RF Welding

Dielectric Heating

Dielectric or insulator materials are capable of transmitting electromagnetic force without conducting current through. Yet most dielectric materials will leak current when molecules are polarized.

The polar molecules of dielectric materials generally are in random orientation, when there is no electric field. An applied electric field will polarize the material by orienting dipole moments of polar molecules, like the compass needle orienting towards the North. 

When the material is in an alternating field (High Frequency or RF Field) some leakage current passes through the material causing a heat called dielectric loss, which results in heating the dielectric material.

RF Welding Press

The RF welding press on the right uses top and bottom RF welding dies (molds) to RF weld and form the final product. The press cylinder extends the upper platen down and applies pressure onto the dielectric material. RF signal is applied to the upper die and the bottom die is connected to the ground. When the dies close and compress the plastic material, a capacitor is formed between the dies (electrodes) and the dielectric material. The leakage current heats and melts the material as the RF energy is applied and forms the product under the pressing force.

Material Properties

  • Must have polar molecules

Radio Frequency - RF Welding Machine

High Frequency - HF Sealing System

Radio Frequency welding is a material joining process on the dielectric loss principle. RF Welding or RF Heat sealing process uses high frequency energy to melt the plastic material with polar molecules placed in between two electrodes (RF Sealing dies). Most commonly used RF frequency is 27.12MHz.

System Comprises

  • RF Generator
  • RF Seal Station
  • RF Heat Sealing Dies

RF Welding Generators are used in the RF Heat Sealing process to melt plastics materials, comprised of polar molecules. Most RF sealing processes use 27.12MHz high frequency. The same frequency is also used in the Ham-Radio Communications Industry hence where the High Frequency (HF) is referred to as Radio Frequency (RF). 

ISM Band Frequencies (Industrial Scientific Medical): The frequency used in the RF welding systems is regulated by FCC and generators must comply with ISM frequency guidelines. The large RF welders will usually use 13.56MHz frequency if the platen size exceeds the 1/4 wavelength of the frequency.

RF Sealing Press: The RF Welding station is a stand-alone pneumatic or hydraulic press, which compresses the plastic material between two molds, referred to as RF sealing dies.

RF Welding Process: In simplest terms, RF welders operate much like microwave ovens that heat food. The Microwave excites the polar water molecules and heats the food.

The RF welder on the other hand heats the plastic material that has polar molecules, which get excited by the high frequency energy.

When RF energy is applied to the compressed material between the RF sealing die surfaces (RF Electrodes), in this case the die surfaces act as capacitors and the plastic material as the insulator dielectric materials, the polar molecules get polarized and start oscillating in the alternating electromagnetic field. This movement or inner molecular friction and dielectric loss material heats up and melts the material. As material softens the press further advances to form the bonded section, defined as RF weld thickness. Once the RF energy is interrupted the material cools down by loosing heat through the dies metal surface. 

RF Welder Compression Force Affect on the RF Seal Thickness

314

The RF welding press compression force is essential to form the plastic material while it's melting, where the shape of seal area is controlled by the shape of RF sealing die.  The sealing area and the material type dictates the compression force.

Larger the seal area the force has to be larger. Yet it is difficult to control the seal thickness by only force or RF power and time. 

There are other factors that can affect the seal thickness, such as material initial temperature, die sealing surface shape or even the humidity level.

It is common practice  to use hard stops to control the seal thickness and have slightly higher pressure to achieve RF seal consistency.

Pressure Related Issues:

  • Low pressure causes arcing or flash burns. When the press come down and energizes the RF power if the materials are not compressed and have slight air gap the material surface will start charging like a capacitor and will start arcing in that area.
  • If the surface area is small and too much pressure is applied by the press the material will collapse, becoming paper thin or even will rip apart or crack which will cause an arc or uneven seal based on the press construction parallelism or surface mismatch.

To insure a uniform RF seal thickness, the pressure must be controlled properly, the RF sealing dies must be flat and parallel, and finally hard stops must be used in carefully engineered locations to maintain evenness around the perimeter of RF sealed surface.  

RF Sealing Power effect on seal quality

46776556_ml

The RF Sealing power is directly proportional to the surface area of the sealing die and material properties.

A general rule for RF sealing 0.012" thick PVC material in 3 second seal time and 3 seconds cool time the process will require about 250-300 watts of RF power.

There are several other factors that can affect the seal quality besides the power over the seal area, are:

  • Die Temperature _ as the material is heated to its melting temperature, it looses some of the temperatures through the sealing die. To prevent die residual heat build-up the process uses thermal control using recirculating water or heated upper platen. this method will give consistent process control. 
  • Buffer Material _ helps to reduce the heat loss of heat plastic and can reduce the cycle time. Also, the buffer materials are used for increasing the voltage breakdown resistance to prevent arcs.
  • Die Material _ It is known that aluminum dies do require up to 25% more RF energy versus Brass Dies.
  • Impedance mismatch _ If there is an impedance mismatch between the generator and load, the reflected RF energy will cancel out the forward RF energy causing weaker seals. 

To ensure a good quality seal, the RF Power must be controlled properly, such that the seal does not come apart due to lack of power or material does not burn due to excess power.

RF Seal Time Effect on Seal Quality

Layer 72@1X

The RF welding process consists of three basic process timers.

  1. Pre seal Time
  2. Main Seal Time
  3. Cool Time

The Pre seal time starts when the press is extended all the way down. A limit switch or sensor detects press down position and timer starts timing.

