Solutions to defects in injection molding products

Background

Various quality defects and abnormal phenomena often occur during injection molding production process; for a long time, most injection molding workers have only relied on experience to deal with it, blindly adjusting machine for a long time, wasting a lot of raw materials, lacking scientific and systematic analysis of some problems. Injection molding is an industry with a wide range of knowledge and strong technicality. Experience alone is not enough. It is necessary to have a considerable degree of knowledge and understanding of properties of polymer materials, processing and molding technology, mold design, injection molding machinery, etc.

Injection molding mechanism

Injection molding (mould for plastics) is a molding method that combines injection and molding, also known as injection molding. It is to put polymer component pellets into material cylinder of injection molding machine, then apply pressure to polymer melt with the help of screw after plasticization (heating, softening, compression, shearing, mixing, conveying, homogenization, melting). High-temperature melt is then injected into closed low-temperature mold cavity through nozzle and runner system, product is ejected after pressure maintenance, cooling and shaping, and mold opening.
Injection molding process includes three major stages (preparation, injection, and post-processing of the product), requires three conditions (heat, pressure, and time) and three elements (molding materials plastics, injection molding machine, and injection mold) to be organically combined and each action is repeated periodically.
Plastic melt flows in gap between pipes or plates of mold with a certain viscosity and pressure loss, and performs shear flow of a non-Newtonian fluid with a certain resistance.
Injection molding machine principle

1 Fixed mold base plate 2 Push rod fixed plate 3 Injection molded parts 4 Melt glue 5 Diverter shuttle 6 Plunger 7 Raw material 8 Ejector 9 Core

Injection molding mold

Injection molding materials

Plastic polymer materials are synthetic resins, which are mainly composed of polymers and polymerized with same or different low-molecular compounds without precipitating low-molecular byproducts. When heated, thermoplastics change their geometric shape through physical changes and can be repeatedly heated to soften and cooled to harden. Material selection should consider its processability, thermal properties, electrical properties, mechanical properties, aging properties, etc.

Plastics expand when heated, and their volume increases, distance between molecules increases, molecular chains become more mobile, viscosity decreases, and anisotropic orientation decreases (directional arrangement increases).
Viscous flow of polymer melts is mainly relative displacement between molecular chains. The larger molecular mass, the better mechanical strength of solid state. Viscosity of viscous flow state is higher and fluidity is poor.
Relative average molecular mass is used to characterize and measure length of polymer molecular chain. After molecular chain breaks (waste from multiple cycles of processing), distribution width of molecular mass is dispersed and mechanical strength decreases.
Under high-pressure injection, melt viscosity increases, shrinkage increases (molecular motion slows down, distance between molecules decreases), and friction heat increases.
Liquid of polymers (except liquid crystal polymers) is amorphous and has an amorphous structure. Thermoplastic polymers are divided into: polymers with amorphous structures and polymers with crystalline structures.
Microscopic molecules in amorphous materials maintain a disordered state in the form of bonding connections.
Common crystalline materials: PP, PE, PA, POM, PTFE, PBT (partially crystalline).
Molecular chains in crystalline materials are folded in an orderly manner along generated crystal nuclei, but surrounding is an amorphous structure. Crystalline materials have crystalline states (solid) and viscous flow states, while amorphous materials have glassy states (solid), highly elastic states and viscous flow states; crystalline materials have large shrinkage, high rigidity and tensile strength, but reduced impact strength.
Three states of polymer thermodynamics: glassy state, highly elastic state, and viscous flow state.
It is necessary to fully understand processability and thermal properties of plastics in order to correctly and effectively control gate freezing time and runner temperature of runner system.
Thermal properties of plastics refer to viscosity value during flow.
Shear pressure τ, shear rate γ, apparent viscosity η, Newtonian fluid μ and non-Newtonian fluid
Plastic melts in relatively long and small flow channels will easily exceed allowable shear rate and shear pressure, causing structure of polymers or additives to decompose, mixtures to separate, fillers and pigments to debond
Influence of additives - improve certain physical properties and processing and molding properties. Due to easier oxidation and decomposition, molten plastics have a time limit in hot state, separation and precipitation of composite components occur during flow, fillers will debond under high shear force, colorants will coagulate or change color, decomposition products of molten plastics will have a corrosive effect on mold, glass fibers will wear screws, nozzles, gates and other parts.

