When designing runner systems, use the same guidelines that apply to most engineering polymers. The runners should be designed for smooth, fully balanced flow. Generously radiused transitions reduce material hang-up and shearing. Cold-slug wells are useful in trapping slugs of frozen material at the flow front. Vent the runners generously.
Eastman polymers can be molded using conventional gate design, including:
- Sprue gating (directly into part)
- Fan gates
- Tunnel or submarine gates
- Flash gates
- Edge gates (tab or fan style)
- Hot runner systems
The size and appearance of the finished part must be considered in selecting the type and location of gates.
Hot runner systems
Hot runner systems are common in applications using polyester materials. When properly designed, these systems can eliminate sprue and runner regrind, mold with lower pressures, reduce cycle times, and improve processing windows. The selection of a suitable hot runner system can vary greatly depending on the size of the part, polyester formulation, and part design. Therefore, it is critically important that runner design and selection be discussed jointly by the molder/end user, tool builder, hot runner supplier, and Eastman to arrive at the appropriate runner-system design to be used.
Uniform heating and good heat control
Excellent thermal control and good cooling at the gate is critical for molding polyester materials. The mold should be designed so that heat is quickly removed from the gate. This is best accomplished when the gate orifice is an integral part of the cavity steel rather than the hot runner system being an insert projecting through the cavity into the part. When the gate is in the cavity, cooling channels (drilled water lines or annular-shaped passages) can be incorporated to provide the cooling needed for the cavity in the gate area. Some hot runner suppliers offer gate-cooling inserts. Drooling, sticking, and stringing may occur if the gate does not cool properly. Steel that is directly heated as part of the hot drop should not contact the part directly; it should be insulated from the cooled portion of the mold.
We suggest separate cooling loops with individual flow and temperature control for cooling a hot drop gate. The additional control is very useful in debugging and optimizing gate appearance and performance.
Eliminating holdup spots
The flow channel for the plastic should be streamlined and uninterrupted. Any crevice or pocket where material can collect and degrade will probably cause defective parts.
Minimizing shear heating
The diameter of the flow path needs to be large enough to minimize the shear heating that can be caused by sharp corners or edges in the flow path at the gate or elsewhere. Mold-filling analyses can show shear heating and indicate potential problems during the design stage.
If possible, a valve system should be used when processing Eastman polymers. This has several advantages when compared with other hot-melt delivery systems. With valve gates, the melt channel is externally heated and the mechanical shutoff feature allows better control of gate vestige. The gate size is generally larger when compared with other available systems. The valve pin is retracted during the filling process, resulting in a less obstructed flow. The end result is less shear heating and pressure drop.
Venting and ejection
Venting allows gas replaced by the melt front to escape from the mold. Short shots, burning, and material degradation can occur if parts are not adequately vented. To prevent this:
Typical venting in molds designed for Eastman polymers
- Provide adequate venting in the proper location.
- Check and clean vents regularly.
- Use ejector pins as vents where possible.
- Avoid vents that require mold disassembly for maintenance access.
A good starting vent depth for molds designed to run Eastman polymers is 0.0005–0.001 in. (0.012–0.025 mm) for small parts or vents close to the gates and 0.001–0.0015 in. (0.025–0.038 mm) for larger parts. A typical land is 0.125–0.250 in. (3–6 mm) long and opens up into a larger channel that allows gas to vent from the mold.
Alloys for mold construction
There are several factors to consider when selecting steel for the mold:
- Wear resistance
- Dimensional stability
Family molds contain two or more cavities that mold different parts. Eastman polymers are being used successfully in family molds. Like any other polymer, the flow into the individual parts must be balanced. All parts should fill evenly and equally; otherwise, uneven packing will occur. In such cases, some parts will be overpacked and highly stressed, leading to warpage, and other parts will be underpacked or not completely filled.
Mold polishing and texturing
Eastman polymers provide excellent gloss and pick up mold finish very well. Keep in mind that surfaces polished smoother than required for ejection only add to mold cost. In most cases, highly polished surfaces can hinder ejection if there is a vacuum drawn in low- or no-draft areas. Where no vacuum is drawn, polished surfaces generally eject better.
Texturing mold surfaces
Texturing is useful in hiding weld lines, flow marks, gate blush, sink marks, and scuffing. There are hundreds of standard patterns available. Basically, anything that can be drawn in black and white can be used as the basis for a texture pattern.
Draft angle guidelines
In most cases, a draft angle of 1 degree per side is suggested to aid ejection. However, ½ degree per side can be used to obtain reasonable dimensions in ribs, bosses, and other design features. Attention to the thickness at the top of ribs or bosses is needed to ensure structural strength.
Mold surface treatment to aid ejection
In some cases, a low draft angle may be required on a part, but the dimensions of the mold may not be suitable for proper cooling. Surface coatings or treatments that can aid in the ejection of parts are available. Eastman has completed an extensive study to determine which coatings and treatments are better release agents for our polyesters. The top three that we suggest follow.