Injection molding defect analysis countermeasures

1. Purpose of lecture

Be familiar with types of injection molding defects
Be familiar with factors affecting injection molding defects
Improve ability to comprehensively analyze injection molding defects and ability to analyze causes
Seek key countermeasures to solve injection molding defects, improve quality and efficiency

2. Importance of analysis of injection molding factors and improvement measures

Improve quality of plastic parts, prevent major plastic parts quality accidents and improve corporate competitiveness
Solve problem of plastic molding, avoid delivery failure or delivery delay
Reduce machine adjustment costs
Reduce material consumption
Reduce defect rate
Improve machine utilization

3. Examples of major plastic parts quality accidents

A company produces toasters. When they were produced and shipped, goods were normal. A few months later, customers complained: 80% of 200,000 toasters were broken:
After investigation: Stress cracking occurred when PC material was used to mold toaster shell.
Cause analysis: PC material sticks to film, and release agent penetrates into PC molecular chain, causing PC molecular chain to break.
Result: 160,000 toasters were shipped back from overseas, disassembled and reassembled, and huge losses were compensated.

4. Types of injection molding defects

Uneven glue flow, shrinkage, shrinkage, bubbles, material flowers, water mouth air marks, flow marks, injection marks, trapped air, burning, black spots, fouling, top height and top depression, top white, white pull, mold sticking, drag, color deviation, different gloss, color mixing, flashing, floating fiber, warping, size deviation, bursting, cold glue, peeling, water line

5. Factors affecting molding defects

6. Steps to deal with molding defects

7. Systematic verification of molds

This chapter details a systematic verification process for molds to expedite identification of the best corrective methods.
material
Before starting verification process of any mold, you must first have information about set of molds, plastic materials and injection molded parts, and understand purpose of verification.
Step
Mold verification process should be implemented according to flow chart above.
Local fill
It is very important to produce local filling parts before each inspection and analysis of cause of defect. During this process, we must ensure that no molten material can enter mold cavity during pressure holding stage, and pressure transfer point should also be visually inspected. In order to avoid adjusting pressure transfer point too high, setting value of holding pressure should be adjusted lower. In addition, there must be sufficient plasticization delay time to ensure that no molten material enters mold cavity during storage.
Where there are surface defects and sudden changes in diameter, proportion of local filling should be reduced to obtain a clearer flow path.
In addition, pressure transfer point or storage stroke must be marked on corresponding partially filled injection molded part, thereby tracking position of flow peak. In order to ensure that injection molded parts can be pushed out by ejector pin, sufficient filling volume must be ensured when producing partial injection molded parts.
Injection stage
In order to obtain stable surface quality of injection molded parts, speed of flow crest should be kept stable. Speed of flow crest is filling time of flow crest travel distance.
Under a stable screw linear speed setting, melt is injected into mold cavity with an uncertain flow rate.

In Figure 2.1, since diameter of flow crest near and far from gate is "smaller", flow crest speed here is higher than that in the middle. In order to achieve a more stable speed, injection speed curve should be set to "slow-fast-slow". After setting diameter and position of flow crest, linear speed of screw must be coordinated to ensure that speed of flow crest remains unchanged.
Transition point
Transition point is change time from injection stage to holding stage. To avoid over-injection and mold damage, transition point should be set when filling amount reaches 98% of the total.
When amount of melt stored is affected by any parameter change, setting of pressure point must be rechecked:
·Pressure (compressing melt)
·Pressure release (changing amount of melt in the front space of screw)
·First injection section (closing of stop valve)
Effective holding time
Effective holding time can be found from relationship curve between weight of injection molded part and holding time.
First, a certain number of injection molded parts are produced using same holding pressure and different holding times, then weight of these injection molded parts is measured to find relationship curve between weight of injection molded part and holding time. Weight of injection molded part will initially increase with holding time, but after reaching a certain holding time, weight will become stable. Time of this transition is "effective holding time". This ensures that no molten plastic can flow back into gate system or into space at the front end of screw, causing deviations in weight and size of injection molded part.

Cooling time
Based on economic considerations, cooling time should be minimized as much as possible. Cooling time for each material is different. You can refer to data obtained using a calculation program (for example: WinCool by Kunststoff-institutLvdenscheid) or make judgments based on experience. Parameters that affect cooling time include material thickness, mold wall temperature and demolding temperature.
It is worth noting that mold wall temperature (i.e. mold wall temperature during production process) has a great influence on cooling time. A 10℃ change in mold wall temperature will change cooling time by 20%. In addition, demolding temperature or mold wall temperature will also affect shrinkage of molded part. Therefore, size of mold can only be modified according to molded part produced under corresponding injection molding cycle.
Processing window
Quality of injection molded parts is only guaranteed within a "certain" parameter setting range. This "certain range" is often called processing window. Only when parameters are set within processing window can injection molded parts with a low scrap rate be produced. Cause of scrap is error of raw materials, injection molding machines and peripheral equipment.
If there is a problem with quality of injection molded parts during production process, the first thing to do is to check injection molding machine and various parts of mold to ensure processing temperature, check drying of materials and compare actual data of set values of each parameter.
Steps to convert parameters
When following following correction method for optimization, only one parameter can be changed at a time and recorded immediately. Especially when changing melt temperature and mold wall temperature, if you want to evaluate injection molded parts, you must first determine that temperature has reached required set value during production process.
Above method is called allowable "error" method to set injection molding process.

