When finished products exhibit various defects, many novice technicians' first reaction is usually to adjust equipment operating parameters—increasing injection pressure, slowing down the overall injection speed, or even directly raising barrel heating temperature.
However, in practice, it becomes clear that root cause of most defects lies within mold itself. Simply adjusting machine parameters can only temporarily stabilize production for a short period and cannot completely solve problem. Scope of stable production is also limited, and over time, it can easily cause new damage to previously intact mold.

 

Flash is a very common defect encountered in daily production. Many people have a rather one-sided view, always believing that flash is only caused by insufficient clamping force. In reality, there are many other contributing factors.
On-site preliminary assessment: After demolding, product surface has flash defects. First, identify specific location of flash and observe its actual thickness. If fine flash appears on outer ring of the entire product, refer to injection molding machine's tonnage specifications and check for any abnormal changes in production process parameters. If flash only appears in a localized area or on a single side, and flash root thickness is unusually large, there's no need for repeated guesswork. It's highly likely due to an abnormal fit between mold surfaces, or the overall mold's clamping strength not meeting production requirements.
General troubleshooting steps: After discovering flash on produced product, first use measuring tools to check flash thickness to confirm whether fault is widespread or localized. If it's a localized, single-sided flash: Use red lead trial molding to check the parting line location, identifying areas of poor fit and gaps. Then check for wear and tear on internal guide pillars and whether slider locking structure has shifted backward. Determined to be a thick-layer localized flash: Carefully inspect support pillars at the bottom of mold core to check for any suspended positions or deformation/bending after prolonged use.
Actual Causes of Fault: Local depressions on parting surface, coupled with surface damage from compression, are the most frequent issue encountered in daily production. These are mostly caused by plastic debris left over from production process, along with some hard impurities that haven't been cleaned up in time. During mold closing, these debris gets trapped in parting surface gap, forcibly squeezing and deforming originally tightly fitted area. Insufficient support at the bottom of mold core can easily lead to sinking and collapse in the middle of moving mold.
Many molds, in their initial manufacturing to save time, have too few support pillars installed under large base plate of moving mold, and some support pillars are at a height inconsistent with mold's square iron, leaving them suspended.
During production, mold needs to withstand extremely high injection pressure. This causes slight elastic bending deformation in the middle area of moving mold, naturally creating gaps. Molten plastic overflows through these gaps, resulting in thick flash. Angle of slider locking block is mismatched. When slider completes locking operation, angle of locking block itself needs to be two to three degrees greater than angle of guide post to meet acceptable standard.
Once locking block wears down, fastening screws loosen or shift, and there are errors in angle machining during initial processing, slider will gradually move backward under external pressure during injection molding. This easily leads to noticeable thick flash at joint between sliders.
On-site practical solutions: Red lead bonding inspection method: First, reduce machine's clamping force to about 30% of the standard force. Apply a thin, even layer of red lead oil to mold parting surface. After closing mold, open it and check for any remaining marks on the surface. Any area without red lead marks indicates that mold is sunken or subsided, meaning mold's fit is not tight enough.
To address this, shims are added to support pillars to adjust their height. After confirming that subsidence of moving mold is causing flash in the middle area, a dial indicator is used to precisely measure subsidence. Mold frame structure is then disassembled, and small precision shims are added to the bottom of support pillars.
After adjustment, ensure support pillar height is 0.03mm to 0.05mm higher than square iron, allowing for a small pre-compression buffer space.
After welding parting surface, a fine mold fitting process is performed. For areas with sunken or damaged areas, welding rods of same material as mold body are used, and argon arc welding is employed for precise repair.
After welding, mold is placed on a machine tool for simple milling, leaving a machining allowance of approximately 0.1mm. Subsequently, using an oilstone and red lead material, fit is manually and slowly adjusted until contact area of parting surface reaches at least 85% to meet normal usage standards.

