4 Common Errors in the plastic injection process, causes, and solutions

Contacting a custom injection mold supplier can save you from many project issues. Many defects can form on the finished plastic product. In this article, we will discuss all the common errors in the plastic injection process, their causes, and how to fix them.

Common mistakes in the plastic injection process

There is a lot of room for costly mistakes when it comes to the injection molding process. Quality problems in injection molded products can range from minor surface defects to more serious problems affecting product safety, performance, and functionality. 

How to get high-quality plastic injection mold

The quality of the plastic custom injection mold supplier is equally important to the molding process. The injection molding company you choose to help you manufacture your product overcomes common mold problems.

When you have a high-quality plastic mold plus a good injection molding process. This will yield the expected results from the final product. Plastic injection molding is a common way to produce all kinds of items. It can be like garden tools, electronics, kitchenware, etc.

Components are easy to make from

• Glass,
• Polymers,
• Metals,
• Plastics, etc.

Working on high-quality molds is the only way to ensure high-quality molded parts.

Common errors in custom injection mold supplier

Common errors are easy to be caused by the type of raw materials common in the plastic injection process, the quality of the plastic mold, the skill of the workers in the injection molding process, the injection molding machine, etc.

Fortunately, the treatment and prevention of common injection mold is also important. And using these methods, manufacturers can make high-quality custom injection molds, because if you make a high-quality mold, all other problems are easily solved. Understanding what defects and what causes them is the first step to ensuring that products developed in the process consistently meet acceptable standards. 
Defects can reduce the loss incurred 

These defects can reduce the loss incurred because of unsold products or defective products returned after the sale. Let’s look at the most common injection molding errors, what causes them, and what you can do to fix them.

Common mistakes in the plastic injection process

Possible defects due to process failure

Sometimes the problem is not with the raw materials or tools common in the process, but with the injection molding process itself. Once you are sure that the defects are caused by the process, all you need to do is adjust certain important factors like wall thickness, which are easy to avoid.

1. Line Mark

Line lines (also called flow lines) are waves, streaks, or lines that change color compared to the area around them.

These are visually visible near the ‘gate’ points or gate locations of the custom injection mold where the molten plastic enters and through the injection molding process. They do not affect the functionality of the finished product but are irritating, especially when found on cosmetic products.

Reason:

The flows are due to the change in the cooling rate of the molten plastic as it flows through the barrel and the mold is heated and solidified. This also happens if there are thin walls and thick walls instead of a uniform wall thickness because the material cools at different rates.

Slow injection rates or low-pressure injection molding can also cause flows, as the plastic cools very quickly and becomes thicker, forming molding defects such as patterns or wavy streaks.

How to fix custom injection, mold supplier:

• Increase injection speed, pressure, and material temperature to ensure the material fills the custom injection mold before cooling
• Rounding mold corners where wall thickness increases to help keep flow rates consistent and prevent flow
• Relocate the mold gates to create more space between them and the mold coolant to prevent the material from cooling too early in the melting process.
• Increase nozzle diameter to increase the flow rate and prevent premature cooling 
• Applying lubricant to the surface will also aid the flow of molten thermoplastic material in the running system.

2. Burns

Burns are dark marks on the edges or on the surface of the molded part produced by a custom injection mold, which is not part of the product design. They are usually harmless unless they burn causes the plastic to decompose.

Reason:

The most common cause behind burn marks is air bubbles or resin trapped in the mold cavity that overheats during the cycle. This may be due to high injection speed or material overheating.

How to fix Burns:

• Lower the melting point and mold temperature to avoid overheating
• Reduce injection speed to limit the risk of air trapped inside the mold
• Widen the air vent and outlet to allow trapped air to escape from the mold
• Shorten custom injection mold’s cycle times so trapped air and plastic have no chance of overheating

3. Warping

Warping occurs when different parts of the product cool and solidify unevenly during plastic processing.

Reason

The main cause of warping in plastic products is the premature cooling of the material. This may or may not be due to overheating and the absence of uniform mold walls.

How to fix:

• Ensures cooling slowly and long enough to avoid uneven stress on the material
• Lower the temperature of the material or custom injection mold
• Try switching to a material that shrinks less during cooling (e.g., granule-filled thermoplastics shrink much less than semi-crystalline or unfilled ones)
• Redesigned mold with uniform wall thickness and part symmetry to ensure more stable part during cooling

4. Vacuum/air bubble

While not considered a major defect, it can lead to a deterioration of the product during product development.

Reason:

One of the most common reasons for vacuum voids in plastic products is insufficient pressure in the mold during low energy production. 

How to fix Vacuum/air bubble:

• Increase injection pressure to push out trapped airbags
• Choose a material with a lower viscosity to limit the risk of air bubbles forming
• Place the gates close to the thickest parts of the custom injection mold to avoid premature cooling where the material is most susceptible to voids

1. Watermark of  the injection tool

Injection tools are small dents that form in plastic products. This usually occurs when the interior of the component solidifies and contracts rapidly, and the outer material inward by the shot.

