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Comprehensive Processing Guide for Undercutting : Best Methods for Molding Undercuts

Views: 0     Author: Site Editor     Publish Time: 2023-11-08      Origin: Site


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Producing plastic parts with undercuts in the injection molding service manufacturing sector presents obvious challenges for manufacturers. Undercuts are protrusions or depressions in a part that prevent the mold from sliding away in the parting direction after the part is molded. These features inhibit the direct removal of the core, so it is often necessary to use additional mold pieces, such as side cores or internal core lifters, to form the shape.

Undercut designs are commonly used in the manufacture of threaded parts such as screw-on bottle caps, snap-on products such as lipstick containers, and a variety of consumer, medical, automotive, and other products. Threaded caps are a good illustration of the complexity associated with undercutting. When a cap is molded, the threads of the part and the threads of the core engage with each other and must be disengaged before the core can be pulled out and the cap removed from the mold.

Manufacturers have developed a variety of methods for molding undercut or threaded parts-some as simple as unscrewing the part by hand or machining the undercut in a separate operation-with a wide range of cost-effectiveness and efficiencies.From this article, ZONZE will present some of the latest technological advances that provide molders with better, more cost-effective methods of producing undercut or threaded parts.


Mold Release Mechanisms

The two most common ways to handle threaded parts are by jumpering the threads or by installing a unscrewing mechanism. Sometimes, if the material is flexible enough, molders can simply pull out the core or strip the part, skipping the threads over each other. If this is not an option, a screw loosening mechanism built into the mold can be used as a secondary action to unscrew the part from the core.

Unscrewing molds are among the most complex of all injection molds and require considerable technical knowledge to build and maintain. They are usually built for multi-year production and are considered a long-term investment for producing large quantities of parts. Twist and Loosen technology has come a long way, but it still has many limitations. It requires frequent maintenance, such as broken drums, damaged frames and problems such as water and oil leaks. Part quality issues such as wear, ovality, flying edges and grease contamination can also occur.

Collapsible Core

A technology that extends undercut molding capabilities more than any other is the collapsible core. Instead of jumpering or mechanically unscrewing the part, bent steel collapsible cores function by folding radially inward during the normal mold sequence. They eliminate secondary operations and complex coring methods while dramatically reducing cycle time - typically 30% faster than spinning mechanisms.

Segments of the collapsible core are attached to the ejector plate, while its tapered internal center pin is attached to the back of the mold. When the mold is opened, the threaded outer core collapses as the ejector plate moves forward. Containing only three moving parts that use conventional mold movement, the collapsible core makes possible the design of parts that were previously considered impossible to mold.

Collapsible cores are compatible with other mold components such as two-stage ejectors and internal latches. These products allow positive control of stroke sequence and distance in two-stage ejector and moldboard locking operations.

• Two-Stage Ejectors: These ejectors are available for a wide range of mold frame sizes and plate thicknesses, and two ejection sequences are available: last ejection and bottom ejection. The travel range of each ejector stage is fixed by a simple program and cannot be tampered with or accidentally changed after installation. Utilizing internally mounted components, the two-stage ejector avoids interference with waterline connectors and externally mounted components.

• Internal latching: This allows measured control of the mold opening sequence on mold frames with stripper molds. A group of plates can be latched together when the first parting line is opened. After a predetermined amount of travel, the latch releases the latched plates to open the remaining parting line or lines. As with two-stage injectors, once installed, the internal latch cannot be accidentally changed and will not interfere with waterline connectors or externally mounted components.
The folded core feature also allows the threads to stop at any point along the length of the molding; the threads do not have to extend to the top of the core as they would if the mold were unscrewed. If a seal is required at the top of the cap, an undercut can often be molded into the part to hold this seal in place.

The folding action also allows a longer threaded area to be formed without increasing cycle time or requiring a long rack and pinion mechanism. In addition to threads, other structures such as pits, notches, or protrusions beyond the capabilities of the spinning release mold can also be successfully molded.

