Single-cavity injection molding has been around for the longest and has been utilized to make various parts. With this injection mold design, each mold cavity and core is designed to make a single part for every shot of the molten plastic.
However, this is nor normally the case as some injection molds have a multi-cavity design. Multiple cavities promote production of multiple components concurrently with each shot.
Nonetheless, multi-cavity molding has both pros and cons. This design usually promotes efficiency thus is crucial for mass manufacturing of identical products.
While this model of cavity is efficient, it has cons which include challenges to make the temperature of molten polymer and pressure remain constant to each of the cavities. Additionally, reduction of defects such as warps is quite challenging as you have to maintain consistency in product output.
In this article, we will explore optimizing multi-cavity molds for enhanced injection molding production!
High Precision Multi-Cavity Mold
The Challenges of Multi-Cavity Molds
In multi-cavity injection molds, the physics encountered when forcing molten plastic through a mold’s sprues, runners, and gates change as with the multiple cavities and complexity of the mold. This impacts molding performance and part quality.
Despite its many advantages, multi-cavity molding can pose challenges, meaning it is not ideal for every molding project. The main disadvantages of multi-cavity molding include lack of part consistency, tooling complexity, and difficulty achieving ideal flow conditions.
Material flow challenges
Multi-cavity molds require a more complex system of sprues, runners, cooling channels, and vents than single-cavity molds. And with material flowing in several different directions, it can be difficult to find the right flow parameters that will lead to consistent filling and batch consistency.
That being said, material flow challenges can be mitigated when using less viscous materials such as liquid silicone rubber.
Mold Cost
Partly due to the design complexities mentioned above, multi-cavity molds are more expensive to produce than single-cavity molds. This makes them unsuitable for low-volume production, as the ultimate reduction in cost-per-part may not be enough to offset the higher upfront costs associated with creating the tooling.
Strategies for Optimizing Multi-Cavity Mold Design
Multi-cavity molds offer increased production volume and reduced cycle times, but they can be expensive and challenging. Here are a few strategies to optimize your multi-cavity mold design.
1. Adjusting the Mold Gate
Proper gate design is vital for smooth plastic flow and even the filling of holes in molds with more than one cavity. Manufacturers frequently utilize tab gates, also known as edge gates, in multi-cavity molds due to their straightforward positioning and ability to handle flow stress effectively.
Pin-style and hot-tip gates, often used in single-cavity molds, may not be suitable for multi-cavity injection molds because they are more complicated and could cause uneven flow.
When transitioning to molds with multiple cavities, adjusting the gate’s original placement might be necessary to accommodate alterations in the part orientations. Working with application engineers early can help determine the best way to place gates on multi-cavity injection molds.
For parts to work well in multi-cavity molds, they may need to be changed or tweaked from how they work in single-cavity molds. Adjust the multiple-cavity mold gate, use side-actions, add pickouts, or choose liquid silicone rubber (LSR) to improve the design of a multi-cavity injection mold.
2. Utilizing Side-Actions
The purpose of side actions is to enable undercut geometry; in other words, to mold parts that could not otherwise be made in a straight-pull mold. In straight-pull injection molding, the A-side and B-side of the mold open and the completed part can be removed by pulling it straight out of either side.
An undercut is a feature in the part design that would prevent the part from releasing out of the mold (part of the mold cavity is undercut in such a way that it grips the molded part). To release a part whose design includes undercuts, the mold surfaces that create the undercut geometry in the part must be pulled out of the part before it is ejected; otherwise the part will be stuck in the mold.
A cam device in the mold is used to pull the side-action mold surfaces away from the undercut features allowing the part to be released from the mold. You can produce parts using linear side-actions that move perpendicular to the mold’s opening and closing axis.
Angle pins (cams) on the A-side guide the cam carriage closer to the mold cavity on the B-side of the mold as the mold closes. When opening the mold, the angle pin pulls the cam carriage away from the cavity and out of the part, allowing the ejector system to advance and push the part off the B-side of the mold.
3. Incorporating Pickouts
In the injection molding process, cams cannot form undercuts on non-exterior surfaces. In many cases, however, undercuts on interior surfaces can be formed using pickouts.
These are inserts that are part of the mold when resin is injected, but are ejected with the part and then removed from the part, leaving undercut features in their place. By filling the undercut and becoming, temporarily, an element of the part, a pickout eliminates ejection problems.
Using a pickout lets designers overcome shape and positioning restrictions, but is more costly than sliding shutoffs or using a side-action.
4. Adopting a Family Mold Approach
Family injection molds are molds used in the injection molding process to produce multiple parts or components simultaneously. Designers create them to accommodate various cavities or impressions within a single mold, enabling the production of different factors in a single molding cycle.
You can use family molds when there is a need to produce a set of related parts that are used together or have similar characteristics. This approach offers efficiency and cost savings by reducing tooling and setup time, maximizing productivity, and ensuring consistent quality across all parts produced.
Family injection molds offer several advantages, making them a preferred choice in manufacturing. They include:
- Increased productivity: Family molds enable the simultaneous production of multiple parts, reducing cycle times and increasing overall productivity.
- Cost savings: By producing multiple parts in one cycle, family injection molds help reduce manufacturing costs.
- Time efficiency: Manufacturers can significantly reduce production lead times with family molds. The ability to produce multiple parts in a single cycle eliminates the need for sequential tooling or mold changes, saving valuable time.
- Consistency and quality: Family injection molds ensure consistent part quality across all cavities. Simultaneously producing components within the same mold ensures uniformity in dimensions, appearance, and material properties.
