Injection molding, a highly efficient manufacturing process, centers on the precise injection of molten materials such as thermoplastics, polymers and elastomers into pre-determined molds, which are then cooled and solidified to form a part that perfectly matches the mold cavity. This process is not only suitable for a wide range of material choices, but also offers significant advantages in mass production due to its low waste output and high reproducibility.
Design Intelligence in Injection Molding
When exploring the endless possibilities of injection molding, design considerations are particularly important. The end use of the product (whether as a stand-alone product or as an assembled part), the dimensional specifications, the mechanical requirements, and even the resistance to chemicals and pressure are all key factors that influence the design and manufacture of the mold. Below are some key design tips to optimize the injection molding process and product quality:
The Art of Material Selection
Each injection molding material contains unique physical and chemical properties that directly determine the performance of the product. For example, some materials excel in dimensional stability, while others are better at bonding with adhesives. When selecting a material, it is important to consider the temperature and pressure conditions of the environment in which it will be used, as well as the biological and chemical interactions between the materials. Thermoplastic resins can be categorized as amorphous or semi-crystalline, with the former being superior in terms of impact resistance and dimensional stability, while the latter is known for its excellent chemical resistance and electrical resistance properties. Therefore, the choice of material will have a direct impact on the tolerance setting and characterization of the product.
At Xometry, we are committed to providing a wide range of injection molding material options, including plastics, elastomers and silicone rubber, to meet the diverse needs of our customers.
Precision in Tolerance Control
Molds are often manufactured to extremely high tolerance standards, with CNC machining accuracy of up to 0.005 mm. However, the shrinkage of plastics during the cooling process cannot be ignored, and the shrinkage rate of different materials varies. For example, the maximum shrinkage of PLA is about 0.5%, while PEEK can be as high as 1.5%. Therefore, when designing injection molded parts and setting tolerances, the shrinkage characteristics of the material must be fully considered to ensure the final dimensional accuracy of the product.
Choosing the right wall thickness
The following are recommended wall thicknesses for different materials:
- ABS: 1.143 mm - 3.556 mm
- Acetal: 0.762 mm - 3.048 mm
- Acrylic: 0.635 mm - 12.7 mm
- Liquid crystal polymer: 0.762 mm - 3.048 mm
- Long fiber reinforced plastic: 1.905 mm - 27.94 mm
- Nylon: 0.762 mm - 2.921 mm
- Polycarbonate: 1.016 mm - 3.81 mm
- Polyester: 0.635 mm - 3.175 mm
- Polyethylene: 0.762 mm - 5.08 mm
- Polyphenylene sulfide: 0.508 mm - 4.572 mm
- Polypropylene: 0.889 mm - 3.81 mm
- Polystyrene: 0.889 mm - 3.81 mm
- Polyurethane: 2.032 mm - 19.05 mm
In addition, the walls should be uniformly thick. Uneven wall thickness will result in sink marks. Sink marks are localized surface depressions caused by slow cooling of the thicker part. Keep the wall thickness as uniform as possible. However, when an uneven wall thickness is unavoidable, the difference in thickness should not be greater than 15% of the nominal thickness. We also recommend the use of tapering or smooth transitions.
Injection Molding Design Essentials: The Art of Applying Draft Angles
When exploring the fine art of injection molding, it's important to incorporate the design considerations of draft angles. While technologies such as CNC machining make it easy to shape vertical walls, injection molding poses unique challenges. Vertical wall designs tend to shrink and lock up the mold during the cooling process, especially in the core area, where forced demolding can jeopardize the safety of the ejector pins and the mold as a whole. Therefore, giving the mold walls a subtle tilt, i.e., a pull-out treatment, is the key to solving this problem.
The incorporation of mold pulling is often the grand finale of a design, and it comes quietly during the fine sculpting phase. Different surfaces have different draft requirements, and textured surfaces call for more pronounced tilts. Below is an overview of several common surfaces in injection molding and their recommended minimum draft angles:
- Near-vertical elegance: 0.5°
- Widely applicable balance: 2°
- A solid choice for closed surfaces: 3
- Subtle treatment of lightly textured surfaces: 3°
- Robust support for medium-grained surfaces: 5°.
Intelligent use of reinforcement and corner support plates
Reinforcing bars and corner braces are essential for strengthening injection-molded parts. They not only add the necessary strength to the part, but also prevent cosmetic imperfections such as warping and denting. The introduction of these structural elements is more efficient and economical than simply thickening the part. However, care must be taken to avoid shrinkage when designing, and the secret is to keep the rib thickness in the range of 50% to 60% of the dependent wall thickness for optimal warpage resistance.
The Art of Rounding and Radii
In injection molding design, the judicious use of rounded corners and radii can elegantly eliminate the negative effects of sharp corners, promote smooth material flow, and improve the overall strength of the part. Sharp corners tend to block material flow and form weak points, while rounded transitions can effectively disperse stress and reduce the risk of rupture. At the same time, the rounded corner design also facilitates part demolding, reducing the risk of mold jamming. It is worth noting that the use of sharp corners should be strictly limited to the parting surface or the necessary closure surface, while the design of the inner and outer radius should follow certain rules, such as the inner radius of not less than 0.5T, the outer radius of not less than 1.5T (T is the thickness of the part).
Innovative practice of snap fit
The key to realizing snap fit lies in the design of undercut. Traditional straight drawing molds are difficult to perform such tasks, while side cores can solve the problem but are accompanied by high costs. For this reason, designers have explored a variety of innovative solutions, such as adopting a transfer core, adjusting the parting line and adjusting the pulling angle accordingly, or utilizing the stripping undercut (bump) technique, but attention needs to be paid to the flexibility of the part in order to adapt to the deformation needs during the demolding process. Additionally, undercuts should be designed with a 30- to 45-degree lead angle to ensure effective demolding.
Robust connection of projections
As a key element in the connection of pins, inserts or self-tapping screws, the design of the projection should not be overlooked. To ensure the structural stability of the part, the projection should be firmly attached to the sidewall or rib. For self-tapping applications, it is recommended that the outside diameter of the protrusion be 2.5 times the diameter of the screw to provide adequate support.
Fine Layout of Gate Positions
The choice of gate location is critical as the gateway for molten material to enter the mold. Improper gate layout may lead to uneven material flow, formation of flow lines and other imperfections. Therefore, gate locations need to be strategically located and combined with measures such as increasing injection pressure to ensure uniform material distribution within the mold cavity. At the same time, gates should be designed to avoid compromising the functionality and aesthetics of the part.
MOLDV's superior choice for injection molding services
In the field of injection molding, MOLDV stands out for its wide selection of materials and efficient quotation system. We offer more than 30 material options, including plastics, synthetic rubbers, silicone rubbers and elastomers, to meet your diverse needs. Simply visit our quoting engine, upload your design files, select your part preferences and get an instant quote. Together, let's create a brilliant chapter in injection molding with superb craftsmanship.