Omar Khalid
In plastic design, undercuts are a crucial aspect of the injection moulding process. They are used to create complex shapes and features in moulded parts, such as indentations, protrusions, holes, cavities, and recessed areas. Undercuts can complicate or even prevent the ejection of the moulded part from the mould, requiring additional techniques and technologies such as side-actions, lifters, and collapsible cores. These solutions enable the creation of intricate and detailed plastic components with enhanced functionalities. While undercuts increase design complexity and cost, they are commonly used in various industries, including consumer electronics, automotive, and medical products.
| Characteristics | Values |
|---|---|
| Definition | Any indentation or protrusion that prohibits part ejection from a mold. |
| Use | Achieve complex plastic part designs for the injection molding process. |
| Examples | Threads on an injection-molded fastener, the hole in a plastic hose barb, the slot for the power switch in a smartphone case, angled bosses, locking tabs, etc. |
| Techniques | Side-action cams, bump-offs, inserts, and other complex injection molding techniques. |
| Challenges | Undercuts can complicate and prevent part ejection. |
| Solutions | Angle lifters, dovetail collapsible cores, flexing steel collapsible cores, etc. |
| Considerations | Part ejection, mold design, material flexibility, and cost. |
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What You'll Learn
Injection moulding
Undercuts in plastic design refer to indentations or protrusions in the moulded part that prevent its direct ejection from the mould. These undercuts can be internal or external, and they require special techniques to be successfully incorporated into the injection moulding process.
There are several ways to address undercuts in injection moulding. One common method is to use a side-action or side pull, which involves adding an extra piece to the mould that moves separately from the two halves. This allows for the creation of features such as holes, vents, slots, and protrusions. However, this method increases the cost and complexity of the process.
Another technique is to use hand-loaded inserts, where metal pieces are manually placed in the mould before injecting the plastic. These inserts prevent the plastic from flowing into specific areas, creating the desired undercut shape. While this method is effective, it increases the injection moulding cost and cycle time due to the manual labour involved.
To optimise the design and minimise costs, it is recommended to eliminate unnecessary undercuts whenever possible. Additionally, choosing flexible materials that can deform enough to exit the mould without damage, such as Low-Density Polyethylene (LDPE) or Thermoplastic Elastomer (TPE), can simplify the process.
In some cases, moving the parting line, which is the intersection plane between the two mould halves, can help address undercuts. By positioning the parting line on the protruding feature, the undercut problem can be avoided, as the part can be ejected without the need for undercuts.
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Complex shapes
Injection molding is a process used to create complex plastic parts with unique shapes and features. An undercut is any indentation or protrusion that prevents the direct removal of the mold from the final product. Undercuts are often used to create threaded parts, such as screw-on bottle caps, snap-on products, and a variety of consumer goods.
Undercuts can be challenging to incorporate into plastic part designs. They increase the complexity of the mold and the overall production costs. The more complicated the arrangement is, the more issues you’ll encounter with the undercut design. For instance, it is essential to control draft angles, cavity angles, and other complex angles for optimal performance. The hardness of the material also affects the ease of removing the mold. Harder materials, like glass-filled plastics, are more challenging to extract from the mold.
To overcome these challenges, designers and molders can employ various techniques. One method is to use a bump-off or small undercut that can be safely removed from a straight-pull mold without side actions. The shape of the undercut is crucial for successful removal. A "ramp" or radius shape allows the undercut to ride over the edge of the groove during ejection, similar to a car riding over a speed bump. The choice of resin is also critical, as it needs to stretch and bend during ejection and then return to its original shape. Resins like TPE or unfilled polyethylene are flexible enough for this purpose.
Another technique is to use side-action cams, inserts, and other complex injection molding methods. Telescoping shutoffs, or sliding shutoffs, are often used to create clip- and hook-style mechanisms. These can eliminate the need for side-actions, inserts, and bump-offs, simplifying the mold design and reducing costs. However, sliding shutoffs must be exceedingly tight to prevent the plastic from developing past the desired shape.
Additionally, advanced technologies like collapsible cores have expanded the capabilities of undercut molding. Dovetail collapsible cores, for example, improve cycle time, reduce costs, and enhance reliability. These cores collapse radially inward during the normal mold sequence, eliminating the need for complex coring approaches and reducing cycle times.
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Ejection techniques
One critical factor in the ejection process is the draft angle, which helps to ease the removal of the moulded component from the mould. Insufficient draft can make it difficult to manually remove the side action, especially when using harder materials like glass-filled plastics. Therefore, it is recommended to incorporate as much draft as possible when working with these materials. Additionally, the mould surfaces should be carefully polished to prevent scratches and indentations that can hinder smooth ejection.
