Precautions for TPE and TPR injection molding process

Time:2021-09-23 08:38:04 / Popularity: / Source:

Before newly purchased injection molding machine is used for the first time, or when product needs to be changed, raw material is changed, color is changed or plastic is found to be decomposed in production, injection molding machine barrel needs to be cleaned or disassembled. 
Cleaning barrel generally adopts heating barrel cleaning method. Cleaning materials generally use plastic raw materials (or plastic recycled materials). For tpr material, transition cleaning material can be replaced with new material being processed.
TPR injection molding process 

TPR molding temperature

In process of injection molding, accuracy of temperature setting is key to appearance and performance of product. Following are some suggestions for temperature setting during TPR processing and injection molding.
Temperature in feed area should be set to be quite low to avoid clogging of feed port and let entrained air escape. In order to improve mixing state when using color masterbatch, temperature in transition zone should be set above melting point of color masterbatch. Temperature in area closest to injection nozzle should be set close to desired melt temperature. Therefore, after testing, usual temperature setting ranges of tpr products in each area are: barrel is 160 degrees Celsius to 210 degrees Celsius, nozzle is 180 degrees Celsius to 230 degrees Celsius.
Mold temperature should be set higher than condensation temperature of injection zone, which will avoid contamination of mold by moisture and appearance of streaks on the surface of product. Higher mold temperature usually leads to longer cycle time, but it can improve appearance of welding lines and products. Therefore, range of mold temperature should be designed to be between 30 and 40.

Filling, holding pressure and cooling of mold

In process of forming product into mold cavity, if filling performance of product is not good, pressure will drop too much, filling time is too long, filling is not full, etc., so that product has quality problems. In order to improve filling performance of product during molding and improve  quality of molded product, following aspects can generally be considered:
1) Change to another series of Keyue products;
2) Change gate position;
3) Change injection pressure;
4) Change geometry of part.
Generally, control of injection pressure is divided into control of primary injection pressure, secondary injection pressure (holding pressure), or more than three injection pressures. Whether timing of pressure switching is appropriate is very important to prevent excessive pressure in mold, prevent flash or lack of material, etc. Specific volume of molded product depends on melt pressure and temperature when gate is closed during pressure holding stage.
If pressure and temperature are same every time when switching from holding pressure to product cooling stage, specific volume of product will not change. At a constant molding temperature, the most important parameter that determines size of product is holding pressure, the most important variables that affect dimensional tolerance of product are holding pressure and temperature. For example: after filling mold, holding pressure will immediately decrease. When surface layer has a certain thickness, holding pressure will rise again. In this way, low clamping force can be used to form large thick-walled products, eliminate craters and flash.
Holding pressure and speed are usually 50%~65% of the highest pressure and speed when plastic is filling mold cavity, that is, holding pressure is about 0.6~0.8mpa lower than injection pressure. Since holding pressure is lower than injection pressure, load of oil pump is low during a considerable holding time, service life of solid oil pump is prolonged, and power consumption of oil pump motor is also reduced.
A certain metering is adjusted in advance, so that near end of injection stroke, there is still a small amount of melt (buffering amount) at the end of screw. According to filling situation in mold, further apply injection pressure (two or three injection pressure) to add a little melt . In this way, product can be prevented from sinking or shrinkage rate of product can be adjusted.
Cooling time mainly depends on melt temperature, wall thickness of product and cooling efficiency. In addition, hardness of material is also a factor. Compared with very soft varieties, harder varieties will solidify faster in mold. If cooling is performed from both sides, 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. 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) Injection pressure is too low, injection time is short, plunger or screw returns too early.
(3) Injection speed is slow.
(4) Material temperature is too low.

2. Overflow (flash)

(1) Injection pressure is too high or injection speed is too fast.
(2) Excessive feeding volume causes flash.
(3) Too high temperature of barrel and nozzle or too high mold temperature will reduce viscosity of plastic and increase fluidity, which will cause flashing under condition of smooth injection.

3. Silver streaks, bubbles and pores

(1) Material temperature is too high, causing decomposition.
(2) Injection pressure is small and pressure holding time is short, so that melt does not adhere to surface of cavity.
(3) Injection speed is too fast, which causes molten plastic to be decomposed by large shearing, resulting in decomposition gas; injection speed is too slow to fill cavity in time, resulting in insufficient surface density of 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 flow and molding pressure of melt, resulting in bubbles.
(5) When screw is pre-molded, back pressure is too low and speed is too high, which makes screw return too fast, air is easily pushed to front of barrel along with material.

4. Burnt dark lines

(1) Temperature of barrel and nozzle is too high.
(2) Injection pressure or pre-plastic back pressure is too high.
(3) Injection speed is too fast or injection cycle is too long.

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