What is good venting in the mold manufacture?

For the injection mold manufacture, mold venting is a very important point.
Mold venting can represent the overall structure of the mold to the greatest extent.
The good mold venting can always a important hallmark of the successful mold structure.
Sometimes,If there is not a good mold venting,it is impossible to make injection products with stable quality.
A good mold should have good air tightness and ensure that the gas is freely vent from the cavity,
instead of the gas being compressed highly .

 

So what is the standard for mold venting?
The first thing should understand is that the unreasonable venting can cause the following pro

blems:

1.  The gas accumulation in the mold cavity will etch the mold surface and the parting line;
2.  The air traps will cause discoloration, sometimes it’s too bright or too dark. The air trap will also cause the texture too bright or fog surface.
3.  If the venting is unreasonable,but you need a prefect surface, you have to increase mold polishing,it will  wast time, money and steel is still easy to be corrosive ;
4.  It is impossible to make the product meet the standard customer’s requirement;
5.  It will cause too many welding Lines (not strong, and not beautiful);
6.  It will waste too much injection pressure, increase the difficulty of the injection molding machine work;
7.  It will need to use too high barrel temperature  (increased the difficulty of the injection machine, temperature control disorders, material performance deviation, increase cycle time);
8.  It will reduce the performance level of materials.
9.  It will reduce the product roundness and cause product deformation;
10.The high-pressure gas in the cavity will increase the instability of the cycle, It will often need to adjust the machine to solve the quality problem;
11. .It will  increase the work of engineers, QA, operators and mold makers.
Let’s take a look at the reasonable venting design requirements:
It is necessary to make the full perimeter venting structure as much as possible. The depth is within the plastic overflow value.
The full perimeter venting structure is best , but it is necessary to leave 1/2 of the steel to compensate the pressure on the parting line.
If it is impossible to do the full perimeter venting due to the irregularity of the parting line surface, the better solution is to use a 12mm wide vent  groove to the mold base, and the gas must eventually be discharged into the atmosphere.
and the vent groove should be machined as much as possible by a flat bottom knife with chamfer.
If you must use a spherical milling cutter, you’d better calculate the depth of the machining and the depth of the flat bottom can be used.
The length of the seal material in the venting is one of the most easily overlooked by most people, and it is related to the proper venting problem;
There are many different viewpoints on the specification on this issue.
The material supplier’s specifications range from 3 to 5 mm, and experience has shown that if the length is greater than 3 mm, it will no longer function well.
Therefore, for safety reasons, we have a standard of 3mm. The vent groove depth is determined according to the molding material.
One of the most common misconceptions is that too much venting leads to flash. In fact, the opposite direction is correct. Unreasonable venting requires excessive pressure. When gas is allowed to escape, the material will go out with the gas.
The only factor of flash is that the vent groove is too deep.
It is usually best to use ejector pin to vent. This is especially important for large plastic parts. This is of course determined according to the diameter of the thimble.
If this is not possible, the surface should be ground with the appropriate length and depth.
To reduce the fit clearance of ejector pin.
The runner, especially the large or long runner, must be done venting.
Another common problem is that when the part cannot be designed full perimeter venting,
The final filling position and the second filling position must be taken into consideration.
 The two must be vented. Deep rib position, boss position and small needle gate position or thin plastic position must be specially considered for venting, or with retangle ejector, Semicircular ejector structure or with breathable Steel , or even vacuum venting.
So what do you need to pay attention to?
A part can’t be vented too much, although the parting line can do it, this will bring the following problems related to the venting:
1.There is not enough steel to withstand the pressure on the  parting line,
2.The venting on the parting surface is pressed ;
Longterm mold engineer team have rich experience in different plastic injection mold ,any project,please feel free to contact us!
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