Runner Weight

Due to this being an engineering cost estimate only and not intended to be used as an actual quoting program, a "rule of thumb" approach of 2 to 25 grams for the sprue plus 1 to 25 grams for each cavity can be used for this input. Designers should be aware that the design of a runner system or the need to use virgin material only can have an impact on part cost. The runner weight is required to determine if the material used in the runner system can be used back as regrind in the molding process. The runner is the feed channel system that is used to get the plastics material from the machine to the cavity in the mold. Most thermoplastics materials can use a percentage of regrind depending on the type of plastics and the end product molded. Products for some industries such as the medical industry, routinely use only virgin materials. In those cases material used in the runner system has to be scrapped and added to the part cost. Calculating runner weight is very difficult without having at least a preliminary tool design. Runnerless molds or hot runner systems, can reduce and often times eliminate the waist of regrind from the runner system. The cost of tooling is higher but often times a financial payback can be realized. The use of a hot runner system can also reduce cycle times in some applications. Using harder flowing materials require larger cross sectional areas of the runner to facilitate flow to the cavity. This increases the volume of material required in the runner system and often times increases cycle time to adequately solidify the material in the runner system prior to opening the mold and ejection of the part. There are numerous types of conventional runner systems used. Two of the most common are full round and trapezoidal. An initial part of the runner system is the sprue. This is the full round channel that the plastics melt travels from the machine tip to the mold before entering the runner system. The primary runner system delivers melt from the sprue to individual cavities in the case of a single of two cavity mold. A secondary runner system branched off the primary is used for multi-cavities. The length and cross section of the runner system is determined by the part geometry and type of material used. A minimum loss of pressure and temperature should be considered in the design of the runner system. Part size and the need for space between cavities determine the length of the runner system.

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