New injection molding materials

New material - wood-plastic composite material, has dual characteristics of wood (appearance) and plastic, anti-corrosion, not easy to deform, good mechanical properties, hard, durable, wear-resistant and other advantages. Molding methods include extrusion (many applications), injection, hot pressing; it has orientation, thickness is greater than plastic, mostly profiles, complex structure (difficult to cool, water cooling)
LCP, Chinese name is liquid crystal polymer. It is a new type of polymer material, generally showing liquid crystal properties in molten state. This type of material has excellent heat resistance and molding processing properties.
In addition, some liquid crystal polymers have certain special functions, such as photoconductive liquid crystal polymers, functional liquid crystal polymer separation membranes and biological liquid crystal polymers. Generally, thermotropic liquid crystal polymers have good fluidity and are easy to process and shape. Its molded products have skin-core structure unique to liquid crystal polymers. Resin itself has fiber properties and is highly oriented in molten state, so it can play a fiber reinforcement effect. Main uses of liquid crystal polymers are: high strength, high rigidity, high temperature resistance, electrical insulation, etc. are very good, and are used in electronics, electrical, optical fiber, automobiles and aerospace fields. Thermotropic liquid crystal polymers can also be made into polymer blends with a variety of plastics. Liquid crystal polymers in these blends play a fiber reinforcement role, which can greatly improve strength, rigidity and heat resistance of material.

Comparison of processing properties of injection molding

Amorphous plastics

Crystalline plastics

Wide range of processing temperatures
Gradual change in viscosity during heating and cooling
Lower heat to be removed during cooling
Lower demoulding temperature required to prevent deformation of plastic parts
Lower mold temperature applied for economic reasons
For technical products, higher mold temperature to reduce internal stress, for which thermal state is controlled
Minor shrinkage, little effect of holding time and pressure
Durability of product depends on frozen internal stress

Strictly defined temperature fluctuation range
Change from viscous flow state to crystalline state, quickly pass crystallization temperature
Higher heat to be removed during cooling and crystallization
High demoulding temperature can be used for ejection
Higher mold temperature to obtain appropriate crystallization
For product quality, mold is essentially in thermal state control
There is a large shrinkage, and a sufficiently high holding pressure is required to improve shrinkage compensation
Performance of product depends on crystallization temperature.

Quality analysis of injection molded parts

In production, we always hope to achieve state of zero-waste production. However, due to influence of various factors such as plastics used for injection molding, molding molds, injection molding machines and auxiliary equipment, molding environment, etc., there will always be problems of one kind or another. Therefore, quality management of injection molded parts is important-to control quality to the best and reduce loss of waste to minimum.
Quality management includes following aspects: correct selection of machine models, control of raw materials, effective management of molds, injection molding process operation and adjustment, implementation of quality inspection and comprehensive quality management, establishment of a complete quality assurance system, selection of appropriate control methods, and realization of network management of quality.
Quality management is a systematic, complex and tedious work. There is no fixed model. Each enterprise should determine its own management ideas and management methods according to its own characteristics. In short, quality must be constantly grasped and persevered, so that parts molds and people can achieve results and quality.
Injection molded parts have requirements for precision and quality (surface and internal). Precision of injection molded parts depends on plastic material, mold, injection process, and product structure.
For large products, error caused by fluctuation of molding conditions accounts for 1/3 of product tolerance; for small products, manufacturing accuracy of mold accounts for 1/3 of product accuracy. Product accuracy of a single cavity is higher, and position accuracy of moving core is lower. Improper design of pouring system, cooling system, demolding force will cause product deformation and affect accuracy.
In mass production, it is necessary to ensure consistency of all cavity flow and curing conditions (time, temperature, and pressure all affect shrinkage) during each injection.
Improving precision of injection molded parts mainly depends on design and manufacture of molds; and ensuring quality of injection molded parts mainly depends on control of injection process, which is closely related to runner system.

