Undercuts perpendicular to the parting line trap parts in standard molds. Side action injection molding solves this with lateral inserts that retract before ejection—unlocking complex geometries without secondary operations. This technology allows you to create sophisticated parts with fewer assembly steps and lower long-term costs.
This guide explains what is side action in injection molding, how cam-driven mechanisms work, and compares various injection molding types of side actions. You will also find essential design rules to help you create reliable, high-quality, and cost-effective tooling for your next project.
What Is Side Action in Injection Molding?
Understanding “what is side action in injection molding” is best accomplished by reviewing how the basic mold opens. A standard mold moves straight down; however, many parts contain “undercuts” (like holes or snaps) that would be trapped by the molding process if it only moved vertically. Side actions are lateral mold inserts that form these undercuts by moving sideways, perpendicular to the main parting line.
These inserts are programmed to retract via mechanical cams, hydraulics, or pneumatics before the part is ejected. This clever movement enables the creation of threads, snaps, and internal pockets that are simply impossible with straight-pull molds. While there is a trade-off in higher initial tool costs, the investment pays off by eliminating expensive secondary assembly steps and manual machining.
How Side Action Works?
The movement of an injection molding side action follows a precise sequence during the molding cycle to ensure the part is formed correctly and released without any damage.
Mold Closure – Engagement
As the mold closes, angle pins or cams drive the side action into the cavity. Manufacturers must ensure precision guidance so the insert sits in the exact spot required to form the undercut. This stage sets the foundation for a dimensionally accurate part before any plastic enters the tool.
Injection – Holding
Once the mold is closed, the side action is locked firmly in place to withstand the massive injection pressure. This prevents “flash,” which is extra plastic leaking into gaps. Molten material flows around the insert, perfectly capturing the complex geometry you designed into your CAD model.
Mold Opening – Retraction
After the plastic cools, the mold begins to open and the cams reverse their movement. The injection molding side action is retracted in the lateral direction – pulled out of the undercut area. Various hydraulic and mechanical actuators are often used to manage the timing of these movements to ensure the insert is completely out of the way of the part before the actual part movement takes place.
Ejection – Release
With the side action retracted, the part is finally freed from the undercut constraint. Standard ejector pins then push the part out of the mold. Because the side action moved out of the way first, the part can be removed smoothly without any tearing or deformation of its features.
Different Types of Side Actions
Understanding the various mechanisms available helps you choose the most efficient way to form your part’s unique features while keeping production costs under control.
Slides – External Undercut Specialists
The most common side action devices used for creating external features (like threads, snaps, or ribs) use slides. These slides are typically driven with cam pins or hydraulic cylinders; they move in a straight line perpendicular to the primary mold opening direction, typically at right angles.
These mechanisms are incredibly robust and ideal for high-volume production. Because the slide is held in place by a “heel block” during injection, it can withstand extreme pressure without shifting. This ensures that the external features on your parts remain consistent and within tight tolerances throughout the entire run.
Lifters – Internal Undercut Solution
When you need to form internal features like tabs or bosses that don’t have a draft, lifters are the preferred choice. These are driven by the vertical movement of the ejection plate. As the plate moves up, the lifter travels at an angle to release the internal undercut.
Lifters are especially helpful when there is limited internal space in the mold. The manufacturer must calculate the angle of travel very carefully to ensure the lifter clears the part geometry completely before final ejection; if not cleared, the part could be damaged internally.
Unscrewing Actions – Threaded Features
If your part requires consistent, high-quality screw threads, unscrewing actions are the professional solution. These mechanisms use motor-driven or hand-cranked rotation to “unscrew” the mold insert from the finished part. This rotation is synchronized perfectly with the mold opening to prevent stripping the plastic threads.
This method provides much better thread quality than simple “strip” molds or lifters. It is essential for parts like bottle caps or technical connectors where thread pitch control is critical. While the tooling is more complex, it guarantees that every part will thread onto its mate perfectly every time.
Collapsible Cores – Circular Internal Features
Collapsible cores consist of segmented pieces that collapse inward toward the center to release a part. They are the ideal choice for large internal threads or deep circular undercuts found in caps and closures. This mechanism allows for a 360-degree undercut that would be impossible with standard slides.
Due to the complicated design, this mechanism requires a larger tooling investment than a standard mold design; however, it can produce parts at speeds unmatched by any other manner, making it desirable for high-volume producers. Manufacturers can produce parts very quickly while still maintaining the integrity of the circular features produced, which must remain perfectly round and dimensionally consistent.
Hand-Loaded Cores – Low-Volume Alternative
For prototypes or small orders under 1,000 units, hand-loaded cores are a great way to save money. These are loose metal pieces that an operator manually places into the mold before each cycle. After the part is made, the operator pulls the insert out of the plastic by hand.
This approach significantly reduces the upfront cost of the mold since no complex cam or hydraulic systems are required. It also offers flexibility, as one tool can often accommodate multiple insert configurations. However, because it requires manual labor, the per-part cost is higher and cycle times are much longer.
| Type | Undercut Location | Actuation | Cost Level | Volume Suitability |
| Slides | External | Cam/hydraulic | Medium | High |
| Lifters | Internal | Ejector plate | Low-Medium | Medium-High |
| Unscrewing | Threads | Motor/manual | High | High |
| Collapsible Core | Circular internal | Mechanical | High | High |
| Hand-Loaded | Any | Manual | Low | Low |
Advantages of Side Action in Injection Molding
Side actions are more than just moving parts; they are strategic tools that improve your product quality and reduce your total manufacturing expenses.
Design Freedom for Complex Geometries
Standard straight-pull molds force you to design simple, blocky shapes. By using injection molding side action, you can include snaps, threads, and interlocking features that would otherwise be impossible. This gives your engineers the freedom to design parts that truly fit their functional requirements.
