TEKNOLOGI PENGELASAN

I. Teknologi Pengelasan

Pengelasan : Proses penyambungan dua buah (atau Lebih) logam sejenis maupun tidak sejenis dengan mencairkan (memanaskan) logam tersebut di atas atau di bawah titik leburnya, disertai dengan atau tanpa tekanan & disertai atau tanpa logam pengisi.

Beberapa keuntungan penggunaan sambungan las (komersial & teknologi) :

• Pengelasan menghasilkan sambungan permanen.
• Sambungan lasan dapat lebih kuat dibandingkan material awal jika menggunakan logam pengisi & teknik pengelasan yang tepat.
• Umumnya pengelasan adalah proses penyambungan yang paling ekonomis ditinjau dari penggunaan material & biaya fabrikasi.
• Pengelasan tidak hanya terbatas di lingkungan pabrik, tetapi juga dapat digunakan dilapangan.

Beberapa keterbatasan & kelemahan sambungan las :

• Umumnya pengelasan dilakukan secara manual & memerlukan biaya operator yang mahal.
• Umumnya proses pengelasan membutuhkan energi besar yang cenderung berbahaya.
• Lasan sulit dibongkar, sehingga jika dibutuhkan pembongkaran produk untuk perbaikan/ pemeliharaan, maka metode pengelasan tidak akan digunakan untuk proses penyambungan produk tersebut.
• Sambungan las dapat menyelubungi cacat sehingga tidak terlihat. Cacat tersebut dapat mengurangi kekuatan sambungan.

Pengelasan sebagai Kegiatan Komersial :

Pengelasan dapat diaplikasikan di berbagai tempat dan di berbagai industri. Sebagai sebuah teknologi penyambungan untuk produk komersial, banyak proses pengelasan dilakukan di pabrik-pabrik. Tetapi beberapa proses pengelasan tradisional seperti Arc Welding (Las Listrik) & Oxyfuel Gas Welding (Las Oksigen) menggunakan perlengkapan yang mudah dipindah-pindah sehingga pengerjaannya tidak terbatas di pabrik saja, tetapi juga pengerjaan konstruksi di lapangan seperti : kapal laut, bengkel perbaikan otomotif, dll.

Secara prinsip pengelasan digunakan utk :

1. Konstruksi (gedung, jembatan, dll).

2. Perpipaan, pressure vessels (ketel uap), boiler, tangki penampungan.

3. Bangunan kapal.

4. Pesawat udara & pesawat ruang angkasa.

5. Otomotif.

Umumnya pengelasan dilakukan oleh operator berpengalaman : Welder (Juru Las), dibantu oleh Fitter (Pembantu Juru Las). Welder bertugas secara manual mengendalikan proses pengelasan untuk menggabungkan komponen satu dengan komponen lainnya. Fitter bertugas mempersiapkan komponen, peralatan las, pemegang komponen yang akan dilas (welding fixture).

Faktor Keamanan

Proses pengelasan pada dasarnya berbahaya bagi manusia :

• Temperatur tinggi logam cair yang merupakan bagian yang akan di sambung.
• Pada las Oksigen, acetylene (sebagai bahan bakar) adalah bahan yang mudah terbakar.
• Proses pengelasan menggunakan energi besar untuk meleburkan bagian permukaan komponen yang akan di sambung.
• Banyak proses pengelasan menggunakan tenaga listrik sebagai sumber energi panas, sehigga ada bahaya listrik beban kejut/ hubungan singkat.
• Pada las listrik, emisi radiasi ultraviolet sangat berbahaya bagi penglihatan, sehingga memerlukan helm/ masker dengan jendela kaca sangat gelap utk melihat.
• Percikan api, loncatan logam cair, asap, & bagian yg melebur menambah resiko proses pengelasan.

Otomasi Pengelasan

Karena bahaya yang ditimbulkan oleh pengelasan manual serta upaya utk meningkatkan produktifitas & kualitas maka di kembangkanlah berbagai variasi mekanisasi & otomasi pengelasan, yang terbagi menjadi :

• Machine Welding (mesin las) : Operator (manusia) secara kontinu mengawasi proses & berinteraksi dengan peralatan untuk pengendalian operasi.
• Automatic Welding (las otomatis) : Peralatan yang memungkinkan untuk melakukan proses tanpa pengaturan kendali oleh operator (manusia), misalnya peralatan untuk mengatur posisi benda kerja (fixture) saat akan dilas.
• Robotic Welding (robot las) : Robot industri atau manipulator terprogram digunakan untuk mengendalikan proses secara otomatis misalnya pergerakan relatif kepala las (welding head) ke benda kerja.

