Side Cores

What are the best methods for releasing severe threads or cuts? Do side cores, splits, or other methods work?

In addition to the added complexity of the mold structure, adding any of these features will significantly increase the cost of the mold (and the cost of the product), not to mention the additional space each mold component takes up, compared to a simple mold design without side cores.

If the mold had the same number of cavities, it may be necessary to make it much larger and thereby larger  the machine to accommodate this extra mold area, despite the clamping forces needed being lower than with a mold without side cores or splits.

If the mold has such features, such as side cores and splits, the cycle time will be longer and the productivity will be lower.

Should the product be redesigned to avoid the use of side cores?

It may be possible to redesign round holes or openings of “odd shape” that arise from using split cavities or side cores without compromising the design of the product, the openings can be created in the side walls (or even in the ribs) with a design that blends the core and cavity together on a “shutoff”. Special inserts may need to be inserted in the cavity or in the core, which may require altering the sidewall’s shape (or draft angle), or even a hole in the bottom.

The end use of the product could be acceptable in many cases, and the mold could be simpler and therefore less expensive . In many cases, a little redesign can result in significant savings on mold and product costs by simply giving more consideration to the product design while planning and designing a mold.

Alternate Parting Lines

A parting line placed in the obvious location may require a side core whereas the same product can often be molded with an up-and-down mold by slanting the P/L. This principle applies to any similar case, such as a simple louver. “Slanted” P/L molds cost more than ordinary P/L molds, but are far less expensive than side core molds.

Study thread and undercut shapes

Often, a design specifies threads or undercuts, on the inside of the product . Is the specified shape of thread or undercut designed with molding in mind? Many such threads or undercuts could be molded without un- screwing, or the need for collapsible cores, by changing the shape of the undercut so that the product can be stripped off the core, i.e., the undercuts can easily slip out of the grooves that created them when pushed by ejectors or a stripper.

The left illustrates the difficulty of unscrewing a mold. Ejecting the cap from the cap mold requires rotating the core out. As a result, cooling the core becomes more difficult and cycle times increase by 30%. An unscrewing mold is more complex than a force ejection  (stripped) mold.

A more simple mold is illustrated in the left  by a thread (and cap) that can be stripped. A very efficient cooling method is available here. Typical (28 mm) HDPE bottle caps with a weight of less than 3 g, molded in a 24-cavity machine powering a 90 t (1,000 kN) machine, cycle in 4.0 seconds, producing 21,600 caps during an hour.

We can see how a small adjustment to the flank of the thread can allow the thread to be stripped from the core, instead of having to unscrew the mold. Having a stripped product can result in significant savings in terms of mold cycle, costs, and maintenance.

Conquering Design Challenges with Side Cores

Side cores are very important in injection molding to create  features like undercuts, recesses, or protruding elements. These features cannot be produced using a simple straight-pull mold design, where the two mold halves separate along a single linear direction. In a straight-pull mold, any design elements that prevent the part from being easily extracted (like an undercut) would trap the molded part inside the mold cavity, making it impossible to eject the finished part.

This is where side cores come into play. When a part’s design includes features that obstruct the direct path of separating the mold halves, side cores are incorporated. These movable core components slide perpendicularly to the direction of the primary mold opening and closing motion. Before the mold fully opens after the injection and cooling stages, the side cores retract away from the molded part, allowing space for features like undercuts that would otherwise lock the part in place.

functionality and aesthetics

The decision to design a product requiring side cores is influenced by several key factors that must be carefully weighed. One crucial consideration is the functionality and aesthetics of the side core-enabled features. If these design elements are essential for the product’s intended function or are critical to meeting aesthetic requirements, then incorporating side cores may be necessary. However, if the undercuts or recesses serve no functional purpose and are purely decorative, it may be worth exploring design modifications to eliminate the need for side cores, reducing manufacturing complexity.

cost

Cost is another significant factor in the decision-making process. Side cores will lead to complexity to the mold design and construction, resulting in higher upfront tooling costs. For products with stringent budget constraints, the additional expense associated with side cores may not be feasible, necessitating a redesign to simplify the molding process. Conversely, for high-volume production runs, the initial investment in a more complex mold with side cores can be justified over the product’s lifecycle, as the per-unit cost amortizes over a larger production quantity.

production volume

Anticipated production volumes also influence whether side cores will prove financially worthwhile. When designing for low volumes or prototype runs, including side cores may not make economic sense and alternative design solutions may be preferable; but with high volume runs where tooling costs are spread out more evenly among various components, including side cores becomes financially more viable.

Space constraints and tolerance

Space restrictions and tolerance requirements within a mold should also be taken into consideration. Side cores require additional space inside of the cavity of a mold for them to slide, making its overall size larger; furthermore, their presence can have an adverse impact on achievable tolerances or dimensions accuracy of molded parts, due to slight misalignments or wear over time that introduce variability into them.

Long-Term Maintenance

Finally, when considering molds with side cores as their long-term maintenance requirements must also be factored in. Side cores may become worn due to sliding motion and require more frequent inspection and repairs or replacements due to wear-and-tear; leading to more frequent maintenance intervals or increased downtime for repairs or replacements and increasing operational costs over the lifetime of a mold; especially with higher cavitation or extended production runs.

Redesigning a product to avoid side cores requires careful consideration and analysis of various factors。Those include functional requirements, aesthetic considerations, production volumes, costs constraints, space requirements tolerance requirements tolerance specifications as well as long-term maintenance implications.

A team composed of product designers, mold designers and injection molding experts must work collaboratively in evaluating these aspects in order to come up with informed solutions which optimize product design while meeting practical manufacturing and cost considerations.