JP3470574B2 - Inspection method for can lid defects - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of inspecting a can lid having a plastic film laminated on the inner surface of the can for defects such as pinholes in the plastic film. The present invention relates to a method for inspecting a plastic film on a can lid for defects by applying a high voltage.

[0002]

2. Description of the Related Art In the case of a can lid used for highly corrosive canned foods, such as canned soup, canned fish meat and canned vegetables, which contains salt, if at least one pinhole is present in the inner protective film, Pitting corrosion may progress from the pinhole portion, resulting in a defective product. Therefore, the inner surface of this kind of can lid is usually laminated with a plastic film such as polyethylene terephthalate. However, the bottom of the rivet for attaching the knob tab and the score for tearing opening (see FIG. 3).
(See) has scratches when the thickness is reduced due to strong working, and pinholes are easily generated. As a method of inspecting such a pinhole, Japanese Patent Laid-Open No. 3-160355 discloses a method of pressing a conductive elastomer body of an electrode into close contact with a portion of a plastic film layer to be inspected of a metal can lid so that the electrode and the can. A defect inspection method for a plastic film layer has been proposed in which a high voltage is applied between the metal plates of the lid and the current flowing between the electrodes and the metal plate is detected. In the case of this inspection method, since it is difficult to bring the conductive elastomer body electrode into close contact with the bottom surface of the rivet portion where pinholes are most likely to occur, there is a problem that inspection leakage easily occurs. Further, according to the experience of the present inventor, when a can lid which passed the inspection is subjected to an enamellator test immediately after the inspection (for the test method, see the right column on page 1 of the above publication), a pinhole is generated. However, there is a problem that it is not suitable for inspecting a can lid for highly corrosive canning. In addition, there is a problem in that pressing takes a little time and is not suitable for high-speed inspection.

[0003]

SUMMARY OF THE INVENTION According to the present invention, the inner surface of a can lid by applying a high voltage is suitable for high-speed inspection of a highly corrosive can lid for canning, which is less likely to cause inspection leakage on the bottom surface of the rivet portion. It is an object to provide a defect inspection method for plastic films. Another object of the present invention is to provide a reliable defect inspection method for the plastic film on the inner surface of the can lid.

[0004]

A method for inspecting a defect of a can lid according to claim 1 is a method for inspecting a plastic film layer for a defect in a can lid formed of a metal plate having a laminated plastic film layer. In the above, the electrodes are opposed to each other in the vicinity of the plastic film layer, an impulse high voltage is applied between the metal plate and the electrode, and the defect is inspected based on the voltage generated between the electrode and the metal plate. At the same time, or immediately after the inspection, the charge charged on the plastic film layer is applied by applying a reverse voltage.
It is characterized by removing by. The term "defect" as used herein mainly means a pinhole penetrating the plastic film layer, and a portion having a thickness of about 5 μm or less and broken by application of a high voltage higher than the dielectric breakdown field strength of the film layer to become a pinhole. Also says. The electrodes are opposed to each other in the vicinity of the plastic film layer, an impulse high voltage is applied between the metal plate and the electrode, and the defect is inspected based on the voltage generated between the electrode and the metal plate. If the plastic film layer is defective, corona discharge will occur and the voltage developed between the electrode and the metal plate will be lower than if there were no defects. Since the occurrence of corona discharge is hardly affected by the slight difference in the distance between the metal plate and the electrode, it is difficult to cause inspection leakage on the bottom surface of the rivet portion or the like. The electrodes are placed close to the plastic film layer for inspection, that is, non-contact inspection, the applied voltage is impulse, and the application time is extremely short, so it is suitable for high-speed inspection. . Since the electric charge charged in the plastic film layer is removed at the same time as the inspection is completed or immediately after the inspection, it is possible to prevent the generation of new defects due to the charged electric charge. Therefore, the inspection of the can lid for highly corrosive cans can be performed. Suitable for Further, by applying a reverse voltage, a charge having a polarity opposite to the charge charged in the plastic film layer is charged, the charges of both types are canceled, and the charge is substantially eliminated from the plastic film layer. Since the time for applying the reverse voltage may be extremely short, it is suitable for high-speed inspection as compared with a method of blowing ionized air to remove charges.

