Note:
1) Principle of EDM, EDM is a thermal processing method that uses electrical energy to convert into workpiece heat energy to make workpiece melt rapidly. During EDM, a transition arc discharge phenomenon occurs in gap between electrode and workpiece, which in turn produces a thermal effect on workpiece. At the same time, liquid in process produces a vaporization explosion phenomenon due to discharge pressure and thermal effect. At this time, molten part of workpiece will be accompanied by liquid vaporization and melted into processing fluid. Workpiece produces a discharge mark due to discharge effect. This is repeated, and shape we want can be processed.
2) Principle of wire cutting, copper wire approaches workpiece (without contacting workpiece), voltage is applied to workpiece and copper wire to generate an arc and high temperature (9000oC-10000oC). After melting, metal debris is blown out, and copper wire continues to move forward. After workpiece cools, a rough cut surface is formed.
Appearance problems and countermeasures of plastic injection molded products

 

Air Trap; Blush; Flash; Flow Line or Flow Mark; Jetting; Short Shot; Sink Mark or Vord; Streak; Weld Line

Definition of Air Trap: Air or gas cannot be discharged in time and is trapped in cavity by sol wavefront.
●Finished product
1) Wall thickness difference is too large, resulting in a race track effect. When wall thickness difference is too large, plastic flow is slow at thin wall, and sol quickly advances along thick wall, which may encircle air or gas in cavity and cause air trap.
2) CAE can predict filling pattern and possible air trap points. Change thickness distribution to keep wall thickness as uniform as possible to avoid air trap.
●Mold
1) Improper gate position: a. When gate position is improper, plastic flow may surround air or gas, forming wind accumulation; b. CAE can predict filling pattern and possible wind accumulation points. Changing gate position can change filling pattern and wind accumulation may be avoided.
2) Improper runner or gate size: a. When designing multiple gates, if runner or gate size is improper, plastic flow may overtake air or gas, forming wind accumulation; b. CAE can predict filling pattern and possible wind accumulation points. Changing gate position can change filling pattern and wind accumulation may be avoided.
3) Poor exhaust: a. If exhaust is poor, air or gas will be drawn into wavefront closing, forming wind accumulation; b. CAE can predict filling pattern and possible wind accumulation points. Add exhaust ports at possible wind accumulation points to avoid wind accumulation.
●Injection molding machine
When injection speed is too high, a jet (Jetting) is generated, which may involve gas and form wind accumulation. Reducing injection speed can stabilize plastic flow, prevent jets, and avoid wind accumulation.

Definition of blush: cloud-like color change near gate. Sometimes it is found at obstruction in plastic flow channel. Reason is sol fracture (Fracture).
●Plastic
Insufficient drying, heavy plastic moisture, heating, mixing, and pushing, steam is involved in sol, when it enters cavity, silver lines are generated, and blushing is often accompanied.
●Mold
1) Mold temperature is too low;
2) When there are sharp angles in sol delivery system (Melt Delivery System), especially when there are sharp angles at gate, blushing is likely to occur. Sometimes, blushing can be eliminated by using a larger fillet radius at injection point.
3) Cold slug well is too small. Diameter of cold slug well should be same as outlet diameter of sprue bushing, and its depth should be same as or exceed diameter.
4) Gate is too small or cavity at injection point is too thin. When sol flow rate is large and short area is small, shear rate is large, and shear stress often increases, resulting in melt fracture and reddening. CAE simulation can predict temperature, shear rate and shear stress of sol when it passes through above narrow area, and CAE generally provides upper limits of various plastic material temperatures, shear rates and shear stresses. CAE engineers can make corresponding adjustments based on analysis results and quickly find appropriate gate size and cavity thickness at injection point.
●Injection molding machine
1) Sol temperature is too low;
2) Injection speed is too fast;
3) Injection pressure is too high;

 

