The mold is one of the most important parts of injection molding. A mold is made up of two halves that have a hollow space between them into which a melted material is injected. This hollow space is shaped by both the core and the cavity when the mold is closed.
Many people confuse the cavity with the core. In injection molding, the cavity is the female portion of the mold that forms a product’s external shape. The core is the male part that is responsible for forming the product’s internal shape.
This article explores cavities of injection molding in more detail, with a specific focus on multi-cavity injection molding.
Single Cavity, Multiple Cavity, and Family Cavity Injection Molding
There are three main types of cavity injection molding a manufacturer may leverage:
No. 1 – Single Cavity Molds
The simplest and most cost-effective option, single cavity molds produce one molded part per production cycle. These molds are easier to produce than multi-cavity molds, which reduces lead times and allows the manufacturer to start production faster. However, they’re not appropriate for parts in high demand as the tool reaches its production limit quickly.
Single cavity injection molding may be a good choice for low-volume production runs. This type of injection molding is often called 1 x 1 to reference the fact that there is one cavity inside the mold producing one part per cycle.
No. 2 – Multiple Cavity Molds
As the name implies, multiple cavity injection molding uses a mold that produces several parts per production cycle. In addition to reducing the cost per part, this speeds up the production process. Creating the mold itself is more expensive. But for high-volume production runs, that expense is quickly accounted for with the faster and more cost-effective production times.
Multiple molds are often denoted as multiplication sums, such as 1 x 2, In this case, the mold produces two identical parts per production cycle. This can extend to 1 x 4, 1 x 8, and so on based on the manufacturer’s needs.
No. 3 – Family Molds
Expanding on the concept of 1 x 1 molds, there are family molds too. Though these tools are more expensive to produce, they can create multiple different molded parts with a single injection mold. Though 1 +1 is the most common, it’s possible to create 1 + 1 + n + ... molds, which would create 'n' different parts per cycle. Examples include:
1 + 1 – The mold produces two different parts per cycle.
2 + 2 –The mold produces 2 different parts with 2 cavities each (4 parts in total). There is also a more modern form of injection molding that involves injection of two different materials into a mold that produces two parts, but this requires valves inside the runners to control polymer flow.
Family molds offer challenges when the parts are of different sizes. The manufacturer needs to balance filling each cavity evenly. Large imbalances lead to product quality issues because of not equal pressure distribution.
Arrangement of the Cavities
After understanding that there are different types of cavity injection molding, a manufacturer needs to determine how to arrange the cavities in the mold. This includes figuring out how many cavities the mold should have. Several factors influence these decisions.
When a mold is closed, it needs to be clamped together to prevent leakage and ensure equal pressure is exerted to each side of the mold. This can be a challenge with larger molds. If a manufacturer clamps the mold at the top and bottom, the center of the mold may not have the appropriate pressure applied to it to produce consistent parts (without flash).
Centric clamping must be used in these cases. This type of clamping ensures pressure is placed on the center of the mold. Centric clamping is fairly simple to accomplish with a single-cavity mold. However, multiple and family molds require forethought in clamping mechanisms due to each having several cavities.
Easy Tool or Multiple Tool
Easy tool is another term for single cavity injection molding. Often, a manufacturer will use a large tool wherein the mold cavity is slightly off-center. This gives the tool larger dimensions but allows for the injection of melt via a side gate. Some manufacturers place the gate directly on the part, allowing a more direct injection that leads to smaller tool sizes.
With multiple and family tools, the manufacturer has to consider sprue distance. This distance must be the same for each part or the mold cavities fill unevenly. The article explores different sprue types later.
According to a Clemson University paper that examined the concept of cavity pressure as a quality indicator in injection molding, cavity pressure is a reliable indicator for part quality and process monitoring.
The research found that creating a cavity pressure curve illustrates the molding cycle’s progression, even in the case of micro-injection molding:
As demonstrated in the graph, the peak cavity pressure line matches the part weight line. It increases as the weight increases, and decreases accordingly. As such, a manufacturer can track pressure increases and decreases in line with part weight. If pressure decreases substantially for a heavier part, this suggests an issue in the mold that could lead to quality problems.
It also found no significant difference in the curves created by different types of materials, leading to cavity pressure being a suitable indicator of product quality.
Cavity Surface Finish
All injection molded parts have a surface finish applied by the tools used to create them. The type of surface finish applied affects the time and effort required to create a part. The finish must be produced, with each step of refinement adding time to this production effort.
Sandblasting and EDM are two common methods for ensuring a stable cavity surface finish. EDM is a subtractive method that uses electrical discharges to create features on a mold. Sandblasting involves forcing solid particles across the part’s surface using compressed air. This serves to clean and smoothen the surface.
Examples of Gates and Sprues
A sprue is a channel through which molten material flows during the injection molding process. Sprues guide the material from the hopper to the desired location inside the mold.
In multiple cavity injection molding, the distance of the sprue is crucial to the production of quality parts. Variances in sprue distances lead to uneven cavity fills that compromise part quality. There are several types of sprue a manufacturer may use in injection molding.
Used for large-area parts, film section sprues and gating involve filling the molded part using a rectangular cross-section. This prevents internal tension and warping, though the sprue has to be mechanically separated from the part after demolding.
The molded part is filled using a cylindrical and conical cross-section. This is also known as a side injection. Tunnel cuts allow automatic separation of the mold and sprue during demolding, though it also leads to pressure loss and material shearing.
Banana cut sprues are curved to enable access to more difficult injection points. It requires the use of easily deformable melt materials, though it does allow automatic ejection of the sprue during demolding.
Used specifically for ring-shaped parts, this sprue prevents weld and flow lines from appearing on the part. It evenly distributes the melt as it flows through, allowing the production of high-quality parts. But this sprue must be manually removed during the rework process, leaving marks behind in the process.
Cone or Bar
Cone and bar sprues offer very little resistance to the melt, which flows straight down into the cavity. This makes it ideal for filling difficult cavities. However, the mold has to be removed manually, again creating visible sprue marks.
Ring sprues are ideal for long tubular parts. They solve the problem of the pressure of inflowing melt leading to bends by providing support to the mold on all sides.
Understanding Cavity Injection Molding
Noting that there are several types of cavity injection molding allows a manufacturer to determine which types of molds serve their products best. For low-volume runs of a single product, a single mold, or easy tool is preferable. In high-volume runs, it’s often best to use a family or multiple mold to produce multiple parts per injection cycle.