There are various appearance issues in plastic products, with the most common being mold parting line problems, which are generally easier to identify and resolve by increasing mold temperature, changing the injection molding gate location, or redirecting flow to non-appearance surfaces. However, there are instances where appearance issues persist despite adjusting parameters and modifying molds. Today, we’ll discuss the issue of white marks/ discoloration/gas patterns.
Why does the surface appear white?
This defect is more noticeable with black-colored material, displaying distinct white marks in the molded product (Figure 1). On-site engineers or designers may have different opinions on the cause of these white marks, attributing them to “gas patterns” or material-induced white haze. Since these white areas are often close to the injection molding gate, some engineers, unable to resolve the issue through parameter adjustments, may request modifications to the mold. If the problem is attributed to “venting,” the following methods can be employed for investigation:
- Shoot air through the material to observe if there is a strong presence of white gas or steam.
- During injection, check whether the injection molding machine barrel occasionally makes popping sounds.
- If gas residue is suspected, wipe the mold surface with a cleaning agent and then shoot 10 shots to see if the defect persists.
If the first two points occur, addressing material issues such as measuring the material’s moisture content may be necessary. However, if none of these three points reveal issues, gas-related problems can be mostly ruled out.
Why does this whitening issue occur? Take the product in Figure 1, for example, with the injection gate located on the non-appearance surface at the top. Short-shot samples reveal that the white marks are present from the beginning of the filling stage, indicating that the problem is unrelated to the subsequent packing stage. Since the issue arises during the filling stage, an understanding of the melt flow behavior in ordinary Spritzguss is needed (Figure 2), referred to as the fountain flow field. The melt in the mold cavity is divided into three layers: solidification layer, shear layer, and central flow layer. As the processing temperature of plastic products is generally between 230~320°C and the Schimmel temperature is between 40~80°C, the melt rapidly cools upon contact with the mold wall, forming a solidification layer. The lower the mold temperature, the thicker the solidification layer, leading to a smaller flow channel for the melt and requiring higher injection pressure.
Assuming the screw barrel advances at a fixed linear speed, the flow rate of the melt in the small-sized injection gate will be much higher than that in the mold cavity. At this point, the cold material at the front end of the material head will rapidly solidify on the Schimmel cavity surface, and the subsequent hot melt cannot remove the cold material, resulting in situation C in Figure 3. If the injection gate speed is faster, it may even lead to issues such as jetting or burning.
From the above theory, two factors causing white marks can be summarized: firstly, there is cold material at the front end of the raw material, a problem that may occur in cold runner designs or poorly controlled hot runners. Secondly, the speed variation from the injection gate into the mold cavity is too abrupt, causing the cold material to solidify rapidly on the mold cavity surface, resulting in color differences due to the large temperature difference with the incoming hot melt.
In terms of molding technology, adjustments can be made through high mold temperature and multi-stage injection speed. High mold temperature can reduce the thickness of the solidification layer and even change the behavior of the melt fountain flow, such as rapid heating and cooling (RHCM). Multi-stage injection speed adjustments are mainly made at the injection gate to decelerate, allowing the melt to transition smoothly before accelerating again. However, making precise speed changes in such a small volume at the injection gate requires a fast-responding injection molding machine (preferably all-electric).
In mold design, it is essential to avoid excessively long cold runners, ensuring that the thickness of the cold runner can transition smoothly. Side injection gates can be designed as fan-shaped injection gates to reduce speed.
In terms of product design, it is advisable to avoid significant differences in product thickness, and transitions should be made at the junction of thick and thin sections.
The above is the analysis and solution direction for white marks on plastic product surfaces. If you need to produce high-quality plastic products, feel free to consult our Grandshine team. We have professional engineering teams in the United States and Singapore, and we have our own factories in China and Malaysia. We welcome discussions at firstname.lastname@example.org.