Definition of Flush: waste formed by molten plastic flowing into parting surface, folding surface of slider or gap of accessories.
●Plastic
Too much or too little fluidity: Plastic fluidity is too large, sol is too thin, and it is easy to penetrate into gaps in cavity, resulting in burrs; plastic fluidity is too small, sol is too thick, high pressure is required to fill mold, mold plate may be stretched, sol overflows, and burrs are generated on the parting surface upstream of pouring system.
●Mold
1) Improper gate position makes flow length too long, high pressure is required to fill mold, mold plate may be stretched, sol overflows, and burrs are generated on parting surface at upstream of pouring system.
2) Improper mold landing (Land), mold landing should extend 12mm outward from outer edge of cavity. Outside, movable and fixed molds are separated from each other (Relieved) to keep mold landing surface close to each other to prevent overflow.
3) If span of support plate is too large, mold plate may be stretched open by high pressure in cavity, sol overflows, and burrs are generated on parting surface in the center of mold plate. A spacer block (Support Block or Support Pillar) can be added between support plate and movable side mounting plate to shorten span.
4) If mold plate is too thin, mold plate may be stretched open by high pressure in cavity, sol overflows, and burrs are generated on parting surface in the center of mold plate.
5) If mold is improperly processed or assembled, a large gap is formed at the edge of cavity, sol overflows, and burrs are generated. Sprue bushing should not be too long, otherwise male and female molds cannot be closed tightly, causing overflow and burrs. Flatness of flat parting surface should be within 0.05mm.
6) Exhaust port is too deep or too shallow: a. Exhaust port is too deep, sol seeps out, and burrs are generated; b. Exhaust port is too shallow, and gas is not easy to discharge. During pressurized exhaust, mold plate may be stretched open, sol overflows, and burrs are generated.
7) Steel is too soft and prone to dents. If dent occurs around cavity, sol penetrates and burrs are generated.
8) Mold surface is unclear, there are foreign objects on mold surface, and mold plate cannot be sealed, causing overflow and burrs.
9) Mold temperature is too high, sol is thin, and it is easy to penetrate into gaps in cavity, resulting in burrs.
● Injection molding machine
1) Clamping force is insufficient, mold plate may be stretched open by high pressure in cavity, sol overflows, and burrs are generated. CAE can predict required clamping force versus time curve, and set clamping force cannot be less than maximum clamping force of curve.
2) Plastic is metered too much, and excess sol is squeezed into cavity. Mold plate may be stretched open by high pressure in cavity, sol overflows, and burrs are generated.
3) Material pipe temperature is too high, sol is too thin, and it is easy to penetrate into gaps in cavity, resulting in burrs.
4) Temperature of material tube is too low, sol is too thick, and high pressure is required to fill mold. Mold plate may be stretched open, sol overflows, and burrs are generated. Material temperature setting can refer to material manufacturer's recommendations. Material tube is divided into four zones: rear, center, front, and nozzle. Material temperature setting from rear to front should be gradually increased, and each zone should be increased by 60℃. If necessary (especially for heat-sensitive plastics), material temperature of nozzle zone and front zone can be set to same as center zone.
5) Injection pressure is too high, and mold plate may be stretched open by high pressure in cavity, and sol overflows, resulting in burrs. Injection can start with a short shot, and increase by 3Bar (50psi) each time until filling pressure is excessive.
6) Shooting speed is too high or too low: a. When shooting speed is too high, sol is too thin and it is easy to penetrate into gaps in cavity, resulting in burrs; b. When shooting speed is too low, sol temperature is reduced, sol is too thick, and high pressure is required to fill mold. Mold plate may be stretched open, and sol overflows, resulting in burrs. Increment of each shot pressure or shot speed adjustment is based on 10% as a principle. After each adjustment, it takes about 10 shots to reach a stable state.
7) Holding time is too long, sol is transferred from high pressure to low pressure, and probability of sol seeping out from gaps in cavity increases, which may result in burrs.
8) Residence time is too long or too short: a. If plastic stays in material pipe or hot runner for too long, it will make plastic thinner, and sol will easily penetrate into gaps in cavity, resulting in burrs; b. If residence time is too short, sol temperature is too low, sol is too thick, and high pressure is required to fill mold. Mold plate may be stretched open, and sol overflows, resulting in burrs. Mold should be installed on an injection molding machine with a volume that is equivalent to its shot. When shot volume is between 20% and 80% of tube capacity, plasticization is appropriate and burrs are not easy to produce.
● Operator
Operators with bad habits open and close door of molding machine too early or too late, and plastic transporters do not transport plastics according to regulations, etc., which will make molding results inconsistent. When tube heater tries to replenish heat in time due to irregular heat loss, plastic temperature is not easy to be uniform, and hot spots are generated. Fluidity near hot spots is good, which may cause burrs.

