Nine major faults in injection molds, nine solutions!

1. Difficulty in material removal from the gate

During the injection molding process, the gate is stuck inside the gate sleeve and is not easily dislodged. During mold opening, cracks and damage occurred in the product. In addition, the operator must use a copper rod tip to knock out the nozzle and loosen it before demolding, which seriously affects production efficiency.

The main reason for this malfunction is: poor smoothness of the sprue taper hole and knife marks in the circumferential direction of the inner hole; Secondly, the material is too soft, causing deformation or damage to the small end of the tapered hole after use for a period of time, as well as the nozzle spherical curvature being too small, resulting in riveting of the sprue material at this location. The taper hole of the sprue sleeve is difficult to process, and standard parts should be used as much as possible. If self processing is required, a special reamer should also be self-made or purchased. Conical holes need to be ground to Ra0.4 or above; In addition, a gate pulling rod or gate ejection mechanism must be installed.

2. Large mold dynamic and fixed mold offset

Due to different filling rates in all directions and the influence of the mold's own weight during mold installation, large molds may experience dynamic and fixed mold displacement. In the above situations, lateral displacement force during injection will be applied to the guide column, causing the surface of the guide column to become rough or damaged during mold opening. In severe cases, the guide column may bend or cut, and even the mold cannot be opened.

To solve the above problems, high-strength positioning keys are added to the parting surface of the mold, one on each side. The simplest and most effective way is to use cylindrical keys. The perpendicularity between the guide column hole and the parting surface is crucial. During processing, after aligning the position of the moving and fixed molds and clamping them, the boring is completed on the boring machine in one go, which ensures the concentricity of the moving and fixed mold holes and minimizes the perpendicularity error. In addition, the heat treatment hardness of the guide post and guide sleeve must meet the design requirements.

3. Damage to the guide column

The guide column mainly plays a guiding role in the mold to ensure that the forming surfaces of the core and cavity do not touch each other under any circumstances. The guide column cannot be used as a load-bearing or positioning component.

In several cases, during injection, the moving and fixed molds will generate significant lateral displacement forces. When the wall thickness of plastic parts is required to be uneven, the flow rate through the thick wall is high, resulting in significant pressure at this location; The side of the plastic part is asymmetric, such as the mold with a stepped parting surface, and the opposite side faces are subjected to unequal back pressure.

4. Dynamic template bending

During injection, the molten plastic in the mold cavity generates a huge back pressure, usually between 600-1000 kg/cm. Mold manufacturers sometimes do not pay attention to this issue and often change the original design dimensions or replace the dynamic template with low-strength steel plates. In molds that use top rods to push materials, the large span of the two side seats causes the template to bend downward during injection. Therefore, high-quality steel must be selected for the dynamic formwork, with sufficient thickness. Low strength steel plates such as A3 should not be used. If necessary, support columns or blocks should be set below the dynamic formwork to reduce the thickness of the formwork and improve its bearing capacity.

5. The top rod is bent, broken, or leaking material

The self-made top rod has better quality, but the processing cost is too high. Nowadays, standard parts are generally used, and the quality is slightly worse. If the gap between the top rod and the hole is too large, there may be material leakage; But if the gap is too small, it may get stuck during injection due to the increase in mold temperature and the expansion of the top rod. What is even more dangerous is that sometimes the top rod cannot be pushed out for a certain distance and breaks, resulting in the exposed top rod not being able to reset and damaging the concave mold during the next mold closing.

To solve this problem, the top rod should be re ground, leaving a 10-15 mm fitting section at the front end of the top rod and grinding the middle part down by 0.2 mm. After assembly, all top rods must be strictly inspected for their fit clearance, generally within 0.05-0.08 millimeters, to ensure that the entire top out mechanism can move back and forth freely.

6. Poor cooling or water leakage in the waterway

The cooling effect of the mold directly affects the quality and production efficiency of the product. Defects such as poor cooling, large shrinkage of the product, or uneven shrinkage leading to warping and deformation; On the other hand, the overall or partial overheating of the mold can prevent it from forming properly and lead to production stoppage. In severe cases, it can cause the top rod and other moving parts to expand and get stuck, resulting in damage.