Elimination of Secondary Assembly
When a part can be molded with all its features in one shot, you eliminate the need for secondary assembly. This reduces the number of parts in your Bill of Materials (BOM) and prevents “tolerance stack-up” issues. You save significant labor costs and reduce the risk of assembly errors.
Improved Part Consistency
Post-mold machining often introduces variability and can lead to handling damage. Features formed directly by the mold maintain much tighter tolerances and better surface finishes. This ensures that every robot part or CNC component you receive meets your exact engineering specifications without deviation.
Reduced Per-Part Cost at Scale
While the initial mold cost is higher, the investment is spread across high production volumes. By avoiding manual labor and secondary machining, the unit cost drops significantly. For long-term projects, side actions are almost always the most economical way to produce complex, high-quality plastic components.
Design Guidelines for Side Action Molds
To get the most out of your tooling, you must follow specific design rules that ensure the side actions operate reliably over time.
Minimize Undercut Depth
Keeping undercuts as shallow as possible is a key goal for a successful mold design. Shallow features require less travel for the slides and smaller angles for the cams, which significantly reduces wear and tear. If your part allows it, consider using simple slots or bypasses to achieve the same function.
When you minimize depth, the mechanical stress on the angle pins drops. This leads to a longer mold life and fewer maintenance headaches over time. Manufacturers often suggest small design modifications that can save you thousands in repair costs without changing how your final product works or fits.
Optimize Parting Line Placement
Injection molding side action intersections should be placed away from visible, cosmetic surfaces. Where the slide meets the main mold, a small line will usually appear on the plastic part. Balancing the flow of plastic is also critical to prevent weak “weld lines” from forming at these mechanical joints.
Smart parting line placement also improves the structural strength of your part. By keeping these lines away from high-stress areas, manufacturers ensure the part won’t break under pressure. Proper placement makes the mold easier to build and keeps any visible marks hidden on non-critical areas of your product.
Ensure Adequate Cooling
Side actions create thick steel sections that trap heat, often leading to sink marks or warping on your plastic parts. To solve this, manufacturers integrate dedicated cooling channels directly into the slide bodies. In high-heat zones, beryllium copper is often used because it moves heat away much faster than standard steel.
Recent research cited in Structural and Multidisciplinary Optimization shows that topology-optimized conformal cooling channels can reduce temperature deviation by up to 43%. This advanced approach can shorten cooling cycles by as much as 70% compared to traditional straight-drilled channels, ensuring better dimensional stability for your complex injection molded components.
Design for Maintainability
All moving parts will eventually wear out, and worn slides are a leading cause of flash and dimensional drift. If designed with accessible wear plates or replaceable inserts, the mold will be able to accommodate wear while being quickly repaired since it uses standardized items to complete these repairs.
A maintenance-friendly design minimizes overall downtime; therefore, if a wear plate needs replacement, it can be done in minutes rather than hours, helping keep the project on schedule. Manufacturers that prioritize maintainability in their mold designs can continue to produce “like new” parts, even after hundreds of thousands of cycles.
Synchronize Movements Precisely
If a side action retracts too late or an ejector pin fires too early, the part—and the mold—can be severely damaged. Manufacturers use interlock controls, sequence valves, and position sensors to ensure perfect timing. This synchronization is the “brain” of a complex side-action injection mold.
Modern sensors can detect even a fraction of a millimeter of misalignment. This safety net prevents catastrophic tool crashes that could take your production offline for weeks. By ensuring every movement is perfectly timed, manufacturers protect your investment and guarantee a smooth, uninterrupted manufacturing process for your order.
Manufacturing of Side Action Molds
The creation of these tools requires a high level of expertise and precision at every stage of the manufacturing process.
Design Validation
Before any metal is cut, manufacturers perform a CAD simulation of the cam kinematics to check for interference. Mold flow analysis is also used to predict where weld lines and air traps might occur. This digital testing catches potential problems early, saving time and preventing expensive mistakes.
Material Selection
Choosing the right steel is vital. Slides are usually made from H13 or S7 tool steel, heat-treated to a hardness of 48–52 HRC. For moving parts, wear plates are often made from bronze-graphite alloys that are self-lubricating. This combination reduces friction and prevents the metal parts from seizing up.
Precision Machining
Manufacturers use high-end CNC milling to cut slide pockets with extreme accuracy—within ±0.01 mm. For very detailed shapes, wire EDM (Electric Discharge Machining) is used to slice through metal with a tiny wire. Finally, the surfaces are ground flat to ensure the mold seals perfectly under high pressure.
Heat Treatment & Surface Engineering
Vacuum hardening is used to strengthen the steel while minimizing any distortion or warping. To further protect the tool, a PVD (Physical Vapor Deposition) coating is often applied to sliding surfaces. This surface engineering reduces friction and prevents “galling,” which is when metal surfaces tear each other during movement.
Assembly & Testing
To ensure perfect timing between side action retraction and part ejection, manufacturers can now leverage real-time monitoring technologies. According to Fraunhofer IST, thin-film sensors combined with machine learning can capture mold data to predict dimensional drift instantly. This high-tech approach enables immediate process adjustments that protect both your part and the expensive side action tooling from damage.
Final Words
Mastering the different types of side actions turns design challenges into a competitive edge for your business. By using these clever mechanisms, you can combine multiple parts into one, improve quality, and get your products to market much faster.
Fecision specializes in high-precision side action injection molding to handle your most complex undercut requirements. We utilize advanced CNC and EDM machining to ensure every slide and lifter operates with zero-play movement. This technical precision prevents flash and ensures that integrated features, like internal threads and side snaps, remain dimensionally perfect throughout high-volume production runs.Contact Fecision today to get a quote and professional DFM feedback for your next complex project.
