Here are five considerations that a hot runner supplier and a moldmaker/molder should discuss when using fiber-reinforced materials, such as those with glass filler, wood fibers (lignite in biomaterials) and wool fibers:
1) Tip selection: The key with tip selection is maintaining appropriate heat as close to the gate as possible, while maintaining a reasonable pressure drop through the tip and balancing the gate vestige requirements. For example, some carbide tips are warrantied for five years (regardless of shot count) against normal wear caused by contact with abrasive and corrosive polymers.
2) Nut selection: The nut detail is how the hot runner interfaces with the tool design. Does the gate detail have to be in the tool steel or can it be in the nut of your hot runner system? If you can live with the witness line on the part (from the temperature variation on the face of the nut relative to the surrounding tool steel), keeping the gate detail in the nut will allow much easier gate size adjustments or future service. If a molded sprue is allowed, consider tipless nuts.
3) Valve gates: Valve gates are frequently used, so it is important to consider service-friendly designs, pin hardness, parallel/cylindrical gate detail and premium guided carbide tips for durable systems that guide the valve pin before it enters the cavity, ensuring long gate life and clean vestige.
4) Gate size and gate land: An important relationship exists between the tip and gate detail, along with the cooling in the mold design. A long gate land will lead to high pressure and gate processing issues. A short gate land can leave a thin steel condition and fast gate wear. Discuss gate life, cosmetic requirements and resin concerns.
Manifold design and construction: The flow channels distribute the molten plastic to the individual nozzles, so carefully consider precise heater control and balanced flow, especially with an engineered-grade application. . If long glass fibers are involved, keep the fiber orientations in the same direction to allow a larger, smoother radius from the flow channel into the nozzle body.
Nozzle Selection and Specification
Get your hot runner supplier involved early in the discussion and know the answers to these questions:
- How are you planning to gate—directly to the part or with a short cold runner?
- What is the exact material grade, total shot weight and material flow length?
- What is the recommended processing temperature and is the material residence time-sensitive?
- Does the molder/moldmaker understand gate size, expected fill time and allowable gate vestige?
For high-temperature applications, use a larger nozzle body to get more steel mass in the nozzle body and a higher watt density heater to help maintain the higher temperature if you’re running PEEK at >700ºF, for example.
When starting up a hot runner mold with a heat sensitive resin, remember to minimize the time the resin is exposed to processing temperatures. With the screw and barrel at operating temperature, start heating up the hot runner system. Make sure that all mold cooling water is turned on and flowing.
First turn on the sprue bushing and manifold heaters only. The sprue bushing should come up to setpoint temperature quickly with the manifold heaters lagging considerably. This is normal. You may see material drooling out of the sprue bushing, this is also normal.
When the manifold temps are at approximately 75% of set point, turn on the drops. The goal is to have the manifold and drops reach set point at the same time.
When the manifold and drops are up to temp, turn on the gate tips. As the tips are coming up, purge and refill the barrel with fresh material (to minimize temperature exposure and residence time).
Move to injection unit forward against the mold after insuring the sprue bushing is free of drool.The entire hot runner should be up to temperature at this point.
In some hot-runner systems, it can be difficult to keep temperatures uniform in the nozzles and tips—areas where evenly distributed heat is needed to prevent flow-channel hot spots.
With many hot-runner systems, and with certain resins, you may need to lower the temperature of the material right at the gate to prevent drooling, stringing, or other part-quality problems. Sometimes, the cause is related to a less-than-optimal match between standard heaters and the nozzles and other components of the system. Special heaters—particularly flexible heaters—that are designed specifically for compatibility with a given system will achieve better distribution of heat. On the heater head, a more heat-retaining material, such as titanium, will also help prevent heat dissipation.
Titanium is beneficial for the same reason in the nozzles and tips. However, when using an abrasive material, you may need to use a more corrosion-resistant metal, such as H13 steel, which conducts more heat. This problem can be fixed by adding a titanium collar to prevent heat dissipation.
Since reinforcing fibers are abrasive, they will erode the surface of the mold over time. The ends of the short-fiber filaments, which are scattered throughout the pellet, act like needles that impact the steel at different angles. Contrary to popular belief, short fibers are harsher on tool steel than long fibers. That’s because there are fewer ends in long fibers with the same weight percentage as short fibers.
For this reason, when running short-fiber materials, it is recommended to use a harder mold steel, such as H-13, a chromium molybdenum hot-work steel; or P-20, a low-alloy steel. It is also advisable to add some type of hard plating to the cavity surface.
When it comes to tool design, radiused (arc-shaped) runners are advisable to avoid 90° angles that can break the fibers. Sharp, 90° turns without fillets or radii should also be avoided. Furthermore, using a full-round runner eliminates dead flow zones, or areas where a solidified layer of plastic does not permit the full volume of flow. All other runner shapes have these areas.
A polymer is generally manufactured by step-growth polymerization or addition polymerization. When combined with various agents to enhance or in any way alter the material properties of polymers, the result is referred to as a plastic. Composite plastics refers to those types of plastics that result from bonding two or more homogeneous materials with different material properties to derive a final product with certain desired material and mechanical properties. Fibre-reinforced plastics are a category of composite plastics that specifically use fibre materials to mechanically enhance the strength and elasticity of plastics.
The original plastic material without fibre reinforcement is known as the matrix or binding agent. The matrix is a tough but relatively weak plastic that is reinforced by stronger stiffer reinforcing filaments or fibres. The extent that strength and elasticity are enhanced in a fibre-reinforced plastic depends on the mechanical properties of both the fibre and matrix, their volume relative to one another, and the fibre length and orientation within the matrix. Reinforcement of the matrix occurs by definition when the FRP material exhibits increased strength or elasticity relative to the strength and elasticity of the matrix alone.
In order to improve the mechanical properties of rigidity, strength and also for surface hardness. A wide range of polymers are now produced in glass-fiber (varieties %), including PA, LCP, etc. On the other hand, it was found that increasing the glass-fiber also increase the complicate of surfaces wear, erosion, and gate frozen problem. It may damage the mold components and hot runner system. Typically Glass fiber grades are not recommended for hot runner applications. But hot-runner systems can provide an array of benefits, including reduced material use, faster cycles, and overall better part quality.