1. Are there any operational differences required with a hot sprue system ?
2. Can HotFlo! be retrofitted to existing dies ?
3. Is any special maintenance required ?
4. Can overflows be used with Hotflo! ?
5. Can shock absorbers be used with Hotflo! ?
6. How do I calculate the diameters of circular runners ?

1. Are there any operational differences required with a hot sprue system ?

Answer: Yes, there are three minor differences.

a. Machine Nozzle

The main difference which must be communicated to die setters and machine operators involves the machine nozzle.

With a traditional sprue system, the bore in the machine nozzle is smaller than the sprue hole in the die. This is required to extract the solidified metal in the end of the nozzle.

With HotFlo! the bore in the nozzle is made the same size as the hole in the hot sprue bush. So, a Series 16 HotFlo!, requires a nozzle bore of 16mm. You need to ensure that a special machine nozzle with a size for size bore, and the correct radius seat is available for Hotflo! dies.

If you prefer to use a machine nozzle with a 90 degree conical seat, this can be achieved by spark eroding, turning or grinding it into the seat of the hardened sprue bush.

b. Nozzle Heat

The heat required on the machine nozzle is usually less than with traditional cold sprue systems.

If you use electrically heated machine nozzles with thermocouple control, no real differences will be evident.

For gas heated nozzles, without thermocouple control, the operator must be made aware that less heat will be required on the nozzle. If the temperature which is registered on the HotFlo! controller, rises above the set point, (say 420 degrees C), then this is usually an indication that too much heat is being applied to the machine nozzle. Simply turn down the gas.

c. Warm up time

The heated sprue bush must be brought up to temperature from ambient at a controlled rate to avoid damage to the bush material. This can be done at the same time that the machine nozzle heaters are switched on to bring the nozzle to operating temperature. For full details see the webpage on "Warm Up Procedure".

2. Can HotFlo! be retrofitted to existing dies?

Answer: Sometimes.

HotFlo! works best if the die is designed around the technology right up front. This is no different to the experiences with hot runner systems in plastic molding dies.

There will always be some existing dies where the position of the sprue bush is conducive to retrofitting a HotFlo! sprue kit, but most likely a special version will need to be made. If the fixed die is very thin, it unlikely to be a worthwhile activity.

If the fixed die is very thick, and the existing runner dimensions are well proportioned, then it may be a candidate for retrofitting.

HotFlo! can offer some guidance if a die drawing, or layout, can be provided.

3. Is any special maintenance required ?

Answer: No.

Traditional sprue posts and bushes require maintenance to keep them functioning correctly. In particular, in high volume production, the mating faces and the hot/cold junction need attention.

With HotFlo!, the metal in the bush remains molten, so there is no longer a concern about undercuts or snapping off the pip at the nozzle tip. However, minor repair at the mating faces with a hand grinder may be required with long running dies.

The spherical radius at the nozzle seat may occasionally need to be re-sparked, just as with a traditional sprue bush.

The heater element may eventually need to be replaced. However, the design and manufacture of these elements has greatly improved due to their wide application in plastic hot runner systems, especially at high temperatures. They have proved to be very reliable in all HotFlo! systems to date.

With the heater element, you will notice that when the Die Casting machine is making shots, the electric element switches on for only a small fraction of the cycle time. This is because the molten metal provides a constant input of heat. The heating element does the most work during start-up (typically 5 to 15 minutes) or when the machine cycle is interrupted.

4. Can overflows be used with HotFlo! ?

Answer: Yes.

There are three main reasons for adding overflows to Die Cast parts.

The first reason is to aid ejection of the part.

If you cannot place an ejector pin on the part then sometimes you can add an overflow containing ejector pins. However, the main requirement in this situation is to ensure that the overflow connects strongly to the part so that it does not break off during ejection. A good method to achieve this is to shorten the ejector pin and connect it to the part with an overflow gate sparked down into the die to increase the strength of the join, and provide guided ejection.

The second reason for overflows is to aid trimming.

Sometimes it is necessary to add a strip of metal to the periphery of the cavity which aids in locating in the trim die and provides a controlled material thickness to remove. In this instance, design the overflow as a narrow ribbon of metal. A thickness of 0.5 to 1.0 mm and a width of 2mm is often all it requires.

The third reason is to improve casting surface quality.

The idea is to heat up the die surface with large amounts of overflow metal and to act as a receptacle for colder metal.

However, in many cases, the reasons for poor surface quality are due to shortfalls in sprue and runner design, insufficient machine shot end power, or insufficient metal in the casting to heat up the die surface.

Whilst overflows are one way to solve the problem, they do so at great cost, and what is more, there is no systematic way to determine where to put them or how large they should be. It is very much a case of "try it and see". There are plenty of examples where dies end up with a raft of overflows, or lots of welding in an attempt to find something that works. Besides that, increasing the overflow volume adds directly to the cavity fill time, so it can help to make the problem worse.

Better solutions to casting surface quality problems are to design the die and metal feed system to fill the cavity with minimum heat loss, to add external die heating, or to improve the machine capability. You will find that the application of HotFlo! and the use of theDC-CALC die design software will help greatly in the improvement of casting surface quality.

5. Can Shock Absorbers be used with HotFlo! ?

Answer: Yes.

"Shock Absorbers" are small discs added to the end of a runner, typically 10 to 15mm in diameter and 2 or 3mm thick. They work by trapping a small bubble of air inside, which is compressed by the metal pressure and cushions the pressure peak at the point when the runners are filled. It prevents a spurt of metal going into the cavity ahead of the main metal front.

There is no problem in adding shock absorbers to circular runners. However it is good practise to trial the die first with no shock absorbers. It may work perfectly well without them and they can always be added later if they are found to be necessary.

If you are using vacuum venting on the die, shock absorbers will not do anything as there is no air to trap.

6. How do I calculate the circular runner diameters ?

Answer: By the same method you use for trapezoidal runners.

The calculations for the AREA of the runners is the same, just convert to a diameter instead of a width and depth.

The area of a GATED runner is calculated so as to give the desired FLOW ANGLE of the metal into the cavity. The area of all FEED RUNNERS is calculated so that there is a gradual reduction in area from the sprue to the gate.

Whether you use Circular or Trapezoidal runners, you will find that the DC-CALC Die Design Software will make all runner and gate sizing a very simple task.