In the design and application for hot runner, many problems are worthy of consideration and special attention. How to solve these problems have a direct bearing on success of hot runner system and it quality. Hence, New Vision pool wisdom and strength of its technical department to work together with professional hot runner company. They give a discussion on hot runner system's malfunction and causes, as well as the do's and dont's in the application of hot runner mould. Undoubtedly, this will contribute to the successful use of hot runner molding technology.
Common fault analysis and its countermeasures of hot runner molding
2.1 Residues around gate,material drooling, or poor appearance
2.1.1 Main Cause
Unreasonable gate structure, incorrect temperature control, and big residual pressure from plastic melt in runner after injecting.
(1) Modification of gate structure. Usually, gate length being exceeded will leave a long gate biscuit on plastic part's surface. And gate diameter being exceeded will lead to material drooling. When the above faults occur, modifying the gate structure is the key consideration.Common gate methods for hot runner are sprue gate,pin-point gate and valve gate.
Sprue gate is characterized by large runner diameter, which makes material not easy to condense around the gate and ensure the plastic melt deep inside the cavity inject successfully. Not fast condensing and least residual stress from plastic material make this kind of gate suitable for mould with deep-cavity and multi-cavities.But this gate is usually associated with problems of material drooling and wire-drawing.It also may leave large gate mark and cylindrical biscuit. So the material temperature around gate can't be too high and must be under stable control. Pin-point gate is similar to sprue gate, expect for smaller gate mark on plastic part. Valve gate's features are less residual stress, moderate condensing rate, and inconspicuous material drooling and wire-drawing problems. As one of most popular gate methods at home and abroad currently, it can be applied to many engineering plastics. It offers high-quality plastic parts and tiny marks on their surfaces. What's more, small marks and low residual stress help valve gate avoid the problems of material-drooling and wire-drawing.But abrasion has an obvious influence on valve port. In case that material-drooling problem occurs again because of fit clearance's increasing,valve spool and valve port should be changed in time.
The selection of gate method is closely related to Plastics' resin performance.Low viscosity resin, easily causing material-drooling, should choose valve gate. Molding temperature range for crystalline resin is limited, and the temperature around gate should be a little high. Resins like POM and PPEX can choose gate method with heating probe. Molding temperature range for amorphous resins like ABS and PS is wide. It can quicken condensing because of Melt insulating barrier at the head of torpedo faucet spool and no heating element at the gate.
(2) Reasonable control on temperature. If cooling water around gate is not enough, thus it will cause heat concentration and further results in material drooling and wire-drawing.If the above phenomenon appears, pls strengthen the region's cooling.
(3) Pressure-relief for resin. Too large residual pressure in the runner is one of the main reasons for material drooling. As a general rule,injection machine should use buffering feedback or buffering device to avoid material drooling.
2.2 Discoloration, Decomposition or Degradation of Material
2.2.1 Main reasons: Incorrect temperature control; Large shear heat caused by undersize runner or gate; Long heating time for Stranded material caused by dead points in runner.
(1) Accurate control on temperature. In order to check temperature fluctuations accurately and swiftly, thermocouple temperature measuring head should directly contact with runner plate or nozzle wall and make sure it be the central position in every independent temperature control area.The distance between temperature feeling point on its head and runner wall should no more than 10mm. Try to make sure heating elements equally distributed at the both sides of runner.
Intelligent fuzzy logic technology under operation of central processing unit can be used for temperature control. It has temperature overrun alarm and automatic adjustment function to make sure temperature variation within the bounds of required precision.
(2)Modify gate dimension. Try to avoid runner dead points and properly enlarge runner diameter within permissive range to avoid too many shear heat. Runner radial temperature difference for melt with internal heating nozzle is very big. This is easy to cause decomposition and degradation. So pay attention to runner radial dimension. It should not be too large.
2.3 Short shot or no material shooting out
2.3.1 Main Reasons
There are barriers or dead points in runner: Runner blocking; thick condensing layer in runner
(1) While designing and processing runner, make sure melt flow to the wall arc at the corner for transition. This will help the whole runner be smooth and no dead-end flow.
(2) Under the circumstance of no affect on plastic part's quality, properly enhance material temperature to prevent gate from untimely condensing.
(3)Properly increase temperature for hot runner to reduce thickness of the internal heating nozzle's condensing layer and pressure loss,thus facilitating the filling of cavity.
2.4 Serious Leakage
2.4.1 Main Reasons
Damage of sealing components; Unequal inflation caused by the overburning of heating components; Transplacement of the center between nozzle and sprue bush; or oversize projective area of the melt insulating barrier decided by leakproof ring projected on nozzle, which leads to nozzle's recession.
(1)Check whether the sealing components and heating elements have any damages. If yes,carefully check whether it's components' quality problem, structure problem, or result of normal service life.
(2) Choose proper method to stop leakage. According to the adiabatic ways of nozzle,there are two structures to stop leakage: leakproof ring and nozzle contact. Note that leakproof contact place should be kept in the status of directly touching.
