hot runner molds analysis of common failures

1. ਜਾਣ-ਪਛਾਣ

Compared with ordinary runner molds, ਗਰਮ ਦੌੜਾਕ ਮੋਲਡ have significant advantages such as time-saving, material-saving, high efficiency, and stable quality. ਹਾਲਾਂਕਿ, it was once easy to cause failures in use, which affected its wide application. With the technological progress of the mold industry, hot runner molding has made great progress in runner melt temperature control, structural reliability, and hot runner component design and manufacturing. This makes hot runner technology regain people’s attention and favor .

In the design and application of hot runner molds, there are many problems worthy of consideration and attention. How well these problems are solved is directly related to the success or failure of the hot runner system and product quality. ਇਸ ਲਈ, discussing the faults and causes of the hot runner system and understanding the matters that should be paid attention to in the application of hot runner molding will undoubtedly contribute to the successful use of hot runner molding technology.

2. Analysis and countermeasures of common failures of injection mold hot runner system

2.1 Protruding residues or drooling material at the gate and poor surface appearance

2.1.1 Main reasons

Unreasonable gate structure selection, improper temperature control, and large residual pressure in the melt in the runner after injection.

2.1.2 Solutions

(1) Improvement of gate structure.

ਆਮ ਤੌਰ 'ਤੇ, the length of the gate is too long, it will leave a long gate material on the surface of the plastic part, and the diameter of the gate is too large, it is easy to cause the phenomenon of drooling and dripping. When the above-mentioned failure occurs, the gate structure can be changed mainly. The common gate forms of hot runners are straight gate, point gate and valve gate.

The sprue gate is characterized by a relatively large diameter of the runner, so it is not easy to condense at the gate, which can ensure the smooth injection of the melt of the deep cavity product; it will not quickly condense, and the residual stress of the plastic part is minimized. It is suitable for forming a mold with multiple cavities Deep cavity products, but this kind of gate is more prone to drooling and wire drawing, and the gate residue is large, even leaving a columnar handle, so the material temperature at the gate should not be too high, and stable control is required;

The characteristics of the straight gate are basically the same as those of ordinary injection molding, but the residual marks on the plastic parts are relatively small; the characteristics of the point gate are that the residual stress of the plastic parts is small, the condensation speed is moderate, and the salivation and wire drawing are not obvious; it can be applied Most engineering plastics are also a type of gate form that is currently used in hot runner molding at home and abroad. The quality of the plastic parts is high, and there are only very small traces on the surface;

The valve gate has small residual marks and low residual stress, and does not produce drooling or wire drawing. ਹਾਲਾਂਕਿ, the valve port wears more obviously. In use, drooling will occur with the increase of the fitting clearance. ਇਸ ਸਮੇਂ ਤੇ, the valve should be replaced in time. Core and valve port body.The choice of gate type is closely related to the properties of the resin being molded. Low-viscosity resin that is prone to drooling, valve gate can be selected. The crystalline resin has a narrow molding temperature range, and the temperature at the gate should be appropriately higher. ਉਦਾਹਰਣ ਲਈ, POM, PPEX and other resins can be in the form of a gate with a heating probe. Amorphous resins such as ABS, ਪੀ.ਐਸ, ਆਦਿ. have a wide molding temperature range. Because the torpedo tip core forms a melt insulation layer, and there is no heating element contact at the gate, the condensation can be accelerated.

(2) Reasonable control of temperature.

If the amount of cooling water in the gate area is not enough, it will cause heat concentration, causing drooling, dripping and wire drawing. ਇਸ ਲਈ, the cooling of this area should be strengthened when the above phenomenon occurs.

• Resin releases pressure.

Excessive residual pressure in the runner is one of the main causes of salivation. Under normal circumstances, the injection machine should adopt a buffer circuit or a buffer device to prevent salivation. 2.2 Material discoloration or degradation

2.2.1 Main reasons

Improper temperature control; too small runner or gate size causes greater shear and heat generation; dead spots in the runner cause the retained material to be heated for too long.

2.2.2 Solutions

(1) Accurate control of temperature.

In order to accurately and quickly measure temperature fluctuations, the thermocouple temperature probe must be reliably contacted with the flow channel plate or nozzle wall and be located at the center of each independent temperature control zone. The distance between the temperature sensing point of the head and the flow channel wall It should be no more than 10mm, and the heating elements should be evenly distributed on both sides of the flow channel as far as possible.

