Visión Artificial con PCL

Often, the design of the feed system is uncertain. One common issue is the capability of an available molding machine to fill the mold with a material whose flow characteristics are unknown. Alternatively, there may be uncertainty as to the exact melt flow rates and pressures that are required to properly balance a family mold or complex multi-gated part.

In uncertain situations, the mold designer should specify feed system dimensions that are

“steel safe”, which means that the design should call for the removal of less mold steel than may ultimately be required. As such, the mold designer may wish to round the feed system dimensions down one or two standard sizes. By doing so, the mold designer will impose a greater pressure drop and use less material than predicted by the analysis. In doing this, there is still a reasonable chance that this smaller feed system design may function properly.

Furthermore, if the feed system requires one or more changes, then the “steel safe” design may be easily machined to improve the mold performance.

Example: Suggest a “steel safe” runner design if the analysis indicated an optimal diameter of 4.6 mm for a cold runner.

If the feed system analysis resulted in a runner diameter of 4.6 mm, then the mold designer may specify a diameter of 4.5 mm or even 4mm for a “steel safe” design. By comparison, if the mold designer had rounded up to 5 mm, the design would have provided a lower pressure drop but consumed unnecessary material throughout the mold’s entire lifetime.

Furthermore, if the molder desired to reduce the 5 mm diameter, then the mold would 6.5 Practical Issues

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require more extensive rework including pocket milling of the old feed system, the manufacture and fitting of an appropriately sized insert, welding and/or the addition of fasteners, and finally the provision of the new, smaller feed system. While this example focused on steel safe design of cold runners, the steel safe concept should also be applied to hot runner designs.

6.6 Chapter Review

The selection of the type of feed system is one of the most critical decisions in a mold’s design, since it determines the type of mold and largely impacts the mold’s purchase and operating costs. Two-plate cold runner molds are the simplest design, are readily produced, and can be quite effective for a small number of cavities. Three-plate and hot runner mold designs provide for increased flexibility in the feed system design, and are more suitable for a greater number of cavities and/or gates. Of all the designs, the hot runner mold provides the least pressure drop, least material utilization, and fastest cycle times. However, the hot runner system requires a significant up-front investment, greater molder capability, and can impede production of small batches of moldings.

All feed systems should minimize the feed system length to reduce both material utilization and pressure drops. The optimization of the diameters along the feed system requires a trade-off between the pressure drop and volume of the feed system. Smaller diameters provide for less material consumption but higher pressure drops. If the pressure drop through the runner is too high, then the molding machine may not be able to complete the filling of all the mold cavities with the available injection pressure. For this reason, the mold designer should perform analysis appropriate to the molding application, and provide a “steel safe”

feed system design that may be readily altered if needed.

When possible, feed system designs should be naturally balanced by using radial, branching, or hybrid layouts. The artificial balancing of melt flow rates, for example in a family mold or complex multi-gated part, can be accomplished by using different diameters to purposefully impose different pressure drops and flow rates through each branch of the feed system.

Depending on the molding application, shut-offs may be placed at multiple junctions in the runner system to direct the flow to different combinations of runners and mold cavities.

After reading this chapter, you should understand:

The objectives to be considered in feed system design including melt conveyance, minimizing pressure drop, minimizing material consumption, and balancing melt flow rates and/or pressures;

The form, function, advantages and disadvantages of two-plate, three-plate, and hot runner mold designs;

The different layouts of feed system designs, including series, branching, radial, hybrid, and custom designs;

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How to analyze pressure drop in a feed system using the Newtonian and power-law model;

How to optimize the feed system diameters to reduce material consumption without imposing excessive pressure drops;

How to artificially balance the melt flow rates in a multi-gated or multi-cavity mold;

How to estimate the cooling time of a cold runner;

How to estimate the residence time in a hot runner;

How to select the runner cross-section, calculate the hydraulic diameter, and estimate the pressure drop in a feed system with a non-circular section;

How to use sucker pins in two-plate and three-plate mold designs;

When and how to use runner shut-offs;

How to adjust analysis results to provide standard and “steel safe” feed system designs.

Chapter 5 focused on the filling analysis and design of the mold cavity. This chapter focused on the filling and design of the feed system. The next chapter connects the feed system to the mold cavity through the design and analysis of gates. Afterwards, the book moves away from the mold filling system to other mold subsystems such as venting, cooling, ejection, and others.

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6.6 Chapter Review