Causes and Solutions for Injection Molding Shortages in Injection Molding Production

 

 

Injection molding shortages are a common problem in the injection molding process. The following are the causes and corresponding solutions:

 

I. Injection Molding Process Parameters

 

Insufficient Injection Pressure

 

Causes:

 

Faulty hydraulic components in the injection molding machine's injection system, such as worn oil pumps or leaking valves, can prevent the injection pressure from reaching the set value, resulting in insufficient flow of the molten plastic in the mold cavity. Poor sealing of the injection cylinder can cause hydraulic oil leakage, affecting the injection pressure of the melt. Furthermore, insufficient understanding of the product requirements and plastic material properties, coupled with an excessively low injection pressure setting, can make it difficult for the melt to fill thin-walled, reinforcing ribs, or undercut areas in complex mold shapes.

Solutions:

 

Use professional equipment to test the injection system pressure. Repair or replace worn oil pumps, leaking valves, and other faulty components. Adjust the injection pressure appropriately, referring to material data sheets and trial molding results to determine the optimal value, and gradually increase the pressure while observing the molding effect. Regularly check the injection cylinder seals and replace worn or aged seals promptly.

 

Injection Speed ​​Too Slow

 

Causes:

 

Injection speed settings are related to product wall thickness and flow path length. Thin-walled products require rapid melt injection; a slow speed will cause the melt to cool too quickly during mold filling, increasing viscosity. A malfunction in the injection speed control system of the injection molding machine, such as a problem with the control circuit, abnormal motor speed, or a faulty transmission component, can also lead to a slow speed. Additionally, an excessively small mold gate size or a narrow runner diameter may limit the injection speed, resulting in insufficient melt flow within the cavity.

 

Solutions:

 

Adjust the injection speed according to the product wall thickness and flow path length. Increase the speed appropriately for thin-walled products and optimize through trial molding. Inspect the injection speed control system and repair or replace faulty components such as circuit boards, drive belts, or gears. Optimize the mold gate and runner design, increasing the gate size or runner diameter, while coordinating other process parameters.

 

Insufficient Holding Pressure or Insufficient Holding Time

 

Causes:

 

Insufficient holding pressure may be due to incorrect pressure settings in the holding system or a malfunction in the hydraulic components of the holding circuit. This results in insufficient compensation for melt shrinkage during cooling, leading to voids or depressions in thick-walled or final-filled areas of the product. The holding time depends on the product thickness, size, and thermal shrinkage characteristics of the plastic material. Insufficient holding time means the melt does not adequately compensate for shrinkage. Improper control of the holding pressure switching point can also cause material shortages. Premature switching before the cavity is fully filled can lead to material shortages.

 

Solutions:

 

Check and readjust the holding system pressure settings, referring to experience with similar products or material supplier recommendations. Check the hydraulic components of the holding circuit to ensure they are functioning correctly. Appropriately extend the holding time based on product characteristics, paying attention to coordination with other process parameters. Check the holding pressure switching point control parameters and sensors, optimizing settings by observing melt flow or using an analysis system.

 

 

Low Melt Temperature

 

Causes:

 

The barrel heating temperature is set too low, the operator is unfamiliar with the material properties, or the parameters are incorrectly adjusted. Barrel heating element malfunction, such as a damaged heating rod or a failed thermocouple, affects accurate barrel temperature control. Excessive residence time of plastic in the barrel, such as during production stoppages, excessively slow screw speeds, or a mismatch between barrel capacity and injection volume, leads to excessive heat loss.

 

Solutions:

 

Understand the processing temperature range of the plastic material, set the barrel heating temperature appropriately, and gradually adjust while observing the plasticizing state and product quality. Regularly inspect the heating elements, replace damaged heating rods, calibrate or replace inaccurate thermocouples, and check the wiring. Optimize the injection molding process, reduce the residence time of plastic in the barrel, select an appropriate barrel capacity, adjust the screw speed, and implement heat preservation measures during production stoppages.

 

II. Mold-related aspects

 

Gate size too small or blocked

 

Causes:

 

An excessively small gate size may be due to insufficient consideration of material flowability and product requirements during the mold design stage. This results in large shear forces and pressure drops when the melt passes through the gate, reducing the flow rate, especially noticeable for high-viscosity plastics or large products. During injection molding, the gate is easily clogged by impurities, plastic debris, or incompletely melted particles. Impurities may originate from the raw materials or the injection environment, while plastic debris may be generated by melt erosion or mold wear.

 

Solutions:

 

Determine the gate size appropriately during mold design, and optimize it through simulation software. For pre-manufactured molds, if the gate size is too small, it can be appropriately enlarged, taking care to avoid other defects. Clean clogged gates promptly, using specialized tools to remove impurities and debris to avoid damaging the gate. Add a fine filter at the hopper to maintain a clean injection environment, and optimize injection process parameters to reduce unmelted particles entering the gate.