Definition of flow mark: flow marks on the surface of molded product.
● Plastic
1) Poor fluidity. Cavity with a large flow length to wall thickness ratio must be filled with easy-flowing plastic. If plastic fluidity is not good enough, sol will move slower and slower, and the slower it is, the colder it will be. Injection pressure and holding pressure are not enough to press condensed skin tightly against mold surface, leaving a depression in sol in vertical flow direction, which looks like an annual ring. (Suggestion: Choose plastic that is easy to flow in nozzle without overflowing)
2) Improper use of molding lubricant. General lubricant content is less than 1%. When flow length to wall thickness ratio is large, lubricant content must be appropriately increased to ensure that condensation layer is tightly attached to mold surface until product is finalized and flow marks are not generated.
●Mold
1) If mold temperature is too low, material temperature will drop too quickly. Injection pressure and holding pressure are not enough to press condensed skin tightly against mold surface, leaving a depression in sol in vertical flow direction, which looks like an annual ring.
2) Injection channel, runner or gate is too small, and resistance is increased. If injection pressure is insufficient, advancement of sol wavefront will become slower and slower, plastic will become colder and colder. Injection pressure and holding pressure are not enough to press condensed skin tightly against mold surface, leaving a depression in sol in vertical flow direction, which looks like an annual ring.
3) Insufficient exhaust will hinder filling of sol, and sol wavefront cannot press condensed surface tightly on mold surface, leaving sol concave in vertical flow direction, like annual rings.
●Injection molding machine
1) Injection pressure and holding pressure are not enough to press condensed surface tightly on mold surface, leaving sol concave in vertical flow direction, like annual rings. Increase injection pressure and holding pressure, so that condensed layer can be pressed tightly on mold surface until product is finalized and flow marks are not generated.
2) Improper residence time, plastic stays in material tube for too short a time, and sol temperature is too low. Even if cavity is barely filled, plastic cannot be compacted during holding pressure, leaving plastic concave in vertical flow direction, like annual rings.
3) If filling time is too long, sol wavefront will get colder and colder, injection pressure and holding pressure are not enough to press condensed surface tightly on mold surface, leaving sol concave in vertical flow direction, like annual rings.
4) When cycle time is too short, plastic is not heated in material tube in time, and sol temperature is low. Even if cavity is barely filled, plastic cannot be compacted during holding pressure, leaving sol concave in vertical flow direction, which looks like an annual ring.
5) When material tube temperature is too low, sol temperature is low, injection pressure and holding pressure are not enough to press condensed surface tightly on mold surface, leaving sol concave in vertical flow direction, which looks like an annual ring.
6) Nozzle temperature is too low, plastic in material tube absorbs heat released by heating belt and friction heat generated by relative movement of plastic molecules caused by rotation of screw, and temperature gradually rises. Last heating zone in material tube is nozzle. Sol should reach ideal material temperature here, but it must be heated moderately to maintain the best state. If nozzle temperature is not set high enough, nozzle and mold are in contact, taking away too much heat, material temperature will drop, injection pressure and holding pressure are not enough to press condensed surface tightly on mold surface, leaving sol concave in vertical flow direction, which looks like an annual ring.

 

Definition of jetting: a curved, folded, snake-like flow mark from a restricted area (such as a nozzle or gate) to a thicker and open area.
● Parts
If wall thickness difference from thin to thick is too large, and plastic flow has no choice but to flow quickly from thin part to thick part, flow will be unstable and jets may be generated.
● Mold
1) Gate position is improper, and plastic flow in cavity flows from thin to thick. If thickness difference is large and flow rate is fast, flow will be unstable and jets may be generated.
2) Gate is not impact type. Impact type gate guides sol into glue to a metal surface to relieve stress, stabilize plastic flow, and avoid jets. Overlap gates and latent gates can be designed as impact gates.
3) From gate to cavity, cross-sectional area suddenly increases, plastic flow is unstable, and jets are easily generated. From gate to cavity, cross-sectional area changes gradually, such as tab gate or fan gate, so that plastic flow can be smoothly transitioned and jet can be avoided.
●Injection molding machine
1) Sol temperature is too high or too low. Jet is related to expansion effect of sol after sol is injected and change of plastic properties. For most plastics, above expansion effect becomes more obvious when temperature is increased, while for some materials, expansion effect becomes more obvious when temperature is increased.
2) Injection speed is too high. Optimal screw forward speed curve is adopted so that molten plastic wave front can pass through gate at a slower speed first. Once tongue-shaped sol is formed downstream of gate, screw speed can be increased.
Note: Number of gates is related to amount of plastic that each gate must bear during filling. The fewer number of gates, the more plastic must pass through each gate per unit time, and the faster plastic advance speed. In order to avoid excessive flow rate and plastic erosion of printed film, flow rate of plastic must be reduced. Number of gates cannot be too small. In addition, when multiple gates are used for injection molding, plastic flow balance must be considered. When flow is unbalanced, amount of plastic passing through each gate will change accordingly. If most of plastic enters cavity through a specific gate, it may also cause plastic film near gate to be eroded by filling plastic.

