Proper maintenance of an injection molding machine is essential for ensuring its longevity, reducing downtime, and optimizing production efficiency. Regular inspections and timely repairs help avoid costly disruptions.

Regular maintenance includes cleaning the machine, lubricating moving parts, checking alignment, and ensuring the hydraulic system is functioning properly. Scheduled maintenance helps improve machine performance and extends its lifespan.

Maintaining an injection molding machine is not just about fixing issues but also proactively ensuring optimal performance. Learn more about each key maintenance task to keep your machinery running smoothly.

What are Black Spots on Injection Molded Parts?

Black spots on injection molded parts are common defects that result from overheating or degradation of the material during the molding process. These imperfections can impact both appearance and functionality.

Black spots on injection molded parts are often caused by excessive heat, trapped air, or contamination. These issues can lead to material degradation, affecting the part’s strength and finish. Proper temperature control and material quality are key to preventing them.

The characteristics of black spots and impurities in injection molded parts are small particles, dark brown, generally non-reflective, and when the particles are large, the impurities are layered, brittle, fragile, and porous after crushing.

What are the Causes of Black Spots on Injection Molded Parts?

Black spots on injection molded parts are often caused by issues like degraded material, overheating, or contamination during the molding process, which affect part appearance and quality.

Black spots on molded parts can result from material degradation, excessive heat, or contamination. Solutions include adjusting processing parameters, using high-quality materials, and maintaining clean molds.

Resin Decomposition

Because plastics are chemical substances, when they are heated above their melting point, they will gradually decompose. The higher the temperature and the longer the time, the faster the decomposition will proceed.

In addition, there are some areas inside the barrel that are easy to retain resin, such as non-return check valves and threads. The resin remaining in these parts will be charred or carbonized, and then fall and mix into the molded product, causing black spots.

Insufficient Cleanin

Another cause of black spots is the resin that remains in the injection molding machine because it wasn’t cleaned well enough. As I mentioned earlier, there are places where resin is easily fixed, like stop rings and threads, so you have to apply the right amount of force and cleaning time to these places when you change materials.

Also, you have to use a cleaning method that’s appropriate for each material. It’s relatively easy to clean similar resins¹, but if you’re cleaning different types of materials, because of the different melting points or decomposition temperatures, and the compatibility between resins, even if you clean it, in many cases, you can’t completely remove it.

Foreign Matter Mixing (Contamination)

Another cause of black spots is contamination. If some pellets in the feed hopper are mixed with other resins with lower decomposition temperatures, the resin will decompose and cause black spots. Also, be careful with recycled plastics. They are easier to decompose after multiple heatings. They can also be contaminated with metal during the recycling process.

Black spots can also be caused by contamination. If some pellets in the feed hopper are mixed with other resins with lower decomposition temperatures, the resin will decompose and cause black spots. In addition, pay attention to recycled plastics. Recycled plastics are easier to decompose after multiple heating. In addition, they may be contaminated by metal during the recycling process.

Material Problems

The raw materials have more impurities, like impurities. The material has too much water. After high-temperature heating, the water evaporates to form pores, and reacts chemically with the molten material to generate black precipitates.

The injection molding material is dirty. When you’re doing injection molding, if your raw material gets contaminated with stuff like oil, plastic material remains , water, etc., you’ll get black spots.

The quality and type of raw materials also directly affect the quality of the product. If the raw materials are mixed with impurities or inappropriate additives are added, black spots may also appear on the product.

Injection Mold Problems

The mold surface is corroded badly, or there are deep grooves and other damages on the mold surface. The cooling water pipeline system of the mold is blocked, and the cooling effect is reduced, which can easily cause the mold temperature to be too high, causing the molten material to deteriorate and generate black substances.The mold is dirty. Dirty mold will make the product surface bad and make black spots .

Injection Molding Process Problems

When the molten material is not fully refined during the heating process, it will cause problems during the injection molding process. For example, if the hot zone of the barrel is not properly controlled, or if the barrel is overheated, it will cause black spots on the surface of the product.