Shrinkage is a notoriously troublesome defect during on-site work. Many operators attempt to fix it by increasing holding pressure, but often shrinkage issue isn't properly addressed, leading to new problems like whitening at the top and strong adhesion to mold surface in other areas of product.
To address these issues, adjustments must be made to two main systems: gating system (where raw material is delivered), temperature control and cooling system.
Actual Cause of Failure: Premature cooling and solidification at gate area: After plastic raw material is heated and melted, it is injected into mold cavity. Filling voids created by cooling and shrinkage relies entirely on continuous material replenishment during holding pressure stage.
If gate is too thin or narrow, plastic at gate will solidify prematurely before holding pressure can be effectively transmitted to product's internal cavity. Subsequent holding pressure will then be unable to reach product's interior.
High internal runner resistance, whether trapezoidal or semi-circular, with excessively rough surfaces or excessively long runners, leads to significant pressure loss as molten plastic flows to end of cavity. In severe cases, this pressure loss can account for more than half of total pressure.
Poor heat dissipation in localized areas results in persistently high temperatures. Excessive wall thickness in certain areas prevents corresponding mold from accommodating sufficient cooling water channels, hindering temperature reduction in these areas compared to other parts of mold.
After outer plastic layer cools and sets, core material gradually shrinks and deforms, pulling surface material inward and creating a clearly visible indentation defect.
On-site troubleshooting and repair methods: Adjust gate specifications and dimensions appropriately. For products exhibiting significant shrinkage, first measure and confirm actual wall thickness.
For commonly used point gates and submarine gates, a thickness of 60% to 70% of the overall product wall thickness is most suitable. If dimensions do not meet usage standards, use EDM equipment or small grinding tools to appropriately widen and deepen gate size. As an aside, it's better to machine gate wider and thicker than to arbitrarily reduce the overall length of runner.
Clean and unclog cooling water channels, checking smooth operation of cooling water channels at mold points corresponding to product shrinkage.
If temperature difference between inlet and outlet exceeds 5℃, it indicates that water flow rate inside channels is too slow, or that a large amount of scale and impurities have accumulated over time. Use a dedicated descaling solvent to circulate and clean channels. For mold areas with thicker walls and poor heat dissipation, you can also install inserts with better thermal conductivity to quickly dissipate excess heat, alleviate problem of localized high temperatures.

Blackened, scorched marks appear at product ends. Small air bubbles, unable to escape properly, frequently remain inside transparent finished product. Ultimately, all these malfunctions are caused by inability of gas inside mold cavity to escape normally.
Gas in a confined space is subjected to both high temperature and high pressure, causing a rapid temperature rise in a short time. This not only causes deterioration and damage to plastic raw materials but also abrades and corrodes steel material on mold surface.
Reference Table for Trapped Air and Scorching Faults

Key Points for On-Site Rectification: Adjust venting groove specifications according to raw material type. Different plastic materials require different standards for venting groove depth.
For commonly used polypropylene and polyethylene materials, venting groove depth should be controlled between 0.02mm and 0.03mm; for ABS and polycarbonate materials, depth should be controlled between 0.03mm and 0.04mm; for nylon materials, which are prone to flash defects, venting groove depth must be strictly controlled within 0.015mm.
After extending venting channels around cavity outwards by 3-5mm, width is adjusted to 0.5mm to 1mm to ensure that gas inside cavity can be smoothly discharged into external air environment.
For gas buildup at the bottom of deep ribs, modify ejector pin components for venting. Remove ejector pin at corresponding installation point and use a grinding tool to symmetrically grind a thin, flat surface around outer ring of ejector pin, extending 20mm downwards from tip. This modification method is relatively low-cost and a practical solution for handling deep cavity gas accumulation issues on-site.
For complex, tricky, and difficult-to-vent areas, simply cut grooves in corresponding locations in mold and insert venting steel components. These steel components have a network of tiny, permeable micropores, allowing gas to escape through them without leakage of molten plastic.
During daily production, be extremely careful not to spray rust-preventive grease or release agents onto the surface of venting steel, as this can easily block internal micropores and render venting function unusable.