8. Cause analysis and countermeasures of molding defects

8.1 Analysis and countermeasures of uneven glue flow

Cause: Due to insufficient glue or excessive resistance, molten glue cannot completely fill mold cavity.
Common locations: far away from gate, trapped air or thin wall thickness.
Incompletely Filled Parts
Injection material cannot completely fill the entire mold cavity. This defect usually occurs at a location far from gate, such as when runner is too long or near a thin wall (such as a rib). This defect also often occurs at other locations due to poor mold venting.

Analysis and countermeasures of uneven glue flow

Product defects	Causes	Countermeasures
Uneven glue flow	1. Insufficient amount of melt glue	Increase amount of melt glue
	2. Wear of screw rubber ring	Replace rubber ring
	3. Blockage of nozzle	Clean nozzle
	4. Injection pressure is too low	Increase pressure
	5. Injection speed is too slow	Increase injection speed
	6. Injection time is too short	Extend injection time
	7. Improper pressure holding switch position	Adjust position
	8. Barrel and nozzle are too low	Increase temperature
	9. Mold temperature is too low	Increase mold temperature
	10. Low back pressure	Increase back pressure

8.2 Analysis of shrinkage causes and countermeasures

Cause: When molten glue turns into solid, volume of thick part shrinks slowly, forming tensile stress. If surface hardness of product is not enough and there is no glue to supplement, surface of product will be pulled down by stress
Common parts: thick area of product, such as the junction between reinforcing rib or column and surface of product.
Sink marks
Mostly appear in area where plastic gathers
Injection molded parts cannot be compensated due to pressure loss, and plastic shrinkage caused by heat.

Analysis and countermeasures of shrinkage causes

Product defects	Causes	Countermeasures
Shrinkage	1. Insufficient amount of melt	Increase amount of melt
	2. Screw rubber ring wear	Replace rubber ring
	3. Nozzle blockage	Clean nozzle
	4. Injection pressure is too low	Increase pressure
	5. Improper injection speed	Adjust injection speed
	6. Holding time is too short	Extend holding time
	7. Improper holding switching position	Adjust position
	8. Barrel and nozzle temperature is too high	Adjust temperature
	9. Mold temperature is too high	Lower mold temperature
	10 . Improper gate position	Gate is set at thick part
	11. Unbalanced gate	Check gate size of multiple cavities in one mold
	12. Runner or gate is too small	Increase runner or gate
	13. Insufficient water transport	Increase number of water transport groups
	14. High hydraulic oil temperature	Clean hydraulic oil heat exchanger
	15. Poor raw material fluidity	Replace raw materials with good fluidity
	16. Too thick glue position	Reduce glue or nitrogen-assisted molding
	17. Large glue position thickness ratio	Reduce thickness ratio
	18. Long process	Add glue ports

Countermeasures

8.3 Analysis and countermeasures of shrinkage holes

Cause: When molten glue turns into solid, volume of thick part shrinks slowly, forming tensile stress. If surface hardness of product is not enough and there is no glue to supplement, stress holes will form inside product.
Common parts: thick area of product, such as intersection of reinforcing rib or column and surface of product:
Voids
Voids, for example: appear where a large amount of material accumulates, caused by condensation shrinkage of plastic not being compensated.

Cause analysis and countermeasures of shrinkage cavity

Product defects	Causes	Countermeasures
Shrinkage Holes	1. Insufficient amount of melt	Increase amount of melt
	2. Screw rubber ring wear	Replace rubber ring
	3. Nozzle blockage	Clean nozzle
	4. Injection pressure is too low	Increase pressure
	5. Improper injection speed	Adjust injection speed
	6. Holding time is too short	Extend holding time
	7. Improper holding switch position	Adjust position
	8. Improper barrel and nozzle temperature	Adjust temperature
	9. Low mold temperature	Adjust mold temperature
	10. Improper gate position	Gate is set at thick wall
	11. Unbalanced gate	Check gate size of multiple holes in one mold
	12. Runner or gate is too small	Increase runner or gate
	13. Insufficient water transport	Increase number of water transport groups
	14. High hydraulic oil temperature	Clean hydraulic oil heat exchanger
	15. Thick wall thickness	Reduce glue
	16. Large thickness ratio of glue position	Reduce thickness ratio
	17. Long process	Add glue ports

8.4 Analysis and countermeasures of bubble causes

Cause: Gas is not removed in time during melting, remains in product during molding, and cannot be released, forming bubbles.
Common places: Transparent products.
Blistering
Identification and classification of bubbles in injection molded parts
Bubbles in injection molded parts must be divided into three categories: trapped air (air bubbles), cavities, and trapped gas. These three categories look very similar and are difficult to distinguish by visual methods. Following properties can help.

