Injection molding, as an efficient and precise manufacturing process, holds a crucial position in modern industrial production. Whether it is automotive components, electronic housings, medical devices, or packaging products, injection molding enables the mass production of complex-shaped parts while ensuring dimensional accuracy and surface quality. As market demands for product performance and appearance continue to rise, manufacturers must rely not only on advanced equipment and processes but also on careful consideration during the design phase.
High-quality injection molded parts begin with thoughtful design. This means that from the earliest stage of product development, material properties, part geometry, wall thickness distribution, draft angles, and mold feasibility must all be thoroughly considered. Ignoring these critical factors can lead to molding defects, manufacturing difficulties, and even increased production costs and mold maintenance challenges.By incorporating Design for Manufacturability (DFM) principles during the design stage, engineers can optimize the injection molding process while ensuring both functionality and aesthetics, reduce development cycles, improve production efficiency, and gain a competitive edge in the market.

Therefore, in the development of injection molded parts, design is not only a reflection of appearance and functionality but also a core factor in ensuring smooth manufacturing, lowering costs, and enhancing overall product quality. Understanding and applying these design-first principles is a key step that every company and engineer aiming to produce high-quality injection molded components must prioritize.

Injection molding product showing high-precision plastic part design with optimized wall thickness and draft angles for manufacturability.

Select the appropriate Injection Molding material

Material selection is a critical aspect of injection molding design. Different thermoplastics, such as ABS, PP, PC, and PA, vary significantly in mechanical properties, heat resistance, transparency, and cost. Designers must match the material to the specific application requirements—for example, high-impact parts require tougher materials, transparent parts need plastics with excellent optical properties, and parts exposed to chemical environments should use corrosion-resistant materials. Additionally, material shrinkage and flow characteristics directly affect mold design and the dimensional accuracy of the final part. Carefully considering these factors not only ensures part functionality and quality but also reduces processing defects, improves production efficiency, and lays a solid foundation for subsequent mold design and mass production.
MaterialShrinkageStrengthFlexural Modulus
ABS0.4-0.7%47MPa2620MPa
POM(Acetal)2.00%64MPa2700MPa
PMMA(Acrylic)0.2-0.6%78MPa3300MPa
Nylon1.4-1.6%83MPa2900MPa
PC(Polycarbonate)0.5-0.7%62MPa2240MPa
PET（Polyester）1.5~2.5%50MPa2100MPa
PP(Polypropylene)1.0-1.5%30MPa1100MP
PEI0.80%105MPa3310MPa
Note: The information provided is for reference only. Actual values may vary depending on the specific material grade, manufacturer, and application.

Selecting the Right Wall Thickness

In injection molding design, wall thickness is one of the most critical factors to consider. With hundreds of available materials, choosing the right material and determining the proper wall thickness can be challenging.

The optimal wall thickness depends on several aspects: whether the part is structural, whether it may become brittle, and—most importantly—the characteristics of the selected material.

Fortunately, designers don’t need to rely solely on trial and error. For most common injection molding materials, well-established recommended wall thickness ranges are available and can serve as a valuable reference during the design process.

MaterialRECOMMENDED WALL THICKNESS
ABS0.045 – 0.140 in/1.14-3.56 mm
POM(Acetal)0.030 – 0.120 in/0.76 – 3.05 mm
PMMA(Acrylic)0.025 – 0.500 in/0.64 – 12.70 mm
PA(Nylon)0.030 – 0.115 in/0.76 – 2.92 mm
PC(Polycarbonate)0.040 – 0.150 in/1.02 – 3.81 mm
PE(Polyethylene)0.030 – 0.200 in/ 0.76 – 5.08 mm
PPS(Polypropylene Sulfide)0.020 – 0.180 in/ 0.51 – 4.57 mm
PP(Polypropylene)0.025 – 0.150 in/ 0.64 – 3.81 mm
PS(Polystyrene)0.035 – 0.150 in/0.89 – 3.81 mm
PU(Polyurethane)0.080 – 0.750 in/2.03 – 19.05 mm
PET(Polyester)0.025 – 0.125 in/ 0.64 – 3.18 mm
Liquid Crystal Polymer0.030 – 0.120 in/0.76 – 3.05 mm
Long-fiber reinforced plastics0.075 – 1.000 in/1.91 – 25.40 mm

Use Radii Strategically in Injection Molding

When designing injection-molded parts, fillets should be added at critical locations to eliminate sharp edges. While some features require right angles or sharp corners to function properly, fillets offer two main benefits:
  • Improve Material Flow — Fillets allow molten material to flow more smoothly through the mold, enhancing molding stability.
  • Increase Part Strength — Reducing sharp corners lowers stress concentrations, decreasing the risk of breakage.
Fillets should be applied selectively: only at user-contact edges, assembly entry points, or critical internal features. Proper internal fillets also help optimize material flow and improve structural strength.

