Experienced product designers possess a deep understanding of the injection molding process and consider numerous factors in plastic part design. This article focuses on key elements in designing plastic molded parts, such as wall thickness, draft angle, ribs, holes, bosses, snap-fits, press fits, and tolerances.
Wall Thickness in Plastic Part Design
Determining the appropriate wall thickness is crucial. Other features, such as ribs and radii, are related to wall thickness. The wall thickness of a plastic product depends on various factors, including external forces it must withstand, support for other components, the properties of the plastic material, weight, electrical performance, dimensional accuracy, stability, and assembly requirements.
Typically, the wall thickness for thermoplastic materials ranges from 1 to 6 mm, with 2 to 3 mm being the most common. For larger-sized parts, the wall thickness can exceed 6 mm. Table 1 lists recommended wall thickness values for various thermoplastic materials.
Uniformity of Wall Thickness
Uniform wall thickness is a key principle in plastic part design. Non-uniform wall thickness leads to inconsistent melt flow and cooling shrinkage, resulting in defects such as sink marks, voids, warpage, and even cracking. Additionally, it can cause shrinkage marks, internal stress, deformation, color differences, or transparency variations. Excessively thin walls may reduce the part's strength and rigidity during use and assembly. From an economic perspective, overly thick parts increase material costs and production time. Thicker plastic areas cool more slowly and are prone to sink marks. Figure 1 illustrates uniform wall thickness design.
If a transition from a thicker to a thinner section is unavoidable, it should be gradual, maintaining a maximum wall thickness ratio of 3:1, as shown in Figure 2.
In many cases, designers can utilize ribs to alter wall thickness. This not only saves material, reduces production costs, but also shortens cooling time. Cooling time is roughly proportional to wall thickness.
Furthermore, designers must also consider the flow path length, which is the distance molten material travels from the gate to various parts of the cavity. Typically, the flow path length is proportional to wall thickness. The greater the wall thickness, the longer the flow path. If the ratio of flow path length to wall thickness is too high, insufficient material or incomplete filling may occur far from the gate. Therefore, in some cases, increasing wall thickness may be necessary.
Sharp Corners
Sharp corners often lead to part defects and stress concentration. These areas are prone to material buildup during post-processing such as electroplating or painting. Stress concentration can cause the part to fracture under load or impact. Therefore, sharp corners should be avoided in design. Figure 3 shows an example of a sharp corner design.
Considerations for Approach Angle and Ejection Direction
Ejection Direction and Parting Line
At the beginning of injection product design, it is crucial to determine the ejection direction and parting line. This minimizes the need for side-action mechanisms and reduces the impact of the parting line on appearance. Once the ejection direction is determined, structures such as ribs, snap-fits, and protrusions should be aligned with it to avoid side-actions, reduce knit lines, and extend mold life. Subsequently, a suitable parting line can be chosen to enhance the product's appearance and performance.
When a part is removed from the mold, it must overcome ejection force and opening force. Opening force refers to the process of the part separating from the cavity. As the part cools in the mold, it shrinks, causing hole walls to tightly grip the core. Friction between the part and the core, vacuum adhesion at the bottom of holes, and other factors make the ejection force much greater than the opening force. Excessive ejection force can cause part deformation, whitening, wrinkling, and surface wear.
Draft Angle
Draft angle is critical in determining the magnitude of the ejection force. Since injection molded parts often stick to the core (male part) after cooling and shrinking, using the same draft angle on both the cavity and core ensures uniform wall thickness and prevents the part from sticking to the hotter cavity after ejection. In special cases, if it is necessary for the part to stick to the cavity after ejection, the draft angle on the adjacent cavity can be reduced, or an intentional undercut can be added to the cavity.
There is no fixed value for the draft angle; it is usually determined empirically. A draft angle as small as 1/8 or 1/4 degree can be used on highly polished outer walls. For deeper parts or parts with textures, the draft angle should be increased accordingly. Typically, for every 0.025 mm of texture depth, the draft angle needs to be increased by 1 degree.
Furthermore, although larger draft angles generally facilitate part demolding, maintaining dimensional accuracy is paramount. Dimensional errors caused by the draft angle must be kept within the precision tolerance. For parts with high shrinkage or complex shapes, a larger draft angle should be considered.
Ribs in Plastic Part Design
The strength of a plastic part is not solely dependent on increasing wall thickness. In fact, increasing wall thickness leads to greater shrinkage, resulting in internal stresses that actually reduce strength. The key to improving the strength of a plastic part lies in its stiffness. This is often achieved by combining a thin-wall structure with strategically placed ribs to increase the section modulus.
Considerations for Rib Design
However, adding ribs increases the thickness at the junction with the main wall. This thickness is typically determined by the diameter of the largest inscribed circle, which in turn is determined by the rib thickness and root radius. For a base wall thickness of 4 mm, varying the rib thickness and root radius changes the diameter of the largest inscribed circle. Figure 4 shows how increased local wall thickness can lead to sink marks on the opposite surface, affecting appearance. A rational design can reduce the likelihood of surface depression, thereby improving part quality.
Analysis shows that the thickness of a rib should be minimized as much as possible within a certain range. If the rib is too thin, its height must be increased to maintain stiffness. However, overly thin ribs can lead to buckling during injection, difficulty in mold filling, and sticking to the mold. The radius at the base of the rib should not be too small to avoid stress concentration.
