RF Welding Systems with Superior Process Control


RF Heat Sealing or RF Welding Process uses Radio Frequency (RF) energy to melt and form

plastic materials, similar to using high frequency energy in microwave oven to heat a chicken

RF Heat Welding Process is like Microwave heating a Chicken.jpg

RF Heat Sealing Theory

Radio Frequency (RF) Welding or High Frequency (HF) Sealing Process Overview

RF heat sealing (also known as dielectric sealing) uses high frequency energy to melt polar plastic material between two electrodes (RF Sealing Dies). Most commonly used RF frequency is 27.12MHz. 

In the simplest terms, RF welder is like an oven that cooks (melts) plastic using high frequency energy similar to Microwave Oven that heats the food at home.

RF welding systems comprises of two main elements the RF Generator that generates the high frequency energy and the mechanical press, defined as RF welding or RF sealing system that compresses layers of plastic material while applying RF energy for bonding or forming the material layers.

The most common types of high frequency welders are the Rotary RF Welders and the Shuttle style RF welders, each style uses different method for handling the material in and out of the RF welding station. The RF welding station uses an air cylinder activated press to compress the plastic materials while applying Radio Frequency energy. As the plastic material heats up under the RF energy, it melts and forms homogeneous bond, under the press force, taking the shape of the RF sealing dies (electrodes).

The RF sealing dies are manufactured using brass or aluminum materials. Due to several factors aluminum dies may consume more RF energy than brass dies, (aluminum may requiring up to 25% more RF energy to seal a particular product compared with brass or copper). 

The RF sealing process may consist of one or two cycles:

  • Single Cycle RF Sealer - Products that require only perimeter seal can be sealed by a single cycle RF welder.
    • The single cycle RF welder is simple and the source RF power is directly connected to the upper platen where the RF sealing die is mounted. The bottom die is connected to chassis ground (which must share the same ground as the generator).
  • Dual Cycle RF Sealer - Most medical bags have tubing between the film layers, defined as ports. The process for RF sealing a bag with ports (e.a. blood, urine or Enteral Feeding bags) will requires dual cycle RF sealer. The dual cycle RF sealer uses a mechanism, called RF Switch, that switches the source RF power to connect to Point A and Point B.
    • Point A - is a connection to a rod (mandrel) to seal the port to the top and bottom film layers. The tubing is assembled over the mandrel and placed between the film layers. The RF is applied to the mandrel the Upper and lower dies are connected to the common ground. 
    • Point B is a connection to the upper platen to seal the perimeter of the product. in this case the Upper die is connected to the source RF power and bottom die is grounded

When High Frequency energy is applied to the electrodes the material polar molecules get excited and start melting the material, which is compressed between the RF sealing dies. Once melted, the RF energy is turned off and by going through a cooling cycle the materials fuse to each other. 
The RF welding process is controlled by the following process parameters: Pre-seal Time, Pre-seal Power, Main Seal Time, Main Seal Power and Cool Time. To learn more about RF Welding System Process Control please click the link for more details.

RF Welding Process Control Screens.png.


Product quality factors:

  1. Material Clamping Force
  2. RF Power
  3. RF ON Time
  4. Cooling Time
  5. Parallelism of the RF sealing press
  6. Parallelism of RF Sealing Dies
  7. Electrode (RF Sealing Die) temperature
  8. Buffer Material
  9. RF connections
  10. Ground connections

Material Clamping Force

During the RF sealing process the force compressing the material between the RF sealing dies is critical to achieve a good bond.

  • Low pressure will cause arcing or surface flashes also can cause poor seal.
  • Too much pressure will "crash" the plastic causing uneven seals.

To ensure a uniform RF seal, the proper pressure must be applied onto the sealed material, uniformly. The HF sealing dies must be made perfectly flat and parallel. To prevent over travel, as the material is melted, the seal thickness is controlled by hard stops evenly distributed around the perimeter of the seal.

Pre-seal Time and Power

Preseal Time is defined as the time required to close the press and build enough pressure to apply RF power. ONEX RF welders sense the press closure and use low RF power to start the melting process during preseal time.

Caution: If the RF energy is applied before full press closure and pressure build up to compress the material, the process will cause an arc or material will flash and burn.