Definition of short shot: plastic fails to fill the entire cavity.
● Plastic
Poor fluidity and large flow length to wall thickness ratio of cavity must be filled with easy plastic. If plastic fluidity is not good enough, plastic wavefront will be overcooled and stop moving halfway, resulting in a short shot.
● Parts
1) Wall thickness is too thin and resistance is high. If injection pressure is insufficient, advancement of sol wavefront will become slower and slower, and before cavity is filled, wavefront will solidify, causing a short shot.
2) Wall thickness difference is too large. Before wall thickness part is filled, plastic flow advances slower and slower at thin wall, which may cause a short shot due to solidification of wavefront.
● Mold
1) If mold temperature is too low, sol wavefront will be overcooled and stop moving before cavity is filled, resulting in a short shot.
2) Sprue, runner or gate is too small, and resistance is increased. If injection pressure is insufficient, advancement of sol wavefront will become slower and slower, and before cavity is filled, wavefront solidifies, resulting in a short shot.
3) Number or position of gate is inappropriate, which will make flow length too long and flow resistance too large. If injection pressure is insufficient, advancement of sol wavefront will become slower and slower, before cavity is filled, wavefront solidifies, resulting in a short shot.
4) Cold well is not set or is improperly designed. A cold well should be added to sprue and end of each runner. Size of cold well should be appropriate, and there should be no obstacles upstream, so as not to affect its function of capturing cold material. Otherwise, any uncaptured cold material will flow down flow ear and may block gate or small runner, resulting in a short shot.
5) Insufficient exhaust will hinder plastic filling and even cause short shots. Consider exhaust at the end of each runner to prevent gas in runner from entering cavity. Cavity exhaust should not be neglected. Consider adding vent holes on parting surface opposite gate, and consider adding venting ejector pins at the end of corresponding product blind hole.
●Injection molding machine
1) Insufficient injection material. Injected material is not enough to fill every corner of cavity. After sol solidifies, an incomplete product is naturally formed.
2) Material tube temperature is too low. Before cavity is filled, sol wave front has already solidified, and molded product is naturally incomplete.
3) Insufficient back pressure. Back pressure can increase group force and friction heat between relatively moving molten molecules. This friction heat helps plasticization and promotes uniform mixing. Insufficient back pressure will prevent sol from obtaining enough heat. Cold material has already solidified before cavity is filled.
4) Injection pressure or injection speed is too low, so that sol is unable to complete task of filling cavity before it is overcooled, and short shots occur.
5) Injection time is too short, and filling action will not come into being, but short shots will follow.
6) Hopper outlet is blocked. Hopper outlet is feed port of material tube, which is the first station where plastic is heated in injection molding machine. If temperature of plastic here is close to softening point of resin, it is possible to combine with each other, forming a roadblock, making it difficult for new material to enter material tube, causing material shortage and short shot.
7) Check valve gap is too large. Check valve prevents sol in front of screw in material tube from flowing back during injection stage. When gap between front end of screw, check valve and material tube is too large, sealing function of check valve is lost, sol at the front end of screw flows back to upstream screw and material tube. Injection volume is insufficient and short shot occurs naturally.
8) Nozzle is too small, flow resistance is increased, and if injection pressure is insufficient, advancement of sol wavefront will become slower and slower. Before cavity is filled, short shot is caused by solidification of wavefront.
9) Capacity of injection machine material tube is too small. Amount of each shot should be between 20 and 80% of material tube capacity. If shot volume is greater than 80% of capacity, next shot will not be plasticized in time, flow resistance will be large, and a short shot may occur.