- Design flexibility: Family molds offer design flexibility, allowing for the production of multiple components with different shapes, sizes, or features within a single tool.
- Reduced material waste: With family molds, material waste is minimized compared to using separate molds for each part.
Multi-cavity Molded Tiny Lab Flasks Molded (Image Source: Istock)
Material Considerations for Multi-Cavity Molds
Thermoplastic elastomer (TPE) is a synthetic plastic that melts upon heating and hardens on cooling without changing the plastic’s chemistry. Liquid silicone rubber (LSR) is synthetic plastic containing polymers joined and structured by chemical bonds that harden permanently after one application of heat.
The main difference between the two is how they react to heat. After TPE is heated, reprocessing is possible. LSR cannot be altered once heat is applied. Let’s have an in-depth look at the two below!
TPE
Plastic pellets are heat-liquified and pressure-molded into components that strengthen and hold shape upon curing without using a chemical bond. Molding can be reversed (regrind/melt) to correct defects and the plastics reused multiple times.
TPE Advantages
- Recyclability and less energy consumption for production
- Re-molding opportunities without chemical change
- Easier molding than thermoset LSR
- Shorter molding cycles
- Less expensive molding process
- Heat sealability
- Easy coloration using most dyes
- Greater number of two-shot molding options
TPE Disadvantages
- Post-cure melting if exposed to high temperatures
- “Creeping” and deformation if exposed to sustained pressure or another stressor
- Tooling can be costly
LSR
Viscous plastic is heated to cause polymer cross-linking, and the resulting chemical bond provides irreversible strength and shape after curing. Another common way to initiate cross-linking in LSR is to mix a catalyst with another component prior to injection into the mold
LSR Advantages
- Better resistance to high temperatures than thermoplastics
- Design flexibility, including thick-to-thin wall constructions
- High tear strength
- Biocompatibility
- High chemical resistance
- Superior compression set
LSR Disadvantages
- No recyclability
- Inability to be re-molded after curing
- Possibility of burning if heated after curing
- Lengthy curing period, which adds to production time and cost
- Potential for production delays as equipment must be disassembled and cleaned if early cure occurs
- Liquid plastic can be difficult to handle
- Bulky or thick appearance
Best Practices for Multi-Cavity Mold Production
To have optimal multi-cavity production, you need to adhere to some of the sector’s best practices. They include:
1. Adjust the Gates
Gates control the flow of plastic from entry to the point of cooling and ejection. Using tab or edge gates can help improve flexibility which is essential with multi-cavity molds. In addition, tab gates reduce or control the stress to the tabbed area during the ejection.
Gate location or placement depends on the specific production expectations. By clearly understanding the expectations early in the process, redesign and issues can be avoided.
2. Design for Wall Thickness
It is important to manage wall thickness to control stress marks and to ensure the design meets minimal wall thickness requirements while maintaining consistency in the thicknesses of adjacent features. The gate is an area where there is high injection pressure. If the wall is narrow, this also acts as a restriction increasing injection pressure.
Consequently, if the gate and wall thickness are not balanced, shearing, flashing and mold damage can occur. A solution for balancing wall thickness and gate pressure is by either increasing wall thickness near the gate, decreasing injection pressure, or both.
3. Consider Side-Actions and Pickouts
It is also important to consider side action and pick-outs during design. Side actions may be more appropriate for use in a single-cavity mold.
Depending on the part being designed, they will not qualify for multi-cavity tooling in certain situations. The same goes for pick-outs. Manually loaded inserts, or pick out, should be carefully considered.
4. Use of Sensor-Based Technologies
The use of cavity pressure sensing technology is a common solution available to avoid flow variations when filling cavities simultaneously. With the use of sensors, molders can get information regarding the cause of problems in flow pressure and flow imbalance.
Once an imbalance is determined, cavity pressure sensing can help with solutions. It can provide information regarding how full the cavity is. Then the molder can use low-velocity injection to ensure the remaining parts are filled with more consistent pressures.
Navigating the Transition from Single to Multi-Cavity Molds
When deciding whether to use a single-cavity vs. multi-cavity mold, you need to think about your desired production run volume, budget, and lead time. Since multi-cavity molds are more expensive and take longer to create, they aren’t ideal for small production runs and limited budgets.
However, they can produce more parts in the same period of time than single-cavity molds, so they’re best for medium or large production runs. Other design modifications might include:
- Determining the number of cavities, because the cooling system in the mold can affect the number of cavities possible for the given
- Machine, depending on the part geometry
- Using symmetrical designs, as they offer better cooling and flow efficiency
- Paying extra attention to gate types and placement, venting, and molten plastic flow as your mold becomes larger and more complex
It’s always smart to know your market and have a clear idea of how much demand there is for your product. This way, you can create and make your product to save money. When it comes to multi-cavity injection molding, this is especially true.
Since making a multi-cavity mold takes a long time and costs a lot of money, your product teams should research and choose the mold that fits the product and market demand the best.
Multi-cavity Injection Mold (Image Source: Istock)
Conclusion
Multi-cavity injection molding is crucial for mass production of goods. This injection molding process brings on board efficiency, helps cut on costs, reduces production times and is very reliable for high volume and quality parts production.
Nonetheless, this form of injection molding can be very costly especially if defects like warps occur in the process. This can lead to substantial losses to you as a manufacturer. To avoid such defects, you need to consider adjusting the mold gate, utilizing side-actions, incorporating pickouts, adopting a family mold approach and using the right material to achieve quality products.
