Beat the Heat When Molding High-Temperature Thermoplastics
Troubleshooting: High-Temperature Molding
Conventional molding techniques are not effective with high-temperature materials. Molders need to be aware of certain conditions and parameters to handle problems sometimes posed by high-heat injection molding.
Increasing market demands for energy efficiency and fuel economy in the automotive industry are generating significant interest among design engineers to replace metals with high-performance thermoplastics. These materials also offer light weight, corrosion resistance, high dimensional stability, and design flexibility for a wide range of applications in the electrical/electronics, medical, aerospace, and telecommunications industries.
The high-performance plastics shown in the table are high-heat materials, meaning they require much higher mold and melt temperatures than typical plastics like polypropylene, nylon, and polycarbonate. Reinforcements such as glass fiber, glass spheres, and other additives are being used in conjunction with the high-performance plastics to achieve strength and stiffness comparable to metal.
As a result, conventional molding techniques—for instance, using hot water for mold cooling and soft steel for prototype tooling—are not effective with these materials. Molders need to be aware of certain conditions and parameters to handle problems sometimes posed by high-heat injection molding.
DEALING WITH MOISTURE ABSORPTION
Most of today’s high-heat plastics are hygroscopic, so they will absorb moisture when exposed to humid air. While the effects of moisture absorption on the performance and dimensional stability of molded parts are minimal, the effects on the molding process can be significant. The severity of these effects will depend on the moisture level, the resin grade, and the particular tool and part.
Higher moisture content generates additional volatiles, thereby increasing the load on the vents. In extreme cases, the molded parts will exhibit burn marks and have poor mechanical properties.
Even relatively small variations in moisture content can affect material viscosity enough to cause dimensional and aesthetic problems. If moisture content is not maintained under rigid control, it will be difficult to establish a stable molding process. If the moisture content varies and parts are molded using a constant hold pressure and hold time, the part density will vary.
The change in part density will be manifested as changes in part dimensions and part weight. In extreme cases, either flash or short shots may occur. Some common problems associated with high-moisture content are brittleness, burn marks, nozzle drools, tool plate-out, poor surface finish, and poor weld-line strength. It is highly recommended to follow the drying guidelines published by the resin supplier.
Typically, the resin is dried in hopper dryers mounted over the molding machine. The dryer system should use a desiccant and be able to maintain a dewpoint of -32 C (-25 F).
Single-bed desiccant systems are usually adequate as long as the desiccant is replaced as needed. Dual-bed systems are generally more reliable because they allow one bed to be regenerated while the other is drying. Even better are rotating-bed systems, because they continuously supply a source of fresh desiccant. Tray drying in air-circulated ovens is practical only for short runs. Plastic resin should be dried within recommended moisture levels prescribed by the resin supplier.
SELECTING PROPER PRESS SIZE
Machine selection is a critical variable when molding high-heat materials. Since the processing temperatures for these resins are relatively high, holding the melt at temperatures above a certain range may result in polymer degradation and unacceptable molded part properties (see Fig.1).
It is generally recommended that the estimated residence time should be no more than 6 min for materials such as polyphthalamide (PPA). Excessive residence time can result in resin degradation, leading to drooling, flash, plate-out, burn marks, and poor mechanical properties of the molded part. If the cycle is to be interrupted for longer than 10 min, it is advisable to remove the resin from the molding machine barrel.
Typical concerns during prototyping are longer residence time due to an oversized machine barrel and longer cycle time due to manual insert loading. It is necessary to purge the resin after a couple of shots to avoid any degradation.
Here is an important formula to keep in mind:
Residence = 1.4 x Max. Shot Size x Specific Gravity x Cycle Time
Time Weight of Shot
Due to the rheology and viscosity of some high-heat plastics, high injection speed and high pressure are required. It becomes necessary to fill the entire mold with primary injection boost pressure, then drop off to a holding pressure. In order to achieve high injection speed, it is common to use injection pressures up to 200 MPa (30,000 psi) for materials such as Solvay’s Torlon polyamide-imide (PAI).
SELECTING THE PROPER TOOL STEEL
The number and quality of parts that a tool is expected to produce will dictate tool-steel selection. For high-volume production, the initial expense of high-quality tooling will be a sound investment to avoid excessive tool wear. Generally, common tool steels such as H-13, S-7, and P20, are acceptable for constructing injection molds for high-performance resins. Molds should be built of P20 or stronger steel to accommodate necessary thermal loads. When molding glass- or mineral-reinforced compounds, abrasion resistance is required and H-13 performs best. Tool steel should be hardened prior to production; however, it is wise to sample the mold before hardening so that final dimensional cuts can be made easily. Aluminum is not recommended for production tooling, but for cost reasons it is used for prototyping.