II. Sambungan Las

Ada 5 (lima) tipe dasar sambungan las :

1. Butt joint

2. Corner joint

3. Lap joint

4. Tee joint

5. Edge joint

Tipe Lasan

Setiap bentuk sambungan dapat dibuat oleh pengelasan. Beberapa tipe lasan berdasarkan bentuk geometri sambungan & proses pengelasannya :

Pengisian Lasan (Fillet Weld) :

1. Pengisian tunggal di dalam untnk corner joint

2. Pengisian tunggal di luar untuk corner joint

3. Pengisian ganda untuk lap joint

4. Pengisian ganda untuk tee joint

Alur/ kampuh las (Groove Weld) :

a. Lasan alur persegi, satu sisi

b. Lasan alur tirus tunggal 

c. Lasan alur V tunggal 

d. Lasan alur U tunggal

e. Lasan alur J tunggal

f. Lasan alur V ganda

Lasan Plug dan Slot :

Proses ini digunakan untuk penyambungan antar pelat secara mendatar, menggunakan satu atau lebih lubang atau slot pada komponen atas & kemudian mengisinya dengan logam pengisi untuk menggabungkan kedua komponen pelat tersebut.

Lasan Spot dan Seam :

Proses ini umumnya digunakan untuk tipe sambungan las Lap Joint

Lasan Flensa (flange) dan Permukaan (Surfacing) :

Flange weld umumnya digunakan untuk pelat lembaran atau pelat tipis. Surfacing weld adalah proses deposit logam pengisi pada permukaan material/ komponen dasar. Tujuannya untuk mempertebal bagian dari material dasar sehigga bagian tersebut dapat digunakan sebagai tumpuan pelat/ pelindung yang disusun di atasnya, atau sebagai pembatas proses pelapisan (coating) pada permukaan material dasar tersebut.

Jenis-jenis proses pengelasan versi American Welding Society :

I. Fusion Welding :

Proses pengelasan dengan menggunakan panas untuk melebur kedua permukaan logam yang akan disambung. Beberapa jenis Fusion Welding menggunakan logam pengisi (filler) yang ditambahkan pada titik logam yang lumer untuk memadatkan & menguatkan sambungan las.

Yang termasuk Fusion Welding :

A. Las Busur (Arc Welding/ AW)

Proses pengelasan dimana pemanasan logamnya terjadi akibat adanya loncatan/ busur listrik (electric arc). Beberapa arc welding juga diikuti oleh penekanan selama proses & umumnya membutuhkan logam pengisi.

Jenis proses AW yang menggunakan Consumable Electrodes :

1. Shielded Metal Arc Welding (SMAW/ Stick Welding)

Pada proses ini menggunakan elektoda (stick) dengan panjang 9 – 18 inch (230 – 460 mm) & diameter 3/32 – 3/8 inch (2,5 – 9,5 mm). Elektroda ini memiliki selubung mengandung fluks (flux) berupa serbuk cotton & kayu yang dicampur dengan serbuk carbon & oksidanya serta kadangkala juga mengandung serbuk logam. Fluks/ selubung berfungsi untuk mencegah udara (atmosphere) dan kotoran (slag) terjebak dlm logam yang mencair saat proses pengelasan. SMAW menggunakan arus 30 – 300 A dng voltage 15 – 45 V, tergantung dari jenis logam yang akan dilas, tipe elektroda & panjangnya, serta kedalaman penetrasi lasan yang diperlukan

2. Gas Metal Arc Welding (GMAW).

Menggunakan kawat elektroda berdiameter 1/32 – 1/4 inch (0,8 – 6,4 mm). Gas yang digunakan sebagai penyekat adalah argon & helium (untuk sambungan las paduan Alumunium & Stainless steels), carbon dioxide (untuk baja karbon/ carbon steels).

