1. Mold Opening Direction and Parting Line
When starting the design of an injection-molded product, determine the mold opening direction and parting line first to minimize the need for core-pulling and sliding mechanisms and reduce the impact of parting lines on the appearance.
- Once the mold opening direction is set, design ribs, snap hooks, protrusions, and other structures to align with this direction as much as possible to avoid core-pulling, reduce joint lines, and extend mold life.
- Based on the mold opening direction, select an appropriate parting line to avoid undercuts in the mold opening direction, improving appearance and performance.
- Applying a suitable draft angle can prevent the product from scuffing. A smooth surface requires a draft angle of ≥0.5°, fine texture (sand surface) requires over 1°, and rough texture needs more than 1.5°.
- A proper draft angle helps avoid damage during ejection, such as whitening, deformation, or breakage.
- For deep-cavity structures, ensure that the draft on the exterior surface is greater than that on the interior to prevent core misalignment and ensure uniform wall thickness and material strength around the opening.
- Each plastic material has a specific wall thickness range, typically 0.5–4 mm. Wall thickness over 4 mm may lead to prolonged cooling times, shrink marks, and may require design adjustments.
- Uneven wall thickness can lead to surface shrinkage.
- Variations in wall thickness can cause voids and weld lines.
- Proper use of ribs can increase rigidity and reduce deformation.
- The thickness of ribs should be ≤(0.5–0.7) times the product wall thickness to avoid surface shrinkage.
- Ribs should have a single-sided draft angle greater than 1.5° to prevent ejection damage.
- Small fillets may lead to stress concentration, causing the product to crack.
- Small fillets can also result in stress concentration in the mold cavity, potentially causing cavity cracking.
- Properly designed fillets improve mold processing by allowing direct milling with R-cutters, reducing reliance on slower electrical discharge machining (EDM).
- Different fillet radii may shift parting lines, so choose radii or sharp edges based on the situation.
- Hole shapes should be simple, generally circular.
- Holes should align with the mold opening direction to avoid core-pulling.
- When the length-to-diameter ratio of a hole exceeds 2, a draft angle is necessary, with the diameter measured based on the smallest size (maximum solid dimension).
- Blind holes should typically have a length-to-diameter ratio of no more than 4 to avoid pin bending.
- Distance between holes and the product edge should generally exceed the hole diameter.
7. Core-Pulling and Slide Mechanisms in Injection Molds
- When the molded part cannot be ejected smoothly in the mold opening direction, design a core-pulling or slide mechanism. However, core-pulling increases mold cost and decreases lifespan due to the joint lines and shrink marks it may cause.
- If possible, avoid core-pulling by aligning hole axes and ribs with the mold opening direction or by allowing core-through in cavity and core components.
- Utilize the toughness of polypropylene (PP) material to design hinges as an integral part of the product.
- The film thickness for hinges should be less than 0.5 mm and kept uniform.
- When injection molding an integral hinge, place the gate on one side of the hinge only.
- Inserts can increase local strength, hardness, dimensional accuracy, or provide threaded holes or shafts to meet special requirements but will add to product cost.
- Inserts are typically made of copper or other metals or plastics.
- Inserts should include features to prevent rotation and pull-out, such as knurling, holes, bends, flattening, or shoulder structures.
- Surrounding plastic should be appropriately thickened to prevent stress cracking.
- When designing inserts, consider their positioning method in the mold (holes, pins, magnetic fixtures).
Product markings are usually placed on flat internal surfaces and designed as raised features. Position them on surfaces aligned with the mold opening direction to prevent scratches.
11. Tolerances of Injection-Molded Parts
Due to the non-uniform and unpredictable shrinkage during injection molding, the tolerances for injection-molded parts are lower than for metal parts. Tolerances should follow established standards, such as China’s GB/T14486-93, based on the plastic material and functional requirements. Designers should also consider the overall production capabilities of the factory and precision levels achieved by similar products.
12. Warpage of Injection-Molded Parts
Increase rigidity and reduce warpage by avoiding flat structures, setting flanges and raised/indented features, and adding appropriate ribs.
- Design snap-fits with multiple snaps in shared use to prevent a single failure from rendering the assembly unusable. Add fillets to increase strength.
- Tolerances for snap-fit dimensions are strict. Too many undercuts can cause snap breakage, while too few make alignment difficult or may cause loose fits. Allow for easy modification by planning ways to add or remove material.
14. Welding (Hot Plate, Ultrasonic, and Vibration Welding)
- Welding can enhance joint strength.
- Welding can simplify product design.
15. Balancing Process and Product Performance
- When designing injection-molded products, consider the balance between appearance, functionality, and manufacturability. Sometimes compromising processability enhances appearance or performance.
- If defects cannot be avoided, try to position them in inconspicuous areas.
16. Screw Post Hole Diameter and Self-Tapping Screw Diameter
- Self-tapping screw M2: Screw post hole diameter 1.7 mm
- Self-tapping screw M2.3: Screw post hole diameter 2.0 mm
- Self-tapping screw M2.6: Screw post hole diameter 2.2 mm
17. Design Principles for Boss
- Avoid standalone bosses; connect them to outer walls or use ribs to strengthen and ensure smooth plastic flow.
- Boss height should not exceed 2.5 times the diameter; overly tall bosses may trap air, causing issues like voids, burning, or incomplete filling.
- For bosses taller than 2.5 times the diameter, especially those away from walls, use ribs for reinforcement.
- Bosses are typically round, as other shapes are harder to process.
- Position bosses at a distance from corners or outer walls.
- Prevent shrinkage by creating “volcano-shaped” indentations around bosses.
- Boss draft angles are generally 0.5° externally and 0.5° or 1° internally.
The OPRO Tech Engineering team includes diversified and experienced system engineers, we can support your project at every stage of development. We have demonstrated our ToolingMold, analysis, and design capabilities over many programs for the medical, household, Automotive, and commercial industries.
Our design engineers are experienced injection mold designers who are continually striving to increase the efficiency and performance of each plastic injection mold we deliver. The injection molding design process optimizes the performance of each custom plastic mold and guarantees each part made in the mold will be to specification. We design using the latest versions of many software programs including Pro-E, Solidworks, AutoCAD and more.