Trapped air:	Cavities:	Gas entrapment:
When injection molded part is filled, air is pushed into melt. This air may come from plasticizing system or poorly vented parts in mold cavity, such as ribs or blind holes.	It is evacuation of cavity. It is caused by shrinkage being blocked during plastic condensation stage.	Bubbles may also form. They are generally caused by release of gas from plastic during a series of degradation.
1. Bubbles are clearly visible in injection molded part: 2. If pressure release is used, it will be improved 3. Changing holding pressure and time has little effect on size of bubbles. 4. These defects can also be clearly seen during local filling.	1. Usually appears near wall thickness or material accumulation. 2. Changing holding pressure and time will affect bubble size. 3. Usually appears during holding stage.	1. Reducing injection speed will affect appearance of defects. 2. Changing melt temperature will affect appearance of defects. 3. Shortening melt residence time will affect appearance of defects

Air entrapments
Air entrapment can be seen on the surface and inside of injection molded part. It is caused by air introduced during injection molding or poor venting in mold cavity. Air entrapment often occurs near mold wall.

Gaseous Entrapments
Gas released by plastic due to heat effect is trapped inside or on the surface of plastic and forms small granular bubbles.

Causes and countermeasures of bubbles

Product defects	Causes	Countermeasures
Bubbles	1. Low back pressure	Increase back pressure
	2. Too fast screw melt speed	Slow down screw melt speed
	3. Undried rubber	Dry rubber
	4. Too high injection pressure	Reduce pressure
	5. Too fast injection speed	Reduce injection speed
	6. Too much loosening	Reduce loosening
	7. Poor mold exhaust	Enhance mold exhaust

8.5 Causes and countermeasures of material flower

Cause: compressed water or decomposed small molecule gas condenses on the surface of product along flow direction.
Common location: gate
Moisture Streaks
Moisture streaks are U-shaped curves that appear on injection molded parts, with opening direction being same as jet direction. In many cases, there are bubbles and surface roughness around silver streaks. Moisture streaks are formed by condensation of water vapor on mold wall and involve a wide range of areas.

Air streaks
Most of air marks appear in the form of matte spots, silver or white marks in recesses, ribs, and locations where thickness of glue changes greatly. Layered marks will appear at starting point near glue mouth, and air marks will also appear at mark or depression.

Product defects	Causes	Countermeasures
Material flower	1. Raw material contains moisture and volatiles	Increase drying temperature and extend drying time
	2. Material temperature is too high	Reduce barrel temperature
	3. Mold temperature is too low	Increase mold temperature
	4. Firing rate is fast	Reduce injection rate
	5. Back pressure is too small	Increase back pressure
	6. Insert is not preheated or temperature is too low	Preheat insert
	7. Cavity exhaust is poor	Increase exhaust or clean exhaust

8.6 Analysis and countermeasures of causes of gate markings

Cause: Molten plastic injected at high speed cools rapidly, making it difficult to completely fuse with molten plastic injected later.
Common locations: Near the straight-injection gate
Dull Spots Near Gate
Small concentric rings appear around gate, appearing as faint corona marks - Dull Spots

Analysis and countermeasures of causes of sprue marks

Product defects	Causes	Countermeasures
Sprue marks	1. Injection speed is too fast	Slow down injection speed
	2. Injection pressure is too high	Reduce pressure
	3. Barrel, nozzle and mold temperature is too low	Increase temperature
	4. Runner or gate is too small	Enlarge runner or gate
	5. Gate is unbalanced	Check gate size of multiple cavities in one mold

8.7 Analysis and countermeasures of flow marks

Cause: When low-temperature glue slowly fills cavity, concentric strip ripples are formed with gate as center.
Common locations: near gate or at transition point of uneven thickness.
Record Grooves effect
Record grooves are extremely fine and record-like grooves on the surface of injection molded part, and concentric grooves are more likely to appear at pin gate. These marks will extend parallel to the end of runner or appear after diaphragm gate.

Causes and countermeasures of flow marks

Product defects	Causes	Countermeasures
Flow marks	1. Low temperature of barrel, nozzle and mold	Increase temperature
	2. Injection pressure is too low	Increase pressure
	3. Injection speed is too slow	Increase injection speed
	4. Runner or gate is too small	Increase runner or gate

8.8 Analysis and countermeasures of causes of injection marks

Cause: Strip-shaped molten plastic injected at high speed cools rapidly, and it is difficult to completely fuse with molten plastic filled later, forming snake marks.
Common locations: Near straight-injection gate
Jetting
Jetting is a rough, snake-like weave that appears on the surface of molded part, usually causing gloss differences and color differences. In some cases, jetting is very similar to record grooves.