3. Flux-cored Arc Welding (FCAW)

Proses pengelasan ini menggunakan elektroda inti fluks. Merupakan bentuk hibrida dari SMAW (menggunakan fluks) dan GMAW (menggunakan gas). Memiliki keunggulan dibanding GMAW yaitu kawat elektroda yg dpt mengumpan secara terus menerus (kontinu) karena tekanan pegas. Ada 2 (dua) tipe FCAW yaitu dengan penyekat sendiri (self shielded dari fluks) & dengan gas penyekat (gas shielded).

4. Electrogas Welding (EGW)

Proses ini umumnya diaplikasikan untuk sambungan Vertical Butt Joint. EGW dapat berfungsi seperti FCAW self shielded yaitu menggunakan elektroda logam berisi fluks yang dapat mengumpan secara kontinu & tanpa gas penyekat, tetapi juga dapat berfungsi seperti GMAW yaitu menggunakan gas penyekat, hanya saja EGW memiliki sepatu cetakan yang mengandung logam cair. Sepatu cetakan (molding shoe) berpendingin air berfungsi untuk mencegah gas masuk lasan. Bersamaan dengan bagian yang tersambung, di dalam sepatu bertambah logam cair (dari elektroda & material). Proses berlangsung automatically, dengan pergerakan kepala las (welding head) secara vertikal naik keatas untuk mengisi sepatu dengan logam cair tersebut dalam satu siklus. EGW di aplikasikan untuk pengelasan baja (karbon rendah & menengah, serta stainless steel) konstruksi tangki penyimpanan besar & bangunan kapal. Tebal elektroda 0,5 – 3,0 inch (12 – 75 mm).

5. Submerged Arc Welding (ASW)

Adalah proses yang menggunakan pengumpan elektroda kontinu, arc shielding (busur penyekat) yang dilengkapi tempat butiran fluks (granular flux). Blanket butiran fluks berfungsi mencegah bunga api (spark), percikan api (spatter), & radiasi, sehingga lebih aman bagi operator dibanding proses arc welding lainnya.

Jenis proses AW yang tidak menggunakan Consumable Electrodes :

1. Gas Tungsten Arc Welding (GTAW)

Pengelasan yang sering diaplikasikan untuk proses ini seperti pengelasan TIG (tungsten inert gas). GTAW dapat digunakan dengan atau tanpa logam pengisi. Elektroda tungsten tidak ikut melebur, tungsten adalah material elektroda yang baik dengan titik leburnya 3410 ºC. Gas penyekat yang digunakan seperti argon, helium, atau campuran keduanya. Proses GTAW umumnya lebih lambat dan lebih mahal dibandingkan proses consumable electroda arc welding, akan tetapi dapat diaplikasikan untuk material yang sangat tipis serta kualitas lasan yang sangat tinggi.

2. Plasma Arc Welding (PAW)

Pengelasan ini adalah bentuk khusus dari GTAW. Pada elektroda tungsten di pasangkan nozzle yang didesain khusus untuk menghasilkan aliran kecepatan tinggi gas inert (argon atau campuran argon hydrogen) yang terus menerus sehigga membentuk aliran plasma. Temperatur PAW dapat mencapai 28000 ºC atau lebih sehigga dapat meleburkan logam apapun. Temperatur ini dapat di capai karena tingginya konsentrasi untuk memproduksi jet plasma dengan diameter kecil & kepadatan energi yang sangat tinggi. Diaplikasikan untuk perakitan mobil, lemari besi, rangka pintu & jendela besi. Sulit digunakan untuk pengelasan perunggu (bronze), besi tuang, timah hitam (lead), & magnesium.

3. Stud Welding (SW)

Proses pengelasan busur (arc welding) yang khusus untuk menyambung stud atau komponen yang sebentuk ke material dasar. Contoh aplikasinya : baut untuk pengikat handle peralatan memasak, baut pemegang pelindung radiasi panas mesin.

Knowledge from: http://tin105.weblog.esaunggul.ac.id/wp content/uploads/sites/803/2015/02/Kuliah-13-PM-Welding.pdf and Internet Literature.

SUMMARY OF MARPOL (International Convention for the Prevention of Pollution from Ships, 1973 as modified by the Protocol of 1978. ("MARPOL" is short for Marine Pollution and 73/78 short for the years 1973 and 1978.)

MARPOL (International Convention for the Prevention of Pollution from Ships, 1973 as modified by the Protocol of 1978. ("MARPOL" is short for marine pollution and 73/78 short for the years 1973 and 1978.)