The quality is unstable and black substances are produced. The inside of the injection molding machine is contaminated. The presence of impurities such as oil inside the injection molding machine will also affect the injection molded products.

There are a few steps in the injection molding process that can cause black spots on the surface of your product. For example, your mold might not be completely filled, or the baking process might fail, which can cause surface defects.

How to Solve the Black Spots on the Injection Molded Products?

Black spots on injection molded products are a common defect caused by various factors, including contamination, improper processing, and equipment issues.

Black spots can be caused by burnt resin, moisture, or foreign particles in the molding process. Proper material drying, machine maintenance, and temperature control can prevent this defect and improve product quality.

Reduce the Resin Melt Temperature

Each resin has recommendedoperating temperatures. You can find this information in the product catalog or on the product packaging. Check if the molding machine temperature is outside the recommended range. If it is, lower the temperature.

Also, the temperature displayed on the molding machine is the temperature of the area where the sensor is located, which is a little different from the actual resin temperature.

If possible, it is recommended to measure the actual temperature using a resin thermometer, etc. In particular, areas where resin retention is prone to occur (such as check rings) may cause black spots, so pay special attention to the temperature nearby.

To avoid carbonized black spots, you need to control the processing temperature. In real life, the temperature you set and the actual temperature are different. Even if you use the same machine, different back pressures² , different cycle times³, different shot sizes, and different thermocouple positions will cause different carbonization and degradation trends at the same temperature.

When the temperature is the same, the back pressure is small, the cycle is short, the one-time shot volume is large, and the thermocouple is inserted in the upper part of the barrel, the melt degradation/decomposition trend is weak, and it is not easy to produce carbonization black spots.

When the temperature is the same, the back pressure is small, the cycle is short, the one-time shot volume is large, and the thermocouple is inserted in the upper part of the barrel, the melt degradation/decomposition trend is weak, and it is not easy to produce carbonization black spots.

Thorough Cleaning

The black spots on the thread surface and the wall of the screw barrel are caused by the degradation and deposition of additives or the deposition of existing carbonized materials. As production continues, these black spots will be separated from their original attachment surface due to the various strong shearing actions in the screw barrel and will enter the molten material and be discharged.

This discharge process is when the product gets black spots and impurities. For example, when you change materials and colors, you can\’t avoid getting impurities in this discharge process, and you have to try your best to shorten this process. It\’s cleaning. Here are two cleaning methods:

Air purge the screw (purge with air after melting).

Melt the resin on the injection molding machine, purge the resin with air, melt the resin on the injection molding machine, purge the resin with air, and repeat this process. Practice has shown that back pressure is very important for purging, and the molten plastic material and the screw are strongly sheared.

The melting injection speed is slow, so the melting time is also long. This has a good effect and foreign matter can be cleaned quickly. Note: There is a principle for back pressure when it is not timed – as high as possible, until the screw barrel does not automatically heat up.

To remove black spots from the mold, the black spots must first adhere to the surface of the cavity, and then be wrapped and fixed on the surface of the product by the molten material, so this method of removing black spots will prevent it from being in the cavity. For molds with poor materials, the cause and location must be determined first.

You can burn all the cavities with relative movement, including the mold cores, sliders⁴, neutrons, ejectors/blocks. After burning, repair the damaged parts, reduce the friction of the sliding parts, and add lubricating oil. In addition, reduce the relative movement speed of the sliding parts, such as pushing in/out, opening and closing the mold, and move the slider as slowly as possible.

There is rust and other stains between the slider and the template. The slider should be removed, the rust and other stains should be cleaned, and the nozzle⁵ should be tied tightly. For oil or water splashing onto the smooth molding surface of the mold cavity, black spots should be frequently wiped, and the oil and water should be prevented from appearing on the molding surface.

Design the mold, clean the ejector and slider. Improve the exhaust system. Clean and polish any dead corners in the flow channel to make sure no material builds up.

Clean the mold surface before injection molding. Check if the inner surface of the barrel and the screw surface are scratched and accumulated by selecting the appropriate injection molding machine tonnage.

