Injection molding is a widely used technique in plastic manufacturing, yet it can be costly in terms of time and money if manufacturers do not understand how to avoid mistakes during the process. There are some typical injection molding defects that will be introduced, along with their solutions. Let’s delve into them for better risk management during production.
1. Overall of Injection Molding
Injection molding refers to a mass plastic manufacturing process in which parts are produced in large quantities. This technique works by injecting molten materials into a mold. These materials can be elastomers, metals, glasses, or confections.
The process of injection molding requires detailed information about several factors: the design, shape, and features of the parts; the materials used for the parts and molds; and the capabilities of the machines. Typically, this sophisticated process involves the following steps:
- Mold preparation: Make sure the mold is cleaned and free from contaminants or debris. Clamp all the mold halves to create a closed mold cavity.
- Plastic material selection and melting: The desired properties of the finished product are the decisive factors in choosing the plastic material. For melting the material in injection molding, start by inserting plastic material in pellet form into the hopper of the machine. As it flows through the barrel, the heating and melting stages take place with the involvement of rotation and pressure created by the screw.
- Injection of molten material into the mold: When the plastic is fully molten, it is injected into the mold cavity under high pressure. The higher and more precise the pressure, the greater the likelihood that the plastic will fill the mold and ensure a complete final shape.
- Cooling and solidification: This is the most crucial stage, determining the structural integrity and dimensional stability of the finished product. Cooling and hardening occur once the molten plastic has been injected into the mold.
- Mold opening and ejection: After the plastic has cooled and hardened, the mold opens to eject the completed product. Ejection mechanisms, typically ejector plates or pins, help remove the product from the mold without damaging its features or shapes.
2. Mistakes in Injection Molding and How to Avoid
Though injection molding brings high efficiency to the mass production process, it requires technical excellence to implement. In the early stages of product development, major problems can arise due to minor mistakes, which can compromise product integrity. These issues can include poor design, ineffective quality control, and production process errors.
To minimize defects in the injection molding process, it is crucial to understand some common mistakes that may occur and how to resolve them.
2.1. Warpage
Warping, which often occurs in injection molding, refers to unexpected bends or twists. This defect results from uneven shrinkage within the material during the mold cooling process, causing internal stresses.
Preventing warpage involves ensuring appropriate cooling time and rate for parts to minimize internal stresses that can damage the piece. More importantly, it is crucial to maintain consistent wall thickness to ensure a uniform flow of plastic within the mold cavity.
2.2. Sink Marks
Sink marks are unwanted dents or craters that can be easily visually detected on the surface of a part, especially on thick sections. It takes longer for thicker sections to cool, resulting in unexpected side effects. As a result, the inside of the part shrinks and contracts at a considerably different rate compared to the outside.
To avoid sink marks, reducing the thickness of sections helps to ensure consistent and even cooling during molding. Additionally, maintaining adequate pressure and appropriate temperatures can contribute to defect prevention.
2.3. Short Shots
Short shots might be one of the most severe defects that can occur during injection molding. It refers to a situation where plastic is not properly injected into the mold, and the mold is not entirely filled with plastic. This can damage the desired appearance and result in an incomplete product after the mold cools.
There are various factors that can lead to this defect:
- The shot is not calibrated accurately, leading to an inadequate amount of plastic to fill the voids.
- The plastic is so viscous that it hardens before reaching all the cavities inside the mold.
- A gate that is too narrow or blocked can also restrict the flow of plastic within the mold.
- Trapped air pockets can hinder injection pressure and the proper flow of material.
There are also many solutions to prevent short shots in the injection molding process, such as:
- Enhancing the plastic flow through the mold cavity by equipping the mold with larger gates, accelerating the injection speed, and increasing the pressure.
- Utilizing less viscous plastic and increasing the temperature to prevent the material from cooling down too quickly.
- Utilize more air vents or widen existing vents as gateways for trapped air to escape.
2.4. Flow Lines
Flow lines often appear on the surface of a part as off-color lines or streaks. Flow lines are caused by inconsistent plastic flow speed through the mold. This issue typically arises from too low injection speed or pressure. Another reason for the occurrence of flow lines is the plastic flowing through areas with different wall thicknesses.
Ensuring even wall thickness is a common solution to reduce flow lines. Positioning the gate at thinner wall sections can also help prevent this issue. In order to guarantee that the material covers the entire mold before cooling, manufacturers also need to increase the injection pressure, speed, and material temperature.
2.5. Discolouration
Unlike the defects mentioned earlier, which mostly affect the product’s strength, discoloration causes the product to lose its desired aesthetics. This defect occurs when abnormal colors appear on a molded object. The presence of leftover pellets or resin in the molds may be the root cause. Additionally, trapped air or the presence of biodegradable materials leaving space for trapped air is another reason for this issue. Discoloration can also result from improper mixing of masterbatch.
The best solution to avoid discoloration is to maintain the cleanliness of the machine, nozzle, hopper, and mold throughout the manufacturing cycles. Manufacturers should also ensure that the masterbatch is evenly blended to achieve consistent color output.
3. Conclusion
Below are some of the most common mistakes in injection molding. Preventing these issues not only helps manufacturers maximize the performance and efficiency of production but also saves time and money.
4. About EuP Egypt
EuP Egypt is a member of EuP, one of the leading filler masterbatch manufacturers worldwide. We provide our customers with tailor-made plastic material solutions to increase their competitiveness in the market.
Why choose EuP Egypt?
- State-of-the-art factory system: We operate 7 factories covering dozens of hectares across 6 provinces throughout Vietnam and Egypt.
- Advanced technology: We own modern equipment and a system of standardized testing machines to ensure the highest-quality products for our customers.
- Professional and passionate team: At EuP, our leading experts are dedicated to delivering the highest quality products to our customers.
We are prepared to discuss and resolve any questions you may have about our offerings. Feel free to contact us today!
