Designing plastic parts for injection molding is like building a house: there are countless ways to approach the problem with each decision impacting function, quality, and cost-effectiveness.
In this blog, we will explore six smart strategies which prioritize cost-effective solutions that are clear and easy to understand without sacrificing the function and quality of your part. From simplifying designs to choosing budget-friendly materials, these practical tips will help you navigate the plastic injection molding journey to an economical outcome. Join us as we break down these strategies to help you create top-notch products without breaking the bank!
Here is what to consider when designing the most cost-effective solution:
- Simplify the Design
- Optimize Wall Thickness
- Use Standardized Components
- Select Cost Effective Materials
- Consider Multi-Cavity and Family Molds
- Conduct a Mold Flow Analysis
Now let’s break each down, and get into the facts!
Simplify the design
Creating a molded plastic part with simplicity in mind involves thinking about how to make it less complicated without losing its usefulness. Look at the design closely and remove any complex patterns or extraneous details that do not contribute to or enhance the part’s function. When designing an injection molded part, consider the manufacturability. Remember that injection molding naturally favors a 'straight draw' for ejection, i.e. part removal, or a uninterrupted linear exit from the mold—adopting this simple design strategy will ultimately simplify the mold design and reduces costs at all stages of production.
Whenever possible it is always preferred design practice to avoid a model that incorporates undercuts. Molded-in undercuts are the result of a part design that does not allow a clean, straight draw ejection and now can complicate the mold-making process by creating intricate, mechanical actions in the mold to allow this ‘designed-in’ trapped plastic to release. This can make it virtually impossible to release the part smoothly during the ejection process, requiring elements like cams, lifters, and sliders to be built into the mold. By minimizing undercuts, you not only simplify the mold design but also enhance the overall manufacturability, ensuring a smoother production flow and reducing the likelihood of challenges during part ejection.
Adding or ensuring proper draft angles to an injection mold part design not only improves how a mold functions, it also helps save money in production. Proper draft angles will increase the longevity of a mold by allowing the molded parts to release from the mold with less resistance, thereby reducing wear and tear on the tool with each ejection. Draft angles also help prevent part defects during production, like part distortion or breakage, which means less wasted material and time fixing errors. Draft angles speed up the process because parts come out of the mold more easily, allowing for faster production. To learn more about Draft Angles, read our blog “Draft Angles in Part Design for Plastic Injection Molding”.
Incorporating rounded, or radiused corners into your design serves as a crucial structural enhancement. Especially in inside corners, ensuring there are small fillet radii throughout rather than sharp corners will eliminate stress concentrations, making it less susceptible to premature part failure. This consideration not only contributes to the overall durability of the product but also streamlines the manufacturing process, showcasing the synergy of design elements in achieving both strength and manufacturability.
Optimize or Even Wall Thickness
With regards to injection molded part design, maintaining a uniform wall thickness as much as possible and wherever possible is most desirable. Uniform wall thickness in plastic part design ensures consistent cooling and shrinkage rates, resulting in more predictable and repeatable part measurement--enhancing cost-effectiveness and reducing the risk of defects. Adjacent walls, intersecting walls, or support ribs should never be more than 60% of the outside, nominal wall thickness. Irregular wall thickness will result in irregular cooling of the molded part, resulting in part distortion and dimensional irregularities. Think about designing wall thicknesses within a range of .050” - .180”, maintaining consistent wall thicknesses throughout. By optimizing wall thickness, you will reduce the amount of raw material and processing time needed, saving you money.
Adding ribs to the design is another way to make plastic part manufacturing even more efficient. Unlike machined components or other types of mass-produced parts which might be designed with solid cross-sectional mass, injection molded parts prefer the use of internal support ribs. Designed-in ribs serve as the internal support structures that help remove excess, unnecessary plastic making the part lighter without compromising its strength. In fact, strategically placed ribs in a given part's geometry can enhance the overall strength considerably.
This not only reduces material costs but also aids in quicker and more even cooling during the molding process, further improving efficiency. Ribs will also help prevent common part defects such as sink marks. Sink marks occur when thicker sections cool unevenly, or at a different rate than the thinner walls, causing an indentation on the surface. Incorporating ribs into the design not only optimizes material usage but also acts as a safeguard against potential part defects, making the manufacturing process more reliable and cost-effective.
Use Standardized Components
If your part includes insert components for insert molding, overmolding or secondary staking, using standardized components—such as commonly sized threaded inserts, nuts, bolts, and screws—can be a favorable approach to designing plastic parts for injection molding. These fasteners adhere to established sizes and specifications, making them easily accessible and cost-effective. Similarly, incorporating standardized bearings for smooth movement, or standardized electrical connectors for seamless integration, adds efficiency to the manufacturing process.
Additionally, relying on widely used inserts, like standardized threaded inserts or knobs, not only streamlines production but also ensures compatibility with readily available parts. These standardized components are often well-tested and work efficiently, making the production process faster. By strategically choosing components like these, companies can harness the benefits of established standards, leading to cost savings and a more straightforward injection molding experience.
