Three Steps for Designing A Complex Molded Part

Pushing the limits of plastic injection molding allows engineers to create innovative solutions to complex issues. A high level of skill and knowledge is required to duplicate these innovations. Many injection molders struggle with complex designs, such as perfect finishes, leak-proof mating, and complex geometries. Designing and manufacturing complex parts is a complicated process requiring planning, experience, and design and manufacturing capabilities. There are many steps in the design process, and making changes after the initial design phase can add to delays and costs. Below are three measures to ensure your part is manufactured to your specifications.

1. Answer these questions to define the project

Many crucial decisions should be made early on in the design process. Understanding your project will ensure that no details are missed, and you don’t incur additional costs by redesigning it later in the process. Below are some questions to consider in your planning stage.

  • What materials will your part be made of? This will likely be determined once the rest of the questions are answered; for example, a particular resin may be chosen for strength properties but can’t hold the tolerances needed. Consider all characteristics of the material, including its shrink rate. In addition to resin, what other materials are required? Is hardware required? Are all materials compatible?
  • What function does the part serve? Is it functional? Is it a critical part? Does it require a cosmetic finish? Will it be assembled with another component, and if so, how?
  • What are physical requirements? Does it require a strength or impact resistance? Are there critical tolerances?
  • What environment will it be exposed to? Will it be exposed to heat, high humidity, saltwater, UV light, or caustic materials? How will this impact the resin, formed part, and hardware? Will functionality be affected?
  • What outside requirements must the part meet? Who are the stakeholders? Is there industry or regulatory requirements?
  • What is the goal for creating the part? Are you creating a new part, modifying a part to change a characteristic (e.g., increase strength), moving from another material, or combining two parts into one? How does this impact the overall project regarding costs and efficiencies?

2. Look out for these challenging design characteristics

When designing an injection molded part, design elements that can impact the manufacturability and the finished part must be considered. Below, we give a rule of thumb value in some instances; however, material selection can change these values. For example, one material may have a higher propensity for shrinkage or warpage than another, and understanding how the resin reacts can help you with your design.

Learn about material selection by reading, Injection Molding: Material Selection Guide for Commodity and Engineered Resins

Design elements that can cause problems if not adequately addressed:

Gates – Gate placement impacts appearance, tolerance, warpage, surface finish, wall thicknesses, and more. Ideally, it should be placed in the center of the part in a non-functional, non-appearance area. Placement in the center means the melt flow has the same flow length to each end. The type of gate will impact the part as well. Gate marks are unsightly, so when the aesthetics of the part matter, the gate can be hidden along the edge of the part or another non-visible area.

Wall thickness – wall thickness must be consistent to avoid warping, twisting, cracking, and shrinking of the part. The nominal wall thickness is determined by the functional performance requirements of the part and the manufacturing process. When determining wall thickness, consider acceptable stress levels, part life expectancy, and resin manufacturing limitations.

Uneven wall thicknesses result in uneven cooling. If two walls of varying thickness connect, the inherent shrinkage will cause stress where the two sections connect, causing warping, twisting, or cracking. When varying thicknesses are required, smooth thickness transitions should be used to reduce the stress.

Draft angle – Without draft angles, the friction created when removing the part would make it difficult to remove and could create scrapes on the part, leave large ejector pin marks, and/or reduce the tool life. Drafts should face the cavity and core sides. The deeper the part, the larger the draft angle should be. For deep cavities, a rule of thumb is to add 1° of angle for every inch of cavity. Texture also plays a role in effective draft angles. The deeper the texture, the higher the draft angle, ranging from 1° to 12+° draft. The rule of thumb is to add 1.5° per 0.001 inches of textured depth.

Undercuts – Avoid undercuts whenever possible. Undercuts prevent the mold from opening in a straight motion and require core pulls or cams. These add substantially to production costs.

Ribs/Gussets – Ribs and gussets provide structural integrity, give strength to a mounting boss, prevent part warpage, and aid in the integration of internal components. If not appropriately designed, sinkage, warpage, and part failure can occur. A fillet can be added where the rib meets the wall to reduce stress, but take care not to make it too small or too large to prevent it from not reducing stress enough or creating sink, respectively. A rule of thumb is that the thickness of the rib or gusset base should be 60 percent of the thickness of the wall it intersects to avoid defects on the outer wall.

Bosses – Bosses (used for mating parts with screws or other fasteners) that are not isolated from a corner can cause sinkage in the nominal wall. They can be strengthened by connecting them with ribs to the walls or gussets to the base. Like ribs, the thickness of the boss should be 60% or less of the part’s nominal wall thickness.

Fillet and Radii – Sharp internal or external corners weaken parts, so corners should be radiused. Fillet radii are found where the walls intersect the floor of the part. A rule of thumb is that a radius equal to at least 0.5 times the adjacent wall thickness should be used. The radii at the top of the wall should be 1.5 times the thickness of the nearest wall.

3. Work with an experienced partner

Not all injection molders have the capabilities and experience to injection mold your custom complex part. Working with an experienced partner early in the design process will ensure that your molds are correctly designed and the company can manufacture your part to your specifications. An experienced partner will streamline the design and production phases of product development, removing unnecessary time and costs.

Your plastic injection molding partner can also prototype your custom molded plastic part to reveal any design flaws that can turn your part into an expensive failure. There are many benefits to prototyping your part, such as evaluating acceptable production tolerances, identifying structural weaknesses, getting an idea of the cost of production, and more. This can save you both time and money in the long run.

Greenleaf Industries for Your Complex Injection Molded Parts

There are many plastic injection molders to choose from, but we have distinct advantages not found elsewhere. There are many reasons our customers have chosen us for their custom injection molding needs.

  • Visually perfect finishes, leak-proof mating surfaces, or speck-free clarity are not problems. We are up for a good challenge and strive to provide the most cost-effective solution.
  • Providing cost-effective solutions to complex challenges is what we do best. We will collaborate with you to reduce upfront and ongoing costs.
  • Competitive pricing doesn’t mean poor quality. We are quality-focused, with quality elements designed into our manufacturing processes. We’ve had an automotive-certified quality system since 2000.
  • One hundred percent on-time delivery of the correct parts is guaranteed every time.
  • We have been in business for 20 years and have solved many tough injection molding challenges. Our experience includes meeting rigorous Japanese OEMs’ service and quality expectations.

When you work with us, you can be assured that from design through delivery, our processes are custom-designed with your quality, on-time delivery, and cost containment expectations in mind.

Contact us to learn more.