How to use Mastip Hot runner System's selection Guide efficiently?

Overview and Detail analysis


Mastip manifold systems and nozzle selection guide.


As an experienced hot runner manifold designer and Mastip hot runner nozzle user and former Application engineer Mastip Technology New Zelaand, supported technically so many Mastip hot runner assisted plastic tools around the world, I would be able to talk about this brillient systems in detail.


I am absolutly sure that next time you can use this system more efficiently.



Mastip Standard Hot Runner Configuration:

http://www.mastip.com/Downloads/SystemSelectionGuide_V2.05.pdf

Page 4 shows the general hot half mould layout

Important points when it come to design your hot runner Plastic tools..
1.
As you can see,hot runner manifold is placed between fixed side clamping plate and Manifold bolster plate or intermediate plate.
2.
Cavity plate machined according with nozzle pocket dimensions

Imporatnt aspects during maching.

Machining the hot runner nozzle sealing area is important.It should bear in mind that boring to the required diameter should allow the nozzle to slide during its expansion so need perfect sliding fit.

It is so eacy to cut the nozzle insulation pockets with CNC assisted ball nose end mill operation. Even tiny gate could be done through similar way with 0.5 -1mm ball nose. or Most of the people spark erode this gate.

We have to make sure that gate surface finish is so important therefore should not have any maching marks as this will lead to pressure loss along the gate area . Good surface finish will assist a better Plastic filling and gate control to obtain better cycle time.

During the operation, nozzle will expand along its axis.So very important to make sure the overall dimension between the gate end to top of the nozzle head pocket surface. This length is Nozzle length + Thermal expansion. Most probably tip side will expand more than its head.

During thermal expansion,we should make sure nozzle has sliding fit and nozzle tip will be in its exact spot during injection.

Manifold also will expand side ways during operation so that we must make sure nozzles top surfaces must have a perfect match (no gap between the nozzle and the manifold).
3.
During the machine start up, we need to heat the manifold and need to do the necessary purging opearation also during color change time purging opeartion is absolutely essential.This all can be done very easily if we design the Mould as per shown on the technical diagram .So that we can separate the cavity plate and manifold plate while mold is on the machine.

Also this will assist the molder to quickly change the dead nozzle heater or Thermocouple, while the mould is on the machine.

4.
Wiring grooves should be machined between these two plates and need a proper clamps so that nozzle heater and Thermocouple wires want be damaged during assembly or opeartion.


5. If there is a thermocouple placed under the manifold surface to monitor the heat, depending on your nozzle selection & manifold design, nozzle head dimensions should be checked and if that is too small than thermocouple overall height then it should be placed within a machined groove to avoid damge during manaifold assembly.

5.
Same with the manifold sprue bush heater wire as well. It has always been located between fixed side clamping plate and manifold bolster plate through a wiring grooves to avoid damage.

6.
There are two spacers one titinium and other steel spacer. Manifold height has had its standards according with its runner configurations. During the milling maching operations and grinding operations always there would be a chances to have dimensional discripancy.And also during operation heated manifold will expand by it is own vertically as well. So we have to calculate this expansion according with its operating temp and allow this clearance as a gap between the steel spacer and the clamping plate so that during an operation.Manifold will not leak.

7
.As you can see the water lines passing very close to the insulation pocket area. This will assist initial insulation as well as control gate freezing during each cycle.

Also there are water lines through clamping plate and manifold bolster plate this would reduce the heat transfer as a result of conduction within a hot runner system.

8
.Mastip has different sprue bush lengths to suit diffrent clamping plate thickness.

9
. Air pocket has been always machined around the hot runner manifold in oreder to reduce convective heat flow in the hot runner manifold block to the cavity plates.

Important Note:

One of the common concerns expreseed by molders about hot runner molds is the threat of a leak of molten plastic causing the manifold pocket to fill.

What causes hot runner system leaks?

Most of the hot runner systems don’t leak because of poor design, but they do, if the hot runner manifold is heated beyond its operating processing temperature or not reached its operating temperature. As I said the most common location for leakage is at the flat seal-off between the nozzle and the manifold. We can calculate the thermal expansion and should allow this as a cold clearance during manifold bolster plate pocket machining.
By having this allowance will protect this system from collapsing at the seal off are due to thermal expansion when the system is at operating temperature.