A method for inspecting a defect of a can lid according to claim 2 is
The defect inspection method according to claim 1, wherein the portion of the electrode that faces the plastic film layer is composed of a large number of protrusions having sharp tips. Since the portion of the electrode facing the plastic film layer is composed of a large number of protrusions having sharp tips, corona discharge is likely to occur. Therefore, it is more difficult to cause inspection leakage on the bottom surface of the rivet portion. It is also more suitable for high-speed inspection.

A method for inspecting a defect of a can lid according to claim 3 is
The defect inspection method according to claim 1 or 2, wherein the impulse high voltage is an induced electromotive force generated when a direct current flowing through the coil is cut off, and in the case of a can lid having a plastic film layer having no defect, between the electrode and the metal plate. It is characterized in that it is determined that there is a defect when a voltage of a predetermined value or lower much lower than the peak value occurs by a predetermined time when the peak value of the voltage has occurred. The induced electromotive force is suitable for high-speed inspection because it has a fast rise. If the plastic film layer is defective, the induced electromotive force will rise rapidly,
In the case of a can lid having a defect-free plastic film layer due to corona discharge, the peak value of the voltage is generated between the electrode and the metal plate. ) Drops sharply. Therefore, the defect can be surely detected.

A method for inspecting a defect of a can lid according to claim 4 is
The defect inspection method according to claim 3, wherein the impulse high voltage is an induced electromotive force generated in the secondary coil when the DC current flowing in the primary coil of the transformer is repeatedly disconnected at a relatively high frequency. To do. Every time the DC current flowing through the primary coil of the transformer is repeatedly cut off,
The induced electromotive force generated in the secondary coil becomes high. Since the DC current is repeatedly cut at a relatively high frequency, the voltage rises rapidly. A higher impulse high voltage can be obtained by increasing the turn ratio n between the primary coil and the secondary coil. Therefore, it is more suitable for high-speed inspection, and reliable inspection is possible.

[0008]

[0009]

[0010]

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIGS. 1, 2 and 3, reference numeral 1 denotes a can lid, which is formed of a metal plate 7 laminated with a polyethylene terephthalate plastic film layer 6 and is on the upper surface side of the drawing, that is, the inner surface of the can. The plastic film layer 6 is provided on the side to be formed. As shown in FIG. 2, the can lid 1 includes a central panel portion 1a, an annular recess 1b, a chuck wall 1c, a seaming panel 1d, and a curl portion 1e. A rivet portion 1a for attaching a tab (not shown) to the central panel portion 1a.
1. A circular score portion 1a2 that is torn when pulling up the knob tab 1a to form an opening (not shown), and a concave portion 1a3 for strengthening the panel portion are provided. The thickness of the plastic film layer 6 is usually 30 μm
The thickness of the bottom surface of the rivet portion 1a1 and the score portion 1a2, which are subjected to heavy working, is reduced to about half, and pinholes are likely to occur.

Reference numeral 2 denotes a turret. As shown in FIG. 5, the turret 2 is provided with a plurality of (eight in the figure) circular holes 3 at equal intervals along the peripheral edge. FIG. 2 shows a state of the hole 3 in the inspection station 51 described later. The seaming panel 1d of the can lid 1 has a step 3 of the hole 3.
The can lid 1 is placed in the hole portion 3 such that the maximum diameter portion of the upper end of the seaming panel 1d is in good contact with the vertical rising surface 3b of the step portion 3a when riding on a. Reference numeral 4 denotes a retainer for preventing the can lid 1 from moving up and down while the turret 2 is rotating and stopping, and the lower end surface of the retainer 1 has substantially the same shape as the chuck wall 1c from the inside and is made of an elastomer such as elastic rubber. A holder 4b that supports the ring body 4a, the annular ring body 4a, and has an inner diameter slightly larger than the maximum outer diameter of the annular recess 1b and is made of an insulator and has a vertical ring portion 4b1 and a holder 4b. The contact tool 5 is made of a conductive material (for example, copper or aluminum) having a ring portion 5a that is in close contact with the outer peripheral portion of 4b1. The lower surface of the ring portion 5a of the contact tool 5 has a curl portion 1
The metal e is in contact with the exposed end surface 1e1. The contact tool 5 is grounded. The annular ring body 4a and the holder 4b have a role of preventing electric discharge from occurring between the contact tool 5 and an electrode 8 described later.

