Runner system multi-cavity

This type of mould, also often referred to as a three plate mould, is used when it is desired to have the runner system in a different plane from the parting line of the moulding. This would be the case in a multi-cavity mould where it was desirable to have a central feed to each cavity (see Fig. 4.38). In this type of mould there is automatic degating and the runner system and sprue are ejected separately from the moulding. 

Balancing the Runner System in Multi-Cavity Injection Molds... 

Inevitably, in multi-cavity injection molds, some of the mold cavity gates are located further than others from the sprue that delivers the melt from the plasticating unit. If the runner system that distributes the melt from the sprue to the individual cavities has a constant... 

Sample balancing problem. Let us consider the multi-cavity injection molding process shown in Fig. 6.54. To achieve equal part quality, the filling time for all cavities must be balanced. For the case in question, we need to balance the cavities by solving for the runner radius R2. For a balanced runner system, the flow rates into all cavities must match. For a given flow rate Q, length L, and radius R, solve for the pressures at the runner system junctures. Assume an isothermal flow of a non-Newtonian shear thinning polymer. Compute the radius R2 for a part molded of polystyrene with a consistency index (m) of 2.8 x 104 Pa-s" and a power law index (n) of 0.28. Use values of L = 10 cm, R = 3 mm, and Q = 20 cm3/s. 

In injection moulding processors often use multi-cavity moulds to make several products in one cycle. The runners required to supply material to each cavity contain a considerable amount of material which, if the product itself is small, may be of the same mass as the parts themselves. This is evidently quite uneconomic. Hot rurmer systems solve this problem by keeping the material in the runners from solidifying between moulding cycles. 

The runner system is used to convey the plastic melt from the sprue to the gate. It is important in producing identical quality of plastic parts. Cost reduction can be carried out by using multi-cavity mold with a balanced runner system and the mold fills in the cavity at the same time. The runners deliver the plastic into the part cavities (usually multiple). 

Direct gating to a cold runner in a multi-cavity mould. In this design, the CR is only partially replaced by an HR system. A simple open HR nozzle eliminates the sprue (Figure 1.3e), thus shortening the reqnired mould opening path and making it easier to separate the sprue protrusion from the moulding on the transporter belt. This method is used when the production volume or product shape does not jnstify the use of HR nozzles for each cavity. 

Gating via a distribution system (indirect) to a cold runner in a multi-cavity mould. 

The use of the hot runner technology allows only certain component geometries and mold concepts. With hot runner systems, gating positions are achieved that cannot be reached conventionally the active movement or complete avoidance of fill lines can be implemented by sequential injection multi-component injection molding can be performed with up to five materials in a mold high-speed stack molds can be built film and fabric can be back injected gas and water injection technology can be implemented multi-cavity molds can be realized clean room production can be optimized and the co-injection technique can be enabled. 

Interesting hot runner solutions have been specifically developed for multi-cavity, gateless injection of smaii and micro injection molded parts in a confined space. As a standard system, solutions with 4, 8, and 16 injection points and injection intervais of only 8,10, or 20 mm are offered. Special gauges are available on request. The minimal installation dimensions of hot runner systems enable tbe use in molds with outer dimensions of 75 x 75 mm. Therefore, even with mold solutions for mini-injection molding machines such as babyplast, modular, multi-cavity hot runner direct connections can be realized. The outer dimensions of mini hot runner systems, illustrated in Figure 2.59, are only 34 x 32 x 100 mm. 

This is the simplest mould design. Mould cavities are formed in one plate only with the stationary half of the mould blank. A central sprue bushing can be placed into the stationary half of the mould or it is possible to have a direct runner system to a multi-impression mould. The moving half of the mould contains the ejection mechanism. This is illustrated in Figure 4.1. 

These are normally used when multi-cavities are involved and semi- or fully-automatic working is required. This type of mould, as its name suggests, has an extra plate (see Figure 4.4). This plate usually continues the gate on one of its sides with the complete runner system, preferably trapezoidal. The opposite side of the plate carries part of the mould form (usually the female part). 

The Mono-Sandwich technique for co-injection moulding was described in Section 10.8.2.3 to which the reader should refer for machinery details. This technique can also be used for over-moulding by using the core back technique, again described earlier. In this technique termed the monosandwich 5 process, an additional valve is required in the runner system that ean release different valves as necessary . Once the melt is layered, the first component is injeeted. The valve is switched within the mould to expand the cavity and then the rest of the shot is injected to ereate a multi-shot component. 

---