Injection molding, an evolved version of metal die casting, is one of the most economical methods for mass production of thermoplastic parts. The success of this efficient process hinges on mold design. Even a minor mistake in injection molding design can cause major defects, making parts non-functional.
To assist designers and product developers, this guide highlights 15 common injection molding design mistakes, their potential consequences, and practical strategies to address them early, before they lead to costly defects or production delays.
Flaws in the Geometric Design of Injection Mold
Physical features are one of the core areas where the chance of injection molding mistakes is higher. Product designers, even experienced ones, can make mistakes if they are unaware of the process’s inherent constraints. Standard injection molding design guidelines must be followed for geometric entities to avoid expensive redesigns.
Inconsistent or Improper Wall Thickness
Wall thickness directly influences material flow, cooling rates, and structural integrity. When wall thickness varies too much within a part, it can cause uneven cooling rates. This results in visible defects such as sink marks, where thicker areas cool more slowly and shrink inward. Warpage is another consequence of the difference in cooling rate.
Design experts suggest to generate a uniform wall thickness. Regarding the minimum thickness of a wall, it must be chosen in accordance with material properties. The table showcases some recommended minimum wall thicknesses for different injection molding materials so you can avoid wall-related design mistakes for injection molding project
| Material | Average Wall Thickness (mm) |
| Polycarbonate | 2.41 mm |
| ABS | 2.35 mm |
| Nylon | 1.84 mm |
| Polyethylene | 2.93 mm |
| Polypropylene | 2.79 mm |
| Polyurethane | 10.55mm |
| Polystyrene | 2.34mm |
Furthermore, there shouldn’t be an abrupt change in the thickness of sections. Ideally, the thin sections should be around 40 – 60% of the thicker sections.
Insufficient Draft Angles
Draft is the slight taper applied to vertical walls. Its primary purpose is a safe ejection of the finished part from the mold. If the draft angle isn’t left, ejection becomes difficult, and the part likely gets damaged during the extraction.
The standard industry practice is to provide at least 1 degree of draft per side for every inch (25.4mm) of cavity depth. For textured surfaces, additional draft is often recommended, an additional 1.5 degrees per 0.025 mm (0.001 inch) of texture depth.
Use of Sharp Corners
Sharp corners should be avoided wherever possible. In the molding process, plastic material flows into the cavity under high injection pressure, and sharp corners disrupt this flow, leading to uneven filling and air traps. Moreover, these corners are stress concentrators and prone to cracking during removal. From the manufacturing perspective, making a mold with sharp corners is also hard.
The remedy is to use fillets to distribute stress evenly and let the plastic flow smoothly in the mold. For internal corners, the fillet radius should be about 0.5 times the adjacent wall thickness. For external ones, it needs to be about 1.5 times the wall thickness.
Improper Rib Design
Ribs are thin, reinforced structures that run perpendicular to the main walls of a part. Their role is to enhance the structural integrity of parts with relatively thin walls. They also reduce the overall material usage.
To avoid any defects, rib height should not exceed three times the nominal wall thickness; ribs that are too tall can cause sink marks, voids, and difficulties in mold filling. And rib thickness, it is recommended to be about 40 – 60% of the nominal wall thickness to prevent excessive shrinkage and stress concentration.
Issue with Undercuts
Undercuts are protrusions or recessed features on the side of a part. While undercuts can add functionality or maybe improve aesthetics, they complicate the mold design and largely increase manufacturing costs. And even ejecting a part becomes difficult when undercuts are used.
Ideally, design a part that eliminates the use of undercuts. However, if they’re to be designed, they should be parallel to the drawing line. And special mechanisms, like lifters or sliders, may be used to push out the molded part. You can read more methods to effectively use undercuts in injection molding design in this post.
Material Selection Issues
Design isn’t only about the physical shape; material selection has its part as well. Here’s how the choice of material can affect the final outcome of the product.
Choice of Incompatible Materials
Since you know the material is injected in a molten state, it has to flow and fill the cavity. The flow characteristics vary largely from material to material. It’s imperative to consider that flow properties and as well shrinkage rate, when designing an injection molded part.
