Product structural design is a critical step in product development process, encompassing transition from concept to actual production. Molds are essential in manufacturing of plastic products, structural design of plastic parts directly impacts mold fabrication and final product quality. Therefore, an excellent product structural design engineer must not only be proficient in plastic part structural design but also have a basic understanding of mold manufacturing process.
This article will delve into 14 key points in plastic product structural design to help readers better understand and master this field. These key points include selecting glue thickness (glue position), setting wall thickness, designing ribs, considering draft angles, applying fillets, designing hole locations, treating support surfaces, matching studs and holes, planning screw holes, strategies for addressing thermal and cold deformation. Through these detailed explanations, readers will be able to more comprehensively grasp key elements of plastic product structural design and contribute to successful product development.
Ideal wall thickness distribution maintains a uniform thickness at all cross-section locations. However, in practical applications, wall thickness often varies due to varying functional requirements. During transition process, transition from thick to thin areas should be as smooth as possible to avoid dimensional instability and surface defects caused by uneven cooling rates and turbulent flow. It is worth noting that plastics with a thickness less than 3 mm often have difficulty filling mold, but some soft and rubber materials can still achieve full fill within a thickness range of 2 to 3 mm. Furthermore, for areas involving exterior surfaces, thickness should be no less than 6 mm to ensure product appearance quality.

Reinforcement ribs can effectively enhance product rigidity and strength without significantly increasing cross-sectional area. This design is particularly suitable for plastic products frequently subjected to pressure, torsion, or bending, effectively preventing deformation. Typically, reinforcement ribs are placed on non-contact surfaces of plastic part, extending in direction of maximum stress and deflection. However, location of reinforcement ribs is also influenced by production factors such as mold cavity filling, shrinkage, and demolding. During design, thickness of rib is generally set at 5 to 7 times the overall rubber thickness. Exceeding 7 times may cause shrinkage issues.
When rib is relatively tall, we typically add a 5 to 1 degree slope to its surface to reduce resistance during demolding. Alternatively, we can create a 25 degree drop on one side to ease demolding. For shorter ribs, a specific slope is generally not necessary. However, in some cases, drafting may still be necessary to ensure smooth demolding of structural mating surfaces.

 

Draft angle is an essential element in mold design, ensuring smooth demolding of part. Its design direction depends on specific dimensions of part. To ensure easy ejection of molded part during mold opening and closing, we apply draft angles to wall surfaces such as side cores and ribs. However, please note that this rule may not apply to shallow parts (such as flat plates) or parts with special requirements. For larger sidewalls without draft angles, other methods, such as slide design, may be necessary. Generally speaking, draft angle ranges from 5 to 8 degrees, with a common value of approximately 3 degrees. However, specific value depends on size, height, and shape of product.
In mold design, draft angle of front mold is often slightly greater than draft angle of back mold, generally with a difference of 5 degrees being appropriate, to ensure that product sits naturally on the back mold after mold opening. Furthermore, key areas such as pillow, insertion, and punch-through areas should all be drafted, with upper and lower runout (i.e., difference between larger and smaller dimensions) exceeding 1 mm per side. If exterior surface requires a paint spray finish, draft angle should be at least 3 degrees; if a material or sun-textured process is used, draft angle should range from 5 to 8 degrees.

In plastic product design, unless a sharp edge is specifically required, edges are typically rounded. This reduces stress concentration, thereby promoting smooth plastic flow and demolding. At the same time, minimum radius on exterior surface should generally be greater than 1 mm, with a radius of 2-3 mm or more generally recommended.

 

From a mold processing perspective, ideal hole pattern is a simple, regularly shaped round hole to avoid processing difficulties associated with complex, irregularly shaped holes. Hole diameter should also be moderate. Generally, for products with a wall thickness of 0 mm, hole diameter should be at least 6 mm. Ratio of hole depth to hole diameter should also be considered to avoid excessive hole depth, which can easily break or deform slender mold core. Furthermore, distance between hole and outer edge of product should be at least 5 times hole diameter, and distance between holes is recommended to be at least 2 times hole diameter to ensure sufficient product strength.
Hole holes parallel to mold opening direction are generally formed using a core (which can be inserted or extended) or a push-through or insert-through method. Holes not parallel to mold opening direction typically require a slide or bevel. Holes in product sidewall should be designed to be capable of push-through or insert-through forming, as long as this does not affect product use and assembly.

