The gap between the lip of the mouth mold and the template of the shaping device not only determines the thickness of the PVC foam board, but also affects its foaming rate. Generally, the gap between the lip of the mouth mold is 1/4 of the thickness of the board (only refers to the low foaming PVC foam board).
Size of the gap between the templates
When the melted raw material is not filled with the space between the templates of the first shaping device, it is easy to have strip folds or collapse in the parts with low pressure. The traction speed should be used as a benchmark to adjust the size of the extrusion speed to adjust the degree of saturation between the templates; the size of the gap between the templates of each shaping is very important for the molding of PVC foam board.
The gap between the first and second plate and the second and third plate is 0.2 -0.5mm. If the gap between the first channel shaped template and other templates is too large, it may not only lead to mold congestion, but also lead to PVC foam board out of the first channel shaped template, into the other shaped template, due to its excessive pressure, resulting in increased density or PVC foam board section of the phenomenon of broken bubble.
For example, for the production of 18mm thick PVC foam board, the gap between the mold lip and the center of the mold is 4.5 mm. The clearance of the first shaped template is 19.6-19.8 mm, the clearance of the second shaped template is 19.0-19.2 mm, the clearance of the third shaped template is 18.4-18.6 mm, and the clearance of the fourth shaped template is determined according to the density and hardness of PVC foam board.
When the density and hardness of the 18 mm thick PVC foam board is high, because the product is subject to the pressure of the shaping template, after traction, in the external force removed, the board back to the free state after a continued expansion process, so the gap should be slightly less than 18 mm;.
When the density and hardness of 18mm thick PVC foam board is low, because the thermal conductivity of foam plastic products is low, after the products are shaped, the core has a continuous cooling, “post-shrinkage” process, so the gap should be slightly more than 18 mm. The specific difference should be determined by production practice.
In a word, each shaping template should have proper pressure on the product to give full play to its shaping role.