The case of 2-color injection molding for the handle shell requires two sets of core cavities of the dynamic mold that cannot be the same, how to solve it?
How to solve this problem? The moving mold adopts movable inserts (the middle plate mold).
Its working principle is: after the injection molding machine finishes the primary injection molding, it pushes out the movable insert in the movable mold first, then makes the movable insert rotate 180°, and then carries out the secondary injection molding.
This way, the two sets of core cavities in the dynamic mold can be made into different shapes to achieve our expected design purpose.
Analysis of the structure of 2-color injection molding parts
The plastic part is a handle on a medical device, which consists of three parts: spring steel coupling piece, PC inner core, and TPE covering layer, and because there are two kinds of plastics, so secondary injection should be made, and the mold adopts two-color injection mold.
During production, the insert spring steel linkage piece is placed first, the first injection molding PC core (two parts) is carried out, and then the soft rubber TPE covering layer is injected.
In this process, a mechanical bond is formed between the metal material, the PC inner core, and the TPE covering material instead of chemical bonding.
2-color injection molding structure analysis
The mold is one out of two, but there are four cavities, two for molding single-color semi-finished products and two for molding two-color handle finished products.
The mold frame adopts a simplified three-plate mold frame, specification model: 3540-DCI-A50-B70-C120-260. i.e., the mold length is 400mm, width is 350mm, fixed mold A plate thickness is 50mm, movable mold B plate thickness is 70mm, square iron height is 120mm, and guide column length is 260mm.
(1) 2-color injection molding pouring system design
There are two pouring systems in this mold, one point gate pouring system and one side gate pouring system, and the main channels of the two pouring systems are perpendicular to each other.
The conventional point gate feeds the PC inner core with the main flow path in the horizontal direction. In contrast, the TPE soft rubber cover is fed from the side with the main flow path facing upwards vertically, and the manifold and gate are on the parting surface.
(2) Design of molded parts for 2-color injection molding
Since the outer surface of the middle part of the two-color handle is covered with a layer of TPE, the two cavities in the mold can’t be identical, and it is impossible to rotate the whole mold 180° during the two-color injection molding like the common two-color injection mold.
In this mold, a movable insert is designed in the moving mold plate so that the moving mold plate does not move during the second injection, but the movable insert is pushed out and then rotated 180°.
The movable insert is pushed out by the top stick of the injection molding machine, and after taking out the two-color handle, the hydraulic cylinder pushes the rack, which drives the gear, and then the gear drives the insert to rotate 180° by the spline shaft.
This mold is a special structure of 2 color injection molding. It does not need two sets of mold frames, reducing the mold size and injection machine size, and reducing the injection machine’s injection action, improving the mold’s productivity.
The movable insert 8 of the mold has a 3° slope around it, which can reduce the friction between the insert and the mold plate. The insert is machined by wire cutting, the movable insert’s shape, and the wire cutting diagram.
(3) Design of fixed pitch parting mechanism for 2-color injection molding
A pointed gate feeds the PC core, and the mold has three parting surfaces, of which the parting surface must be opened first to allow the pouring system to disengage the condensed material from the PC core.
The parting face II will be opened next, and the casting system will automatically fall off. When the parting surface is opened, the mold releases the two-color handle, and the single-color semi-finished product is rotated 180° with the movable insert.
To ensure the above sequence, the mold is designed with a built-in fixed-distance parting mechanism consisting of nylon plugs, small tie bars, and small screws.
(4) Design of 2-color injection molding release system
The handle’s surface is demanding, and no push rod trace is allowed. The PC core casting system’s solidified material is automatically demolded under the fixed pitch parting mechanism. In contrast, the PC core and metal insert are not separated from the mold but rotated 180° into another cavity.
The finished two-color handle is pushed out with the movable insert after the TPE injection is completed and taken out by hand.
Note: The fixed plate of the push rod and spline shaft cannot be added with a reset spring. Otherwise, the two-color handle product can not be taken out, and the single-color semi-finished product cannot be rotated 180° before the movable insert is returned to the moving template.
2-color injection molding working process
(1) Before closing the 2-color injection molding mold, put the spring steel linkage piece in the dynamic mold cavities 1 and 2.
(2) After the 2-color injection molding mold is closed, the PC melt passes through the horizontal mainstream channel and enters the cavity by the point gate pouring system to form the PC core.
At the same time, the TPE melt enters the cavity and covers the PC core of the last injection molding by the side gate pouring system through the vertical mainstream channel.
When the melt in both cavities is filled and cooled, and cured to sufficient rigidity, the injection molding machine pulls the moving mold to open the mold.
(3) Under the fixed distance parting mechanism, the 2-color injection molding mold opens from the parting surface I at a distance of 80mm, which is controlled by a small tie bar.
When parting surface 1 is opened, the PC pouring system is separated from the PC core by the action of the puller bar and is separated from the fixed mold insert and the fixed mold plate.
(4) Then the 2-color injection molding is opened from the parting surface II. The opening distance is 10mm, controlled by the limit screw.
When the parting face II is opened, the stripping plate forces the PC runner to condense away from the pulling bar. Then it will fall off automatically under gravity, realizing automatic de-pouring.
(5) The 2-color injection molding mold is opened from the parting surface I.
The opening distance must ensure that the movable insert 8 rotates 180° after being pushed out and the finished product of the two-color handle is easily removed. According to the insert size and the finished product, the opening distance of the parting surface can be 200mm, which is controlled by the injection molding machine.
(6) After completing the 2-color injection molding opening stroke, the top stick of the machine pushes the push rod base plate 21, which in turn pushes the spline shaft. The spline shaft pushes the movable insert of the mold, and the movable insert is pushed out together with two single-color PC core semi-finished products and two two-color handles finished with a TPE coating layer. The push-out distance is 40 mm, controlled by a limit post.
The finished products in cavities 3 and 4 are manually removed after the movable insert is pushed out. After that, the hydraulic switch of the cylinder is turned on manually, the piston of the cylinder pushes the rack, the rack pushes the gear, and the gear drives the spline shaft and the movable forging of the die to turn 180° counterclockwise through the spline.
(7) Put in the insert, close the 2-color injection molding mold, and the movable mold insert is reset under the push of the fixed mold insert. The next injection follows the 2 color injection molding.
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