Hot runner systems are a key component of multi-cavity injection molds. Conversely, hot runners are perhaps the least understood mold component from the mold manufacturer's point of view. In reality, hot runner systems are complex and include various aspects of mechanical action, heating technology and precision manufacturing. The successful application of hot runner systems in the injection molding environment requires a detailed analysis of the injection port design form, cavity geometry, resin and mold worker.
The three main steps involved in the application analysis include: computer-aided engineering analysis, resin testing, and design of the hot runner concept.
Computer-aided engineering analysis tools, including MoldFlowTM analysis software and others, can be used to analyze the flow of molten plastic in a mold system. These programs are often used by mold designers to predict injection pressure, flow lines, porosity, shrinkage, and warpage of parts.
Computer-aided analysis is applied to mold production projects at the earliest possible stage. Even a rudimentary analysis at the initial stage has the potential to address important issues such as injection molding possibilities, production cycle times, pressure requirements, and predictions of flow lines.
With the help of computer engineering software, different scenarios can easily be repeated, which helps to reduce production costs and test time.
The use of flow modeling procedures for part analysis is very common practice, but the addition of hot runner manifolds to the analysis process is a recent development. Modeling the geometry of the manifold melt channel will facilitate more accurate predictions. Also, these software programs will be very useful when it is necessary to balance a mold with multiple cavity sizes.
Resin testing is used to evaluate the processing performance of a resin configured to a certain composition. Resin testing can be performed in real applications where only a short portion of the resin has been hot runner treated or not treated at all. The purpose of designing a resin test is to evaluate the process window of the resin and each combination of components. The basic steps of the test are as follows:
(i) Evaluate the injection rate and the corresponding pressure to determine the optimal injection time for this test;
(ii) Establishing a temperature window for the hot runner system that will facilitate the production of parts that meet the requirements;
(iii) Vary the holding time and pressure experimentally to determine the baseline required for part production with the best size and best injection port quality;
(iv) Evaluation of the sensitivity of the resin to the residence time;
(v) Optimizing each mold parameter and evaluating the total application cycle time.
In addition to the standard test procedures, specialized tests can be performed to evaluate special processing parameters or requirements. Prior to completing the mold design, a resin test allows the customer to obtain the actual process data required prior to the initial mold test and then provide samples of the molded part.
Another important step is the design concept of the hot runner. A detailed design concept, including manifolds and platens, which will become an important part of the mold audit.
Hot runners are a complex and somewhat superior mold part. In a mold production project, CAE computer aided engineering analysis, resin testing and design concepts can be done by the hot runner system supplier. By having the hot runner supplier work together at the beginning of a project, the mold designer is able to further optimize the final product.
Manifolds are used to ensure that the melt channels can be arranged in the most efficient way. Ideally, the melt channels are designed symmetrically, with the same flow length and number of turns in all downstream runners.
In the case of a multi-cavity mold or an asymmetrical mold, the melt channels may include artificial lengths and turn points to be able to properly balance this system. This concept is helpful for both the mold designer and the hot runner designer to ensure the best manifold design.
In addition, platen technology is used to ensure that the closure height and key features required by the customer can be designed. Since hot runner nozzles are included in the nozzle, the mold designer also has to verify that the proximity and cooling of the injection port will meet the hot runner manufacturer's requirements.