Wrong mold temperature
When molding semi-crystalline engineering plastics such as POM (acetal), PA (nylon), PBT, and PET (polyester), it is important to ensure that the temperature of the mold surface is appropriate. To achieve satisfactory processing, the mold design needs to be fully considered. Only when the mold design is in place can the processor produce a high-quality molded product with temperature control equipment. In order to avoid production problems in the subsequent process, it is required to consider the mold design in combination with the design at the early stage of design.
Possible negative consequences.
The most common problem is the rough surface finish of molded parts. This is usually caused by a low surface temperature of the mold.
Molding shrinkage and post-molding shrinkage of semi-crystalline polymers depend primarily on the temperature of the mold and the wall thickness of the part. An uneven temperature distribution in the mold will result in different shrinkage, making it impossible to guarantee that the part meets the specified tolerances. In the worst-case scenario, the shrinkage exceeds the correctable value, regardless of whether the unreinforced resin or the reinforced resin is processed.
In high-temperature use conditions, if the size of the part becomes smaller, it is generally caused by the temperature of the mold surface being too low. This is because if the mold surface temperature is too low, the mold shrinkage is also low, but the post-mold shrinkage is higher.
If the start-up process is too long before the dimensions are stabilized, this is an indication that the mold temperature is not well controlled, which is due to the long time it takes for the mold to reach thermal equilibrium.
Uneven heat dispersion in some parts of the mold can lead to a significant increase in the production cycle, which can increase the cost of molding.
In some cases, incorrect processing temperatures can be known by analyzing molded parts, such as structural analysis of POM and differential scanning calorimetry (DSC) of PET.
Recommendations for determining the correct mold temperature
Nowadays, molds have become more and more complex, so it is becoming more and more difficult to create the right conditions to effectively control the molding temperature. Molding temperature control systems is often a compromise beyond simple parts. Therefore, the following recommendations are only a rough guideline.
In the mold design phase, the temperature control of the appearance of the machined part must be considered.
When designing a mold with a low shot volume and large mold size, it is important to consider good heat transfer.
Design the cross-sectional dimensions of the fluid flow through the mold and feed pipe with margins. Do not use fittings, as this will cause a serious obstacle to the flow of fluid controlled by the mold temperature.
If possible, use pressurized water as a temperature control medium. Use flexible pipes and manifolds (8 bar, 130°C) that can withstand high pressures and temperatures.
Give a detailed description of the performance of the temperature control equipment that matches the mold. The datasheet given by the mold maker should provide some necessary figures about the flow rate.
Please use an insulating board at the overlap between the mold and the machine formwork.
Different temperature control systems are used for moving and fixed molds
On either side and center, use an isolated temperature control system that allows for different start-up temperatures during the molding process.
Different temperature control system circuits should be connected in series, not in parallel. If the circuits are connected in parallel, the difference in resistance will cause a difference in the volumetric flow rate of the temperature-controlled medium, resulting in a greater temperature change than if the circuits were connected in series. (It will only operate well if the series circuit is connected and the temperature difference between the inlet and outlet of the mold is less than 5°C)
It is an advantage to have a display of the supply temperature and the return temperature on the mold temperature control equipment.
The purpose of process control is to add a temperature sensor to the mold so that temperature changes can be detected in actual production.
Thermal equilibrium is established in the mold through multiple injections throughout the production cycle, typically with a minimum of 10 injections. The actual temperature in reaching thermal equilibrium is influenced by many factors. The actual temperature of the mold surface in contact with the plastic can be measured with a thermocouple inside the mold (reading at 2mm from the surface). The most common method is to hold a pyrometer in hand, and the probe of the pyrometer should respond quickly. The determination of the mold temperature is measured at many points, rather than at a single point or on one side. It can then be corrected according to the set temperature control standard. Adjust the mold temperature to the appropriate value. The recommended mold temperatures are given in the list of different raw materials. These recommendations are usually based on the optimal configuration of high surface finish, mechanical properties, shrinkage, and cycle time.
For molds to be machined for precision components and molds to meet strict appearance conditions or certain safety standards, higher mold temperatures are usually used (which can lead to lower post-mold shrinkage, brighter surface, and more consistent performance). For parts with low technical requirements and production costs as low as possible, lower processing temperatures can be used for molding. However, manufacturers should understand the disadvantages of this option and carefully inspect the parts to ensure that the produced parts can still meet the customer's requirements.
