When designing a plastic part, the ultimate goals are correct functioning and long wear, as well as parts that are cost-effective. Because a finished product will always reflect the planning and expertise that goes into design, finding the appropriate tolerance for plastic components requires a specific process — one step at a time — to help you successfully reach your goals.
- Define Usage Requirements
The first step is defining how the part will be used and the requirements it must meet. This includes determining structural load and physical and mechanical properties as well as the environmental conditions the part must withstand such as exposure to chemicals, UV radiation, or temperature extremes. It’s also important to investigate whether part specifications are dictated by regulatory or industry standards to ensure compliance.
- Material Selection
Once you have defined specifications and requirements, the next step is to select the right materials. Factors that will help you make a final decision on material include:
- Type of plastic resin: Will amorphous or crystalline allow you to produce the tolerance you need?
- Additives: Will fillers or additives be needed that can enhance the properties of the polymer to increase performance?
- Cost feasibility: Can the materials you are considering allow you to produce the part at a cost within your budget?
Narrow your options to a range of materials that can be used for the application, and then begin to determine which is the best choice. Tolerance range for the specific resin being selected can often be found by speaking to the resin supplier and/or consulting with SPI data.
The third step in the plastic part design process is the use of computer simulation. Finite Element Analysis (FEA) will show if anticipated loads and other factors will be acceptable with the design and materials selected. The designer will look at results and adjust if needed.
From there, Mold Filling Simulation can be performed to understand what the selected material(s), part design, and proposed mold design elements will yield. This data will be used to adjust part design and will guide mold design and injection molding process parameters. Factors such as non-uniform shrinkage can be evaluated to determine effects on ability to meet tolerance objectives.
The fourth step to finding the appropriate tolerance for plastic components is tooling based on the data from the simulation. A variable in this step that can make or break your project is the plastic injection molding company you choose for the job. You should partner with a company that has extensive skill and expertise, uses robust tooling design practices to ensure long-term viability, and has the ability to protect sensitive areas of the part design with “steel safe” areas to groom the part to tolerance specified in the design. Tooling should also result in tolerances within at least of 10% of the product design requirement nominal.
The final step is processing. Again, the choice of a plastic injection molding partner will be a large factor in success of this step. To find the appropriate tolerance for plastic components, it’s necessary to make sure the company you select has the equipment that will provide consistent pressure, position and temperature to successfully turn out quality parts. Ask questions about their molding processes, decoupled molding including fill, pack and hold phases, and shot molding as well as how quality is ensured in each process. Also research the company’s tolerance monitoring processes and how they identify and reject products that don’t meet tolerance.
Following a systematic plastic part design process will help you ensure the finished product is a part that will perform as needed and that can be produced cost-effectively and with minimal waste. Partnering with the right plastic injection molding company is vital to successful completion of your project in the tooling and processing phases, but you can also rely on their expertise as a resource to consult with as you select materials and analyze results of simulations or when you are researching best practices in general.
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