SIGMASOFT VIRTUAL MOLDING INCREASES PROFITABILITY IN LSR APPLICATION In a four-cavity LSR mold, CVA Silicone, the French leader of Liquid Silicone Rubber Injection Molding, based in Saint Vidal, France, was able to reduce scrap production, optimize thermal layout and avoid costly quality issues by using SIGMASOFT® Virtual Molding.
The demand for liquid silicon rubber (LSR) products is growing. Particularly in the medical and baby care markets, its high thermal stability and very good physiological properties make LSR the material of choice for an ever increasing number of applications. However, molding LSR can be challenging: because it is a low viscosity reactive material, the processing window can be narrow and the scrap produced cannot be re-processed.
Also, proper mold venting is paramount to avoiding air traps. The position of weld lines and filling patterns, such as jetting, can affect the quality of the final product. In addition, proper mold tempering must be guaranteed during the entire molding cycle, in order to ensure both a cost-efficient cycle time and a quality product.In order to maximize profit and reduce scrap, it’s important to have a thorough understanding of the complete process and to anticipate possible problems, including the flow and curing behavior, as well as the tempering conditions through the complete molding process.
CVA Silicone, reached out to SIGMA to assist the decision making process regarding a new four-cavity mold to produce silicon nipples. SIGMASOFT® Virtual Molding was used through each stage of the mold development, to evaluate the runner design, the mold tempering and the overall efficiency of the molding system.
Finding the best runner In a first approach, an X-shaped runner configuration, as shown in Figure 1a, was designed. The channels are cylindrical with a minimum diameter of 4 mm. Still, the runner volume represented 52% of the total shot volume.
This configuration showed that weld lines and air traps would appear along the part, as seen in Figure 2, an undesirable outcome both for mechanical integrity and part aesthetics.The alternative of using a cold runner was then considered, shown in Figure 1b. In a cold runner, no material reticulation takes place, and therefore no scrap is produced. However, due to the additional investment required, the feasibility of this approach had to be carefully evaluated.SIGMASOFT® Virtual Molding was used to assess different cold runner layouts.
Several virtual iterations were investigated where the parameters of balanced filling, pressure drop, flashing, clamping force and curing time were evaluated. In this iterative approach, the mold was considered just as it works in reality, with all its components, each one with its own material properties. The optimal runner configuration, which minimized pressure drop and flashing while ensuring an optimum thermal performance, was found to be as presented in.
The mold configuration used is presented in Figure 3. The cycle time, energy efficiency and pressure required confirmed the alternative as economically viable, and an important reduction of 52% in volume per shot was achieved.Curing: the result of a complex thermal interactionOnce the optimum filling layout was found, an analysis of the thermal mold performance was needed.
In this case, the objective was to obtain the real mold temperature, at each point and each step over the molding process to predict the energy available for curing the LSR material.The mold temperature gradient was reproduced in SIGMASOFT® Virtual Molding (as seen in Figure 3) over several molding cycles.
The production parameters were considered as they would be in reality; filling and curing conditions, even including the non-productive times between cycles. The mold starts at room temperature, and is tempered prior to the first molding cycle by the heating cartridges present. Several cycles are “run” virtually, just as in the injection molding machine, until the system achieves a quasi-stationary thermal state – the same that in reality would produce consistent part quality.In Figure 4, the separation between the hot and cold regions of the mold is evident. By being able to visualize this behavior it was possible to improve the geometries of heater bands and to plan the positioning of sensors in the system.In Figure 5, the temperature distribution in the movable mold half is presented.
This is the real temperature found after several molding cycles, produced by the interaction of the heating system, the mold material, the cold runner and theincoming cold melt. As seen, there is a significant temperature gradient in each cavity.
The bottom of the cavity’s temperature is 170°C, while the nipple tip it was almost 20°C lower. This large temperature gradient induced variations in the curing behavior and would have increased the cycle time.For comparison purposes, the curing results achieved with a homogeneous mold temperature of 160°C are presented in Figure 6a. Under this ideal scenario, a curing time of 30s is required.
In Figure 6b, the curing results produced with the real mold temperature are presented, making it evident that at 30s only 43% of curing has been achieved on the partЂs tip. Without this evidence the mold performance would have been compromised, leading to costly production.
Several mold trials would have been required to find the solution.
COMPANY PROFILE CVA Silicone, located in Auvergne, France, was founded in 1960 as one of the French pioneers in liquid injection molding. The company processes more than 200 tons of LSR for the sectors of baby care, medical, cosmetics, connections and optics, and is nowadays the most important LSR transformer of the French market.