Defects	Cause Analysis	Countermeasures
Injection molded parts lack material, unsaturated mold	Plastic melt does not completely fill the cavity. Plastic material has poor fluidity.	Product is not properly matched with injection molding machine, and plasticizing capacity or injection volume of injection molding machine is insufficient. Material temperature and mold temperature are too low, plastic has difficulty flowing under current pressure, injection speed is too slow, or holding pressure is too low. Plastic is not fully melted and has poor fluidity, resulting in a large loss of injection pressure. Increase number of gates, gate position should be arranged reasonably, and multi-cavity unbalanced layout is filled. Cold material well in runner is not reserved enough or improperly, cold material head enters cavity and hinders normal flow of plastic, increasing cold material hole. Nozzle, runner and gate are too small, process is too long, and plastic filling resistance is too large. When mold is not vented well, air cannot be removed.
Burring & Flashing	When plastic melt flows into parting surface or insert mating surface, Burring will occur. Clamping force is sufficient, but there is a film of excess plastic material at junction of main runner and branch runner, which is Flash.	Clamping force is insufficient, and high-pressure plastic injected into cavity creates a gap between parting surface or insert mating surface, and plastic melt overflows into this gap. Mold (fixed side) does not fully contact machine nozzle, and a gap is created between male and female molds. (Not tightened) Influence of mold temperature on crankshaft clamping system. Improve strength and parallelism of mold plate. Mold guide sleeve is worn/mold mounting plate is damaged/tie rod (coring column) is not strong enough and bends, causing parting surface to shift. Foreign matter adheres to parting surface, and exhaust groove is too deep. Cavity projection surface is too large/the plastic temperature is too high/over-pressure is maintained.
Surface shrinkage, shrinkage holes (vacuum bubbles)	Phenomenon of depression on the surface of product. It is caused by volume shrinkage of plastic, and is common in local thick areas, such as intersection of reinforcing ribs or column positions and surfaces. Vacuum bubbles caused by volume shrinkage of local thick parts of product during cooling process are called shrinkage holes (Void). When plastic melt contains air, moisture and volatile gases, air, moisture and volatile gases enter interior of product during injection molding process, and remaining cavities are called bubbles.	Insufficient injection pressure and holding pressure, insufficient shrinkage compensation of plastic melt. Insufficient holding time of holding pressure, insufficient shrinkage compensation of plastic melt, and easy to cause backflow (backfow) Injection speed is too slow, and shrinkage compensation of plastic melt is insufficient. Insufficient injection volume. Material temperature and mold temperature are too high, cooling is slow, plastic shrinks completely and shrinks and sinks. Runner and gate size are too small, pressure loss increases, gate solidifies too early, and shrinkage compensation is poor. Local meat is too thick. When CUSHION VOLUME residual of injection molding machine is insufficient or check valve does not work smoothly, uniform wall thickness of product will also shrink, and there will be waves on the surface of product. Increase size of gate and runner to effectively apply pressure to thick part of molded product. If necessary, position of glue port can also be adjusted. Increase holding pressure and extend holding time. Increase filling speed to achieve full compression before plastic cools and solidifies. Injection to holding pressure too fast: Make thickness change smoother and improve cooling efficiency of this part. Pre-dry plastic pellets to remove moisture. Barrel temperature should not be set too high to effectively prevent generation of plastic decomposition gas. Replace a small screw or machine to prevent screw from over-shearing. Increase back pressure so that gas can be discharged from cylinder. Appropriately reduce filling speed so that gas has enough time to be discharged.
Silver Streak	Silvery white stripes appear on the surface or near surface of product along direction of plastic flow. Silver streaks are generally caused by vaporization of moisture or volatiles in plastic or moisture attached to surface of mold. Sometimes silver streaks are produced when screw of injection molding machine is entrained in air. Material has impurities.	Plastic contains moisture, volatiles, and insufficient drying. Plastic melt is overheated or stays in barrel for too long and decomposes, producing a large amount of gas, which is not completely discharged, and silver streaks are produced during solidification. Mold temperature is too low, and plastic melt solidifies quickly, resulting in incomplete exhaust. Oil, moisture, or mold release agent is attached to mold surface, which evaporates into gas and liquefies as plastic melt cools and solidifies. Screw is drawn into air, and lower part of hopper is cooled sufficiently. Temperature on hopper side is low, and there is a temperature difference with barrel. Rubber particles often scratch screw and easily bring in air. Poor exhaust at the beginning of injection. Plastic melt ejected at the beginning solidifies quickly, so gas is not completely discharged, and silver streaks occur. Injection pressure is too high and injection speed is too fast. When thickness changes dramatically, compressed plastic melt in flow is rapidly decompressed and expanded, and volatile decomposition gas liquefies after contacting mold cavity.
Burn Mark	So-called burn mark generally includes discoloration of product surface due to plastic degradation and charring of filling end of product. Burning is air trapped in cavity that cannot be quickly discharged (idle air) when plastic melt is filled, is compressed and significantly heated, burning material. Poor exhaust.	Enhance exhaust in trapped air area to allow air to be discharged in time. Reduce injection pressure, but it should be noted that injection speed will slow down after pressure drops, which is easy to cause flow marks and weld marks to deteriorate. Adopt multi-stage controlled filling, and use multi-stage deceleration at the end of molding process to facilitate gas discharge. Use a vacuum pump to extract air in cavity so that cavity is filled under vacuum. Clean exhaust groove to prevent blockage. Gate is too thin or too long, resulting in plastic degradation, Exhaust groove, exhaust inserts, etc.
