Plastic welding is the process of creating a molecular bond between two or more compatible thermoplastics.
Welding and other fabrication techniques are used when a complex shape cannot be made by molding alone. Common applications of plastic welding include packaging and consumer products with intricate curves and cavities.
Plastic Molded Concepts provides advanced welding services for high performance applications. In some cases, prototypes and low volume production is better served by plastic welding than a more complicated mold.
PMC offers a number of welding techniques to fulfill your specifications, including:
- Application-based Welding
- Contact (Spot) Welding
- Hot Plate Welding
- High Frequency Plastic Welding
- Ultrasonic Plastic Welding
- Vibration or Friction Welding
- Plastic Spin Welding
- Laser Welding
- Solvent Welding
Plastic Molded Concepts offers several different welding techniques to address the specific needs of our customers. In some cases, a part needs to be welded for manufacturability. Our molders make suggestions to your design process to help you select the best combination of molding and welding. We also recognize that the bonded part needs to withstand the effects of stress and other environmental exposures. We can provide guidance on the physical and chemical characteristics of the weld.
Contact, or spot welding is used in bonding sheets of thermoplastic material. Two plastic pieces are brought together by a metal pincher. The heated tips of the pincher melt the plastic, joining the parts in the process. The process is similar to metal spot welding, except that the heat is supplied by the heated metal rather than by electrical conduction.
Hot plate welding is used to weld larger parts, or parts that have a complex weld joint geometry. The parts to be welded are placed in the tooling attached to the two opposing platens of a press. A hot plate, with a shape that matches the weld joint geometry of the parts to be welded, is moved in position between the two parts. The two opposing platens move the parts into contact with the hot plate until the heat softens the interfaces to the melting point of the plastic. When this condition is achieved the hot plate is removed, and the parts are pressed together and held until the weld joint cools and resolidifies to create a permanent bond.
Certain plastics with chemical dipoles, such as polyvinyl chloride (PVC), polyamides (PA) and acetates can be heated with high frequency electromagnetic waves. High frequency plastic welding uses this property to soften the plastics for joining. The heating can be localized, and the process can be continuous. This process is also known as dielectric sealing or radio frequency (RF) heat sealing. This is the same concept as induction plastic welding.
Ultrasonic plastic welding uses high frequency (15 kHz to 40 kHz) low amplitude vibration to create friction between to pieces of plastic, melting them together. This type of welding creates a strong, uniform bond. When this welding is specified for an assembly, it is important that the parts are designed to concentrate energy at the joint. We do this by considering the mass of the resin, the thickness of the part and the rigidity of the individual components. This helps achieve maximum weld strength.
In vibration or friction plastic welding, the two parts to be assembled are rubbed together at a lower frequency (typically 100-300 Hz) and higher amplitude (typically t-2 mm) than ultrasonic welding. The friction caused by the vibration motion combined with the clamping pressure between the two plastic parts creates the heat to melt the contact interface between the two parts, and results in a strong welded bond. At the completion of the vibration motion, the parts remain held together until the weld joint cools and the melted plastic resolidifies. The friction movement can be linear or orbital, and the joint design of the two parts has to allow this movement.
Plastic spin welding a specific type of frictional welding. With this process, one part is held stationary, while the other one is rotated at high velocity. The rotating part is then pressed against the fixed part with significant force. The friction heats the components at the joint to achieve the weld. Plastic spin welding works when at least one piece is symmetric about an axis.
Laser welding uses a laser beam to create heat in a precise point or line between the parts to be welded. This welding technique requires one part to be transmissive to a laser beam. The other part must be either absorptive to the beam or coated at the interface to be made absorptive. The two plastic parts are put under pressure while the laser beam passes through the first part and is absorbed by the other one or its coating. This generates enough heat to soften the interface creating a permanent weld.
Semiconductor diode lasers are typically used in plastic welding. Wavelengths in the range of 808nm to 980nm can be used in the welding of various plastic material combinations. Power levels from less than 1W to 100W are needed depending on the materials, thickness and desired process speed.
Diode laser systems have the following advantages when welding plastic materials:
- Cleaner than adhesive plastic bonding
- No micro-nozzles to get clogged
- No liquid or fumes to affect surface finish
- No consumables
- Higher throughput
- Can access work-piece in challenging geometry
- High level of process control
Requirements for high strength laser welded plastic joints include:
- Adequate transmission through upper layer
- Absorption by lower layer
- Material compatibility – wetting
- Good joint design – clamping pressure, joint area
- Lower power density
Plastic materials that can be joined by laser welding include:
Specific laser plastic welding applications include sealing /welding /joining of catheter bags, medical containers, automobile remote control keys, heart pacemaker casings, syringe tamper evident joints, headlight or tail-light assemblies, pump housings, and cellular phone parts.
In solvent plastic welding, a solvent is applied which can temporarily dissolve the polymer at room temperature. When this occurs, the polymer chains are free to move in the liquid and can entangle with other similarly dissolved chains in the other component. Given sufficient time, the solvent will permeate through the polymer and out into the environment so that the chains lose their mobility. This leaves a solid mass of entangled polymer chains which constitutes a solvent weld.