A mold is usually divided into two sides according to their function.
The side which is moved for mold opening is called movable side.
This side contains the ejector pins, therefore another common name is “ejection side”.
“Parts, which have been produced from melt by non-cutting molding (e.g. by pressing, compression molding or injection molding) in all side closed molds” are referred
to as molded parts according to DIN 7708, sheet 1 [freely translated].
Thus in injection molding, the end product, which is produced by filling the cavity with molten plastic, is called a molded part.
It should be noted that a possibly existing cold runner is not a component of the molded part.
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An injection mold consists in general of two halves: injection side and ejection side.
The touching faces between these two halves are called parting line.
Through this, the air is transported from the cavity to the outside.
When the mold is opened, part and cold runner are separated from the mold at this line and ejected.
The parting line emerges in general as a surrounding seam on the part.
If a cold runner is used to gate directly on a part, there is only one way to separate runner and part automatically.
This is by using a pin gate.
The mold needs to have 3 plates in this case and will get more complex.
During design phase of a pin gate it needs to be kept in mind to have the gate height between 0,5mm to 1mm and the gate diameter in between 40%to 80% of the part wall thickness.
This is to make sure the automatic separation will work properly during mold open.
In injection molds with conventional cooling, the cooling system is manufactured with as little as possible drill holes.
A change in direction is realized by intersecting drill holes.
These auxiliary holes need to be closed by a plug to avoid the coolant flowing out of the mold.
Additionally plugs need to be inserted to define a clear direction of the coolant stream.
This way the coolant can flow through a drill hole in multiple areas.
The plugs prevent the coolant to flow into uncertain directions and support with the enforced flow direction a good cooling system of the mold.
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The primary molding process mainly used in the plastic industry is injection molding.
With the help of the injection molding machine and the influence of heat, the plastic material is brought to a viscous state.
Subsequently, the melt is injected under pressure into the cavity in the mold.
The shape and surface structure is determined by the geometry of the cavity.
Cooling or cross-linking reaction (for elastomers) solidifies the part and it can be ejected after opening the two mold halves.
The injection molding process is particularly suitable for large quantities.
This is due to the fact of high mold costs which need to be payed proportionally by the produced parts.
Injection molded parts can be used directly after ejection.
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A mold is usually divided into two sides according to their function.
The side with the ejector pins is called ejection side.
This side is moved for mold opening, therefore another common name is "movable side".
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A mold is usually divided into two sides according to their function.
The side, which is not moving during production is called fixed side.
This side contains the runner system, therefore another common name is "injection side".
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A group of experts in the plastic industry founded the company PLEXPERT, 2006 in Germany, in order to save resources, providing a positive impact on budget and environment.
There are two divisions within the company. One is focusing on finite element simulations (FEM) like injection molding simulation and structural analysis.
Based on decades of practical experience DFM conform results are given by the consultant service of PLEXPERT to decrease part costs and increase quality in the production of plastic parts.
The second division is focused on the development and supply of soft- and hardware to the plastic industry. With the state of the art products PLEXPERT contributes to secure production and reduce the need of precious resources.
Today PLEXPERT has offices in Canada, Germany, Thailand and China in order to support its customers fast and effectively.
Due to a close network of plastic experts, partners are available in many other countries as well and offer PLEXPERT products and services.
PLEXPERT Company Logo.
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For the production of injection molded parts a mold is necessary to be placed on the injection molding machine.
It is commonly build out of steel or aluminum and is designed as a permanent form.
Depending on the plastic material used millions of parts can be created with one mold.
The complexity of a mold range from a simple two-plate molds up to a complex stack mold with sliders, etc.
To avoid additional costs it is important to think about the mold concept early in the concept phase of part design.
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To get the parts homogeneously and with as least as possible tension out of the cavity, ejectors are used.
Additional ejectors can be set as optimization for the venting system.
The ejectors are mainly axial pins, which are located on the ejector plate and fixed from the ejector holding plate.
The movement of the ejector plate takes place most of the time by hydraulic.
When moving forward, the pins press against the parts and throw them out. The backwards movement is done by hydraulic or spring force.
The movement of the ejectors has a big influence on the wear of the mold.
The unheated melt channel from machine nozzle / hot runner up to the cavity is called cold runner.
In the injection molding cycle the cold runner is cooling down and freezes.
The gate geometry is very flexible, like umbrella gate, cashew gate, ring gate.
The cold runner is ejected together with the part.
The separation from the part is done automatically or manually.
The collected cold runners will be recycled or disposed as waste.
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To display the chronological history of the filling behavior of an injection molded part, the dosage volume is extended in the injection molding machine from shot to shot until the cavity is completely filled.
The different filling percentages explain, which area is filled at which time and give hints on the process and possible optimization.
All shots with different filling percentage collectively are called filling study.
In injection molding simulation the filling study is an inherent part of the displayed results.
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"Parts, which have been produced from melt by non-cutting molding (e.g. by pressing, compression molding or injection molding) in all side closed molds" are referred
to as molded parts according to DIN 7708, sheet 1 [freely translated].
Thus in injection molding, the end product, which is produced by filling the cavity with molten plastic, is called a molded part.
It should be noted that a possibly existing cold runner is not a component of the molded part.
Related topics:
The space between the two mold halves, which should be filled with melt, is called cavity.
The shrinkage of the material needs to be considered for the production of the cavity to get dimensionally accurate parts.
In the injection molding simulation, it is important to use the cavity size for the warpage calculation.
