Heating temperatures for plastic can 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 9is molded determines the property reactions. That is why the melt temperature setting of the molding machine is so important.
Table 1 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.
Table 1 Recommended Melt Temperatures.
|Nylon (6)||500||Polystyrene (HI)||390|
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 Figure 6 A and B.
Figure A Amorphous Material Figure B Crystalline Material
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.
These are the temperature values that the material supplier tests for when developing a material, and these are the values that get published in their data sheets.
INFLUENCE OF RESIDENCE TIME
Residence time can be defined as the total amount of time a material resides in the heating cylinder of the molding machine (with heat on) before it is injected into a mold. The material temperature settings on the barrel must be made determined by the residence time. The normal settings would be made based on 50% of the heating cylinder (barrel) being emptied every cycle. Based on this concept, if we have a total shot size of three ounces (including the runner system) we should run the mold in a machine with a six-ounce barrel. This is an ideal situation because it means we are injecting one batch of material for a cycle while preparing the next batch for the next cycle. By doing this, the residence time is kept at an ideal value.
However, due to the variation in size of all the different parts we will mold, the amount of material being injected each cycle may not equal 50% of the barrel size for every production run. We use a rule-of-thumb that states we can use from 20% to 80% of the barrel capacity depending on the heat sensitivity of the plastic being molded. A material that is very heat sensitive (such as PVC) should be run in a machine using up to 80% of the barrel capacity every cycle or the plastic will degrade in the barrel due to too much residence time. But, a material that is thermally forgiving (such as polypropylene) can be run in a machine using only 20% of the barrel capacity every cycle because the material will not degrade easily due to excessive residence time.
Therefore, 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. The temperature that you set for the mold will depend on manufacturer's specifications and the properties that you want for the final product.
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