Mold fouling of products
Almost all thermoplastic materials experience this problem. Due to the increasing demand for finished products, the requirements for the amount of additives such as modifiers and flame retardants are also increasing. These additives often produce mold scale on the surface of the mold cavity.
There are also some other reasons that may cause mold fouling during forming and processing. Some of the most common reasons are:
• Thermal degradation
Excessive shearing
Insufficient exhaust
The generation of these mold scales is usually caused by the combined action of different factors. The fault should be identified, and the causes and prevention methods of mold fouling should be accurately analyzed. The problem is that mold scale usually appears after a few days
Types of sedimentation
Each type of additive produces a specific type of mold scale. Flame retardants can degrade at high temperatures, and the degradation products may form mold scale. Impact modifiers are not only affected by temperature, but also by excessive shear. Under unfavorable conditions, the modifier can be separated from the plastic and form mold scale on the surface of the mold cavity. Engineering plastics require pigments with high melting temperatures, which can reduce the thermal stability of specialized materials for engineering plastics. The result is the formation of mold scale composed of plastic degradation products and pigment decomposition products. Some parts of the mold are particularly hot (such as molds), and modifiers/stabilizers and other additives may adhere to the surface to form mold scale. In this case, certain steps must be taken to improve the temperature control of processing and molding, or special stabilizers must be used. The table lists the possible causes of mold scale and the methods to prevent mold scale formation.
If mold scale suddenly appears, it may be due to changes in the molding conditions or changes in different batches of plastic. The following suggestions may be helpful.
Firstly, in order to search for mold scale, the melt temperature should be measured and visually examined for signs of material decomposition, such as the presence of burnt particles. You can check whether the plastic has been contaminated by foreign substances, whether incompatible cleaning materials have been used, and also check the exhaust situation of the mold. Next, use primary or colored (non black) plastic., Turn off the machine after about 20 minutes of driving, disassemble the nozzle, machine head, and even screw. Checking the composition of the coking particles and comparing their color with the original plastic will quickly identify the root cause of the problem.
In some cases, this technology may reveal unexpected issues, but it is only applicable to small machines (i.e. injection molding machines with screw diameters not greater than 40mm). The elimination of these issues will result in some improvement in quality when processing other materials. Similar programs are also applicable to machines with hot runners.
Maintenance of molds
If the methods described above cannot prevent the generation of mold scale, special attention should be paid to the mold.
It has been found that the scale on the surface of the mold cavity is relatively easy to remove in the early stages of its formation. Therefore, the mold cavity and exhaust holes should be cleaned at specific time intervals, such as at the end of each shift. Once the mold scale is already quite thick, the removal work will be very difficult and time-consuming.
Because the chemical composition of mold scale varies greatly, multiple experiments must be conducted to determine the most suitable solvent. In addition to traditional solvents, some non-traditional products can also solve this problem, such as oven spray or caffeinated lemonade. Another method is to use clean rubber used in traditional model railways
Suggestions to prevent the formation of mold scale
If using a mold with a hot runner to process thermosensitive plastics, please remember that in such cases, the material will stay in the mold for a longer time, so the possibility of mold fouling due to material decomposition is higher.
Plastic that is sensitive to shear should be processed using molds with large-sized runners and gates. Multiple gates will reduce the flow distance, thereby reducing the injection speed and achieving good results
Generally speaking, effective exhaust during molding processing can reduce the possibility of mold scale formation. When designing the mold, attention should be paid to providing sufficient exhaust holes. It is best to use self-cleaning exhaust holes or mold scale capacity as the exhaust holes for cleaning. The improvement of the exhaust system often reduces mold fouling in the mold
In some cases, it may be necessary to apply a special non stick coating on the surface of the cavity to prevent the formation of mold scale, and testing should be conducted to evaluate the effectiveness of the coating. Titanium nitride often reduces the speed of fouling in the mold
