Hot manifold / runnerless molds

Runnerless molds differ from cold runner molds by extending the injection molding machine’s melt chamber and acting as an extension of the machine nozzle. A portion or all of the polymer melt is at the same temperature and viscosity as the polymer in the barrel of the injection molding machine. There are two general types of runnerless molds  - the insulated system and the hot runner system.

Insulated runner system

Insulated runner molds have oversized passages formed in the mold plate. The passages are of sufficient size that, under conditions of operation, the insulated effect of the plastic (frozen on the runner wall) combined with the heat applied with each shot maintains an open, molten flow path. The insulated runner system should be designed so that while the runner volume does not exceed the cavity volume all of the molten material in the runner is injected into the cavity during each shot. This helps prevent excessive build-up of the insulating skin and minimizes any drop in melt temperature.

Advantages of an insulated runner system compared to a conventional cold runner system include:

• Reduction in material shear
• Faster cycle times
• Elimination of runner scrap
• Decreased tool wear
• Improved part finish
• Less sensitivity to the requirements for balanced runners
• Shorter cycle times

Disadvantages include:

• (Generally) more complex tool design
• (Generally) higher tool costs
• Higher maintenance costs
• Start up procedure is more difficult
• Possible thermal degradation of material
• More difficult to change color

Insulated runner mold.

Hot runners
More commonly used than insulated runners are hot runners which fall into two categories: internally and externally heated.

Hot runners retain the advantages of the insulated runner system over conventional cold runner system and eliminate a number of the disadvantages. The major disadvantages compared to a cold runner mold are:

• More complex mold design, manufacture operation and maintenance
• Substantially higher costs
• Thermal expansion of various components needs to be taken into account

These disadvantages are a result of the need to install a heated manifold, balance heat generated by the manifold and the minimization of polymer hang-ups. The figure below shows a schematic cross-section of a hot runner system. It is often cost effective to produce large volumes with hot runner molds, in spite of high investments. These systems are used for a wide range of applications.

Cross section of a basic hot runner system.

The electrical/electronic industry uses small components, like connectors and bobbins that are molded in multi-cavity molds. On the other hand, large multi-gated parts are used in the automotive industry, e.g. bumpers and dashboards. Yet both can benefit from the cost and technical advantages of hot runners.

Cycle time reduction is possible when cooling of a cold runner would determine the cycle time. Following are typical advantages and disadvantages of hot runner systems:

Advantages
Production increase (cycle)
Material saving
Quality improvement
No waste
Automatic degating
Energy saving
Flexible choice gate location

Disadvantages
Higher investment
Critical molding conditions
Critical temperature control
Start-up problems (tailing)
Color change problems
Abrasion (reinforced plastics)
Critical mold design (no dead spots)

In selecting a hot runner system, the following factors have to be taken into account:

Economy
Investment
Number of parts
Cycle times
Material waste
Energy
Regrinding

Product
Dimensions
Shot weight
Gate/sink marks
Reproducibility
Required tolerances/warpage
Fiber orientation

Process
Start up
Total flow path
Pressure distribution
Melt homogeneity
Residence time (esp tailing)
Color change

Material
Flow behavior
Melting temperature/range
Process window
Thermal stability
Reinforcement
Additives

Taking all these factors into consideration, there is still a choice between many types and variations of hot runner manifolds and nozzles. General recommendations cannot be given. The best option depends on the thermoplastic and the requirements of the specific application.

The following guidelines should be respected:

• Natural runner balancing
• Minimal pressure-losses
• Sufficient heating capacity for manifold and each single nozzle
• Accurate, separate temperature controls for manifold and nozzle
• Effective insulation between manifold and mold
• Optimal mold temperature control
• No dead spots and flow restrictions in manifold and nozzles
• Limited residence time of melt in the hot runner
• Adequate sealing of runners

The figure below shows various basic types of nozzle configurations with their typical advantages and disadvantages.

With respect to externally and internally heated manifolds the same conclusions are applicable as for nozzles.

A relatively cheap and robust alternative for hot runners is the hot runner/ cold sprue. The hot runner manifold is followed by a short cold sprue that eliminates the use of expensive nozzles.

Advantages and disadvantages of basic hot runner configurations.