Add Draft Angles for Easier Part Ejection

When designing injection-molded parts, it is important to include draft angles (also called tapers) on part surfaces. A draft angle means that the sides of the part are slightly inclined rather than perfectly vertical. Proper draft angles offer several benefits:

  • Facilitate Ejection — Inclined surfaces make it easier to remove cooled parts from the mold.
  • Reduce Defects — Draft angles help minimize part deformation and other molding issues.
In general, the deeper the cavity, the larger the required draft angle. A common guideline is approximately 1° per inch of cavity depth. However, the optimal angle depends on factors such as material type, wall thickness, and shrinkage rate. Consulting an injection molding expert is recommended to determine the best draft angle for a specific design.

Core-Out Design to Reduce Costs

While injection-molded parts can be fully solid, a more cost-effective approach is to use a core-out design, creating a hollow interior while maintaining structural strength through wall thickness and ribs.

Hollowing parts reduces material usage and part weight, yet properly designed walls and ribs can ensure strength comparable to solid components.

A common guideline for rib design is that rib thickness should be 40%–60% of the part’s nominal wall thickness, roughly half the thickness of the solid section.

In addition to saving material and reducing weight, core-out designs help minimize sink marks and stress concentrations, resulting in higher-quality injection-molded parts.

Retain or Eliminate Undercuts?

In injection molding design, simple structures are easier to mold, reducing tooling complexity and production risk. However, removing critical features such as undercuts solely to simplify the design can compromise part strength, functionality, or usability. A common example is snap-fit connections: without undercuts, assembly stability may be reduced, undermining the design intent.

Undercuts are typical “difficult-to-eject features.” Due to their shape (e.g., internal grooves, protrusions) or position, parts with undercuts cannot be directly ejected from the mold. Additional side-action mechanisms such as slides, lifters, or core pins are required. This increases mold complexity and adds costs in two ways: extended development time (to validate the reliability of side actions) and higher manufacturing expenses (due to multi-component machining and tighter assembly tolerances).

Designers generally face two strategic options:

  • Simplify the structure — Adjust feature placement or adopt alternative designs to eliminate undercuts, reducing molding difficulty and overall cost.
  • Retain undercuts — Preserve necessary features and optimize mold design with appropriate ejection mechanisms, balancing performance requirements with tooling feasibility and cost.

Mold Opening Direction and Parting Line Considerations

In injection molding design, selecting the proper mold opening direction and locating the parting line are critical. The mold opening direction determines whether the part can be ejected smoothly, while the parting line directly affects both mold construction and part appearance.
  • Minimize Undercuts and Complex Mechanisms — Align key features with the mold opening direction to avoid undercuts and reduce reliance on side actions such as slides or lifters.
  • Optimize Aesthetics and Function — Position the parting line away from critical cosmetic or tactile surfaces to minimize visible seams and flash. When necessary, adjust the part geometry to relocate the parting line to a less noticeable area.
  • Ensure Structural Simplicity — Follow the natural contours of the part when defining the parting line. Avoid overly complex or irregular parting paths to simplify mold manufacturing and improve alignment precision.
  • Support Production and Assembly — Consider downstream requirements such as assembly and functional performance when planning the mold opening direction and parting line placement.
By carefully defining the mold opening direction and parting line early in the design process, engineers can reduce tooling complexity and cost while ensuring optimal part performance and appearance.

Choosing the Right Partner

When injection-molded part designs become too complex, consulting an experienced molding specialist is often the best solution.
Horizon has years of experience delivering high-quality injection molding solutions for clients across various industries. We work with a wide range of plastic materials, including polycarbonate, nylon, and thermoplastic elastomers, ensuring the right balance between performance and cost.

Latest Post

  • Precision laser-cut metal decorative panels with complex patterns

    Sheet Metal Laser Cutting: Principles, Applications, Industry Standards, and CNC Machining Integration

  • Why Surface Treatment Is Essential for Metal Manufacturing

  • Aluminium 6061 Alloy Material

    How to correctly select CNC machining materials?

  • why choose 3D printing？