Injection molding is one of the most widely used manufacturing processes in the world, responsible for producing everything from automotive components and medical devices to consumer electronics and packaging. The global injection molding market was valued at approximately USD 298 billion in 2024 and is projected to reach over USD 462 billion by 2033, growing at a CAGR of 5.0%. With that kind of scale, even small design inefficiencies can translate into significant cost overruns and quality issues across production runs.
The reality is that many of the most common defects in injection molding, warping, sink marks, short shots, flash, can be traced directly back to decisions made at the design stage. Poor part design doesn’t just create headaches during production; it leads to expensive mold modifications, wasted materials, extended cycle times, and delayed time-to-market. On the other hand, a well-designed part that follows proven injection molding principles can reduce tooling costs, improve part quality, and streamline the entire manufacturing process.
This article covers the essential design guidelines that every engineer should understand before sending a part to production. Whether you’re a seasoned product engineer or new to designing for injection molding, these principles will help you create parts that are manufacturable, cost-effective, and built to perform.
Maintain Uniform Wall Thickness
If there is one rule that sits at the foundation of every successful injection-molded part, it is this: keep your wall thickness consistent. Uniform wall thickness is the single most impactful design decision you can make because it directly influences how molten plastic flows into the mold, how evenly the part cools, and how dimensionally stable the finished component will be.
When wall thickness varies significantly across a part, sections cool at different rates. Thicker areas take longer to solidify, while thinner areas cool quickly. This differential cooling creates internal stresses that manifest as warping, sink marks, voids, and dimensional inaccuracies. Thick sections are particularly problematic because they can form internal voids as the outer skin solidifies before the core, pulling material inward and leaving hollow pockets that weaken the part.
As a general guideline, wall thicknesses for most injection molded parts should fall within the range of 0.080 inches to 0.160 inches (2.0 mm to 4.0 mm), though this varies by material.
Polypropylene, for example, can be molded with walls as thin as 0.025 inches, while polycarbonate typically requires thicker walls of at least 0.040 inches due to its higher viscosity. The key is not the absolute thickness but the uniformity across the part. If thickness transitions are absolutely necessary, use gradual tapers or fillets, never abrupt steps, to smooth the change and prevent stress concentrations that lead to defects.
Apply Proper Draft Angles
Draft angles are the slight taper applied to vertical surfaces of a part to allow it to release cleanly from the mold during ejection. Without adequate draft, parts can stick to the mold, causing surface scuffing, drag marks, or even damage to the tool itself. Despite being one of the most fundamental requirements in mold design, insufficient draft remains one of the most common design oversights engineers make.
The general rule of thumb is to apply 1 to 2 degrees of draft per side for most surfaces, with a baseline of 1 degree of draft per 1 inch of cavity depth. Textured surfaces require additional draft, typically an extra 1.5 degrees per 0.001 inches of texture depth, because the rough surface increases friction during part ejection. For very deep draws or tall features, draft angles of 3 to 5 degrees may be necessary.
As a general principle, always remember: any draft is better than no draft. Even 0.5 degrees offers a meaningful improvement over zero draft. If your design constraints make it difficult to incorporate standard draft angles, discuss the limitations with your molding partner early in the process. They can often suggest creative solutions, such as adjusting parting lines or using side actions, to accommodate the design while maintaining manufacturability.
Design Ribs and Bosses Correctly
Ribs and bosses are among the most common structural features in injection molded parts. Ribs add stiffness and strength without increasing overall wall thickness, while bosses serve as mounting points for screws, inserts, and assembly hardware. However, improperly designed ribs and bosses are a leading cause of cosmetic defects, particularly sink marks, and can introduce structural weaknesses if not sized appropriately.
For ribs, the established guideline is to keep rib thickness at 50–60% of the adjacent wall thickness. A rib that is too thick relative to the wall creates a localized mass of material that cools more slowly than its surroundings, resulting in visible sink marks on the opposite side. Rib height should generally not exceed 3 times the rib thickness, and the spacing between ribs should be at least 2 times the nominal wall thickness. A base fillet with a radius of at least 25% of the rib thickness should be added at the junction with the wall to reduce stress concentrations and improve material flow during filling.