Typically, the root radius of a rib should be at least 40% of the rib thickness. The rib thickness should be 50% to 75% of the base wall thickness, with higher percentages applicable only to materials with low shrinkage. The rib height should be less than five times the base thickness. Ribs must have a draft angle, and their orientation should be aligned with the ejection direction, or movable mold components can be used. The spacing between ribs should be more than twice the base thickness.
To achieve uniform stiffness in all directions, the simplest method is to add ribs both longitudinally and transversely, with the ribs intersecting perpendicularly. However, this increases the wall thickness at the intersection, leading to greater shrinkage. A common solution is to add a circular hole at the intersection to form a uniform wall thickness, as shown in Figure 5.
Design Considerations for Holes in Plastic Parts
1. Hole Location and Strength
It is common practice to incorporate holes in plastic parts for ease of assembly or functional purposes. Ideally, the size and location of these holes should not compromise the product's strength or increase manufacturing complexity. Key considerations:
The distance between adjacent holes, or the distance from a hole to the nearest edge, should be at least equal to the hole's diameter. This is especially important for holes near edges to prevent breakage. For threaded holes, the distance from the hole to the product edge is typically greater than three times the hole diameter.
2. Types of Holes
Holes come in various types, such as through holes, blind holes, and stepped holes. From an assembly perspective, through holes are more common and easier to machine than blind holes. In terms of mold design, the structure for through holes is simpler. They can be formed by placing cores on both the moving and fixed parts of the mold, or by using a single core in only one part. The former creates two cantilever beams under the action of molten plastic, but due to the short beam lengths, deformation is minimal. The latter typically forms a simply supported beam, with also minimal deformation. When two cores are used, their diameters should be slightly different to prevent misalignment and ensure a smooth mating surface. Blind holes formed by cantilever core pins are more prone to bending under the impact of molten plastic, leading to irregular hole shapes. Typically, the depth of a blind hole should not exceed twice its diameter. For blind holes with a diameter of 1.5mm or less, the depth should not exceed the diameter. The wall thickness at the bottom of a blind hole should be at least one-sixth of the hole diameter to avoid shrinkage.
3. Side Holes
Side holes are typically formed using side cores, which increases mold cost and maintenance, especially when the side core is long and prone to breakage. If feasible, the design can be improved as shown in Figure 6 to mitigate these issues.
Bosses in Plastic Part Design
Bosses typically protrude from the main wall thickness and are used for product assembly, spacing objects, and supporting other components. Hollow bosses can accommodate inserts or screws. These applications require bosses to have sufficient strength to withstand pressure without cracking. Bosses are usually cylindrical, as this shape is easier to mold and offers better mechanical properties.
Integration with Structure
Ideally, a boss should not be designed as an isolated cylinder, but should be connected to the outer wall or used in conjunction with ribs. This approach enhances the boss's strength and aids the smooth flow of plastic material. The connection to the outer wall should be a thin-wall connection to avoid sink marks.
The fillet radius at the base where the boss connects to the substrate should be 0.4 to 0.6 times the substrate thickness. The boss wall thickness should be 0.5 to 0.75 times the substrate thickness. The top of the boss should be chamfered to facilitate screw installation. Bosses must also have a draft angle. These design requirements are similar to those for ribs, so a boss can be considered a variation of a rib. Refer to Figures 7 and 8 for these relationships.
Threaded Bosses for Self-Tapping Screws
Many bosses are used for connecting self-tapping screws. The internal threads on these bosses are formed by a cold-flow molding process, which shapes the plastic through deformation rather than cutting. The dimensions of threaded bosses must be sufficient to withstand screw insertion forces and the loads they carry. The bore diameter on the boss should ensure the screw remains secure under specified torque and vibration conditions.
The outer diameter of the boss must withstand the circumferential forces generated during screw tightening without fracturing. To facilitate screw insertion, a countersink is usually provided at the top of the boss, with a diameter slightly larger than the nominal screw diameter. Calculating boss dimensions can be complex.
A simplified estimation method, suggested on foreign websites and based on the nominal screw diameter, is recommended. First, determine the material of the screw, then, based on the corresponding coefficient in the table, substitute the coefficient into the nominal screw diameter to determine the appropriate size.
Snap-Fit Connections in Plastic Part Design
Snap-fit assembly is a convenient, economical, and environmentally friendly connection method. Snap-fit components are molded simultaneously with the product, eliminating the need for additional fasteners like screws. Assembly involves simply snapping the corresponding parts together.
The principle of a snap-fit connection involves pushing a protruding part of one component over an obstruction on another. This process involves elastic deformation; once the obstruction is cleared, the part snaps back to its original shape and locks in place, as shown in Figure 9. Snap-fit connections can be permanent or detachable.
Structurally, snap-fits can be classified into cantilever, annular, and spherical types, as shown in Figure 10.
Key Angles and Calculations
Critical Angles
The two key angles in snap-fit design are the return angle (or retention angle) and the lead-in angle. Generally, a larger return angle favors a more secure fit. When the return angle approaches 90 degrees, the snap-fit connection becomes permanent, as shown in Figure 11.