ONEX RF recommends applying minimal RF power like 30-50% of the main-seal power to start melting the material in 1-2 seconds.

After the Pre-seal time, the Main-Seal timer starts timing and RF power is increased to the Main-seal set power.

Some companies would like to apply high RF power to achieve good quality seal in the shortest possible time and experience many inconsistencies.

We have learned the opposite is true. In order to achieve a consistent process control, it is much easier to rely on controlling seconds versus milliseconds, where even the PLC scan time may have an effect on the sealing cycle when using higher RF power.  

 

Cool Time Effect on RF Seal Quality

Cooling Time 49930699_s

The cooling cycle is time can affect the seal quality if the RF seal station retracts the press up prematurely while the material is solidifying.

As soon as the RF Main-Seal timer times out the Generator stops the RF energy and material is kept compressed between the dies. The plastic material slowly dissipates heat through the die surfaces. If the process uses insulator materials the cooling cycle will be longer compared with a direct metal contact.

 

Material Sealability Chart for RF Sealing Process in Alphabetical Order

Material Type

Excellent, Good, Fair, Poor or None 

ABS polymers

Good

Acetal (Delrin)

Poor almost None / Used as an Insulator

Acetal co-polymer

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

Good

EVOH (Ethyl Vinyl Acetate)

Fair

Melamine-formaldehyde resin

Good

Methyl acrylate

Fair

Nylon (Polyamide)

Fair

Pellethane

Good

PET (Polyethylene Terephthalate)

Good

PETG (Polyethylene Terephthalate Glycol)

Excellent

Phenol -formaldehyde resin

Good

Pliofilm (Rubber Hydrochloride)

Excellent

Polyamide

Fair

Polycarbonate

Poor

Poly-chloro-trifluoro-ethylene

Poor

Polyester

Poor

Polyethylene

None

Polyimide (Kapton)

Poor

Poly-methyl (Methacrylate)

Fair

Polypropylene

None

Polystyrene

None

Poly-tetrafluoro-ethylene (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 (fabric, cloth or paper)

Excellent

PVC adhesive emulsions

Excellent

Rubber

None

Rubber, compounded

Fair

Rubber, hevea

Poor

Saran (Polyvinylidene Chloride)

Excellent

Silicone

None

Teflon

None

Urea-formaldehyde resin

Good

Excellent RF Weldable Materials List

Barex 210

Excellent

Barex 218

Excellent

PETG (Polyethylene Terephthalate Glycol)

Excellent

Pliofilm (Rubber Hydrochloride)

Excellent

Polyvinyl Chloride (PVC) flexible, clear

Excellent

PVC color

Excellent

PVC flexible glass-bonded

Excellent

PVC coated material (fabric, cloth or paper)

Excellent

PVC adhesive emulsions

Excellent

Saran (Polyvinylidene Chloride)

Excellent

Good RF Weldable Materials List

Material Type

Excellent, Good, Fair, Poor or None 

ABS polymers

Good

APET

Good

Butyrate

Good

Cellulose acetate (clear)

Good

Cellulose acetate (color)

Good

Cellulose acetate butyrate

Good

EVA

Good

Melamine-formaldehyde resin

Good

Pellethane

Good

PET (Polyethylene Terephthalate)

Good

Phenol -formaldehyde resin

Good

Polyurethane-vinyl film

Good

Polyvinyl Acetate

Good

PVC opaque

Good

PVC semi-rigid

Good

Urea-formaldehyde resin

Good

Fair RF Weldable Materials List

Material Type

Excellent, Good, Fair, Poor or None 

Acrylics

Fair

Aclar

Fair

Cellulose nitrate

Fair

Cellulose triacetate

Fair

Epoxy resins

Fair

EVOH (Ethyl Vinyl Acetate)

Fair

Methyl acrylate

Fair

Nylon (Polyamide)

Fair

Polyamide

Fair

Poly-methyl (Methacrylate)

Fair

Polyurethane

Fair

PVC rigid

Fair

Rubber, compounded

Fair

Poor RF Weldable Materials List

Material Type

Excellent, Good, Fair, Poor or None 

Acetal (Delrin)

Poor almost None / Used as an Insulator

Acetal co-polymer

Poor

Diallyl phthalate polymer, glass-filled

Poor

Polycarbonate

Poor

Poly-chloro-trifluoro-ethylene

Poor

Polyester

Poor

Polyimide (Kapton)

Poor

Polyurethane foam

Poor

Rubber, hevea

Poor

Non RF Weldable Materials List

Material Type

Excellent, Good, Fair, Poor or None 

Cellophane

None

CPET

None

Ethyl cellulose

None

Polyethylene

None

Polypropylene

None

Polystyrene

None

Poly-tetrafluoro-ethylene (Teflon)

None

Rubber

None

Silicone

None

Teflon

None

Portfolio of ONEX RF Welders

RF Welder Brochure Cover Page

RF Welding Solutions

ONEX RF offers various RF welding systems for the medical and automotive products manufacturers. 
Some of the products we have sealed in the past are blood, urine or other solution collection bags, and car seat air bladders.
View the brochure for more system options.

Other Services:

  • RF Welding Die Design and Fabrication
  • Sample Runs for Evaluation
  • Process Development
  • Validation Support


View RF Welder Brochure

CONTINUE LEARNING

RF Welder Insight
RF Welder Setup