Precautions for TPE and TPR injection molding process

Need to clean the barrel before injection

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 material, the transition cleaning material can be replaced with the new material being processed.


TPR injection molding temperature

In the process of injection molding, the accuracy of the temperature setting is the key to the appearance and performance of the product. The following are some suggestions for temperature settings during TPR processing and injection molding.

The temperature in the feed area should be set to be quite low to avoid clogging the feed port and let entrained air escape. In order 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 usual temperature setting ranges of the products in each area are: 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 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.

Filling, holding pressure, and cooling of the mold

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. In order 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 the product;

2) Change the gate position;

3) Change the injection pressure;

4) Change the geometry of the part.

Generally, the control of injection pressure is divided into the control of primary injection 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 every time when 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 size of the product is the holding pressure, and the most important variables that affect the dimensional tolerance of the product 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 and eliminate craters and flashes.

The holding pressure and speed are usually 50%~65% of the highest pressure and speed when the plastic is filling the mold cavity, that is, the holding pressure is about 0.6~0.8mpa lower than the injection pressure. Since the holding pressure is lower than the injection 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 further injection pressure (secondary or third injection pressure) is 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.

The cooling time mainly depends on the melt temperature, the wall thickness of the product and the 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 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.

Influence of injection molding process conditions

1. Incomplete plastic molding

(1) Improper feed adjustment, lack of material or excessive material.

(2) The injection pressure is too low, the injection time is short, and the plunger or screw returns too early.

(3) The injection speed is slow.

(4) The material temperature is too low.

2. Overflow (flash)

(1) The injection pressure is too high or the injection speed is too fast.

(2) Excessive feeding volume causes flash.

(3) Too high temperature of the barrel and nozzle or too high mold temperature will reduce the viscosity of the plastic and increase the fluidity, which will cause flashing under the condition of smooth injection.

3. Silver streaks, bubbles and pores

(1) The material temperature is too high, causing decomposition.

(2) The injection pressure is small and the pressure holding time is short, so that the melt does not adhere to the surface of the cavity.

(3) The injection speed is too fast, which causes the molten plastic to be decomposed by large shearing, resulting in decomposition gas; the injection speed is too slow to fill the cavity in time, resulting in insufficient surface density of the product to produce silver streaks.

(4) Insufficient amount of material, too large feeding cushion, too low material temperature or too low mold temperature will affect the flow and molding pressure of the melt, resulting in bubbles.

(5) When the screw is pre-molded, the backpressure is too low and the speed is too high, which makes the screw return too fast, and the air is easily pushed to the front of the barrel along with the material.

4. Burnt dark lines

(1) The temperature of the barrel and nozzle is too high.

(2) Injection pressure or pre-plastic back pressure is too high.

(3) The injection speed is too fast or the injection cycle is too long.