Sofia Martinez
Joining plastic moulds is an important step in creating a finished product, and there are a variety of ways to do so. The process of joining plastic moulds can be done by an individual for a fun, inexpensive way to create unique pieces or replicas, or by manufacturers to create complex projects. The method chosen depends on the product's assembly requirements, whether it needs to be disassembled, and the type of plastic being used. Common methods include mechanical fasteners, ultrasonic welding, spin welding, and bonding.
| Characteristics | Values |
|---|---|
| Joining techniques | Mechanical fasteners (pins, screws, nuts, rivets), ultrasonic welding, spin welding, solvent bonding, over-molding, two-color injection molding, secondary injection molding, nano-injection molding |
| Factors influencing technique choice | Required strength, need for disassembly, product structure, aesthetics, electronics insulation, chemical resistance, cost, volume of production, type of plastic |
| Mold preparation | Cleaning, application of release agent, coating with bubble buster, use of clay bed and alignment keys |
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What You'll Learn
Solvent bonding
The choice of solvent is critical in solvent bonding. The appropriate solvent depends on the solubility of the chosen thermoplastic and the processing temperature. Polymers are generally more soluble in solvents with similar solubility parameters, and increasing the temperature promotes dissolution and interdiffusion bonding. Therefore, it is essential to consider the specific plastic being used and select a compatible solvent.
There are several methods for applying the solvent in solvent bonding:
- Brush-on method: The solvent is brushed onto the surfaces to be joined, followed by the application of pressure until the bond reaches full strength after evaporation.
- Capillary action method: Commonly used with acrylic components, this method utilizes a narrow gap between the parts, allowing the solvent to flow via capillary action. A hypodermic needle ensures precise application.
- Dip-dab method: One surface is dipped into a vat of solvent for a controlled amount of time. Excess solvent is removed using a screen mesh or foam pad before pairing the bonding surfaces.
- Solvent dispenser method: A dispenser is used to precisely control the amount of solvent applied to each surface.
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Mechanical fasteners
When selecting a mechanical fastener, it is important to consider the type of plastic being used and the specific requirements of the application. For example, conventional machine screws are rarely used except with extremely strong plastic. Threaded fasteners work best on thick sections of plastic, while push-on locknuts and clips may be more suitable for thinner sections. Thread-forming screws are recommended for softer plastics, while thread-cutting screws are ideal for harder plastics. If the fastener will need to be removed frequently, metal inserts are often the best option.
One common type of mechanical fastener is the snap fit, which is a low-cost and efficient method for joining plastic parts. Snap fits are moulded features that fasten when two or more components are pressed together. They are commonly used in injection-moulded parts, such as cell phone cases and battery covers. Another variation is the annular snap fit, which does not need to be a complete circle and can be designed according to specific requirements. For example, a bottle cap may have four evenly divided inner snap fits to allow for ease of assembly and a good hand feel.
Other types of mechanical fasteners include interference fits, which involve forcing one part to deform elastically, such as press-fitting a dowel pin into a hole that is slightly smaller. Additionally, ultrasonic assembly techniques, such as welding, staking, spot welding, and ultrasonic inserts, use high-frequency mechanical vibrations to melt mating surfaces and create permanent joints.
When using mechanical fasteners with plastics, it is important to consider the fastener's head design. Countersunk heads should be avoided as they can produce undesirable tensile stress in mating parts. Instead, bolt or screw heads with a flat underside, known as pan or cheese heads, are preferred as they produce less harmful compressive stress in plastics. Flat washers should also be used under both the nut and fastener heads to distribute the assembly force over a larger area.
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Ultrasonic welding
In ultrasonic plastic welding, it's important to focus the energy in as small a region as possible. This means incorporating an "energy director" into your sonic welding plastic design. An energy director is a raised region with a triangular cross-section. The triangle tip is pressed against the mating plastic part and ultrasonic energy is applied. With friction occurring over a very small area, the local temperature rises quickly, causing the plastic components to join together.
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Spin welding
The spin welding process involves two main phases of heat generation. In the initial solid phase, the bulk of the heating occurs due to the friction between the two solid parts rubbing against each other. The second phase is the steady-state phase, where a constant film of molten plastic forms at the interface, and viscous heating becomes dominant.
Inertia welding machines use a motor to spin the parts to a set RPM, and then disengage the motor, relying on friction to slow down the machine. The inertial energy is transferred to the weld interface through the parts. Continuous drive machines, on the other hand, use a motor to spin the part at a user-determined RPM throughout the entire welding cycle. The rotation is then stopped via a mechanical braking system.
The overall spin welding process involves loading parts into a holding fixture, activating drive motors to spin the parts, and then engaging the parts to be welded. The welding step involves generating enough heat from friction to melt the parts at their interface, forming a strong bond. Once a steady state is reached, the spinning is stopped, and the parts are held in contact under pressure while the joint cools to ensure a solid mating.
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Two-colour injection moulding
There are two common methods for two-colour injection moulding. The first method involves using two independent injection moulds, each with its plastic colour. The second method uses a single mould with two injection points, each connecting to a different coloured plastic. In some cases, it is possible to inject one colour into a single cavity and then inject another colour into the same cavity to create whirlpool effects or other interesting patterns. This process requires careful planning and execution, considering the design of the moulded parts, the characteristics of the chosen plastic, and the injection moulding process.
To achieve the desired results, several factors must be considered when choosing the colours for double shot injection moulding. Firstly, consider the final look of the product and choose a colour that complements it. Secondly, take into account the production process and select colours that work well with the chosen materials and manufacturing techniques. Finally, think about the end use of the product and choose a colour that is durable and easy to clean.
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Frequently asked questions
It depends on the specific requirements of your project, including the desired strength of the bond, the need for disassembly, and the volume of production. Common methods include mechanical fasteners, ultrasonic welding, and solvent bonding.
Solvent bonding involves using a solvent to soften two plastic parts, which are then pressed together. As the solvent evaporates, the plastic molecules mix and form a strong bond.
Ultrasonic welding is a process that uses vibrational energy and pressure to generate heat, melting and joining the plastic parts without the need for solvents or glue. It is useful for joining plastics that are difficult to bond otherwise.
Yes, mechanical fasteners are a common method for joining plastic molds. However, the plastic must be able to handle the strain and stress of fastener insertion. This method is ideal for low- to medium-volume production and allows for future disassembly.
Circular plastic parts require a specific welding procedure called spin welding. This technique uses surface friction to create a circular welding joint by rotating the parts at high speeds and applying pressure to help the materials bond.