Reason:

Sinking occurs because the molten plastic material cools too slowly in the mold. As a result, the outer material inject before it has had a chance to cool completely, causing material degradation.

This usually occurs in the thickest parts of the injection tool due to the uneven cooling of the plastic material.

   

How to fix the watermark of the injection tool:

• Increase holding pressure and increase time to allow material near the part surface to cool
• Increase cooling time to limit shrinkage
• Design your mold with thinner components to allow for faster cooling near the surface.

2. Welding line

Welds may appear on the surface of the injection tool part, where the molten material has converged after separating into two or more directions in the mold. A hair-like weld seam results from weak material bonding, which reduces the strength of the part.

This leads to a weakening of the plastic component in the weld

Welding occurs when molten plastic converges from different channels at a certain point in the mold and weakly bonds with each other. This leads to a weakening of the plastic component in the weld.

How to fix it:

• Increase material temperature to prevent partial solidification
• Increase injection speed and pressure to limit cooling before the material fills the mold
• Redesign the injection tool to remove the baffles
• Switch to a material with a lower melting point or viscosity to allow faster flow and prevent premature quenching.

Type of deformation in a molded part that can occur

The injection typically appears as a squiggly line on the surface of the finished part, usually leading from the original nozzle. This visible flow pattern can lead to partial weakening.

3. High pressure of the injection tool

This may be due to the high injection pressure, which results in the material injecting through the nozzle into the injection tool instead of gradually filling the mold.

This stream of molten plastic begins to cool before the rest of the cavity fill, resulting in the formation of ripples on the surface of the finished product.

How to reduce injection pressure

• To prevent material from rapidly injecting into the mold cavity
• Raise the material and mold temperature to keep the initial jet material from solidifying prematurely

4. Common Defects Relating to the Use or Care of the Material

Defects in injection tools can often stem from the material itself or the way the manufacturer stores and treats the material before manufacturing. These defects can range from minor cosmetic problems to compromise the durability of the finished part. Serious safety concerns may also result, depending on the intended application of the affected product.

5. Discoloration occurs when a part has different colors

Discoloration, also known as “staining,” occurs when a part has a different color than it was intended. Often the discoloration is limited to a localized area or a few unusual streaks of color on the injection molded part. This defect usually affects the surface of the part without reducing the quality of the product.

Reason:

One possible cause of discoloration is pellets of plastic material leftover from a previous run in the machine. It is also possible that some residual material is stuck in the nozzle or mold due to low pressure.

Other reasons may be the colorants not being mixed well with the ingredients.

How to fix:

• Ensure that workers clean hoppers, nozzles, and molds properly between production runs to remove residual pellets or base material.
• Consider using a bleach compound to remove excess color from the machine
• Make sure you or your supplier are using a colorant with the proper heat stability

6. Surface delamination of the injection tool

If you see thin layers on the surface of a part that easily separate or peel off from the underlying material, you are seeing a casting defect called delamination. Delamination is a defect characterized by a flaky surface layer, similar to what you would normally see on flake mica. This is generally considered a relatively serious defect because of the reduced strength of the material.

Reason for Surface delamination of the injection tool

Usually, surface delamination occurs because the raw material is with particles of foreign matter such as sheet metal. The product’s surface begins to peel when the materials cannot bond together properly. And very dangerous if the finished product is a critical ingredient in terms of safety.

Another reason for surface flaking because the material has not been properly dried before use, and there is moisture on the surface.

How to fix Surface delamination of the injection tool:

• Raise the injection tool temperature or pre-dry the appropriate material if excess moisture is an issue
• Ensure workers are properly storing and handling plastic pellets or base materials to prevent contamination
• Consider redesigning the mold with a focus on the nozzle to limit your reliance on release agents

7. Injection Molding Defects Due to Poor Mold Design

Short shot

If the molten plastic material does not fill the mold cavity during the running process, the final product is defective and incomplete after cooling and solidifying. This is a crucial defect that greatly affects the functionality of the product and increases the cost of production.

Block lines or ports in the machine are one of the main causes of short shots. This can especially happen when the nozzles are narrow.  If the material is too viscous or the mold is not at a high enough temperature, the injection tool will not fill before the material solidifies.

Material can also affect the flow of the molten plastic

Air trapped in the material can also affect the flow of the molten plastic. Another reason could be insufficient pressure during material injection.

How to fix:

• Redesign mold with wider runners or gates for better flow
• Increase injection speed or pressure or choose thinner base material to improve the flow
• Increase the mold temperature to prevent the material from cooling down too quickly

Extrusion or injection: which method is better?