The temperature control can choose the intelligent fuzzy logic technology under the operation of the central processing unit, which has the function of temperature over-limit alarm and automatic adjustment, which can control the temperature change of the melt within the required accuracy range.

• Correct the gate size.

Try to avoid the dead point of the runner, and appropriately increase the gate diameter within the allowable range to prevent excessive shearing and heat generation. The internally heated nozzle has a large temperature difference in the radial direction of the flow channel, which is more likely to cause scorching and degradation. ਇਸ ਲਈ, it is necessary to pay attention to the design of the radial size of the flow channel not to be too large.

2.3 Shortage of injection volume or no material injection

2.3.1 Main reasons

Obstacles or dead corners appear in the runner; the gate is blocked; a thicker condensate layer appears in the runner.

2.3.2 Solutions

(1) During the design and processing of the runner, it should be ensured that the melt flows to the arc transition of the wall at the corner, so that the whole runner is smooth and there is no dead corner of the flow. ਨੂੰ

(2) Properly increase the material temperature without affecting the quality of the plastic parts to avoid premature condensation of the gate. ਨੂੰ

(3) Appropriately increase the temperature of the hot runner to reduce the thickness of the condensate layer of the internally heated nozzle and reduce the pressure loss, so as to facilitate the filling of the cavity.

2.4 Serious leakage

2.4.1 Main reasons

The sealing element is damaged; the heating element is burnt to cause uneven expansion of the runner plate; the nozzle and the center of the sprue sleeve are misaligned, or the projection area of ​​the melt insulation layer determined by the stop ring on the nozzle is too large, causing the nozzle to retreat.

2.4.2 Solutions

(1) Check whether the sealing element and heating element are damaged. If there is any damage, carefully check whether it is the result of component quality, structure, or normal service life before replacement.

(2) Choose an appropriate way to stop leakage. According to the heat insulation method of the nozzle, two structures can be used to prevent leakage of the material, the leak-proof ring or the nozzle contact. Attention should be paid to keep the leak-proof contact parts in a reliable contact state. ਨੂੰ

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Within the allowable range of strength, it is necessary to ensure that the projection area of ​​the melt between the nozzle and the sprue sleeve is as small as possible to prevent excessive back pressure during injection to cause the nozzle to retreat. When the leak-stop method is adopted, the direct contact area between the nozzle and the sprue sleeve must ensure that no resin leakage occurs when the centers of the two are misaligned due to thermal expansion. ਹਾਲਾਂਕਿ, the contact area should not be too large to avoid increased heat loss.

2.5 The hot runner cannot be heated normally or the heating time is too long

2.5.1 Main reasons

Insufficient lead channel spacing, leading to breakage of the lead; short circuit, leakage and other phenomena when the lead intersects when assembling the mold.

2.5.2 Solutions

Choose the correct processing and installation technology, ensure that all wires can be placed, and use high-temperature insulating materials as required, and regularly check the damage of the wires.

2.6 Poor material change or color change

2.6.1 Main reasons

The method of changing the material or color is improper; the flow channel design or processing is unreasonable, resulting in a large amount of retained material inside.

2.6.2 Solutions

(1) Improve the structural design and processing method of the runner. When designing the runner, the dead point of the runner should be avoided as much as possible, and the arc transition should be sought at each corner. Within the permitted range, the size of the flow channel should be as small as possible, so that there is less retained material in the flow channel and the flow rate of new material is large, which is conducive to rapid cleaning.

When processing the flow channel, no matter how long the flow channel is, it must be processed from one end. If the two ends are processed at the same time, it is easy to cause the center of the hole to not overlap, which will inevitably form a retained material part. ਆਮ ਤੌਰ 'ਤੇ, the external heating nozzle can easily clean the flow channel because the heating device does not affect the melt flow, while the internal heating nozzle is easy to form a condensation layer on the outer wall of the flow channel, which is not conducive to rapid material change.

(2) Choose the correct refueling method. In the hot runner system material change and color change process, the new material directly pushes out all the retained materials in the runner, and then moves the retained materials on the wall of the runner forward as a whole, so the cleaning is easier to carry out. ਇਸਦੇ ਵਿਪਰੀਤ, if the viscosity of the new material is low, it is easy to enter the center of the retained material and separate the retained material layer by layer, which makes cleaning more troublesome. If the viscosities of the new and old materials are similar, quick material change can be achieved by speeding up the injection speed of the new material. If the viscosity of the retained material is more sensitive to temperature, the material temperature can be appropriately increased to reduce the viscosity to speed up the refueling process.