Analysis and countermeasures of causes of injection marks

Product defects	Causes	Countermeasures
Injection marks	1. Injection speed is too fast	Slow down injection speed
	2. Injection pressure is too high	Reduce pressure
	3. Improper injection position	Adjust slow injection position
	4. Gate is too small	Enlarge gate
	5. Improper gate position	Choose appropriate gate position
	6. Improper gate method	Use slow gate

8.9 Analysis and countermeasures of trapped air

Cause: Flowing molten plastic blocks exhaust duct, resulting in inability to exhaust gas in some parts of cavity.
Common locations: flow end or flow dead corner
Analysis and countermeasures of trapped air

Product defects	Causes	Countermeasures
Trapped air	1. Too fast injection speed	Slow down injection speed
	2. Too high injection and holding pressure	Reduce injection and holding pressure
	3. High melt temperature	Lower melt temperature
	4. Improper gate position	Use Moldflow to select appropriate gate position
	5. Poor cavity venting	Enhance venting or clean up venting

8.10 Analysis and countermeasures of burning causes

Cause: Rubber material is burned black and yellow due to high temperature or trapped air decomposition.
Common locations: Gate or trapped air location.
Diesel Effect / Burner
Black spots (caused by burning) appear and are visible on the surface of injection molded part. Usually these locations are not fully filled.

Causes of scorching and countermeasures

Product defects	Causes	Countermeasures
Scorching	1. Injection speed is too fast	Slow down the injection speed
	2. Injection pressure is too high	Reduce pressure
	3. High barrel temperature	Lower barrel temperature
	4. Long retention time in barrel	Shorten retention time
	5. Poor cavity exhaust	Increase exhaust

8.11 Analysis of causes of black spots and countermeasures

Cause: Black spots appear on the surface of product due to mixing with dirt or high temperature degradation
Dark Spots
Dark spots or dark spots caused by wear, thermal damage or contamination

Causes and countermeasures of black spots

Product defects	Causes	Countermeasures
Black spots	1. Plastic decomposition and carbonization	Reduce barrel temperature
	2. Long machine downtime	Slow down screw speed. Reduce back pressure
	3. Dead corners in injection system	Reduce residence time or set insulation state
	4. Raw material contamination or impurities	Clean dead corners, or replace broken screws and rubber rings, etc.
	4. Raw material contamination or impurities	Clean mixing hoppers and drying hoppers, etc.
	5. Many dust spots in the environment	Use pure water inlet materials

Build a dust-free room or use a drying hopper dust collector, etc.

8.12 Analysis of causes of fouling and countermeasures

Cause: Small molecules with poor compatibility with rubber material precipitate and gather on mold surface to form dirt.
Common locations: gates and air traps
Plate-out
After several injection molding cycles, low molecular weight substances in plastic will be separated and adhere to surface of mold cavity to form deposits. They usually appear near gate or venting position. These deposits prevent injection molded part from accurately replicating surface of mold cavity, resulting in defects or differences in brightness on the surface of injection molded part.

Causes of fouling and countermeasures

Product defects	Causes	Countermeasures
Fouling	1. High barrel temperature	Lower barrel temperature
	2. Too fast melt speed	Slow down melt speed
	3. High back pressure	Reduce back pressure
	4. Too fast injection speed	Slow down injection speed
	5. Poor cavity exhaust	Increase exhaust
	6. Excessive use of lubricant or diffuser	Appropriately reduce or not use

8.13 Analysis and countermeasures of causes of top bulge and top depression

Cause: When product has a large adhesive force and ejection part is not strong enough, it will cause top bulge or top depression.
Common parts: complex structure, deep cavity, small demolding slope,
Visible Ejector Marks
Ejector marks can be either sunken or protruding. They cause thickness of molded part to vary. These variations can cause gloss differences and visible sunken areas on the surface of molded part. In some cases, molded part may be penetrated by ejector due to excessive demolding force.

Analysis and countermeasures of causes of top height and top depression

Product defects	Causes	Countermeasures
Top height	1. Injection and holding time are too long	Reduce injection and holding time
	2. Injection and holding pressure are too high	Reduce pressure
	3. Improper injection and holding position	Adjust position
	4. Injection speed is too fast	Slow down injection speed
	5. Mold temperature is too high	Reduce mold temperature
	6. Cooling time is too short	Extend cooling time
	7. Unbalanced gate	Check gate size of multiple cavities in one mold
	8. Inner surface is not smooth	Save inner surface
	9. Demoulding slope is small	Increase demoulding slope
	10. Ejector pin is too small	Change to a larger ejector pin
	11. Unbalanced ejection	Balanced ejection

8.14 Analysis and countermeasures of top whitening

Cause: When product has a large adhesive force and ejection part is not strong enough, top whitening marks will be produced.
Common parts: complex structure, deep cavity, small demoulding slope.
Stress-Whitening
Stress whitening occurs when injection molded part is affected by internal stress and external stress (such as excessive stretching). Location affected by stress will turn white.

