Heating temperatures for plastic should be based on manufacturers' specifications or by testing.
The manufacturer of any specific plastic molding compound supplies data with each shipment of that material and provides processing recommendations designed to allow the molder to produce high quality molded parts in the fastest possible cycle time.
One of the pieces of information available is the recommended melt temperature. This is the temperature the material should be as it leaves the molding machine (through the nozzle) and enters the mold (through the sprue bushing). This temperature is different for every material made, and should be held as close as possible by the molder to attain parts with expected physical, mechanical, thermal, and electrical properties. Materials are chosen for how their combination of properties reacts to a specific product design requirement. The temperature at which the material is molded determines the property reactions. That is why the melt temperature setting of the molding machine is so important.
The Table below shows the recommended melt temperatures (as determined by the suppliers) for some common materials. Please remember to check the melt temperature as it leaves the nozzle. This is accomplished by pulling the sled back and purging to the air. Then, immediately plunge a pointed probe from a fast-acting pyrometer into the purged material and record the temperature. An average of three readings is recommended.
Recommended Melt Temperatures.
The method used to determine the proper molding temperature of any plastic is based on the use of test equipment called the differential scanning calorimeter (DSC). In a simplified explanation of the DSC process, a small amount (20 milligrams) of plastic material is placed in a chamber of the test unit and the DSC begins applying heat to that sample of plastic. The DSC must determine how much heat is required to get all of the molecules of the sample moving. It displays a graph showing this movement. Typical graphs for both amorphous and crystalline materials are shown in the table above
The amorphous graph (on the left) shows that as soon as heat is applied the plastic molecules begin moving. The amorphous material goes through a series of phases during which it changes from a hard substance to a softer substance, finally to a liquid, and eventually degrades. At the point at which it became liquid is called a glass transition phase. That is the temperature at which we should mold the material to obtain the highest quality level of property values.
The crystalline graph shows that the molecules do not begin to move until the temperature is close to what is called the melting point of the material. This is due to the molecular structure order of crystalline materials. When the maximum molecular movement is attained the material is at its melt point. That is the temperature at which we should mold the crystalline material.
The material suppliers use these temperature test values when testing their production runs, and these are the values that get published in their data sheets.
The melt temperature stated by the material supplier for a specific material is based on averages and 50% barrel capacity usage. It should be used as a guideline but you must be flexible in establishing the final melt temperature based on the conditions under which you are molding.