When it comes to design, multi-cavity molds for rubber injection involve a number of critical considerations that can significantly impact production in terms of efficiency, quality of the items to be molded, and overall cost of the project. In today’s article, we are going to share with you a list of key factors to consider and the main challenges a mold maker needs to face associated with multi-cavity molds.
1. Material Flow Management
First and foremost, one has to keep the compound at the heart of the project. When considering the compound, we are not only talking about the properties that may improve the functionalities of the final product but also its chemical structures and other physical factors, e.g., how it interacts with other materials, i.e., the steel of the mold plate, and hydraulics, that is, how the compound behaves when flowing through the mold runners under a certain amount of pressure. When designing multi-cavity molds, we therefore consider flow paths and viscosity very carefully.
Here’s why.
Complex Flow Paths
Multi-cavity molds require intricate designs for sprues, runners, and gates to ensure that molten rubber flows evenly into each cavity. The complexity increases as the number of cavities rises, making it challenging to achieve uniform filling and pressure across all cavities
Viscosity Considerations
Using materials with lower viscosity, such as liquid silicone rubber (LSR), can help mitigate flow challenges. These materials tend to fill cavities more uniformly, reducing the risk of defects caused by uneven flow. However, they require a specific press machine and unique processing, not to mention that it may just not be the right material for your current project.
2. Temperature Control
Temperature is another essential aspect to consider in the design phase of our mold. Hence, it is paramount to design a mold so that the operator can fully control temperature and ensure even heating to avoid lumps, compound burning, or flow issues that can severely compromise the final output, if not damaging the tool. Since each compound has its own unique curing times, which depend not only on its chemical and structural properties, as highlighted above, but also on the mold geometry itself, one wants to avoid thermal variations at all costs. Depending on the project (which can include press machine specifications as well), a mold designer may want to consider how to achieve the best results, calculating the diameter and depth of each runner, cavity paths (in terms of length and bends), material to be used, surface treatment, and other features of the tool and the press machine where it is going to be installed (e.g., heating plate, thickness of the mold plates, and consequent temperature propagation).
As a consequence, when dealing with temperature control, there are two crucial aspects you want to keep under control, that is, thermal variations and the cooling of the channels.
Thermal Variations
Maintaining consistent temperatures across all cavities is crucial. Variations can lead to differences in curing times and item quality, resulting in defects such as warpage, bubbles, incomplete filling, or even clogging the machine that may lead to cumbersome extraordinary maintenance with subsequent production downtime.
Cooling channels
Effective cooling channels must be designed to manage heat distribution evenly throughout the mold. One may want to consider the possibility, if within budget, to install a custom CRB. CRB stands for Cold Runner Block, a specific thermoregulated injection system that allows to keep the compound at a constant temperature. Did you know it? Our company has its own CRBs, and our staff of professional experts is at your disposal to develop a custom CRB, fully compatible with your mold and project specifications.
3. Gate Design
Gate design is maybe one of the most intriguing yet underestimated steps of a mold design. This tiny little part is key to achieving outstanding results or ending up with a complete failure. Injection gates are extremely important as they can help better distribute the compound within the mold cavity according to the dimension, compound flow, orientation, and angle between the cavity and runners, among other things. As a consequence, not only are gate type and geometry essential, but their strategic placement can play a pivotal role while molding.
Gate Placement and Type
Proper gate design is essential for smooth compound flow. When it comes to gate type and placement, there are several techniques, which vary greatly depending on the material processed (plastic vs. rubber compounds) and unique project specifications. For example, designing a mold from scratch is different from changing an existing one, for instance, when transitioning from single-cavity to multi-cavity molds, in which case existing gates may need readjustments to accommodate changes in item orientation and flow dynamics. There are many different types of gate design, but this is a complex issue, a thorough discussion of which goes beyond the scope of this article. We are going to dedicate a full article on gate types, positioning, and their pros and cons according to project specifications another time.
4. Tooling Complexity and Cost
When embarking on a journey towards the perfect multi-cavity mold for your project, it is fundamental to keep in mind that it can be quite an investment, so choose wisely according to your production needs and volumes. While multi-cavity molds offer significant advantages in terms of production efficiency and cost savings for high-volume batches, they also introduce complexities that require careful planning and design. Other than the initial investment for the actual design and construction of the tool, one needs to keep in mind other variable costs, such as maintenance. Also, given the complexity multi-cavity molds entail, you may want to consider the cost to develop an effective quality control procedure to keep consistency throughout your production cycle.
Higher Initial Investment
Multi-cavity molds typically require a greater upfront investment compared to single-cavity molds due to their complexity and the materials needed for construction. This makes them less suitable for low-volume production runs where the cost per part may not justify the initial expense. Rigorous quality control measures are quintessential to monitor all the factors described above during production, so you’ll always know where to make adjustments when needed. This results in a longer lifespan of your mold, reduced maintenance costs, waste minimization, and faster lead time.
5. Quality consistency & maintenance
Achieving consistent quality across all parts produced by a multi-cavity mold can be challenging. Variability in filling, cooling rates, and material properties can lead to defects such as sink marks or dimensional inaccuracies. Also, maintenance can reveal a real challenge. The maintenance of multi-cavity molds can be more demanding than the one you do on your average single-cavity mold. If one cavity develops a defect, it can affect the performance of the entire mold, leading to increased downtime and repair costs.
A trusted partner in mold making
Are you considering purchasing a multi-cavity mold for your next project or adapting your existing single-cavity mold to boost production? In either case, our staff is here to help you to give you advice for dimensional adjustments, wall thicknesses, implementation of ejector systems and punching tools, or installation of a custom CRB.
Got questions? Do not hesitate; contact us now! Your next greatest project may be just one dream away.