Surface flow lines (flow marks), water ripples	Traces of plastic melt flow, with gate as the center, present a striped wave shape. Countless fine lines occur on the surface in a vertical direction, resulting in fingerprint-like ripples on the surface of product.	Flow marks are caused by plastic melt that initially flows into cavity cooling too quickly, forming a boundary with plastic melt that flows in later. If cold material remaining at the front end of injection molding machine nozzle directly enters cavity, it will cause flow marks. If temperature of plastic melt is low, viscosity will increase and flow marks will occur. If mold temperature is low, a large amount of heat from plastic melt will be taken away, causing temperature of plastic melt to drop, and viscosity will increase, resulting in flow marks. If injection speed is too slow, temperature of plastic melt will decrease and increase during filling process, and viscosity will increase, resulting in flow marks. During mold filling process, temperature of plastic melt in cavity drops, and it is filled in a high viscosity state. Plastic melt that contacts mold surface is pressed in a semi-solidified state, and countless fine lines in vertical flow direction occur on the surface, resulting in ripples similar to fingerprints on the surface of product. When temperature of plastic melt drops further, it solidifies if filling is not complete, resulting in insufficient filling. Ripples often occur near edge of product and at the end of filling.
Water-trapping lines (weld marks), jet lines.	When mold adopts a multi-gate pouring scheme, flow fronts of rubber material merge with each other; in hole position and obstacle area, flow front of rubber material will also be divided into two; uneven wall thickness will also cause welding marks. Plastic melt passing through gate at high speed directly enters cavity, then contacts cavity surface and solidifies, and is then pushed by subsequent plastic melt, leaving snaking marks. For side gates, when there is no stagnation area or insufficient stagnation area after plastic passes through gate, spray marks are easily generated.	Reduce number of gates. Add a material overflow well near fusion part, move fusion line to overflow well, then cut it off. Adjust gate position. Change gate position and number, and move location where fusion line occurs to another place. Strengthen exhaust in fusion line area, quickly evacuate air and volatiles in this part. Increase material temperature and mold temperature, enhance fluidity of plastic, and increase material temperature during fusion. Increase injection pressure and appropriately increase size of gating system. Increase injection speed. Shorten distance between gate and fusion area. Shorten distance between gate and weld area. Reduce use of mold release agent. Adjust gate position so that plastic melt hits pin or wall after passing through gate. Change gate form, use overlapping gates or lug gates, and set up sufficient material retention areas in gate area. Slow down initial injection speed of plastic melt, Increase gate thickness / cross-sectional area, so that flow front is formed immediately Increase mold temperature to prevent material from solidifying quickly.
Surface cracks (cracking), internal stress cracking, top whitening (top explosion)	Severe and obvious cracks on the surface of product are called cracking; hair-like cracks on the surface of product, which often appear at sharp corners of product, are called crazing, also often called stress cracking.	Poor demoulding; excessive internal stress; weld line location; gate cracking. Shrinkage difference leads to excessive internal stress. Overfilling. Excessive injection pressure leads to overfilling, excessive internal stress of product, and cracks during demoulding. At the same time, deformation of mold accessories also increases, making demoulding more difficult, and ribs often crack. Interface cracking in insert-molding. Expansion coefficient of plastic is several times that of metal. After molding, shrinkage generates stress, causing crazing in this part, and in severe cases, cracking. When mold surface temperature is too low, fiber solidifies in a stretched state, and stress caused by orientation often causes cracking. Chemical substances and ultraviolet rays. Annealing treatment-product shrinks more after molding, and cracking often does not occur immediately, but occurs after a period of time. The incubation period of stress cracking is about 21 days.
Surface color difference, color mixing, poor gloss, insufficient transparency	Surface of product loses its original gloss, forming a milky white film or a blurred state, which can be called poor surface gloss.	Partial degradation; uneven plasticization; uneven mixing of recycled materials. Uneven cooling; plastic degradation. Poor polishing of mold surface. Mold temperature is too low. Using too much release agent or greasy release agent is also a cause of poor surface gloss. Material moisture absorption or contamination with volatiles and foreign substances is also one of reasons for poor surface gloss. When there is too much lubricant or high volatile content, plastic will gasify after passing through gate due to pressure drop, condense on the surface of mold cavity, and produce milky white blur. When lubricant particles are too large, thick white stripes will occur. Raw materials contain impurities; material degradation; low temperature and poor plasticization. High temperature mold helps improve transparency; insufficient drying; poor polishing.
Black streaks and spots on the surface	Product has black stripes. Main reason for this is thermal decomposition of plastic material; this is common in materials with poor thermal stability.	Product is small, size of material cylinder is large, and plastic is retained for too long and decomposed; Recycled material is added in an improper proportion, and it is decomposed by repeated heating. Screw is partially damaged or gap of check ring is large. Special attention should be paid to materials with high viscosity. Abnormal temperature rise of plastic causes partial decomposition of plastic. Screw rubber material bites poorly and too much air is involved. When plastic lubricant is insufficient, friction is severe, too much shear heat is generated, and poor exhaust causes combustion. Add appropriate lubricant, but at a dosage of 0.2%, flammable volatiles of lubricant make combustion easy and produce black streaks.
Warpage	Deformation can be divided into two phenomena: warpage and torsion. Deformation of parallel sides is called warpage; deformation in diagonal direction is called torsion.	Uneven thickness and uneven cooling. Cooling speed of plastic is different. Places where it cools faster shrink less, and places where it cools slower shrink more, thus causing deformation. Material temperature is high, shrinkage is large, and thus deformation is large. Difference in molecular orientation; inward bending of side wall. Deformation caused by internal stress during demolding of product is caused by product being ejected from mold before it is fully cooled and solidified. Generally, in order to prevent deformation of product, product can be fixed with a fixture after ejection to correct deformation or prevent further deformation. However, if product encounters high temperature again during use, it will recover again, so special attention should be paid to this point.
Dimensional Variation	Size of plastic parts depends on: plastic model, including additives; mold shrinkage index; molding conditions. Since mold and plastic material have been determined, molding operator can only adjust size of plastic parts by changing molding conditions. (1) Excessive shrinkage between or within plastic batches; (2) Fluctuation in molding process conditions; (3) Out-of-tolerance mold cavity size; (4) Large differences in product wall thickness; (5) Temperature errors during measurement and use.	The overall shrinkage of plastic part is unreasonable. Difference in dimensional variation in length and width. Change in internal dimensions of plastic part. Dimensional difference between cavities.

