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Mold Cavity

Mold cavity is one of the most important elements in injection molding and has a huge impact on the structural design of the molded part which must be impeccable for manufacturing process to proceed as stated in the project. This article introduces to the injection molding process for different types of cavity molds, their advantages and disadvantages, as well as the cost of injection molding.

 

 

Injection molding process

Injection molding technology is used for mass production and it takes an excessive amount of time for manufacturing, thus it is essential for the process to be as rapid as possible. This could be done by ensuring excellent technical properties of the machinery. Therefore, determination of how the plastic will be injected into the mold is a must. The mold consists of the cavity and the core. The process of injection molding starts with placing raw materials (usually in pellets) into a molding machine. Then, through nozzles, the material is being shot into the mold through the injector gate and, lastly, reaches the mold cavity. Cavity is attached to the fixed side of the press while the core is on the moving side of the clamp. In other words, the core makes the internal shape and cavity is the external shape of the part. After the plastic reaches right place, the cavity and the core close and allow the part to cool. Later the core is being pulled out and the component ejected.

Types of molds in injection molding

Even though the main concepts may be generalized, the process of injection molding may differ depending on the types of mold cavities. There are three main types of molds used in injection molding:

  • Single cavity molds. These molds produce a single part per cycle and are mainly used for micro injection molding when low-volume production is demanded. The machinery for single cavity molds is lighter and smaller, therefore, results in reduced tooling costs and waste minimization. Accordingly, single cavity mold injection molding is usually quite cheap to perform, however not that efficient and thus not widely applicable for mass manufacturing.

  • Multi-cavity molds. The main difference between single and multi-cavity molds is that multi-cavity molds have more than one cavity. Logically, this allows to scale the production and manufacture more parts through a single cycle. Multi-cavity molding increases productivity and enables more efficient use of resources. This type of manufacturing is rather expensive compared to the single cavity molding due to higher machining costs. However, if used for high-volume production, it minimizes price per piece while maximizing efficiency.

  • Family injection molds. This type of molds, as multi-cavity molds, has more than one cavity cut into the mold and several parts are formed from the same material in one cycle. As each cavity may form a different component, family molds are an ideal choice for prototype molds. In this way, not only the processes are more efficient, but also much simpler.

 
Cavities and cores

Cavity pressure measurement

Cavity pressure is one of the key parameters indicating the quality of the micro molded components, as the sensor indexes the pressure in the mold which overpowers the polymer melt resistance and pushes it into cavities. Then the hot runner systems may be controlled along with the cavity pressure measured. 

The cavity pressure is usually calculated by the formula below and actually measures pressure per area: 

 

Cavity pressure (P) = 400 kgf/cm2

Cavity pressure sensors

Specific observations on cavity pressure are made by using cavity pressure sensors that are most usually placed along the polymer melt flow paths to convert the pressure into the measure of the piezoelectric effect. There are two main types of cavity pressure sensors:

  • Direct cavity pressure sensors are directly inserted into the measurable area. Under pressure, a sensor delivers an electrical signal in the pC (Picocoulomb) units and then converts it into pressure units which indicate change in pressure divided by Picocoulombs (bar/pC).

  • Indirect cavity pressure sensors, also called force sensors because pressure is caused by force through ejector pins, are placed outside the cavity, usually behind the ejector pins. Indirect sensors are sensitive to a change in force. Therefore, similarly to direct, indirect sensors transmit electrical signal (pC) after change in pressure, but in comparison, they turn the measurement into change in force units divided by Picocoulombs rather than change in pressure units.

Mold cavity space

Mold cavity spacing is of great relevance when optimizing injection molding processes. Minimizing mold cavity space may lead to a significant cost reduction due to the following reasons:

  • Retainer plates are less needed to support the nozzles

  • Operating costs decrease because lower cavity spacing prevents the heat loss and nozzles and manifolds connection on the surfaces

  • Mold cavity spacing minimization shortens the injection cycle because the heat is removed faster. 

However, even though due to mold cavity spacing minimization the processes are cheaper, they might become ineffective. The reason for that is that the surface temperature differences (when cooling) may negatively affect the quality of the components, especially if the component parts cool differently due to uneven wall thickness or deformation which sometimes might be caused by attempts to minimizing mold cavity spaces.

 
 
 
Cores
 

 

Cavity shape and shrinkage

While solidifying, the volume of the molten plastic in the cavity tends to shrink because of the variations of polymers density between melt and rigid stages. Sometimes a warpage may occur - if some parts of a component shrink and cool unequally, this might lead to component deformation and defects. Both equal and unequal shrinking and deformation have the same impact on mold cavities. 

Shrinkage is signified as a rate and is a crucial indicator in material choice because different materials shrink differently and have specific intervals of allowance. Most usually, the interval has a range between 0.2% and 2%. Not only is shrinkage dependent on material, but also on other factors, such as temperature, duration under pressure, wall thickness, the shape of the gates and additive materials (if present).

 

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