 

Molds are fixedly assembled on injection molding machines for continuous production, constantly in a dynamic reciprocating motion. Each mold opening and closing action generates significant mechanical impact forces. Jawed moving parts and broken or damaged components are major causes of production line downtime.

Sudden jamming and malfunction of slider during its push-pull movement, accompanied by frequent burrs, scratches, and wear marks on sliding track surface, are mostly caused by inadequate routine lubrication and maintenance, uneven stress distribution on components, and insufficient clearance between components.
Actual Causes of Fault: A hard impact occurs between inclined guide post and pre-drilled mounting holes on slider. When inclined guide post is inserted normally into slider hole, if elastic spring at the bottom of slider responsible for positioning and fixing fails, it loses its fixing and limiting force. Under influence of machine vibration and its own weight, slider is prone to sliding forward and deviating from its original position.
During the subsequent mold closing process, inclined guide post cannot accurately align with hole, resulting in a strong impact on slider body structural components. Thermal expansion and deformation can cause jamming. When producing products such as nylon and fiber-reinforced plastics that require higher mold temperatures, exposed area of slider dissipates heat quickly and has a lower overall temperature, while temperature inside mold core is relatively higher. Originally reserved 0.02mm to 0.03mm single-sided clearance disappears due to different heating rates of various components, leading to sliding damage and complete jamming.
On-site inspection and repair procedures: After shutdown, prioritize a reset safety check. Manually complete mold opening process, carefully inspecting slider positioning screws and elastic positioning components for breakage, damage, or loss of elasticity.
For sliders with an inverted top-bottom structure, relying solely on springs for positioning and fixation is insufficient; additional mechanical locking components are necessary to ensure smooth and reliable production operation. To repair scratched or worn surface structures, avoid using coarse sandpaper to sand track surface if scratches are visible. Use a fine-grained oilstone dipped in machine oil lubricant, gently sand down any raised burrs and edges along normal sliding direction of the slider. Simultaneously check hardness parameters of wear-resistant components. Standard wear-resistant blocks maintain a Rockwell hardness range of 52 to 56, with an overall hardness two to three degrees higher than slider body. This hardness difference reduces wear and tear between components.
Re-manufacture lubrication oil reservoir structure. If wear-resistant parts do not have pre-existing oil reservoirs, disassemble them and mill cross-shaped or diamond-shaped oil reservoirs using machining tools. Groove depth should be controlled to 0.5mm. After machining, grind edges of grooves smooth and rounded. Remember not to machine straight-through oil reservoir structures. During back-and-forth sliding of slider, internal lubricating grease will be quickly lost and consumed, failing to achieve actual effect of long-term lubrication and protection.

During product demolding and ejection process, a sharp and piercing abnormal sound is emitted. Ejector pin components show bending and deformation, and in severe cases, they become stuck inside mold hole and cannot move.
These types of failures are basically caused by poor balance performance of ejector plate during back-and-forth movement and significant temperature differences between different areas of mold.
Actual cause of malfunction: After prolonged use, ejector guide sleeves experience wear and tear. The entire ejector mechanism relies on four guide pillars for stable movement. Once guide sleeve wears down and clearance increases, ejector pins pushing ejector plate are prone to skewing and shifting, causing all ejector pins to tilt and move along with it. Small ejector pins with a diameter of less than 1.5mm, when forced to eject finished product while skewed, have a significantly increased probability of breakage.
When ejector pins are heated, they undergo unilateral bending deformation. Hot water circulating inside mold core raises the overall mold temperature. Since ejector pins themselves lack dedicated heat dissipation and cooling structures, repeated sliding friction will continue to increase temperature, leading to thermal expansion. If mating section between ejector pin and mold hole is designed to be too long, under normal operating conditions, an effective mating length of 15 to 20mm is sufficient. Without sufficient clearance for movement, ejector pins can easily become stuck inside hole after thermal expansion.
Practical Solutions on-site: For small ejector pins that frequently jam, after disassembling mold core components, use a tool 0.5mm to 1mm larger than ejector pin diameter from back of mold to enlarge hole. Only retain a sealing fit section of about 15mm on the surface of cavity; enlarge hole in remaining areas to leave room for movement. This avoids material leakage and flash during production and provides sufficient room for ejector pin to move due to thermal expansion and slight deformation.
Add auxiliary elastic spring components to return pins to prevent ejector plate from performing mold closing action before fully returning to its initial position, which could damage mold cavity structure. Springs with matching spring force parameters should be installed on all four return pins. After springs are assembled, pre-compression length should ideally be 10% to 15% of their free length. With spring force, ejector plate can autonomously complete most of return stroke after mold opening.

