Temperature-related problems can occur when molding semi-crystalline engineering polymers such as POM (acetal), PA (nylon), PBT and PET (polyesters). However, simple solutions exist that will allow you to mold final parts with the proper characteristics. This article will look at how the wrong melt temperature and the wrong tool temperature can affect your process, and offer ways to deal with both.
Wrong Melt TemperatureChoosing the right melt temperature is vital for part quality when molding semi-crystalline engineering polymers such as polyoxyphenylene (POM), polyamide (PA), polybutylene terephthalate (PBT) and polyethylene terephthalate (PET). As a rule, the margin of tolerance is less than when processing amorphous resins. The molder at his machine directly influences the properties of the end-product.
Possible Negative Consequences. Melt temperature (table 1) can be too high or too low: both are wrong. Even temperature distribution in the melt also is an important factor.
Temperatures that are too high degrade the polymer by destroying the molecular chains (figure 1). Another consequence may be that additives in the melt such as pigments and impact modifiers also decompose. The results are poorer mechanical properties (as a result of the shorter molecular chains), surface defects (caused by decomposition products) and unpleasant odors.
When the temperature is too low, the structure fails to achieve the required homogeneity. This drastically reduces impact resistance and leads, in most cases, to considerable variations in physical properties.
Apart from the melt temperature, the polymer's dwell time in the injection unit also plays an important role. Experience has shown that dwell times of between two and nine minutes are normal. If the dwell time is longer, thermal decomposition may take place in certain circumstances, even if the melt temperature is correct. If the dwell time is very short, the melt usually does not have time to become fully homogeneous.
Signs of the Wrong Melt Temperature. In the case of POM, excessive thermal stress generates decomposition products, causing bubbles to form in the melt. This can be observed clearly in the melt when it is purged. Other symptoms are increased mold deposits and an unpleasant odor. The physical properties of POM homopolymer are, however, hardly affected by too high melt temperatures.
PA discolors under extreme conditions, including if overheating occurs as a result of injection nozzles that are too hot (figure 2). Thermal decomposition can be recognized in all PA types through reduced mechanical properties. In the laboratory, thermal decomposition can be established by measuring solution viscosity, but as a rule, molders are not in a position to apply this method.
PBT and PET react even more strongly to overheating, leading to reduced toughness. Faults are scarcely discernible during processing. If no suitable quality-control measures are carried out, the damage usually becomes apparent only at the assembly stage or when the part is in use. Discoloration indicates an unusually high degree of damage. In practice, there are tests on random samples with which certain toughness-related properties can be measured. Tests on the viscosity of molded parts are time-consuming and expensive to carry out.
In the case of unreinforced PA or PBT, if unmelted particles are observed in the purge, it is a sign of too low melt temperature or, in extreme cases, excessive shot size.
The Right Melt Temperature. The data sheets for engineering polymers indicate the optimum melt temperature range for each. In general, the temperature setting of the barrel heating zones alone is not reliable because, apart from the temperature rise due to the heater bands, friction from the screw rotation also generates heat. How much heat is generated this way depends on screw geometry and rate of rotation (rpm) as well as on back pressure.
The following recommendations can help to achieve accurate temperature measurements:
- Keep the diameter of the melt temperature probe less than ~0.05" (1.5 mm).
- Preheat the probe.
- Collect the melt in a thermally insulated container.
- Stir while taking measurements.
When taking initial temperature measurements or when there are no known values to rely on, a temperature profile should be selected that is 10 to 15 K above the melting point in the feed section and about 5 to 10 K under the required melt temperature in the metering zone. The temperature can be fine-tuned according to the measured melt temperature. In the case of long dwell times and short metering strokes, rising profiles usually are recommended. For short dwell times and long metering strokes, flat profiles generally give the best results. A temperature zone should never be set at less than the melting point of the polymer.
Wrong Tool TemperatureWhen molding semi-crystalline engineering plastics, it is important to make sure that the surface temperature of the tool is correct. The basic requirements for optimum processing are in the design of the tool. Only if the tool design is right can the molder produce good quality parts with the help of temperature control equipment. This calls for close cooperation in the tool design and planning phase, in order to avoid production problems at a later stage.