Within the limits of strength,make sure the projective area between nozzle and sprue bush be as small as possible, in case that overlarge backpressure make nozzle draw back while injecting.While choosing the leakproof way, direct contact area of nozzle and sprue bush must make sure no resin leakage even if thermal expansion causes their center slant. Of course, the contact area cannot be too large in case of increasing heat loss.
2.5 Hot runner cannot heat up normally or heating up time is too long.
2.5.1 Main Reasons
The space between wire channels is not enough, which leads to wire's break; Wires are intersecting while assembling the mould, which causes short circuit, leakage of electricity, and etc.
Use correct processing and assembling technology to ensure that all wires are placed safely and heat insulation materials are used by rules. Check wire damage at regular intervals.
2.6 Poor material changing or color changing
2.6.1 Main Reasons: Methods of changing material or color is not proper; unreasonable runner design or process resulting in many stranded material inside.
(1) Modify runner structural design and processing methods. Try to avoid runner dead points and use circular arc for transition at corners while designing runner.Make runner size minimum in the allowed range.Few stranded material and high flow velocity of new material will contribute to rapid cleaning. The processing of runner must start from one side, no matter how long the runner is. If processing from two sides at the same time, it's easy to happen that centers don't coincide and then will form stranded material place.Usually, exterior heating nozzle will not affect the melt flow since it has heating equipment.The runner can be easily cleaned. But, for internal heating nozzle, condensing layer is easy to be formed on the outside of runner.This is not good for quickly changing material.
(2) Choose correct way to change material. Usually, the procedure for hot runner system to change material and color is using new material to directly push all stranded material out of the runner. Thus the cleaning will be very easy. Instead, if the new material has a low viscosity, it will be easy to enter the center of stranded material, separating stranded material layer by layer. This will make cleaning harder. And if the viscosity between new and old materials, we can quicken the changing of material through increasing the injecting speed of new material. If the viscosity of stranded material is sensitive to temperature,properly raising the material temperature to lower its viscosity and quicken material's changing.
3. Notes for Selecting And Applying Hot Runner
In order to remove or reduce the malfunction during usage as much as possible, pay attention to the following things while selecting and applying hot runner system.
3.1 Selection of Heating Methods
(1) Internal Heating Method. Internal Heating Nozzle's structure is complicated and cost is high. It's hard to change components and has a high requirement on heating element.Putting heating element into the runner center will engender circular flow, thus increasing melt's frictional area and lowing the pressure more than 3 times that of external heating nozzle.But, because of internal heating element is in the nozzle's torpedo body, all of heat is provided to the material. It has very small heat loss and saves electric energy. If using pin-point gate,keep the tip of torpedo in the center of runner,helpful for cutting the gate after injection and leaving a low residual stress benefited from slow gate condensing.
(2) External Heating Method. External heating method can eliminate cold film and reduce pressure loss. At the same time, it's widely applied in production on account of its advantages like simple structure, convenient processing and accurate temperature control via thermocouple fixing in the nozzle center. However, external heating nozzle is not power saving as internal heating nozzle,since it causes large heat loss.
3.2 Selection of Gate Methods
The design and selection of gate directly affect plastic parts' quality. We should choose suitable gate method based on resin flow property, molding temperature and product quality requirement, in case of material drooling, material dropping, poor color changing and etc.
3.3 Temperature Control Method
After deciding the gate method, the control of melt temperature fluctuations will play a critical role in plastic parts' quality. Mostly, burnt material, degradation and runner choking are caused by incorrect temperature control,especially for heat-sensitive plastic materials, since they need reflect temperature fluctuation promptly and accurately.
Therefore, we should reasonably install heating element to keep off local overheat. And make sure the fit clearance between heating elements and runner plate or nozzle to reduce heat loss as much as possible. Meanwhile,choose advanced electronic temperature controller for meeting requirement on temperature control.
3.4 Calculation Contents After Deciding Structure of Hot Runner System
(1) Balance Calculation Between Temperature and Pressure for Every Branch Runner. The purpose of hot runner system is to inject thermoplast from injection machine's nozzle and make sure them go through runner under same temperature and allocated to every mould gate under balanced pressure.That's why we need make calculations on the temperature distribution of runners' heating zones and melt pressure entering every gate.
(2) Calculations For Center Excursion Between Nozzle And Gate Bush Caused By Thermal Expansion. That is, we should make central lines of hot nozzles (inflated) and cold gate bushing (not inflated) accurately align.
(3) Heat Loss Calculation. Internal heating runner is surrounded and supported by cooling mould sleeve, so we should precisely calculate the heat loss caused by thermal radiation and direct contact (conduction). Otherwise, the actual runner diameter will be smaller with the condensing layer on runner wall becoming thicker.
3.5 Installation Of Hot Runner Plate
We should responsibly take into account heat insulation and injection pressure bearing. Usually placing cushion block and supporter between runner plate and mold plate can bear injection pressure against leakage caused by deformation of runner plate, on the other hand, it also can reduce heat loss.
3.6 Maintenance of Hot Runner System
During the usage of mould with hot runner, it's very important to do preventive maintenance for hot runner components at fixed period. This includes examining electrical test, sealing element and connecting wire, cleaning dirt on components and so on.