Drew Jelgerhuis & Bill Reed, Life Sciences at Extol Inc.10.09.14
Higher-performing engineered polymers and more advanced product designs have led to an increase in the use of plastics in the medical device market. Single-use disposable (SUD) medical devices often are constructed from injection-molded plastic components. Typically, the complexity of the device requires that it be molded of multiple components and then permanently assembled. Popular joining methods used to assemble plastic components are: adhesive, solvent bonding, ultrasonic welding, laser welding, infrared (IR) welding, vibration welding, spin welding, radio frequency (RF) welding, and hot-plate welding (HPW).
This article will examine two joining processes for plastics: infrared welding and hot-plate welding. Both recently have benefited from significant process advancements and related growth in popularity. The medical device SUD sector should take note of the advances. Precise control of tooling position, velocity, force and acceleration are improvements that provide a new level of control and process data acquisition. Additionally, with the offering of smaller and more efficient machines, the user has more practical options for consideration.
Non-contact IR welding has been a viable joining method for a while. Its increase in popularity in the United States has been a relatively recent occurrence. The “non-contact” premise of this method addresses a significant challenge of “contact” HPW and that is the prevention of plastic sticking to the heated platen surface and the deposit of plastic material on the heated tooling surface.
The non-contact benefits of IR, however, are negated to some degree with other, less desirable realities. Let’s compare and contrast the HPW and IR processes further.
Plastics Welding Processes: Hot-Plate vs. Infrared
There are three rather generic modes of heat transfer to contemplate when discussing plastics joining: conduction, convection and radiation. The principal heat transfer method for HPW is conduction, while the primary method with non-contact IR welding is radiation.
The hot-plate welding process consists of three key steps: the melt phase, the open phase and the seal phase.
The sequence of operations takes place in this way:
Hot-Plate Welding, Material Heating Theory
As mentioned above, conduction is the primary method of heat transfer in HPW. Heat conducts from the heated tooling directly to the weld surface (melt rib). From the weld surface, it continues to conduct and “soak” deeper into the rib. An appropriate amount of heat soak into the weld rib is crucial for achieving high-bond strength and a tight, hermetic seal. The goal is to optimize the heat soak time. This will help ensure that when the two heated components are brought together in the seal phase, there is an optimal depth of heat-softened material on the two opposing surfaces.
This combined depth is called the heat-affected zone and it actually can be measured under magnification. If the heat-affected zone is too small, it means that excessive displacement of the semi-molten material has taken place and the likely result is a weak weld with poor sealing performance. If the heat affected zone is too large, the question of insufficient pressure applied during the seal phase is considered with similar poor weld results.
Non-Contact Infrared Welding, Material Heating Theory
IR heating of the weld rib surface is achieved through radiation. The IR source emits the energy which is absorbed at the surface by the components. Conduction, within the plastic resin, now transfers the heat within the weld rib. A critical parameter to be aware of is the rate of heating. Thermoplastic materials melt and re-flow when they are heated. If they are heated too quickly, there is a risk of material degradation, burning, or charring. Many resins absorb the initial IR energy very rapidly, but most of them do not conduct that surface heating very quickly. Yet, just as with HPW, it is still necessary to achieve a sufficient depth of heating into the weld rib to execute a robust welding process.
The rate at which a material absorbs IR radiant heat is characterized by three properties: absorptivity, transmissivity and reflectivity.
These material properties are affected by the material type, pigment/color, fillers, etc. Additionally, the emissivity of the IR emitter (its ability to emit radiation) also contributes to the effectiveness of radiant heat transfer and impacts the rate of heating.
Further yet, the density of the IR light as it reaches the surface that is being heated affects the rate of heating as well. This density is affected by the distance between the emitter and the heated surface and in how the infrared energy is focused. All of this is to say that there are many different factors that affect how quickly a plastic surface will absorb IR energy.
It is necessary to control this rate of heating to avoid some of the issues mentioned earlier. For example, an opaque, black material with a high percentage of glass filler content will absorb radiant energy at a very high rate but it will not conduct that surface heating as aggressively and might be susceptible to overheating on the outer surface. On the other side of the spectrum, a white material with no filler might conduct or transfer the surface heating more efficiently, but it will not absorb the initial IR energy quickly. It would not be at much risk for overheating, but the overall cycle time likely would be longer.
