Ultrasonic Plastics Welding: The Influence of Base Materials

Ask the Expert is a resource sponsored by SPE's Decorating & Assembly Division.

by Ken Holt, senior applications engineer, Dukane, IAS
Material properties determine the ability of plastic to convert vibrational energy to heat, which melts parts. Photo courtesy of Dukane.

One of the most common questions in the plastic welding industry is, “Can you weld this type of plastic part?” What follows is typically a discussion regarding the part’s material, final application, requirements and design state. If the plastic is a commonly used thermoplastic and the design is per common industry design practices, the answer is typically yes. We then dive deeper into the exact material properties, as well as the design requirements, of ultrasonic plastic welding before going further.

This article is meant to be a primer for the “weldability” of polymers, and my intention is to provide real-world observations on what is typically seen in the industry and, in layman’s terms, how these materials work. I have worked thousands of applications and have a great deal of successful experience, but some applications have required design changes, and some needed material changes to work well. At the end of this article is a list of materials and a loose ranking as to which materials are good and which are problematic – and may require special design details.

For all the topics we’ll be discussing, the reader is urged to get more factual, preferably published, information on the subject for a deeper understanding. Beware of the “they do it on another continent, so we should be able to do it here” mentality. Get the facts first.

The best text I have come across for all things plastic welding is the “Plastics and Composites Welding Handbook,” published by Hanser and edited by a group of experts in the field (David Grewell, Avraham Benatar and Joon Park). The ISBN number is 1-56990-313-1. I’ve used this book, recommend it strongly and will reference it throughout this article.

Springs in my material?

I’ll assume the reader has a basic understanding of the ultrasonic plastic welding process, but if not, most ultrasonic welding equipment manufacturer’s websites have a section devoted to a description of the process. A very basic and simplified description of this type of welding is that it is the application of high frequency, reciprocating and mechanical vibrations, under force, acting on one plastic part and driving it against the other part. The force and vibrations are being introduced perpendicular to the weld joint or joining line.

These vibrations melt the interface and cause molecules to flow between the two parts. The interface between the parts is the most important design detail, capable of “making or breaking” the success of the welding process. There is much more to it than that, but a complete description is outside the scope and intention of this article. A good article recently was written in this publication by Brian Gourley of Sonics & Materials, Inc. in the July/August 2016 issue: “Five Factors Influencing a Successful Ultrasonic Weld.”

I appreciate simple analogies when explaining technical matters and would like you to consider this as we progress: we have a hammer to swing at a spring. The stiffer the spring the less energy we need to apply to impart energy through the spring and into the material underneath. More energy is transmitted through the stiffer “spring.” A stiffer spring – higher spring constant (k) – would be analogous to a material with a high stiffness or storage modulus, which is able to transfer the vibrations with little attenuation of the vibrations. That is to say, the vibrations introduced on one end go through the plastic part with little attenuation. Polycarbonate is a good example of such a stiff material. Conversely, a spring with a lower k will attenuate/absorb much more of the vibration and transfer less through the part. Polypropylene is a good example of this. Think of this as we progress, and you’ll soon understand why styrene is much easier to weld than polypropylene.

Can it melt and flow?

The chemical formulas for polystyrene and polypropylene are shown.

Another factor that influences our discussion is the ability of the plastic to convert this vibrational energy to heat, which is what we really need to melt the parts. This is related to a property of materials called loss modulus. The higher this number, the more heat that can be produced at the weld area. Alas, the loss modulus seldom is listed on material data sheets. But we know which materials work well, as we’ll discuss.

It’s somewhat more difficult to find information about the final properties to be considered: how well a material will “wet out” and flow, and how the molecules of the two parts to be welded intermix or entangle themselves. This is somewhat related to the melt flow property. This behavior is what produces the ultrasonic weld’s strength.

Material considerations

So, we reach the purpose of this article: to communicate what we in the industry see as commonly used materials, what works well and what to avoid.

Plastics come in two basic types: thermosets and thermoplastics. Ultrasonics cannot process thermoset materials because they do not melt and reflow when heated. So, check those off the list.

Thermoplastics, on the other hand, can be melted and reflowed/formed; hence, the standard injection molding, extrusion, thermoforming and similar processing techniques. Most thermoplastics can be welded using ultrasonic welding if certain critical design criteria are met. Weld joint designs are readily available from industry resources or books specific to ultrasonic welding designs. The reader is urged to consult on what would work well for the particular application.