Definition of Sink Mark: Local collapse on the surface of molded product (or dimple or groove shape); Definition of Void: Shrinkage inside molded product.
●Product
1) Rib is too thick, intersection of rib and bottom plate is also thick. Plastic is concentrated here. When cooling, surrounding ribs and plates solidify first. Center of intersection of rib and plate remains liquid. The later solidified plastic shrinks on the first solidified plastic, which has effect of sucking surrounding plastic. If any condensation layer is relatively weak (usually on mold surface opposite to rib), it is possible that place will form a sink due to collapse. If condensation layer is strong enough, a shrinkage hole will form in the center of intersection of rib and plate. Thickness of rib should be 50% of thickness of bottom plate, or even thinner.
2) Stud hole is shallow, and bottom of stud (where it meets bottom plate) is too thick. Plastic is concentrated here. When cooling, this place solidifies last, and it has effect of sucking plastic that solidifies first around it. If any overflow condensation layer is relatively weak (usually at mold surface opposite to stud), it is possible that this place will form a depression due to collapse. If condensation layer is strong enough, a shrinkage cavity will form in the center of above intersection.

 

●Mold
1) Temperature of mold surface opposite to rib is too high (easy to form a depression). If temperature of mold surface opposite to rib is higher than that of surrounding area (generally true, because sol is concentrated nearby, heat load is large, and mold temperature remains high), condensation layer here is thin and rigidity is insufficient. When sol in the center solidifies, it is possible to pull thinner condensation layer inward to form a depression.
2) Use ceramic or plastic inserts to prevent dents
3) If sprue, runner or gate is too small, flow resistance will increase. If injection pressure is insufficient, cavity cannot be filled, sol density is low, and probability of dents or shrinkage holes is high.
4) If number or position of gate is inappropriate, flow length will be too long and flow resistance will be too large. If injection pressure is insufficient, cavity cannot be filled, sol density is low, and probability of dents or shrinkage holes is high.
●Injection molding machine
1) Temperature of material tube is too high, and sol density is low. When cooling, sol close to surface of cavity solidifies into a condensation layer first, plastic volume shrinks, and sol density in the center of cavity is even lower. When sol in the center gradually solidifies, center of cavity will shrink into a hole. Inner wall of shrinkage hole is full of tensile stress. If rigidity of condensation layer is not enough, it will collapse inward to form a dent. If rigidity of condensation layer is sufficient, shrinkage hole will still remain in product.
2) Insufficient cooling time (easy to form dents), plastic condensation layer is not thick enough to resist tensile force generated when internal sol solidifies and shrinks, forming dents.
3) Insufficient buffering or pressure holding. Plastic in cavity is not filled solidly due to low pressure or insufficient supplementary material, and density is low, which makes it more likely to have dents or shrinkage holes. When buffer becomes 0, screw reaches bottom and no longer moves forward. When sol cools and shrinks, pressure decreases, but screw cannot increase pressure, resulting in insufficient pressure holding, a high probability of dents or shrinkage holes.
4) Failure of the check valve. Check valve prevents sol in front of screw in material tube from flowing back during injection stage. When screw pushes a certain amount of material forward, if check valve is worn, broken or improperly seated, sol may slide through gap between front end of screw and check valve loading tube, resulting in reflux, causing screw to be pushed to the bottom, buffer disappears, and probability of dents or shrinkage holes is high.

Definition of streaks: Splash-like lines formed on the surface of molded products along flow direction. Streaks can be divided into: 1) Burn streaks; 2) Moisture streaks; 3) Color streaks; 4) Air streaks; 5) Glass fiber streaks.

Shortening of molecular chains produces silver streaks, and deterioration of molecules into blocks produces brown streaks. Both can be called burn streaks or burn streaks.
● Plastics
Temperature is too high or residence time is too long during drying.
● Mold
(1) Gate is too small;
(2) Gate or flow channel is not smooth;
(3) Insufficient exhaust.
●Injection molding machine
(1) Sol temperature is too high;
(2) Residence time is too long;
(3) Injection speed is too fast and injection pressure is too high;
(4) Screw speed is too fast and shear rate is too high during plasticization.

