The Best Way For Processing Undercuts In Injection Molding

Molders face a significant challenge when making plastic parts with undercuts. Undercuts are indentations or protrusions in the parts that prevent mold from slipping away from the injection molding part’s direction after it has been molded. Due to these characteristics, the desired shape is achieved by adding mold components like side cores internal core lifters.

Six examples of parts designs with undercuts: TE cap, CT cap, threaded ring, prescription, dosage, and long-thread designs.

Undercut design is commonly used in the manufacture of threaded parts, such as screw-on bottle caps, lipstick containers, snap-fit products, as well as various consumer goods, medical, automotive, and other products. Threaded caps illustrate well the complexity associated with undercuts. After the cap is molded, the threads of the part mesh with the threads of the core, which must be disengaged before the core can be removed and the cap taken out of the mold.

Demolding mechanism

The two most common methods for handling threaded parts are thread stripping or installing unscrewing mechanisms. Sometimes, if the material is sufficiently flexible, mold manufacturers can simply pull out the core or strip the part, skipping the threads past each other. If this isn’t an option, an integral unscrewing mechanism built into the mold can act as an auxiliary motion to unscrew the part from the core.

“Unscrewing molds are highly complex and require considerable technical knowledge for construction and maintenance. Tailor-made collapsible core molds (left) are simpler and more compact.

Unscrewing molds represent some of the most intricate molds in all of injection molding, demanding extensive technical expertise for their construction and upkeep. Typically built for years of production, they are viewed as long-term investments for manufacturing large volumes of parts. While unscrewing technology has advanced significantly, it still carries numerous limitations. Regular maintenance is necessary for issues such as broken threads, damaged racks, leaks, and oil seepage. Quality issues can also arise with parts, including wear and tear, ovality, flash, and grease contamination.”

Foldable Mold Core

A technology that extends undercut molding capabilities more than any other is the collapsible core. Unlike thread stripping or mechanical unscrewing, collapsible cores function by radially collapsing inward during the normal mold sequence. They eliminate secondary operations and complex core extraction methods, while significantly reducing cycle times—often up to 30% faster than unscrewing mechanisms.

The segmented connection of the collapsible mold core attaches to the ejector plate, while its tapered internal center pin connects to the back of the mold. When the mold opens, the threaded outer core collapses as it moves forward with the ejector plate. With only three moving parts using traditional mold motions, the collapsible core allows designs for parts previously deemed impossible to mold.”

The collapsible core is compatible with other mold components such as two-stage ejectors and internal locks. These components allow active control over stroke sequence and distance in two-stage ejection and mold locking operations.

Two-stage ejectors:

These ejectors are suitable for various mold frame sizes and plate thicknesses, offering two ejection sequences to choose from: final and final-last. The stroke range for each ejection stage is fixed through simple programming and remains unchanged or accidentally altered once installed. Using internally mounted components, two-stage ejectors avoid interference with waterline connectors and externally mounted components.

Internal locking:

This allows measured control over the opening sequence of mold plates on a mold frame with mold release. When the first parting line opens, a set of plates can lock together. After a predetermined stroke, the locking plate releases to open the remaining parting lines or multiple lines. Similar to two-stage ejectors, once installed, internal locking prevents accidental changes and does not interfere with waterline connectors or externally mounted components.

The functionality of collapsible cores also allows threads to stop at any point along the mold length; threads do not need to extend to the top of the core as in unscrewing molds. If a seal is required at the top of a cap, for example, an undercut can typically be molded into the part to secure this seal in place.

Collapsible action also enables the formation of longer thread areas without increasing cycle time or requiring long gear rack mechanisms. Apart from threads, other features such as recesses, notches, or protrusions beyond the capability of unscrewing molds can also be successfully molded.

Mini Foldable Mold Core

Foldable mold cores have many advantages, but sometimes they are too large for certain applications. That’s when manufacturers turn to foldable mini cores. These extend the application range of foldable core molds to enclosures with diameters as small as 10.8 millimeters. Due to the smaller diameters involved, these mini cores use three larger foldable sections and three narrow non-folding blades, which are part of the center pin. This allows for molding up to 80% of full threads or undercuts.