Causes and countermeasures of top whitening

Product defects	Causes	Countermeasures
Top whitening	1. Injection and holding time are too long	Reduce injection and holding time
	2. Injection and holding pressure are too high	Reduce pressure
	3. Improper injection and holding position	Adjust position
	4. Injection speed is too fast	Slow down injection speed
	5. Mold temperature is too high	Reduce mold temperature
	6. Cooling time is too short	Extend cooling time
	7. Unbalanced gate	Check gate size of multi-cavity mold
	8. Inner surface is not smooth	Save inner surface
	9. Small demoulding slope	Increase demoulding slope
	10. Small ejector pin	Change ejector pin
	11. Unbalanced ejection	Balance ejection

8.15 Analysis and countermeasures of whitening causes

Cause: When product has a large adhesive force and rib strength is insufficient, whitening marks will be produced.
Common locations: complex structures, deep cavities, and locations with small demoulding slopes.
Stress whitening occurs when injection molded part is affected by internal stress and external stress (such as excessive elongation). Location affected by stress will change color.

Causes and countermeasures of whitening

Product defects	Causes	Countermeasures
Whitening	1. Improper injection and holding time	Adjust injection and holding time
	2. Improper injection and holding pressure	Adjust holding pressure
	3. Improper injection and holding position	Adjust position
	4. Too fast injection speed	Slow down injection speed
	5. High mold temperature	Lower mold temperature
	6. Short cooling time	Extend cooling time
	7. Unbalanced gate	Check gate size of multi-cavity mold
	6. No light saving on inner surface	Save inner surface
	9. Small demoulding slope	Increase demoulding slope

8.16 Analysis and countermeasures of sticking film

Cause: Product sticks to mold with great force and cannot be removed smoothly.
Common locations: Complex structure, deep cavity, and small demolding slope.
Analysis and countermeasures of sticky film causes

Product defects	Causes	Countermeasures
Mold sticking	1. Holding pressure is too high	Reduce holding pressure
	2 Holding pressure time is too long	Reduce holding time
	3. Cooling time is too short	Increase cooling time
	4. Improper gate position	Find the best gate position
	5. Insufficient smoothness of mold side	Increase side smoothness
	6. Unreasonable mold separation	Product structure should be as small as possible in cavity without ejection
	7. Too small demoulding slope, difficult to demould	Increase demoulding slope
	8. Unreasonable ejection position structure	Add ejector

8.17 Analysis and countermeasures of causes of dragging

Cause: Due to burrs on mold surface, surface of rubber part is too rough or demoulding slope is too small, side of rubber part is scratched during demoulding.
Common parts: Side of rubber part with a small demoulding slope and rough surface
Demoulding Grooves
When injection molded parts are demolded, grooves (drag patterns) will be generated to damage their surface. This phenomenon mainly occurs on structural surfaces.

Causes and countermeasures of dragging

Product defects	Causes	Countermeasures
Dragging	1. Too high injection and holding pressure	Reduce pressure
	2. Too long injection and holding time	Reduce injection and holding time
	3. Improper injection and holding position	Adjust position
	4. Small demoulding slope	Increase demoulding slope
	5. Rough texture	Check whether texture surface meets requirements of drawing
	6. Wrong glue port position	Select appropriate glue port position
	7. Unbalanced gate	Check gate size of multiple cavities in one mold
	8. Burrs on mold edge	Polish mold

8.18 Analysis and countermeasures of color difference

Causes: 1. Temperature-intolerant color powder and rubber material degrade and change color at high temperatures.
2. Caused by color powder difference or contamination.
Coloured Streaks
Coloured Streaks are caused by uneven colouring and distribution of parts or by arrangement of colourants following the flow direction of melt. Thermal damage and severe deformation of injection moulded parts, such as using excessive demoulding force, can also cause uneven colours and coloured streaks.

Analysis and countermeasures of color difference

Product defects	Causes	Countermeasures
Color deviation	1. Long drying time	Shorten drying time
	2. High drying temperature	Reduce drying temperature
	3. Too high back pressure	Reduce back pressure
	4. Too high barrel temperature	Lower barrel temperature
	5. Mixer, crusher, hopper and barrel not cleaned up.	Clean up
	6. Different batches of raw materials	Color adjustment
	7. Different batches of toner	Color adjustment

8.19 Analysis and countermeasures of gloss difference

Causes: 1. Non-serious air entrapment, gas compression, sudden temperature rise, mild burns, local surface of the product is shiny,
2. Uneven mold temperature or pressure, resulting in different degrees of melt adhesion to mold cavity surface, resulting in different gloss on the surface of plastic part
3. Caused by color powder formula
Gloss/Gloss differences
When evaluating gloss of injection molded parts, two defects can be distinguished, namely, too high or insufficient gloss on injection molded part, and gloss differences on the surface of injection molded part. Gloss differences often appear at locations where thickness of glue position varies greatly.

