Metal detection in a garment is not only an essential quality assurance requirement but also provides a product safety solution which minimizes risk of legal action because of customer injury from a broken needle embedded in the garment. There are various levels of technologies available for needle detection depending on needs and scales of operations.
Sarablin Kaur, MF Tech from NIFT and Professor Prabir Jana, Chairperson, Dept. of Fashion Technology, NIFT, analyse the operation of dart sewing and the technology involved at the basic and highest levels. examine various options, with a favourable return on investment.
An industrial metal detector consists of four main components: the sensor, the control, the signal processor, and the output device. The sensor reacts to the proximity of metal which is then transmitted to the filter, an electronic device that interprets the sensor signal. If the filter determines that the sensor has detected metal, it activates the output device, which may be a simple display or alarm to alert the operator. The operation of all these components is governed by the control unit.
In addition to these four components, which are embedded in all metal detectors, there are five auxiliary components frequently found in a metal detection system as we move up the technology scale: the feed device, the reject device, the alarm device, the record keeper – for further analysis, and the power supply. The feed device feeds the product, which may be contaminated, to the sensor. Conveyor belt and hanger system are commonly used feed devices in metal detectors used for garments. Reject devices are designed to remove the metal pieces from the product after the detection. Such devices include trap doors, pusher arms, air jets, retracting head pulleys and many others.
Basic level technology
In the most basic technology the detector is manually moved in close proximity over a garment laid out on a table. Such equipments are handy and portable, enhancing the ease of operation. Electricity or an electromagnetic charge is sent through the coils to the ground and back to the coils. Metallic objects interrupt the signal, which results in the unit creating an audible sound. This type of needle detector machine is fitted with a microprocessor which enhances the detecting ability.
The operational area at any point on a garment is 25(W) X 50 (D) mm, equipped with a buzzer alarm the detector warns of the exact location of the broken needle as it is moved inside the garment and all over it. It is easy to see the location on the LED screen which comes with the equipment. Metal pieces are detected using magnetic induction with the option of choosing between high and low detecting modes. The modes are selected depending on the type of garment being detected, for example for a suit, the operator would select high sensitivity mode which can detect as minute as 0.8 mm steel at a height of 5 mm from the detecting surface and for a basic knit T-shirt, will opt for low sensitivity mode which can detect as minute as 1.0 mm steel at a height of 5 mm. The Hashima HN-30 and DST ST-25 are some of the best examples of this category of needle detectors.
Intermediate level technology
In a conveyor type needle detector, each garment is passed through a tunnel-like machine. The feed device here is a conveyor belt. Alarm devices include flashing lights, sirens, flag drops and computer displays, designed to draw the attention of a human operator to the detected presence of metal. This technology is widely used in many garment units nowadays. The state-of-the-art equipment is characterized by microprocessor-controlled detection equipment, generally incorporating sophisticated embedded signal processors, or networked to remote computers (or both).
The machine, at this level, is equipped with an LCD screen which indicates position of the piece found, sensitivity level, language mode, etc. Mostly the detecting width ranges between 400 mm to 600 mm with height ranging between 100 mm to 300 mm. The machine has the ability to detect metal as minutely as 0.8 mm (high sensitivity mode) and 1 mm (low sensitivity mode) at the lowest belt speed of 30 metres/min., 40 metres/min. being the maximum speed. The results can be stored on to the computer by optional software embedded in the metal detector control and/or in networked computers. The collected data can be used for analysing the number of garments detected over a period of time for quality management.
The Hashima HN-6770G and TEC-QD are the most widely used mid-level needle detectors.
Advanced level technology
Hanger type needle detector is the most advanced detection system, where inspection of finished garments is performed on the hangers, passing through a tunnel like structure fitted with sensors. This is a fully automated product handling system which minimizes manual handling and increases the rate of performing the task and can detect as minute as 1.0 mm steel, ensuring total quality control. The technology has adopted a newly designed detecting sensor and can cover trousers, jacket, and coats with great ease. The process is such that after putting the garment on the hanger, the system automatically positions and carries it through the tunnel on a moving hanger, detecting the impurity and then assorting the defected garment from the rest of the batch without stopping the machine. This facility of automatically assorting the defected garments is not available in the conveyor system where the belt stops on detecting the metal and piece has to be removed manually. It is easy to connect the hanger type needle detector to manufacturing processes using hanger system. The system is equipped with a display screen to indicate detected needle position and a counter, which keeps a check on the number of detected pieces providing aid in quantity management.