Defects	Cause analysis and countermeasures
Stick mold & drag hurt	Insufficient demoulding angle. Insufficient ejector pins. Poor polishing. Too many ribs and undercuts. Overcharge and over-pressure holding. Improve cooling. Too much pressure, overcharge and over-pressure holding. Conversion from injection to pressure holding is too slow.
Insufficient strength (brittle fracture)	Degradation, molecular chain breaks and becomes smaller. Weld line. Poor crystallization. Excessive residual internal stress. Incompatible materials. Too much recycled material. Overdrying.
Surface cold material spots, peeling (delamination)---Delamination	Unmelted plastic enters cavity and appears on the surface of product. Surface of product shows a thin layer of mica-like cracks. Temperature of plastic melt is too low to melt rubber particles entering the cavity. Too much recycled material brings in too much air, making rubber material unevenly mixed. Incompatible materials are mixed; screw speed is too low. Mixing of different materials or improper molding conditions. Shear stress is too large. When mold temperature is too low, interface inside flowing material will also cause peeling. Excessive use of release agent; plastic degradation; insufficient plastic drying.
Bad metal inserts, blind holes, broken needles, uneven ejector positions	Poor mold matching. Flash and burrs. Poor concentricity. Unreasonable length. Ejection too early.
Nozzle drooling (running), glue leakage, nozzle drawing	R angle does not match. Mold is not installed correctly, nozzle is not aligned. Nozzle damage. Screw is not loose enough: Insufficient drying (nylon + glass fiber) Material temperature is too high, loosen amount appropriately (5-6mm).
Nozzle is blocked, mold opening is difficult	Iron cuttings, small screws and other debris are mixed into material: Temperature is too low. Properly withdraw turret Guide pin and guide sleeve are not well vented, prevent vacuum, High pressure, overcharge and over-pressure maintenance
Screw slippage, plasticizing noise	Temperature is too high. Hopper outlet is frozen or blocked. Temperature is not high enough (noise), such as PC, PMMA.

