In order to ensure that designed mold and injection moulding machine have a good match, it is necessary to check relevant data between these two, thereby design mold and select appropriate injection moulding machine model.
Areas to be checked for data are: injection volume, injection pressure, clamping force, mounting size, mold opening stroke, and ejection device.
Injection volume check
Whether mold cavity can be filled with maximum amount of injection allowed by injection machine. If finished product is partially deficient, the most likely cause is insufficient injection volume. Therefore, when designing mold, the total amount of melt required by mold should be within range of actual maximum injection volume of injection machine, which is equivalent to 80% of rated injection volume of injection machine, so there are: nm1+m2≦80%m
M1—mass or volume of a single finished product
M2—weight or volume of plastic required for gating system
m — maximum injection volume allowed by injection machine (g or cm3)
n — number of cavities
Note: n is related to plasticizing rate of injection machine, maximum injection volume and clamping force, precision of finished product, economics of production, etc. Generally speaking, size accuracy of finished product is 4%~8% lower for each additional cavity. For example: gears or blades can only be designed with a maximum of 1*2.
Injection pressure check:
This is to verify whether maximum injection pressure of injection machine can meet needs of molding plastic part. Injection pressure required for finished product is determined by type of injection machine, nozzle form, plastic fluidity, pouring system and flow of cavity. Insufficient injection pressure will actually form a short shot.
Generally speaking, whether injection pressure meets requirements of finished product can be controlled by organic adjustment of machine. It cannot be adjusted to be too large to prevent pressure at the gate or strength of mold from being damaged or mold cannot be divided.
Checking clamping force:
When injection molding, projected area of plastic part on mold parting surface is main factor affecting clamping force, and clamping force is proportional to projection surface. If clamping force provided by injection moulding machine is not enough, mold parting surface will not be closed tightly, shut off and kiss off will be insufficient, so that burr or flashing phenomenon is directly formed. Tonnage model of injection moulding machine is determined by clamping force, and formula for quickly calculating tonnage required for clamping force of injection moulding machine is:
F1=(n A1+A2) P
A1—Maximum projected area of finished product, total length * total width, Unit: cm2
A2—Projection area of gating system Unit: cm2
P — Pressure coefficient (high pressure: 0.75 tons / cm2 general pressure: 0.5 tons / cm2)
F1—resin pressure unit: ton
F1≦80% F F—Clamping force of injection moulding machine
Check installation size:
In order to ensure smooth installation of mold, it is very important to check installation size. In this part, designer often neglects problem of over-reliance on computer operation. As a result, mold cannot be tested on time.
1 Nozzle size:
Between aperture nozzle dn and aperture sprue bushing dp, and between convex spherical radius Rn of nozzle and radius Rp of bushing surface, following relationship must be satisfied:
Dp=dn+1.0mm, dn, Rn value, please refer to specific injection machine data
Effective length of injection of nozzle (length that can be extended into mold) and mold (especially hot runner system), sinking depth of sprue bushing should be well matched to prevent nozzle head from being able to abut bushing.
2 Positioning ring size:
Purpose of positioning ring is to ensure that center line of mold can be overlapped with nozzle and center rod of injection machine after mold is installed, so that injection molding must be carried out smoothly. A certain gap is used between positioning ring dH and center hole dj of injection setting plate: dj=dh+(0~0.2mm). Positioning ring has a thickness of 10mm and contact with center of pallet.
3 Mold thickness:
Mold thickness H (also known as closing height) must meet: Hmin < H <Hmax
Hmin—minimum opening distance between moving and fixed plates
Hmax—Maximum mold thickness allowed by injection machine
Thickness of clamping die of injection machine is a range, and its value can be adjusted.
4 Molding and fixing dimensions of mold:
Mold is placed between columns of injection machine on injection machine. Safety relationship columns between total width Wm of all molds and total width Wj is:
If Wm value of mold is affected by Wj and there is no larger model to choose from, you can try to change “work” model into a straight mold design.
There are two common forms of fixing molds:
One type is fixed by screws (when plate is in the form of a screw hole in a fixed position);
The other is fixed (tray is in the form of “T” groove) by screws and pressure plate (or automatic pressure plate). When fixing with pressure plate, thickness difference between front and rear plates must be less than 5mm, so that same type of pressure plate can be used.
5 Check of mold opening stroke:
Mold opening stroke refers to distance that movable mold fixing plate moves during opening and closing of mold, and its size directly affects height of finished product that can be formed by mold. If it is too small, finished product cannot be taken out.
Simple method of weighing: When mold opening stroke of injection machine is less than 2 times of total height of finished casting system, finished product is difficult to take out. In actual calculation, mold opening stroke should contain safety margin of finished product 30~50mm, which is suitable for robot movement, and mold opening stroke is independent of mold thickness.
6 Checking ejector:
1. Verify position of KO hole opened on mold according to specific machine, and whether hole diameter matches it.
2. Verify ejection distance of injection machine, whether it meets need of mold ejection action, especially in secondary ejection mold structure, amount of ejection is easy to be insufficient, length of ejector is not enough, and supporting columns must be added on the first two ejector plates.
3. In ejector mechanism of forced return, verify that mold screw matches screw hole of top of injection ram.