Plastic temperature control in injection molding usually uses rotation of screw and heating plate on barrel to heat low-temperature solid plastic particles into high-temperature liquid melt. Melt temperature will affect quality of product. If it is too high, there will be problems such as material cracking and yellowing; if it is too low, it will reduce fluidity of material and increase flow resistance. Temperature of melt cannot be directly controlled by injection machine, but is indirectly affected by plasticizing process. There are three main conditions in plasticizing process that will affect melt temperature, including heating plate temperature, screw speed and back pressure. How plasticizing process affects material temperature will be explained below.

Rotation of screw will drive plastic to flow, and shear heat generated during process will increase temperature of melt. Too fast screw speed will cause material temperature to deviate too much from heating plate temperature; too slow speed will increase metering time and reduce production efficiency.

 

(Figure 1: Relationship between screw speed and material temperature)
It can be seen from Figure 1 that when screw speed is faster, temperature at the center is higher; while the two sides are maintained at temperature of heating plate, resulting in uneven melt temperature. When increasing speed to reduce metering time, it is necessary to pay attention to effect of ensuing temperature rise on plastic.

Generally, there are several different settings for heating plate, which can be set according to sensitivity of material to temperature. Before plastic has melted, heating plate provides main heat. At this time, temperature of plastic will be lower than that of heating plate. As plastic melts and enters rear section of screw, shear heat gradually increases, and temperature of melt may be higher than temperature of heating plate at this time.

 

(Picture 2: Relationship between heating plate and material temperature)

During plasticizing process, phase change of plastic needs to be considered physically; spatial change of screw rotation is geometrically required. These two factors make simulation quite complicated, and generally model needs to be simplified. Melting process of plastic from solid particles can be divided into three parts: solid bed, melt pool, melt film. Simulation of phase change is based on these three areas. For complex geometry of screw, de-winding method can be used to straighten spiral channel between screw and barrel into a two-dimensional or three-dimensional channel, then assume that screw is stationary and barrel rotates in opposite direction. Dynamic calculation domain can be transformed into a fixed space, which greatly reduces complexity of calculation. Taking Moldex3D as an example, you only need to fill in dimensions of screw in window, and software can automatically complete model required for simulation. Simplified model takes about three minutes to analyze each time, which can quickly provide data as a modeling reference.

 

(Figure 3 screw parameter setting)

Moldex3D will display results for each position of screw. Taking Figure 4 as an example, it presents temperature distribution from surface of screw to surface of barrel under various cross-sections. What we are most concerned about is usually final material temperature. From figure, we can see that the highest temperature here is about 2℃ higher than that of heating plate. Users can use this to confirm whether temperature of material is controlled within appropriate range.

 

(Figure 4: Plastic temperature distribution map)
Figure 5 and Figure 6 respectively show average temperature and average pressure of plastic at each position. Average temperature can be used as basis for adjusting temperature of each heating plate; average pressure can be used as a reference for screw speed and back pressure setting.

 

(Figure 5 Plastic average temperature distribution map)

 

(Figure 6: Plastic average pressure distribution map)
Figure 7 shows percentage of solid plastic at each location. When value drops to zero, it means that plastic is completely melted; picture can be used to check whether plastic can be completely melted under current conditions.

 

(Figure 7: Distribution map of plastic melting ratio)
Table below shows comparison between simulation and experiment. Molding conditions are screw speed 150RPM and back pressure 4.5MPa. Maximum temperature rise represents difference between the highest temperature of melt and heating plate. In general, the smaller temperature change, the better, which means that temperature of plastic is uniform and almost equal to temperature of heating plate. Plasticizing time is time required for screw to reach designated metering position. This period of time is preferably close to and must be shorter than cooling time, because previous shot is also cooling in mold while plasticizing process is in progress. If plasticizing time is too much shorter than cooling time, it means that plastic stagnates in material tube for a long time, and plastic may deteriorate; if plasticizing time is longer than cooling time, it means that measurement cannot be completed before mold is opened, which will affect production of next mold.

 

(Table 1 HIPS60 simulation and experiment comparison table)

Plasticizing analysis function of Moldex3D can be used to evaluate action behavior of screw, as well as changes of pressure and temperature in various parts inside barrel. At the same time, we can observe temperature rise and pressure drop behavior changes caused by these settings when different materials are plasticized through difference settings of parameter settings and geometric structures, so as to quickly grasp molding characteristics of plastic injection.