In injection molding, wall thickness is one of the most crucial design elements. Selecting the appropriate injection molding wall thickness ensures the manufacturer optimizes the part’s appearance and moldability. It also improves the mold’s performance, leading to lower manufacturing costs.
Most manufacturers use the prototyping stage to test different wall thicknesses and their impacts on the resulting part. Nevertheless, there are some best practices most manufacturers follow to ensure they select the appropriate injection molding wall thickness. This article examines those best practices, the issue of wall uniformity, and explains why uniformity is so important when designing a mold.
The Importance of Uniformity in Injection Molding Wall Thickness
Uniformity ensures that every wall used for a part is of the same thickness. The most important benefit is that uniform wall thickness ensures consistency when making multiple parts. Manufacturers don’t have to worry about changing flow or cooling rates when the melt moves from thick to thin walls or vice versa. Instead, they get consistent flow and cooling.
Maintaining uniform thickness also improves the part’s stress distribution, minimizes shrinkage, and generally allows manufacturers to save money when creating their molds.
What Can Happen if Injection Molding Wall Thickness Isn’t Uniform?
If a manufacturer doesn’t maintain uniform wall thickness, they must account for variations in thickness in their design and the way they inject melt into the mold. Failure to do that can lead to an array of issues:
· Warping may occur if the part experiences uneven shrinkage as a result of varying wall thickness. This usually presents as twists and bends in the part.
· Sink marks tend to form on the thickest sections of uneven walls. These small crates occur due to the melt not being able to fully cool inside the mold.
· Uneven wall thicknesses can cause varying flow rates, leading to the creation of flow lines.
· If the melt is intended to flow from a thin section into a thick one, short shots may cause issues. A short shot happens when the melt cools prematurely in the thin section of the mold, which prevents the melt from flowing into the thicker section.
· Uneven wall thicknesses create cooling challenges. The melt in the thicker areas of the part takes longer to cool than that in the thinner areas. The aforementioned short shot issues can occur because of this. However, inconsistent cooling times can create other visual defects and may lengthen the manufacturing process.
The Best Practices for Selecting the Appropriate Wall Thickness
It isn’t always possible to maintain uniform injection molding wall thickness for a part. But even in those cases, manufacturers have several best practices to follow that can account for variances and help them achieve as much uniformity as possible.
Understand the Melt Material’s Wall Thickness Guidelines
The material used for the melt plays a huge role in determining a part’s ideal wall thickness. Every material is different, with some having lower tolerances for thickness than others. The following is a useful at-a-glance guide for some of the most common melt 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 Plastics: 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
Staying within these recommendations ensures the manufacturer avoids cooling and flow rate issues.
If Wall Thickness Varies, Make It Gradual
Uniformity isn’t always possible, especially if some areas of a part are thicker than others. In these cases, manufacturers must manage the transition from thick to thin or vice-versa carefully. Changes in wall thickness should be gradual because sudden changes can cause cooling issues. The example of shifting straight from a thin wall to a thick wall applies, as this sudden change can lead to the previously mentioned short shots.
Follow the Wall Uniformity Best Practices
There are several general best practices that manufacturers use to achieve uniform wall thickness:
· Regardless of variances, all walls should fall within the recommended thickness range for the material used.
· Use ribs to strengthen tall walls.
· Place radiuses on inside corners to strengthen them and alleviate the stress that causes warping.
· Avoid sharp corners, long unsupported areas, and poorly designed bosses.
· Keep draft consistent to avoid creating unnecessary internal stress. A good rule of thumb is to use one degree of draft for every inch of cavity depth.
Use Coring or Gussets to Avoid Sink Marks and Shadowing
Intelligent tweaks to a part’s geometry can alleviate the stress created by suboptimal wall thickness. For example, a manufacturer may use coring for parts shaped like dumbbells. This technique is similar to coring an apple, as it involves removing cross-sections of the material while keeping the sturdy core in place. Coring can help to prevent sink marks and reduces material wastage.
Furthermore, using gussets reinforces long and thin walls, such as those required for box lids. This prevents shadowing, which happens if one area of the part cools faster than another.
Understand the Material’s Key Properties
The material’s recommended injection molding wall thickness isn’t the only material consideration a manufacturer must account for. They must also consider the attributes of the intended product when selecting the material to use. Key questions for these considerations include:
· Can the material be painted or can the manufacturer add colorant to the melt before injection?
· Does the part need a high ultraviolet light or chemical resistance?
· Is the part intended to be subjected to extreme temperatures?
· Should the part be able to flex when under a heavy load?
· Will the part be used in an electromagnetic environment?
Knowing the answers to these questions ensures the manufacturer selects an appropriate melt material. With the material selected, they can then adapt the part’s design to account for that material’s recommended wall thickness.
Follow the Adjacent Wall Thickness Rule
The adjacent wall thickness rule states that the thickness of one wall should be no less than 40-to-60% of any wall adjacent to it. Going below this percentage range creates thin sections of a part that may be subject to short shots.
Furthermore, manufacturers must follow this rule while staying within the melt material’s recommended injection molding wall thickness guidelines.
Avoid Wall-Related Problems
here are many issues for a manufacturer to consider in injection molding. Wall thickness is among the most important because sudden variations in thickness can create visual and structural defects. Maintaining uniformity throughout is ideal. When that isn’t possible, the manufacturer must design for manufacturability by using the best practices in this article to account for wall thickness variations.