Injection weight = The maximum weight of a single injection
The injection weight of an injection molding machine is the maximum amount of injection that can be achieved during a single maximum injection stroke of the screw.
PS (polystyrene) plastic is used as the standard
Due to the variety of plastics, their densities also vary. Therefore, for the convenience of statistics, the weight of PS (polystyrene) plastic is used as the standard.
How to calculate the injection weight of injection molding machine ?
During the entire process of injection molding, the density of plastic raw material changes correspondingly with changes in temperature and pressure.
Different plastics have different density changes:
Amorphous plastics: (PC,ABS,PS) density change of about 7%,
Crystalline plastics: The density change of (PE, PP, PA) is even greater, approximately 15% -20%.
Therefore, there is a difference in the measured volume of raw materials under plasticization compared to the volume of products at room temperature and pressure.
In addition, during injection, the molten material flows back into the screw groove, check ring, etc. Therefore, the way to calculate the injection weight of the machine should be:
W=Vc* αβ P= Vc* K * P= Vc* * P’
In the formula:
W – Injection weight (g)
Vc – Theoretical injection volume
α– Density correction coefficient, determined by plastic type and process conditions
β– Leakage correction coefficient of plasticizing device
P-Density of plastic products at room temperature
K-injection coefficient
P ‘– Corrected calculated density
Density changes during injection molding of polystyrene glycol:
| Different Stage | Temperature °C | Pressure（MPA） | Density（G/CM3） |
| Heating and melting | 230 | 0 | 0.97 |
| Injection into mold | 230 | 84 | 1.02 |
| Cooling time | 25 | 0 | 1.05 |
Density of raw material
| Name | HPVC | SPVC | PS | ABS | PE | PA |
| Products Density | 1.35 | 1.1-1.4 | 1.05 | 1.06-1.1 | 0.92-0.95 | 1.1-1.2 |
| Calculated density | 1.12 | 1.02 | 0.91 | 0.88 | 0.71 | 0.91 |
| Name | PP | PC | PMMA | CA | POM | CAB |
| Products Density | 0.95 | 1.2 | 1.18 | 1.3 | 1.41-1.42 | 1.2 |
| Calculated density | 0.73 | 0.97 | 0.94 | 1.02 | 1.15 | 0.97 |
The injection weight of injection molding machine mainly depends on the stroke and diameter of the screw.
The injection weight of Haichen machine mainly depends on the stroke and diameter of the screw.
The stroke (S) is directly related to the axial temperature difference of the molten material in the barrel, and the diameter is directly related to the injection pressure.
According to the summary of practice, for general screws, the maximum stroke should generally be around 3.5D. For example, while ensuring plasticization quality, the injection stroke should be taken as a larger value.
This allows the use of smaller diameter screws to achieve higher injection pressure and compact structure.
At present, some injection devices with good plasticization performance have a screw stroke of about 5D.
After selecting the ratio of screw stroke to diameter K (x=S/D), referring to the injection volume of similar international machines, the diameter of the screw can be estimated, calculated, and according to the R. The expression is as follows:
D3=4Vc/π*K=4W/a*β*ρ*π*K=4W/ρ’*π*K
When using an injection molding machine, the weight of plastic products processed is generally within the range of 1/4-4/5 of the injection volume of the machine, and the minimum should not be less than 1/10. Because a small injection volume not only hinders the full utilization of the machine’s capabilities, but also reduces the performance of the product due to the prolonged retention of plastic in the material barrel, which can easily lead to thermal decomposition. On the contrary, being too large can sometimes fail to fill all of the cavity, and sometimes it prone to problems such as insufficient fullness.
