Injection Molding Design Guide: Snap-fit and Part Wall Thickness

In my previous article, I introduced key Concepts and Draft Angles that form the foundation of good injection molding design. Building on that, this article dives deeper into two critical design considerations for custom injection molded parts: snap-fit structures and part wall thickness.

Whether you're engineering custom machined plastic parts or designing complex assemblies with snap-fit connections, understanding these principles will help you avoid costly mold defects and quality failures.

Reserve Sufficient Clearance for Lifters (Slides) in Snap-fit Structures

Snap-fits are one of the most widely used assembly methods in plastic part design. They are formed using side-core pulling structures in the mold — specifically lifters  or slides — which move laterally to release the undercut geometry during ejection.

When the part is ejected from the mold, the lifter or slide must travel through a defined stroke to withdraw from the snap-fit feature. If insufficient space is provided for this motion, two problems can occur:

• The lifter or slide cannot retract— making demolding impossible.
• The lifter or slide collides with adjacent features(such as bosses or ribs) during retraction — causing mold damage or part defects.

Design Guidelines

When designing snap-fit structures for custom injection molded parts, engineers must:

1. Map the full retraction pathof the lifter or slide during mold opening and part ejection.
2. Ensure no other features— such as bosses, ribs, or walls — fall within the lifter's travel envelope.
3. Maintain adequate clearancebetween the snap-fit and surrounding features so that the side-core mechanism can move freely without interference.
4. Communicate with your mold designer early— snap-fit geometry should be reviewed alongside the mold action plan before tooling begins.

Neglecting lifter clearance is one of the most common and expensive mistakes in the design of custom machined plastic parts and injection-molded components. It often requires costly mold modifications or even complete tooling redesigns.

Avoid Thin Steel Sections (Thin Walls in the Mold) and Low-Strength Mold Designs

When two features on a plastic part are placed very close together, the corresponding area in the mold becomes an extremely thin steel section — commonly referred to as "thin steel". This is a critical mold design concern.

Thin steel sections in a mold suffer from:

• Reduced mold strength— thin sections are prone to bending, cracking, or breaking under injection pressure.
• Shortened mold life— frequent stress cycling causes fatigue failure in thin areas, requiring premature mold repair or replacement.
• Inconsistent part quality— a damaged mold produces flash, dimensional variation, or surface defects on the finished part.

Design Guidelines

To avoid thin steel conditions when designing custom injection molded parts:

1. Maintain sufficient spacingbetween adjacent features such as ribs, bosses, holes, and snap-fits. As a general rule, the distance between two features should allow for a mold steel wall that is robust enough to withstand repeated injection cycles.
2. Avoid parallel walls that are too close— features like twin bosses or paired ribs should be spaced far enough apart to leave adequate mold steel between them.
3. Review the mold steel section at all critical areasduring the DFM (Design for Manufacturability) analysis phase.
4. Consult your mold makerwhen features must be positioned close together — they can advise on minimum steel thickness based on the mold material and expected shot count.

This principle applies equally to custom machined plastic parts produced via CNC machining from plastic stock, where thin section geometry can similarly affect structural integrity and machining accuracy.

Part Wall Thickness

Wall thickness is arguably the most fundamental parameter in plastic part design. It directly affects:

• Mechanical performance— strength, stiffness, and impact resistance
• Part appearance— surface quality and sink marks
• Injectability— how easily molten plastic fills the mold cavity
• Production cost— cycle time and material consumption

The wall thickness decisions made during the design phase largely determine whether a custom injection molded part will succeed or fail — both technically and commercially.

Wall Thickness Must Be Within an Appropriate Range

Due to the characteristics of plastic materials and the nature of the injection molding process, wall thickness must fall within a suitable range — not too thin, and not too thick.

Too Thin

• High flow resistance during injection makes it difficult for the melt to fill the entire cavity.
• Requires higher-performance injection equipment to achieve the necessary fill speed and pressure.
• Risk of short shots, surface defects, and weak weld lines.

Too Thick

• Cooling time increases significantly — statistically, doubling wall thickness increases cooling time by approximately 4×.
• Longer cycle times reduce production efficiency and increase manufacturing cost.
• Excessive thickness is a primary cause of sink marks, voids (gas pockets), and warpage.

