Overcoming The Problem of Marks And Voids in The Table Mould
In any part of the Table Mould(FURNITUREMOULD), when the part suddenly changes and locally increases the volume of the material, the surface will be inhaled when the material shrinks, creating a dent, or if the surface becomes hard enough, it will not sink, shrink It will occur in the body of the material that causes the void. Overcoming this problem is not simple.
One method that is often recommended by the manufacturer to reduce or prevent sink marks and voids in the literature is to increase the injection hold time (plunger residence time). This may have some impact, especially when using large gates to make thick molded parts, but there is the pressure that is not evenly distributed throughout the material in the table mould and some parts will be more stressed than others. danger. Therefore, the molding may appear to be completely flawless but will decompose in use under adverse environmental conditions. This technique requires great care to ensure that the holding pressure is not too great without causing excessive stress.
Another recommended method is to reduce the temperature of the material in the cylinder (based on less shrinkage of the cooler material). This may only have a limited impact, as it is highly unlikely to withstand a few degrees of reduction, and it should be remembered that the forming should always be carried out at the lowest possible temperature.
For a temperature change of 10 ° C, the average heating volume of the crystalline plastic varies by about 1% (about half of the amorphous material). If the reduction of 10.C can be tolerated, then when the effect of the consideration is a 14-18% change in volume, this will result in a shrinkage of only 1%.
In addition, a decrease in temperature also leads to a decrease in the ease of filling because the viscosity of the melt is slightly higher. Therefore, more pressure is required, resulting in greater residual stress.