According to the characteristics of the material and the feeding situation, the appearance and process performance of the material should generally be tested before molding. The pellets supplied often contain different degrees of moisture, fluxes and other volatile low molecular substances, especially the TPR moisture content with hygroscopic tendencies always exceeds the allowable limit of processing. Therefore, it must be dried before processing and the moisture content determined. The moisture content of TPR at high temperature is required to be below 5%, or even 2% ~ 3%, so vacuum drying ovens are commonly used to dry at 75 °C ~ 90 °C for 2 hours. The dried material must be properly sealed and preserved to prevent the material from absorbing moisture from the air and losing the drying effect, for this reason, the use of drying chamber hopper can continuously provide dry hot material for the injection molding machine, which is beneficial to simplify the operation, keep clean, improve quality, and increase the injection rate. The loading capacity of the drying hopper is generally 2.5 times the amount of material per hour used by the injection molding machine. SBC-based TPEs are superior in color to most other TPR materials. Therefore, they only require a small amount of color concentrate to achieve a specific color effect, and the color produced is purer than other TPRs. Generally speaking, the viscosity of the color concentrate should be lower than the viscosity of TPR, because the melting index of TPR is higher than that of the color masterbatch, which will facilitate the dispersion process and make the color distribution more uniform.
For SBS-based TPEs, polystyrene-based color carriers are recommended.
For harder SEBS-based TPR, polypropylene (PP) color carriers are recommended.
For softer SEBS-based TPR, low-density polyethylene or ethylene vinyl acetate copolymers can be used.
For softer varieties, PP color carriers are not recommended because the hardness of the composite material will be affected.
For some overmolding applications, the use of polyethylene (PE) color carriers may adversely affect adhesion to the substrate.
Before the first use of the newly purchased injection molding machine, or when it is necessary to change the product, change the raw material, change the color or find that there is decomposition in the plastic, the injection molding machine barrel needs to be cleaned or disassembled.
The cleaning barrel generally adopts the heating barrel cleaning method. Cleaning materials are generally made of plastic raw materials (or plastic recycled materials). For TPR materials, the transition cleaning material can be replaced with the new material processed.
In the process of processing and injection molding, whether the temperature setting is accurate is the key to the appearance and performance of the product. Here are some suggestions for temperature settings when performing TPR injection molding.
The temperature of the feed area should be set fairly low to avoid blockage of the feed port and allow entrained air to escape. When using a color masterbatch, in order to improve the mixing state, the temperature of the transition zone should be set above the melting point of the color masterbatch. The temperature of the area closest to the injection nozzle should be set close to the desired melt temperature. Therefore, after testing, usually the temperature setting range of TPR products in each area is: 160 degrees Celsius to 210 degrees Celsius for the barrel, and 180 degrees Celsius to 230 degrees Celsius for the nozzle.
The mold temperature should be set high and the condensation temperature of the injection molding area, which will avoid the contamination of the mold by moisture and the appearance of streaks on the surface of the product. Higher mold temperatures usually result in longer cycle times, but it can improve the appearance of welding lines and products, so the mold temperature range should be designed to be between 30 and 40.
In the process of product molding and filling mold cavity, if the filling performance of the product is not good, the pressure reduction is too large, the filling time is too long, the filling is not satisfied, etc., so that the product has quality problems. In order to improve the filling performance of products during molding and improve the quality of molded products, it can generally be considered from the following aspects:
1) Change to another series of products;
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. The appropriate timing of the pressure switch is very important to prevent excessive pressure in the mold, prevent overflow or lack of material, etc. The specific volume of the molded part depends on the melt pressure and temperature when the gate is closed during the packing stage. If the pressure and temperature are the same each time from the holding pressure to the cooling stage of the article, the specific volume of the article will not change. At a constant molding temperature, the most important parameter that determines the size of the article is the holding pressure pressure, and the most important variables that affect the dimensional tolerance of the article are the holding pressure pressure and temperature. For example, after the end of mold filling, the holding pressure is immediately reduced, and when the surface layer forms a certain thickness, the holding pressure rises again, so that the thick-walled large products can be formed with low clamping force to eliminate collapse pits and flashing.
The holding pressure and speed are usually 50%~65% of the highest pressure and speed when filling the mold cavity of the plastic, 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 the considerable holding time, the service life of the solid oil pump is extended, and the power consumption of the oil pump motor is also reduced. A certain amount of pre-adjustment is used so that near the end of the injection stroke, there is still a small amount of melt (buffer amount) remaining at the end of the screw, and the injection pressure (secondary or tertiary injection pressure) is further applied according to the filling situation in the mold to supplement a little melt. In this way, it is possible to prevent denting of the article or adjust the shrinkage of the article.
The cooling time mainly depends on the melt temperature, the wall thickness of the article and the cooling efficiency. In addition, the hardness of the material is also a factor. Compared to very soft varieties, harder varieties will solidify faster in the mold. If cooling is done from both sides, the cooling time required for every 0.100' wall thickness will typically be about 10 to 15 seconds. Coated products will require longer cooling times because they can be cooled efficiently with a smaller surface area. The cooldown required per 0.100' wall thickness will be approximately 15 to 25 seconds.
1. Plastic molding is incomplete
(1) Improper feeding adjustment, lack of material or multiple materials.
(2) The injection pressure is too low, the injection time is short, and the plunger or screw is returned too early.
(3) The injection speed is slow.
(4) The material temperature is too low.
2 spills (flashing)
(1) The injection pressure is too high or the injection speed is too fast.
(2) The amount of feeding is too large to cause flashing.
(3) If the temperature of the barrel and nozzle is too high or the mold temperature is too high, the viscosity of the plastic will decrease, the fluidity will increase, and the flash will be caused in the case of smooth mold feeding.
3 silver lines, bubbles and pores
(1) The material temperature is too high, causing decomposition.
(2) The injection pressure is small and the holding time is short, so that the melt and the cavity surface are not closely adhered to.
(3) The injection speed is too fast, so that the molten plastic is decomposed by large shear, resulting in decomposition gas; The injection speed is too slow, and the cavity cannot be filled in time, resulting in insufficient surface density of the product and silver lines.
(4) Insufficient material amount, excessive feeding buffer pad, too low material temperature or too low mold temperature will affect the flow and molding pressure of the melt, and produce bubbles.
(5) The back pressure is too low and the speed is too high when the screw is preshaped, so that the screw retreats too quickly, and the air is easy to push to the front end of the barrel with the material.
4. Scorched dark lines
(1) The temperature of the barrel and nozzle is too high.
(2) The injection pressure or preplastic back pressure is too high.
(3) The injection speed is too fast or the injection cycle is too long.