The definition of quality according to DIN 55 350/T11 is "the nature of a unit as its suitability to meet specified and required needs".
Different factors are used to evaluate the quality of plastic parts.
The easiest is the optical evaluation.
Visible surfaces should not contain streaks, burn marks, sink marks and/ or air traps.
The dimensional examination is used to control the functionality of the part.
Shrinkage and Warpage are main quality criteria.
The technical drawing shows the necessary tolerances of the part for an acceptable quality.
Mechanical testings are necessary to check the different applied forces on the part dependent on its usage.
Fiber orientation, weld lines, voids and wall thickness may lead to this quality failure.
The choice of the material and the processing, both influence the mentioned quality factors decisively.
Injection molding simulation and additional structure analysis help to check the quality of a part in advance.
A geometrical difference between target and actual geometry is called warpage.
In the plastic industry this is caused by the developing stresses during the process.
Stresses arise due to the different shrinkage degrees in the part.
Varying Cooling rates, Orientation of the molecules, in-mold constraints (limitations for the shrinkage), uneven wall thickness/packing pressure are responsible for the differences.
These effects can cancel each other out or enhance one another.
A mold is usually divided into two sides according to their function.
The side with the runner system is called injection side.
This side is not moved, therefore another common name is "fixed side".
Related topics:
An injection mold consists in general of two halves: injection side and ejection side.
The touching faces between these two halves are called parting line.
Through this, the air is transported from the cavity to the outside.
When the mold is opened, part and cold runner are separated from the mold at this line and ejected.
The parting line emerges in general as a surrounding seam on the part.
One of the most basic gates for injection molding is the edge gate.
The gate normally has a rectangular cross-section along the parting line of the mold to connect the runner system with the mold cavity.
The lapped edge gate is slided onto the top of the cavity.
It can be easily manufactured by a basic machining process.
The risk of jetting is reduced with this version.
The disadvantage of the edge gate is the remaining of the connection of the molded part to the runner after ejection. Additional mechanical action is needed for separation.
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If a cold runner is used to gate directly on a part, there is only one way to separate runner and part automatically.
This is by using a pin gate.
The mold needs to have 3 plates in this case and will get more complex.
During design phase of a pin gate it needs to be kept in mind to have the gate height between 0,5mm to 1mm and the gate diameter in between 40%to 80% of the part wall thickness.
This is to make sure the automatic separation will work properly during mold open.
Related topics:
If a cold runner is used and an automated separation between part and runner is wanted, a tunnel gate can be used.
In this case the gate will be on the side of the part or a rib.
The tappered shape leads to a small gate size on the part surface and allows a good removal of the cold runner during mold open.
A baffle wall at the end of the tunnel is used to collect cold melt at the start of injection.
The separation between runner and part is possible by using a standard two plate mold.
Often there is a positive impact to the parts appearance due to the less visible gate on the side of the part.
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One of the most basic gates for injection molding is the edge gate.
The gate normally has a rectangular cross-section along the parting line of the mold to connect the runner system with the mold cavity.
The common edge gate is positioned on the side of the cavity.
It can be easily manufactured by a basic machining process such as milling.
The disadvantage of the edge gate is the remaining of the connection of the molded part to the runner after ejection.
Additional mechanical action is needed for separation. Furthermore jetting might occur with this kind of gate.
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The cooling system in the injection mold is usually inserted into the steel by drilling.
By adding plugs the flow path for the coolant is clearly defined.
The conventional cooling is cheap compared to other cooling technologies and for most molds satisfying.
The injection mold is brought up to a specific temperature by the cooling system, which consists of the cooling device and the cooling channels.
Water or oil is used as a coolant flowing through the channels.
The better the temperature layout, the shorter the cooling time.
The aim is a uniform temperature at the cavity wall.
This way one factor for the warpage can be reduced.
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In the injection mold, in general, every cooling circuit stays at one temperature over the complete cycle.
For parts with a high demand on surface quality it is important that the melt reproduces the cavity perfectly.
A variotherm process is used to achieve this.
Hereby one or more cooling channels will be processed with a lot higher temperature during filling phase (A in the picture below).
As this results in a longer cycle time, the coolant temperature is decreased during cooling phase (B in the picture below) to speed up the process.
The phase of mold movement (C in the picture below) is used to heat up the coolant of the specific circuits to the higher temperature for the next cycle.
A uniform cooling of the part is important in the injection molding process in order to reduce shrinkage and warpage.
Especially in long and narrow areas the cooling layout is often difficult.
The reason is insufficient space to drill holes next to each other.
When there is only enough space for one hole, it is recommended to use a bubbler.
A tube is inserted into the hole.
The coolant flows inside the tube to the end of the hole and back on the outside.
In injection molds with conventional cooling, the cooling system is manufactured with as little as possible drill holes.
A change in direction is realized by intersecting drill holes.
These auxiliary holes need to be closed by a plug to avoid the coolant flowing out of the mold.
Additionally plugs need to be inserted to define a clear direction of the coolant stream.
This way the coolant can flow through a drill hole in multiple areas.
The plugs prevent the coolant to flow into uncertain directions and support with the enforced flow direction a good cooling system of the mold.
Related topics:
A uniform cooling of the part is important in the injection molding process in order to reduce shrinkage and warpage.
Especially in long and narrow areas, the cooling layout is often difficult.
This is because the space is insufficient to drill holes next to each other.
Therefore, when there is only enough space for one hole, it is recommended to use a baffle.
It is inserted into the hole and redirects the coolant from one side into the hole and out on the other side.
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