[Abstract] Structural characteristics of manifold of multi-cavity hot runner assembly are introduced, design and calculation points are given in combination with actual case of a mold multi-cavity open hot runner.

At present, application of injection molds in medical packaging industry is becoming more and more extensive. From perspective of cost, proportion of molds using hot runner systems is also increasing. In addition, output of plastic parts in medical packaging industry is relatively large. Spacing of plastic parts is small, application requirements for hot runner system are also high. Demand for stable multi-cavity hot runner systems in market is increasing.

Figure 1 shows runner scheme of a 8-cavity injection mold.
Scheme A: Common parting surface mold structure, using cold runner plan, with cold runner.
Scheme B: Mold uses an open single nozzle, which eliminates waste of sprue and can also shorten length of sprue. This scheme can reduce waste of sprue and runner by about 40%, injection molding cycle can be shortened by about 10 %. Most of cooling time of molding is at the position where cooling main channel and flow channel are combined, which is usually more than 2.5 times the thickest part of plastic part.
Scheme C: Mold uses a hot runner plate combined with two open nozzles. Compared with Plan A, main runner waste is reduced by about 60% to 70%.
Scheme D: Each cavity of mold is filled with hot runner, eliminating cold runner. Advantage is that it allows injection temperature to be lowered, which can further shorten cycle. Plastic parts are injected at the top of middle, and there is no need to recycle waste materials, which can save a lot of costs. This part of cost can be added to mold, but mold cost is relatively high. Usually this kind of multi-cavity mold should be designed for large demand for plastic parts. Short-term value created by use of multi-cavity hot runners can make up for increase in mold costs, so value of existence of multi-cavity hot runners is more obvious.

 

Figure 1 8-cavity injection mold runner scheme
a — — Plan A b — Plan B c — — Plan C d — — Plan D

For multi-cavity hot runner system, plastic melt must be transferred to each cavity under same pressure and same temperature to achieve principle of balanced filling. In multi-cavity mold, shape and size of a product are two types that are completely consistent and inconsistent, but both need to be injection molded at the same time. In order to ensure production quality, a hot runner system is required to complete melt transfer.

Flow balance in hot runner system can enable a wider process window during injection processing, can improve quality of finished product. In design of hot runner system, following two ways can be used to achieve filling balance.

(1). Design runner system with equal flow length, consider balance of geometric parameters, which is called natural balance, as shown in Figure 2. With this design, flow distance from main nozzle and main runner to each nozzle is equal. As long as symmetrically arranged corresponding runners have same radius of the circular section, balanced pouring of each gate can be realized.

 

Figure 2 Natural balance

(2). Design runner system with same pressure drop at each injection point. Different flow path lengths are compensated for cross section of flow path, balance calculated by principle of rheology is provided, which is called rheological balance, as shown in Figure 3. In this design, flow distance from main nozzle to gate of each nozzle is different, different flow length ratios will lead to differences in filling pressure of melt of each injection gate. However, after rheological calculations and adjusting radius of each runner, balanced filling of each gate can also be achieved.

 

Figure 3 Rheological balance

On runner plate of a multi-cavity hot runner system, layout of runners and design of runner dimensions must meet balanced filling of injected plastic melt to each cavity to ensure stable quality and consistent accuracy of plastic parts. However, there are also defects in natural balance of runner: First, this kind of balance will cause flow length of naturally balanced runner to be too long, pressure loss in hot runner system, the longer residence time of plastic at injection temperature in hot runner, leading to risk of decomposition of material; second is same as balance runner of cold runner mold, resulting in same flow length ratio of each gate, but balance effect may not be achieved. As shown in Figure 4, due to different distribution of shear rate of flow channel section, it is not necessary that each cavity is filled or filled at the same time (memory effect of plastic flow). In order to avoid this phenomenon, many factors need to be considered in design. Pay attention to distance between nozzle and nozzle of injection molding machine, distance between nozzle and nozzle. In a mold with a large number of cavities, multi-level runners are assembled together to achieve a natural balance and a short process.

 

Figure 4 Shear rate distribution of flow channel section

In order to achieve a natural balance of multi-cavity mold, mold adopts arrangement as shown in FIG. 5, adopts a symmetrical equal path distribution cavity, and also uses a multiple of 3 to arrange cavities in an equal path.

 

Figure 5 Arrangement of natural balanced runners
a — — Symmetrical equal path arrangement b — — 3 multiples of equal path arrangement
Geometric balance is based on layout of flow path of each injection point. Length of runners in all cavities is equal and diameter of runners at all levels is same. Gate is same. Common natural balance layout of multi-cavity hot runner system is arranged symmetrically according to length of runner. Runner branch is usually divided into two by one, three by one, and four by one to reduce runner reasonably.

The overall length of non-balanced layout runner is shorter than length of balanced runner, but different melt pressure from gate may affect quality of molded plastic part, it can be matched by size of runners and gates at all levels. Adjustment to achieve same pressure at all output points of gating system. Figure 6 shows natural balance flow channel design. Flow channel plate adopts a double-layer design. Purpose is to have same flow length from main nozzle to nozzles at all levels. Figure 7 shows arrangement of unnaturally balanced runners. Straight rheologically balanced runner plate is a single-layer design. Compared with geometrically balanced runner plate, length of runner is shortened, height of runner plate is also reduced by about half.

