(Updated May 2019)
There are several methods for joining custom injection molded plastic components to produce a bigger and more complex part but each one has equipment, cost and labor to consider.
The joining method chosen depends on the strength required of the final product and whether the component will need to be disassembled at any point. The simplest way to join plastic parts is to design a fastening element such as a hinge or latch into the part. Stronger plastics are required for these because the joint must survive the repeated use, load and strain of assembly.
When working with custom injection molded components, mechanical fasteners like screws, rivets, pins, or nuts tend to be the most common joining methods and are a great option for products that may need to be disassembled for servicing or replacement. If the fasteners need to be removed a number of times for disassembly because of servicing or replacement, metal inserts are recommended. Mechanical fasteners are a good choice for low to medium volumes where the initial capital cost far outweighs the consumables expense.
The fasteners can be molded in place, forced, glued or expanded into holes. They can also be inserted ultrasonically or with heated probes. Mechanical fasteners require that the plastic use can withstand the strain of fastener insertion along with the high stress around the fastener.
Threaded fasteners work best on parts with thicker sections. Thread-forming screws are preferred for softer materials and thread-cutting screws work best on harder plastics. Push on lock nuts or clips may be suited better for thinner section parts.
For more information on how to design threaded fastners in your custom part, register to receive Injection Molded Part Design Guidelines.
Solvent and Adhesive Bonding
There are two common types of bonding: solvent and adhesives. In adhesives, two parts are bound together via a chemical that is applied and attaches to the surface of both parts. In solvent bonding, plastics are softened by coating them with a solvent, then clamping them or pressing them together. The plastic molecules mix together and the parts bond when the solvent evaporates; this process is limited to thermoplastics.
Bonding techniques are used when a permanent bond is desired. The amount of pressure used is critical, as too much pressure causes parts to distort. A day or more at room temperature or several hours at elevated temperatures are also sometimes needed to help cure the bond. The drying and curing steps required for bonding results in a longer cycle time.
Bonding methods require either solvent or adhesive consumables, which can become costly to supply over time; however, the initial upfront investment for bonding is low (compared to other techniques such as welding). Bonding requires on-going maintenance of the applicators and associated equipment. This technique provides flexibility in that this process works well for a wide range of materials and shapes; additionally, if the design or size of the parts being bonded is modified or the component includes optional parts, the adhesive process remains unchanged and is easily adapted.
This is a process that utilizes ultraviolet curing with high-intensity ultraviolet light to instantly cure or dry inks, coatings or adhesives. UV bonding works well to bond plastic to non-plastic materials such as glass and metal. Offering many advantages like increased production speed, reduced rejection rates, and improved scratch and solvent resistance, along with the facilitation of superior bonding. This method is limited to clear materials.
Ultrasonic Welding is the process where sonic pulses are transmitted to the plastic part by a resonant vibrating tool called a horn, which causes two plastic materials to vibrate against each other. Vibration both heats and fuses the part together. No glues or solvents are required. When the horn vibrates, it stretches and shrinks in length by a small amount. This motion is referred to as the “amplitude” of the horn. The shape and size of the parts to be welded typically dictates the horn face.
Custom plastic injection molded components and products - including blends or alloys of different resin families - can be joined together by ultrasonic welding. The differing materials can be welded if their melting temperatures are within 30°F and their composition is compatible. To ensure the plastic assemblies are adequately joined, the injection molded parts should have a suitable joint design from the beginning. There are several factors to take into consideration when designing a part to be ultrasonically welded like material, end use of product, cost, ease of injection molding and the location of the joint surface relative to the horn.
There are many benefits to ultrasonic welding. This technique is fast and offers lower cycle times. Assembly rates of more than 25 parts-per-minute are possible with a single station. There are no secondary operations like coating, inserting or cleaning. This method is flexible since a single welder and associated tooling can perform welding activities for various parts along an assembly line. Additionally, there are no consumables with ultrasonic welding and once the initial tooling costs are incurred, the maintenance and operation costs are minimal.
Ultrasonic welding is widely used for various component assembly applications that require a permanent bond. Some of these applications include liquid bearing vessels, IV components, hearing aids, filter assemblies, monitors and diagnostic components. Ultrasonics can be used to insert metal fasteners into thermoplastic material.
In order to provide a single source manufacturing solution for your custom injection molded components, Crescent Industries offers several secondary operations such as product assembly to complete your project. Our experience includes simple or complex operations depending on customer requirements, as well as designing and building of custom fixtures to aid in the assembly.
For additional information, please click below to get our white paper “How to Overcome Common Design Concerns for Injection Molded Components”. This white paper addresses some of the key concerns in the design step that are a 'must' to consider.