Metal parts play a critical role in automobiles, as their structural strength impacts both the lifespan and safety of the vehicle. Consequently, South Korean automobile manufacturers impose strict standards on the procurement of metal parts. With a high demand for metal parts in the automotive industry and local South Korean MIM companies unable to meet the large-scale supply needs, these manufacturers often turn to overseas suppliers like China’s EMITECH.
So, how do we design the wall thickness structure of injection-molded parts for South Korean automobile companies? EMITECH presents an insight into the structural design of wall thickness in injection-molded parts.
Definition of Wall Thickness Wall thickness is a fundamental structural characteristic of a part. It refers to the thickness value between the outer surface (outer wall) and inner surface (inner wall) of a part. The value entered into the software during structural design when extracting the shell can be referred to as the wall thickness.
Function of Wall Thickness
- For the product’s outer wall: The outer wall of a part is like its skin, while the inner wall serves as the structural skeleton. Tratamientos superficiales on the outer wall can achieve various aesthetic effects. The inner wall simply connects the structure (ribs, screws, clips, etc.) together, providing the part with necessary strength. During the proceso de moldeo por inyección, additional structures can be filled in. There are no specific requirements for the internal and external walls (cooling, assembly). Typically, they are made as a single unit to ensure sufficient strength to protect internal components from environmental damage or interference.
- For internal parts of the product: Acting as bearings or connecting brackets, there are no strict requirements for the inner and outer walls. Other structures (ribs, screws, clips, etc.) can be designed on the outer wall as needed. For ease of manufacturing (especially concerning the separation of front and back molds), the outer wall at the front end of the mold should be as simple as possible. If not feasible, adjust the ejection angle of the front and back molds, or incorporate ejector pins in the front mold or a small groove in the back mold. Additional structures may also be designed on the inner wall.
Whether for external casing parts or internal components, wall thickness is crucial for the successful ejection of the part from the mold.
Design Principles of Wall Thickness: Al diseñar partes de metal, wall thickness is a priority. It’s akin to the foundation of a building and vital for building other structures upon it. Wall thickness also affects the mechanical properties, moldability, aesthetics, and cost of metal parts. Thus, it should be designed considering these factors.
Principles Based on Mechanical Performance: Both external and internal parts need certain strength levels. Apart from other factors, part molding should also consider resistance to dislodgement. Too thin, and the part can deform. Generally, the thicker the wall, the higher the strength (a 10% increase in wall thickness roughly equates to a 33% increase in strength). However, if the wall thickness exceeds a certain range, increasing it further can reduce part strength due to shrinkage and porosity. Increased wall thickness also adds weight, prolongs the moldeo por inyección cycle, and raises costs. The optimal approach is to use geometric features to enhance rigidity, such as ribs, curves, corrugated surfaces, reinforcement, etc.
In some cases, due to spatial and other constraints, the strength of certain parts is primarily achieved through wall thickness. Therefore, if strength is a significant factor, it’s advisable to determine the appropriate wall thickness through mechanical simulation.
Principles Based on Moldability: The actual wall thickness refers to the thickness of the cavity between the front and back molds. After the molten polvo de metal fills the mold cavity and cools, the wall thickness is formed.
- Flow of molten metal powder during injection and filling: The flow of molten metal powder in the mold cavity can be considered laminar. According to fluid dynamics, laminar flow involves adjacent liquid layers sliding over each other under shear stress.
- During injection, the molten polvo de metal contacts the mold walls, causing the liquid layers to first adhere to the cooled mold walls. This results in zero velocity and frictional resistance with adjacent layers. The middle layer moves the fastest, with the velocity decreasing towards the mold walls.
- The middle layer is fluid, while the outer layers solidify. As cooling progresses, the solidified layers grow, reducing the cross-sectional area of the fluid layer, making filling more challenging and requiring greater injection force.
- Therefore, the size of the wall thickness greatly impacts the flow and fillability of the injected part and should not be too small.
- Viscosity of molten polvo de metal also significantly impacts flowability: Viscosity is the internal friction that arises when layers of fluid move relative to each other under external influence. It’s measured by dynamic viscosity or the viscosity coefficient – the ratio of shear stress to shear rate in the melt.
The viscosity of the melt indicates the ease of flow of the metal melt and is an index of flow resistance. The higher the viscosity, the greater the resistance and the more difficult the flow. Factors affecting melt viscosity include molecular structure, temperature, pressure, shear rate, additives, etc. (These can change during the injection process, altering the flowability of the metal powder).
In practice, the melt flow index indicates the flowability of metal materials during processing. The higher the index, the better the flowability, and vice versa.
- Calculating using the Flow Length Ratio: The flow length ratio of metal refers to the ratio of the length of flow (L) of the metal melt to the wall thickness (T). Therefore, for a given wall thickness, the greater the flow length ratio, the further the metal melt can flow. Conversely, for a given length of flow, a greater flow length ratio allows for a smaller wall thickness. This ratio directly affects the quantity and distribution of metal feed in the product. It also influences the wall thickness of the metal.
- For greater accuracy, the specific range of wall thickness can be obtained by calculating the flow length ratio. However, this value depends on factors like material temperature, mold temperature, and polish level. It’s a rough range and varies under different conditions, so while not precise, it can serve as a reference. We often refer to the methods used in moldeo por inyección de plástico for preliminary estimates.
Shrinkage or Porosity: Shrinkage or porosity typically occurs in thicker-walled areas. Mechanism: According to the principles of material solidification in injection molding, internal porosity and surface shrinkage are caused by continuous contraction during cooling. When shrinkage concentrates at later solidification sites without immediate compensation, internal shrinkage and porosity are more likely to occur.
The above principles of wall thickness design mainly cover mechanical performance, moldability, appearance, and cost. In a nutshell, the wall thickness of an injection-molded part should be as small and uniform as possible while meeting mechanical and processing requirements. Otherwise, it should transition uniformly.