RF Sealing Power versus Seal Area

The power required for a good seal is directly proportional to the seal area of the product. After the pre-seal cycle the main seal power is applied to the dies. Nice controlled rise of power can bring consistency in the RF sealing process and prevent flashes or material burns.

  • If too much energy is applied, the material will burn.
  • If the energy is not enough, the material layers will not bond evenly or at all.

As the material temperature rises, a state of equilibrium is achieved between the generated heated material and the heat loss through the dies. The heat loss is greater when sealing thinner materials, thus buffer material is used to reduce the heat loss specially with ultra-thin materials' (less than 0.004”). Another method to reduce the heat loss is to heat the RF sealing dies. Thin materials may require higher RF power to seal the thinner materials.

The sealing chart below shows the required energy based on the seal area. and material thickness.  Also, the amount of RF power to a achieve a good quality seal is affected by the material type.

Sealing Area Chart

Total Sealing Area in Square Inches:

Material Thickness   Generator Power Required
of Vinyl 1 kw 4 kw 6 kw 10 kw 15 kw 20 kw 30 kw 40 kw 50 kw
0.008 2.0 8.0 12.0 20 30 40 60 80 100
0.012 3.0 12.0 18.0 30 45 60 90 120 150
0.016 3.5 14.0 21.0 35 52.5 70 105 140 175
0.020 3.8 15.2 22.8 38 57 76 114 152 190
0.024 4.2   17.0 25.0 42 63 84 126 168 210
0.032 4.6 18.4 27.6 46 69 92 138 184 230
0.040 5.0 20.0 30.0 50 75 100 150 200 250
0.060 5.5 22.0 33.0 55 82.5 110 160 220 270
0.080 6.0 24.0 36.0 60 90 120 180 240 300

 

The average RF Sealing Power is 1 KW/3sq.in area

Main Seal Time

Main Seal Time is for applying the set RF energy ON for the set time for melting the plastic (passing the material's glass transition temperature) to bond the two materials. When RF sealing two different materials, it's almost impassible to get homo genius bond since materials don't mix because of the melt temperature difference, for example PVC to TPU.

Selecting new process parameters

When setting up a new sealing job the first test should be done with minimum power, moderate time and around 80 PSI pressure. If the seal is weak, power should be increased gradually. Usually the RF Sealer has a manual operations screen to manually activate the mechanical motions and rf power. Usually the material is pressed on the bottom die and press is lowered manually applying full force, then RF is energised for a set time to seal the product. This process is repeated several times by increasing time and power until the proper sealing parameters are reached. 

The dies must be held parallel to produce even pressure around the seal area. If there is too much extrusion or if the seal is too thin, the press sealing stops need to be adjusted. To set the stops, adjust the press to half the total thickness of the material. Close the press and adjust the stops finger tight. Then insert the full thickness of material in the press and make a seal. Check the results and lower or raise the stops as required. Lock the stops in place.

If the seal is weak at a particular area, the dies are not level. The leveling screws should be checked and adjusted. If these adjustments are still unsatisfactory, the use of a shim may be required or the dies must be surface ground.

After several RF seal cycles the dies warm up and process parameters (time and power) may require re-adjustment. To eliminate parameter adjustments, the upper platen temperature must be regulated using circulating distilled water or heated using cartridge heaters. This process is used in tear seal applications.

Pre-Seal time and heated platens can change these factors. Heated platens help maintain consistency.