Collapsible Mini Cores

Collapsible cores have many advantages, but sometimes they are too large for the application. That's when molders turn to collapsible mini-cores. These extend the application range of collapsible core molds to closures with diameters as small as 10.8 mm. Because of the small diameters involved, these mini cores feature three larger collapsible sections and three narrow non-collapsible inserts that are an integral part of the center pin. As a result, it is possible to mold up to 80 percent full threads or undercuts.

Collapsible Core Challenges

The collapsible core is designed to fold independently when the center pin is withdrawn. Controlling the fit between the segments to allow for fretless molding means that the position of the core on its pin is critical. The distance between the rear of the core flange and the front of the center pin flange (known as headspace) must be accurately maintained. Otherwise, inaccurate headspace will result in unsatisfactory operation and possible permanent damage to the core.

Collapsible cores are designed to operate without lubrication. Plating of the core is not recommended, although the core may be treated with an alloying process to reduce wear and resist corrosion.

The sections of the collapsible core are self-cleaning, which will tend to carry any dirt or deposits to the outer surface of the collapsible core. As a result, the first 50 to 100 injections may show foreign deposits on the inside of the molded part. The core should be thoroughly degreased and cleaned prior to final mold assembly. It is often a good idea to lightly wipe the tapered end of the center pin with grease or PTFE lubricant to aid in its insertion.

You should also make sure that the folded core is free to rotate when installed into the ejector plate. This slight clearance will allow the core flange to "float" slightly, helping it to 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. Core ejection travel varies from model to model.

The stripper plate actuation must be sequenced so that the cylinder returns to the stripper plate before the ejector plate returns. This will avoid interference of the stripper ring with the core and possible core damage. Collapsible cores are individually mounted to matching numbered pins and are not interchangeable.

Collapsible cores and mini cores set the bar high for efficiency and cost savings. But even these technologies have room for engineering advances. The latest improved version of this technology, the Dovetail Collapsible Core, enhances the strength and functionality of the traditional design.

Dovetail Collapsible Core

Dovetail Collapsible Cores offer the most compact and simple way to shape challenging internal undercut features. With a mechanism for folding the segments, Dovetail cores add versatility to handle a wider range of diameters and undercut depths.

Manufacturers are sometimes hesitant to use standard collapsible cores, in part because the product design utilizes steel bending segments that are integrated with each other. For example, segments of conventional collapsible cores can be damaged or broken if the machine is clamped to the part. While the root cause is incorrect forming operations or mold design, this type of error can create an undeservedly negative reputation for bent steel collapsible cores.

Because of their strength, dovetail joints are commonly used to lock parts together in woodworking and other industries. Again, Dovetail collapsible cores are much stronger than their traditional counterparts. While standard collapsible cores use steel tubing slotted into 12 separate sections, dovetails use six separate sections that are larger, stronger, and easy to repair when necessary.

Traditional collapsible cores work well in the B half of the mold, but can create design problems in the A half, and Dovetail's segmented design allows it to work equally well in either half of the mold. This means molders can save more money by using fewer, smaller templates and smaller molding machines.

The Dovetail Collapsible Core also allows for front and side closure. This is a major advantage for molders over traditional collapsible cores, which typically require modifications to the part design to handle folded segments or mold closure. However, both types of collapsible cores can be used to mold protrusions or cutouts into the sidewalls of a part.

The direct actuation of Dovetail collapsible cores allows manufacturers to design and build molds that require only a "mold open/mold close" command to operate. In most cases, there is no need for special core extraction circuits or even the usual ejector plate sequence. The potential cycle time reduction is tremendous. The dovetail collapsible core also incorporates a patent-pending quick-lock system that allows molders to quickly remove components from the mold without removing the mold from the machine.

Another significant advantage of the dovetail design is that molders can use a standard fixture to grind the threads onto the outer diameter of the core. This is not an option with the traditional version, which requires molders to purchase or fabricate special grinding rings.


The importance of forming undercutting for a specific product is crucial, and it is important to understand the full range of design elements that need to be considered during the design process. ZONZE gives you a detailed insight into the best methods for forming undercutting in this article; specialized manufacturers are particularly important, and ZONZE can provide you with an exclusive on-demand manufacturing service, so please contact us if your project is underway.

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