The International Convention for the Prevention of Pollution of Ships, 1973 (MARPOL 73/78)

Summary:

• Annex I: Pollution by Oil

• Annex II: Pollution by Noxious Liquid Substances

• Annex III: Pollution by Harmful Substances in Packaged Form

• Annex IV: Pollution by Sewage from Ships

• Annex V: Pollution by Garbage from Ships

• Annex VI: Prevention of Air Pollution from Ships

Summary

The International Convention for the Prevention of Pollution of Ships, 1973 was adopted in 1973. This Convention was subsequently modified by the Protocol 1978 relating thereto, which was adopted in 1978. The Protocol introduced stricter regulations for the survey and certification of ships. It is to be read as one instrument and is usually referred to as MARPOL 73/78.

This IMO Convention is the most important global treaty for the prevention of pollution from the operation of ships; it governs the design and equipment of ships; establishes system of certificates and inspections; requires states to provide reception facilities for the disposal of oily waste and chemicals. It covers all the technical aspects of pollution from ships, except the disposal of waste into the sea by dumping, and applies to ships of all types, although it does not apply to pollution arising out of the exploration and exploitation of sea-bed mineral resources.

Regulations covering the various sources of ship-generated pollution are contained in the six Annexes of the London Convention and are updated regularly. Annexes I and II, governing oil and chemicals are compulsory but annexes III, IV, V and VI on packaged materials, sewage, garbage and air pollution are optional.

Annex I: Regulations for the Prevention of Pollution by Oil

Entry into force: 2 October 1983

Details the discharge criteria and requirements for the prevention of pollution by oil and oily substances. It maintains predominantly the oil discharge criteria prescribed in the 1969 amendments to the 1954 Oil Pollution Convention. Beside technical guidelines it contains the concept of "special areas" which are considered to be vulnerable to pollution by oil. Discharges of oil within them have been completely prohibited, with minor well-defined exceptions.

Annex II: Regulations for the Control of Pollution by Noxious Liquid Substances in Bulk

Entry into force: 6 April 1987

Details the discharge criteria and measures for the control of pollution by noxious liquid substances carried in bulk. It subdivides substances into and contains detailed operational standards and procedures. Some 250 substances were evaluated and included in the list appended to the London Convention. The discharge of their residues is allowed only to reception facilities until certain concentrations and conditions (which vary with the category of substances) are compiled with. In any case, no discharge of residues containing noxious substances is permitted within 12 miles of the nearest land. More stringent restrictions apply to "special areas".

Annex III: Regulations for the Prevention of Pollution by Harmful Substances Carried by Sea in Packaged Form

Entry into force: 1 July 1992

Contains general requirements for the issuing of detailed standards on packing, marking, labeling, documentation, stowage, quantity limitations, exceptions and notifications for preventing pollution by harmful substances. The Annex should be implemented through the International Maritime Dangerous Goods (IMDG) Code, which has been amended to include marine pollutants. The amendments entered into force on 1 January 1991.

Annex IV: Regulations for the Prevention of Pollution by Sewage from Ships

Entry into force: 27 September 2003"

Contains requirements to control pollution of the sea by sewage from ships.

Annex V: Regulations for the Prevention of Pollution by Garbage from Ships

Entry into force: 31 December 1988

This deals with different types of garbage and specifies the distances from land and the manner in which they may be disposed of. The requirements are much stricter in a number of "special areas" but perhaps the most important feature of the Annex is the complete ban imposed on the dumping into the sea of all forms of plastic.

Annex VI: Regulations for the Prevention of Air Pollution from Ships and NOx Technical Code

Not yet into force: 12 months after being ratified by 15 States whose combined fleet of merchant ships constitute at least 50% of the world fleet. Contains requirements to control the air pollution from ships. It provides guidelines and provisions for the emission of different substances and specifies the requirements for the testing, survey and certification of marine diesel engines to ensure they comply with the NOx limits. Amendments are made regularly. Generally they facilitate the implementation of annexes, extend the concept of "special areas", establish more sea areas as "special areas", replace list of substances, design new construction standards for ships, precise reporting requirements and reduce amount of oil which can be discharged into the sea from ships.