Manifold bolster pocket height= Manifold stack height (Nozzle skirt height +manifold height + total spacer heights + (crush allowance roughly 0.02mm) + expansion at its correct material processing temperature)

Since the correct preload on nozzles and the manifold is so critical it is understandable that the dimensions and tolerances provided by the hot runner supplier must be strictly be look at in order to avoid leakage of the system.

Note:
Mould makers should be carefully inspect all the stack heights and bolster pocket dimensions against the hot runner manufacturer’s manifold drawings.

“What causes my hot runner system leaked during our mould trial?” Some of the most common questions expressed by the toolmakers and injection molders during their trialing period.
It is a strange feeling to see your newly designed plastic mould leaking. Manifold bolster pocket around the new manifold filled with plastic material. It is a kind of threat in the feature to the second trial without knowing the exact problems.
We will see generally, what was caused for this leak.
1. As I mentioned, the most common location for any leakage is at the seal between the nozzle solid skirt top surface and manifold. “Cold clearance”, Mould designers usually forget to apply this clearance. This clearance is the calculated thermal expansion with allowable crush at the systems operating temperature. There is always some variation in the plastic melt temperature coming from the barrel of the injection machine. In exceptional cases up to 40 deg variation has been measured using a high speed thermo-couple.
In these situations, the whole systems would possible be overheated, if the hot runner manifold systems were designed with recommended plastic material melt temperature.
Always refer the supplied manifold drawing and find out the designed operating temperature. This would give you some operating window to do your injection mould trials and eventually can eliminate the permanent damage and expensive replacement with hot runner nozzles and new manifold.

The fact that there is no positive seal in the cold conditions is the main root for the almost every hot runner leakage. In order to maintain this seal to resist the injection forces which would push, the nozzle head and manifold apart from each other at the max injection pressure, Machined bolster pocket dimension should have this cold clearance. Calculation is so easy but when you working by yourself you must check the correct melt temperature and outside ambient temperature
And manifold material coefficient of expansion and exact height of your manifold stack.

For an example I will show you one simple calculation. Standard Mastip manifold thickness = 44mm. Nozzle head = 15mm, Titanium spacer + steel spacer (used to overcome machining tolerances through height adjustment) =11.5mm
Total stack height = 70.5mm

Coefficient of thermal expansion for P20 steel is 0.0000132
Find out the manifold operating temperature say 230 deg and outside ambient temperature say 30 deg.

Thermal expansion= stack height x (230-30) x 0.0000132 = 0.186mm

At hot conditions this stack height will expand by 0.186mm. We must allow this cold clearance to prevent Nozzle, manifold deformation.
Say during the tool trial manifold is heated by mistakenly 330 deg instead of 230 deg.

Thermal expansion= 70.5 x 0.0000132 x (330-30) = 0.279mm.
Components will grow additionally by 0.1 mm. This means that the forces reached roughly almost double, which may well above the yield strength of the nozzle body and manifold steel. Once the overheated systems returns back to 230 deg, System will not develop the necessary sealing pressure.

Externally heated hot runner systems are the majority of the hot runner systems in use. Sometimes inexperienced operators may even forget to turn these heaters on before material injection. Material within the manifold only heated by the help of nozzle coil heaters, sprue bush heater and melt temperature along. This temperature is not sufficient as manifold looses heat as well. Hot runner mould designed with cold clearance that has not reached its expected shut height at the hot conditions, will not have enough surface pressure between the nozzle and the manifold and will eventually leak.

Usually this cold clearance is adjusted in the last mint by the tool makers simply grinding the supplied steel spacers. Once the hot runner systems fully assembled then it is too late.

2. Another silly but the worst scenario would be faulty assembled manifold end plugs; it could be from hot runner manufacturer. But this type of error cannot occur because the entire hot runner manufactures products used to go through their Quality inspection. But still there is a possibility or chance once the end plugs faulty assembled.