As you design your plastic injection molded components, the need to join parts together becomes a detailed part of the finished product.  The question is how you plan to create the perfect blend of fastening and cost-effectiveness.  Each method will require additional cost, and whether that comes in the form of labor or machinery depends on what you need from the final product.  Each method will have benefits and disadvantages, and we’ll give each a review so you can choose the best for each product to ensure you exceed your customer’s expectations.

Ultrasonic Welding

Ultrasonic welding is an assembly process that uses high-frequency ultrasonic acoustic vibrations to create a solid-state plastic weld.  The parts are held together with pressure in a fixture or a holding jig while the vibrations melt the plastic substrate to create a permanent molecular bond.  This process is often used in food and drink selections, electronic assemblies, medical devices, and automotive parts.

Ultrasonic welding has proven to offer excellent reliability, long-term durability, and corrosion resistance in harsh environments.  Another benefit is that you don’t need to worry about stripped fasteners during initial or reassembly of components or eventual corrosion in the field due to dissimilar materials being used.  No extra materials are needed, such as solvents, adhesives, or mechanical fasteners.  That can reduce the assembly line set-up time for production, reduce processing cost, and assist in keeping work areas clean.  The post-weld time for cooling on an ultrasonic weld is faster than a joint using an adhesive or solvent, which can also reduce labor and processing costs for each assembly.

In-process by-products from Ultrasonic Welding are very minimal.  The process doesn’t contaminate our product or require additional cleaning to remove adhesives or solvents.  Welding equipment doesn’t require extensive maintenance costs or labor-intensive programs.  Simple initial checks can be done with a TPM schedule, and your assembly personnel will be ready to go in minutes.  The best option of Ultrasonic Welding offers flexibility to join plastics, and can offer the added benefits of threaded inserts for additional mechanical fastening options.

While ultrasonic welding has many great applications in many industries, it doesn’t offer a benefit for applications that require disassembly as an option.  If your product needs the option for disassembly, or component replacement, using a mechanical fastener with a threaded insert may be the best option.

From a design standpoint, two things need to be considered upfront: the melting temperatures of joining plastics should be within approximately 30°F of each other, and each should have an appropriate joining amount of material to melt and mold together.

Adhesive and Solvent Bonding  

When bonding plastics together, two applications are the primary options: adhesives or solvents.  The adhesive process involves a material that bonds to each plastic and chemically combines them to form a permanent bond.   The adhesive acts as a permanent barrier combining between the parts but does not chemically alter either plastic in the process.  Solvents are put on as a liquid material that coats each plastic part until each are soft enough to press together with a clamp or press.  Once the solvent evaporates, the plastics are mixed together enough to stay permanently bonded.  The solvent application should only be used with thermoplastics, while adhesive bonding can be used for most plastics.

Some of the disadvantages of adhesive or solvent bonding involves the secondary materials (the consumables) required to form the permanent bond.  Some can be quite expensive, which can drive up the cost of the product over time.  It can be a lower cost initially because of the minimal equipment required, but it will be a cost that is required for the life of the product.  As mentioned with ultrasonic welding, if your product requires any disassembly, these two options of permanent bonding of plastic may not be the best solution.

UV Bonding

UV (ultraviolet) bonding involves using high-intensity ultraviolet lighting to cure and dry a coating or adhesive.  This process can more costly initially due to the equipment investment, but it has an advantage of bonding plastics to non-plastic materials such as glass or metal.  It does require a secondary material (the coating or adhesive), but can increase process speed and throughput, increase production quality, and better resistance to solvents and scratches.  It should only be used for clear materials, so keep that in mind during the design phase as you select your perfect plastic resin.

Mechanical Fastening

The three previous options involved permanent bonding of assembled parts.  Once you adhere plastic parts with solvents or adhesives, there isn’t a chance to take them apart again.  If your product requires the option of disassembly, or may not require a permanent bond, mechanical fasteners may be the best option.

Mechanical fasteners typically include screws, nuts, bolts, rivets, and pins or clips.  One could argue that rivets are a permanent bond, but they offer a unique case in that they can be removed from the plastic without damaging the plastic itself.  The other bonding applications will damage the plastic.  Fasteners that require multiple disassembly opportunities should add a threaded metal insert to increase the durability of the threads.  Plastic threads, that may or may not be formed via the fasteners, will have a limited amount of times they can accept a fastener torque before they become damaged or fatigued.

While the disassembly option can be quite an advantage over the other options, one disadvantage be be the continued cost of the fasteners over the life of the product assembly.  The initial cost of the equipment may be reduced by having the inserts molded into place, glued or expanded into holes as a secondary option.  The secondary process may benefit from ultrasonic or heated application of the insert, which can offset human labor required to control the process.  Just keep in mind that the plastic parts should be designed for the addition of the inserts initially rather than added as a later after-thought.