3. Precautions for selecting and applying hot runner

In order to eliminate or reduce the faults in use as much as possible, the following matters should be paid attention to when selecting and applying the hot runner system.

3.1 Selection of heating method

(1) Internal heating method. The internal heating nozzle has a relatively complicated structure, ਉੱਚ ਲਾਗਤ, difficult parts replacement, and high requirements for electric heating elements. Placing the heater in the middle of the flow channel will produce an annular flow, increase the friction area of ​​the container, and the pressure drop may be as much as three times that of the externally heated nozzle.

ਹਾਲਾਂਕਿ, since the internally heated heating element is arranged in the torpedo body in the nozzle, all the heat is supplied to the material, so the heat loss is small and the electric energy can be saved. If a point gate is used, the tip of the torpedo body is kept at the center of the gate, which is conducive to the gate cut after injection and the residual stress of the plastic part is lower due to the late condensation of the gate.

3.2 External heating method.

The use of externally heated nozzles can eliminate cold film and reduce pressure loss. ਇੱਕੋ ਹੀ ਸਮੇਂ ਵਿੱਚ, due to its simple structure, convenient processing, and the thermocouple installed in the middle of the nozzle to make temperature control accurate, it has been widely used in production at present. ਹਾਲਾਂਕਿ, the heat loss of the externally heated nozzle is relatively large, which is not as energy-saving as the internally heated nozzle. 3.2 Selection of gate form

The design and selection of gates directly affect the quality of plastic parts. When applying the hot runner system, the appropriate gate form should be selected according to the resin’s flow properties, molding temperature and product quality requirements to prevent drooling, dripping, material leakage and poor color change.

3.3 Temperature control method

When the gate type is determined, the control of melt temperature fluctuations will play a key role in the quality of plastic parts. In many cases, the phenomenon of burnt material, degradation or flow channel blockage is mostly caused by improper temperature control, especially for heat-sensitive plastics, which often require rapid and accurate response to temperature fluctuations. ਨੂੰ

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ਇਸ ਕਰਕੇ, the heating element should be set reasonably to prevent local overheating, ensure the matching gap between the heating element and the runner plate or nozzle, and minimize the heat loss. ਇੱਕੋ ਹੀ ਸਮੇਂ ਵਿੱਚ, the more advanced electronic thermostat should be selected as far as possible to meet the temperature control Require.

3.4 The calculation content after the structure of the hot runner system is determined

(1) Calculation of temperature and pressure balance of each branch. The purpose of the hot runner system is to inject the hot plastic from the nozzle of the injection molding machine through the hot runner at the same temperature and distribute the melt to the gates of the mold with a balanced pressure, so the temperature distribution of the heating zone of each runner should be dealt with. And calculate the melt pressure flowing into each gate.

(2) Calculation of the offset of nozzle and sprue bushing center caused by thermal expansion. ਜੋ ਕਿ ਹੈ, it should be ensured that the center lines of the hot (expanding) nozzle and the cold (no expansion) sprue bushing can be accurately positioned and aligned.

(3) Heat loss calculation. The internally heated runner is surrounded and supported by the cooling mold sleeve, so the heat loss caused by heat radiation and direct contact (conduction) should be calculated as accurately as possible, otherwise, the actual runner diameter will be due to the runner The wall condensate layer thickens and becomes smaller.

3.5 Installation of runner plate

Full consideration should be given to the issues of thermal insulation and withstand injection pressure. ਆਮ ਤੌਰ 'ਤੇ, spacers and supports are arranged between the runner plate and the template, which can withstand injection pressure to avoid material leakage due to deformation of the runner plate, ਅਤੇ ਦੂਜੇ ਪਾਸੇ, it can also reduce heat loss. ਨੂੰ

3.6 Maintenance of the hot runner system

For ਗਰਮ ਦੌੜਾਕ ਮੋਲਡ, it is very important to regularly perform preventive maintenance of hot runner components during use. This work includes electrical testing, inspection of sealing components and connecting wires, and cleaning of component dirt.

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