 

Bosses are commonly used in plastic products for shaft-hole assembly or self-tapping screw assembly. If boss height is moderate and mold uses sleeve ejection, bevel design can be omitted. However, when boss height is taller, a cross rib (rib) is often added to outside with a 1-2 degree bevel. Boss itself may also be beveled, depending on situation. When boss mates with a column above or below (or with another boss), a clearance of 0.5-10 mm per side is typically set to accommodate positioning errors during boss machining. If boss is used to attach a self-tapping screw, inner hole size should be 2-5 mm smaller than screw thread diameter per side to ensure a secure tightening. For example, when using an M0 self-tapping screw, inner hole size of boss is typically designed to be Ø50-70 mm.

In design of plastic products, it is sometimes necessary to insert a pre-existing metal or plastic part into mold for remolding. These pre-existing parts are called inserts. When designing inserts, it's crucial to ensure complete, accurate, and reliable positioning within mold. Furthermore, connection between insert and molded part must be secure, especially when overmolding is thin. Furthermore, adhesive leaks should be prevented during design process.

 

Plastic products can be finished in a variety of ways. These include a smooth surface, typically achieved through a polish-reducing process on mold surface; a spark pattern, created through copper electro-discharge machining within mold cavity; and etched or engraved surfaces with various patterns. Deeper or more numerous textures increase resistance to mold release, so draft angle should be appropriately increased to ensure smooth demolding.

 

Text engraving on the surface of plastic products can be either raised or recessed. In mold manufacturing, raised characters correspond to concave cavities and are relatively easy to create; concave characters, on the other hand, require a raised profile, which is often more challenging. However, if you want a glossy finish on lettering, concave engraving is necessary, with a depth generally limited to around 2 to 5 mm.

Since threads on plastic parts often have low precision, specialized thread removal mechanisms are required. If precision requirements are less stringent, simplifying demolding mechanism and employing forced demolding can be considered.

 

In plastic products, the entire surface is typically not used as a support surface. Instead, localized structures such as bosses, raised points, or ribs are used for support. This is because plastic materials inherently struggle to maintain a perfectly flat surface over large areas without deformation or warping. This localized support design can effectively improve product stability and durability.

Ultrasonic Wire Bonding: This method is relatively easy to implement in mold production, but requires specialized ultrasonic equipment, which increases costs and prevents disassembly after assembly. Cross-sectional design of ultrasonic wire typically adopts a triangular structure with a width of 30 mm and a height of 3 mm, with interruptions of 2 mm at intervals of 3 to 6 mm along length.
Self-tapping screw assembly: This method is relatively simple in mold production, but it increases assembly steps, leading to higher costs and a more cumbersome disassembly process.
Hook-and-clip assembly: Although this method requires more complex mold production, it offers convenient assembly and allows for repeated disassembly for multiple uses. When designing hook, avoid excessive adhesive thickness in certain areas and consider mold manufacturing convenience. Hook should also have a moderate fit to ensure smooth assembly and disassembly. Mating surfaces should fit tightly, while allowing for appropriate clearance in other areas.
BOSS shaft-and-hole assembly: This method is relatively simple in mold production, offering both easy assembly and convenient disassembly. However, its disadvantage is that assembly is not secure enough and may become loose.
Groove Design: Grooves are typically designed at mating surfaces of two plastic parts. Depth of grooves is generally between 8 and 5 mm, with approximately 1 mm of clearance left on the sides. When tooth depth is deep, a bevel of 1 to 5 degrees is used, with 2 degrees being the most common choice. Shallower depths generally do not require a bevel. Upper and lower mating surfaces of tooth should be designed to fit snugly, with zero clearance.

 

A aesthetic line is often added to mating surfaces of two plastic parts. Width of aesthetic line typically ranges from 2 to 0 mm, and specific size varies depending on the overall dimensions of product. This design enhances product's aesthetic appearance.