According to characteristics of material and supply situation, appearance and process performance of material should generally be tested before molding. Supplied pellets often contain varying degrees of moisture, flux and other volatile low-molecular substances. In particular, moisture content of TPR, which has a tendency to absorb moisture, always exceeds allowable limit for processing.
Therefore, it must be dried and moisture content determined before processing. At high temperatures, moisture content of TPR is required to be below 5%, or even 2% to 3%, so vacuum drying ovens are commonly used for drying at 75℃ to 90℃ for 2 hours.
Dried material must be properly sealed and stored to prevent material from absorbing moisture from air and losing drying effect. For this reason, drying chamber hopper can continuously provide dry hot material to injection molding machine, which is beneficial to simplifying operations, keeping clean, improving quality, and increasing injection rate. Loading capacity of drying hopper is generally 2.5 times hourly consumption of injection molding machine.

 

SBC-based TPE is superior in color to most other TPR materials. Therefore, they only require a smaller amount of color masterbatch to achieve a specific color effect, and colors produced are purer than other TPRs. Generally speaking, viscosity of color masterbatch should be lower than that of TPR. This is because TPR has a higher melt index than color masterbatch, which will facilitate dispersion process and make color distribution more uniform.
For SBS-based TPE, polystyrene-based vehicles are recommended.
For TPR based on harder SEBS, polypropylene (PP) vehicle is recommended.
For TPR based on softer SEBS, low density polyethylene or ethylene vinyl acetate copolymer can be used. For softer varieties, PP vehicles are not recommended as hardness of composite will be affected.
For some overmolding applications, use of polyethylene (PE) vehicles may adversely affect adhesion to substrate.
Barrel needs to be cleaned before injection molding
Before newly purchased injection molding machine is used for the first time, or when product needs to be changed, raw materials are replaced, color is changed during production, or decomposition is found in plastic, barrel of injection molding machine needs to be cleaned or disassembled.
Barrel cleaning method generally uses heated barrel cleaning method. Cleaning materials generally use plastic raw materials (or plastic recycled materials). For TPR materials, transitional cleaning material can be replaced with new processed materials.

During injection molding process, whether temperature setting is accurate is the key to appearance and performance of product. Following are some suggestions for temperature settings when performing TPR processing and injection molding.
Temperature in feed area should be set fairly low to avoid clogging of feed opening and to allow entrapped air to escape. In order to improve mixing state when using color masterbatch, temperature of 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, temperature setting ranges of TPR products in each area are usually: 160 ℃ to 210 ℃ for barrel, and 180 ℃ to 230 ℃ for nozzle.
Mold temperature should be set high to condensation temperature of injection molding area, which will prevent moisture from contaminating mold and causing streaks on the surface of product. Higher mold temperature usually results in longer cycle times, but it can improve appearance of welding line and product. Therefore, mold temperature range should be designed to be between 30 and 40℃.

During process of molding and filling mold cavity, if filling performance of product is not good, pressure will drop too much, filling time will be too long, filling will not be full, etc., which will cause quality problems in product. In order to improve filling performance of products during molding and improve quality of molded products.
Generally, it can be considered from following aspects:
1) Change to another series of Keyue products;
2) Change gate position;
3) Change injection pressure;
4) Change geometry of part.
Control of injection pressure is usually divided into control of primary injection pressure, secondary injection pressure (holding pressure) or three or more injection pressures. Whether timing of pressure switching is appropriate is very important to prevent excessive pressure in mold, overflow 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 consistent each time when switching from pressure holding 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 mold filling is completed, holding pressure decreases immediately. When surface layer forms a certain thickness, holding pressure rises again. In this way, large thick-walled products can be formed with low clamping force, eliminating pits and flash.
Holding pressure and speed are usually 50% to 65% of maximum pressure and speed when plastic fills mold cavity, that is, holding pressure is about 0.6 to 0.8MPa lower than injection pressure. Since holding pressure is lower than injection pressure, load on oil pump is low during considerable holding time, service life of solid oil pump is extended, and power consumption of oil pump motor is also reduced.
A certain amount of measurement is pre-adjusted so that a small amount of melt (buffer amount) remains at the end of screw near end of injection stroke. Further apply injection pressure (second or third injection pressure) according to filling situation in mold, and add a little melt. In this way, product can be prevented from sinking or shrinkage 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. Harder varieties will set faster in mold than very soft varieties. If cooling is done from both sides, required cooling time will typically be approximately 10 to 15 seconds per 0.100' of wall thickness.
Encapsulated articles will require longer cooling times because they can be cooled efficiently through a smaller surface area. Required cooling time will be approximately 15 to 25 seconds per 0.100' of wall thickness.

(1) Improper feeding adjustment, lack of material or excess material.
(2) Injection pressure is too low, injection time is short, and plunger or screw returns too early.
(3) Injection speed is slow.
(4) Material temperature is too low.

 

(1) Injection pressure is too high or injection speed is too fast.
(2) Excessive feeding amount causes flash.
(3) If temperature of barrel or nozzle is too high or temperature of mold is too high, viscosity of plastic will decrease, fluidity will increase, and flash will occur when mold is smoothly advanced.

(1) Material temperature is too high, causing decomposition.
(2) Injection pressure is small and pressure holding time is short, so that molten material does not adhere closely to the surface of cavity.
(3) Injection speed is too fast, causing molten plastic to decompose under large shear, producing decomposition gas; injection speed is too slow, and mold cavity cannot be filled in time, resulting in insufficient surface density of product and resulting in silver streaks.
(4) Insufficient material quantity, too large feeding buffer, too low material temperature or too low mold temperature will affect flow of melt and molding pressure, resulting in bubbles.
(5) When screw is pre-molded, back pressure is too low and rotation speed is too high, causing screw to retract too quickly, and air is easily pushed to the front of barrel along with material.

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