the same frequency that is repeated multiple times. A chirp wave changes the frequency of a pulse wave (ie, modulation). For example, it is described in JP2009-276319A.

When burst wave ultrasonic waves or chirped wave ultrasonic waves are applied to a non-destructive inspection method, it is possible to transmit a foam, which is difficult with general ultrasonic waves, because it has a characteristic of excellent SN ratio.
In general, a signal obtained from a reception probe includes noise. The ratio between the signal (Signal) and noise (Noise) is expressed as the SN ratio. Here, if the S / N ratio is low, it means that a meaningful signal is buried in noise and cannot be measured.

That is, the ultrasonic wave emitted from the transmission probe is attenuated by passing through the foam, but if it becomes a weak signal exceeding the limit, it cannot be distinguished from noise and measurement is impossible. Noise is an electrical signal that occurs irregularly, but the ultrasonic signal generally used for measurement is a pulse signal (that is, a one-time wave). When the ultrasonic signal becomes weak, it is generated many times, It is difficult to improve the signal-to-noise ratio even if the average is taken.
Signals called burst wave ultrasound and chirp wave ultrasound are targeted for the entire regularly repeating wave, so by calculating the correlation of the entire wave that fluctuates regularly between input and output, It becomes easy to eliminate irregular noise. Therefore, in the burst wave ultrasonic wave and the chirp wave ultrasonic wave, the SN ratio is improved, that is, the ultrasonic signal is easily transmitted.

  As mentioned above, burst wave ultrasonic waves and chirp wave ultrasonic waves can pass through the foam to be measured, so internal defects are detected by evaluating the attenuation rate when the foam is passed through. It becomes possible to do.

That is, when there is no defect inside the foam, the walls of the bubbles constituting the foam are connected to each other and are restricted from vibrating freely. In this case, most of the ultrasonic waves cannot be transmitted because most of the ultrasonic waves are reflected on the surface of the wall. Therefore, the attenuation rate increases.
On the other hand, when there are large cavities or beads are not fused together in the in-mold foam molding method, the vibration of burst wave ultrasonic waves and chirped wave ultrasonic waves may be attenuated due to resonance of the cavity portions. Since it decreases, it becomes easy to transmit.

  In the nondestructive inspection method for a foam according to the present invention, a calibration curve is created by measuring in advance the relationship between the attenuation rate and the fusion rate of burst wave ultrasonic waves and chirp wave ultrasonic waves that have passed through the foam. Subsequently, the fusion rate can be detected nondestructively by measuring the attenuation rate of the ultrasonic wave transmitted through the foam to be inspected and referring to the calibration curve.

Here, in the nondestructive inspection method for foam according to the present invention, the ultrasonic wave transmitted through the foam is called “echo”, and the intensity of the echo waveform is called “echo height”.
If the attenuation rate of the ultrasonic wave is small, the echo height is high, and if the attenuation rate is large, the echo height is low. Therefore, the echo height is measured as an index reflecting the ultrasonic attenuation rate. Therefore, in the nondestructive inspection method for foams of the present invention, a calibration curve relating to the relationship between echo height and fusion rate is used. With the obtained calibration curve, the fusion rate can be estimated from the measured echo height.

  In the nondestructive inspection method for foam according to the present invention, burst wave and chirp wave ultrasonic waves are converted into electric signals in an ultrasonic wave transmitting / receiving device (also referred to as “pulser / receiver”) 2 as shown in FIG. Emitted from the transmission probe 3 as ultrasonic waves. The ultrasonic wave transmitted through the foam is received by the receiving probe 4, converted into an electric signal, amplified by the preamplifier 5, and input to the ultrasonic wave transmitting / receiving device 2 to perform necessary signal processing. The echo wave (that is, the received wave that has passed through the molded body sample) is displayed on the display device 6.

  In order to obtain an echo waveform accurately in the inspection method of the present invention, the frequency of the ultrasonic wave used for the inspection is preferably any frequency within the range of 16 to 200 KHz, and any one within the range of 30 to 150 KHz. The frequency of is more preferable.

In general, when ultrasonic waves are transmitted through the inside of the foam, if the frequency of the ultrasonic waves is high, most of the ultrasonic waves are scattered on the wall surfaces of the foam bubbles and do not pass through the inside of the foam. Therefore, the ultrasonic frequency should be set low.
On the other hand, in order to detect a defect, it is necessary to make the wavelength of the ultrasonic wave the same as or less than the size of the defect. This is because if the wavelength is longer than the object to be measured, ultrasonic scattering and diffraction phenomena cannot be ignored, and it is difficult to obtain an echo waveform with high accuracy.

