Must read! Causes and solutions for frequent problems in injection molds

Time：2021-04-26 11:27:18 / Popularity： / Source：

1. Difficulties in gate stripping

During injection molding process, gate is stuck in sprue sleeve and is not easy to come out. When mold is opened, finished product shows crack damage. In addition, it is necessary for operator to knock out top of copper rod from nozzle so that it can be demolded after it is loosened, which severely affects production power.
Main element of this shortcoming is poor brightness of gate taper hole and knife mark in circumferential direction of inner hole. Second is that material is too soft, small end of taper hole is deformed or damaged after a period of use, curvature of nozzle spherical surface is too small, causing riveting of gate material to occur here. Taper hole of sprue bushing is more difficult to process, so standard parts should be used as much as possible. If you need to process it yourself, you should also suppress yourself or buy a special reamer. Taper hole needs to be ground to a Ra0.4 or more. In addition, it is necessary to set gate pull rod or gate ejection.

2. Large-scale mold moving and fixed mold offset

Due to different filling rates of large molds, influence of weight of mold during mold installation, movable and fixed molds will deviate. In above-mentioned situations, lateral deflection force will be added to guide post during injection, guide post will be roughened and damaged when mold is opened, guide post will be bent or blocked when mold is severe, and mold cannot be opened.
In order to deal with above questions, a high-strength positioning key is added on each of four sides of mold parting surface. The most concise and useful one is to use a cylindrical key. Straightness of guide pin hole and parting surface is the most important thing. In processing, moving and fixed molds are aligned and clamped in azimuth, boring is completed at one time on boring machine. This ensure concentricity of moving and fixed mold holes, minimizes straightness error. In addition, heat treatment hardness of guide post and guide sleeve must meet requirements of plan.

3. Damage to guide post

Guide post mainly plays a guiding role in mold to ensure that molding surfaces of core and cavity do not collide with each other under any circumstances, guide post cannot be used as a force-bearing part or a positioning part.
In several cases, moving and fixed molds will have infinite lateral deflection force during injection. When wall thickness of plastic part is not uniform, material flow passes through thick wall at a high rate, and greater pressure occurs here; side surface of plastic part is asymmetrical, such as back pressure on opposite sides of mold of stepped parting surface is not equal.

4. Movable mold plate twists and turns

When mold is being injected, molten plastic in mold cavity generates infinite back pressure, usually 600~1000 kg/cm. Mold makers sometimes don't pay attention to this question, and usually change original plan standard, perhaps replace movable mold plate with a low-strength steel plate. In mold with ejector rod ejection, due to large span between two side seats, mold plate will bend when injection is made.
Therefore, it is necessary to use excellent steel for movable mold base, thickness must be met, and low-strength steel plates such as A3 cannot be used. When necessary, support columns or support blocks should be installed under movable mold base to reduce thickness of mold base and adjust forward load.

5. Ejector rod twists, cracks or leaks

Quality of self-restrained ejector rod is better, but processing cost is too high. Nowadays, standard parts are usually used, and quality is worse. If gap between ejector pin and hole is too large, material will leak, but if gap is too small, ejector pin will become stuck due to increase in mold temperature during injection.
What's more risky is that sometimes ejector pin is pushed out and breaks normally at intervals, exposed ejector pin cannot be reset when mold is clamped once, and concave mold is damaged. In order to deal with this question, ejector rod was re-ground, and a 10~15 mm cooperative section was stored at the front end of ejector rod, and base was slightly reduced by 0.2 mm. After all ejector pins are installed, it is necessary to strictly check cooperation gap, usually within 0.05~0.08 mm, to ensure that all ejector arrangements can move forward and backward freely.

6. Poor cooling or water leakage

Cooling effect of mold directly affects quality and output of finished product, such as poor cooling, large shortening of finished product, or uneven shortening, resulting in warping deformation and other shortcomings. On the other hand, whole mold may be overheated, so that mold cannot be formed normally and production is stopped. In severe cases, movable parts such as ejector rods may be damaged due to thermal expansion.
Cooling system plan depends on shape of product. Don't omit this special system due to messy structure of mold or difficult processing. Especially large and medium-sized molds must be adequately considered for cooling.