Bosses follow similar principles. The outside diameter of a boss should be 2.0 to 2.4 times the outer diameter of the screw or insert it is designed to receive. The internal diameter for self-tapping screws should be approximately 0.8 times the nominal screw diameter. Importantly, bosses should not be merged directly into sidewalls, as this creates excessively thick sections. Instead, connect bosses to adjacent walls using thin ribs or gussets that maintain structural support without concentrating material.
Use Generous Radii and Fillets
Sharp corners are a serious concern in injection molding for two reasons. First, they act as stress concentrators in the finished part, creating weak points where cracks are most likely to initiate under load or impact. Second, sharp internal corners impede the flow of molten plastic during filling, causing pressure spikes, hesitation marks, and incomplete packing of the mold cavity.
The standard recommendation is to use internal corner radii of at least 0.5 times the adjacent wall thickness. External radii should match the internal radius plus the wall thickness to maintain uniform cross-sections around corners. Avoid 90-degree internal corners wherever possible. The small investment of adding fillets pays significant dividends in part strength, surface finish quality, mold longevity, and reduced cycle times by promoting smoother material flow and more even cooling.
Optimize Gate Location and Design
The gate is the small opening through which molten plastic enters the mold cavity, and its location has a profound effect on part quality. Gate placement influences how material fills the cavity, where weld lines form, how pressure distributes during packing, and where aesthetic blemishes appear on the finished part. Understanding the injection molding process at this level is essential for producing defect-free components.
Gates should be positioned to promote balanced, uniform filling of the mold cavity. Ideally, the gate directs material flow into the thickest section of the part first, allowing it to flow progressively toward thinner areas. This prevents the material from freezing prematurely in thin sections before thicker areas are adequately packed. Gates should also be placed on non-cosmetic or hidden surfaces whenever possible, since the gate will leave a vestige mark on the finished part.
Weld lines, the visible seams that form where two flow fronts meet, should be positioned away from high-stress areas and cosmetic surfaces. Engineers and mold designers often use mold flow simulation software to predict fill patterns, pressure distributions, and weld line locations before cutting steel. This kind of upfront analysis can prevent costly mold revisions and production delays down the line.
Minimize and Manage Undercuts for Parts Design
Undercuts are features that prevent a part from being ejected straight out of the mold in the direction of the draw. Common examples include holes, hooks, snap-fit features, and internal threads that run perpendicular to the mold’s opening direction. While undercuts are sometimes necessary for functional reasons, they add significant complexity and cost to mold tooling because they require side actions, lifters, or collapsing cores to release the part.
The first rule of undercut management is to eliminate them from the design wherever possible. Often, a slight redesign, such as repositioning a feature, adjusting the parting line, or incorporating pass-through holes that can be formed by opposing core pins, can remove an undercut entirely without sacrificing the part’s function. When undercuts are truly essential, work with your injection molding partner to determine the most cost-effective tooling solution. Side actions are common but increase both mold complexity and cycle time, so their use should be deliberate and justified.
Account for Material Shrinkage
All plastics shrink as they cool and transition from a molten state to a solid one. The degree of shrinkage varies significantly by material type and is one of the most critical factors in achieving dimensional accuracy. Amorphous polymers like ABS and polycarbonate typically shrink between 0.4–0.7%, while semi-crystalline materials like polypropylene and nylon can shrink anywhere from 1.0–2.5% or more.
Failing to account for these differences during the design phase leads to parts that don’t meet dimensional specifications, create assembly problems, or require expensive mold modifications. Selecting the right material for injection molding is therefore not just about mechanical properties; it’s about understanding how the material will behave throughout the entire molding cycle.