 

Figure 6 Arrangement of natural balanced runners

 

Figure 7 Unnaturally balanced runner arrangement

Rheological balance design of16-point single-layer straight drainage channel in this example is shown in Figure 8.
(1) Last stage of 2*2*4 in this case is an unbalanced flow channel layout, and front 2*2 stages are a balanced layout.
(2) Rheological balance is achieved by adjusting diameter of flow channel to achieve same pressure loss in AB and AC processes.
(3) AC process of rheological balance calculation has a shorter length on manifold. In order to achieve pressure balance with AB process, diameter of lower flow channel is also added and adjusted together with diameter of flow channel plate.
(4) Non-geometrically balanced runner plate with 16 cavities is only a single layer. Compared with geometrically balanced runner plate, length of runner plate is shortened, height of runner plate is reduced by half, steel used for runner plate is saved, installation is convenient, and heat dissipation area is also reduced.

 

Figure 8 Rheological balance design of a single-layer straight drain channel
Some suggestions on rheological balance design of a multi-cavity unbalanced runner system:
(1) Unbalanced design of subtle parts of runner system with one mold and multiple cavities can achieve rheological balance through simple calculations, shorten length of runner, reduce stratification of runner, and compress thickness of runner plate. As a result, the overall thickness of hot runner part of mold is improved.
(2) For completely unbalanced design of runner system with multiple cavities, flow balance equation can be used for preliminary calculation and design, then rheological balance method can be used for correction, so that pressure of each gate is basically equal.
(3) Balance design of a runner system with multiple cavities also needs to design runner size according to principle of rheological balance, so that shear rate and shear force of plastic melt in manifold pipes at all levels are basically unchanged, so as to achieve uniform temperature distribution of entire runner plate, and there is no local high temperature that affects flow of plastic melt.
(4) Non-equilibrium design of multi-cavity runner system reaches rheological equilibrium. After rheological equilibrium is reached, melt shear rate and shear force in various runners will change, and transmission of plastic melt will cause instability. In order to prevent such a situation, it is necessary to calculate in advance fluid shear rate limiting flow passages of each section.
(5) Flow balance design at all levels, calculate the best shear rate and allowable pressure loss, you can use DIMH hot runner system design program to optimize design of runner system (Design of Injection Mold for Hot Runner System).

For balanced design of a multi-cavity runner system with one mold, double-layer runner plate design cannot be avoided. Let’s analyze advantages and disadvantages of double-layer runner plate design.
(1) Shortcomings of double-layer runner plate design, 1 mold with 8 cavities or less, generally adopts a single-piece splitter plate with double-layer runner design, which will not increase thickness of mold, but runner is slightly longer. If manifold design of 1 mold 16 cavities usually adopts multi-layer manifold design.
With a 1-mold 16-cavity splitter plate design, runner plates are usually two-layered, so that fixed mold of mold will increase thickness accordingly, as shown in Figure 9.
(2) Multi-layer splitter plate design, runner plates are superimposed on it. Temperature layout of two splitter plates is relatively complicated, it is impossible to avoid temperature interference, and there may be local high temperature phenomena. It is recommended not to use such a design for heat-sensitive plastics to avoid risk of plastic degradation.
(3) Multi-layer runner design of manifold can not avoid problem of plugging during processing. Due to balanced design, number and shape of runners are relatively complicated, plugging requires space, so that design of splitter plate must leave space, resulting in a corresponding increase in thickness and width of splitter plate. Blocking position also needs to be left with a lateral platform to facilitate positioning and cause a waste of mold space.

 

Figure 9 Double-layer runner plate

Design and processing of multi-layer runner plates need to be carefully calculated, processing of corners and bifurcations of runners, butt joint of intermediate cross parts are relatively complicated.

(1). Inlaid manifolds, because there are cross flow channels in multiple directions, in order to avoid thin iron sharp corners in the middle, an inlaid design is usually adopted, which not only facilitates processing of flow channel, but also eliminates two end faces of flow channel. Plug can also ensure curved surface accuracy of bends and forks, as shown in Figure 10.

 

Figure 10 Inlaid manifold

(2). Plug-type manifold is mainly suitable for a row with two or more points, usually used for straight rows of more than two points. After runner is machined, two ends are blocked by forming plugs and cock-type blind caps. Plastic melt has low turning resistance and will not be stagnated, as shown in Figure 11.

 

Figure 11 Plugged manifold

Hot runner technology has now entered a mature stage, and injection molds using hot runners have accounted for more than 60%. Hot runner design of a mold with multiple cavities must pay attention to residence time of fluid in each runner, consistency of pressure and temperature from each runner to cavity, also consider machining and assembly errors. In short, we need to think from multiple dimensions such as energy saving, material saving, space saving, we must be good at summarizing and learning from.