7. Slider is tilted and reset is not smooth

Some molds are bound by area of mold plate, length of guide groove is too small, and slider is exposed outside guide groove after core-pulling action is completed. This will simply constitute slider tilt during period after core pulling and initial period of mold resetting, especially when mold is clamped, slider is not reset smoothly, causing slider to be damaged, and even bending damage. According to experience, after slider finishes core-pulling action, length left in chute should not be less than 2/3 of total length of guide groove.

8. Fixed distance tightening arrangement fails

Fixed-distance tensioning arrangements such as swing hooks and buckles are usually used in fixed mold core pulling or some secondary demolding molds. Because this type of arrangement is set in pairs on both sides of mold, its actions must be synchronized, that is, mold is closed together to buckle, and mold is opened to a certain position to release hook together.
Once synchronization is lost, mold plate that constitutes pulling die must be tilted and damaged. Parts of these arrangements must have higher rigidity and wear resistance, adjustments are also difficult, arrangement life is short, try to prevent use. You can switch to other arrangements.
In the case that core pulling force is relatively small, a stretch spring can be used to push out fixed mold. In the case of a large core pulling force, core sliding when movable mold retreats can be selected, core pulling action is completed first and then mold is split. For large molds, a hydraulic cylinder can be used for core pulling. Oblique pin slider core pulling arrangement is damaged.
Disadvantages of this arrangement are mostly inadequate processing and too small materials. First two questions are as follows:
Oblique pin inclination angle A is large, and advantage is that a larger core pulling distance can occur in a shorter mold opening stroke. However, an excessively large inclination angle A is adopted. When extraction force F is a certain value, bending force P=F/COSA encountered by oblique pin during core pulling process is larger, oblique pin deformation and oblique hole wear are prone to appear.
At the same time, upward thrust N=FTGA of oblique pin on the slider is also greater. This force increases positive pressure of slider on guide surface in guide groove, and then increases resistance when slider slides. It is easy to form that sliding is not smooth, and guide groove is worn. According to experience, inclination angle A should not be greater than 25.

9. Poor exhaust in injection mold

Gas often occurs in injection molds. Why does this happen?
Air in pouring system and mold cavity; some materials are rich in moisture that has not been swept away by drying, and they will vaporize into water vapor at high temperatures; due to high temperature during injection molding, certain unstable plastics will differentiate and generate gas; certain increasers in plastic materials may evaporate and generate gas that chemically reacts with each other.
At the same time, cause of poor exhaust is also required to find out quickly. Poor venting of injection mold will cause a series of damages to quality of plastic parts and many other aspects. The first embodiment is that melt will replace gas in cavity during injection process. If gas is not discharged in time, it will be difficult to fill melt, so that injection volume is short and cavity cannot be filled; poorly swept air will cause high pressure in cavity, enter inside of plastic under a certain degree of contraction, forming quality defects such as voids, pores, sparseness and silver streaks;
Because gas is highly compressed, temperature in cavity rises sharply, which in turn causes surrounding melt to differentiate and burn, which makes plastic parts appear to be carbonized and burnt. It is mainly present at the junction of two melts and flange of gate; gas is not cleaned smoothly, which makes speed of melt entering each cavity different, so it is easy to form movable marks and fusion marks, reduce mechanical function of the plastic part; due to obstruction of gas in cavity, filling speed will be reduced, molding cycle will be affected, tax power will be reduced.
Dispersion of air bubbles in plastic part, air bubbles generated by accumulation of air in mold cavity, are often scattered on position opposite to gate; air bubbles generated by differentiation or chemical reaction in plastic material are scattered along thickness of plastic part; air bubbles caused by residual water vaporization in plastic material are scattered irregularly on all plastic parts.

Gud Mould Industry Co., Ltd