Nowadays, there are too many injection molding machines on the market. It is only considered that the injection machine is too simple and the clamping force is not comprehensive. It is necessary to combine the two to make a choice.

Injection weight is an important parameter of an injection molding machine, and it is the most useful and critical parameter for selecting an injection molding machine. Expressed in gram or ounze, although this parameter is simple and easy to understand, it can also be easily misunderstood. the reason is simple. When the plastic material is determined, the weight of the plastic part can be determined. Therefore, it is easy for them to use the injection weight to select an injection molding machine. In fact, the definition of injection weight refers to the maximum plastic injection weight that the injection device can reach when the screw performs the maximum injection stroke under the screw. Air shot state (not injected into the mold); this parameter reflects the processing capacity of the injection molding machine to a certain extent; it can be measured or calculated theoretically (theoretical value and measured value may be different). The material used in the test is usually ordinary PS with a density of 1.05.

Steps to determine the injection weight

  1. Calculate the product weight first: W = product weight + gate system weight, where: W is the weight required for injection. In actual situations, there are two situations to consider.
  2. When the injection molded product is polystyrene (PS), the injection volume of the injection molding machine should be Wps; Wps = (1.3 ~ 1.1) × W (product weight + total gate system weight), when the product requirements are high, The coefficient in the above formula should take a large value, otherwise, it can take a small value.
  3. When the product is the imitation method of other plastics (named X plastic), first calculate the theoretical injection volume of the plastic should have Wx; Wx = (1.3 ~ 1.1) × (product weight + total weight of the gate system); then according to the plastic The density (X) is converted into the actual weight of the PS material Wps, the conversion formula is: Wps=Wx×1.05/Vx
  4. According to the calculation results of the Wps and LOG manuals, select the appropriate injection molding machine.

Remarks: The relationship between injection weight and injection volume

  1. The injection weight is more than just the injection volume multiplied by the density of PS. Measure the injection weight, and the injection volume is a theoretical value. Because the plastic material will flow into the gap between the barrel and the screw during injection, and the non-return ring needs to move backwards to reach the closed position, the injection volume is doubled because the density of PS is greater than the injection weight.
  2. Products with low requirements, such as toy dolls: the total weight should be 85% of the injection weight; in high-demand injection molding; such as crystal products, 75%.
  3. Do not use an injection molding machine with an excessive injection weightThe total weight of plastic and runner plastic should be between 35% and 85% of the injected weight. The lower limit is due to the following three considerations: the bending of the plate; the residence time of the plastic in the barrel and the power consumption per kilogram of injection molded parts. The use of small molds for small injection molded parts will cause excessive bending of the pressure plate, resulting in warping (affecting product quality), and even cracking of the pressure plate. Use an oversized injection molding machine to inject small plastic parts. If the residence time of the molten material is too long, it may cause the melted plastic in the barrel to decompose.
  4. Injection speedFor ultra-thin-walled products, high fluidity materials are used to make the molten plastic flow at an ultra-high speed to ensure that the thickness l of the surface layer formed by cooling and solidification after the molten material contacts the mold wall is as thin as possible, which is to delay the generation time of the solidified skin as much as possible. So it involves the issue of injection speed. In terms of experience, the injection speed can be 1 to 1.5 times the flow length ratio.