Analysis and countermeasures of gloss difference

Product defects	Causes	Countermeasures
Gloss difference	1. Difference in injection and holding pressure	Adjust pressure
	2. Difference in injection and holding time	Increase injection holding time
	3. Temperature deviation of barrel, nozzle and mold	Adjust temperature
	4. Unbalanced gate	Check gate size of multi-cavity mold
	5. Difference in surface texture	Etch texture as required
	6. Difference in pigment formula	Adjust formula

8.20 Analysis and countermeasures of color mixing

Causes: 1. Pigments with poor compatibility or mixed with multiple pigments, resulting in uneven pigment dispersion
2. Mixed with different color materials
Coloured Streaks

Analysis and countermeasures of color mixing

Product defects	Causes	Countermeasures
Color mixing	1. Low temperature of barrel, nozzle and mold	Increase temperature
	2. Too low back pressure	Increase back pressure
	3. Too low injection pressure	Increase pressure
	4. Too slow injection speed	Increase firing speed
	5. Mixer not cleaned	Clean up mixer
	6. Cannon barrel not cleaned	Clean up cannon barrel
	7. Crusher not cleaned	Clean up crusher
	8. Poor compatibility of color powder	Use color powder with good compatibility or add diffuser or pull particles

8.21 Analysis and countermeasures of flashing causes

Cause: Flowing molten plastic enters gap of mold under a certain pressure, forming an excess plastic film
Common locations: parting surface, penetration surface or insertion surface, and sliding position.
Oversprayed Parts-Flashes
Flashes often occur near parting line of mold, sealing plane, exhaust or ejector pin. Burr looks like a thin film of glue flying out from edge of injection molded part. Thin burrs are not very noticeable, but large-area thick burrs (flashes) will overflow from edge of injection molded part for more than a few centimeters.

Analysis and countermeasures of flashing causes

Product defects	Causes	Countermeasures
Flashing	1. Too high injection pressure	Reduce pressure
	2. Too fast injection speed	Reduce speed
	3. Too long holding time	Reduce holding time
	4. Too high temperature of barrel, nozzle and mold	Reduce temperature
	5. Improper holding switch position.	Adjust position
	6. There are debris on mold surface	Clean mold surface
	7. Insufficient clamping force	Increase clamping force
	8. Deformation of machine mold plate	Replace mold plate
	9. Insufficient clamping force of machine	Replace large-tonnage machine
	10. Too good fluidity of raw materials	Replace raw materials with low fluidity
	11. Large gaps between parting surface, insertion surface and punching surface, ejector pin of sleeve	Check and repair gaps

8.22 Analysis and countermeasures of floating fiber

Causes: 1. Injected molten plastic has cooled down a lot when it contacts mold wall, and glass fiber is difficult to soak into molten plastic, forming fiber marks.
2. Difference in shrinkage between glass fiber and plastic causes local protrusions.
Common parts: small gate and large area or slender glass fiber products:
Glass Fibre Streaks
When glass fiber is used to fill reinforcement, there is a chance of matte surface marks and rough surface. Glass fiber that reflects like metal appears on the surface of injection molded part and forms marks.

Analysis and countermeasures of floating fiber causes

Product defects	Causes	Countermeasures
Floating fiber	1. Insufficient feed amount	Increase amount of melt
	2. Injection pressure is too low	Increase pressure
	3. Injection speed is too slow	Increase injection speed
	4. Barrel, nozzle, and mold temperature is low	Increase temperature
	5. Gate is too small	Increase gate
	6. Incorrect gate position	Choose appropriate gate position
	7. Glass fiber is too long	Use short glass fiber or glass beads

8.23 Analysis and countermeasures of warping deformation

Cause: Product changes shape when subjected to internal stress (caused by temperature difference, pressure difference) or external force.
Common deformed products: large area or slender products made of glass fiber or easily oriented rubber.
Warpage
Warpage refers to shape of injection molded part being inconsistent with original requirements. They are usually caused by uneven shrinkage of injection molded part, but do not include deformation of injection molded part caused by demolding.

Thermal difference causes uneven shrinkage and deformation

Different thickness of glue causes uneven shrinkage and deformation

Pressure difference causes uneven shrinkage and deformation

Analysis and countermeasures of warping deformation causes

Product defects	Causes	Countermeasures
Warping deformation	1. Difference in mold temperature	Adjust to appropriate mold temperature
	2. Large difference in product thickness	Adjust wall thickness or cooling
	3. More and deeper local ribs	Increase slope or number of ejectors at rib position
	4. Insufficient cooling time	Increase cooling time
	5. Incorrect gate position	Find the best position
	6. Improper ejection position, uneven force	Balance ejection
	7. Local rib pattern is not light-saving	Mold saving
	8. Large positioning effect of macromolecules in plastic	Add filler or change material
	9. Creep deformation due to extrusion	Review packaging method

8.24 Analysis and countermeasures of causes of dimensional deviation

Product defects	Causes	Countermeasures
Dimension deviation	1. Unstable injection	Check and repair whether rubber ring and meson are worn
	2. Uneven raw material particles	Check whether gate material particles are too large
	3. Too high barrel and nozzle temperature	Adjust to appropriate temperature
	4. Too low or too high injection pressure	Adjust to appropriate injection pressure and holding pressure
	5. Improper holding time	Set appropriate holding time
	6. Uneven gate size	Multi-hole product gate is consistent
	7. Too low or too high mold temperature	Adjust to appropriate temperature
	8. Inaccurate mold size	Repair to accurate size
	9. Unstable electrical or hydraulic system of injection machine	Check and repair electrical or hydraulic system

8.25 Analysis of causes of rupture and countermeasures

Reason: due to degradation, excessive stress or mixing of incompatible impurities, strength is extremely poor and rupture occurs.
Common locations: column position or gate.
Flaking of Surface Layer
Layers in material have not been completely fused together, and peeling has begun to occur. Location is at injection port or on product. Different degrees of peeling may occur due to different fusion strengths, such as large pieces or small and thin pieces.

