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Welding rigid thermoplastics—why use ultrasonics?
WELDING RIGID THERMOPLASTICS - WHY USE ULTRASONICS 1
run to hours, possibly to days. Solvents tend to attack the edges of a plastic part at stress points and cause stress cracks in the part after the joint has finished curing. AIl much as an extra 3 hours' annealing may therefore have to be Table 1 gives part of the answer straight away. Several of added to the curing time. Apart from the reject rates and the the plastics that can be welded ultrasonically have no conve- delay, boxes of curing parts occupy valuable space and annient solvent, often no known solvent. Formic acid and phenol nealing ovens cost money. So does extra ventilation. will dissolve nylons but each has its difficulties and hazards. By disposing of the need for solvents, ultrasonics reduces Special ventilation is usually necessary and the operator must wear protective clothing. Polypropylene resists almost welding to a single process of less than 1 min with reject rates of O. 1-1 It opens the way to automation for producevery known chemical, which is why vessels are moulded tior. rates of 800-2000 per hour from a single welder. from it, and it cannot therefore be joined by solver.ts. Why should a fabricator of rigid thermoplastics bother with ultrasonic welding when there are plenty of solvents on the market with a good many years of experience behind them?
0 /0 ,
Though we often hear how quickly an ultrasonic weld can be made (0. 5-10sec including hold time) the real savings of ultrasonic welding only become apparent when it is compared to a solvent process. An adhesive for joining two rigid plastics parts may be either a pure solvent or a solvent with a small quantity of the parent polymer already dissolved in it. Perspex is a good example of the latter. To get it onto the face of the joint, the adhesive can be spread with a brush or sqUirted from a small nozzle or hypodermic syringe; or the component can be pressed onto a spongy pad soaked in adhesive. Whichever method is used, solvent is continuously evaporating. Toxic or inflammable solvents such as methylene chloride or toluene, respectively, are subject to definite safety regulations. Even if it is safe an evil-smelling solvent may irritate the operator. Therefore, ventilation. However neat-handed the operator is, there is always the chance of a drip of solvent marking the part, either directly or via his hands. Reject rates of up to 20~. because of drips and weeping from a joint are not unknown.
After being placed in contact, the solvent-bonded parts must be clamped together for a curing period that will certainly
QUALITY OF THE JOINTS Per unit area, ultrasonic joints are stronger than adhesive bonds (70-80~. of the strength of the parent material at least) but because adhesive joints are usually designed with greater surface area, they may have a greater breaking strain. Most ultrasonic joints are hermetic seals also. Correct design is fundamental to a good ultrasonic weld, and consists chiefly in providing energy concentrators of the right volume and flat surfaces near the weld for the ultrasonic horn to bear on. The best energy concentrator is a ridge of triangular crosssection protruding :rom one surface and rUlU1ing the length of the weld. Its point should rest in a mating groove and its volume should be such that when it melts it fills the groove with a little to spare. The surplus liquid spreads over the whole interface, very like the polymer dissolved in a solvent but much more closely calculated. Careful design can make the concentrator of just such a volume as to fill the joint and extrude a small flash line along one side or other, if a flash line is wanted as evidence of a strong joint. (continued on p. 16)
Table 1 Ultrasonically weldable materials
Material
Solvents
Ultrasonic Welding
Polystyrene Styrene acrylonitrile ABS
C
Weld excellently even when horn is applied some distance from the joint. Hermetic joints possible.
Glass-filled polystyrene Rubber-modified polystyrene
C
Weldability depends on amount of regrind or rubber added. Low impact fairly easily welded. High impact very difficult.
Polycarbonate
c
Good joints, hermetic if necessary. Horn should be close to joint to give sufficient energy.
Polypropylene filled/unfilled
None
Unfilled or filled can be welded. Horn must be close to joint.
Polyethylene (high and low density)
None. Can be made to swell but not dissolved.
AIl above.
Acetal (Delrin and Kematal)
None
Good but needs high energy. Therefore horn close to joint and top power. Must be welded dry (as moulded).
Nylons
Formic aCid, phenol
Acrylics
C
AIl for acetal. Glass reinforced nylons are difficult except for inserts, which can be made quite easily. Injection-moulded welds easily (hermetic seal): cast or extruded welds badly. Must be welded dry.
Cellulosics
None. Trichlorethylene attacks slightly.
Vary. Cellulose acetate impossible as yet.
Rigid pvc
C
Good.
C = Soluble in one or more of the common plasUcs solvents (methylene chloride, trichIOl'ethylene, perchlorethylene, toluene, and various esters and ketones). ULTRASONICS January 1967
13
APPLICATIONS The answers to five questions will decide whether ultrasonic welding is likely to be suitable. 1
What is the material?