By the way, there exists a relationship of (1) Formula between the wavelength (lambda) of an ultrasonic wave, the frequency f, and the sound speed v.
λ = v / f (1)
Since the sound velocity v in the air at 15 ° C. is 340 (m / sec), the relationship between the frequency and the wavelength is as shown in Table 1 when calculated by substituting into the equation (1).

  When the frequency of the ultrasonic wave exceeds 200 KHz, the wavelength is shortened, so that a fine defect of about 1.7 mm can be detected, but the ultrasonic wave itself tends to be difficult to transmit inside the foam. Moreover, if the frequency of the ultrasonic wave is lowered to less than 16 KHz, the ultrasonic wave is easily transmitted, but the wavelength becomes long, so that a fine defect of 20 mm or less may be overlooked.

  Although affected by the base resin of the foam and the shape of the bubbles, as a condition that satisfies both the permeability and resolution, the ultrasonic frequency is most often in the range of 30 to 150 KHz. Suitable for detection.

  In general, in non-destructive inspection using pulse wave ultrasonic waves, it is necessary to close the probe to the inspection target, or to interpose water or a gel-like substance to eliminate air between the inspection target object and the probe. There is. This is because if the density difference between the air and the object to be inspected is large, ultrasonic waves are reflected at the interface between the two and do not transmit.

On the other hand, burst wave ultrasonic waves and chirp wave ultrasonic waves can be transmitted to some extent even if they are emitted from the air toward the inspection object. Therefore, when using burst wave ultrasonic waves or chirp wave ultrasonic waves, it is not necessary to use a medium such as water or gel, or to closely contact the probe to the inspection object, that is, non-contact inspection is possible. Become.
Therefore, the non-destructive inspection method for foam according to the present invention is a very advantageous method in that the structure of the inspection facility is simplified and the operation is quick.

In the inspection method of the present invention, as shown in FIG. 2, the transmitting probe 3 is placed on one surface of the inspection target sample 1 and the receiving probe 4 is placed on the opposite side in a state where it is not in direct contact with the inspection target sample. Deploy. Then, the ultrasonic transmittance at each point is measured while moving the transmitting probe and the receiving probe along the surface of the sample.
The obtained transmittance data is input to the computer for data processing, and if there is an abnormal value (that is, a transmittance indicating that the fusion rate is low beyond the limit), an alarm is issued to detect defective products. can do. In addition, by displaying the transmittance of the inspected surface by contour lines or color coding, the distribution of the fusion rate of the sample is displayed, adjustment of molding conditions, abnormalities of the mold, feeder (resin that supplies resin into the mold) ) Can also be monitored.
Therefore, the inspection method of the present invention is an effective inspection method for process management.

In the inspection method of the present invention, as shown in FIG. 3, the probe 3 is disposed at an oblique position with respect to the sample 1 to be inspected, ultrasonic waves are transmitted obliquely, the inside of the sample is transmitted, and the same It is also possible for the reception probe 4 to receive a transmitted wave that has returned to a position distant from the surface side.
According to this method, since a portion located between the transmitting probe 3 and the receiving probe 4 that are arranged at a distance can be inspected at a time, if the product size to be inspected is large, the presence or absence of defects can be detected in a short time. Can be inspected.
In the inspection method of the present invention, depending on the characteristics of the inspection object, the measurement accuracy can be improved by bringing the probe into contact with the inspection object.

  The object of inspection in the present invention is a foam. Preferably, it is a foam molded by an in-mold foam molding method using a bead method using a thermoplastic resin.

  The in-mold foam molding method using the bead method is filled with resin beads that have been foamed to the final foaming ratio of the molded body in advance and foamed by heating with steam, and the beads are fused together to foam. This is a molding method for obtaining a molded body. In the in-mold molding method, the gap between the beads is filled by the foaming pressure, and at the same time, the beads whose temperature has increased are pressurized and fused.

