An injection mold is a mold into which the plastic ma […]
An injection mold is a mold into which the plastic material is introduced from an external heating or plasticizing cylinder.
This mold runs at a constant temperature; cool, or slightly warm.
Injection molds are mainly used for thermoplastic materials, which are shaped as they cool.
The main components are the cavity, the cavity fixing plate, the cavity back plate, the ejection mechanism, the adapter plate of the suitable press, and the gate, runner and gate system for introducing the material into the cavity.
The mold shown in the figure below is a simple thimble type mold for molding ink bottle caps. Two views of the mold are shown, one is a cross-sectional view and the other is a view facing the injection half.
The parting line is shown (detail 12). The cavities are arranged in a circle, so they are equidistant from the sprue in the center.
The six cavity blocks (detail 18) are rolled from low-carbon mold steel.
After milling, their diameter and length are processed, and the stock is allowed to be ground. Then they are carburized, hardened, ground and polished. The core block (detail 23) is turned from alloy die steel and is hardened, ground and polished.
Both the cavity and the core block have shoulders at the rear end to be fixed in the fixing plate. 'Polishing is limited to the forming part, while grinding is only performed on the accessories part.
All six cores are ground to the same length, and the cavity blocks are treated in the same way.
The cavity blocks are treated with pins or shot peened on the back to prevent them from rotating.
This is to align the gate with the runner. The gate is ground in the cavity block after assembly, and is made large enough through experiments to make the cavity fill correctly.
In this case, since the magnetic core is round, it does not need to be fixed with pins, and rotating the fixed plate will not cause damage.
The cavity fixing plate (detail 13) is made of alloy mold steel, but it can be kept soft to avoid the possibility of deformation during the hardening process.
Water pipes (details lit for drilling, and inlet and outlet (details 30 and 31) are tapped for pipe fittings. Plate tapping is used for 8 fixing screws (detail 14).
The guide pin bushing (detailed figure 20) is installed on this plate.
They are hardened and ground and have a fixed shoulder on the back.
Grind the cavity fixing plate to the same thickness as the cavity block, and then rotate the relief (detail 12) outside the area required to close the runner and land the ejector reset pin (detail 7).
The area of land left is as small as possible to increase the unit pressure to keep the mold closed.
Although the alloy steel plate is not hardened, it is strong enough to withstand some impact and extrusion when the mold is closed.
Drill a hole in the center of the cavity fixing plate to accommodate the sprue bushing. The holes for the plate, cold material and sprue thimble (detail 26) are drilled in the center of the plate.
The hole is slightly undercut to hold the cold block in place until the mold opens to pull out the gate block.
The core fixing plate (detail 10) is made of alloy die steel and is as soft as its mating plate. It also bore holes for the waterline at the same relative position.
It has 4 fixing screws (detail 8) to fix it on the back plate (detail 9).
Clearance holes are provided for the three ejector reset pins (detailed figure 7).
The thimble (detail 24) is installed a short distance back from the molding edge of the board, and then the hole is opened a bit to create a gap and make grinding easier.
Three guide pins (detailed figure 21) are installed on this board. They are hardened and ground and have a fixed shoulder on the back.
The six runners cut in the core holder (detail 17) are the first material that enters the mold.
It is easy to be too cold and unable to flow or weld normally, so it is allowed to bypass the provided space, and the subsequent hotter material is in better molding conditions.
The groove on the iron core fixing plate is the same as the groove on the mating plate.
It is also ground to the proper height.
The two fixing plates are drilled together to ensure the correct alignment of the cavity and core.
When the plates are clamped together, the hole of the guide pin bushing is also drilled.
The back plate is used to fix the cavity and the core block in the fixed plate.
They also hold the guide pins and bushings.
In this mold, a special back plate (detail 9) is provided, and the cavity is supported by the injection adapter plate.
If the mold is large, or the projection cavity area is large, these plates may harden.
This is to prevent bending and prevent the block from sinking under molding pressure.
In less severe cases, the back plate may be made of alloy steel and remain soft.
For light services, they may be made of cold rolled or mechanical steel.
In any case, they should be smoothed to make it fit.
If the core fixing pad is to be hardened, the holes for the thimble and fixing screws must be completed before hardening.
Ordinary holes should have enough clearance for the pins to operate freely. Threaded holes can be provided by matching the gaps in the bore of the component.
A good process requires a fairly good fit even in the clearance hole, although it does not require the same precision as a proper cavity.
There is no need to increase the size and weight of the mold in order to provide a lot of extra clearance space for careless processes.
This applies especially to holes near the waterline.
The ejection mechanism is composed of a casting thimble, a sprue thimble or thimble, a thimble reset pin, a thimble fixing plate, a thimble assembly plate, an ejection box and an ejection rod.
The thimble is made of drill rod and is upset at the fixed end, preferably straight, without turning. They should be hardened.
In this mold, 3 thimble pins are used for each casting (detail 24).
A sprue thimble is used (detail 26). Three ejector reset pins are used (detail 7).
When the mold is closed, they push the ejector assembly back to the position shown in the figure.
The pin is fixed in the fixing plate (detail 27), which is fixed to the assembly plate (detail 28) with screws. These boards should be flat, but not necessarily polished or hard.
The ejector components move back and forth in the ejector box; the stroke is shown in detail 25.
When the press is opened, the ejector rod (detail 1) hits a stop and pushes the assembly forward to the limit of its stroke. The stroke is sufficient to push the workpiece away from the core.
When the mold is closed, the return pin (detail 7) protrudes from the fixed plate (detail 10) by the amount of stroke.
They hit the surface of the fixed plate (detail 13) and push the component back into the forming position.
The ejector components can be hung on the pins, and the plates (details 27 and 28) can be removed from the sides of the box (detail 6). In this mold, the ejector box is square, and the box body is formed by four partitions.
The screw (detail 5) is used to fix the washer to the adapter plate (detail 3). This is to facilitate assembly.
If the mold is not too big, the ejector box can be made into a circle and drilled from a solid block to include the adapter plate.
The ejector end adapter plate (detail 3) in the mold can be made of cold-rolled steel.
There is a through hole for ejecting the rod in its center. There is a circular mounting boss (detail 2) on the back.
One type of pressure plate has a device for clamping the adapter plate, and the clamping groove is shown in detail 4.
As shown in the picture, drill and tap the board for the fixing screws (details 5 and 29).
The injection end adapter plate (detailed figure 19) is also used as the cavity back plate and should be made of alloy steel to adapt to the harsh environment.
It has a mounting boss (detail 15) to install the pressure plate.
It is drilled to allow the injection nozzle to contact the sprue bushing (detail 16).
Drill holes for fixing screws (detail 14).
Both adapter plates should be polished to ensure parallel installation in the press.
Sprue, runner and gate
The material is introduced into the mold through the sprue; the tapered hole in the sprue bushing is shown as detail 16.
It flows from the runner through six runners (detail 17) and gates, as indicated by a small notch in the cavity block at the end of the runner.
The sprue bushing is made of alloy die steel and is hardened.
At the head end, a small recess is machined to fit the specific type of nozzle preferred by the molder.