Creating the most effective hot runner system design that includes manifolds, nozzles and full hot-halves demands a collaborative approach among the moldmaking supply chain: mold builders, molders, original equipment manufacturers and technology suppliers. The first step is reviewing and understanding the vital information that the hot runner manufacturer needs to quote and build the appropriate hot runner system. Those details include part weight, plastic material, number of cavities, nozzle style, gating style, mold size and mold material requirements. Only then can a standard or customized solution be identified with the proper design process as the right hot runner solution.
Additionally, these key hot runner selection criteria can help hot runner system suppliers decide between a standard and a custom hot runner solution:
Part weight. Calculate the part weight in grams for a single molded part by multiplying the specific gravity (g/cm3) of the plastic material by its volume (cm3). This data is available within the 2D drawing or 3D data of the molded part.
Part weight in volume will help determine the proper melt flow channel sizing within the system. Your supplier will calculate this information and use it to correctly build your manifold supply channels and system.
Plastic material. Obtain data sheets and design information on the plastic material that will be processed from your material supplier/manufacturer. If this is not available, or if you are only in the quoting stage, then you can access several plastic material resources online for similar data. For example, see the Plastics Technology Universal Selector.
The required data should include the plastic material’s specific gravity in g/cm3, melt volume flow rate in g/10 minutes and molding shrinkage in percentage. The mold builder needs mold temperature in °C while the hot runner manufacturer needs to verify melt processing temperature in °C versus the mold temperature. Some data sheets omit data and/or go further into the mechanical properties of the material. For example, the data sheets may include properties like ABS (acrylonitrile butadiene styrene) specific gravity = 1.2 g/ cm3, melt flow rate = 8 cm3/10 minutes, shrinkage = 0.1 to 0.4 percent, and Nylon 66 PA (Polyamide Nylon 66) specific gravity = 1.44 g/ cm3, melt flow rate = N/A, shrinkage in flow direction = 0.007 cm/cm and shrinkage in transverse of flow direction = 0.010 cm/cm.
Cavities. Address the number of cavities next. The customer provides this data based on the required production level, which dictates the number of molded parts and nozzles. Hot runner manufacturers may recommend ways to reduce the number of nozzles per cavity or cavities, decreasing mold design and component cost and assembly size—for example, a single cavity with a single hot runner nozzle to multiple cavities on a single hot runner nozzle or multiple cavities with multiple hot runner nozzles. This is the area where your hot runner supplier can work with you directly to keep your project in or under budget while still producing valid molded parts.
When considering nozzle size, note that most molded parts less than 1 gram require specially-sized nozzles, while standard nozzle systems work most effectively with molded parts that are more than 1 gram in size.
Nozzle style. Evaluate proper nozzle options. Hot runner nozzles can be used alone, as in mono shot nozzles, or they can be assembled with a standardized manifold system to supply many hot runner nozzles at once. A mono shot nozzle is a single hot nozzle that moves melt directly from the molding machine supply nozzle through to the cavity.
Determine how the nozzles will gate into or supply the molded part or parts in the system assembly. With direct gating, each cavity will have a single nozzle. For example, an eight-cavity mold will use eight nozzles. When multiple gating into a cold runner, a single nozzle will supply many cavities. When direct multiple gating, a single nozzle will supply many gates in one cavity or a single nozzle will supply many cavities.
Gating style. Determine the gate style that the molded part requires. Your customer’s molded part requirements or the plastic material that is molded generally define the gate style. For example, some materials work better with valve gates at larger diameters because of the material shear rate and stringing, while other materials work better without valve gates.
Gate styles include pinpoint clean that shows on the part but is not raised; protruding pinpoint that shows on the part and is raised; and smooth and sprue gate point with or without circular gate markings that shows a circular mark on the molded part. The gate point will determine which nozzle tip to use on the nozzle body assembly and what mark, if any, the hot runner system will make at the gating point on the molded part.
Mold size. Calculate mold size. The mold builder will create a preliminary mold layout that shows cavity spacing and factors in the overall thickness of the stack-up plates that are required in the final mold base assembly. This helps to determine the appropriate mold base size (length, width and height), and in turn, determines whether a standard metric mold base size, as shown in Table 1, or a custom mold base size is required.
Remember also to include sufficient space for insulation around the manifold, the cable channels required for the nozzles and the block-heating units, and the control connection plug wiring slots.
Mold material. Choose proper steel types for manifolds and mold plates. Toolox 33, 1.2312 or #2 holder block steel are routinely used for general-purpose plastic materials. Stainless steel types like 1.2085, 1.2099 or #7 are commonly used for corrosive or high-wear plastic materials.
Hot runner technology, with its advantages of low scrap rats, shorter the molding cycle times, has become an important direction of development of plastic injection molding process. In western countries, hot runner system are widely employed in injection molding process, in our local mold making industry, hot runner system is also a trend since our local injection molders face labor cost and raw materials increase.
Hot runners inject plastic directly into the cavities, as opposed to the sprue and runner system used in a cold runner system. One of the biggest advantages to a hot runner system is that the plastic in the runners will never solidify. This decreases the cycle time, allowing for faster processing. Another benefit of this system is a reduction in plastic waste, as the material does not harden until the mold is filled. The lack of a sprue system substantially cuts down on the amount of trimming required to smooth the final product. Hot runners operate through an additional manifold which is bolted to the mold assembly.
Although innovation in design is very important, you need to consider its positive and negative effects on your specific application. A good rule of thumb is to keep it simple and direct. For example, use a direct approach to keep the manifold and nozzles at the desired temperature, and the plastic material flowing evenly. Over-simplified or over-designed system features could be signs of potential problems. The system’s design should have everything it needs and nothing more.
Possible red flags may include belabored heater design, heaters wound too tight, heaters with low contact area, heaters with wattage that is much higher or lower than comparable systems, the use of exotic metals, components/manifolds that are too thick or too thin, and features that appear unnecessary.
The mold designer should communicate with the professional hot runner engineer to avoid mistakes when using hot runner system