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HANDBOOK OF PLASTICS JOINING
A Practical Guide
By Michael J. Troughton Elsevier
Copyright © 2008 William Andrew Inc.
All rights reserved.
ISBN: 978-0-8155-1976-8
CHAPTER 1
Heated Tool Welding
1.1 Process Description
Heated tool welding, also known as hot plate, mirror, platen, butt or butt fusion welding, is a widely used technique for joining injection molded components or extruded profiles.
The process uses a heated metal plate, known as the hot tool, hot plate, or heating platen, to heat and melt the interface surfaces of the thermoplastic parts. Once the interfaces are sufficiently melted or softened, the hot plate is removed and the components are brought together under pressure to form the weld. An axial load is applied to the components during both the heating and the joining/cooling phases of the welding process.
Welding can be performed in either of two ways: welding by pressure or welding by distance. Both processes consist of four phases, shown in the pressure versus time diagram in Fig. 1.1.
In welding by pressure, the parts are brought into contact with the hot tool in Phase I, and a relatively high pressure is used to ensure complete matching of the part and tool surfaces. Heat is transferred from the hot tool to the parts by conduction, resulting in a local temperature increase over time. When the melting temperature of the plastic is reached, molten material begins to flow. This melting removes surface imperfections, warps, and sinks at the joint interface and produces a smooth edge. Some of the molten material is squeezed out from the joint surface due to the applied pressure. In Phase II, the melt pressure is reduced, allowing further heat to soak into the material and the molten layer to thicken; the rate at which the thickness increases is determined by the heat conduction through the molten layer. Thickness increases with heating time—the time that the part is in contact with the hot tool.
When a sufficient melt thickness has been achieved, the part and hot tool are separated. This is Phase III, the changeover phase, in which the pressure and surface temperature drop as the tool is removed. The duration of this phase should be as short as possible (ideally, less than 3 seconds) to prevent premature cooling of the molten material. A thin, solid "skin" may form on the joint interface if the changeover time is too long, affecting the weld quality.
In Phase IV, the parts are joined under pressure, causing the molten material to flow outward laterally while cooling and solidifying. Intermolecular diffusion during this phase creates polymeric chain entanglements that determine joint strength. Because the final molecular structure and any residual stresses are formed during cooling, it is important to maintain pressure throughout the cooling phase in order to prevent warping. Joint microstructure, which affects the chemical resistance and mechanical properties of the joint, develops during this phase.
Welding by pressure requires equipment in which the applied pressure can be accurately controlled. A drawback of this technique is that the final part dimensions cannot be controlled directly; variations in the melt thickness and sensitivity of the melt viscosities of thermoplastics to small temperature changes can result in unacceptable variations in part dimensions.
In welding by distance, also called displacement controlled welding, the process described earlier is modified by using rigid mechanical stops to control the welding process and the part dimensions. Figure 1.2 shows the process steps.
In Step 1, the parts are aligned in holding fixtures; tooling and melt stops are set at specified distances on the holding fixture and hot tool (heating platen), respectively. The hot tool is inserted between the parts in Step 2, and the parts are pressed against it in Step 3. Phase I, as described for welding by pressure, then takes place. The material melts and flows out of the joint interface, decreasing part length until, in this case, the melt stops meet the tooling stops. Melt thickness then increases (Phase II) until the hot plate is removed in Step 4, the changeover phase (Phase III). The parts are then pressed together in Step 5 (Phase IV), forming a weld as the plastic cools; tooling stops inhibit melt flow. The welded part is then removed in Step 6.
1.2 Advantages and Disadvantages
Heated tool welding is a simple economical technique in which high strength, hermetic welds can be achieved with both large and small parts. Joints with flat, curved, or complex geometries can be welded, and surface irregularities can be smoothed out during the heating phases. Dissimilar materials that are compatible but have different melting temperatures can be welded using hot tools at different temperatures. The welding process can be easily automated with full monitoring of the processing parameters. Since the process does not introduce any foreign materials into the joint, defective welded parts can be easily recycled.
The major disadvantage of the process is the long cycle time compared with other common techniques such as vibration or ultrasonic welding. Welding times range from 10–20 seconds for small parts, and up to 30 minutes for large pipes. For the welding of smaller parts, production efficiency can be improved by the use of multiple-cavity tools, allowing simultaneous welding of two or more components.
A second disadvantage is the high temperatures required for melting. Heat is not as localized as in vibration welding, and in some cases can cause plastic degradation or sticking to the hot plate. When the molten surfaces are pressed against each other, weld flash is produced. For certain applications, this must be hidden or removed for cosmetic reasons. In welding by pressure, part dimensions cannot always be controlled reliably due to variations in the molten film thickness and sensitivity of the melt viscosities of thermoplastics to small temperature changes.