Since the beginning of the 20th century, when Injection mold makers started studying polymers, we had a huge exploration to search for cheaper substitutes for natural materials. Several methods of transforming the raw material into the desired product emerged. We know it as extruder and injection.

The most common processing methods in the industry today are extrusion. And injection, corresponding to more than 90% of the production of plastic materials. These processes have similar characteristics, such as increasing the temperature at the beginning to bring the polymer to a liquid state.

Factors that influence polymer processing!

Extrusion

The most common Injection mold-making method in the industry, extrusion processing, mainly generates linear products. These are widely common in everyday life, such as hoses, sheets, and plastic bags. Unlike most polymer processing methods, extrusion has the great advantage of being a continuous process.

And then cooling it until it solidifies again (therefore, it is important to know the material’s properties and the factors that influence its processing). But extrusion or injection? What are the differences and advantages of each of these types of processing?

Screw: A most important part of the whole machine

Initially, the raw material adds (with or without additives and other loads) into the equipment by the Injection molding machine, in the form of small solid grains, arriving in the cylinder or cannon. At that moment, the material is transported through the entire cylinder with the action of the most important part of the whole machine: the screw.

Whether it has two joints, the thread rotates in such a way as to force the material along with the cylinder.

The solid to liquid process

In a second moment, heaters heat the raw material beyond its boiling temperature, changing it from solid to liquid. Thus, the molten plastic continues advancing along the cylinder path until it passes through the mold, which will shape the material as it exits the machine.

The matrix is ​​another component of the equipment that is extremely important, as it has a unique profile that will be responsible for shaping the final product. In this way, the same extruder can produce different products, changing only its matrix.

Finally, when passing through the matrix, the material begins a cooling process and solidifies again. Thus, you get a continuous product that can be cut into different sizes, according to needs and other interests.

On the other hand, injection is also an alternative. Although not as common as extrusion. It is the second most common method to produce polymeric products. And it also has its advantages. The injection does not have continuous production. But because it uses molds, it can generate both simple and complex products and various formats.

Injection

The injection is a molding process and has steps analogous to extrusion from a hopper.

These grains of polymeric material also heat until they melt and travel through the machine’s interior with the rotation of a screw.

After this path, they pass through the injection nozzle to fill the mold. This process is easy to repeat several times to obtain the same part in a large quantity and with the quality maintains.

The injection can produce various kinds of products

As an Injection mold’s molding process, the injection can produce kitchen utensils, construction parts, toys, support devices, and many other products with high quality and low variation.

Which method is better?

Once you understand what polymer processing is and understands how both extrusion and injection molding work, you might be asking yourself: which one should I choose to make my products?

Injection molding allows the conformation of more complex shapes

However, it is not so simple to point out just one as absolutely superior to the other. We know that extrusion allows for continuous production. But injection allows the conformation of more complex shapes.

And it is still necessary for the Injection mold maker to analyze each case, know the material to work on, and understand the desired properties for the final product.

Do you want to improve your company’s polymer processing? We can help you! We identify areas for improvement, such as the composition and process of the product, analyzing which is the most suitable method and which will guarantee a better cost-benefit ratio for your reality.

Important details about this manufacturing technique and industry.

It is possible to say that plastic injection is a casting process that, instead of metal, uses polymers. It is a process widely common for the creation of industrial parts, among other applications, such as packaging and toys.

In this article, you will understand more about how plastic injection comes and other important details about this manufacturing technique and industry.

Injection molding as it happens?

The material converts into its liquid state and is poured into molds. Over time, solidification occurs and then an item in the desired shape is achieved.

It is extremely common to use a plastic injection machine to carry out this process. The machine has the following components:

• Hopper;
• Barrel;
• Heating unit;
• Pumping nozzle;
• Adjustment unit;

For the Injection molding machine, the function of this unit is to melt the plastic through heat.

The pump nozzle is responsible for injecting this liquid into the molds. Then, the adjustment unit is where solidification takes place and, finally, the ejector expels the molten materials from the machine.

The manufacture of injection molds is a crucial process for plastic injection. After all, it is common for the customer to order a series of plastic products that need to-measure with a custom mold.

Generally, the chosen material of the injection molds is metal, and the manufacturing process runs in three main steps.

The virtual version must be evaluated.

This virtual version evaluates by the customer and considers the type of injection machine that will be common. Once approved, the project sends to the production team, which transforms the idea into reality. Remember that the mold also includes the internal gaps and recesses.

The final part is to finish the mold for plastic injection, ensuring that there are no leftovers and that the result will be satisfactory. It is important to note that there are more and less rigid injection designs in precision.

Conclusion

Perhaps you have wondered what technique is common to produce these metal molds. Injection mold maker uses another very common manufacturing process for producing parts: machining services.

Machining is changing the shape of a material by removing chips (irregular portions of material).