Material Displacement Comparison
Another big difference between HPW and IR welding is the displacement of material during the welding cycle. For this discussion, displacement is defined as any material that is moved from its original location at the melt rib to another location (flash).
In contact HPW there is physical contact between the heated tooling and the molded weld rib on the plastic components. As a result, there is displacement of the molded weld rib. As the press tooling positions the weld rib surfaces into intimate contact with the heated tooling, the material is displaced, which is a good thing. During this process the weld ribs conform to the heated tool surfaces, which are presumed to be highly accurate as machined. This ensures that the weld rib surfaces are formed precisely to design while they are being sufficiently heated.
During the melt cycle, the displacement is occurring on both components simultaneously, creating matched weld rib surfaces. This precise contact and related material displacement is a significant element in the HPW process as it prevents some issues otherwise associated with typical molding inconsistencies. Because the surfaces are uniform with each other, when welding pressure is applied in the seal phase, it evenly is applied along the entire length of the weld rib. Balanced welding pressure is imperative in creating high strength, tightly sealed welds while displacing minimal material.
With non-contact IR welding, there is no physical contact between the weld rib and the IR emitters. Therefore, there is no controlled material displacement to correct molded weld rib inconsistencies. For this reason, it is very important that the dimensions of the two molded components are precisely controlled in molding to achieve “as designed” surfaces.
When contemplating contoured weld rib surfaces with the non-contact IR welding process, the placement and distance of the IR emitters from the weld rib surface becomes particularly critical. To assure even heating and ultimately a robust process with minimal plastic displacement, the tool design must be critically assessed and proven to heat the contoured surfaces evenly. If uneven heating occurs during the melt phase there likely will be an uneven application of pressure through the weld joint during the seal phase which could result in poor weld quality.
Sensitive Internal Components in Welded Assemblies
Product designers cleverly secure sub-assemblies and internal devices within the subject components to be welded. These components are exposed to the welding environment and depending on the sensitivity of these components, this needs careful consideration.
Whether these internal devices are batteries, printed circuit boards, valves, filters, O-rings, or LED lights, one must have the assurance that the energy from the welding process does not cause any damage.
In contact HPW, the exposure duration of the heated platen is quite limited but the proximity to the devices requires evaluation.
Non-contact IR welding exposes these internal components to high-energy radiation for an extended duration. This exposure could be very problematic and damaging to these internal features and devices. Tools that mask or shield these vulnerable details from the IR energy may be required.
With either method, simulating the heating energy and the exposure duration should be relatively easy to perform and this testing is highly recommended.
Heated Tooling Challenges
Non-contact, IR welding, inherent in its function, completely eliminates the challenges of heated tool wear, plastic sticking to the heated tool, and resin buildup on the heated tool. The absence of contact between the IR emitters and the molded weld ribs removes the possibility that semi-molten plastic material will wear, stick to or build up on the tooling.
Advancements in durable, anti-stick coatings significantly have reduced much of this issue for the HPW process. But even with high-performance materials and coatings, the tool coatings must be considered an item that requires regular service and maintenance.
A noteworthy finding in the effort to eliminate material sticking and buildup on the heated platen has to do with the advent of servo-controlled press platens in HPW. There is an important benefit realized with high acceleration rates as the press platens remove the components from the heated surface during the melt phase.
Cost Comparisons
In addition to the processing differences between the two technologies, there are operating and maintenance cost differences as well.
Energy Use: IR welding machines draw a large amount of power during the melt phase; however the IR emitters are switched off through the rest of the process. As a result, the machine draws less power overall. Contact HPW draws a higher amount of power because the heated platen must be maintained at the programmed temperature throughout production. In addition, platen pre-heating is required at the outset of the production shift.
Cycle Time: The welding process cycle time significantly is longer with IR welding due to the rate that the materials can be consistently heated with radiation and their ability to conduct that heat deep into the molded weld bead. Typical cycle times for IR welding vary between 20 to 60 seconds. On the other hand, contact HPW cycle times can be significantly shorter due to the speed and efficiency of conduction heat transfer. Typical cycle times for contact HPW vary between 8 to 30 seconds.
Equipment & Tooling Cost: As a general comparison, capital equipment costs of the two processes are reasonably comparable.