The electrode 8 is made of a conductive material such as tool steel, and the main portion 8a has a cylindrical shape having a diameter substantially the same as the deepest portion of the annular recess 1b of the can lid 1. The portion of the electrode 8 that faces the can lid 1, that is, the lower portion in FIG. 1 and FIG. 2, the counter specimen portion 8b is composed of a large number of protrusions 9 having a sharp top 9a. The protrusion 9 is formed of a pyramid or a cone, and in the case of FIG. 4, is formed of continuous small regular pyramids 9b. The tops 9a are distributed at equal intervals on a virtual coplanar plane perpendicular to the axis of the main part 8a, and have sharp edges or spherical surfaces with a very small radius of curvature (for example, 0.3 mm or less). preferable. This is to facilitate corona discharge. Such a protrusion 9 is formed by, for example, electric discharge machining. The protrusions 9 are preferably gold-plated to prevent rust and improve conductivity. The electrode 8 and the can lid 1 are separated and are not in contact with each other. Since the can lid 1 has unevenness, the distance between the top portion 9a and the plastic film layer 6 is not uniform, but the distance between the smallest portions is about 0.1 mm or more, and the electrode 8 and the can lid are in this range. It is preferable that 1 is as close as possible. This is because if the interval is too large, corona discharge is less likely to occur and the reliability of inspection is reduced.

In FIG. 1, reference numeral 10 is an electric circuit for applying a voltage between the electrode 8 and the metal plate 7 and removing electricity.
Reference numeral 11 is a DC constant voltage source whose set voltage is relatively low. 12
Is a transformer, 12a is a primary coil, and 12b is a secondary coil. Secondary coil 12b, secondary coil 12b
The diode 13, which is a rectifying element whose positive side is connected to the positive terminal 12b1 of the switch, the switch 14, and the electrode 8 form a series circuit. The number of turns of the secondary coil 12b is n times (n> 1) the number of turns of the primary coil 12a. Therefore, a voltage n times as large as that of the primary coil 12a is generated in the secondary coil 12b. The negative terminal 12b2 of the secondary coil 12b is grounded and connected to the end surface 1e1 of the can lid 1 (metal plate 7). A set voltage (for example, about 2 to 4 k) is set in parallel with a series circuit including the secondary coil 12 b, the diode 13, and the switch 14.
V) is relatively high, a series circuit including a DC constant voltage source 15, a resistor 16 and a switch 17 is provided. The positive side of the power source 15 is grounded, and the terminal of the switch 17 on the side opposite to the power source 15 is the electrode 8. Connect to. Switches 14 and 1
Each of 7 is ON / OFF-controlled at a constant timing by relay devices 19 and 20 in the controller 18 (see FIG. 6).

The electrode 8 has a resistor 21 and a capacitor 22.
It is grounded through a parallel circuit composed of a parallel circuit composed of a resistor 23 and a variable capacitor 24. The capacitors 22 and 24 are provided to prevent danger. Resistance 23
The voltage between both ends of the voltage is a voltage divided between the electrode 8 and the metal plate and is input to the signal reading unit 26 in the controller 18. From the time t2 when the impulse high voltage is applied to the signal reading unit 26, to the time t5 when the reverse voltage is applied,
The divided voltage is input and read at a predetermined timing as described later (see FIGS. 6 and 7). That is, it is input and read after an extremely short time (for example, 150 μsec) from the time when the switch element 34 is turned off (for example, time t12). This extremely short time is from the time when the switch element 34 is turned off (for example, time t12).
Next, when the switch element 34 is turned on (for example, at time t
It is much shorter than the time to 21). Power supply 11
Form a series circuit with the primary coil 12a, the resistor 33 for current adjustment, and the switch element 34 in the controller 18, the negative electrode of the power supply 11 is connected to the primary coil 12a, and the positive electrode is a switch element such as a semiconductor. Connect to 34. Two
A timing signal generator 7 outputs a signal to the switch element 34 at a predetermined timing to output the signal to the switch element 3.
4 is turned on and off (see FIG. 7).

