Laser Welding of Plastics

Materials, Processes and Industrial Applications

Rolf Klein

• Publisher: Wiley-VCH
• Year: 2012
• Print ISBN: 9783527409723
• eBook ISBN: 9783527636976
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

This is the first detailed description in English of radiation and polymeric material interaction and the influences of thermal and optical material properties. As such, it provides comprehensive information on material and process characteristics as well as applications regarding plastic laser welding.
The first part of this practical book introduces the structure and physical properties of plastics, before discussing the interaction of material and radiation in the NIR and IR spectral range. This is followed by an overview of the physical foundations of laser radiation and laser sources used for plastic welding. The third part describes the main processes of laser welding thermoplastics, as well as possibilities of process control, design of joint geometry, material compatibilities and adaptation of absorption of plastics to NIR radiation. Finally, the author explains applications of laser welding plastics using several industrial case studies from the automotive industry, household goods, and medical devices.
Tailored to the needs of everyone dealing with laser welding of plastics, especially engineers in packaging, component manufacturing, and the medical industry.

Chapter 1

Material Properties of Plastics

1.1 Formation and Structure

The basic structure of plastics (or polymers) is given by macromolecule chains, formulated from monomer units by chemical reactions. Typical reactions for chain assembling are polyaddition (continuous or step wise) and condensation polymerization (polycondensation) [1] (Figure 1.1).

• Polyaddition as chain reaction: Process by chemical combination of a large number of monomer molecules, in which the monomers will be combined to a chain either by orientation of the double bond or by ring splitting. No byproducts will be separated and no hydrogen atoms will be moved within the chain during the reaction. The process will be started by energy consumption (by light, heat or radiation) or by use of catalysts.
  Figure 1.1 Processes for generating plastics and examples [1].

  
• Polyaddition as step reaction: Process by combination of monomer units without a reaction of double bonds or separation of low molecular compounds. Hydrogen atoms can change position during the process.
• Polycondensation: Generation of plastics by build up of polyfunctional compounds. Typical small molecules like water or ammonia can be set free during the reaction. The reaction can occur as a step reaction.

The monomer units are organic carbon-based molecules. Beside carbon and hydrogen atoms as main components elements like oxygen, nitrogen, sulfur, fluorine or chlorine can be contained in the monomer unit. The type of elements, their proportion and placing in the monomer molecule gives the basis for generating different plastics, as shown in Table 1.1.

Table 1.1 Examples of some common plastics and their monomers.

The coupling between the atoms of a macromolecular chain happens by primary valence bonding [2]. The backbone of the chain is built by carbon atoms linked together by single or double bonding. Given by the electron configuration of carbon atoms, the link between the carbon atoms occurs at a certain angle, for example, for single bonding at an angle of 109.5°. Atoms like hydrogen, which are linked to the carbon atoms, hinder the free rotation of the carbon atoms around the linking axis. The “cis”-link of carbon atoms has the highest bonding energy while the "trans"-link has the lowest (Figure 1.2) [3].

Figure 1.2 Potential energy for rotation of ethylene molecules around the carbon-linking axis [3].

Depending on the type of bonding partners several chain conformations are possible. Examples of such conformations are zig-zag conformation (e.g., PE or PVC) or helix conformation (e.g., PP, POM or PTFE) (Figure 1.3) [2].

Figure 1.3 Conformation types of macromolecules.

The chain length and by this also the molecular weight of macromolecules have a statistical distribution [4] (Figure 1.4). By influencing the conditions of the polymerization process, the average molecular weight and the width of the distribution function can be controlled within certain limits.

Figure 1.4 Statistical distribution of macromolecule chain length using polyvinylchloride (PVC) as an example [4].

During the polymerization process, depending on the type of polymer, side chains can be built to the main chain in a statistical way [5]. As for the length of the main chain, frequency and length of the side chains depend on the macromolecular structure and the physical/chemical conditions of the polymerization process [6].

An example for the order of size of macromolecules is the length and width of polystyrene molecules with an average molecular weight of 10⁵. Corresponding to the molecular weight the macromolecular chain consists of a number of approximately 2 × 10⁵ carbon atoms. The average distance between each carbon atom is 1.26 × 10⁻¹⁰ m. Using this distance and the number of atoms in the chain takes to a length of 25 × 10⁻⁶ m and 4–6 × 10⁻¹⁰ m width for a stretched chain.

The statistical forming of the macromolecular structure of plastics results in the fact that physical properties of plastics, like temperatures of phase changes, can only be given as average values. Unlike materials like metals, phase changes of plastics occur in certain temperature ranges. The width of such temperature ranges is dependent on the homogeneity of the materials structure [6].

The physical and chemical structure of the macromolecule is given by the primary valence bonding forces between the atoms (Figure 1.5) [1]. The secondary valence bonding forces, like dispersion bonding, dipole bonding or hydrogen bridge bonds, have a direct influence to the macroscopic properties of the plastic like mechanical, thermal, optical, electrical or chemical properties.

Figure 1.5 Context of molecular and macroscopic material properties [1].

The secondary valence forces are responsible for the orientation of the macromolecules among themselves [6–8]. During processing of plastics the orientation of molecule segments can result in an orientation of segments of the macromolecular chain. Under suitable conditions, like specific placements of atoms in the monomer structure and by this within the macromolecular chain, a partial crystallization of the plastic is possible. The strength of the secondary valences is directly correlated with the formation of the...

Table of contents

• Cover
• Related Titles
• Title Page
• Copyright
• Introduction
• Chapter 1: Material Properties of Plastics
• Chapter 2: Laser Sources for Plastic Welding
• Chapter 3: Basics of Laser Plastic Welding
• Chapter 4: Process of Laser Plastic Welding
• Chapter 5: Case Studies
• Index