Materials Sealability Chart

Material Excellent Good Fair Poor None
ABS polymers   Good      
Acetal (Delrin)       Poor  
Acetal copolymer       Poor  
Acrylics     Fair    
Aclar     Fair    
APET   Good      
Barex 210 Excellent        
Barex 218 Excellent        
Butyrate   Good      
Cellophane         None
Cellulose acetate (clear)   Good      
Cellulose acetate (color)   Good      
Cellulose acetate butyrate   Good      
Cellulose nitrate     Fair    
Cellulose triacetate     Fair    
CPET         None
Diallyl phthalate polymer, glass-filled       Poor  
Epoxy resins     Fair    
Ethyl cellulose         None
EVA (Ethyl Vinyl Acetate)   Good      
EVOH (Ethyl Vinyl Alcohol)     Fair    
Melamine-formaldehyde resin   Good      
Methylacrilate     Fair    
Nylon (Polyamide)     Fair    
Pelathane   Good      
PET (Polyethylene Terphthatate)   Good      
PETG (Polyethylene Terphthatate Glycol) Excellent        
Phenol-formaldehyde resin   Good      
Pliofilm (Rubber Hydrochloride) Excellent        
Polyamide     Fair    
Polycarbonate       Poor  
Polychlorotrifluoroethylene       Poor  
Polyester       Poor  
Polyethylene (All)         None
Polymide       Poor  
Polymethyl (Methacrylate)     Fair    
Polypropylene         None
Polystyrene         None
Polytetrafluoroethylene (Teflon)         None
Polyurethane     Fair    
Polyurethane foam       Poor  
Polyurethane-vinyl film   Good      
Polyvinyl Acetate   Good      
Polyvinyl chloride (PVC) flexible, clear Excellent        
PVC color Excellent        
PVC opaque   Good      
PVC semi rigid   Good      
PVC rigid     Fair    
PVC flexible, glass-bonded Excellent        
PVC coated material (cloth & paper) Excellent        
Polyvinyl chloride (PVC) (adhesive emulsions) Excellent        
Rubber         None
Rubber, compounded     Fair    
Rubber, hevea       Poor  
Saran (Polyvinylidene Chloride) Excellent        
Silicones         None
Teflon (Tetrafluoroethytene)         None
Urea-formaldehyde resin   Good      
           

(x) = Response of the materials in the 20 to 30 Mc/sec range


Arc Prevention

Arcing may occur in several cases:

  • If there is not enough pressure or gap between the die and material surface.
  • Uneven die surface
  • When the die overlaps the edge of the material.
  • Too much power.
  • Dirty materials or carbon build up on the die surface
  • Sharp corners on the dies 

The die edges should always be rounded and smooth.

When an arc occurs, the dies must be carefully cleaned with an emery cloth and solvent.


Surface Flash

This is sometimes confused with arcs. A flash occurs on the surface of the material during the sealing cycle. It leaves smoke and/or a layer of black carbon on the die. Clean all traces of carbon off. Surface flash is caused by:

  • Press pressure too low
  • Hard stops set too high
  • Power set too high
  • Die too cold

The arc suppressor usually does not stop the flash unless it burns the material before the cycle ends.


Arc Suppression

Most dies are complex and expensive not counting the fact that down time can reach hours if die damage occurs.  It is essential to protect the dies from arc damage. Although dies are repairable, the loss of production time for repairs is extremely expensive.

Most sealing equipment comes with an arc suppression device. The function of this device is to sense the possibility of an arc and then turn off the RF power before the arc can damage the dies.

ONEX RF Arc detect circuit is one of the fastest the industry.

ONEX RF uses low voltage Arc detect circuit. As soon as the arc starts the system detects current rush and instantly stops the oscillator circuit to interrupt RF energy to the die. The stoppage is very quick and simply saves the dies from melting under the current discharge. 

The Arc-Detect circuit functionality can be tested by shortening the upper and lower dies without presence of RF power. If the Arc detected message appears on the screen then it functions properly.

Buffer Materials

When the material is heated between the two electrodes, heat loss occurs through the die surface. In order to minimize the loss, a special insulating material(s) known as “Buffer Material” are used for isolation.

In most cases sealing is improved by using a thin layer of Buffer between one or both layers top and bottom dies and article.

The Buffer Material does several things:

  • It lowers the heat loss from the materials to the dies
  • Compensates for small dents in the die surface
  • Decreases the tendency to arc when thin material is used.
  • In general, it makes a better seal with less arcing.

Buffer Material should have good heat resistance and high voltage breakdown such as:

  • Mylar (one of the best and most used Buffer materials)
  • Bakelite (Bakelite grade XXX about .010 to .030” is used quite often)
  • Teflon (Because it is the least RF conductive material, it can cause issues)
  • Silicone fiberglass
  • Glassine
     

ONEX RF offers RF Sealers and RF Welding Seminars

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The best RF sealers in the industry:

  1. Basic Rotary RF Sealers
  2. Rotary RF Sealer with two stations
  3. Automation RF Sealers 
Gain real knowledge by attending one of our RF training seminars.

Contact us for your RF Sealing training needs. 

Request Consultation

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