Shrinkage also tends to be non-uniform. Parts with varying wall thickness, reinforcing fibers, or complex geometries may shrink differently in different directions, leading to warpage. Engineers should work closely with their material suppliers and molding partners to obtain accurate shrinkage data for the specific grade and conditions being used, and incorporate those values into the mold design from the outset.
Design for Proper Venting
As molten plastic fills a mold cavity, it displaces the air that was already inside. If that air has no way to escape, it becomes trapped and compressed, leading to a range of defects, including burn marks, short shots (incomplete filling), and surface blemishes. Proper venting is a critical but often overlooked aspect of both part design and mold engineering.
While vent channels are primarily the responsibility of the mold designer, the part designer can influence venting outcomes by avoiding extremely thin features or dead-end geometries that trap air. Smooth, gradual flow paths that direct air toward the parting line or designated vent locations help ensure complete cavity filling. For complex multi-cavity molds or parts with intricate geometries, computational flow analysis is invaluable for identifying potential air traps before the mold is manufactured.
Set Realistic Tolerances
Tolerances define how much a finished part’s dimensions are allowed to deviate from the nominal design. In injection molding, the typical achievable tolerance is ±0.010 inches (±0.25 mm), though tighter tolerances of ±0.002 to ±0.005 inches are possible with precision tooling, controlled processing, and careful material selection.
Over-specifying tolerances is one of the most common and costly mistakes in injection molding design. Tighter tolerances require more precise mold machining, more controlled processing conditions, and more rigorous quality inspection, all of which increase cost. Engineers should critically evaluate which dimensions are truly functional and specify tight tolerances only where they are genuinely needed for fit, function, or assembly. Non-critical dimensions should be left at standard tolerances to keep manufacturing costs reasonable.
It is also important to understand that plastic parts naturally exhibit more dimensional variation than machined metal components. Material behavior, mold temperature, and process conditions all influence the final dimensions. Collaborative discussions between the design engineer and the injection molding team about which tolerances matter most for the application will lead to better outcomes for everyone.
Leverage Simulation and Mold Prototyping Early
Modern mold flow simulation tools allow engineers to virtually test part designs before committing capital to tooling. These simulations predict how molten plastic will fill the cavity, where pressure drops and weld lines will occur, how the part will cool, and where it is most likely to warp or develop sink marks. Investing in simulation during the design phase is one of the most cost-effective decisions an engineering team can make, because changes to a CAD model cost a fraction of what mold modifications cost once steel has been cut.
Rapid prototyping through 3D printing or CNC machining provides another critical validation step. Prototypes allow engineers to verify form, fit, and function in production-equivalent materials before committing to full production molds. This iterative approach, designing, simulating, prototyping, and refining, is standard practice in industries with stringent quality requirements, such as medical device manufacturing, where even small design oversights can have serious consequences.
For engineers working across multiple industries, early collaboration with an experienced injection molding partner can surface manufacturability issues that might otherwise go undetected until production. A good manufacturing partner will provide Design for Manufacturability (DFM) feedback that helps optimize the part design for both quality and cost before the first mold is built.
Bringing It All Together for Injection Molding
Designing for injection molding is not about memorizing a checklist; it is about understanding how material behavior, mold mechanics, and part geometry interact to determine the quality and cost of the final product. The guidelines covered in this article are the building blocks of a manufacturable, high-quality part.
The engineers who consistently produce the best results are those who engage with their manufacturing partners early in the design process. Injection molding is a collaborative discipline, and the best outcomes come from open communication between design, tooling, and production teams.
KS Manufacturing offers full-service injection molding solutions backed by over 45 years of manufacturing expertise across industries including medical devices, automotive, electronics, and consumer products. Our end-to-end capabilities span design consultation, material selection guidance, precision mold fabrication, validation testing, and full-scale production.
Whether you are developing a new product or looking to optimize an existing component, our engineering team can provide the DFM feedback and production expertise needed to bring your design to market efficiently and at the highest quality standards. Contact our team today to discuss your project.