1.3 Applications
Hot tool welding can be used to join parts as small as a few centimeters to parts as large as 1600 mm (63 inches) in diameter, such plastics pipes (Section 1.9.2). It can also be used for the continuous welding of lining membranes (Section 1.9.3).
The heated tool welding method is widely used in the automotive sector, where one of the most common applications is the welding of vehicle tail lights and indicators ( Fig. 1.3 ). The housing, usually made of acrylonitrile-butadiene-styrene (ABS) is welded to the colored lens which is made from either polymethylmethacrylate (PMMA) or polycarbonate (PC). These represent one of the few material combinations that are compatible for heated tool welding. ABS to PMMA lights can be welded using a single hot plate, because their melting points are similar. Dual hot plates are necessary for ABS to PC. Vacuum fixtures with suction cups are employed to limit any scuffing to the lens.
Custom-built heated tool welding machines are used in the manufacture of blow-molded high density polyethylene (HDPE) fuel tanks. These can require as many as 34 parts to be welded onto the tank, such as clips, filler necks, vent lines, and brackets.
Other automotive components welded by heated tool include battery casings, carburetor floats, coolant and screen wash reservoirs, and ventilation ducts.
Domestic appliance components welded by heated tool include dish washer spray-arms, soap powder boxes ( Fig. 1.4 ), and steam iron reservoirs.
Miscellaneous items welded by the process include lids on HDPE barrels, sharps boxes for medical needle disposal, polypropylene (PP) transport pallets, and the corners of polyvinyl chloride (PVC) window frames.
1.4 Materials
Heated tool welding is suitable for almost any thermoplastic, but is most often used for softer, semicrystalline thermoplastics such as PP and PE. It is usually not suitable for nylon or high molecular weight materials. The temperature of the molten film can be controlled by regulating the hot tool temperature so that plastics that undergo degradation at temperatures only slightly above the melting temperature can be welded.
The properties of the plastics to be welded affect the strength of the weld, including melt viscosity and density. Lower melt index polymers produce higher melt viscosities and can tolerate higher heating temperatures without melt sticking to the hot tool. As a result, the size of the heat affected zone (HAZ)—the area of the part affected by heat—can be larger, resulting in a higher strength joint. For a constant melt index, increasing polymer density results in joints with lower tensile strength. Higher density polymers have a greater proportion of crystalline regions, which melt in a narrower temperature range than polymers of lower crystallinity. As a result, a thinner HAZ and more brittle welds are obtained.
In hygroscopic materials such as PC and nylon, absorbed water may boil during welding, trapping steam and lowering the weld strength. High weld strengths can be obtained by pre-drying materials; alternatively, processing parameters can be adjusted to compensate for absorbed water.
Dissimilar materials having different melting temperatures can be welded by heated tool welding, provided they are chemically compatible; instead of a single plate with two exposed surfaces, two plates are used, each heated to the melting temperature of the part to be welded. Different melt and tooling displacements and different heating times for each part may be necessary, and due to different melt temperatures and viscosities, the displacement of each part will be different. High strength welds equal to the strength of the weaker material can be achieved.
1.5 Weld Microstructure
Weld quality is determined by the microstructure of the HAZ of the weld. The HAZ consists of three zones in addition to the weld flash. The stressless recrystallization zone consists of crystals with a spherulitic shape, indicating that crystallization occurred under no significant stress. This zone results primarily from reheating and recrystallization of the skin layer and the molten layer near the joint interface. The columnar zone consists of elongated crystals oriented in the flow direction; lower temperatures in this zone lead to an increase in melt viscosity, and crystals formed during melt flow aligned with the flow direction. In the slightly deformed zone, deformed spherulites are present, resulting from recrystallization under the joining pressure. Higher heating temperatures result in larger HAZs and greater bond strength; however, too high a temperature or pressure results in void formation at the joint interface.
1.6 Equipment
Depending on the components to be welded, heated tool welding machines can be standard models or specialized, custom units. Standard machines have the capability of welding different components by means of interchangeable hot plates and tooling fixtures. These tend to be more labor-intensive, requiring manual loading and unloading of the components. Custom machines are usually dedicated to one particular component and may form part of a high volume, integrated production line. These will often feature a high degree of automation including conveyor feeding and component removal devices, typically with robotic assistance.
The key components of a heated tool welding machine are the hot plate assembly with two exposed surfaces, fixtures for holding the parts to be welded, and the actuation system for bringing the parts in contact with the hot plate and forming the weld. Dual platen hot tool welding machines are used for welding dissimilar materials.
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Excerpted from HANDBOOK OF PLASTICS JOINING by Michael J. Troughton. Copyright © 2008 William Andrew Inc.. Excerpted by permission of Elsevier.
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