Stress cracks
Stress cracks are cracks that are very close together. They usually appear days or even weeks after production.

Ragged Film Hinge
Flake breakage is mainly caused by excessive stress in plastic. They may tear partially or completely. Excessive stress also causes stress whitening.

Causes of rupture and countermeasures

Product defects	Causes of occurrence	Countermeasures
Rupture	1. Plastic decomposition	Optimize back pressure
		Lower barrel temperature
		Optimize baking temperature and time
		Shorten long retention time
	2. Glass fiber becomes shorter	Lower barrel temperature
	2. Glass fiber becomes shorter	Optimize back pressure
	3. Poor welding	Increase mold temperature and injection speed
	4. Excessive stress	Reduce holding pressure and time
	5. Plastic mixed with impurities	Select impurities
	6. Too many plastic returns	Recovery rate does not exceed 25%
	7. Sharp-angle notches	Increase R angle
	8. Insufficient plastic thickness around metal inserts	Increase wall thickness or preheat inserts

Internal stress detection
After cooling at room temperature, soak product in carbon tetrachloride solution for 90 seconds (room temperature 20°). If there is a serious crack, it means that internal stress of product is too large and unqualified.
After soaking, if there is no crack, internal stress control meets standard.
Annealing treatment at 120℃ in oven and then soaking in carbon tetrachloride solution for 90 seconds (room temperature 20°), there are very slight cracks, indicating that after heat treatment, internal stress has been released a lot.

8.26 Analysis and countermeasures of cold glue

Cause: After cold glue in runner is injected into cavity or splashed out at thin ribs of product, it cannot be fused with surrounding hot glue and has a low temperature, so it has a poor fit to cavity wall and looks like a separate glue block.
Cold slugs
When a relatively cold plastic melt is injected into mold cavity through nozzle, it will produce a comet-like tail. Cold slugs can appear near injection nozzle or on the entire injection molded part.
Cold slugs may also block flow channel, causing molten glue flow to branch and eventually form a fusion line

Analysis and countermeasures of cold glue

Product defects	Causes	Countermeasures
Cold glue	1. Uneven plasticization	Increase back pressure, check whether screw is worn
	2. Too low mold temperature	Increase mold temperature
	3. Refractory materials mixed in material	Select miscellaneous materials
	4. Too low nozzle temperature	Increase nozzle temperature
	5. No main or branch channel cold material hole	Add cold material well
	6. Too fast or too slow shooting speed	Adjust shooting speed

8.27 Analysis of peeling causes and countermeasures

Cause: Delamination due to presence of miscellaneous materials or low-molecular additives with poor compatibility
Flaking of Surface Layer
Layers in material have not been completely fused together, and peeling has begun to occur. Location is at injection port or on product. Different degrees of peeling may occur due to different fusion strengths, such as large pieces or small and thin pieces.

Analysis of causes of peeling and countermeasures

Product defects	Causes	Countermeasures
Peeling	1. Mixing of different plastics	Avoid mixing
	2. Mixing of different grades of same plastic	Avoid mixing
	3. Uneven plasticization	Increase back pressure and check whether screw is worn
	4. Raw material contamination or foreign matter	Select contaminated materials
	5. Poor compatibility of fillers (too much diffusant, etc.)	Reduce amount of fillers

8.28 Analysis and countermeasures of causes of water line

Cause: Two cold melts meet to form water line
Common locations: holes, obstacles, uneven thickness or multi-gate products
Weld line
In most cases, weld line is optical and mechanical weakness of molded part, and may cause a gap or discoloration. Gap is particularly noticeable on dark or smooth molded parts or those with a high polish. Discoloration is particularly noticeable when using metallic masterbatches.

Analysis and countermeasures of water line causes

Product defects	Causes	Countermeasures
Water line	1. Improper injection speed	Adjust injection speed
	2. Material temperature is too low	Increase barrel temperature
	3. Mold temperature is too low	Increase mold temperature
	4. Injection pressure is too small	Increase shot pressure and holding pressure
	5. Short holding time	Extend holding time
	6. Poor cavity venting	Increase venting
	7. Improper gate position	Find the best gate position
	8. Too many gates	Reduce number of gates
	9. Raw materials are contaminated	Select dirty materials
	10. Poor plastic fluidity	Select raw materials with good fluidity

9 Abnormalities specific to injection molding process

Wire drawing at nozzle, screw slippage, glue leakage, difficulty in opening mold, poor material feeding, nozzle blockage, nozzle sticking to mold, mold compression, plasticization noise, drooling, ejector breakage
Analysis of wire drawing causes and countermeasures
Cause: Nozzle or hot runner nozzle temperature is high. When nozzle is removed from mold, fluidity of melt is very good and melt is in the form of a thread.
Common locations: main runner inlet or hot nozzle