Thermoplastics themselves come in two types – either amorphous or semicrystalline – and these categories pertain to the molecular structure within. Is it a random structure (amorphous), or is it more orderly (semicrystalline)? Both families come in a wide variety of types and grades that are tailored to the specific requirements of the end use. For example, a PP (polypropylene) can come in grades made for extrusion, injection molding or a number of other processes, as well as grades made for a specific end use. Each has its own unique properties, and these properties can be changed and/or increased with the use of additives. Generally speaking, amorphous polymers are easier to weld than semicrystalline polymers, due to their stiffness.

What’s inside?

Most additives are fine for ultrasonic welding if they are used properly and are distributed through the parts in a homogenous matrix. High concentrations of any additives are problematic. Some additives, (such as flow enhancers) and lubricants can indeed be problematic if they are not properly distributed or at a proper concentration: They affect the heating and flow of the materials in welding.

Colorants, while they do change the required weld parameters, are not overly problematic. Slight changes in welding parameters are typically all that is required when a color change is made.

Reinforcements, such as glass or talc, can increase the stiffness of semicrystalline materials to an extent that they become more weldable. Care must be taken with glass, and molding parameters should ensure a resin-rich surface. Glass, talc and minerals cannot be melted with ultrasonic welding. Typically, a hotter mold surface will cause the surface of the part to be resin rich. Again, many articles have been written regarding this subject, and the reader is urged to do further research.

The “listing”

I have compiled a short list of the most common polymers welded and their relative ease of welding. These are seen in the industry regularly. A huge number of types of plastics is on the market, but these can be distilled into a somewhat manageable number below by looking at the “basic” types.

Amorphous Materials

  1. Acrylic (pmma)
  2. Styrene
  3. ABS (acrylonitrile butadiene styrene – a mixture of SAN and rubber)
  4. SAN/NAS/ASA (various mixtures styrene, nitrile and acrylic)
  5. PVC (polyvinyl chloride)
  6. PC (polycarbonate)
  7. PSU (various polysulfones)
  8. PEI (polyetherimide)

General notes:

  • Stiff, with a higher storage modulus.
  • Large softening range and range of Tg (glass transition temperatures) and have larger processing windows.
  • Engineering grades hold tight tolerances to avoid dimensional changes.
  • The easier flow (higher MFI) gives better flow; i.e., more wetting occurs.
  • More freedom of design from a geometrical standpoint, far field /near field.

Semicrystalline Materials

  1. PA (nylons) – Many types, 6/6 easiest, frequently GR for automotive under hood.
  2. PP (polypropylene)
  3. HDPE (high density polyethylene)
  4. PBT/PET (various types of polyesters
  5. PEEK (polyetheretherketone)
  6. PPS (Polyphenylene sulfide)
  7. Fluoropolymers/ UHMW HDPE (ultra-high molecular weight polyethylene) – very difficult, if not impossible, to weld as they take so long to flow and dissipate that the surface becomes damaged. Most crosslinked polymers cannot be welded well.

General notes:

  • Less stiff, lower storage modulus.
  • Commonly reinforced to stiffen and hold better tolerances.
  • Can’t typically hold as tight a tolerance without such reinforcements.
  • Higher melt temperatures, and specific melt temperatures rather than Tg, necessitate a tighter parameter set.
  • Typically require more amplitude than amorphous due to the lower storage modulus.
  • Near field only – horn must be directly over joint and less than 6mm vertical distance.
  • Some flow very fast when melted necessitating higher weld velocities.
  • Shear joints are more applicable.

Finally, the reader is cautioned about mixing materials, as compatibility between various types may not be possible. Again, consult industry information.

I wanted to give the uninitiated a brief, but comprehensive, view of the most common polymers used in industry, why and how they weld, and some insight into why a certain material is easy or difficult to weld. Readers are urged to get much more information than this to make any decisions on a specific project, but we are eager to make you successful.

Ken Holt has been involved in the plastics industry for more than 30 years, 24 of which have been focused on the welding of plastics using ultrasonic welding technology. He has worked for Dukane in many aspects of the field of joining, including the development of applications using this technology, plastic part design to optimize welding, ultrasonic tooling design, process development and education about the process. He is a senior member of the Society of Plastics Engineers, is a board member of the Decorating and Assembly Division and has presented papers regarding ultrasonic welding technology at many of that society’s technical meetings. Currently, Holt is a senior applications engineer at Dukane, IAS, focused on both servo welding technology and the support of Dukane’s automotive customer base. For more information, visit www.dukane.com/us.