Injection molding is one of the most important manufacturing processes for plastic products and is widely used in the automotive, electronics, home appliances, toys and other industries. Although the technology is very mature, quality problems sometimes occur in the actual production process, including the whitening of the product surface. The whitening of the surface not only affects the appearance, but also affects the product performance and reduces production capacity. Therefore, it is important to solve this problem because it will hinder product quality and production efficiency.

Causes of Whitening in Injection Molded Products

The degree of whiteness depends on the material used in injection molding, the structure of specific molds, the parameters of the process, specific treatments for the products made, and conditions in the production area. As for these aspects, we will provide separate analysis below.

Material Selection

Injection molding material selection has a significant impact on the surface quality of injection molded products. It is for this reason that different kinds of plastics do exhibit unique physical and chemical properties during the injection molding process, which in turn affect the surface finish of the final product.

1. Material Properties

Some plastics, such as polystyrene (PS), polypropylene (PP) and polyvinyl chloride (PVC), produce gases or residues when heated, which will remain on the surface of the product after solidification and form white spots. In addition, certain fillers and whitening agents used in production can also cause whitening. For example, the use of titanium dioxide can increase the whiteness of the product, but improper use can cause white deposits to form on the surface.

2. Material Mixing

It is also important to note that when there are several materials mixed for injection molding, the incompatibility between these materials can cause phase separation, which can lead to whitening of the surface. For example, when different types of plastics are used, the different materials have different melting points and flow properties, so they separate from each other during cooling and cause white spots to form. The solution is to use compatible materials or add appropriate compatibilizers to the mixture to reduce the likelihood of such events.

3. Use of Recycled Materials

Using recycled materials in injection molding production can save procurement costs. However, when using recycled materials, the materials may contain impurities or other degradation products, which may change the surface quality and cause whitening. Therefore, the proportion of recycled materials should be strictly limited, and they should be thoroughly cleaned and screened before use.

Mold Design

Mold design is an indispensable step that has a direct influence on the surface quality of molded products made of an injection molding process. The Whitening of the surface of the component is also caused by an unreasonable mold design.

1. Mold Surface Treatment

The finish and surface treatment of the mold surface will affect the finish of the final product. If there are scratches, dents, dirt, etc. on the mold surface, the product will feel the impact of these defects during injection molding, resulting in an uneven surface and whitening. It is recommended to properly clean and polish the mold.

2. Gate Design

The gate is the entrance for plastic to enter the mold, which largely determines the molding quality. An unreasonable gate design will also affect the distribution of plastic flow, cause stress concentration, and may cause surface whitening. Optimizing the gate design to ensure uniform plastic flow can reduce the whitening phenomenon.

3. Cooling System Design

The design of the cooling system will affect the mold temperature control process. If the mold cooling is inconsistent, it will cause different cooling rates of the plastic, generate internal stress, and cause the surface to whiten. Uneven cooling can be minimized through reasonable cooling system design.

4. Vent Design

Air and other gases must be discharged during injection molding, otherwise sediment or white spots will form on the surface of the product during the production process. Unreasonable exhaust port design and insufficient number of exhaust holes will cause gas retention and affect the surface quality. Optimizing the exhaust port design to ensure smooth exhaust can reduce white spots.

Process Parameters

All the factors of control that are used in injection molding involving temperature, injection speed, pressure as well as time are instrumental in determining the quality of the product. Inappropriately set parameters affect the system and cause the surface to become white.

1. Temperature Control

Temperature management is very important during the injection molding process. When the temperature is too high, the plastic used will decompose to release gas or produce residue, causing a whitening surface. On the other hand, too low a temperature can reduce the plastic flow rate and produce cooling marks. Accurate control of barrel temperature and mold temperature can reduce whitening.

2. Pressure Control

Defects on the surface of the molded product also depends on the injection pressure as well as the holding pressure. Very high injection pressures may result in over fill of the mold, development of internal stresses and therefore whitening. Properly reducing the injection pressure to a suitable level and maintaining the pressure can help improve the surface quality.

3. Time Control

Injection time, holding time and cooling time must be maintained at an optimal level. Short injection time will result in insufficient filling of the plastic material into the mold cavity, resulting in various forms of defects. Short holding time will lead to shrinkage and whitening of the product. Therefore, precise control of injection time, holding time and cooling time will help improve surface quality.

Post-treatment Methods

Another factor that determines Surface Quality of the injection moulded products is the post-treatment methods of injection moulded products. One of the drawbacks that can be observed when the correct post-treatment is not followed is the whitening of the surface.

1. Surface Treatment

Some injection molded products can be sold simply by taking them out of the mold, while others must be painted or electroplated after molding. When the surface treatment process is not appropriate, the surface will turn white. For example, too much or uneven painting will cause whitening. Optimizing the surface treatment process can improve the surface quality.