Product defects	Causes	Countermeasures
Wire drawing	1. High nozzle temperature	Lower nozzle temperature
	2. High hot nozzle temperature	Lower hot nozzle temperature
	3. Short holding and cooling time	Extend holding and cooling time
	4. High melt temperature	Lower barrel temperature and back pressure

Causes and countermeasures for poor material feeding

Causes for poor material feeding	Improvement methods
1. Particles of recycled water inlet material are too large (uneven in size)	1. Re-crush larger particles of water inlet material (reduce gap of crusher blade)
2. Too much diffusant, raw materials are agglomerated	2. Reduce amount of diffusant
3. Raw materials in hopper have a "bridge" phenomenon	3. Check/dredge raw materials in drying barrel
4. Temperature of melting tube feeding section is too high	4. Reduce material temperature of feeding section or check cooling water at feeding port
5. Feeding port of drying barrel or feeding port of the machine is too small	5. Increase diameter of feeding port or replace machine

Analysis of causes of plasticizing noise and countermeasures

Reasons for plasticizing noise	Improvement methods
1. Excessive back pressure	1. Reduce back pressure
2. Too fast screw speed	2. Reduce screw speed
3. Too low barrel (compression section) temperature	3. Increase temperature of compression section
4. High viscosity of plastic (poor fluidity)	4. Use plastic with good fluidity
5. High screw compression ratio	5. Use a screw with a small compression ratio

Analysis and countermeasures for screw slippage

Causes of screw slippage	Improvement methods
1. Temperature of rear section of material pipe is too high, and material particles are lumpy (no material falling)	1. Check cooling water at inlet and reduce temperature of rear section
2. Excessive diffusant or lubricant	2. Reduce amount of diffusant or lubricant
3. Back pressure is too large and screw speed is too fast	3. Reduce back pressure and screw speed
4. Resin in hopper is agglomerated (no material falling)	4. Repair heating system of drying barrel and replace new materials
5. Particles of recycled water inlet material are too large, resulting in a "bridge" phenomenon	5. Pick out excessive water inlet particles and crush them again
6. Lack of material in hopper	6. Add plastic to drying barrel in time
7. Inner wall of barrel and screw are seriously worn	7. Check or replace barrel/screw

Causes of nozzle blockage and countermeasures

Causes of nozzle blockage	Improvement methods
1. Metal and other infusible materials in nozzle	1. Remove nozzle to remove foreign matter in nozzle
2. Plastic impurities with high melting points are mixed in nozzle material	2. Remove plastic impurities with high melting points in nozzle material
3. Nozzle temperature of crystalline resin (such as PA, PBT) is too low	3. Increase nozzle temperature
4. Heating ring at the nozzle head is burned out	4. Replace heating ring at nozzle head

Analysis of causes of drooling and countermeasures

Reasons for drooling	Improvement methods
1. Melt temperature or nozzle temperature is too high	1. Reduce melt temperature or nozzle temperature
2. Back pressure is too high or screw speed is too high	2. Reduce back pressure or screw speed
3. Insufficient glue extraction	3. Increase glue extraction (extract glue before or after melting)
4. Nozzle hole is too large or nozzle structure is improper	4. Use a nozzle with a smaller aperture or a self-locking nozzle
5. Plastic viscosity is too low	5. Use a plastic with a higher viscosity

Analysis of causes of glue leakage and countermeasures

Causes of glue leakage	Improvement methods
1. Focus of nozzle and mold nozzle is not accurate	1. Re-align nozzle or check whether nozzle head is smooth
2. Nozzle is larger than mold nozzle	2. Replace small-hole nozzle
3. Spherical surface of nozzle or mold nozzle is damaged	3. Process spherical surface of nozzle or mold nozzle
4. Back pressure is too large or screw speed is too high	4. Reduce back pressure or screw speed

Analysis of causes and countermeasures for nozzle sticking

Causes of nozzle sticking	Improvement methods
1. Injection pressure or holding pressure is too high	1. Reduce injection pressure or holding pressure
2. Melt temperature is too high	2. Reduce melt temperature
3. Ball diameter of mold entrance is smaller than ball diameter of nozzle	3. Increase ball diameter of mold entrance or reduce ball diameter of nozzle
4. Inner surface of main channel is not smooth or has demoulding chamfers	4. Save light on main channel or improve its demoulding chamfers
5. Damage on outer side of runner or burrs around nozzle	5. Repair mold and repair damaged parts
6. No buckle on main runner	6. Make front end of nozzle ejector pin into a "Z" shaped buckle pin
7. Main runner size is too large or cooling time is insufficient	7. Reduce size of main runner or extend cooling time
8. Demoulding slope of main runner is too small	8. Increase demoulding slope of main runner

Analysis of causes of needle breakage and countermeasures

Causes of needle breakage	Improvement methods
1. Excessive holding pressure	1. Reduce injection pressure and holding pressure
2. Needle is too thin	2. Replace needle
3. Small demoulding slope	3. Increase demoulding slope
4. Unbalanced needle distribution	4. Balance needle distribution
5. Insufficient mold saving	5. Sufficient mold saving