Another technique to consider is the use of hand-loaded inserts. In complex designs with sharp angles or awkwardly placed features, machinists may insert metal pieces into the mould before injecting the plastic. These pieces create cavities by preventing the plastic from flowing into certain spaces. After the moulding is complete, the operator must manually remove the inserts.
Side-action cores are another type of insert used in ejection techniques. These are introduced before injecting the plastic into the mould, and the plastic cannot fill the volume occupied by these inserts. The machinist then slides the insert out after the moulding process is complete. Side actions work well with rigid materials that do not easily adhere to the mould surface. However, each insert must be specifically designed, making the procedure more complex.
The choice of ejection system depends on specific production requirements. Some common ejection systems include mechanical, pressure-driven, pneumatic, and hybrid systems. The mechanical ejection system is the most commonly used type, employing tools like pins, sleeves, blades, valves, or stripper plates/bars to push or pull the product out of the cavity. Pin ejection is generally less expensive than hydraulic and pneumatic systems, which require higher initial investments and have higher maintenance needs.
In summary, successful ejection techniques in plastic injection moulding require careful consideration of draft angles, surface polishing, and the use of inserts or side actions to facilitate the removal of the moulded component from the mould. The choice of ejection system depends on production requirements, and proper design and selection of ejection components are crucial to ensure product quality and efficiency.
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Design solutions
Injection molding undercuts are features in a molded part that can complicate or prevent the release of injection-molded parts. They are commonly used in medical parts and consumer electronics, automotive, and other products. Undercuts can be indentations or protrusions, holes, cavities, or recessed areas. They can be used to incorporate complex shapes, details, or additional functionalities in the design of plastic parts.
- If an undercut is not essential to your design, it is best to eliminate it. This is because undercuts increase complexity and cost.
- If an undercut is necessary, consider using side-action cams, bump-offs, inserts, and other complex injection molding techniques.
- To prevent damage to the part during ejection, use a stripping plate or ring instead of ejector pins.
- To create angled or undercut features, use an angle lifter, which is a movable component within the mold that can be lifted or angled after the initial mold opening.
- To improve cycle time, cost savings, and reliability, use a dovetail collapsible core. This eliminates secondary operations and complex coring approaches.
- To create threaded parts, use a collapsible mini-core, which can mold up to 80% full threads or undercuts.
- To ease removal from the mold, add enough draft to your undercut.
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Cost-effectiveness
Injection molding undercuts can increase complexity and costs, but they can also enhance a product's functionality. Undercuts are used to create interlocking parts, snap-on products, and parts with moving components. They can also be used for coring, which eliminates plastic material from thick sections to avoid longer cooling times and minimise the risk of sink marks and warping.
Undercuts can complicate the assembly process and increase the chance of part damage. They can also cause complexity, which raises production costs due to labour, tooling, and other additional operations. Therefore, it is recommended to use undercut molding only when necessary.
Designers must determine whether the advantages of intricate features outweigh the extra costs and difficulties. They should select materials that can endure the strains and alterations that undercuts may cause, striking a balance between cost and material performance. High-performance materials can cost more, so designers should consider whether the advantages are worth the extra expense.
There are several ways to reduce costs when using undercuts. One way is to use secondary operations, such as a drill press or milling machine, instead of altering the mold. This can be more cost-effective during the prototyping and low-volume production phases. Another way to reduce costs is to use collapsible cores, which have proven to improve cycle time, cost savings, and reliability. Dovetail collapsible cores, in particular, provide a simple and compact way to mold challenging internal undercut features.
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Frequently asked questions
An undercut in plastic design refers to a protruding or recessed area on a molded part that prevents the part from being easily ejected from the mold.
Undercuts are used to incorporate complex shapes, details, or additional functionalities in the design of plastic parts. They can be used to create threaded parts, such as screw-on bottle caps, and snap-on products like lipstick containers.
Some techniques to release undercuts from molds include using side-actions, lifters, strippers, and angle lifters. Side-actions, or side pulls, are mechanical features that enable the mold to create complex shapes by moving in directions other than the standard mold opening and closing. Lifters and strippers are used to address features that may interfere with the ejection process. Angle lifters are movable components within the mold that can be lifted or angled to free the molded part from any undercut features.
Undercuts increase the complexity and cost of injection molding. They can also present challenges during the demolding process, especially when there is insufficient draft or taper, which can make it difficult to remove the side action manually. It is important to consider the functionality of the design and choose the appropriate technique for releasing the undercut.
