Figure 1 shows plastic cover with internal thread, material is ABS, internal thread specification is M54*2mm, and length is 7mm. There are 12 grooves evenly distributed on outer surface of plastic part. When threaded core rotates, these grooves are embedded in pusher plate, which can prevent plastic part from rotating with threaded core.

 

Since plastic part has a tubular structure with a through hole in the middle, parting surface of plastic part is divided into an inner parting surface and an outer parting surface, inner parting surface is located at step of inner surface, and outer parting surface is located at the largest contour of plastic part. Fixed mold core and movable mold core adopt bump-through structure, fixed mold core adopts structure of plastic part to be formed, movable mold core adopts form of insert, and inner thread of plastic part is located on movable model core. In order to make groove on outer surface of plastic part come out from pusher plate during demolding, taper direction of outer surface of plastic part must be opposite to taper of inner surface. Parting structure of plastic part is shown in Figure 2.
Plastic part has a simple structure, no buckles and ribs, and is suitable for production with a two-plate mold. It adopts a single-point latent gate for feeding, as shown in Figure 3.

 

Type structure
1. Moving mo core 2. Push plate 3. Plastic parts 4. Fixed mold plate

 

Latent gate
1, moving mold plate 2, plastic part 3, gate sleeve 4, fixed mold plate

The overall dimensions of plastic parts are φ62mm*14mm, and molds are arranged in a rectangular array according to layout of 4 cavities. A hydraulic motor is installed next to mold base, as shown in Figure 4. This arrangement can not only ensure same injection pressure of each plastic part to be molded, but also balance force on gears when main gear drives four threaded cores to rotate. Internal thread of plastic part is unscrewed from pusher plate through rotation of thread core, so mold can be demolded without use of a push rod and a push rod fixing plate. Mold structure is shown in Figure 5.

 

Mold cavity layout
1, plastic part 2, core gear 3, limit screw 4, main gear 5, hydraulic motor

 

Mold structure
1. Moving die seat plate 2, backing plate 3, gear pressing plate 4, core gear 5, main gear 6, deep groove roller bearing 7, movable die plate 8, spring 9, limit screw 10, threaded core 11, plastic 12, fixed die seat plate 13, sprue sleeve 14, positioning ring 15, fixed die plate 16, fixed die insert 17, guide sleeve 18, guide post 19, push plate 20, tapered roller bearing 21, hydraulic motor 22 , Motor support 23, hydraulic sprocket 24, chain 25, driven sprocket 26 deep groove roller bearing guide column 27, main gear shaft.

Mold transmission mechanism is shown in Figure 6. A hydraulic motor is installed on the side of mold, then main gear in the middle is driven by chain to rotate, and main gear drives 4 core gears to rotate at the same time, finally 4 core gears drive 4 threaded cores to rotate. There are grooves on the outer surface of plastic parts, which can prevent plastic parts from rotating together with threaded core, and when threaded core rotates, plastic parts can be pushed out of push plate.

 

(1) When injection is completed, mold is separated between fixed platen and pusher plate (at P1), and the first mold opening is completed, as shown in Figure 7(a). Because plastic part is fastened by threaded core and remains on pusher plate, pusher plate cannot be separated from movable mold plate by plastic part.
(2) When fixed die and movable die are separated, hydraulic motor drives main gear to rotate through chain, then main gear drives 4 threaded cores to rotate. Due to multiple grooves on outer surface of plastic part, plastic part can be prevented from rotating with threaded core. Threaded core rotates out of inner thread of plastic part and pulls out plastic part from pusher plate. At this time, under action of spring, pusher plate and moving mold plate are separated (P2). Under action of limit screw, pusher plate is separated from movable mold plate by 5mm and then stops moving, completing second mold opening, as shown in Figure 7(b).
(3) When plastic part is unscrewed from threaded core, since force exerted by threaded core on internal thread of plastic part disappears, plastic part will automatically eject from mold, as shown in Figure 7(c).
(4) Travel switch sends a signal, and hydraulic motor stops rotating. When plastic parts are taken out, under action of slider of injection molding machine, movable and fixed mold plates and push plate are reset, and then next injection cycle starts.

 

Distance between pusher plate and moving platen should be shorter than length of thread, otherwise there will be two drawbacks: ① When rotation of thread core ends, force exerted by spring on pusher plate will be concentrated at the end of thread, which will easily lead to damage to thread at the end and a high rejection rate; ② Even if thread is not damaged, after thread core is separated from plastic part, plastic part remains in pusher plate, and it is difficult for operator to remove plastic part from pusher plate.
Thread pitch of inner thread of plastic part is 2mm, and thread length is 7mm. A limit screw is set on push plate. Under action of limit screw, push plate moves 5mm and stops to separate from moving mold plate. At this time, there is still a thread with a pitch in core. When threaded core continues to rotate, elastic force of spring on limit screw is no longer borne by inner thread of plastic part. When threaded core is unscrewed with last thread, thread is in a state of free unscrewing and will not damage thread.