2
What shape are the mating surfaces?
3
How long is the weld line?
4
How near the weld line can the welding horn be placed?
5
How strong a joint is needed? Must it be a hermetic seal?
c.
A B
This display contains most of the weldable materials. More important, shape is no bar to ultrasonic welding Toy pistol in polystyrene. The whole circumference is welded in one operation
C
Rigid pvc toy blocks need a square-ended hollow horn but present no other problems
D
Screws inserted into Perspex with a single application of the welding head. Inserts can be made in materials difficult to weld because in making an insert the ultrasonic enelogy is conducted by the metal sc;rew
E
Toy boats. A circumferential weld holds all four parts together. Weld takes less than lsee 14
ULTRASONICS January 1967
D
E
K
H
F
F Switch to be welded contains a spring tending to force the halves apart. Welding is therefore followed by a lsec hold time without ultrasound. Impact grade polystyrene G Cosmetic tube in polystyrene is welded in 0.5 sec K
Standard wedge horn with two spot-welding points. For safety, operator has to depress both handles
H The straightforward weld of inner to outer of a simple shape
like a bowl is well established. What the photograph doesn't show is that the machines are on shift work 24hr per day 7 days per week ULTRASONICS Janmu'y 1967
15
MOULDS Large in the economics of plastics moulding bulks the cost of the mould, but the cost of moulds varies so much that generalizations can't be made. A cheap mould of cast kirksite for a very complicated part might be written off over a dozen items only, because it is cheaper than machining them. A costly beryllium copper mould might run for five million operations. However, moulds can often be modified for ultrasonic welding quite easily. A small hollow can easily be made in the end of an ejector pin to produce a spike-shaped concentrator. Grooves can often be machined in the face of the pin to produce ridge concentrators along surfaces to be welded. Flat surfaces for the ultrasonic horn to bear on are less easily incorporated, even if the mould is designed from scratch with ultrasonics in mind. Fortunately it is possible to profile the horn to the shape of the part but here again the cost of the special horn has to be balanced against the cost of the mould, the final appearance of the job and so on. Experimental horns can be made cheaply in aluminium and it is l' easonable to expect sixty to seventy thousand operations from them. I
J
ACKNOWLEDGEMENT We are grateful to the following firms for information and photographs: Dawe Instruments Ltd (R. D. Stafford), Radyne Ltd (G. King) and Soag l'4achine Tools Ltd (J. Craw) REFERENCES 1
'Welding rigid thermoplastics-joint design', Ultrasonics, 4, p47 (January 1966)
2
Obeda, E. Modern Plastics (November 1964)
3
Maher, A. D. British Plastics (JUly 1965)
4
Rolb, D. J. SPE Journal (November 1966)
MANUFACTURERS OF EQUIPMENT
I
Rattles in GP polystyrene welded in O. 3sec. 625JID1 wall. Static pressure 35 Ib/in 2 over a lin 2 area. 1750-2000 per hour J Special double-pronged horn with six welding tips for welding rigid pvc honeycomb
16
ULTRASONICS January 1967
Branson Sonic Power, Danbury, Connecticut, U.S.A. Dawe Instruments Ltd, Western Avenue, London W3. Dohm Plastics Machinery Ltd, 167 Victoria Street, London SW1 Kerry's (Ultrasonics) Ltd, Hunting Gate, Wilbury Way, Hitchin Hertfordshire, England. Distributors to the plastics industry: Soag Machine Tools Ltd, Transport Avenue, Brentford, Middlesex, England. Dr. Lehfeldt & Co. GmbH, Heppenheim, near Mannheim, Germany (FDR) Radyne Ltd, Wokingham, Berkshire, England.
ULTRASONIC WELDING: SUMMARY
Transducer (magnetostrietive or piezoelectric) Control asci llator
Frequency control
Power suppLy
Timer and timing control
t
Workhorn Velocity transformer
The work horn can be of steel, aluminium alloy or, most commonly, titanium. It vibrates at a frequency near 20kHz. The horn changes shape towards the lower end. The crOBS section of the working surface is smaller than that of the upper end of the horn so as to increase the amplitUde of vibration. The increase depends on the ratio of their areas. The amplitude of a typical horn varies over 10-50 !lm. Not all systems include the intermediate coupling section shown in the diagram: the horn may be fixed directly to the transducer.