Possible Negative Consequences. The symptom that is easiest to recognize is poor surface finish of molded parts. The cause is often too low surface temperature in the tool (figure 3).
Mold shrinkage and post-molding shrinkage of semi-crystalline polymers are strongly dependent on tool temperature and the wall thickness of the part (figure 4). Uneven heat dissipation in the tool can thus lead to differential shrinkage. This in turn can lead to the inability to maintain part tolerances. In the most unfavorable circumstances, shrinkage can be beyond correction, whether working with unreinforced or reinforced resins.
When the dimensions of parts in high-temperature applications become smaller with use, this is generally due to mold surface temperatures that are too low. This is because with too low mold surface temperatures, mold shrinkage may be lower, but post-molding shrinkage is substantially higher.
If a long startup phase is needed before the dimensions settle down, it is a sign of poor temperature control in the tool: the tool temperature is probably rising for a long time until equilibrium is reached. Poor heat dissipation in some regions of the tool also can cause substantial lengthening of the cycle time, leading to increased cost of the molding.
Incorrect tool temperatures also can sometimes be established from the molded parts by means of analytical methods such as structural analysis (for example, in the case of POM) and differential scanning calorimetry (DSC) examination (for example, with PET).
Recommendations. Tools are becoming more and more complex, and as a result, it is getting ever more difficult to create the proper conditions for effective mold temperature control (table 2). Except in the case of simple parts, mold temperature control systems are always a matter for compromise. For this reason, the following list of recommendations should be seen as rough guidelines only.
- Temperature control of the shape to be molded must be taken into consideration at the tool design stage.
- When designing molds that have a low shot weight and large mold dimensions, it is important to allow for good thermal transfer in the construction.
- Be generous when dimensioning flow cross-section in the tool and in the feed pipes. Do not use fittings that cause a major restriction to the flow of the mold temperature control fluid.
- Use pressurized water as the temperature control medium, if possible. Provide flexible pipes and manifolds that are capable of withstanding high pressures and temperatures (up to 8 bar and 266°F [130°C]).
- Specify the performance of the temperature control equipment to match the tool. The tool-maker's data sheets should supply the necessary figures for flow rates.
- Use thermal insulation plates between both halves of the tool and the machine platens.
- Use separate temperature control systems for the moving half and the fixed half of the mold.
- Use separate temperature control systems for any side actions and the core, so you can work with different startup temperatures to get the mold running.
- Always connect different temperature control circuits in series, never in parallel. If circuits are in parallel, small differences in the flow resistance cause different volumetric flow rates of the temperature control medium, so bigger temperature variations can occur than with circuits connected in series. This series connection will work properly only if there is less than 9°F (5°C) difference between mold inlet and mold outlet temperatures.
- It is an advantage to have a display showing the supply temperature and return temperature on the mold temperature control equipment.
- For process control, it is recommended to have a temperature sensor built into the tool, so you can check its temperature during actual production.
Thermal equilibrium is established in the tool after a number of shots on cycle. Normally, the minimum is 10 shots. The actual temperature at equilibrium will depend on many factors. This actual temperature of the tool surfaces in contact with the plastic can be measured either by thermocouples within the tool (reading ~0.75" [2 mm] from the surface) or more commonly by a hand-held pyrometer. The surface probe of the pyrometer needs to be fast-acting, and the tool temperature needs to be measured in a number of places, not just once on each side. Corrections may then be made to the set temperatures of the control units to adjust the mold temperature to what it should be.
The data sheets for the various raw materials always give the recommended tool temperature. These recommendations represent the best possible compromise between a good surface finish, mechanical properties, shrinkage behavior and cycle times.
Molders of precision parts and of parts that have to meet exacting optical or safety-oriented specifications generally tend to use higher tool temperatures (giving lower post-molding shrinkage, shinier surface and more uniform properties). Technically, less critical parts that have to be produced at the lowest possible cost can probably be molded at somewhat lower tool temperatures. However, molders should be aware of the drawbacks of this option and they should test the parts thoroughly to be sure they still meet the customer's specifications.