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HOT RUNNER SYSTEMS AND SUPPLIES

Hot Runner Systems and Supplies

Hot runners deliver the material into the mold to make the part. These systems consist of hot halves, nozzles, gates, and temperature controllers-all of which must be monitored, maintained, and cleaned to guarantee a quality end product.

FAQ: Hot Runner Systems and Supplies

What are the advantages of hot runner systems?

The key advantages are:

Elimination of the Runner
With materials becoming more sophisticated and expensive, the cold runner becomes, in turn, expensive scrap. Whether or not the runner can be reused - and in many medical and food applications they can't - regrinding and storing the runners is costly, noisy and dusty. In high-speed, high-cavitation molding operations, the scrap and regrind would be huge problems. When using hot runners, these regrind problems and their associated costs are non-existent.

Cycle Time Reduction/Lower Processing Costs
In many cases, the weight of the cold runner is up to half the total shot weight and usually much thicker in section than the molded part wall thickness. Doing away with the cold runner will reduce the cycle time since cooling time is controlled by the thickest section. In addition, there will be further cycle timesavings in the injection screw recovery and injection times because of the smaller shot size.

Improved Molding System Efficiency
Hot runner molds are generally easier to start because there aren't any solidified runners to be removed after each under-filled shot until a full mold shot is achieved and automatic cycling can begin. In hot runner systems, the mold is ready to run when the system reaches operating temperature. Also, with hot runner systems, lower injection pressures can generally be used, which will reduce mold and platen deflection and keep flash caused by mold component movement to a minimum.

Optimal Part Quality
Eliminating the cold runner will give better filling and packing conditions. When plastic is flowing through the cold runner, it loses heat to the mold plates - causing pressure drops that can result in sink marks and under-filled parts. Hot runner systems, on the other hand, maintain a balanced melt flow at a constant temperature from the machine nozzle all the way to the gate to fully fill and pack the cavities. Hot runners, therefore, help molders take full advantage of highly accurate and interchangeable cavities to achieve plastic part dimensional accuracy and quality. Should a cavity get damaged or be out of specification for whatever reason, it is very easy to reduce the heat to its hot runner nozzle and stop the cavity from producing faulty parts.

Molding Automation
An ever-increasing number of companies are automating their plastics molding and assembly. This, in turn, is driving the need for very accurate, flash-free plastic parts. Hot runner molds have a definite advantage when used with automation because, in addition to part dimensional consistency, there are no runners present to get tangled in the mold mechanisms, robots, conveyors, assembly machinery, etc.

Design Flexibility
A major hot runner advantage of which very little is written is the ability to locate the gate at a wide variety of points on the part. With the use of hot tip gating, valve gating or edge gating, hot runners allow the gate to be placed at the most favorable location for optimum filling and/or part aesthetics. This flexibility that hot runners give the part design also extends to the mold, where flexibility in gate location can help optimize cavity orientation, cooling and mold simplification.

Balanced Melt Flow
The flow advantage that hot runners have is that the melt channels are in separate, externally heated manifolds, which are insulated from the surrounding mold plates. Different to a cold runner plate, the hot manifold can be designed to have flow channels on multiple levels to ensure that the resin flows the same length from the molding machine nozzle to each cavity with the same channel profiles and diameters, number of turns, heat soak and pressure loss. In other words, the plastic reaches each cavity - whether there are two or 96 - with the same pressure and thermal history. This designed-in temperature and pressure control is particularly favorable for resins that have narrow processing windows.

(Source: 7 Key Advantages of Hot Runner Systems)

What should moldmakers consider when purchasing a hot runner system?

Evaluating the cost justification for a hot runner mold requires careful consideration of cycle time, material type, annual volume, available press size and power consumption, as well as the cost of utilities, resin pricing, allowable regrind percentage and labor rates. Here are three key areas to consider:

1. Cold Vs. Hot Process Considerations

Cycle time is the primary cost measure of the molding process, and cooling represents about 80% of the molding cycle. The thicker the part, the longer the cycle time. However, the runner could determine the cooling time, and increase cycle time more than expected if the runner is thicker than the part.

The minute the resin leaves the machine barrel, the material starts to cool and solidify. This, in turn, causes injection pressures to climb and can yield unfilled parts, stress and warp. Limit the l/t ratio (length of flow versus part thickness) to avoid this outcome. Generally, a ratio under 100 is considered general-purpose molding and does not require increased injection pressures.

In a cold runner mold, the l/t ratio is measured from the start of the cold sprue, where the material leaves the heat source and begins to cool, which continues through the entire runner to the furthest point in the part (last point of fill). For example, 1 mm (0.039 inch) thick resin can flow easily 100 mm (3.94 inch). However, if a hot runner is used, the l/t ratio starts at the gate, reducing flow length and the injection pressure required to fill the part.

The higher the ratio, the more injection pressure is required to fill the part. A cold runner is included in the l/t calculation when the molder measures at the point of no added heat. This is one reason hot runners are appealing and can make the difference between molding a good or bad part.

2. Material Considerations

Most polyolefin resins can be reused without issue. Engineered materials can be limited in regrind content because each time the resin is processed, it experiences another heat history that can degrade melt quality and impact performance, color, rigidity, tensile strength, etc.

Keep in mind, not every runner makes it to the granulator, and not every pellet makes it back to the hopper. While eliminating the cold runner can reduce scrap, a complete hot runner system might not be cost-justified based on low annual volume. The cost of an entire hot runner system may take longer than a year for payback. It might be wise to consider a hot/cold combination or at least a heated sprue bushing. The sprue is typically the thicker portion of the runner, and eliminating that and/or a portion of the runner could have an impact on the cycle time, resin consumption, scrap, etc.

In some applications, a hot runner system might eliminate scrap from unusable runners, but based on the required shot size volume (total volume of parts and runners) relative to the barrel volume, and it might reduce the consumption causing the barrel of the molding machine to be too large. Residence time on engineered materials would increase, which may cause other issues. For example, too small a shot can be challenging to process even with olefins.

3. Energy Considerations

Hot runner systems run on electricity like granulators. An injection molding machine takes roughly 1 kiloWatt (kW) to process 1 kg (454 lbs) of resin. Reducing the shot size to parts only can have a significant impact on power consumption. It stands to reason that only heating, melting and processing parts takes less energy than parts and runners.

When deciding between a cold or hot runner system, consider all of these variables, which combined can increase productivity and energy savings, improve material utilization and reduce floor space and noise requirements on the shop floor.

(Source: How to Make an Informed Hot Runner Decision)

Why are valve-gated hot runners advantageous?

Valve gate hot runner nozzles were introduced to the injection molding industry shortly after the first hot runner systems appeared on the market. Until recently, valve gates had been chosen primarily for applications where thermal gate vestige is unacceptable. However, valve gates offer several additional part quality and processing benefits to the injection molder including:

  • Elimination of drool and gate string.
  • Improved physical properties with lower molded-in stress.
  • Cycle time reduction.
  • Ability to balance family molds and control weld line location with sequential valve gating.
  • Superior molding processes for thinwall parts.

Although higher in cost than thermal gates, valve gates offer tremendous value. In addition to the superior gate quality, valve gate nozzles can improve part quality and increase productivity. All of the benefits - not just gate quality alone - should be carefully considered when determining if a valve gate nozzle is best for your mold. In addition, valve gate applications should be reviewed with your hot runner supplier to ensure that the most appropriate nozzle style is used in your mold.

(Source: Why Choose a Valve-Gated Hot Runner?)

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