Horns are shaped to suit the partiCUlar job
ted depends on the amplitude of the motion and on the static force with which the transducer bears down on the workpiece. Too great a force stops the motion. Too small a force fails to bring the surfaces properly into contact. Either way, little or no heat is generated and no weld isformed. All the time the ultrasound is switched on, the horn is pressed against the workpiece with up to 1001b!in 2 static pressure, usually applied by pneumatic cylinders. The ultrasound lasts 1-10sec after which there may be an equaUy long hold time without ultrasound during which the weld sets hard.
When the tip is applied to the plastics workpiece it transmits The controls on most welders are those for static force, ulultrasound through the plastic with greater or smaller loss trasonic power, weld time and hold time. In addttion there depending on the elastic modulus of the plastic. Rigid brittle may be a manual frequency tuning knob which should be materials usually transmit best. Transmission distances vary over 1-20cm but depend on power input and the material. checked at intervals, say daily. Some welders tune themselves automatically to the co rrect frequency. When the ultrasound reaches the joint it causes the upper surface of the joint to Vibrate while the lower surface reSuccessful welding depends on correct setting up, using a suitable horn and designing the joint for ultrasonic welding mains stationary. The relative motion generates frictional heat, which melts the surfaces. The amount of heat generawherever p.ossible.
A selection of well designed joints
ULTRASONICS January 1967
17
WELDERS FOR RIGID THERMOPLASTICS Kerry's (tntrasonic s)/Soag 150W electrical. Controls: power, on/off, weld time (0-5sec), hold time (0-5 sec), air pressure Automatic timing. Nominal 20kHz Stove enamel finish Lead zirconate titanate sandwich transducer Type No. KS220-223
Radyne 300 W and 1000W Coritrols: on/off, power, hold time (0.4-10 sec) weld time (0. 4-10 sec). Pneumatic pressure control. Manual tuning. Nominal 19kHz Magnetostrictive transducer 300 W model SG3 and SW3 Lehfeldt-Nirona/Dohm 2500W and lOOW Controls: on/off, power, lime, hydraulic pressure on large model, manual on small Manual tuning. Nominal 22kHz on large model, 40kHz on small Types: M40/100 and H22/2500. Four intermediate models Dawe Instruments
250 W. electrical Controls: on/off, power, weld time (0-10 sec) hold time (0-10 sec) air pressure. Automatic timing. Nominal 20kHz. Lead zirconate titanate sandwich transducer Type-: Sonicwelder 1133A
18
ULTRASONICS .January 1967
run to hours, possibly to days. Solvents tend to attack the edges of a plastic part at stress points and cause stress cracks in the part after the joint has finished curing. AIl much as an extra 3 hours' annealing may therefore have to be Table 1 gives part of the answer straight away. Several of added to the curing time. Apart from the reject rates and the the plastics that can be welded ultrasonically have no conve- delay, boxes of curing parts occupy valuable space and annient solvent, often no known solvent. Formic acid and phenol nealing ovens cost money. So does extra ventilation. will dissolve nylons but each has its difficulties and hazards. By disposing of the need for solvents, ultrasonics reduces Special ventilation is usually necessary and the operator must wear protective clothing. Polypropylene resists almost welding to a single process of less than 1 min with reject rates of O. 1-1 It opens the way to automation for producevery known chemical, which is why vessels are moulded tior. rates of 800-2000 per hour from a single welder. from it, and it cannot therefore be joined by solver.ts. Why should a fabricator of rigid thermoplastics bother with ultrasonic welding when there are plenty of solvents on the market with a good many years of experience behind them?
0 /0 ,
Though we often hear how quickly an ultrasonic weld can be made (0. 5-10sec including hold time) the real savings of ultrasonic welding only become apparent when it is compared to a solvent process. An adhesive for joining two rigid plastics parts may be either a pure solvent or a solvent with a small quantity of the parent polymer already dissolved in it. Perspex is a good example of the latter. To get it onto the face of the joint, the adhesive can be spread with a brush or sqUirted from a small nozzle or hypodermic syringe; or the component can be pressed onto a spongy pad soaked in adhesive. Whichever method is used, solvent is continuously evaporating. Toxic or inflammable solvents such as methylene chloride or toluene, respectively, are subject to definite safety regulations. Even if it is safe an evil-smelling solvent may irritate the operator. Therefore, ventilation. However neat-handed the operator is, there is always the chance of a drip of solvent marking the part, either directly or via his hands. Reject rates of up to 20~. because of drips and weeping from a joint are not unknown.