Next, the operation of the voltage application / static elimination circuit 10 will be described. As shown in FIG. 2, the electrode 8 faces the can lid 1, and as shown in FIG.
N, when the inspection start signal 25 that rises at time t1 when the switch 17 is in the OFF state is input to the timing signal generation unit 27, the timing signal generation unit 27 outputs the timing signal for turning the switch element 34 ON and OFF to the switch element 34. Output. As shown in FIG. 7, the timing signal is set at the time when the switch element 34 is turned on, for example, t11, t2.
The time T between 1, t31 and t41, that is, the period is constant and extremely short (for example, T = 600 μsec), and the time during the ON state, for example, the time between time t11 and time t12 or time t.
The time between 21 and the time point t22 gradually becomes extremely long (for example, becomes 10 μsec longer). The timing signal is output until the time t3 when the voltage V reaches the peak value Vp. The number of times the timing signal is turned on is usually about 10
It is a predetermined number of times within the range of 30.

The switch element 34 uses the inspection start signal 2
At time t1 when 5 is turned on, it is first turned on (hence time t1 = time t11), and the current I1 flows from the power supply 11 to the primary coil 12a of the transformer 12. Along with this, an induced current I2 flows through the secondary coil 12b. At time t12 when the current I1 reaches the peak value (that is, time t2), if the switch element 34 is turned off to cut off the current I1, the current I2 of the secondary coil 12b is also cut off, and a large induction is generated in the secondary coil 12b. An electromotive force is generated, and the impulse high voltage V (V is the primary side coil 12a based on this induced electromotive force.
N times the induced electromotive force) is applied between the electrode 8 and the metal plate 7 (in contact with the contact 5) via the diode 13 at time t2. In this way, the switch element 34
, The impulse voltage V increases every time it is turned on and off, and when the plastic film layer 6 does not include pinholes and is normal, the voltage V is turned on as shown in FIG.
It is saturated until it is turned off a predetermined number of times at the time t3.
Rapidly rises to a substantially flat peak value Vp, and the plastic film layer 6 is charged with positive charges. Even after the time point t3 when the ON / OFF operation of the switch element 34 is stopped, the peak value Vp continues as it is. Switch 14 at time t4
Is turned off, and switch 17 is turned on at time t5 immediately after that.
N (time point t5 is, for example, 20 msec from time point t2)
Later time point: FIG. 6, FIG. 8) and the positive electrode of the power supply 15 is the metal plate 7.
Then, the negative electrode is connected to the electrode 8, the reverse voltage acts on the metal plate 7, the voltage V becomes negative, the plastic film layer 6 is negatively charged, and the plastic film layer 6 is positively charged. And the negative charges cancel each other out to remove the charges. After that, for example, at time 50 msec (at time t
6: The electrode 8 and the retainer 4 are lifted to the position shown in FIG. 6) and separated from the can lid 1.

When the plastic film layer has a pinhole, corona discharge occurs between the top 9a of the protrusion 9 and the metal plate 7 through the pinhole, so that the curve 2 in FIG.
As shown in, the voltage V does not reach the peak value Vp. The signal reading unit 26 reads the voltage V from the time point t12 (t2) to the time point t5 (at the end of the inspection), and when a voltage below a predetermined voltage (for example, 2 kV) much lower than the peak value Vp is read, The can lid 1 is rejected as a defective product. As shown by the curve 1 in FIG. 8, the can lid 1 in which the voltage V read by the time t5 (at the end of the inspection) is higher than the predetermined voltage is sent as a good product. If the power supply 15 is not provided, when the switch 14 is turned off and the switch 17 is turned on while the peak value Vp continues, the electrode 8 is grounded via the resistor 16, and the electrode 8 and the metal plate 7 are grounded. The voltage V of V decreases rapidly as shown by the curve 3 in FIG.
However, in this state, due to the positive charges charged in the plastic film layer 6, as shown by the curve 3, a voltage of about 2 to 3 kV remains for a while. A considerable amount of positive charge is stored in the plastic film layer 6 which is not in contact with the electrode 8. This causes the generation of new pinholes. In the case of the above-described embodiment, the positive voltage is first applied to the electrode 8, but the voltage application and static elimination circuit 10 is applied so that the negative voltage is applied.
The configuration of may be changed. The switch 17 may be turned on immediately after the reading of the voltage V is finished, that is, immediately after the reading of the voltage V is finished, that is, immediately after the reading of the voltage V is finished.