Crystalline materials like polypropylene (PP) and polyethylene (PE) typically have higher shrink rates (1.5% to 3%) compared to amorphous materials like ABS or polystyrene (0.2% to 0.7%). If the design part does not account for these shrinkage values, the part dimensions can be off, leading to poor fits, warpage.
Another serious issue arises when impurities or incompatible materials are introduced. Contaminants such as dust, moisture, oils, or recycled (regrind) material mixed with virgin resin can degrade the polymer matrix, leading to weak spots and surface defects.
Gate and Vent Design Flaws
Sizing and location of gates & vents are another key aspect among different injection molding design mistakes that directly impact the end results. Any error in the design of gates is often underestimated during the early stages of product development. Here’s how they should be designed:
Improper Gate Sizing
Gate size determines how quickly and efficiently the molten plastic enters the cavity. If the gate is too small, it can restrict flow, leading to incomplete filling (short shots), high shear stresses, and visible knit lines. On the other hand, an oversized gate can cause excessive flash (where material seeps into parting lines). Size gates to ~50–80% of the part’s nominal wall thickness. For example, a 2 mm wall typically uses a 1–1.6 mm gate.
There are different types of gates as well, whose right selection is critical too. Choose based on the geometric properties of the mold and the material being used. For instance, edge gate is the most economical option for thicker cross-sections and works pretty well with most types of resins.
Positioning of the gate also matters. Place gates away from critical cosmetic surfaces to avoid vestige marks.
Insufficient Venting
Venting allows trapped air and gases to escape from the cavity as it fills the mold. Without adequate venting, air pockets can become trapped, causing burn marks, voids, incomplete filling, or even part ejection problems. Sometimes the trapped my ignite and discolor parts.
The best design approach is to add vents at the end of fill areas (e.g., ribs, corners) and along parting lines. Use 0.01–0.03 mm deep vents for most materials.
Tooling and Manufacturability Obstacles
Failing to involve the manufacturing service provider or account for industry-standard injection molding practices during the design phase leads to significant manufacturability issues. Two of the most common pitfalls in this area include:
Non-Consideration of Parting Line Placement
The parting line is the dividing surface where the two halves of the mold, core, and cavity meet. Poor placement of this line can negatively impact the cosmetic appearance of the part and introduce defects. One common issue is flash, where molten plastic escapes through the small gap between the mold halves, forming thin, unwanted fins on the finished part.
To minimize visual defects, parting lines should ideally be positioned along sharp edges or natural transitions in the geometry, where they are less noticeable. Modern CAD tools often include parting line analysis features that can help designers optimize placement early in the design process.
Creating Features Impossible to Mold or Machine
Problems also arise when designers include features that are impossible or extremely difficult to mold or machine. For instance, if you include deep or complex undercuts, intricate internal geometries, or extremely thin walls that cannot be reliably formed or ejected. Such features may require advanced tooling solutions like side actions, lifters, or collapsible cores. The idea is to stay close to standard options and avoid any non-critical features to both save cost and time.
Tolerance and Precision Complications
In the quest for perfect parts, designers often fall into the trap of demanding extremely tight tolerances and complex geometries. Over-optimization of precision can cause issues in manufacturability, resulting in several injection molding mistakes.
Specifying Unnecessarily Tight Tolerances
Injection molding achieves dimensional tolerances in the range of about ±0.1 mm (0.004 inches) for most features. Designing parts with tolerances tighter than this standard not only drives up tooling and manufacturing process expenses but also increases the risk of part rejection.
When tolerances are set too narrowly, molds become more complex and costly to manufacture, and maintaining those tolerances consistently during production becomes challenging.
Therefore, it is best practice to design parts with realistic tolerances that align with the capabilities of injection molding. Reserving tighter tolerances only for critical features or mating surfaces. Otherwise, most parts can essentially function with the standard tolerances.
Surface Finish & Aesthetic Defects
We have already undergone multiple design issues that ultimately cause aesthetic issues in injection molded parts. Here are some critical ones that need a separate explanation.
Sink Marks on Cosmetic Surfaces
Sink marks are shallow depressions/dimples that appear on the surface of molded parts, particularly in areas where the material is thicker. These marks are a direct result of uneven cooling, i.e., when the outer surface solidifies before the inner material has fully cooled and contracted, the surface can cave in, leaving a visible blemish.
