Heat plays a pivotal role in injection molding. While it’s understood that heat is required to melt and form plastic components, other less apparent uses are critical for producing functional, reliable components that meet specifications. To consistently mold high-quality components, specialized equipment like dryer banks, high-temperature oil heaters, and annealing and post-curing ovens are required. This post focuses on this equipment and how it is used in the injection molding process.
The Role of Dryer Banks
Understanding Moisture Control Over Engineering Plastics
Controlling moisture in engineering and high-performance plastics is critical, as even slight variations can affect the mechanical and physical properties of the component. Hygroscopic materials, such as nylon, ABS, acrylic, PET, PBT, and polyurethane, absorb moisture from their environment. Nylon can absorb as much as 9% of its weight in water. Non-hygroscopic plastics do not absorb moisture internally but may collect on the pellet’s surface.
Moisture within the raw plastic can create defects during processing, such as splay marks (visible streaks on the surface), bubbles, and reduced impact strength. In more extreme cases, moisture can degrade the polymer itself, causing molecular chain scission, which results in brittleness and poor mechanical performance. Additionally, residual moisture can contribute to dimensional instability, which is especially problematic for high-tolerance applications where precision is expected.
By understanding and controlling moisture levels, engineers can avoid these quality issues and ensure that the final product meets performance and aesthetic standards.
How Dryer Banks Work
Drying ovens are specifically designed to reduce the moisture content of the polymer feedstock before it is molded. These ovens use a combination of heat and airflow to drive off moisture absorbed by or on the surface of the resin. Typically, a drying oven is set to a temperature above the water’s boiling point and below the material’s thermal degradation point. Steady air flow promotes even moisture removal throughout the plastic pellets. Some drying ovens incorporate desiccants to lower the humidity in the chamber, which enhances drying efficiency and can reduce the time required to achieve a target moisture level.
The drying cycle’s duration depends on factors such as the plastic type, initial moisture content, and desired final moisture level. Once removed from the oven, it must be processed quickly as it will begin to absorb moisture again. However, integrated hopper dryers are sometimes used, allowing plastic pellets to be dried immediately before they are fed into an injection molding machine. Properly dried materials lead to more consistent and reliable manufacturing outcomes, reducing defects and ensuring specifications are met.
High-Temperature Oil Heaters Are Essential
How High-Temperature Oil Heaters Work
Temperature control is essential for producing plastic injection molded components. While it’s understood that plastic must be heated in the barrel to be moved into the mold, consistent temperature must also be maintained in the mold. High-temperature oil heaters are specialized heating systems used in injection molding to maintain the consistent, elevated mold temperatures required during the production process. High-temperature oil, with excellent thermal conductivity, is heated and pumped through a closed-loop system. The heated oil is circulated through channels within the mold, ensuring that each area of the mold reaches and maintains the precise temperature needed for optimal plastic flow, part quality, and cycle times. After transferring the heat, the oil is returned to the heater, where the cycle continues. Unlike water-based systems, oil heaters can reach much higher temperatures, often up to 300°C (572°F), making them suitable for engineering plastics that require elevated mold temperatures to prevent premature cooling and improve material properties. High-temperature oil heaters have high thermal efficiency and offer precise temperature control and uniform heating.
Why Controlling Temperature Matters with High-Performance Plastic Molding
Controlling mold temperature is crucial in injection molding because it directly impacts the flow and cooling behavior of the plastic material. When mold temperatures are too low, the plastic may cool too quickly, causing issues like incomplete filling of the mold cavities, poor surface finish, or internal stresses. Internal stress within the component can result in warping and distortion, cracking and crazing, sink marks and voids, and dimensional instability. Conversely, temperatures kept too high can lead to excessive cycle times and potential material degradation. High-temperature oil heaters allow manufacturers to achieve high-quality parts with minimal defects by maintaining precise control over mold temperature. This control also enables better dimensional stability, surface finish, and mechanical properties, particularly in high-performance or engineering plastics applications. Additionally, consistent mold temperatures help to stabilize the production process, reducing scrap rates and optimizing production efficiency.
Ovens for Annealing and Post-Curing
How They Work
In injection molding, annealing and post-curing ovens play crucial roles in enhancing the performance and stability of plastic parts. Annealing is a controlled heat treatment used primarily for thermoplastics. Annealing ovens operate by gradually heating the plastic part to a temperature below its glass transition temperature (Tg), holding it there to allow internal stresses to dissipate, and then cooling it in a controlled manner to prevent the reintroduction of stress. The oven must maintain precise temperatures throughout the process to ensure optimal results.
For thermoset plastics, post-curing, rather than annealing, involves a chemical crosslinking process at elevated temperatures to solidify the material and stabilize its properties. Molded items are placed in a forced-air oven and subjected to a sequence of elevated temperatures held for specified durations. This sequence of times and temperatures is known as the cure schedule or cure cycle. During post-curing, water vapor or alcohol may be released as a byproduct and must be managed as voids, bubbles, or weak points within the material can occur if trapped inside. Post-curing ovens for thermosets may be designed to ensure good ventilation or include desiccants or inert atmospheres to manage and remove any byproducts, thus ensuring a solid, defect-free final structure.
Benefits in Injection Molding
The benefits of annealing and post-curing are significant, particularly for applications that require high dimensional stability, chemical resistance, or mechanical strength. Annealing is particularly beneficial for crystalline and semi-crystalline thermoplastics, as the heat allows polymer chains to realign, enhancing crystallinity and thus improving mechanical properties like tensile and flexural strength. The process reduces brittleness and can even improve optical clarity for amorphous plastics, such as polycarbonate. This process helps relieve internal stresses and minimizes the risk of part warping, cracking, or dimensional changes, making it essential for high-precision components. In addition to stress relief, annealing can enhance impact and wear resistance, extending the part’s operational life.
For thermosets, post-curing is required to expedite the crosslinking of the polymer’s chains, which means the molecular chains interconnect through strong covalent bonds to create a rigid three-dimensional network. The thermoset part will continue to cure at room temperature for the next 1-2 weeks. Post-curing achieves a final, rigid structure, giving thermosets their characteristic durability and heat resistance. Post-curing is extremely important for parts that will have secondary machining. These processes create high-quality, reliable plastic components suitable for demanding applications in aerospace, automotive, and industrial equipment.
Discover Ensinger’s Solutions for High-Performance Plastic Components
At Ensinger, we are committed to providing quality solutions for all your plastic injection molding needs. We employ dryer banks, high-temperature oil heaters, and specialized ovens for annealing and post-curing as needed to ensure the highest quality and performance of your plastic components.
Don’t compromise on quality. Contact us today to learn how we can work together to ensure your products meet specifications and continue to perform throughout their lifecycle.