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.

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.

After changing the color or material on plastic processing machines, the new production often causes problems due to material residues within the plasticizing unit or the hot runner.
In order to avoid this, cleaning granulates are used, which make the material switch easier.
As a result of a chemical or mechanical cleaning action, residues, for example in the cylinder of the injection molding machine or on the screw, are dissolved and rinsed out.
Through the use of cleaning granules, the wasted material during production of molded parts and the time for material or color changes is reduced. The duration of the cleaning process depends essentially on the manufacturer's specifications. While some manufacturers specify reaction times, cleaning granules exist entirely without residence times and can be injected directly after plasticizing. As a rule of thumb two to three times of the maximum dosage volume are required as quantity. The notes of each manufacturer about the quantity should be considered as a reference.

The pressure inside the cavity is called cavity pressure. It is usually smaller than the specific pressure due to pressure losses.
Sensors inside the mold are used to measure the cavity pressure and to compare it with the injection molding simulation.
The time-pressure curve of a complete cycle helps to validate the vp-switch, as well as the effect of the packing phase.
The part quality can be evaluated easier.

The length of the screw is specified exclusively in relation to the screw diameter.
Doing it like that allows to compare different screws much better.
The process-engineering effective length of the screw is specified as L.
This value represents the distance the granules/melt travels along the screw.
For the determination, the length from the front edge of the hopper to the backflow barrier is always used.
The effective length is set in relation to the diameter D of the screw.

In injection molding, the resin to be processed is fed via a hopper into the injection molding machine.
The shape of the hopper can be rounded or squared. The hopper leads into a cylindrical drain inlet connected to the screw cylinder.
The hopper can be moved in axial direction before a material change.
This way its content can be released before it is filled with another material.
The filling of the machine hopper is either be done manually or automatically via a central material supply.
Depending on the application, the hopper is often also equipped with dryers or mixers.

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".

If the clamp force is too high for the chosen injection molding machine, multiple cavities with identical projected areas can be set behind each other inside the mold.
Despite the multiple parting plane, the clamp force is used only from one.
The reason for this is the neutralization of the cavity pressures of opposite positioned cavities.
The middle plate synchronizes by tooth gear, hydraulic or an articulated lever.
A mold that is designed in this way is called a stack mold.

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".

A standard injection mold is split into the injection side and the ejection side.
When the mold is closing, the two halves of the mold are centered through the guide pillars.
In most cases, the guide pillars are fixed on the injection side.
The ejection side moves along with the guide pillars till the mold is completely closed.
In order to make the movement of the other half of the mold easier, guide pillars can have oil grooves.
Depending on the mold layout, guide pillars can have a centering collar to increase stability.
Four guide pillars will be used in one mold.
It has to be noted that the diameter of the guide pillars varies.
Three of them have the same diameter, one of them has a different one.
This is to make sure the mold cannot be mounted in the wrong way.

One parting line is not enough to eject complex geometries of injection molded parts.
Undercuts in the direction of ejection are the main difficulty.
So called slider are used to build an additional parting line.
The movement of the slider diagonally to the opening direction of the mold frees the undercuts and allows the ejection of the part.
Slider can be placed inside the fixed as well as the movable side of the mold.
The angle of the slider compared to the other parts of the mold should be between 83° and 97° to avoid a blocking of the slider.
The buoyancy inside the mold needs to be considered for the correct size of the slider.
It need to be ensured that the sliders are adequately locked.

In injection molding the plastic melt shrinks during cooling down.
Therefore after a volumetric filling of the cavity additional material is pushed into the cavity to compensate the shrinkage.
The phase where the screw still apply pressure to the melt is called packing phase.
Packing pressure (and packing time) is hereby essential.
Too much packing pressure leads to overmolding and flash formation.
Not enough packing pressure results in sink marks, voids and higher shrinkage.
Often a profile is used to apply pressure during the packing phase.

The machine is pressing the mold halves against each other to counteract the pressure of the melt in injection and packing phase.
The needed force for this is called clamp force.
Most injection molding machines contain this value in their name.
The unit might be displayed in kN or t.
If the clamping force is too low, the mold halves will be pushed away from each other by the melt and flash appears.
The injection molding simulation helps to determine the necessary clamp force in advance to choose a machine with the correct size.
The clamp force can be estimated by multiplying the projected surface with the cavity pressure.

