Injection weight is an important parameter of injection device of Injection molding machine . It is the most commonly used parameter when selecting an injection molding machine and the key parameter, expressed in grams (g) or ounces (oz) (note: 1 gram = 0.035 ounces, 1 ounce = 28.35 grams). Although this parameter is simple and easy to understand, it is also easy to misunderstand. The reason is very simple, after the plastic is selected, the weight of the plastic parts can be determined, so they are easy to use the injection weight to select the machine. In fact, the definition of injection weight refers to the maximum plastic injection weight that the injection device can reach when the injection screw makes a maximum injection stroke under the condition of air injection (without injection into the mold); This parameter reflects the processing capacity of injection molding machine to some extent. It can be measured or calculated from theory (theoretical values and measured values will have a certain degree of difference). The adhesive used for the test is usually a general PS hard adhesive with a specific gravity of 1.05.
1, first calculate the weight of the product: W = product weight + gate system weight, where: W injection required weight. In Haitian's product sample, all actual injection parameters are calculated using polystyrene (PS) as an example, so in the actual situation, there are two cases to consider.
2, when the injection product is polystyrene (PS), the injection molding machine should have the injection amount of Wps; Wps = (1.3 ~ 1.1) ×W (product weight + total weight of gate system), where, when the quality requirements of the product is high, the coefficient in the above formula should be taken as a large value, and vice versa.
3, when the product is other plastics (named X plastic) to imitate the above method, first calculate the theoretical injection volume of this plastic should be Wx; Wx = (1.3 ~ 1.1) × (product weight + total weight of gate system); Then convert the density of this plastic (X) into the actual weight of PS material Wps, the conversion formula is:Wps = Wx 1.05/Vx
4, according to the calculation results Wps and product sample comparison, choose the appropriate injection molding machine.
The relationship between injection weight and injection volume
1, the injection weight is not the injection capacity multiplied by the proportion of PS, the injection weight is measured and the injection capacity is theoretical. Because the plastic will flow into the gap between the cylinder and the screw during injection molding, and the check ring needs to move back to reach the closed position, the injection capacity multiplied by PS is larger than the injection weight.
2, choose the injection molding machine, should not choose the injection weight just equal to Wx injection molding machine. In injection molding with low requirements, such as toy figures: the total weight should be 85% of the injection weight; In demanding injection molding; For crystal supplies, use 75%.
For example, the plastic parts and the flow channel plastics weigh 113g when the toy figure is moulded with a specific gravity of 1.38 rigid polyvinyl chloride (UPVC); What weight injection molding machine is needed?
Answer: The injection weight expressed in PS is Wps =113g× 1.05/1.38 =86g.
Use 85% injection efficiency.
The injection weight of the injection molding machine required =86g / 0.85=102g.
If the mold size allows, choose HTF58XC.
3, can not use the injection weight is too high injection molding machine
The total weight of plastic parts and runner plastics should be between 35% and 85% of the injection weight. The lower limit is due to the following three considerations: the bending of the template; The resident time of plastic in the cylinder and the power consumption per kilogram of injection molded parts. Small injection parts Using a small mold will make the template have excessive bending, so that he warped (affecting product quality), and even make the template crack. Using a
Large Injection Molding Machine for injection of small plastic parts, the dwell time of molten material is too long, which will cause the melt plastic in the cylinder to decompose.
4. Injection speed
For ultra-thin-wall products, a material with high fluidity is used, and the molten plastic flows at an ultra-high speed, so that the thickness of the molten material in contact with the mold wall and cooling solidification (skin layer) is as thin as possible, that is, the formation of the cured skin layer is delayed as much as possible. So it comes down to the speed of injection. As a rule of thumb, the injection speed can be 1-1.5 times the flow length ratio.
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