Keep an eye on the cleanliness of your production equipment and the area around it. Make sure everything stays clean. Before you package and store your finished products, check the surface of the material particles to make sure they’re clean. Look for any dust or dirt. Don’t let anything that’s not up to snuff get through.

When molding, black spots are formed due to foreign matter mixed into the material. We must strictly control the cleanliness of each link from production, packaging, storage, transportation, opening, mixing to the barrel.

Black spots tend to stay in the stop ring and screw in the barrel. If black spots appear, it is estimated that the cause of the black spots is likely to remain in the barrel. Therefore, after the black spots appear, the barrel must be thoroughly cleaned and countermeasures must be taken (otherwise the black spots will never disappear).

Clean and remove the resin used in the last injection molding process. Do the work. When using recycled plastics, check if there are foreign objects in the pellets. Keep the production environment clean and dry to effectively reduce the generation of black spots. Reduce the temperature of the barrel and nozzle. Clean each link of the injection molding process.

Don\’t reuse stuff that already has black spots/black lines.

Reduce Residence Time

Even if the setting temperature of the molding machine is within the recommended temperature range of various resins, long-term retention may cause the resin to deteriorate and thus black spots. If the molding machine provides a delayed setting function, make full use of it and choose an injection molding machine that is suitable for the mold size.

Choose the Right Raw Material

Check the injection molding material⁶. First, you need to check if the raw material is contaminated.If there are impurities, you need to replace it with new raw materials. Use pollution-free raw materials, put the materials in a relatively closed storage bin, and increase the thermal stability of the materials.

When you buy heat sensitive materials, you should choose plastic materials with stable quality, few impurities, and low moisture absorption rate, and pay attention to controlling the water content in the raw materials. It can effectively reduce the occurrence of black spots.

If you’re getting black spots because of particles, you can try using better raw materials. If you use clean particles and screen them, you can reduce the chance of getting black spots from particles.

To avoid black spots, choose the right raw materials. Use high-quality raw materials and choose the right type for your product.

Maintaining the Injection Molds

Check the mold surface regularly for damage, and strictly implement the cleaning and maintenance of the cooling water pipeline to ensure the cooling effect and reduce the mold temperature.

Clean the impurities such as oil stains inside the injection molding machine, and use special cleaning agents or solvents for cleaning. Clean the dirt and oxide layer on the surface of the mold, and use professional cleaning tools and efficient cleaning agents.

Cleaning and maintenance of the mold are also very important. Regular cleaning and inspection of the mold can avoid the appearance of black spots caused by discovering mold blockage or wear problems too late.

Maintenance of injection molding machine equipment is also very important. Enterprises should perform regular maintenance on equipment to ensure that the surface of the equipment is flat and pollution-free, and the inside of the equipment is clean and free of impurities.

Adjust the Injection Molding Process

Tweak the settings. Reasonably adjust the injection molding process parameters such as adjusting the hot zone temperature of the injection molding machine barrel, the barrel temperature and other parameters, and operate after the adjustment is correct. Add filters. In order to prevent impurities in the raw materials from entering the injection molding machine, filters can be added to reduce the number of impurities.

To reduce black spots, optimize the process flow. Different processes can lead to different results for the same product. To reduce black spots, monitor and adjust pressure and temperature. Also, check the filling of the product.

If black spots that cannot be completely eliminated appear during the process, you can consider surface treatment methods. For example, you can polish the surface of the product or use chemicals to treat it, which can reduce the black spots to a minimum.

Conclusion

In summary, the solution to the problem of black spots in injection molding products is mainly to avoid problems by cleaning thoroughly, trying to reduce the resin temperature, reducing the residence time, selecting suitable raw materials, maintaining the mold, and adjusting the injection molding process⁷.

Additionally, to prevent black specks on injection molded parts, it is critical to maintain consistent conditions during the injection pressure, including monitoring material degradation, ensuring uniform melt flow rate⁸, and controlling injection pressure . At the same time, companies should also strengthen monitoring and quality management to ensure product quality and the competitiveness of companies.