During storage and molding process, plastic absorbs moisture and evaporates into water vapor in sol. When water vapor approaches wavefront, it forms bubbles and gradually expands. When bubbles reach wavefront, they burst and roll to mold surface, being stretched into silver stripes, leaving a shadow on the surface of product.
● Plastics
(1) Insufficient drying;
(2) Improper material storage;
● Molds
(1) Water leakage from mold temperature control system;
(2) Condensation formed on the mold surface;
● Injection molding machine
(1) Sol temperature is too high;
(2) Injection pressure is too high and injection speed is too fast;
(3) Screw speed is too fast and shear rate is too high during plasticization;
(4) Residence time is too long;

● Plastics
(1) When coloring, pigment agglomerates into lumps, resulting in different color concentrations. Plastics, molding parameters, adhesives and other additives may cause this uneven distribution phenomenon;
(2) When dyeing with dyes in factory, dye particles are not completely dissolved in sol.
● Injection molding machine
Color is sensitive to excessively high molding temperatures and excessively long residence times, resulting in discoloration. If this color change is due to thermal degradation, it can be classified as burning streaks.

●Mold
(1) Air cannot escape in time during filling, is brought to surface of product and stretched in direction of flow;
(2) Air accumulated in floating or concave characters, ribs, arches and downstream of concave parts may be covered by sol that arrives later, forming air streaks or groove-shaped air pockets;
●Injection molding machine
(1) When screw moves back to release pressure, air is sucked in. During injection, air is rolled through gate, enters cavity, is pushed to mold wall (especially near gate), and is surrounded by condensed plastic, forming air streaks;
(2) Screw rolls in air;

●Plastic
(1) When glass fiber is rolled to mold wall by wave front, there is not enough sol around glass fiber;
(2) Shrinkage ratio of glass fiber to brown plastic is 1:200. Glass fiber hinders shrinkage of plastic, making surface uneven.

Definition of weld line: line formed when sol wavefronts meet.
●Plastic
(1) For cavities with poor fluidity and large flow length to wall thickness ratio, they must be filled with easy-flowing plastic. If plastic fluidity is not good enough, sol wavefront will move slower and slower, and the slower it moves, the colder it will be. When weld line is formed, wavefront temperature has dropped too low, resulting in poor bonding and obvious lines.
(2) Too much reinforcement is added. When percentage of reinforcement increases, strength of weld line decreases. Weld line formed by plastic without reinforcement can maintain 80-100% of strength of raw material. Weld line formed by plastic with reinforcement often cannot maintain 80% of strength of part without weld line. For PP with 30% glass weave, weld line formed by its opposite wavefront is only 34% of strength of part without weld line. For PC with 30% glass weave, weld line formed by its opposite wavefront is only 64% of strength of part without weld line.
●Parts
(1) Wall thickness is too thin or wall thickness difference is too large;
(2) Wavefront meeting angle is too small. When wavefront meeting angle is less than 135°, a weld line is formed. When it is greater than 135°, a fusion line is formed. Compared with fusion line, molecules on both sides of weld line diffuse less and quality is poor. When meeting angle is between 120° and 150°, surface traces of weld line gradually disappear.
●Mold
(1) Sprue, runner or gate is improperly positioned or too small or too long. When sprue, runner or gate is improperly positioned, weld line will be generated at appearance or strength sensitive part. If sprue, runner or gate is too small or too long, flow resistance will increase. If injection pressure is insufficient, temperature will drop too low when sol wavefront forms a weld line, resulting in poor bonding and obvious lines.
(2) Mold temperature is too low. When sol wavefront forms a weld line, temperature will drop too low, resulting in poor bonding and obvious lines. Increasing mold temperature can improve quality of weld line.
(3) Poor exhaust, air or volatiles will be drawn into wavefront, and weld line will be obvious. Sometimes an overflow well can be added at weld line, then cut off after molding to improve quality of weld line.
●Injection molding machine
(1) Temperature of material tube is too low. When sol wavefront forms a weld line, temperature is too low, connection is poor, and line is obvious.
(2) Insufficient back pressure. Back pressure can increase resistance and friction heat between relatively moving sol molecules. This friction heat helps plasticization and promotes uniform mixing. Insufficient back pressure will prevent sol from obtaining enough heat. Weld line formed by low-temperature sol wavefront has obvious lines due to poor connection. Increasing back pressure can improve quality of connection line.
(3) Injection pressure or injection speed is too low. When sol wavefront forms a weld line, temperature has dropped too low, connection is poor, and line is obvious. Increasing injection pressure or injection speed can naturally improve it.