Product design structure

Introduction to hot runner mold

Hot runner injection mold is the most common type of runnerless condensate injection mold. Pouring system adopts insulation or heating methods to keep plastic melt in a molten state without solidification. Its equipment mainly includes runner plate, nozzle and temperature controller.
Advantages of hot runner: 1) Reduced raw material loss, 2) Runner condensate does not need demolding, easy to realize automated production, 3) Long process runner is possible, which also ensures consistency of multi-cavity injection molded parts and improves precision of products, 4) It is conducive to transmission of melt pressure, pressure loss in runner is small, and filling balance can be achieved, 5) It is easy to realize injection processing of large thin-walled products, 6) Strengthen function of injection molding machine and improve injection process.
Disadvantages of hot runner: 1) High technical difficulty in mold manufacturing and injection production, operation experience is required, and maintenance is difficult (mold installation is not only mechanical assembly but also instrument and electrical installation), low one-time injection success rate of new mold, 2) High mold cost, special materials, professional design, 3) Plastics that are sensitive to heat and shear are easily damaged by heat, and rubber must be clean and free of debris, 4) Precise temperature control is required, 5) Designed for a certain rubber (for different nozzles), 6) Due to diameter of nozzle, distribution and number of some small cavities are limited, 7) Colored plastics and large viscosity changes are difficult to replace.
Hot runner brands: American Incoe Hard Shell (First Home), Shanghai Kron KN, etc.
Feed system--Insulation uses ceramics, titanium alloy, insulation and radiation sheets; heating uses beryllium copper or molybdenum alloy as heat-conducting elements to keep plastic melt in a molten state without solidification.
Gate--There are two types (main runner type straight gate and ejector gate). Melt temperature of point gate is higher (compared with cold runner), and gate diameter should be carefully considered: There are two setting methods (end of nozzle shell, on insert of fixed mold plate), filling shear rate near point gate is high, and curing residual stress is large (easy to crack), so wall thickness opposite gate can be appropriately increased locally.
Runner plate--To withstand force of high-pressure melt in runner and thermal expansion of each nozzle, it must have sufficient rigidity and strength. connection, fastening and sealing of runner plate (and nozzle, etc.) should reliably prevent melt leakage. Runner usually has a circular cross-section, with internal heating and external heating (mostly tubular elbow type and less round rod type), and is made of hot-working mold steel; runner plate pressure ring must be made of materials with good rigidity and poor thermal conductivity.
Nozzle-There are main runner nozzles and gate nozzles (they must be insulated from cold mold), which must have heating (coil type) and temperature measuring elements (thermocouples), some with filter core sleeves, pneumatic and hydraulic types, and hot-working mold steel (molybdenum-titanium alloy)
Temperature controller (heating and temperature measuring elements)--Heating system has its own control circuit, and thermocouples are set at two important points (gate and high temperature) to prevent melt decomposition.
Replaceable parts are well surface treated (chrome plating, nickel plating or titanium nitride treatment) to ensure that gate has a suitable temperature and is reliable and leak-free (sealing is mechanically matched and sealed with stainless steel sealing rings).
Before producing thermosensitive plastics (PBT, PET, POM, PVC, etc.), attention should be paid to establishing insulating skin layer of nozzle. Thermostable plastic (such as PA66) can be used for injection filling to establish a stable insulating skin layer: process parameters during processing are prohibited from changing: runner plate and nozzle should not be heated and cleaned with flame of a blowtorch to reduce hardness of parts; injection of thermosensitive plastics cannot be interrupted (it is easy to cause nitriding separation), and runner should be cleaned stably before shutting down. Use plastics with a temperature close to that of good thermal stability (such as HDPE). For high-temperature plastics, use PC materials to cool down first and then use HDPE or PP materials; plastics of same type and viscosity are easier to change color, and natural color material is used to fill sealed insulating skin layer at low pressure and slowly.
Heating element is easily affected by moisture, and insulation of wire is easily invalid when it is above 100℃. Wires and wires should be above mold.