After being placed in contact, the solvent-bonded parts must be clamped together for a curing period that will certainly
QUALITY OF THE JOINTS Per unit area, ultrasonic joints are stronger than adhesive bonds (70-80~. of the strength of the parent material at least) but because adhesive joints are usually designed with greater surface area, they may have a greater breaking strain. Most ultrasonic joints are hermetic seals also. Correct design is fundamental to a good ultrasonic weld, and consists chiefly in providing energy concentrators of the right volume and flat surfaces near the weld for the ultrasonic horn to bear on. The best energy concentrator is a ridge of triangular crosssection protruding :rom one surface and rUlU1ing the length of the weld. Its point should rest in a mating groove and its volume should be such that when it melts it fills the groove with a little to spare. The surplus liquid spreads over the whole interface, very like the polymer dissolved in a solvent but much more closely calculated. Careful design can make the concentrator of just such a volume as to fill the joint and extrude a small flash line along one side or other, if a flash line is wanted as evidence of a strong joint. (continued on p. 16)
Table 1 Ultrasonically weldable materials
Material
Solvents
Ultrasonic Welding
Polystyrene Styrene acrylonitrile ABS
C
Weld excellently even when horn is applied some distance from the joint. Hermetic joints possible.
Glass-filled polystyrene Rubber-modified polystyrene
C
Weldability depends on amount of regrind or rubber added. Low impact fairly easily welded. High impact very difficult.
Polycarbonate
c
Good joints, hermetic if necessary. Horn should be close to joint to give sufficient energy.
Polypropylene filled/unfilled
None
Unfilled or filled can be welded. Horn must be close to joint.
Polyethylene (high and low density)
None. Can be made to swell but not dissolved.
AIl above.
Acetal (Delrin and Kematal)
None
Good but needs high energy. Therefore horn close to joint and top power. Must be welded dry (as moulded).
Nylons
Formic aCid, phenol
Acrylics
C
AIl for acetal. Glass reinforced nylons are difficult except for inserts, which can be made quite easily. Injection-moulded welds easily (hermetic seal): cast or extruded welds badly. Must be welded dry.
Cellulosics
None. Trichlorethylene attacks slightly.
Vary. Cellulose acetate impossible as yet.
Rigid pvc
C
Good.
C = Soluble in one or more of the common plasUcs solvents (methylene chloride, trichIOl'ethylene, perchlorethylene, toluene, and various esters and ketones). ULTRASONICS January 1967
13
APPLICATIONS The answers to five questions will decide whether ultrasonic welding is likely to be suitable. 1
What is the material?
2
What shape are the mating surfaces?
3
How long is the weld line?
4
How near the weld line can the welding horn be placed?
5
How strong a joint is needed? Must it be a hermetic seal?
c.
A B
This display contains most of the weldable materials. More important, shape is no bar to ultrasonic welding Toy pistol in polystyrene. The whole circumference is welded in one operation
C
Rigid pvc toy blocks need a square-ended hollow horn but present no other problems
D
Screws inserted into Perspex with a single application of the welding head. Inserts can be made in materials difficult to weld because in making an insert the ultrasonic enelogy is conducted by the metal sc;rew
E
Toy boats. A circumferential weld holds all four parts together. Weld takes less than lsee 14
ULTRASONICS January 1967
D
E
K
H
F
F Switch to be welded contains a spring tending to force the halves apart. Welding is therefore followed by a lsec hold time without ultrasound. Impact grade polystyrene G Cosmetic tube in polystyrene is welded in 0.5 sec K
Standard wedge horn with two spot-welding points. For safety, operator has to depress both handles
H The straightforward weld of inner to outer of a simple shape
like a bowl is well established. What the photograph doesn't show is that the machines are on shift work 24hr per day 7 days per week ULTRASONICS Janmu'y 1967
15
MOULDS Large in the economics of plastics moulding bulks the cost of the mould, but the cost of moulds varies so much that generalizations can't be made. A cheap mould of cast kirksite for a very complicated part might be written off over a dozen items only, because it is cheaper than machining them. A costly beryllium copper mould might run for five million operations. However, moulds can often be modified for ultrasonic welding quite easily. A small hollow can easily be made in the end of an ejector pin to produce a spike-shaped concentrator. Grooves can often be machined in the face of the pin to produce ridge concentrators along surfaces to be welded. Flat surfaces for the ultrasonic horn to bear on are less easily incorporated, even if the mould is designed from scratch with ultrasonics in mind. Fortunately it is possible to profile the horn to the shape of the part but here again the cost of the special horn has to be balanced against the cost of the mould, the final appearance of the job and so on. Experimental horns can be made cheaply in aluminium and it is l' easonable to expect sixty to seventy thousand operations from them. I
