Injection Molding Process - Most Common Defects and Remedies

The plastic injection molding process is one of the most frequently used production processes in industry. As in other processes, product quality is crucial and may constitute a competitive advantage. It can be divided into two categories: quantitative quality and quality relating to aesthetic defects.

Quantitative quality refers to the dimensions, weight, and mechanical properties of the product, while aesthetic quality refers to the appearance, gloss, color, and surface.

A few words about the materials used in the injection molding process

We are all perfectly aware of the significant presence of plastics in the modern world. The automotive industry is also increasingly investing in the use of plastics for the production of various car components, with the share of plastics in production gradually increasing from year to year.

Statistically, currently produced cars contain from 40 to 100 kg of various types of plastic products. An average passenger car weighs 1,500 kg, making the weight of plastic elements relatively small. However, each car contains as many as 300 different plastic products. An interesting fact is that in Formula 1 cars, plastics can constitute up to several dozen percent of the vehicle’s weight!

We can produce elements from plastics such as:

– various types of pillars (A, B, C) in the car,

– seat adjustment elements

– dashboards, glove box and interior lighting

– door panels

– steering wheels

– rear elements of the front seats

– fuel tanks

The main advantages that make the injection molding process in the automotive industry so attractive are:

Weight

As it was mentioned earlier, parts made of polymer materials are relatively light. The higher their share in the production of a car, the lighter the cars and the less energy they need to drive.

Durability

Applying appropriate production technology can result in high durability of materials. Depending on its use, we can obtain elements that are highly durability to mechanical damage and environmental factors at a similar level to steel, and can even achieve better results in tests.

Resistance to external factors

Plastics are resistant to, among other things, high temperatures and humidity.

As you can see, the share of plastics in the automotive industry is increasing every year. This will result in more plants using injection molding. Therefore, such machines and processes will require more quality engineers to work with them.

Injection Molding Process – Most Common Defects and Remedies

Let’s get to the heart of this article, which is describing the most common defects during production using injection molding and remedies. It is thanks to them that we will be able to effectively eliminate a given defect and bring the product into compliance.

Short fill

In the case where the sprue has too small a cross-section, especially in areas far from the injection point, the cavity may not be completely filled, resulting in a defect as shown in the picture below. Possible causes of short fill may also include a leaky check valve of the screw tip, or too low injection pressure.

Overfilling

Since in the previous point we had short filling, it’s time to address another common defect in plastic products, which is overfilling. They can occur in the form of large protruding points, or as a small, palpable roughness. One of the reasons for their occurrence may be, for example, too low mold closing force.

Visible Flow Line during Injection Molding

Such defects are very well visible on dark or transparent molds with well-polished surfaces. In places where streams of the melt (two or more) meet during the filling of the cavity, areas similar to scratches or incisions appear, differing in gloss or color.

In combination with insufficient plasticization of the too cool melt front, this can cause a deterioration in the mechanical strength of the product.

Incorrect Gloss

The automotive industry frequently measures the gloss of plastic elements in products. Very often, the release of production is based, among other things, on the measurement of gloss, color, and dimensionality. In 90% of the products that are visible to the car user, an exact value and tolerance for gloss are specified.

Therefore, there should also be a place for gloss measurement on the element in such a document.

Defects related to gloss can appear on the entire element (the entire element is outside the tolerance) or only on a specific part of the element where the surface does not have a uniform gloss. In general, it can be stated that for cavities with a matte surface, we obtain a mold with a lower gloss, and for polished surfaces, a higher one.

What remedies can we apply here:

– increase the mold temperature

– increase the clamping pressure

– determine the best time to switch the injection phase to the clamping phase. This point should occur just before the cavities completely fill.

Improper part weight

The weight of the component strictly depends on the parameters of plasticizing the material and the amount of injected material into the mold. The correct operation of the screw return valve (material retraction) is important here. Another important element is the lack of material leakage between the nozzle and the nozzle or from a leaky channel.

Injection Molding Process and Quality Issue Limitation – Summary

As can be seen with a high probability, it can be stated that the injection molding process and injection molding machines will become increasingly common in automotive industry facilities. The defects I described are commonly occurring defects.

You will surely encounter them while working in a company that produces plastic components. Consequently, there are definitely more types of defects that may appear on molded parts.

We also cordially invite you to the CQI-23 – Injection Process Assessment training.

Kamil Ochał