The tooling cost of non-contact IR welding equipment, depending on the style of emitters used, can be more expensive—possibly as much as 30 to 40 percent more than HPW, but this is highly application dependent, as both size and geometry are factors.
Maintenance Considerations: IR emitters must be replaced every few years in a typical installation and they can be quite expensive, even for standard emitters. Custom emitters cost even more and may have an impractical lead time necessitating the inventory of expensive spares. The cartridge heaters used in contact HPW also require replacement, but they are much more robust and less expensive than IR emitters. The life and durability of the non-stick coatings on the heated tooling of a contact HPW is finite and these heated tools must be stripped and recoated occasionally. In certain installations, the user would be wise in securing backup heated tooling if the tooling is custom or prone to high wear.
A Winner?
The two welding technologies discussed in this article are both viable, robust processes with certain advantages and disadvantages. As with all plastics joining challenges, it is the application’s unique characteristics and requirements that should drive the recommended process. When considering how best to assemble an SUD product requiring plastics joining, consider the factors discussed in here and evaluate specific product requirements and priorities against these factors to make an informed decision.
During the past 30 years, Drew Jelgerhuis has held a variety of engineering and technical leadership roles in the plastics industry. He joined Extol in 2001 as a business unit manager. He has contributed to the growth of Extol Inc. in a technical leadership capacity, including Life Sciences Development since 2009. Jelgerhuis was a key member of the team that developed the company’s infrared joining process called InfraStake and InfraWeld. This patented plastics joining process has been recognized with two international automotive awards. Bill Reed has served the plastics joining business for three decades. From ultrasonic tooling maker to automated-systems concept developer and sales executive, Reed’s extensive industry experience and observations have influenced and refined Extol’s business strategy. Reed currently serves as sales and marketing director for Extol and has been with the company since 2005. Extol’s life-science business provides exceptional plastics joining solutions for applications such as pharmaceutical filtration, medical device, single-use disposable, catheters and tubing. Extol manufactures a standard line of equipment, as well as custom assembly and leak test equipment—along with assembly consultation, application lab testing, prototyping and pre-production services. Products include patented infrared staking and welding, servo-controlled hot-plate and spin-welding machines, and ultrasonics integration.
This article will examine two joining processes for plastics: infrared welding and hot-plate welding. Both recently have benefited from significant process advancements and related growth in popularity. The medical device SUD sector should take note of the advances. Precise control of tooling position, velocity, force and acceleration are improvements that provide a new level of control and process data acquisition. Additionally, with the offering of smaller and more efficient machines, the user has more practical options for consideration.
Non-contact IR welding has been a viable joining method for a while. Its increase in popularity in the United States has been a relatively recent occurrence. The “non-contact” premise of this method addresses a significant challenge of “contact” HPW and that is the prevention of plastic sticking to the heated platen surface and the deposit of plastic material on the heated tooling surface.
The non-contact benefits of IR, however, are negated to some degree with other, less desirable realities. Let’s compare and contrast the HPW and IR processes further.
Plastics Welding Processes: Hot-Plate vs. Infrared
There are three rather generic modes of heat transfer to contemplate when discussing plastics joining: conduction, convection and radiation. The principal heat transfer method for HPW is conduction, while the primary method with non-contact IR welding is radiation.
The hot-plate welding process consists of three key steps: the melt phase, the open phase and the seal phase.
The sequence of operations takes place in this way:
- An operator (or automated machinery) loads the components to be welded into opposing press tooling;
- At cycle start, the machine positions the heated platen between the two press platens;
- The machine positions the press platens sufficient to present the molded weld rib features on the component surfaces to the heated tooling to perform the melt phase;
- The machine retracts the platens removing the components from the heated tooling;
- The machine then retracts the heated platen (open phase); and
- The seal phase occurs where the machine activates the press platens to present the two semi-molten weld rib surfaces on each component in contact with each other to perform the actual bond, the weld.
Hot-Plate Welding, Material Heating Theory
As mentioned above, conduction is the primary method of heat transfer in HPW. Heat conducts from the heated tooling directly to the weld surface (melt rib). From the weld surface, it continues to conduct and “soak” deeper into the rib. An appropriate amount of heat soak into the weld rib is crucial for achieving high-bond strength and a tight, hermetic seal. The goal is to optimize the heat soak time. This will help ensure that when the two heated components are brought together in the seal phase, there is an optimal depth of heat-softened material on the two opposing surfaces.
