Front bumper of a car is one of the most important exterior parts of a car. It must not only have sufficient strength and rigidity, but also play a buffering role in the event of a collision and protect car body. It must also pursue harmony and unity with car body shape, and achieve its own lightweight. In order to achieve this purpose, main body of front bumper of current car is made of plastic, commonly known as plastic bumper.
This article introduces design key points and advanced technology of a large thin-wall injection mold for a plastic bumper.

 

Shape of front bumper of a car is similar to that of a saddle. Specific structure is shown in Figure 1. Material is PP+EPDM-T20, and shrinkage rate is 1.0095. Among them, EPDM can improve elasticity of bumper, T20 means that 20% of talcum powder is added to material, purpose is to improve stiffness of bumper.
Characteristics of plastic parts are:
(1) Shape is complex, size is large, and wall thickness is relatively small. It is a large thin-walled plastic part.
(2) Plastic parts have many collisions and insertion holes, many reinforcing ribs, and large melt flow resistance.
(3) There are three undercuts on the inside of plastic part, and it is quite difficult to pull core laterally at each point.

 

Injection mold for main body of front bumper adopts an internal parting surface, passes through a hot runner, and is controlled by a sequence valve to enter glue. Inverted buckle on both sides adopts structure of large lifter sleeve, horizontal lifter and straight top, maximum external size is 2500*1560*1790mm, and detailed mold structure is shown in Figure 2.

 

In automobile front bumper injection mold, there are two types of parting surfaces: outer parting surface and inner parting surface. Outer parting bumper has same parting method as ordinary mold, mold structure is relatively simple, but parting clamping line can be seen, which affects appearance. After inner parting bumper is assembled, parting clamp line cannot be seen, and parting clamp line is hidden on non-appearance surface of plastic part. Technical difficulty and structure of inner parting bumper are more complicated than that of outer parting bumper, and technical risk is higher. Mold cost and mold price will be much higher than that of outer parting bumper, but appearance is beautiful, it is widely used in middle and high-end automobiles. This mold forming part adopts an advanced and complex design scheme of internal parting, and has achieved good results, as shown in Figure 3.

 

In addition, this plastic part has a large number of through holes, some of which are larger in area. Venting groove and avoiding groove are designed at collision point, and insertion angle is greater than 7°, which can increase service life of mold and prevent flash .
Front bumper injection mold molding parts and mold plate are integrated, mold plate material can be P20 or 718.

Mold casting system adopts an integral hot runner system. Its advantages include convenient assembly and disassembly, low machining accuracy requirements, no risk of glue leakage, reliable assembly accuracy, no need for repeated disassembly and assembly, low maintenance and repair costs.
Front bumper is an exterior part, and surface is not allowed to have weld marks. Weld marks must be driven to non-appearance surface or eliminated during injection molding. This is one of key and difficult points of mold design. Although traditional simultaneous multi-point pouring can fill the entire cavity with melt, it is difficult to achieve desired product quality due to existence of weld marks. For this reason, mold adopts 8-point sequential valve hot runner gate control technology, namely SVG technology. This is another advanced technology used in this mold. It controls opening and closing of eight hot nozzles through drive of a cylinder. As a result, ideal effect of no weld marks on the surface of plastic part is achieved. Location of hot runner gate of front bumper injection mold is shown in Figure 4.

 

Since front bumper adopts an internal parting surface, parting line at undercut of fixed mold A plate is located under lifter of movable mold side. In order to avoid risk of damaging mold during operation, core-pulling step must be strictly controlled when opening mold. For details, see mold working process. This mold adopts a straight top and a lower lifter design, and a complex structure of horizontal lifter is designed in lifter. In order to pull core smoothly, there must be enough space between lifter and straight roof, contact surface of lifer and straight roof shall be designed with a slope of 3°～5°. Large lifer and large straight roof on both sides of injection mold of inner parting bumper should be designed with cooling water channels. Side hole of fixed mold of inner parting bumper should be designed with a fixed mold elastic pin structure, as shown in enlarged view at E in Figure 2. What I want to explain here is: inner parting bumper injection mold is different from general injection mold. When mold is opened, plastic part is not left in movable mold and then ejected, but is ejected synchronously by hook during mold opening process. Side core 43 of mold is ejected during mold opening process, and plastic part will follow fixed mold for a certain distance.