Sink marks result from inconsistent wall thickness, poor rib design, or inadequate packing pressure during molding. To avoid this defect, the design tip is to move with a uniform wall thickness, minimize thick sections, and ensure that the process parameters are optimized for even cooling and sufficient packing.
Gate or Ejector Pin Marks
Gate and ejector pin marks are small imperfections left on the part where the molten plastic entered the mold (the gate) or where pins pushed the part out of the mold (ejector pins). In most cases, that’s unavoidable; however, their visibility can be minimized with thoughtful design.
They can be considered design defects in the sense that their position was not accounted for in the design. Selection of the right gate and positioning it on non-cosmetic surfaces could fix the problem.
Gate Vestige
Gate vestige refers to the small protrusion or scar left on a part after the gate is trimmed or broken off following molding. While sometimes minor, an obvious vestige can be unsightly or interfere with the fit and function of the part, especially in assemblies with tight tolerances or visible surfaces.
Gate vestige is a design defect when the gate type or location has not been chosen with consideration for post-molding appearance and usability. To minimize gate vestige, designers can use gate types that allow for automatic trimming (such as tunnel or sub-gates), position gates on hidden or non-critical surfaces, and refine process parameters to ensure clean separation during ejection.
Post-Processing Difficulties
The journey of an injection molded part doesn’t end once it leaves the mold. It has to undergo some assembly, maybe finishing or packaging, to reach the consumer. The mold design should be made considering these post-processing steps.
Overlooking packaging and shipping requirements
Packaging and shipping are critical stages that protect parts from damage before they reach the customer. Designs that do not account for packaging constraints, such as part fragility, stackability, or susceptibility to abrasion, result in damaged goods. For example, thin-walled or delicate features may break during handling if not adequately supported or cushioned.
Summary Table: Common Mistakes in Injection Molding Design & Recommendations
A slight deviation in design parameters or a small mistake in injection molding design can cause failure in the functionality of molded items. The table below summarizes the common mistakes and counter-recommendations.
| Design Mistake | Counter Recommendation |
| Inconsistent or Improper Wall Thickness | Maintain uniform wall thickness & 40–60% if there is a thick and thin transition |
| Insufficient Draft Angles | minimum 1° draft per side |
| Sharp Corners | Use fillets (internal: 0.5×wall thickness; external: 1.5×wall thickness). |
| Improper Rib Design | Height ≤3× wall thickness & Rib thickness 40–60% of wall thickness |
| Undercuts | Use lifters/sliders if necessary |
| Choice of Incompatible Materials | Consider material properties like flowability, shrinkage, etc. |
| Improper Gate Sizing | Gate size 50–80% of wall thickness |
| Gate Positioning | Position gates away from cosmetic areas |
| Insufficient Venting | Maintain 0.01–0.03 mm deep vents. |
| Parting Line Placement | Place parting lines on edges or use natural transitions |
| Manufacturability of features | Stick to moldable geometries and standard tooling limits. |
| Tight Tolerances | Manufacturable tolerances (±0.1 mm typical) unless it is absolutely necessary. |
| Sink Marks | Design walls with consistent thickness and ensure that mold can be cooled uniformly |
| Visible Gate/Ejector Pin Marks | Place pins/gates on hidden surfaces |
| Packaging & Shipping Flexibility | Design parts to resist damage, allow stacking, and meet packaging needs. |
How RapidDirect Can Help Avoid Costly Errors in Injection Molding Design
If you’re a product owner or developer with a design project, you can avoid mistakes for injection molding design by following proven design guidelines and avoiding the common injection molding design mistakes many others make.
However, it’s even more effective to get technical support from a service provider like RapidDirect, which has hands-on experience in Injection Molding Services, turning designs into functional products, hundreds of times over.
RapidDirect can be your trusted injection molding partner, whether you need a few batches of prototypes or full-scale production. We have a dedicated team of engineers who are well-versed in every stage of the injection molding process, from early design validation to final production. By working with us, you can catch potential issues early, reduce rework, and bring your product to market faster and with greater confidence.
