Optimum Injection Speed in Plastic Injection Molding

Optimum Injection Speed 

      The optimum injection speed allows us to maintain minimum injection pressure under given conditions. Injection speed should vary with cavity thickness. Compared with the thick ones, the thin ones possess a smaller effective flow cross-section. As a result, we need to raise injection speed, improve overall deformation rate and increase frictional deforming force, so as to stabilize flow temperature and minimize injection pressure.

Injection speed also varies greatly with the heat capacity, thermal conductivity and viscosity of resin materials. For materials (PC, PMMA) whose viscosity changes dramatically with temperature, the U-curve is smaller. For materials (PP, ABS) without this feature, the U-curve is much larger. Therefore, some materials are sensitive to injection speed, while some others are not.

injection molding speed

Injection pressure for plastic injection molding

 Injection Pressure

 Injection pressure refers to the force that helps molten resin overcomes the resistance to flow.

When the mold screw reaches changeover to holding pressure, the pressure on screw front end is referred to as injection pressure.

Fill the entire cavity with a pressure lower than the max. injection pressure of the injection molding machine, or molding failure might occur.

Out of considerations for safety, during mold design and injection molding condition setting, the molding pressure is set to be lower than 80% of the max. injection pressure of the injection molding machine.

injection pressure

Design standard for the runner in plastic injection mold

1.The runner type

the runner design type for plastic injection mold

2.Series diameter(mm)2    2.5   3   3.5    4     4.5     5     6      7      8       (10      12 for reference)

3.How to design the runner diameter for different material

For PE,PA .etc. we can choose smaller diameter,but for PC,PMMA,we choose bigger diameter.

runner design size for plastic injection mold

4. Helpful Tips:
(1). For a U-shaped runner, the size of the runner can be changed by adjusting the H value;
(2). Try to select and use a circular runner;
(3). The sectional area of the runner has to be larger than that of its sub runners. See the following diagram which shows a runner divided into two sub runners. If the no. of sub runners is increased, D1 can be reduced appropriately.

sub runner design


The sliders for plastic injection mold

Due to the special requirements of some products, the mold release of some part is not consistent with the mold opening direction of the injection machine, which needs the side parting core pulling mechanism to eject the product smoothly.
The side parting core pulling mechanism comes with two types: slider and lifter.
1. Slider travel calculation:
To ensure smooth product release, the travel distance of the slider has to be sufficient. Usually, the shortest travel distance that can guarantee smooth product release is 2 – 3mm:
AB = AC + (2 – 3)
   sliders stroke
2. All core sliders adopt the press plate + guide pin + spring structure as shown in the diagram (sometimes, when the slider is wider than 100 and yet it is not convenient to adopt the structure, T-plate structure may be considered). However, when the slider is vertically placed and restricted by pin position/mold size, press plate will not be necessary – an integrated mold base may be the option.
 sliders press block
The press plate is a standard self-built part of the company, which shall be located with a locating pin.
3. No matter whether the slider sides are sealed, both of its sides need a gradient design. Usually, the angle of a single side is 3 – 5 °; but when two sliders which travel in vertical direction join with one another, the angle will be 45°. During the design process, if there are sliders joining with one another on the four sides of a product, the ear of one of the sliders may stick out to guarantee accurate location.
4. The ratio of slider height to its length should be no greater than 1, or slider movement will be affected by overturning moment, leading to movement failure. General requirement: L≧1.5H.
5. Usually, the angle of a slider guide pin is 15° – 25°, with the biggest no larger than 25°. The angle of a guide pin is usually 2° smaller than that of the slider. In general, try not to use small guide pins, so as to ensure smooth slider movement.
6. The hole of a guide pin is 1/64" larger than a single side of it, about 0.4. When a guide pin goes through a slider, enough clearance should be kept on the mold plate.
7. Identify the location of a guide pin in a slider: try to place the guide pin in the center of a slider. See the diagram for specific measurements:
guide pin hole for sliders
8. It is required that the wedge surface matched with the slider should be higher than 2/3 of slider height, and the screws used on wedges should as big as possible. The following diagrams show wedges of two different structures. Try to avoid the structure shown in diagram b.
the wedge for sliders in plastic injection mold
9. Identification of slider spring length: Guarantee a sufficient space for spring, so as to avoid spring failure.
Assume slider travel is M and total spring length is L; assume the spring is compressed by 40%, and after the slider quits completely, the spring still bears 10% of the pressure, then:
Space for spring is 0.6L.
When L is too small, to prevent spring failure, spring length is often to be increased.
10. To ensure smooth slider movement, there cannot be obstacles to movement around it, such as pointed angles. Generally, chamfers of R3 – R5 should be designed around it.
11. When a spring is needed to be mounted under a slider (see the diagram for dimensional requirements), to prevent screws from being seized by the spring, Table 4-1 should be referred to for selection of springs and screws.
sliders spring
Spring Matching Screws
ø3/8” M3/16”
ø1/2” M1/4”
ø5/8” M5/16”
ø3/4” M3/8”
ø1” M1/2”
ø1-1/4” M1/2”
12. Large sliders should be cooled separately, and wear blocks should be fitted on the slider or the wedge. At this point, there should be a 0.5 clearance between the slider and the wedge.
See the diagrams.
clearance between slider and wedge