Design of temperature control system for injection mold of front bumper has a great impact on molding cycle of mold and quality of product. Mold temperature control system is in the form of "straight-through cooling water pipe + inclined cooling water pipe + cooling water well". See Figure 3 for details. Cooling water pipe should be arranged along shape of plastic part as far as possible to improve cooling effect.
Main points of design of cooling water channel of this mold are as follows:
(1) Structure of movable mold is more complicated, and heat is more concentrated. Cooling should be focused on, but cooling water channel must be kept at least 8mm away from push rod, straight top and lifer hole.
(2) Distance between water channels is 50-60mm, distance between water channels and cavity surface is 20-25mm.
(3) Cooling water channel can be made straight holes instead of oblique holes. Oblique holes with an inclination of less than 3 degrees can be directly changed to straight holes.
(4) Length of cooling water channel should not be too different to ensure that mold temperature is roughly balanced.

This mold is a large-scale thin-walled injection mold. Design of guiding positioning system directly affects accuracy of plastic part and life of mold. This mold adopts square guide pillars and 1 ° precise positioning guide positioning. Among them, 4 square guide pillars 80*60*700 (mm) are used on movable mold side, 4 square guide posts 180*80*580 (mm) are used between movable and fixed molds. Location is shown in Figure 2 and Figure 3.
In terms of parting surface positioning, mold adopts two conical surface positioning structures (also called internal mold tube positions) at both ends, inclination angle of conical surface is 5°.

Plastic parts are large thin-walled parts, demoulding must be stable and safe. Middle position of mold adopts straight ejector and push rod ejection, diameter of push rod is 12mm. Because contact area is small, it is difficult to return, it is easy to cause putter to collide with cavity surface of fixed model, so inner parting bumper is designed to be straight as possible, and putters are used less.

 

Due to large number of push parts, demolding force and pusher resetting force are relatively large, so demolding system uses 2 hydraulic cylinders as power source. Position of oil cylinder is shown in Figure 5.
Due to uneven surface of moving model core, fixed ends of all push rods and push tubes must be designed with anti-rotation structures.

Because this bumper injection mold uses internal parting technology, parting line of undercut position of A plate is located under lifer of movable mold side. In order to avoid risk of damage to mold during operation, mold’s working process is very strict. Let's talk about steps and precautions starting from clamping.
①Fixed mold A board must be ensured that push rod plate is in 50mm state before mold is closed, so as to ensure that undercut of A board does not touch horizontal small lifer protruding from large lifer, and A plate can smoothly press reset lever to complete mold clamping action. See Figure 6(a).
②Fixed mold A plate returns push rod plate and lifer pressure, as shown in Figure 6(b).
③A plate and push rod plate must be opened 60mm synchronously to ensure that plastic parts and horizontal small lifer are all separated from inverted surface of A plate. Before opening mold, pressure must be applied to ejection cylinder in advance to ensure that the entire ejection system and A plate can be opened simultaneously, as shown in Figure 6(c).
④Fixed mold A plate continues to open mold, movable mold keeps ejection state of 60mm unchanged to achieve effect of separating A plate from straight top, as shown in Figure 6(d).
⑤After fixed mold is opened to required space, movable mold continues to be ejected to a state of 164mm. At this time, horizontal small lifer guide rod reaches inflection point of changing angle of guide rail, inverted surface of plastic part is separated from mold. If plastic part has a small sloping top at this time, directly pull plastic part out of mold by hand, otherwise, continue to push out to final position of 210mm, as shown in Figure 6(e).
⑥If product is slightly sticky to small lifer, ejection is completed when product reaches 164mm. Remove product and directly cycle to step ① to prevent product from being pulled back by horizontal small lifer, resulting in inability to remove product, as shown in Figure 6 ( f).
⑦Non-stick horizontal small lifer of plastic part continues to be ejected to 210mm. After ejection is completed, plastic part is removed, and cycle goes to step ①, as shown in Figure 6(g).

 

(1) This mold uses internal parting technology to ensure beautiful appearance of plastic parts.
(2) Mold adopts secondary core-pulling structure of "composite lifer", which solves problem of lateral core-pulling in complex parts of plastic part.
(3) Mold adopts an eight-point needle valve sequence valve hot runner pouring system, which solves melt filling problem of large thin-walled plastic parts.
(4) Mold uses hydraulic pressure as power of demolding system, which solves problems of large demolding force and difficult resetting of pusher.
Practice has proved that mold is advanced and reasonable in structure and accurate in size. It is a classic among automobile molds. Since mold was released from production, lateral core-pulling action has been coordinated and reliable, quality of plastic parts has been stable, meeting customer requirements.