The design and processing of the cooling system depend on the shape of the product. Do not omit this system due to the complexity of the mold structure or difficulty in processing, especially for large and medium-sized molds. Cooling issues must be fully considered.

7. The slider tilts and the reset is not smooth

Due to the limitation of the template area, some molds have too small a guide groove length, and the slider is exposed outside the guide groove after the core pulling action is completed. This can easily cause the slider to tilt during the post core pulling stage and the initial stage of mold closing and resetting, especially during mold closing, where the slider cannot be reset smoothly, resulting in damage to the slider and even bending and breaking.

According to experience, after the slider completes the core pulling action, the length left in the groove should not be less than 2/3 of the total length of the guide groove.

8. Failure of the fixed distance tensioning mechanism

Fixed distance tensioning mechanisms such as hooks and buckles are generally used in fixed mold core pulling or some secondary demolding molds. These mechanisms are set in pairs on both sides of the mold, and their actions must be synchronized, that is, they must be fastened while closing the mold, and unhooked when the mold is opened to a certain position. Once the synchronization is lost, it will inevitably cause the template of the pulled mold to tilt and be damaged. The parts of these mechanisms need to have high rigidity and wear resistance, and adjustment is also difficult. The lifespan of the mechanisms is short, so try to avoid using them as much as possible and switch to other mechanisms.

When the pumping force is relatively small, the method of using a spring to push out the fixed mold can be used; When the core pulling force is relatively high, a structure can be adopted where the core slides when the moving mold retreats, and the core pulling action is completed first before the mold is separated; On large molds, hydraulic cylinders can be used for core pulling.

The inclined pin slider core pulling mechanism is damaged. The common problems with this type of mechanism are inadequate processing and small material usage, mainly due to the following two issues: a large inclination angle A of the diagonal pin, which has the advantage of producing a larger core pulling distance within a shorter mold opening stroke. However, with an excessively large inclination angle A, when the pulling force F is a certain value, the bending force P=F/COSA experienced by the diagonal pin during the core pulling process is also greater, which is prone to deformation of the diagonal pin and wear of the diagonal hole; At the same time, the inclined pin generates an upward thrust N=FTGA on the slider, which increases the positive pressure of the slider on the guide surface inside the guide groove, thereby increasing the frictional resistance when the slider slides, which can easily cause unsmooth sliding and wear of the guide groove. According to experience, the inclination angle A should not exceed 25 degrees.

9. Poor exhaust in injection molds

Gas is often generated in injection molds. What causes this?

(1) The air present in the pouring system and mold cavity;

(2) Some raw materials contain moisture that has not been dried out, and they will vaporize into water vapor at high temperatures;

(3) Due to the high temperature during injection molding, some unstable plastics may decompose and produce gases;

(4) Some additives in plastic raw materials evaporate or react with each other to produce gases.

At the same time, the cause of poor exhaust needs to be identified as soon as possible. Poor exhaust of injection molds will bring a series of hazards to the quality of plastic parts and other aspects, mainly manifested in:

(1) During the injection molding process, the melt will replace the gas in the mold cavity. If the gas is not discharged in a timely manner, it will cause difficulty in filling the melt, resulting in insufficient injection volume and inability to fill the mold cavity;

(2) Removing obstructed gases will create high pressure in the mold cavity and infiltrate into the interior of the plastic under a certain degree of compression, causing quality defects such as voids, pores, loose tissue, and silver lines;

(3) Due to the high compression of gas, the temperature inside the mold cavity rises sharply, causing the surrounding melt to decompose and burn, resulting in local carbonization and burning of the plastic parts. It mainly appears at the confluence of two melts and at the flange of the sprue;

(4) Poor gas removal results in varying melt velocities entering each cavity, leading to the formation of flow and fusion marks and a decrease in the mechanical properties of the plastic parts;

(5) Due to the obstruction of gas in the mold cavity, the filling speed will be reduced, affecting the molding cycle and lowering the molding efficiency.

The main distribution of bubbles in plastic parts is:

(1) The bubbles generated by the accumulation of air in the mold cavity are often distributed at the position opposite to the gate;

(2) Bubbles generated by decomposition or chemical reactions in plastic raw materials are distributed along the thickness of the plastic part;

(3) The bubbles generated by residual water vapor in plastic raw materials are irregularly distributed throughout the entire plastic part.