Establishing a uniform wall is one of the most important decisions an engineer makes when designing a plastic part. The nominal wall influences the overall dimensions of the part, the part performance, and the manufacturing requirements. Proper wall thickness is just one of the key design elements that must be held to a high standard for the injection molding process to work properly. This article discusses the importance of maintaining uniform wall thickness as well as the drawbacks of not being able to maintain this design feature.
Why Maintain Uniform Wall Thickness?
Once the uniform wall thickness is established, it’s important that it remains consistent. Maintaining uniform wall thickness allows for the most uniform flow of the plastic through the part, allowing the best processing. Also, plastics are essentially poor heat conductors. By maintaining a uniform wall thickness, the part will cool as uniformly as possible.
Injection molding is a pressure-driven process, and the molten polymer will take the path of least resistance. When there is variation in wall thickness, molten polymers take preferential flow to the thicker walls. This leads to air trapping, formation of weld lines, and unbalanced filling pattern.
Effects of Non-Uniform Wall Thickness
Non-uniform wall thickness can cause serious design, cosmetic, and structural problems, as well as overall failure of the part. Designers need to take a holistic approach on how to reduce the wall thickness of the part before making decisions. Not considering the uniform wall early in the design phase can greatly disrupt the manufacturing process: narrowing the processing window or inability to manufacture the part. Below are the features that non-uniform wall thickness can have on a part.
Varying wall thickness has a great effect on the appearance of the part. Varying wall thickness can result in undesirable physical features such as warpage, sinks, and cosmetic issues like flow lines.
It’s important to gate into the thickest section and then flow into thinner areas: The flow path of molten material must remain open, so the plastic materials can continue to flow into the part details during the cooling phase. Flowing into an area to get materials to a thicker area, may create flow irregularities. The thinner area may solidify, preventing the additional material in the pack phase from reaching the thicker section of the part.
Shear stress in the flowing plastic can also be affected. At a constant fill rate, thin areas force the flowing plastic to move faster, increasing shear stress in the process. Different shear stress across a part will promote warpage.
Varying wall thickness can have an impact of the cooling rate: Thicker areas take longer to solidify. The entire part must stay in the mold until it is cooled sufficiently to be ejected, which can extend the cycle time in the manufacturing process.
Of all the various design aspects, uniform wall thickness has the most significant impact on the cost, production, and final quality of a part. Midstate Mold & Engineering is proud to be a leader in the plastic injection molding industry that is able to handle all phases of your project, including mold design, prototype development, high and low volume production, and custom assembly. Contact us today.