Recommended Wall Thickness by Material

Different plastic materials have different optimal wall thickness ranges. Even different grades of the same material from different suppliers can vary. The table below provides typical recommended ranges for common plastics used in custom injection molded parts:

Material	Recommended Wall Thickness (mm)
ABS	1.5 – 4.5
PP (Polypropylene)	0.8 – 3.8
PA (Nylon)	0.8 – 3.0
PC (Polycarbonate)	1.0 – 4.0
POM (Acetal)	1.5 – 5.0
PMMA (Acrylic)	1.5 – 5.0
PE (Polyethylene)	0.8 – 3.0
PVC	1.5 – 5.0

Note: When wall thickness approaches the upper or lower limits of the recommended range, designers should consult the material supplier for specific guidance.

Minimize Wall Thickness Where Possible

The minimum usable wall thickness for custom injection molded components is determined by a range of practical engineering factors, which are outlined below:

1. Structural performance – Increasing wall thickness can effectively boost the overall rigidity and durability of molded parts. However, excessive thickness will trigger manufacturing flaws including internal voids and surface sink marks, which will instead compromise the structural stability of finished products.
2. Demolding stress tolerance – Components with overly thin walls are prone to bending, distortion or cracking when being released from injection molds during mass production.
3. Assembly stress resistance – Part walls need to endure external mechanical force generated in product assembly processes, including clamping fixation, screw installation and snap-fit connection without damage.
4. Embedded insert stability – For parts fitted with metal inserts, the plastic material wrapping these inserts requires sufficient thickness. This offsets concentrated mechanical stress brought by the inconsistent shrinkage rates of metal and plastic materials during molding and cooling.
5. Impact force dispersion – Proper wall thickness enables molded parts to evenly spread external impact force across the component structure, avoiding local fracture or deformation.
6. Structural robustness of holes and weld lines – Pre-fabricated holes weaken the mechanical strength of adjacent plastic materials. This structural weakness becomes more prominent at weld lines formed by the convergence of molten plastic flows during injection.

Within the constraints above, wall thickness should be minimized — thinner walls mean less material, lighter parts, shorter cooling times, and lower overall production costs.

Design Tip: Many designers instinctively increase wall thickness to improve part strength. In practice, a better approach is to add ribs, incorporate curved cross-sections, or use corrugated profiles — these techniques improve stiffness and strength without the cost and quality penalties associated with excessive wall thickness. This is equally relevant for custom machined plastic parts, where material efficiency directly impacts cost.

Maintain Uniform Wall Thickness

The ideal plastic part has uniform wall thickness throughout every cross-section. Non-uniform wall thickness leads to uneven cooling and shrinkage, which causes:

• Sink markson the surface opposite thick sections
• Internal voids(gas pockets) in thick areas
• Warpage and distortionof the overall part
• Dimensional inconsistencythat makes tight tolerances difficult to achieve

Design Strategies for Uniform Wall Thickness

• Taper thick sections graduallyinto thinner areas rather than making abrupt thickness transitions.
• Core out thick sections— remove material from the interior of thick walls to bring thickness within the recommended range while preserving the outer profile.
• Avoid adding material to one side only— if extra thickness is needed for strength, distribute it symmetrically.
• Use ribs instead of solid thick walls— ribs provide stiffness without the penalties of a solid thick section.

When designing custom injection molded parts, wall thickness uniformity should be reviewed at every cross-section of the part — not just at the most visible or structurally critical locations.

Summary

Successful injection molding design requires attention to multiple interdependent factors. The three principles covered in this article — snap-fit lifter clearance, avoidance of thin mold steel, and wall thickness design — are among the most impactful for both custom injection molded parts and custom machined plastic parts.

Design Principle	Key Risk if Ignored	Best Practice
Snap-fit lifter clearance	Mold damage, demolding failure	Map full lifter travel path; clear all surrounding features
Avoid thin mold steel	Short mold life, part defects	Maintain adequate spacing between adjacent features
Appropriate wall thickness	Short shots, sink marks, warpage	Stay within material-specific recommended range
Minimize wall thickness	High cost, long cycle times	Use ribs and curved profiles instead of thick walls
Uniform wall thickness	Warpage, voids, dimensional errors	Core out thick areas; taper transitions gradually

In the next article, I will explore how to avoid sharp corners in injection molding design — another common source of stress concentration, mold wear, and part failure.To learn more about injection molding, please contact LVMA.