The problem at that time is when the gaps between the beads seem to be filled and integrated, but they are in contact but not fused.
In such a case, when the user cuts the foamed molded body or when a force is applied during use, the beads are separated, and a defect of poor fusion becomes apparent for the first time. When inspecting during the manufacturing process, it is not possible to determine whether or not there is a defect of poor fusion unless the foamed molded product is broken.

  According to the inspection method of the present invention, such a defective fusion defect can be detected in a non-destructive manner, and the entire product can be inspected, which is an extremely useful inspection method.

In the inspection method of the present invention, when transmitting ultrasonic waves of burst waves and chirp waves to the in-mold foam molded body by the bead method, if the beads are fused together, the ultrasonic waves are not easily transmitted and obtained. The echo height is lowered. On the other hand, when the fusion between the beads is poor, the ultrasonic wave is easily transmitted, and the echo height obtained is high.
In addition, when transmitting ultrasonic waves, if there are voids inside the foam, it is also easy to transmit, resulting in a higher echo height and detection of defects due to voids.

  An inspection apparatus and measurement conditions according to an embodiment of the foam inspection method will be described with reference to FIG.

The foam sample 1 to be inspected is a plate-like sample having a thickness (thickness of 60 mm), and the transmitter probe 3 (Japan probe) is spaced at a distance of 50 mm between the sample 1 and the opposite position. The receiving probe 4 (manufactured by Japan Probe Co., Ltd., 0.05K50 × 50A) is arranged on the opposite side.
A burst wave ultrasonic signal generated by an ultrasonic transmission / reception device 2 (Japan Probe Co., Ltd., JPR-IC) having a frequency of 47 kHz and a repetition of 5 waves is applied to the sample 1 as an ultrasonic wave from the transmission probe 3, The ultrasonic wave transmitted through the sample 1 is received by the reception probe 4 and amplified by the preamplifier 5 (Japan Probe Co., Ltd., PR-60A), and the ultrasonic transmission / reception device 2 (Japan Probe Co., Ltd., JPR). -IC), signal processing is performed, the received waveform (that is, echo wave) is displayed on the display device 6 (notebook personal computer, OS: Windows (registered trademark) XP), and the echo height is read.
The used foam sample 1 is a foamed polypropylene molded body (height 295 mm × width 393 mm × thickness 60 mm, expansion ratio 45 times) molded by an in-mold foam molding method using a bead method, and has a fusion rate of 0%. 20%, 60%, and 100% foams were prepared.
In the measurement, burst wave ultrasonic waves were transmitted so as to transmit in the thickness direction of the foam sample.

The echo height in the four-level sample was read from the display device, and the echo height of the transmitted wave was recorded. In FIG. 4, the echo height when the fusion rate is 0% is set to 100%, the relationship between the fusion rate and the echo height is shown (in the figure, ▲), and a calibration curve is shown.
Separately, for foamed polypropylene moldings with the same shape and expansion ratio and a fusion rate of 0%, 30%, 70%, and 100%, the same operation was performed, and the resulting echo height (● in the figure) When plotted, as shown in FIG. 4, it almost coincided with the calibration curve.

DESCRIPTION OF SYMBOLS 1 Foam sample 2 Ultrasonic transmission / reception apparatus 3 Transmission probe 4 Reception probe 5 Preamplifier 6 Display apparatus

Claims (6)

A non-destructive inspection method for detecting internal defects in foam,
Transmits burst wave or chirp wave ultrasonic waves toward the foam, receives the ultrasonic waves transmitted through the foam, and detects internal defects of the foam from the attenuation rate of the intensity of the transmitted ultrasonic waves A non-destructive inspection method for a foam.   2. The foam non-destructive inspection method according to claim 1, wherein the ultrasonic transmission probe and the reception probe are not in contact with the foam to be inspected.   The nondestructive inspection method for foam according to claim 1 or 2, wherein the frequency of the burst wave ultrasonic wave or the chirp wave ultrasonic wave is any one of 16 kHz to 200 kHz.   The nondestructive inspection method for foam according to claim 1 or 2, wherein the frequency of the burst wave ultrasonic wave or the chirp wave ultrasonic wave is any one of 30 kHz to 150 kHz.   The foam according to any one of claims 1 to 4, wherein the foam is a foam formed by an in-mold foam molding method using a bead method using a thermoplastic resin as a base resin. Non-destructive inspection method.   6. The nondestructive inspection method for a foam according to claim 5, wherein the internal defect of the foam is a poor fusion of the raw material foam beads.

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