J
ACKNOWLEDGEMENT We are grateful to the following firms for information and photographs: Dawe Instruments Ltd (R. D. Stafford), Radyne Ltd (G. King) and Soag l'4achine Tools Ltd (J. Craw) REFERENCES 1
'Welding rigid thermoplastics-joint design', Ultrasonics, 4, p47 (January 1966)
2
Obeda, E. Modern Plastics (November 1964)
3
Maher, A. D. British Plastics (JUly 1965)
4
Rolb, D. J. SPE Journal (November 1966)
MANUFACTURERS OF EQUIPMENT
I
Rattles in GP polystyrene welded in O. 3sec. 625JID1 wall. Static pressure 35 Ib/in 2 over a lin 2 area. 1750-2000 per hour J Special double-pronged horn with six welding tips for welding rigid pvc honeycomb
16
ULTRASONICS January 1967
Branson Sonic Power, Danbury, Connecticut, U.S.A. Dawe Instruments Ltd, Western Avenue, London W3. Dohm Plastics Machinery Ltd, 167 Victoria Street, London SW1 Kerry's (Ultrasonics) Ltd, Hunting Gate, Wilbury Way, Hitchin Hertfordshire, England. Distributors to the plastics industry: Soag Machine Tools Ltd, Transport Avenue, Brentford, Middlesex, England. Dr. Lehfeldt & Co. GmbH, Heppenheim, near Mannheim, Germany (FDR) Radyne Ltd, Wokingham, Berkshire, England.
ULTRASONIC WELDING: SUMMARY
Transducer (magnetostrietive or piezoelectric) Control asci llator
Frequency control
Power suppLy
Timer and timing control
t
Workhorn Velocity transformer
The work horn can be of steel, aluminium alloy or, most commonly, titanium. It vibrates at a frequency near 20kHz. The horn changes shape towards the lower end. The crOBS section of the working surface is smaller than that of the upper end of the horn so as to increase the amplitUde of vibration. The increase depends on the ratio of their areas. The amplitude of a typical horn varies over 10-50 !lm. Not all systems include the intermediate coupling section shown in the diagram: the horn may be fixed directly to the transducer.
Horns are shaped to suit the partiCUlar job
ted depends on the amplitude of the motion and on the static force with which the transducer bears down on the workpiece. Too great a force stops the motion. Too small a force fails to bring the surfaces properly into contact. Either way, little or no heat is generated and no weld isformed. All the time the ultrasound is switched on, the horn is pressed against the workpiece with up to 1001b!in 2 static pressure, usually applied by pneumatic cylinders. The ultrasound lasts 1-10sec after which there may be an equaUy long hold time without ultrasound during which the weld sets hard.
When the tip is applied to the plastics workpiece it transmits The controls on most welders are those for static force, ulultrasound through the plastic with greater or smaller loss trasonic power, weld time and hold time. In addttion there depending on the elastic modulus of the plastic. Rigid brittle may be a manual frequency tuning knob which should be materials usually transmit best. Transmission distances vary over 1-20cm but depend on power input and the material. checked at intervals, say daily. Some welders tune themselves automatically to the co rrect frequency. When the ultrasound reaches the joint it causes the upper surface of the joint to Vibrate while the lower surface reSuccessful welding depends on correct setting up, using a suitable horn and designing the joint for ultrasonic welding mains stationary. The relative motion generates frictional heat, which melts the surfaces. The amount of heat generawherever p.ossible.
A selection of well designed joints
ULTRASONICS January 1967
17
WELDERS FOR RIGID THERMOPLASTICS Kerry's (tntrasonic s)/Soag 150W electrical. Controls: power, on/off, weld time (0-5sec), hold time (0-5 sec), air pressure Automatic timing. Nominal 20kHz Stove enamel finish Lead zirconate titanate sandwich transducer Type No. KS220-223
Radyne 300 W and 1000W Coritrols: on/off, power, hold time (0.4-10 sec) weld time (0. 4-10 sec). Pneumatic pressure control. Manual tuning. Nominal 19kHz Magnetostrictive transducer 300 W model SG3 and SW3 Lehfeldt-Nirona/Dohm 2500W and lOOW Controls: on/off, power, lime, hydraulic pressure on large model, manual on small Manual tuning. Nominal 22kHz on large model, 40kHz on small Types: M40/100 and H22/2500. Four intermediate models Dawe Instruments
250 W. electrical Controls: on/off, power, weld time (0-10 sec) hold time (0-10 sec) air pressure. Automatic timing. Nominal 20kHz. Lead zirconate titanate sandwich transducer Type-: Sonicwelder 1133A
18
ULTRASONICS .January 1967