Flow channels inside mold that transport plastic raw materials are a core component in producing qualified finished products. Cooling water circuit is a crucial foundation for ensuring stable daily production and reducing probability of malfunctions. When these two systems malfunction, it's difficult to detect them directly from surface. Accurate troubleshooting requires data obtained through actual measurements.

During daily use of cooling water circuit system, two main types of malfunctions are frequently encountered. First is accumulation of thick scale and dirt over time, significantly reducing actual cooling effect. Second is leakage and cross-contamination at water circuit connections and sealing points.
Simple On-site Troubleshooting Method: Temperature Difference Assessment: Use a temperature measuring instrument to measure actual temperature values at inlet and outlet.
In standard product manufacturing processes, water temperature difference should be controlled within 1℃ to 2℃. For products with higher precision requirements, temperature difference should be controlled within 0.5℃. If temperature difference exceeds 5℃, it generally indicates a blockage in water system, or the overall water supply flow may not meet production requirements.
To assess water pressure difference, install pressure gauges at both inlet and outlet. If inlet pressure reaches 0.4MPa, but outlet pressure is less than 0.1MPa, it indicates severe blockage in certain areas of water system. It could also indicate an excessively long overall water circuit design, resulting in insufficient water flow.
On-site rectification methods: Clean water system of scale and impurities safely and compliantly. Never pour strong acid directly into water system for cleaning, as this can easily corrode and damage mold steel and sealing rings. Prepare a standard concentration of specialized neutral descaling solvent and use a small water pump to circulate solvent within water system for one to two hours.
After cleaning, neutralize any remaining chemicals inside pipes with an alkaline agent, use high-pressure airflow to dry any remaining moisture, finally inject anti-rust grease for protection. For emergency handling of leaking seals, leaks in molds are often not due to substandard seal quality, but rather to misalignment of seal installation groove dimensions during fabrication.
Reference standards for seal assembly: For movable parts, seal compression should be between 10% and 15%; for fixed sealing points in mold, seal compression should reach 20% to 25%. If installation groove is cut too deep, seal may not fit properly, leading to leaks. In an emergency, a thin layer of sealing material can be placed at the bottom of groove before assembling and securing seal.

Molds equipped with hot runner systems generally have higher power consumption. Common faults fall into two categories: one is burnt-out or damaged internal heating components; the other is molten plastic leaking out through gaps and accumulating in various corners inside mold.
On-site Troubleshooting and Repair Procedures: For power-on tripping and leakage fault troubleshooting, immediately trip power supply as soon as hot runner temperature control equipment is turned on. First, unplug power cord and use professional testing instruments to measure insulation resistance between heating components and mold grounding terminal. If value is below two megohms, temperature control equipment will automatically activate trip protection mechanism.
Most of these faults are caused by moisture seeping into equipment during mold cleaning operations, combined with damp and cold workshop environment, causing heating elements to absorb moisture and leading to problems. Temporary on-site handling can be achieved by switching to low-pressure dehumidification mode, relying on a small amount of heat generated by a weak current to gradually dry and remove accumulated moisture. Once insulation value returns to acceptable standard, it can be switched back to regular automatic temperature control mode for operation.