Knowledge from: MARPOL 

Procedures are to ensure the weathertightness of structures/shipboard

General 

These test procedures are to ensure the weathertightness of structures/shipboard outfitting, the watertightness of tanks and watertight boundaries and structural adequacy of tanks. Tightness of all tanks and tight boundaries of the ships at the new construction and, when major conversions or repairs* have been made, those relevant to the major conversions/repairs should be confirmed by these test procedures prior to delivery of the ship.

* Major repair means a repair affecting structural integrity.

Application 

All gravity tanks** and other boundaries required to be watertight or weathertight should be tested in accordance with this Guideline and proven tight and structurally adequate as follows:  

• - Gravity Tanks for their tightness and structural adequacy
• - Watertight Boundaries Other Than Tank Boundaries for their watertightness, and
• - Weathertight Boundaries for their weathertightness 

** Gravity tank means a tank having a design working pressure not greater than 70 kPa at the top of the tank.  

The testing of cargo containment systems of liquefied gas carriers should be in accordance with standards deemed appropriate by the Administration.

Testing of structures not listed in Table 1 or 2 should be specially considered.  

Types of Tests and Definition of Test

The following two types of test are specified in this requirement:

Structural Test: A test to verify the structural adequacy of the construction of the tanks. This may be a hydrostatic test or, where the situation warrants, a hydropneumatic test.  

Leak Test: A test to verify the tightness of the boundary. Unless a specific test is indicated, this may be a hydrostatic/hydropneumatic test or air test. Leak test with remark *3 in Table 1 includes hose test as an acceptable medium of the test.

Definition of each type of test is as follows:

Hydrostatic Test: (Leak and Structural) A test by filling the space with a liquid to specified head. 

Hydropneumatic Test: (Leak and Structural) A test wherein space is partially filled with liquid and air pressure applied on top of the liquid surface. 

Hose Test: (Leak) A test to verify the tightness of the joint by a jet of water. 

Air Tests: (Leak) A test to verify the tightness by means of air pressure differential and leak detection solution. It includes tank air test and joint air test, such as compressed air test and vacuum box test. 

Compressed Air Fillet Weld Test: (Leak) An air test of fillet welded tee joint and leak indicating solution applied on the fillet welds. 

Vacuum Box Test: (Leak) A box over a joint with leak indicating solution applied on the fillet or butt welds. Vacuum is created inside the box to detect any leaks. 

Ultrasonic Test: (Leak) A test to verify the tightness of a sealing by means of ultrasonic. 

Penetration Test: (Leak) A test to verify that no continuous leakages exist in the boundaries of a compartment by means of low surface tension liquids. 

TEST PROCEDURE

General

Tests should be carried out in the presence of the Surveyor at a stage sufficiently close to the completion of the work with all hatches, doors, windows, etc. installed and all penetrations including pipe connections fitted, and before any ceiling and cement work is applied over the joints.

Structural Test Procedures

Type and Time of Test 

Where a structural test is specified in Table 1 or Table 2, a hydrostatic test in accordance with 4.4.1 will be acceptable. Where practical limitations (strength of building berth, light density of liquid, etc.) prevent the performance of a hydrostatic test, a hydropneumatic test in accordance with 4.4.2 may be accepted as an equivalent method. Provided the results of a leak test are confirmed satisfactory, a hydrostatic test for confirmation of structural adequacy may be carried out while the vessel is afloat. 

Number of Structural Test 

1. Structural test should be carried out for at least one tank of same construction (i.e., same design and same workmanship) on each vessel provided all subsequent tanks are tested for leaks by an air test. However, where structural adequacy of a tank was verified by structural testing required in Table 1, the subsequent vessels in the series (i.e., sister ships built in the same shipyard) may be exempted from such testing for other tanks which have the structural similarity to the tested tank, provided that the water-tightness in all boundaries of exempted tanks are verified by leak tests and thorough inspection should be carried out. For sister ships built several years after the last ship of the series, such exemption may be reconsidered. In any case, structural testing should be carried out for at least one tank for each vessel in order to assure structural fabrication adequacy. 
2. For watertight boundaries of spaces other than tanks (excluding chain lockers), structural testing may be exempted, provided that the water-tightness in all boundaries of exempted spaces are verified by leak tests and thorough inspection should be carried out. 
3. These subsequent tanks may require structural test if found necessary after the structural testing of the first tank. 
4. Tanks for structural test should be selected so that all representative structural members are tested for the expected tension and compression. 