The Hashima HN-5000 and Nissin NS-5107 are the two most famous advanced level needle detection technologies and can detect from 650 to 4800 pieces per day.
Working formulas
• Finishing cost = 0.20 x CM of the garment
• CM price per Operation = Finishing cost x (SAM for single operation/Total SAM)
• Production per day (shift of 8 hours) = 480/SAM value for single operation (min)
• Production per annum = Production per day x Working days/month x No. of months/year
• Cost output/annum = Production per annum x CM price per garment inspected
• No. of machines required = Roundup (Production Target/Production per day, 0)
• Cost of total machine (Rs.), T = No. of machines required x Cost of one machine
• Depreciated value of machine after one year (Z) = Cost of total machine x [1 – Annual Depreciation (%)]
• Total operators salary (Y) @ 12 months = Cost of one operator per month x No. of operators x 12
• CM price from total machine (X) = Cost output per annum x No. of machines required
• Total kWh consumption = kWh consumption of one day (8 hour) x 26 x 12 x No. of machines; 1 kWh = 1 unit
• Total electricity consumption (W) = Total No. of units x 5 (price of one unit)
• Cash Inflow (I) = X – Y
• ROI for the first year = (X-Y-W)/Z = I/Z
• Payback Period = T/I
ROI for technology in Broken Needle Detection... |
||
| CM price of the garment (Rs.) assumed | 80 | |
| Finishing cost (20% of Rs. 80) | 16 | |
| SAM value for single operation (sec) | 15 | |
| Total SAM (minutes) assumed | 5 | |
| CM price per operation (Rs.) | 0.80 | |
| Intermediate Level of Technology | Advanced Level of Technology | |
| SAM value for single operation (sec) | 15.00 | 6.00 |
| Production per day (shift of 8 hours) | 1,920.00 | 4,800.00 |
| Working days in a month | 26.00 | 26.00 |
| Months in a year | 12.00 | 12.00 |
| Production per annum (pieces) | 5,99,040.00 | 14,97,600.00 |
| CM price per garment inspected (Rs.) | 0.80 | 0.80 |
| Cost output/annum (Rs.) | 4,79,232.00 | 11,98,080.00 |
| Production target (pieces per day) | 8,000.00 | 8,000.00 |
| No. of machines required | 5.00 | 2.00 |
| Cost of one machine (Rs.) | 5,50,000 | 24,45,000 |
| Cost of total machines (Rs.) (T) | 27,50,000.00 | 48,90,000.00 |
| Annual depreciation (%) | 15.00 | 15.00 |
| Depreciated value of machine after one year (Z) | 23,37,500.00 | 41,56,500.00 |
| Cost of one operator per month (minimum wage) | 4,800.00 | 5,500.00 |
| Total operators salary (Y) @ 12 months | 2,88,000.00 | 1,32,000.00 |
| Total KwH consumption per year | 1,747.20 | 998.40 |
| Electricity cost per year (Rs.) (W) | 8,736.00 | 4,992.00 |
| CM price from total machines (X) | 23,96,160.00 | 23,96,160.00 |
| Cash Inflow (I = X-Y) (Rs.) | 21,08,160.00 | 22,64,160.00 |
| ROI for the 1st year (X-Y-W)/Z (%) | 89.81 | 54.35 |
| ROI Calculation for 2nd year | ||
| Cost of total machine (Rs.) (T) | 23,37,500.00 | 41,56,500.00 |
| Depreciated value of machine after one year (Z) | 19,86,875.00 | 35,33,025.00 |
| Total operators salary (Y) @ 12 months | 2,88,000.00 | 1,32,000.00 |
| Electricity cost per year (Rs.) (W) | 8,736.00 | 4,992.00 |
| CM price from total machine (X) | 23,96,160.00 | 23,96,160.00 |
| Cash Inflow (I = X-Y) (Rs.) | 21,08,160.00 | 23,96,125.00 |
| ROI for the 2nd year (X-Y-W)/Z (%) | 105.66 | 67.68 |
| Pay Back after 2 years (%) | 195.48 | 122.03 |
