The most important plastic injection molding parameters – Optimization for maximum efficiency

The plastic injection molding process is a core of modern manufacturing technology and permits an immense range of high-quality components for a wide variety of industries. But the path from granulate to the finished component presents numerous challenges. What critical components and parameters play a role? What are the most common problems that can occur and how can they be efficiently remedied? Find answers to these questions in this blog.

The plastic injection molding process

Plastic injection molding is a widely used plastics processing method that is used to produce complex precision components. This process melts and injects plastic granulate into a mold under high pressure. The methods permits mass production of parts with high accuracy and repeatability.

There are different types of plastic injection molding methods that are used depending on the specific requirements of the product and the materials used. When molding with thermosets, for example, the materials cure irreversibly during the process and cannot be re-melted, which makes them particularly resistant. By contrast, the material remains meltable during thermoplastic injection molding, which allows the material to be processed and recycled multiple times. However, various parameters also play an important role in addition to the materials and methods. Some of the most important parameters in plastic injection molding are presented below for a better understanding of the plastic injection molding principles.

The most important plastic injection molding parameters

Precise machine parameter settings are required for plastic injection molding to ensure the quality and efficiency of the manufacturing process. These parameters determine not only the material processing, but also the properties of the end product. From the melting temperature to the injection pressure to the cooling time: each of these parameters influences how well the material fills the mold, how quickly the process runs, and the mechanical properties of the finished injection molded part.

• Melting temperature: Determines how much the plastic granulate is heated before it is injected into the mold.
• Injection pressure: The pressure at which the melted material is injected into the mold.
• Injection speed: The speed at which the material is injected into the mold.
• Tool temperature: The temperature of the mold influences the cooling behavior and the surface quality of the injection molded part.
• Holding time and holding pressure: After the injection step, pressure is maintained to compress the material in the mold and to prevent shrinkage.
• Demolding force: The force required to release the part from the mold after solidification.

Frequent problems in the injection molding process

The plastic injection molding process is one of the most efficient and accurate methods for mass producing complex components. However, despite advances in technology and automation, plastic injection molding remains susceptible to errors and problems that can significantly impair the quality of the manufactured parts. These problems can be caused by material properties, machine components, tools and the processing parameters. Continue reading to learn how products from MISUMI can help to prevent or correct frequently occurring sources of errors.

Sink marks

Sink marks are depressions or dents on the surface of an injection molded part. These are often caused by material shrinkage during the cooling phase. They mainly occur in areas with thicker wall thicknesses, since the material cools more slowly in these areas and shrinks. Correcting this problem requires, among other things, adjustments to the parameters and tool components.

• By increasing the holding pressure or lengthening the holding pressure time, the material is pressed better into the cavity and shrinkage is compensated.
• Lengthening the cooling time can ensure that the material is sufficiently cooled before the mold is opened.
• A larger size of the sprue bushing can improve the material flow and ensure that the holding pressure is transferred better to fill the cavity completely and evenly. If the gate is too small, the material flow can be uneven, which increases the likelihood of sink marks.
• Optimized mold temperature control, which ensures uniform cooling in all areas of the component, prevents thicker areas from cooling more slowly thus causing shrinkage. The use of precisely placed cooling channels or concentric cooling can help to dissipate heat evenly.

Incomplete filling of the mold

Incomplete filling occurs when the available material is insufficient to fill the entire mold or certain regions of the mold (usually thinner sections). This happens because the polymer melt cools prematurely and solidifies before the mold cavity is completely filled.

• Incomplete filling often occurs because the material does not enter the mold quickly enough, causing it to cool off in thinner areas and preventing it from flowing further. By increasing the injection pressure and the injection speed, the polymer melt is pressed into the mold more quickly and can completely fill the cavity before the material cools too much.
• If the melt temperature is too low, the material may not be sufficiently free-flowing to completely fill the mold. An increase in the melt temperature makes the material more flowable and increases the probability that thin areas of the shape are also reached.
• Insufficient or uneven cooling of the mold can lead to the material cooling too quickly in thinner regions of the cavity. The temperature of the mold can be regulated more evenly by using optimized cooling pipes or cooling systems close to the contour. As a result, the material remains free-flowing for longer in thinner regions.
• The injection point plays an important role for the material flow. If the gate is too small during injection molding, the material cannot flow quickly enough into the mold, which also leads to incomplete filling. Increasing the gate size can improve the material flow and ensure that the cavity is filled evenly and completely.

Voids (air pockets)

Voids are cavities or air pockets in the interior of an injection molded part caused by shrinkage of the material during cooling. Insufficient holding pressure or incorrect cooling can lead to the material not being completely refilled in the cavity, causing cavities to form.

• Shrinkage occurs frequently because the material is not completely refilled during shrinking. By increasing the holding pressure and extending the holding pressure time, the material is pressed more firmly into the cavity, which prevents cavities and ensures that the material is compacted.
• Even and controlled cooling reduces the likelihood of shrinkage and air pockets. The material can cure more evenly by adjusting the cooling time or by more efficiently cooling the tool.
• Poor mold venting is a common cause of air pockets. The precise placement of venting inserts allows trapped air to escape from the mold during the injection process.
• Ejector pins, which are normally used to demold the component, can also act as additional venting points. These can be designed to assist with more efficient air venting from the mold.

The importance of the right parameters and components

A variety of defects occur during injection molding, which can affect the quality and functionality of the end product. In addition to the problems already explained in detail, such as voids, sink marks and incomplete filling, there are also numerous other sources of error that must be taken into account to ensure production efficiency and component quality, such as cracks, seams, matte spots, vacuoles, burr formation or warpage. In order to eliminate these problems, it is important to optimize the plastic injection molding tools and components in addition to the methods and parameters.

Core pins, for example, help to precisely mold components with cavities and ensure uniform cooling and shape stability. They contribute toward minimizing warpage and improve mold filling, especially for complex geometries. Centering bushes ensure that the mold halves are perfectly aligned. Incorrect alignment can cause burrs to form because material flows unevenly. They also help to minimize warpage by aligning the mold more accurately.

Undercuts can often not be avoided, particularly on complicated geometries. Movable components such as slanted ejectors, block core pins or mold inserts are essential in these cases. These permit molding complex components without causing problems with demolding or filling, and ensure precise shaping and uniform material distribution. Optimized mold opening monitoring ensures that the component is only removed from the mold after sufficient cooling and curing. Premature demolding can cause warping and surface defects. Monitoring also helps prevent burrs from forming by correctly closing and holding the mold.

The quality and efficiency of the plastic injection molding process depends largely on the correct parameters and tools. In a production environment that requires precise and repeatable results, even a small deviation in the parameters can have significant effects on the end product. The correct MISUMI equipment can help to effectively avoid frequent sources of errors during injection molding and to sustainably increase production quality.