The pressure at different positions inside the cavity/runner is called “Pressure Distribution”.
Ideally in injection molding process, the pressure distribution should be uniform throughout the cavity after the molten plastic completely fills the cavity. In this case the molded part shrinks uniformly.
In reality due to the different cooling behaviour of the part there is always a difference in pressure distribution.

Injection molding simulation is a great tool to predict the pressure inside the cavity, which can help to achieve uniform pressure distribution as good as possible.

In injection molding, during filling and packing phase the two mold halves are hold closed with the clamping force against the pressure of the melt. If the driving force of the cavity pressure is higher than the clamping force, the mold is pushed open by a few thousandths to hundreds of millimeters. This is known as the so-called mold breathing. Too large mold breathing can lead to flash formation, while too low mold breathing results to air compression with burn marks.
In order to optimize the required clamping force, dial gauges can be attached to the mold to determine the mold breathing. Permissible respiration depends on the quality requirements of the injection molded part, the material and the size of the machine.

The pressure of the melt pushes the mold to open due to a too low clamp force.
On the parting line, the mold opens a little and the outcoming melt freezes and a flash remains.
The risk of the flash formation can be reduced by the use of a machine with a higher clamp force or the reduction of the specific pressure in the cavity.
Another reason for flash formation can be the wrong positioning of the touching surfaces at the parting line.

If the air can't escape, it will be compressed by the melt.
This leads to an increase in temperature and builds burn marks (black marks on the polymer).
A better venting or slower injection at lower temperatures can solve this issue in most cases.

Length or differences in length are measurend with a dial gauge. It can be used for comparative, flatness, positional or concentricity measurements.
Dial gauges exist with analogue or digital display. Setup and function are standardized by DIN regulation 878.

Dial gauges with round scale and analogue display transmit the longitudinal movement of the measuring probe with a toothed rack and gear to the pointer. As a result, the pointer of the dial gauge rotates several times in a circle. The number of revolutions is indicated by a second pointer, similar to the hour hand of a watch.
The advantage of the gear rack transmission is the relatively large measuring range. The disadvantage is, that if the measuring distance greatly exceeds the travel range of the rack, it might lead to gear damage or rack damagein case of a rigid locking of the dial gauge.
Alternativly there are dial gauges with slave pointer with a delayed point of return. However this kind of dial gauges cannot carry out measurements with value differences, like concentricity or flatness.

In addition a great advantage of a dial gauge is, that it can be used in pressure and tension mode and that the return of the measuring rod is done with a spring. Therefore the applied measuring force is almost constant.
Most dial gauges achieve an accuracy level of approximately 10 µm and have a measuring range of 5 to 60 mm. A few dial gauges have an accuracy of 1 µm.

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.

The cavity contains air, which is pushed out by the arriving melt.
Therefore, venting is necessary to allow the air to escape the cavity.
Otherwise a compression of the air occurs, which leads to a rise of the temperature (burn mark).
The venting consists of small gaps, which are situated for example at the parting line, at the sliders and at the ejector pins.
Depending on the used polymer, the height of the gap needs to be adjusted.
If the gap is too small, it prevents the air from escaping.
If the gap is too big, flash formation occurs at the part.

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.

The development of designs using a computer is called CAD (short for Computer Aided Design).
This includes all computer-aided activities in a design process.
There is a differentiation between 2D and 3D CAD systems according to the available software and the aim.
3D CAD systems can be differentiated into wireframe- surface- and solid modelling tools.
In common CAD systems a mixture of all of the discussed modules can be found.
The design is mostly done in 3D while a 2D drawing can be extracted based on the given model.
Typical data formats are Step (.stp or .step) to exchange models between CAD systems as well as STL (.stl) if it comes to simulation or rapid prototyping.

Plates, which contain the part-cavity in the injection mold are called mold plates.
Usually there is one mold plate for the injection side and one for the ejection side.
Dependent on the complexity of the part, the cavity is carved directly into the mold plate or embeded with inserts into the mold plate.
The primary cooling system of the cavity is located in the mold plates as well.

An injection mold can contain one or more cavities.
The number of cavities is often named cavity count.
In general it is considered that all the cavities are identical if we talk about cavity count.
The more cavities in one mold, the more parts can be produced at once.
Therefore production time and part costs can be reduced.