2. Cleaning and Packaging

Some injection molded products need to be cleaned and packaged after molding. If the surface is not cleaned thoroughly, residual cleaning fluid or moisture will cause the surface to turn white; improper packaging may cause the surfaces to rub against each other, which will cause the surface to turn white. Improving the cleaning and packaging process can reduce whitening.

Environmental Factors

The components that affect the injection molded component include environmental factors, which can also affect the surface type of injection molded products. For example, the humidity in the production workshop is relatively high, moisture can condense on the surface of the product, causing white spots and whitening, and dust and pollutants in the air can adhere to the surface of the product, causing the product to turn white. Improving the production environment and controlling the humidity and cleanliness of the workshop can reduce the whitening problem.

Solutions to Whitening of Injection Molded Products

Having understood the causes of whitening in injection molded products, let’s discuss some solutions to this problem.

Choose Appropriate Materials

Choosing right materials can help in addressing the problem of whitening in the injection molded products. Select materials that can resist heat and have high decomposition points for the plastics. Besides, do not use materials that exhibit phase separation; if the combined material is being used, incorporate compatible fillers.

1. Select High-Quality Materials

Before injection molding, raw material inspection and select high-quality raw materials to make them pollution-free, uniform and consistent. High-quality materials are not easy to degrade or leave residues during processing, which can avoid the occurrence of whitening to a certain extent.

2. Control Recycled Material Proportion

Ensure that the utilization of recycled material should be limited to a certain percentage and the materials should be cleaned and screened before used to minimize on impurities and degradation products. This can also help minimize the effects of the recycled material on the surface finish of the item to be produced.

Improve Mold Design

Improving the quality of mold design can also reduce white spots. In order to keep the mold surface smooth, it is necessary to clean and polish it regularly, ensure that the gate design is reasonable, and the plastic is evenly distributed. The cooling system should also be designed to allow uniform cooling.

1. Mold Surface Treatment

Clean and polish the mold regularly to ensure surface smoothness and reduce injection molding defects that cause whitening. Ensure that mold materials and surface coatings meet high standards to improve mold durability and product surface finish.

2. Optimize Gate Design

The gate design should ensure that the plastic is evenly distributed without stress concentration. The gate position and number should be reasonably determined based on the product shape, wall thickness and size to avoid whitening caused by overfilling.

3. Improve Cooling System

Give proper design to the mold cooling system to get proper solidification and to minimize the case of internal stress. Use multi-stage cooling circuits or variable temperature control systems to enhance cooling process effect and reduce whitening.

4. Improve Vent System

Reduce the bubble and white spots by improving the design of the mold vent for better gas venting. Increasing the number of exhaust holes is a feasible method, or micro-exhaust technology can be used to improve exhaust efficiency.

Optimize Process Parameters

Whitening can also be avoided through fine adjustment of process factors and control strategies. Accurately control the barrel and mold temperature of the injection molding machine to ensure that the plastic is molded at the appropriate temperature, appropriately adjust the injection and holding pressure to avoid internal stress, and control the injection, holding and cooling time according to the molding process.

1. Temperature Control

Precisely control the barrel, mold and material temperature to ensure that the plastic is molded at the ideal temperature. In order to better control the temperature, a complex temperature control system, such as a PID controller, can be used.

2. Pressure Control

Adjust the injection pressure to an appropriate value and adjust the holding pressure to avoid internal stress. To improve the surface quality of the product, use pressure sensors and controllers to control the injection pressure in real time and effectively control the injection pressure.

3. Time Control

Properly control injection, holding, and cooling times to ensure uniform molding. Use automated control systems to precisely control each time parameter, enhancing production efficiency and product quality.

Improve Post-treatment Methods

Increasing the efficiency of post-treatment approaches can continue to contribute to a further decrease in whiteness. Enhance the painting or electroplating systems to cover all injection molded parts with equal competence; enhance the cleaning and packaging methods to decrease the rubbing effect and external aggression.

1. Optimize Surface Treatment Processes

During surface treatment, moderate change of process parameters should be practiced. For instance, when painting, ensure that product quantity and application is well monitored so that it does not result to whitening since too much paint application or an uneven one can lead to this.

2. Improve Cleaning and Packaging Processes

Thoroughly clean products after molding to remove residues of cleaning agents and moisture. Use suitable packaging materials and methods to avoid friction and contamination during transportation and storage.

Improve Production Environment

Improve the production environment and control the relative humidity and sanitary conditions of the workshop. Use vacuum cleaners and dehumidifiers to ensure that the workshop is clean and dry to limit the impact of moisture and contaminants on contact surfaces.

Conclusion

This article aims to help industry practitioners overcome the problem of surface whitening of injection molded products and contribute to the development of the industry. The whitening problem of injection molded products is a complex problem that is affected by materials, mold design, process conditions, post-processing and the environment. By understanding the causes and implementing appropriate solutions, the plastic injection molding process can be analyzed and optimized to improve the quality of the final product and the production speed.