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Defects On Plastic Co-extrusion Profile Surface And Its Solution
Oct 18, 2017

In addition to the common defect when manufacturing profile, defects on surface of CO extrusion profile in the production process are: CO extruded layer thickness is not uniform; coextrusion layer appears cusp thickening or thinning phenomenon in partial area; color coextrusion layer with dark or bright band; imitation wood grain extrusion molding surface layer coextrusion is not obvious; the scratches, bubbles, black spots; easy to appear the phenomenon such as white edge coextrusion profile welding.

1. Defects in uneven thickness of coextrusion layer

A. in the confluence section, the co extrusion runner and the main material flow passage form a gap for the coextrusion material. The cause of this gap is generally the pin positioning or the misalignment of the die assembly, or the gap caused by the die repair. In order to solve this defect, the gap between the extruded materials should be readjusted to ensure the consistency of the coextrusion in each face.

B. in the formation of uneven thickness of the two surfaces, CO extrusion material melt flow velocity distribution or uneven distribution of pressure. In order to solve this defect, the co extrusion runner shall be trimmed and the co extrusion material, the melt flow rate or the pressure shall be adjusted so that the distribution is uniform.

C, in the form of two surface uneven thickness of the material, the melt pressure and density of different export mode after the die swell is unequal, so the main material extrusion pressure is inconsistent, resulting in uneven thickness of coextrusion. This phenomenon often occurs when the material thickness is different, the feeding mode of the main material is different, and the compression ratio and compression angle of the feed section are different. In order to solve this defect, the compression ratio, compression angle and length of the main material should be adjusted so as to keep the density of each surface and the pressure in the melt as consistent as possible.

2. there are two reasons and Countermeasures of coextrusion layer layer on the cusp of partial thickening or thinning phenomenon: one is due to the boundary effect profile materials; the other is due to the main profile in the local density and melt pressure is too large or too small. The Cusp thickening caused by A. boundary effect. the appearance of the co extrusion layer at the local cusp point is at the corner of the work in process or the intersection with the inner tendon. At the inflection point due to the boundary effect of melt, melt pressure at the inflection point of the main material than others and slightly smaller than the material flow properties of extruded material is much better, the boundary effect on the melt extrusion melt is smaller, therefore, CO extrusion materials can fill into the corner, the corner formed local point thickening. In order to solve this defect, the inflection point of the die can be designed as a small arc and the boundary effect can be eliminated as much as possible so as to effectively improve the thickening of the local cusp at the inflection point. Thickening and thinning of B. material caused by local density and melt pressure. because of special material ingredient in the local density and melt pressure is too large or too small, resulting in local layer coextrusion cusp thickening or thinning of this phenomenon, mostly in the main meeting place and inner rib. The confluence of the inner ribs may lead to local cusp thickening or local thinning, which is mainly determined by the feeding of the inner ribs.  

3. The reasons and Countermeasures of the color coextrusion layer with dark stripes or bright band: extruded materials of high transmittance and mold vacuum system design is not reasonable and co extruded material precipitates on the polished coextrusion surface. Different solutions can be adopted for the different reasons mentioned above.

A. co extrusion material, especially when PMMA is used as co extrusion material, may have dark streaks or bright band problems. Light of different colors, different manufacturers PMMA material rate vary greatly, ranging from 5% to 80%, when the light transmitting material extrusion rate of more than 35%, the thickness of the extruded nuances it may lead to surface layer coextrusion stripes. To solve this problem, on the one hand is the choice of PMMA co extrusion materials quality, or you can add PMMA in the material in the opaque coextrusion inorganic filler to reduce light co extruded material rate; on the other hand to strictly control the coextrusion layer thickness can also improve the coextrusion layer to a certain extent, dark lines and bright with this defect.

B. The vacuum system design of the calibration die is not reasonable, and it may cause dark lines or bright bands. The location of the vacuum hole and the width of the vacuum groove have a great influence on the quality of the surface of the coextrusion layer. The larger the vacuum hole and the wider the vacuum groove, the greater the vacuum adsorption is near the vacuum hole, and the more friction between the co extruded profile products and the molding die, resulting in a bright band near the vacuum hole.

C. coextrusion material has precipitation, precipitates increase the friction between coextrusion layer and mold. The vacuum system during the molding process, the adsorption surface crust core layer micro foam products, will lead to precipitation of formula lubricant, these precipitates will not adsorb evenly in the vacuum groove, lubrication and friction between mould and products increased, resulting in the defect of bright band.

In addition, with the longer use of time, the mold and co extrusion products friction parts easily lead to wear and tear, resulting in wear and tear of the place can not touch the products, it will form dark lines.

4. Causes and Countermeasures of wood grain forming is not obvious: one is due to wood fiber reinforced PVC resin extruded material cause; the other is the imitation wood co extrusion mold flow caused by the unreasonable design. For the former, the composition of the melt may affect the flow rate of the melt. For the latter, the channel can be trimmed to increase the diffraction pattern of the co extrusion material, so that the wood grain forming is more obvious.

5. The causes and Countermeasures of scratches, bubbles and black spots in the coextrusion layer. For PMMA and ASA, it is more prone to scratches; for PVC surface CO extrusion materials, it is more likely to have black spots and bubbles, which is due to poor thermal stability of PVC resins. In addition, the lack of CO extrusion raw material is also possible to bring bubble defects to the coextrusion layer. Strengthening the drying of CO extrusion material can effectively solve the defects caused by moisture.

For scratches, mainly through the mouth mold coextrusion, the flow of the inverted round, the model cavity and vacuum groove polishing, increasing the shape of the block, the entrance inverted circle, clean water quality and other methods to solve.

In addition, due to the influence factors of surface quality of coextrusion profile is more, to get the surface quality of intact coextrusion profile is difficult, the calibrator and traction machine is arranged between the polishing machine, mechanical polishing of coextrusion profile surface, in order to improve the surface quality of extruded profiles, eliminate the surface quality defects caused by various factors.

6. White edge phenomenon in the actual use of color in the coextrusion profile coextrusion profile welding, when welding assembling doors and windows are prone to be crowded, white profile material out of the welded seam, and the extrusion mixture color together with a white white corner after the edge of the phenomenon, affect the overall appearance. This is because the welding seam of ordinary profiles is generally fixed between 0.2 ~ 2mm. In order to solve the defects of white edge when welding, it is recommended to adopt parallel welding technology. Considering the higher welding angular strength and smaller weld limit, some technical options can be used in parallel welding. For example, the weld seam of the color coextrusion layer can be adjusted to 0, and the weld limit of the white base material can be adjusted to 2.0mm, which is a compromise two bit optimum to obtain the best appearance effect and the optimum corner strength.