This article talks about the TPR plastic material and TPR plastic material injection molding process.
TPR plastic material
According to the characteristics of the material and the supply situation, the appearance and process performance of the material should be tested before injection molding.
The pellets supplied often contain varying degrees of moisture, solvents, and other volatile low-molecular-weight substances, especially the moisture content of TPR plastic material, which tends to absorb moisture and always exceeds the allowable limit for processing.
Therefore, it must be dried before processing and the water content must be measured. The dried material must be properly sealed and stored to prevent the material from absorbing moisture from the air and losing the drying effect.
TPR plastic material features
For this reason, the drying chamber hopper can be used to continuously provide dry hot materials for the injection molding machine, which is useful for simplifying operations, maintaining cleanliness, and improving quality. It is advantageous to increase the injection rate.
The filling amount of the drying hopper is generally 2.5 times the hourly consumption of the injection molding machine. TPE based on SBC is better than most other TPR plastic material in color.
Therefore, they only need a small amount of color masterbatch to achieve a certain color effect, and the color produced is purer than other TPR plastic material.
Generally speaking, the viscosity of the color masterbatch should be lower than that of TPR plastic material. This is because the melt index of TPR plastic material is higher than that of the color masterbatch, which will facilitate the dispersion process and make the color distribution more uniform.
For TPE plastic material based on SBS, polystyrene vehicles are recommended.
For TPR plastic material based on harder SEBS, a polypropylene (PP) vehicle is recommended.
For TPR plastic material based on softer SEBS, low-density polyethylene or ethylene-vinyl acetate copolymer can be used. For softer varieties, PP vehicle is not recommended because the hardness of the composite material will be affected.
TPR plastic material cleaning
For some overmolding applications, polyethylene (PE) vehicles may adversely affect the adhesion to the substrate.
Before the newly purchased injection molding machine is used for the first time, or when the product needs to be changed, the raw material is changed, the color is changed or the plastic is found to be decomposed in the production, the injection molding machine barrel needs to be cleaned or disassembled.
The cleaning barrel generally adopts the heating barrel cleaning method. Cleaning materials generally use plastic raw materials (or plastic recycled materials). For TPR plastic material, the transition cleaning materials can be replaced with newly processed materials.
TPR plastic material product injection molding process
Injection molding temperature
In the process of injection molding, the accuracy of the injection molding temperature setting is the key to the appearance and performance of the product. The following are some suggestions for temperature settings during TPR plastic material processing injection.
The temperature in the feed area should be set to be quite low to avoid clogging the feed port and let entrained air escape.
To improve the mixing state when using color masterbatch, the temperature in the transition zone should be set above the melting point of the color masterbatch. The temperature in the area closest to the injection nozzle should be set close to the desired melt temperature.
Therefore, after testing, the general temperature setting range of TPR plastic material products in each area is: the barrel is 160 degrees Celsius to 210 degrees Celsius, and the nozzle is 180 degrees Celsius to 230 degrees Celsius.
The mold temperature should be set higher than the condensation temperature of the injection zone, which will avoid the contamination of the injection mold by moisture and the appearance of streaks on the surface of the product.
Higher mold temperature usually leads to a longer cycle time, but it can improve the appearance of welding lines and products. Therefore, the range of mold temperature should be designed to be between 30 and 40.
In the process of forming the product into the mold cavity, if the filling performance of the product is not good, the pressure will drop too much, the filling time is too long, the filling is not full, etc., so that the product has quality problems.
To improve the filling performance of the product during molding and improve the quality of the molded product, the following aspects can generally be considered:
1) Change to another series of products;
2) Change the gate position;
3) Change the injection pressure;
4) Change the geometry of the part.
Injection molding pressure
Generally, the control of injection molding pressure is divided into the control of primary injection molding pressure, secondary injection pressure (holding pressure), or more than three injection pressures.
Whether the timing of pressure switching is appropriate is very important to prevent excessive pressure in the mold, prevent flash or lack of material, etc. The specific volume of the molded product depends on the melt pressure and temperature when the gate is closed during the pressure holding stage.
If the pressure and temperature are the same whenever switching from the holding pressure to the product cooling stage, the specific volume of the product will not change.
At a constant molding temperature, the most important parameter that determines the product size is the holding pressure, and the most important variables that affect the product dimensional tolerance are the holding pressure and temperature.
For example, after filling the mold, the holding pressure will immediately decrease. When the surface layer has a certain thickness, the holding pressure will rise again. In this way, the low clamping force can be used to form large thick-walled products, eliminating craters and flash.
The holding pressure and speed are usually 50% to 65% of the highest pressure and speed when the plastic is filled in the mold cavity, that is, the holding pressure is about 0.6 to 0.8 MPa lower than the injection molding pressure.
Since the holding pressure is lower than the injection molding pressure, the load of the oil pump is low during a considerable holding time, the service life of the solid oil pump is prolonged, and the power consumption of the oil pump motor is also reduced.
Certain metering is adjusted in advance so that near the end of the injection stroke, there is still a small amount of melt (buffering amount) at the end of the screw, and the injection pressure (secondary or third injection pressure) is further applied according to the filling situation in the mold. Add a little melt. In this way, the product can be prevented from sinking or the shrinkage rate of the product can be adjusted.
Injection mold cooling
The cooling time mainly depends on the melt temperature, the product’s wall thickness, and cooling efficiency. In addition, the hardness of the material is also a factor. Compared with very soft varieties, harder varieties will solidify faster in the injection mold.
If cooling is performed from both sides, the cooling time required per 0.100′ wall thickness will usually be about 10 to 15 seconds.
Encapsulated products will require a longer cooling time because they can be effectively cooled through a smaller surface area.
The required cooling time per 0.100′ wall thickness will be approximately 15 to 25 seconds.
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