 

During mold opening and closing process, a dull impact sound can be heard. Processed products have a large deviation in thickness on one side, and the overall concentricity does not meet qualified standards specified in production process.
Actual Causes of Fault: Slight deformation problems can easily occur during mold hoisting, handling, and prolonged static placement. Moving mold with hoisting equipment, or placing mold on machine table for a long time without a level surface, coupled with large weight of mold itself, will cause guide pillars to be subjected to unilateral extrusion forces for a long time.
Lacking precise positioning auxiliary structural components, many mold manufacturing stages rely solely on four circular guide pillars for mold opening and closing guidance and positioning. These guide pillars only meet basic, rough guiding requirements. Lateral extrusion forces generated during injection molding are often high, especially in molds with single-sided injection structures. Circular guide pillars have limited load-bearing capacity and are prone to bending deformation, leading to severe wear and tear on one side after prolonged use.
On-site troubleshooting and corrective measures: Diagonal dimension measurement and verification: Measure distance between any two pairs of diagonal dimensions of four guide pillars. If difference between two sets of diagonal dimensions exceeds 0.05mm, it indicates that mold base has undergone diamond-shaped twisting deformation. Replace severely worn guide pillars and bushings with brand new parts. After replacement, recalibrate center positions of moving mold and fixed mold core. In addition, install square precision positioning blocks around mold parting surface to distribute lateral extrusion impact forces generated during production, allowing circular guide pillars to only perform basic guiding functions and no longer bear lateral extrusion load pressure.

Product inexplicably develops flash defects, which are not improved even when clamping force is adjusted to maximum setting. Actual measurements of the overall mold height reveal a noticeable decrease in height of central area.
Actual Causes of Fault: Prolonged high-intensity clamping and impact during production, coupled with the use of steel with low hardness in mold core, or a rough, uneven surface on base plate with obvious tool marks or small raised spots, can cause mold core to gradually press against mold base plate, creating indentations. This type of fault is commonly referred to in industry as "mold core sinking."
On-site Solution: Re-grind and smooth base plate structure. Disassemble the entire problematic mold base plate, use a surface grinder to grind and smooth it, eliminating surface indentations and machining residue. For severe cases of mold base plate denting and deformation, after grinding and smoothing, install a high-temperature hardened precision steel shim at the bottom of mold core mounting slot. This shim absorbs and distributes all extrusion impact force generated during subsequent production operations, preventing mold base plate from denting and deforming again at its source.

Faced with continuously operating injection molding equipment that consistently produces defective products, this simple four-word troubleshooting approach allows for practical application, clarifying general direction of fault within minutes.

Inspect actual condition of broken or fractured parts of product. When product shows whitening or cracking at the top, carefully observe true appearance of fracture surface.
A whitish fracture surface usually indicates product is tightly adhered to mold surface, causing excessive pulling force during ejection. A clean fracture surface without other colored marks generally indicates excessive ejection speed or use of ejector pins with too small a size, resulting in instantaneous impact that shatters product.
Check for residual oil stains on parting surface. After opening mold, observe the entire parting surface area. Large areas of shiny oil stains indicate excessive lubricating grease used in daily ejector pin application. During high-pressure mold closing, grease is squeezed and flows to parting surface. Excessive oil buildup can also clog small venting channels, leading to defects such as localized scorching and blackening in produced product.

If there are jamming or unusual noises during mold opening and closing, first check if slider core-pulling structure has fully returned to its original position and whether inclined guide pillar components are cracked or damaged. A sharp, grinding sound during product ejection indicates a lack of lubrication between ejector pins and mold cavity, leading to dry friction and potential damage to components.
After removing ejector plate, apply high-temperature resistant lubricating grease to base of ejector pins for maintenance. A noticeable vibration sound during mold closing phase could be due to improperly adjusted low-pressure mold closing protection parameters, misalignment of mold positioning components, or forced impact after contact, resulting in abnormal vibration and noise.

 

Inquire about form and state of malfunction, and consult with operators whether such problems suddenly appear or gradually worsen during production process. Sudden malfunctions are mostly caused by damaged parts, broken elastic springs, loose fastening screws, or blockage of gate by debris. Malfunctions that gradually worsen are generally caused by gradual increase in mold operating temperature, gradual accumulation of scale and dirt in water channels, gradual blockage of venting channels by oil, gradual wear and tear of various guide parts.