This combined depth is called the heat-affected zone and it actually can be measured under magnification. If the heat-affected zone is too small, it means that excessive displacement of the semi-molten material has taken place and the likely result is a weak weld with poor sealing performance. If the heat affected zone is too large, the question of insufficient pressure applied during the seal phase is considered with similar poor weld results.
Non-Contact Infrared Welding, Material Heating Theory
IR heating of the weld rib surface is achieved through radiation. The IR source emits the energy which is absorbed at the surface by the components. Conduction, within the plastic resin, now transfers the heat within the weld rib. A critical parameter to be aware of is the rate of heating. Thermoplastic materials melt and re-flow when they are heated. If they are heated too quickly, there is a risk of material degradation, burning, or charring. Many resins absorb the initial IR energy very rapidly, but most of them do not conduct that surface heating very quickly. Yet, just as with HPW, it is still necessary to achieve a sufficient depth of heating into the weld rib to execute a robust welding process.
The rate at which a material absorbs IR radiant heat is characterized by three properties: absorptivity, transmissivity and reflectivity.
These material properties are affected by the material type, pigment/color, fillers, etc. Additionally, the emissivity of the IR emitter (its ability to emit radiation) also contributes to the effectiveness of radiant heat transfer and impacts the rate of heating.
Further yet, the density of the IR light as it reaches the surface that is being heated affects the rate of heating as well. This density is affected by the distance between the emitter and the heated surface and in how the infrared energy is focused. All of this is to say that there are many different factors that affect how quickly a plastic surface will absorb IR energy.
It is necessary to control this rate of heating to avoid some of the issues mentioned earlier. For example, an opaque, black material with a high percentage of glass filler content will absorb radiant energy at a very high rate but it will not conduct that surface heating as aggressively and might be susceptible to overheating on the outer surface. On the other side of the spectrum, a white material with no filler might conduct or transfer the surface heating more efficiently, but it will not absorb the initial IR energy quickly. It would not be at much risk for overheating, but the overall cycle time likely would be longer.
Material Displacement Comparison
Another big difference between HPW and IR welding is the displacement of material during the welding cycle. For this discussion, displacement is defined as any material that is moved from its original location at the melt rib to another location (flash).
In contact HPW there is physical contact between the heated tooling and the molded weld rib on the plastic components. As a result, there is displacement of the molded weld rib. As the press tooling positions the weld rib surfaces into intimate contact with the heated tooling, the material is displaced, which is a good thing. During this process the weld ribs conform to the heated tool surfaces, which are presumed to be highly accurate as machined. This ensures that the weld rib surfaces are formed precisely to design while they are being sufficiently heated.
During the melt cycle, the displacement is occurring on both components simultaneously, creating matched weld rib surfaces. This precise contact and related material displacement is a significant element in the HPW process as it prevents some issues otherwise associated with typical molding inconsistencies. Because the surfaces are uniform with each other, when welding pressure is applied in the seal phase, it evenly is applied along the entire length of the weld rib. Balanced welding pressure is imperative in creating high strength, tightly sealed welds while displacing minimal material.
With non-contact IR welding, there is no physical contact between the weld rib and the IR emitters. Therefore, there is no controlled material displacement to correct molded weld rib inconsistencies. For this reason, it is very important that the dimensions of the two molded components are precisely controlled in molding to achieve “as designed” surfaces.
When contemplating contoured weld rib surfaces with the non-contact IR welding process, the placement and distance of the IR emitters from the weld rib surface becomes particularly critical. To assure even heating and ultimately a robust process with minimal plastic displacement, the tool design must be critically assessed and proven to heat the contoured surfaces evenly. If uneven heating occurs during the melt phase there likely will be an uneven application of pressure through the weld joint during the seal phase which could result in poor weld quality.
Sensitive Internal Components in Welded Assemblies
Product designers cleverly secure sub-assemblies and internal devices within the subject components to be welded. These components are exposed to the welding environment and depending on the sensitivity of these components, this needs careful consideration.
Whether these internal devices are batteries, printed circuit boards, valves, filters, O-rings, or LED lights, one must have the assurance that the energy from the welding process does not cause any damage.
