Early on in manufacturing, it was common to produce an injection mold for every single component required for a plastic assembly. Products were designed without the thought of crossover, and the idea of using the same component in multiple products wasn’t commonplace. Over time, and especially in the current global production environment, it’s easy to find where plastic components are used in multiple products from one brand. With brand partnerships in place, many times you will find partnering brands using the same components inside plastic assemblies. In order to accomplish a modular product that can be utilized anywhere, the components and assemblies must be designed specifically for an assembly process. Let’s take a look at ways to design your plastic parts for an assembly operation and keep your options open for more utilization across your product line.
Choosing Your Best Material Options
Before you finalize your design, you’ll have to choose what materials are being used in the plastic assembly. Plastics don’t have the corrosion problem that dissimilar metals can have in the field, but plastics can contract and expand at different rates when exposed to extreme heat and cold. While there are 1,000s of plastics to choose from, each component in your final assembly should be similar in temperature tolerance to prevent failures in the field.
Your material selections will also need to take into consideration the performance characteristics required for the design. Will they see shock loads in the customer’s hands? Will they be under extreme tension or compression in service? The small details can and will affect which material is selected and will also help define how the product is assembled.
Lastly you will have to take into consideration how all components will be assembled. Will they use fasteners for permanent assembly, should it be serviceable, or does it just go together by welding with no option for disassembly? These answers will direct your designer to introduce different materials for assembly and performance based on their product function.
Define Your Initial Assembly Process
It sounds simple to just write down the assembly process, yet the process of writing every step down can reveal forgotten or overlooked steps from the initial concept. For every step in the assembly process you’ll want to review what is required, can it be simplified, and can the step be redesigned to reduce potential mistakes. There a few different ways that designs can incorporate error proofing. Your in-house designers, or your design partner, should have a solid foundation in ‘designing for assembly’, which will keep the mistakes to a minimum.
Many industries require some form of documentation to identify error prevention and a reaction plan if an error does occur. One such document that is used across multiple industries is a DFMEA document. It outlines all potential errors in every step of the assembly process, and how the design will prevent that error from occurring. It’s primarily listed in a matrix form with assembly errors listed by failure mode type, then ranked by projected severity, and their projected occurrence. The document should indicate which assembly error would be most annoying or harmful to your end customer, and where the design team should focus their effort to prevent assembly issues in manufacturing.
Using Datum Strategies & Alignment Features to Reduce Errors
Your design team needs to focus on how to take all guess work out of assembly and make them extremely intuitive to put together. A datum strategy should quickly identify how an assembly comes together without an error. A datum scheme, for reference, pinpoints which feature of an assembly to connect first, second, third, and beyond. When it references assembly, a datum scheme may translate into hole sizes being round, oblong, or oversized based on their sequence in the assembly process. The smallest round hole will dictate a primary assembly point, an oblong could be used a secondary point to limit rotation, and an oversized third hole just simply used for the connection of the components. These holes are used typically to cause a misalignment when completed out of sequence. The final assembly shouldn’t fit or operate as designed, and it should clearly indicate a problem at or before the final inspection that it is not ready for the next step in the manufacturing process.
Define How Your Plastic Components Will Be Attached Together
This is the point where your design team will need to know if your assembly needs to use fasteners for permanent assembly, should any component be serviceable, or does it just go together permanently with no option for disassembly? Some designs may require fasteners as they need to be serviceable by the end customer or a dealer for the assembled product. If you don’t require the option of disassembly, you may choose to eliminate any or all fasteners and go with a simpler option like heat staking for permanent attachment.
The question is how you plan to create the perfect blend of fastening and cost-effectiveness. Assembly options have varying cost depending on if it comes in the form of technician labor or extra machinery. Each attachment method will have benefits and disadvantages. Here are some of the options we offer as part of our turnkey manufacturing operations:
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 connection. 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. No extra materials are needed, such as solvents, adhesives, or mechanical fasteners, and in-process by-products are very minimal.
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
The adhesive process involves a material that bonds to each plastic component and chemically combines them to form a permanent bond. 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 plastic assemblies.
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 as a consideration during the design phase.
If your plastic injection molded assembly requires service or disassembly, a mechanical fastener may be required for assembly. Mechanical fasteners typically include screws, nuts, bolts, rivets, pins and/or clips. One sub point of mechanical fastener attachment should include how many times the fasteners need to be removed. If the disassembly requirement is just once or twice during the life of the product, you may not need a threaded insert for durability. If the plastic assembly requires servicing multiple times over the life of the product, your design team should consider adding a threaded insert to increase the durability of the threads. If a threaded insert is chosen as a requirement, the plastic parts should be designed for the addition of the inserts initially rather than added as a later after-thought.
Designing for Assembly (DFA) is a complete methodology to identify the assembly method for a product, regardless if it is plastic or metal. You will identify how to manufacture a product through a specified assembly process, reduced the potential for making assembly errors, and made it efficient and simple to assemble. Some Computer Aided Design (CAD) software includes tools that can aid designers in prediction, but a lot of the industry know-how will come from years of experience. SEA-LECT Plastics has an elite staff of designers that specialize in design for assembly. Our staff keeps us in the top ranking for turn-key manufacturers that can offer industry-specific design options, material selection for competitive cost and performance, and turn-key assembly options to produce it in-house in Everett, Washington. If you have a new idea, call us (425) 339-0288 or email us at email@example.com. We can offer you advice on the best technology to use, the best materials to meet your product demands, and how to navigate through each development stage with ease.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.