The turret 2 shown in FIG. 5 is indexed by an index in the direction of arrow A, for example, every 45 degrees for 0.2 sec (90 msec rotation, 110 msec stop; in this case, 300 can lids 1 can be inspected per minute). Rotate intermittently. 5
Reference numeral 0 is a feeding station of the can lid 1, 51 is an inspection station, 52 is a reject station, and 53 is a sending station. When the hole 3 is stopped at the feeding station 50, the can lid 1 is vacuumed downward, and the seaming panel 1d rides on the step 3a of the hole 3 as shown in FIG. Placed on the part 3. When the can lid 1 reaches the inspection station 51 and stops, the holding tool 4 descends and the annular ring body 4a makes light contact with the chuck wall 1c, and at the same time, the contact tool 5 comes into contact with the end surface 1e1 of the can lid 1. At the same time, the electrode 8 also descends until the shortest distance between the top portion 9a of the protrusion 9 and the can lid 1 becomes close to 0.1 mm. In this state, the inspection is performed as described in paragraphs 0016, 0017 and 0018. Here, the can lid 1 that is determined to have a defect, that is, a defective product, is pushed upward at the reject station 52 and rejected. The normal can lid 1 is sent from the sending station 53 to the next step.

[0020]

EXAMPLE A can lid 1 to be inspected has a metal plate 7 having a plate thickness of 0.2.
It was made of a 5 mm aluminum alloy for cans, and a polyethylene terephthalate film 6 having a thickness of 30 μm was laminated on the inner surface side with an adhesive, and the diameter between the deepest portions of the annular recess 1b was 70.70 mm. . The thickness of the film 6 at the bottom of the rivet part 1a1 and the score part 1a2 was about 15 μm. The diameter of the main portion 8a of the electrode 8 is slightly smaller than the diameter between the deepest portions 70.5.
The diameter of the lower end of the sample portion 8b, that is, the entire top portion 9a of the protrusion 9 was 69 mm, which was slightly smaller than the diameter of the main portion 8a. The protrusion 9 has a bottom side length of 1.5.
mm, the height is 5 mm, and is composed of a regular square pyramid 9b, and the top portion 9a
Had a spherical shape with a radius of curvature of about 0.2 mm. The size of the regular quadrangular pyramid 9b is determined so that, when the can lid 1 and the electrode 8 are coaxially arranged, one of the top portions 9a is located above the center of the rivet portion 1a and above the score portion 1a2. . The protrusion 9 was plated with gold.

In the voltage application / static elimination circuit 10, the power source 1
The set voltages of 1 and 15 were 17.5V and -3.2kV, respectively. Resistors 16, 21, 23 and 3
The resistance values of 3 are 4 MΩ, 5000 MΩ and 3.9, respectively.
It was kΩ and 1 KΩ. Capacitors 22 and 24
Had a capacitance of 300 pF and 0.033 μF, respectively.

8 and 9 show a recorder 55 for testing.
And the results of investigating the relationship between the inspection time and the voltage V (value converted into the voltage generated between the electrode 8 and the metal plate 7) by connecting both ends of the resistor 23 to the recorder 55. Is. The timing signal T in FIG. 7 is 600 μse
It was c. The curve 1 in FIG. 8 is the plastic film 6
When there is no pinhole in and the power supply 15 is provided, the switch 14 is turned off at a time of 18 msec (time t4) and the switch 17 is turned on at a time of 20 msec (time t5) to apply a reverse voltage to the can lid 1. An example of the relationship between time and voltage when applied (when charge is removed) is shown. Time 0 ms
ec is the time point t2 in FIG. Almost 32 mse
It can be seen that the voltage becomes 0 kV at the time of c. Reverse voltage,
That is, it is considered that the positive charge is neutralized by the newly generated negative charge by applying the negative voltage. Curve 2 in FIG. 9 is a similar example when the plastic film 6 has a pinhole (however, the time is 20 msec.
The switch 17 was not turned on at that time), which reached a peak value of 5 kV after about 7 msec, and then rapidly decreased. In this case, the peak value Vp in the curve 1
It can be seen that the voltage V is almost 0 during the period.
Curve 3 in FIG. 9 shows a time of 18 ms when the plastic film 6 has no pinhole and the power supply 15 is not provided.
ec with switch 17 turned off, switch 14
Is turned off, and switch 17 is turned on for 20 msec.
An example of the relationship between time and voltage (kV) at the time of the inspection performed as described above is shown. It can be seen that the voltage V rises rapidly in about 15 msec, reaches the peak value of 7 kV, and then the voltage V rapidly decreases when the switch 17 is turned on in 20 msec, and the voltage of about 2 to 3 kV continues until about 50 msec. . The present inventor has found that a new pinhole may be generated in a short time when the voltage of about 2 to 3 kV continues.