Leak Test Procedures

For leak test specified in Table 1, tank air test, compressed air fillet weld test, vacuum box test in accordance with 4.4.3 through 4.4.6, or their combination will be acceptable. Hydrostatic or hydropneumatic test may also be accepted as leak test provided 4.5 and 4.6 are complied with. 

Hose test will also be acceptable for the locations as specified in Table 1 with the foot note *3. 

Joint air test may be carried out in the block stage provided all work of the block that may affect the tightness of the joint is completed before the test. See also 4.5.1 for the application of final coating and 4.6 for safe access to joint and their summary in Table 3. 

DETAILS OF TEST

Hydrostatic Test

Unless other liquid is approved, hydrostatic test is to consist of filling the space by fresh water or sea water, whichever is appropriate for testing of the space, to the level specified in Table 1 or Table 2. In case a tank for cargoes with higher density is to be tested with fresh water or sea water, the testing pressure height should be specially considered.

Hydropneumatic Test

Hydropneumatic test where approved should be such that the test condition in conjunction with the approved liquid level and air pressure will simulate the actual loading as far as practicable. The requirements and recommendations for tank air tests in 4.4.4 will also apply to hydropneumatic test.

Hose Test

Hose test should be carried out with the pressure in the hose nozzle maintained at least at 2·105 Pa during the test. The nozzle should have a minimum inside diameter of 12 mm and be at a distance to the joint not exceeding 1.5meters. Where hose test is not practical because of possible damage to machinery, electrical equipment insulation or outfitting items, it may be replaced by a careful visual examination of welded connections, supported where necessary by means such as a dye penetrant test or ultrasonic leak test or an equivalent. 

Tank Air Test

All boundary welds, erection joints and penetrations including pipe connections should be examined in accordance with the approved procedure and under a pressure differential above atmosphere pressure not less than 0.15·105 Pa with a leak indication solution applied. It is recommended that the air pressure in the tank be raised to and maintained at about 0.20·105 Pa for approximately one hour, with a minimum number of personnel around the tank, before lowered to the test pressure of 0.15·105 Pa. A U-tube with a height sufficient to hold a head of water corresponding to the required test pressure should be arranged. The cross sectional area of the U-tube should be not less than that of the pipe supplying air to the tank. In addition to U-tube, a master gauge or other approved means to verify the pressure should be approved. 

Compressed Air Fillet Weld Test 

In this air test, compressed air is injected from one end of fillet welded joint and the pressure verified at the other end of joint by a pressure gauge on the opposite side. Pressure gauges should be arranged so that an air pressure of at least 0.15·105 Pa can be verified at each end of all passages within the portion being tested. Note: Where the leak test is required in way of the fabrication applying the partial penetration weld, compressed air test is also applied in the same manner for fillet weld where the root face is sufficiently large, i.e., 6 – 8 mm.  

Vacuum Box Test

A box (vacuum tester) with air connections, gauges and inspection window is placed over the joint with leak indicator applied. The air within the box is removed by an ejector to create a vacuum of 0.20·105 – 0.26·105 Pa inside the box.

Ultrasonic Test 

An arrangement of an ultrasonic echoes sender inside of a compartment and a receiver outside. A location where the sound is detectable by the receiver displays a leakage in the sealing of the compartment.

Penetration Test

A test of butt welds by using of a low surface tension liquid at one side of a compartment boundary. If no liquid were detected on the opposite sides of the boundaries after expiration of a definite time this means the verification of tightness of the compartments boundaries.

Other Test

Other methods of testing may be considered by each society upon submission of full particulars prior to commencement of the testing.

APPLICATIONS OF COATING

Final Coating 

For butt joints by automatic process, final coating may be applied anytime before completion of leak test of the space bounded by the joint. For all other joints, final coating should be applied after the completion of leak test of the joint. The Surveyor reserves a right to require leak test prior to the application of final coating over automatic erection butt welds.  

Temporary Coating

Any temporary coating which may conceal defects or leaks should be applied at a time as specified for final coating. This requirement does not apply to shop primer. 

Safe Access to Joints 

For leak tests, a safe access to all joints under examination should be provided. See also Table 3. 

Knowledges From : IACS Guideline for Procedures of Testing Tanks and Tight Boundaries