Select Cost-Effective Materials
Due to the vast array of plastic materials available in the marketplace, choosing a particular plastic resin can often have you “scratching your head”. The material selection process usually involves a series of discussions as you narrow down what the priorities are. Selecting the right materials can be crucial for companies aiming to be cost-effective when designing plastic parts for injection molding. The materials chosen play a big role in how much the manufacturing process will cost. Using materials that offer good performance and relatively affordable helps save money without compromising the final product's strength.
In plastic injection molding, some cost-effective materials include:
Polypropylene (PP): A versatile and budget-friendly option used in various applications.
Polyethylene (PE): This plastic comes in different types, like high-density polyethylene (HDPE) and low-density polyethylene (LDPE), and is known for its affordability.
Polystyrene (PS): An economical choice, PS can be purchased in clear and easy to mold. Be aware that it can be brittle and will not have the impact strength of higher performing engineering resins.
Acetal: Known for its very good lubricity, a good choice for mechanical applications. Also possesses good chemical resistance with many solvents and chemicals.
Acrylonitrile Butadiene Styrene (ABS): Combining toughness with affordability, ABS is common in consumer goods and automotive components.
Acrylic: A good lens material and often used in light pipe applications as well. Not as strong or impact resistant as PC, but a more economical choice. Be sure your part design has adequate draft for part release from the mold.
PC/ABS: Combining polycarbonate and ABS gives you the best of both worlds, or a slightly less costly alternative to straight PC. Good overall mechanical properties with good cosmetics and moldability
By carefully picking these or similar cost-effective materials, companies can make sure their injection molding projects are both high-quality and budget-friendly. To learn more about plastic resins, read our blog on “Choosing the Right Plastic Resin for Injection Molded Plastic Parts”.
Consider Multi-Cavity Molds
When evaluating your costing targets, your company might consider multi-cavity tooling as a strategy to drive down unit costs given the ability of multi-cavity tooling to produce numerous identical parts in a single molding cycle. This approach significantly lowers per-unit costs, especially in the context of high-volume manufacturing scenarios. Conversely, of course, you will need to budget for a higher capital expenditure for this larger and more complex tooling.
In addition to multi-cavity tooling, companies could also explore the benefits of family tooling. Family tooling is similar to multi-cavity molds, except—instead of multiple cavities of the same part—all the cavities are different. This strategy offers cost advantages by streamlining production processes and minimizing tooling expenses. The versatility of family tooling contributes to efficient manufacturing, making it a valuable consideration for companies aiming to achieve optimal cost-effectiveness in their production operations.
For lower-volume projects, companies may find that Modular Unit Die (MUD) tooling presents a cost-effective solution. MUD tooling involves a standardized mold base with interchangeable mold cavities, allowing for more flexibility and quicker turnaround times, which can be advantageous for smaller production runs. However, it's crucial to acknowledge that MUD tooling comes with certain design constraints. The standardized nature of the mold base may limit the complexity and customization of part designs, making it less suitable for intricate or highly-specialized components. Therefore, while MUD tooling can be a cost-effective choice for lower-volume projects, careful consideration of design constraints is essential to ensure compatibility with specific project requirements.
Conduct Mold Flow Analysis
When a company designs plastic parts for injection molding, they often will consult with the injection molder to conduct a Mold Flow Analysis, a sophisticated tool that's a bit like a crystal ball for manufacturing. A mold flow analysis is a computer generated simulation of the part design virtually placed into its tooling to predict how the part will actually mold. It will accurately demonstrate how the melted plastic will form the designed part through an injection molding cycle in a given mold. It's like having a virtual test run to catch any potential issues early on. This proactive approach allows companies to fine-tune their designs, ensuring a smooth production process and avoiding costly surprises down the line.
Think of Mold Flow Analysis as a super-powered sidekick, helping companies dodge pitfalls and make smart decisions. Detecting problems in the digital realm allows for adjustments before physical production begins, saving both time and resources. This powerful tool not only ensures the final plastic parts meet high standards of strength and aesthetics but also helps companies be environmentally conscious by optimizing material usage, minimizing waste, and promoting sustainability in their manufacturing practices.
By thinking through these criteria of part design early on, you can make sure that your molded plastic part is not just easy to manufacture but also budget-friendly. Remember: part design is not DIY! Even with all of this in mind, teaming with competent and experienced designers who know how to optimize for injection molding is the only way to ensure your part will be ready for production.
How Can I Learn More?
Stelray Plastic Products offers numerous advantages for device manufacturers, including high-quality and precise products, cost-effective manufacturing, and design flexibility. With the ability to produce complex and intricate designs with tight tolerances, Stelray’s injection molding service is the preferred choice for your company’s molding solutions.
To learn more about our injection molding services, including our medical molding solutions, please call or email our Sales team below!