Using the turret 2 shown in FIG. 5, the plastic film 6 has no pinholes and the curve 1 of FIG.
By the method described in 1), 1600 can lids 1 were inspected and discharged. Among them, there were 1500 good products and 100 rejected products (defective products). As a result of performing an enamel lator test on non-defective products, the total test value is 0.000.
It was mA. As a result of performing an enamellator test on the rejected product, the test value is 0.00xmA (x is 1
Was an integer of ˜9).

For comparison, except that the charge was not removed, the inspection was carried out in the same manner as described above.
There were 94 and 6 rejected products. As a result of performing an enamellator test on non-defective products, the test value of 1592 was 0.000 mA, but the test value of 2 was 0.00.
It was xmA (x is an integer of 1-9). As a result of the enamellator test of the rejected product, the test value is 0.0.
It was yxmA (x is an integer of 1-9, y is an integer of 0-9).

[0025]

According to the method for inspecting the plastic film layer on the inner surface of the can lid according to the first to fourth aspects of the present invention, the leakage of the inspection is unlikely to occur on the bottom surface of the rivet portion, and the can lid for canning is highly corrosive. It is suitable for high-speed inspection and has the advantage of being able to reliably inspect defects.

[Brief description of drawings]

FIG. 1 is an explanatory drawing of an electrode, a can lid and an electric circuit for carrying out the method of the present invention.

2 is a II-II of the inspection station of the turret of FIG. 5 when the electrodes and the can lid are in the state of FIG. 1;
It is a longitudinal cross-sectional view along a line.

FIG. 3 is an enlarged view of a P portion of FIG.

FIG. 4 is a bottom view seen from the line IV-IV in FIG.

FIG. 5 is a plan view for explaining an essential part of a turret for industrially carrying out the inspection method of the present invention.

FIG. 6 is a timing chart showing an example of the operation of the electric circuit of FIG.

FIG. 7 is a partially enlarged detailed view of the switch element signal and the read signal of FIG.

FIG. 8 is a diagram showing an example of a relationship between time and voltage when a reverse voltage is applied after repeatedly applying an induced electromotive force as an impulse high voltage in an extremely short cycle.

FIG. 9 is a diagram showing a first example and a second example of a relationship between time and voltage when an induced electromotive force is repeatedly applied as an impulse high voltage at an extremely short cycle.

[Explanation of symbols]

1 can lid 6 plastic film layers 7 metal plate 8 electrodes 9 protrusion 9a Top (tip) 12 transformers 12a Primary coil 12b Secondary coil (coil) 15 Power supply (means for applying reverse voltage) 34 switch (means for cutting off the current flowing through the coil)

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-48-68296 (JP, A) JP-A-3-160355 (JP, A) JP-A-2-62978 (JP, A) JP-A-8- 240569 (JP, A) JP-A-8-298770 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/20 G01N 27/92 G01M 3/40

Claims (4)

(57) [Claims] 1. A method of inspecting a can lid formed from a metal plate having a laminated plastic film layer for defects in the plastic film layer, the electrodes being opposed to each other in close proximity to the plastic film layer, and the metal plate and the electrode. Impulse high voltage is applied between the electrodes and the defect is inspected based on the voltage generated between the electrode and the metal plate, and the charge charged in the plastic film layer is reversed by the reverse voltage at the end of the inspection or immediately after the inspection. Add
Defect inspection of can lids characterized by removal by
Method. 2. A method of inspecting a defect of a can lid according to claim 1, wherein the portion of the electrode that faces the plastic film layer comprises a plurality of protrusions having sharp tips. 3. An impulse high voltage is an induced electromotive force generated when a direct current flowing through a coil is cut off, and in the case of a can lid having a defect-free plastic film layer, the peak value of the voltage between the electrode and the metal plate is The defect inspection method for a can lid according to claim 1 or 2, wherein it is determined that there is a defect when a voltage equal to or lower than a predetermined value lower than a peak value is generated by a predetermined time. 4. The impulse high voltage is an induced electromotive force generated in a secondary side coil when a direct current flowing through the primary side coil of a transformer is repeatedly cut at a high frequency.
The method for inspecting a defect of a can lid according to Item 3 .