In contact HPW, the exposure duration of the heated platen is quite limited but the proximity to the devices requires evaluation.
Non-contact IR welding exposes these internal components to high-energy radiation for an extended duration. This exposure could be very problematic and damaging to these internal features and devices. Tools that mask or shield these vulnerable details from the IR energy may be required.
With either method, simulating the heating energy and the exposure duration should be relatively easy to perform and this testing is highly recommended.
Heated Tooling Challenges
Non-contact, IR welding, inherent in its function, completely eliminates the challenges of heated tool wear, plastic sticking to the heated tool, and resin buildup on the heated tool. The absence of contact between the IR emitters and the molded weld ribs removes the possibility that semi-molten plastic material will wear, stick to or build up on the tooling.
Advancements in durable, anti-stick coatings significantly have reduced much of this issue for the HPW process. But even with high-performance materials and coatings, the tool coatings must be considered an item that requires regular service and maintenance.
A noteworthy finding in the effort to eliminate material sticking and buildup on the heated platen has to do with the advent of servo-controlled press platens in HPW. There is an important benefit realized with high acceleration rates as the press platens remove the components from the heated surface during the melt phase.
Cost Comparisons
In addition to the processing differences between the two technologies, there are operating and maintenance cost differences as well.
Energy Use: IR welding machines draw a large amount of power during the melt phase; however the IR emitters are switched off through the rest of the process. As a result, the machine draws less power overall. Contact HPW draws a higher amount of power because the heated platen must be maintained at the programmed temperature throughout production. In addition, platen pre-heating is required at the outset of the production shift.
Cycle Time: The welding process cycle time significantly is longer with IR welding due to the rate that the materials can be consistently heated with radiation and their ability to conduct that heat deep into the molded weld bead. Typical cycle times for IR welding vary between 20 to 60 seconds. On the other hand, contact HPW cycle times can be significantly shorter due to the speed and efficiency of conduction heat transfer. Typical cycle times for contact HPW vary between 8 to 30 seconds.
Equipment & Tooling Cost: As a general comparison, capital equipment costs of the two processes are reasonably comparable.
The tooling cost of non-contact IR welding equipment, depending on the style of emitters used, can be more expensive—possibly as much as 30 to 40 percent more than HPW, but this is highly application dependent, as both size and geometry are factors.
Maintenance Considerations: IR emitters must be replaced every few years in a typical installation and they can be quite expensive, even for standard emitters. Custom emitters cost even more and may have an impractical lead time necessitating the inventory of expensive spares. The cartridge heaters used in contact HPW also require replacement, but they are much more robust and less expensive than IR emitters. The life and durability of the non-stick coatings on the heated tooling of a contact HPW is finite and these heated tools must be stripped and recoated occasionally. In certain installations, the user would be wise in securing backup heated tooling if the tooling is custom or prone to high wear.
A Winner?
The two welding technologies discussed in this article are both viable, robust processes with certain advantages and disadvantages. As with all plastics joining challenges, it is the application’s unique characteristics and requirements that should drive the recommended process. When considering how best to assemble an SUD product requiring plastics joining, consider the factors discussed in here and evaluate specific product requirements and priorities against these factors to make an informed decision.
During the past 30 years, Drew Jelgerhuis has held a variety of engineering and technical leadership roles in the plastics industry. He joined Extol in 2001 as a business unit manager. He has contributed to the growth of Extol Inc. in a technical leadership capacity, including Life Sciences Development since 2009. Jelgerhuis was a key member of the team that developed the company’s infrared joining process called InfraStake and InfraWeld. This patented plastics joining process has been recognized with two international automotive awards. Bill Reed has served the plastics joining business for three decades. From ultrasonic tooling maker to automated-systems concept developer and sales executive, Reed’s extensive industry experience and observations have influenced and refined Extol’s business strategy. Reed currently serves as sales and marketing director for Extol and has been with the company since 2005. Extol’s life-science business provides exceptional plastics joining solutions for applications such as pharmaceutical filtration, medical device, single-use disposable, catheters and tubing. Extol manufactures a standard line of equipment, as well as custom assembly and leak test equipment—along with assembly consultation, application lab testing, prototyping and pre-production services. Products include patented infrared staking and welding, servo-controlled hot-plate and spin-welding machines, and ultrasonics integration.