 

Inadequate Runner System Design

 

Causes:

 

Excessively long runners increase melt flow pressure loss. When the melt flows through a long runner, the pressure decreases due to friction and viscous resistance, potentially leading to material shortages at the thin-walled or complex structural parts of the cavity. Insufficient runner diameter limits melt flow rate and velocity. According to fluid mechanics principles, the smaller the pipe diameter, the greater the resistance, which has a more significant impact on high-viscosity plastics or high-speed injection molding processes. Rough runner surfaces increase melt flow resistance, potentially caused by insufficient machining precision, mold wear, or corrosion. Uneven temperature control in the hot runner system leads to inconsistent melt viscosity, affecting flow balance.

 

Solutions:

 

Shorten runner length and optimize layout during mold design, employing a balanced runner design. For long runner molds, auxiliary runners can be added or the runner direction can be changed. Determine the runner diameter reasonably based on the plastic material's fluidity and injection volume, referring to supplier recommendations or simulation software analysis. Modify excessively small runners, ensuring coordination with other parts of the mold. Polish the runners using mechanical, chemical, or electrochemical polishing methods, strictly controlling machining precision. Repair or re-machine worn or corroded runners. Regularly inspect the heating elements and temperature sensors of the hot runner system, optimizing the structural design by using zoned heating and adding thermal compensation devices.

 

Poor Venting

 

Causes:

 

If air cannot be expelled from the cavity during injection molding, it will occupy space and hinder melt filling. Compressed air forms high-pressure areas, especially accumulating in blind holes, deep grooves, and reinforcing ribs, leading to material shortages. Inadequate mold venting design, such as insufficient venting channels or unreasonable channel location, size, and quantity. Injection process parameters also affect venting effectiveness; excessively fast injection speed or pressure prevents air from escaping or traps it within the cavity.

 

Solutions:

 

During mold design and manufacturing, rationally design venting channels, creating venting grooves at appropriate locations within the cavity, determining their depth, width, and length. Special materials such as permeable steel can be used, or venting can be achieved through the parting line or ejector pin gaps, paying attention to the gap size. Optimize injection process parameters, appropriately reducing injection speed and adjusting injection pressure. Observe product quality during trial molding to optimize parameters and venting design. For large or complex molds, vacuum venting can be used.

 

III. Plastic Raw Materials

 

Poor Plastic Flowability

 

Reasons:

 

The molecular structure and rheological properties of different plastics determine their flowability. Some plastics filled with large amounts of inorganic fillers or high-performance engineering plastics have high melt viscosity; fillers or reinforcing materials interfere with molecular chain movement. Plastic moisture affects flowability; hygroscopic plastics, after absorbing too much water, vaporize at high temperatures, producing bubbles that affect intermolecular forces. Uneven size and distribution of plastic particles, as well as irregular particle shapes, can lead to inconsistent melt viscosity and increased flow resistance.

 

Solutions:

 

Select plastic raw materials with good flowability. If using high-viscosity plastics, choose modified varieties. Thoroughly dry highly hygroscopic plastics, using drying equipment to control humidity within a suitable range. A dryer can be installed at the injection molding machine hopper, or the raw material can be pre-dried. Screen or replace raw materials with uneven particle size to ensure relatively uniform particle size and shape.

 

A table of common injection molding material flowability standards, expressed as the ratio of maximum filling length (L) to cavity thickness (T), L/T:

 

IV. Equipment Aspects

 

Screw Wear

 

Causes:

 

The screw is a key component for plasticizing and conveying plastic. After long-term use, the screw threads, diameter, and other parts may wear. A worn screw cannot effectively shear and convey the plastic, leading to uneven plasticizing, decreased melt quality, and affecting the molding quality of the product, resulting in material shortages.

 

Solutions:

 

Regularly check the screw wear. Replace the screw promptly when wear reaches a certain level. The degree of wear can be assessed by measuring key screw dimensions, such as thread height and diameter changes. Simultaneously, select high-quality, wear-resistant screw materials, such as alloy steel. During operation, avoid prolonged idling of the screw under high temperature and pressure. Set the screw speed and injection cycle appropriately to reduce unnecessary wear.

 

 

Barrel Wear or Uneven Temperature

 

Causes:

 

Wear on the inner wall of the barrel alters the flow state of the plastic within the barrel, causing uneven melt flow. Malfunctions in the barrel's heating system, such as aging heating rods or thermocouple failure, can lead to uneven temperature, preventing some plastic from fully plasticizing and affecting melt quality.

 

Solutions:

 

Inspect the barrel for wear. Replace or repair severely worn barrels using methods such as welding repair or internal lining repair. Regularly calibrate the barrel's temperature sensor to ensure accurate temperature control and uniform temperature throughout the barrel. Replace aging heating rods promptly and check the circuit connections of the heating system to ensure there are no open circuits or poor contacts.