Injection molding of TPR/TPE thermoplastic elastomer
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Depending on the characteristics of the material and the feeding conditions, the appearance and process properties of the material should generally be tested prior to molding. The supplied pellets often contain varying degrees of moisture, fluxes and other volatile low molecular weights. In particular, the TPR moisture content with a tendency to absorb moisture always exceeds the limits agreed by the process. Therefore, it is necessary to carry out a drying treatment before processing and measure the water content. The moisture content of the TPR is required to be below 5%, or even 2% to 3%, at a high temperature, so that the vacuum drying oven is usually dried at 75 ° C - 90 ° C for 2 hours. The dried material must be properly sealed and stored to prevent the material from absorbing moisture from the air and losing the drying effect. For this reason, the drying chamber hopper can continuously provide the drying machine with dry hot material, which simplifies the operation, keeps the cleaning, improves the quality, Increasing the injection rate is advantageous. The amount of charge for the drying hopper is generally 2.5 times that of the injection molding machine per hour.
TPR staining SBC-based TPE is superior in color to most other TPR materials. Therefore, they only need a smaller amount of color masterbatch to achieve a certain color effect, and the resulting color is more pure than other TPR. In general, the viscosity of the masterbatch should be lower than the viscosity of the TPR, since the melt index of the TPR is higher than that of the masterbatch, which will facilitate the dispersion process and result in a more uniform color distribution.
For SBS-based TPE, polystyrene-based vehicles are recommended.
For TPR based on a harder SEBS, a 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 the hardness of the composite will be affected.
For some overmolded applications, the use of polyethylene (PE) vehicles may adversely affect the adhesion to the substrate.
Before the injection molding machine needs to clean the new injection molding machine before the initial use, or in the production needs to change the product, replace the raw materials, change the color or find decomposition in the plastic, the injection molding machine barrel needs to be cleaned or removed.
The cleaning barrel is generally cleaned by a heated barrel. Cleaning materials are generally made of plastic raw materials (or plastic recycled materials). For TPR materials, the transition wash can be replaced with the new material being processed.
The molding temperature of TPR is the key to the appearance and performance of the product during the processing of injection molding. Below are some suggestions for temperature setting during TPR processing.
The temperature in the feed zone should be set relatively low to avoid blockage of the feed port and allow entrained air to escape. In order to improve the mixing state when using the color masterbatch, the temperature of the transition zone should be set above the melting point of the color masterbatch. The temperature in the zone closest to the injection nozzle should be set close to the desired melt temperature. Therefore, after testing, the TPR product usually sets the temperature in each zone as follows: the cylinder 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 high and the condensing temperature of the injection zone, which will avoid the contamination of the mold by moisture and the streaks on the surface of the product. Higher mold temperatures typically result in longer cycle times, but they improve the appearance of weld lines and articles, so the mold temperature range should be set between 30 and 40.
Filling, holding and cooling of the mold During the process of molding the mold cavity, if the filling performance of the product is not good, the pressure drop is too large, the filling time is too long, the filling is not full, and the like, so that the product exists. Quality topic. In order to improve the filling performance of the product during molding and improve the quality of the molded article, it can generally be considered from the following aspects:
1) Change the products of another series of Keyue;
2) Change the gate position;
3) changing the injection pressure;
4) Change the geometry of the part.
The control of the injection pressure is usually divided into one injection pressure, a secondary injection pressure (holding pressure), or a control of injection pressure of three or more times. Whether the pressure switching timing is appropriate is very important to prevent excessive pressure in the mold, prevent flashing or lack of material. The specific volume of the molded article depends on the melt pressure and temperature at which the gate is closed during the holding phase. If the pressure and temperature of the product are switched from the pressure holding to the cooling stage of the product, the specific volume of the product will not change. At a constant molding temperature, the most important parameter determining the size of the article is the holding pressure. The most important variables that affect the dimensional tolerance of the article are the holding pressure and temperature. For example, after the filling is completed, the holding pressure is immediately lowered. When the surface layer is formed to a certain thickness, the holding pressure is increased again, so that the thick-walled large product can be formed by the low clamping force to eliminate the collapse pit and the flash.
The holding pressure and speed are usually 50%-65% of the maximum pressure and speed when the plastic is filled into the cavity, that is, the holding pressure is about 0.6-0.8 MPa lower than the injection pressure. Since the holding pressure is lower than the injection pressure, the load of the oil pump is low, the service life of the oil pump is prolonged, and the power consumption of the oil pump motor is also reduced during a considerable holding time. Pre-adjusting a certain amount of metering, so that a small amount of melt (buffering amount) remains at the end of the screw stroke around the end of the injection stroke, and the injection pressure (secondary or tertiary injection pressure) is further applied according to the filling condition in the mold. Add a little melt. In this way, it is possible to prevent the product from being depressed or to adjust the shrinkage rate of the article.
The cooling time depends primarily 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. Harder varieties will solidify faster in the mold than very soft varieties. If cooling is performed from both sides, the cooling time required for every 0.100' wall thickness will typically be about 10 to 15 seconds. Encapsulated articles will require longer cooling times as they can be effectively cooled by a smaller surface area. The cooling time required for every 0.100' wall thickness will be approximately 15 to 25 seconds.
Influence of injection molding process conditions
1 plastic molding is incomplete (1) improper feed adjustment, lack of material or more material.
(2) The injection pressure is too low, the injection time is short, and the plunger or screw is retracted too early.
(3) The injection speed is slow.
(4) The material temperature is too low.
2 flash (flash)
(1) The injection pressure is too high or the injection speed is too fast.
(2) The amount of feed is too large to cause flash.
(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 mobility will increase, and the flash will be caused in the case of smooth advancement.
3 Silver, bubbles and pores (1) The material temperature is too high, causing decomposition.
(2) The injection pressure is small, and the dwell time is short, so that the melt and the surface of the cavity are not closely attached.
(3) The injection speed is too fast, so that the molten plastic is decomposed by large shearing action to generate decomposition gas; the injection speed is too slow, and the cavity cannot be filled in time to cause the surface density of the product to be insufficient to produce silver streaks.
(4) Insufficient material, excessive packing cushion, too low material temperature or too low mold temperature will affect the activity of the melt and the molding pressure, resulting in bubbles.
(5) When the screw is pre-molded, the back pressure is too low and the rotation speed is too high, so that the screw is retracted too fast, and the air is easily pushed along with the material to the front end of the barrel.
4 burnt dark lines (1) barrel, nozzle temperature is too high.
(2) The injection pressure or pre-plastic back pressure is too high.
(3) The injection speed is too fast or the injection cycle is too long.