Why is injection speed important?

Table of Contents

Injection speed is a critical factor when creating a plastic product. It can affect the adhesion, orientation, and shrinkage of a finished product.

This article will cover the factors that influence speed. We’ll also discuss the advantages and disadvantages of different injection speeds. Depending on the type of product, injection speed can make a big difference.


Influence of injection speed on adhesion

Injection speed affects the degree of orientation and adhesion between two materials. It also determines the degree of shrinkage and composite strength.

Higher injection speeds lead to higher shear heating and shorter pressure delay times. In addition, higher injection speeds lead to higher molecular orientation, which hinders bonding.

Adhesion in the first stages of injection molding process can be explained by the theory of adsorption and diffusion, as well as by van der Waals forces on the roll surface.

However, to observe these processes properly, a thorough study of the physical properties of the material is necessary. Moreover, a complete understanding of the adhesion properties of a substance is necessary to optimize the process.

Injection speed and barrel temperature have a strong impact on interfacial adhesion. The original film was tested with 13 N of load and 120-150 mm of elongation.

The resulting load-displacement curves are shown in Fig. 9. Each curve represents different failure modes. In Type 1, the film will fail to adhere to the substrate, resulting in peeling.

Influence of injection speed on orientation

Injection speed plays a critical role in the molecular orientation of composite materials. Moreover, it influences the strength of the composite, adhesion, and shrinkage of the component.

Higher injection speeds result in higher temperature, shorter pressure delay time, and stronger composites. In addition, high injection speeds decrease the likelihood of stress peaks and notch formation.

During the injection molding process, the material undergoes a pseudoplastic laminar profile. This results in chains that are stretched out during the mold filling phase while remaining in a coil configuration at the core. This orientation continues throughout the process.

The injection speed can be increased or decreased to achieve a desired orientation. High-molecular-weight polymers and fiber-reinforced polymers are especially vulnerable to orientation problems.

Injection speed also affects the thickness of the core region. A higher injection speed results in a thicker core layer (37%) than that of a low-speed injection. On the other hand, a low-speed injection speeds result in a thinner core layer (21%) and a lower shear rate.

Numerous researchers have examined the fiber orientation distribution of injection-molded SFRP parts. Some have developed numerical methods to predict the orientation distribution of SFRP parts based on reliable experimental results. This can be useful in the part design phase.

Influence of injection speed on shrinkage

If shrinkage is a concern in your injection molding process, you should understand the relationship between injection speed and shrinkage. The lower the injection speed, the lower the melt temperature, and the slower the injection time, the more likely your part will shrink. If shrinkage is an issue, you may need to increase the injection pressure or lengthen the injection time.

The SN ratios between post-molding shrinkage and warping are a measure of how these two factors interact. The SN ratios between the two factors are calculated using Eq. 1. The response table of average SN ratios is used to determine the optimal process parameter combination. The optimal combination is the one that exhibits the highest SN ratio.

Moreover, injection speed also affects the core region thickness. At high injection speed, the relative core thickness is greater than in low-speed injections.

This is because a thinner core region experiences higher shear rates. As a result, it has a flatter velocity profile and a greater pseudoplasticity. In short, a smaller cavity has a thinner core region.

The shrinkage changes are logarithmic in both PP20 and PP80 samples. These trend lines are represented by the trend line equations in Figure 2.

The highest primary shrinkage occurs when polymeric parts are processed at higher mold temperatures. This is not desirable in industrial practice, but it is possible to reduce it by adjusting injection molding process parameters. For instance, extending the holding phase can reduce the primary shrinkage.

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