While an excellent surface appearance may not be required, it is necessary to remove all machining marks from the mold to ensure proper part ejection. All surfaces should be polished in the direction of ejection. Textured surfaces are allowable for cosmetic parts, but undercuts are not permitted.
MOLD COOLING IS CRITICAL
It is very important that mold temperature be controlled properly. Typically, circulating oil is required for high-heat plastics because the recommended tool temperature is higher than 125 C (257 F), which is well above the boiling point of water. However, pressurized water can be heated above the 100 C (212 F) boiling temperaure. There are commercially available systems designed specifically for temperature control of injection molds with pressurized water at up to 204 C (400 F).
Thermally conductive copper-beryllium pins may be inserted into critical areas to facilitate heat transfer. Heat-transfer channels in the mold should be located equidistant from each cavity, and the flow should be designed so that each cavity is exposed to the same amount and temperature of fluid. The flow pattern past the cavities should be designed to be concurrent (parallel) rather than sequential.
Electric cartridge heaters have been used for decades in heating injection molds, but they are not recommended. They can heat the mold, but they cannot remove heat from the mold. Since the melt being injected into the mold is considerably hotter than the cavity, the excess heat must be removed somehow. This is particularly true in thermally isolated areas such as core pins, where heat may build up and cause ejection problems. Also, cartridge heaters tend to have hot and cold spots along their length that can cause inconsistent mold surface temperature. That, in turn, can lead to inconsistent shrinkage, especially with semi-crystalline plastics.
MANAGING HEAT IN HOT RUNNERS
Thermal management of hot-runner systems for crystalline materials is critical. Each drop in the mold should have its own thermocouple and heat source. Placement of the thermocouple should be between the heat source and the melt channel, allowing accurate thermal management without degradation of the resin.
Manifold channels should be free-flowing with no sharp corners or dead spots. Molten resin tends to stagnate in such areas, resulting in degradation, which eventually contaminates the parts. Excessive residence time in the manifold should be avoided because it can result in material degradation. In general, mold design should be as simple as possible.
TIPS FOR FUNNEL OR SUBMARINE GATING
Tunnel or submarine gating is another popular method because it is self-degating. A potential disadvantage is the possibility of an irregular gate vestige. Gate inserts are strongly recommended for use with tunnel gates.
Tunnel gates employ a conventional parting-line runner system similar to a standard edge gate. In close proximity to the mold cavity, however, the runner tunnels beneath the parting line and gates into the part below the mold parting surfaces, as illustrated in Fig. 2.
Upon ejection, the molded part and runner/gate are separated by the tool steel itself. The angle of the drop is critical in ensuring that the runner will eject properly and not become stuck in the mold. Due to the high modulus of reinforced grades of high-heat plastics, a maximum angle of 30° perpendicular to the parting line of the mold is recommended for those grades. Unfilled grades with a lower modulus may use less severe drop angles.
GETTING A HANDLE ON CRYSTALLINITY
In the case of semi-crystalline polymers, the modulus gradually decreases with increasing temperature. At or near the glass-transition temperature (Tg), the modulus decreases rapidly to a lower but still useful level. Continuing to increase the temperature causes the modulus to remain at or near this new level (the crystalline plateau) until the melting-point temperature (Tm) is reached. At Tm, the modulus decreases rapidly again. Semi-crystalline polymers are often used at temperatures above their Tg but below their Tm, particularly when they are modified with glass fiber and/or mineral fillers.
The properties of the molded material depend upon the percentage of crystallinity, which in turn depends upon the mold temperature. Figure 3 shows the relationship between tensile strength and percent crystallinity. If the tool temperature is below the recommended level, the part will not achieve full crystallinity and could potentially fail in end use. The level of crystallinity of the material also affects the dimensional stability at higher temperatures. The dimensions of a part not fully crystallized will change at service temperatures higher than the Tg. This could present a big problem for a part used in a slip-fit or interference-fit design.
Producing plastics parts with undercuts presents distinct challenges for molders.
It may seem like a dull topic, but it will overcome the emotional experience that follows when you put a new mold into a machine and you find out there is not enough barrel capacity to make a full shot.
Modifications to the common core pin can be a simple solution, but don’t expect all resins to behave the same. Gas assist is also worth a try.