Introduction to gas-assisted forming

Gas-assisted injection system--is to inject molten plastic into mold cavity with the help of gas (nitrogen) (compressed air or oil pressure <350bar can be used). All pressurized gas expands inside plastic part to form a cross section of plastic part but maintains a complete shape. It combines advantages of foam structure and injection molding.
Features of gas-assisted injection: 1) Reduce molding pressure (and evenly distribute pressure), reduce residual internal stress due to pressure concentration, thereby reducing deformation of finished product, improving dimensional stability, solving many injection molding difficulties, making mold design more flexible, and replacing injection molding machine's pressure holding procedure. Pressure holding can be distributed locally or evenly. Gas can be injected from parting line or surface of finished product and multiple points, and segmented pressure (rise, rise, high pressure holding, decrease, low pressure holding and decrease) is controlled, and nitrogen can be recycled. Number of gates and runners of mold can be reduced (improving joint line and idle gas). 2) Reduce injection pressure. 3) Reduce plastic weight. 4) Eliminate shrinkage marks in thick-wall molding (improve appearance and finish). 5) Shorten molding cycle. 6) Reduce production costs (reduce plastic by 30%). 7) Applicable to both thick-walled and thin-walled plastic parts (all thermoplastics, general engineering plastics and some thermosetting plastics).
Injection cycle:
1) Injection period - required amount of melt must be found through experiments to ensure that gas will not break surface of finished product during inflation period.
2) Inflation period - gas can be injected at different times before, during and after injection period. Gas pressure must be greater than plastic pressure to make product hollow. It can be triggered by screw stroke or time.
3) Gas pressure holding period - when interior of finished product is filled with gas, it can reduce shrinkage and deformation of finished product.
4) Demolding period - air pressure in mold drops to atmospheric pressure.
Precautions:
1) Prevent trapped air
2) Prevent gas from entering thin wall of finished product
3) Prevent gas from breaking through surface of finished product
4) Prevent plastic from advancing slowly and causing surface of finished product to be uneven
5) Determine position and size of fusion line
6) Ensure uniform gas filling and select direction of gas flow
7) Keep distribution of airway as uniform as possible to avoid slow shooting
8) Slow shooting can easily cause gas to break through surface (especially for reinforced plastics)

Introduction to CAE Technology

Using the principle of finite element method, computer software and hardware are used as tools to assist engineering analysis, simulate melt flow and mold filling state, and conduct flow, pressure, stress, temperature analysis, cooling and solidification, shrinkage and thermal deformation analysis; it also uses injection molding numerical simulation technology to quickly and effectively solve injection molding site problems; optimize product design, gate location determination, and improve injection process parameters.
CAE technology includes a comprehensive software system of numerical calculation technology, computer graphics, engineering analysis and simulation, and numerical library. For injection molding processing, its theoretical basis is rheology and heat transfer of polymers.
Help users to perform diagnostic work to solve existing or potential problems in engineering. When materials, designs or conditions change, it can help users understand impact of these changes on product quality and productivity. CAE analysis can be performed at various stages of concept design, product design, mold design, mold opening, mold trial, production to shorten time of each stage, reduce errors and waste, increase success rate, and enhance competitiveness of enterprises.

Gud Mould Industry Co., Ltd