Foldable mini mold cores offer many of the advantages of foldable cores for parts with diameters as small as 10.8 millimeters.

The Main Challenge of foldable mold core

Foldable mold core designs are made to independently collapse as the center pin retracts. The fit between control segments is managed to allow for flash-free molding, meaning the position of the core on its pin is crucial. The distance between the back of the core flange and the front of the center pin flange, known as the top space, must be precisely maintained. Inaccuracies in the top space can lead to unsatisfactory operation and potentially permanent damage to the core.

They are also designed to operate without lubrication. Although alloying processes can be used to treat the core parts to reduce wear and corrosion, plating the core parts is not recommended. The various parts of the foldable core have a self-cleaning function, which tends to bring any dirt or deposits to the outer surface of the foldable core. As a result, the first 50 to 100 injections may show foreign material deposits inside the molded parts. The cores should be thoroughly degreased and cleaned before final mold assembly. It’s generally best to lightly wipe the tapered end of the center pin with grease or PTFE lubricant to help it insert smoothly.

You should also ensure that the foldable core can rotate freely when mounted on the ejector plate. This slight clearance will allow the core flange to “float” slightly, helping it find its center and balance the wear on the center pin. To fully collapse the core, the center pin must be pulled out a specified amount. Different models of cores have different ejection stroke lengths.

The ejector plate drive must operate sequentially, so the cylinder returns to the ejector plate before the ejector plate returns. This prevents interference and possible damage to the core from the demolding ring. Each foldable core is installed on its matching numbered pin and cannot be interchanged.

Foldable cores and mini cores set high standards for efficiency and cost savings. However, even these technologies have room for engineering advancements. The latest improvement in this technology, the dovetail foldable core, enhances the strength and functionality of the traditional design.

Dovetail foldable mold core

The dovetail foldable mold core provides the most compact and straightforward method for molding challenging internal undercut features. With the mechanical device used for the folding segments, the dovetail core increases versatility to handle a wider range of diameters and undercut depths.

The dovetail foldable core provides the most compact and straightforward way to mold challenging internal undercut features.

Manufacturers sometimes hesitate to use standard foldable cores, partly because the product design involves interlocking steel bending segments. For example, if the machine clamps onto the part, the segments of a traditional foldable core might get damaged or break. Although the root cause is incorrect molding operation or mold design, such errors can undeservedly give a negative reputation to bending steel collapsible cores.

Due to their strength, dovetail joints are commonly used to lock parts together in woodworking and other industries. Similarly, dovetail foldable cores are much stronger than their traditional counterparts. Standard foldable cores use a steel tube slotted into 12 separate sections, whereas the dovetail core uses six larger, sturdier sections that are easier to repair when necessary.

Traditional foldable cores work well in the B half of the mold but can cause design issues in the A half. The segmented design of the dovetail core allows it to work equally well in either half of the mold. This means manufacturers can use fewer, smaller templates and smaller molding machines, resulting in more cost savings.

The dovetail foldable core can also close from the front and sides. Compared to traditional foldable cores, this is a significant advantage for mold makers, as traditional foldable cores often require part design modifications to accommodate folding segments or mold closures. However, both types of foldable cores can be used to mold projections or notches into the sidewalls of parts.

The direct drive of the dovetail foldable core allows manufacturers to design and produce molds that only need “mold open/mold close” commands to operate. In most cases, special core pulling circuits are not needed, nor are common ejector plate sequences. The potential reduction in cycle time is significant. The dovetail foldable core also includes a patent-pending quick-lock system, enabling manufacturers to quickly remove components from the mold without taking the mold out of the machine.

Another notable advantage of the dovetail design is that mold makers can use standard fixtures to grind threads onto the core’s outer diameter. This is not an option with traditional versions, which require mold makers to buy or fabricate special grinding rings.

Undercut parts present many challenges for designers and mold makers, but technology is constantly evolving to meet these challenges. While some methods for molding undercuts have been tried and tested, they may not always be as cost-effective or efficient as more advanced technologies. The dovetail foldable core has proven to be an excellent solution for various applications—reducing cycle times, saving costs, and improving reliability. This product is available from DME.