What is Plastic Welding?
Plastic welding is the process of creating a molecular bond between two compatible thermoplastics. Welding offers superior strength, and often drastically reduced cycle times, to mechanical joining (snap fits, screws) and chemical bonding (adhesives). There are three main steps to any weld: pressing, heating, and cooling. The application of pressure, which is often used throughout both the heating and cooling stages, is used to keep the parts in the proper orientation and to improve melt flow across the interface. The purpose of the heating stage is to allow intermolecular diffusion from one part to the other across the faying surface (melt mixing). Cooling is necessary to solidify the newly formed bond; the execution of this stage can have a significant effect on weld strength.
There are several possible methods of plastic welding: Ultrasonics, Vibration, Spin, Hot Plate, Laser / Infrared, Radio Frequency, and Implant are the most common. These plastic welding processes are primarily differentiated by their heating methods. The application of pressure and allowances for cooling are mechanical considerations may vary from machine to machine within the general process category.
The use of pressure during the weld serves multiple purposes:
- Flattens surface asperities to increase part contact at joint
- Maintains orientation of part
- Compresses melt layer to encourage intermolecular diffusion between the two parts
- Prevents formation of voids from part shrinkage during cooling
Historically, pressure has been applied for plastic welding through the use of pneumatic presses. Recently, however, servo motors have been employed for at least a few of the common processes. Pneumatic welders are economical and well-suited to most simple applications. The precision of servo motion, however, offers greater control and precision which is desirable for more difficult applications or when the equipment is used for a wide variety of applications.
It is crucial to the plastic welding process to form a melt layer at the
faying surface to allow intermolecular diffusion for formation of a
molecular bond. In the solid state, polymer chains will not flow.
Therefore, the joint surface on both of the parts must be melted to
allow the plastic molecules to diffuse across the interface and bond
with molecules of the other part. The hotter the melt is, the more
molecular movement is achieved, and a weld can be made in a shorter
cycle time. Amorphous polymers must be heated to above their glass
transition temperature while semi-crystalline polymers must be
heated to above their melting temperature.
In all types of plastic welding, only a thin layer of the parts are melted near the joint. It would be impractical to heat the entire part for several reasons:
- Heating only a small area takes less time, and reduces cooling time
- Limiting the melt also reduces the heat affected zone
- Maintains the molded microstructure of the bulk of the part
- Prevents excess shrinkage or warpage during cooling
- Allows rigid support of the part during welding
This process works through the translation of AC current into mechanical vibrations using piezo-electric ceramics. The vibrations are then passed through the part to the joint at which point they cause stress and strain in the contact area between the two parts which leads to localized heating and melting of the polymer. Ultrasonic welding typically has weld times from one-tenth to two seconds and Dukane's ultrasonic welding equipment can handle parts from approximately one-eighth inch diameter at high frequencies up to twelve inches square at low frequencies.
This process works through either linear or orbital motion of one part relative to the other. This results in surface friction that leads to heat generation and the formation of a melt layer at the joint. Vibration welding typically has weld times from one to five seconds and Dukane's vibration welders can handle parts from approximately two inch diameter up to five feet long.
This process works through rotational motion of one part relative to the other. This results in surface friction that leads to heat generation and the formation of a melt layer at the joint. Spin welding typically has weld times from one-half to five seconds and Dukane's spin welding equipment can handle parts from approximately one-half inch diameter using high speed motors up to nine inches diameter using high torque motors.
In this process, the facing surfaces of the two parts are heated through conduction, convection and/or radiation from an actual heated platen. The two parts are either pressed against the hot plate, or held next to it for some period of time, then the hot plate is removed and the parts are pressed together to form the weld. Hot welding typically has weld times from ten to twenty seconds and Dukane's hot plate welding equipment can handle parts from approximately one inch diameter up to five feet long.
There are two means of using laser / infrared energy to create a weld. One way is to heat the joining surface of both parts to create a melt layer, then press the parts together to form a weld. The second way is to use one part that is transparent to the laser / infrared wave length, and another that absorbs it. This way, the laser / infrared beam can be passed through the transparent, or transmitting, part to the absorbing part which will then heat. The molten polymer of the absorbing part then heats the surface of the transmitting part. Laser / infrared welding typically has weld times from three to five seconds and Dukane's laser welding equipment can handle parts from approximately ten-thousandths inches thick to one-quarter inches thick.
Heat Staking Thermal Press
In thermal heat staking, light pressure and localized heat are used to control the molten flow of plastic material to capture and/or retain another component to provide a strong mechanical bond between the two components. A thermal tool at a user defined temperature contacts the plastic boss or post and heat is thermally transferred from the tool to the plastic. Once the plastic material has reached its melting point, force is introduced to the process to reform the plastic typically to the shape of the custom designed thermal tooling cavity.
This type of welding only works with thermoplastics that have a high dielectric constant. In this process, an electric field is generated near the joint, the direction of which is alternated approximately twenty-seven million times a second. Every time the electric field changes, the dipole molecules of the polymer attempt to flip around to realign themselves to the new field orientation. Because this movement is impaired by inertia and friction, the molecules do not flip in unison. This constant relative movement of the molecules results in intermolecular friction which results in heat and, subsequently, melting. Radio Frequency welding typically has weld cycle times from two to five seconds and can handle parts from approximately one-thousandths inches thick to fifty-thousandths inches thick.
This type of welding relies on heat generated by a foreign substance implanted into the weld joint. There are two methods of generating this heat, resistance and induction. In resistance implant welding electrical current is passed through a conductive implant (typically a wire) that is located in the weld joint. The implant then heats due to electric resistance, which causes heating and melting of the surrounding polymer. In induction implant welding a gasket (typically made of the same polymer to be welded filled with conductive or ferromagnetic material) is placed at the joint. The parts are then placed in an electromagnetic field and the gasket is heated though induction. As the polymer in the gasket heats and melts, the plastic of the parts nearby is heated through conduction and convection, creating a weld between the parts and the gasket. Implant welding typically has weld cycle times from one-half to one minute and can handle parts from approximately one inch diameter up to eight feet long.
During the cooling stage, the bonded polymer hardens into one solid part, completing the weld. For semi-crystalline materials, the cooling stage, generally called the "hold" phase, provides the opportunity for the polymer to re-crystallize. The rate of cooling will affect the final microstructure. For amorphous polymers, the cooling stage solidifies the microstructure into the orientation created by the melt flow. The pressure applied during this stage, the time allotted for it prior to putting the part under stress, and the rate of cooling all have significant effects on the final weld strength.
A wide variety of plastic parts can be welded, some are listed below:
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