Injection Molding products China manufacturer

Fingering, gas bubbles, hesitation lines, burning of resin, witness lines, cold slug, and gas blow-out are typical defects normally encounterted in GAIM.
Fingering, or gas permeation, is a common problem encountered in GAIM. In fingering, gas escapes from the gas channel and migrates into undesired areas of the part. Severe gas fingering can results in significant reduction in part stiffness, impact strenghth and reliability of the final molded parts. During the has holding phase, the transitional region between the gas channel and the flat area is possible for fingers to form within the flat area. In this case, the main cause if the fingering effact is the thickness of the flat area of the part. The thicker flat area, the higher its shrinkage potential, and hence the greater danger of the fingering effect. In order to largely exclude the fingering effect through design, it is necessary to implement the following criteria: a basic wall thickness of 4mm or greater should be avoided for flat areas, a material with favorable solidification behavior sould be selected, and the lowest possible gas pressure should be applied.
Gas bubbles are caused by fingering. When fingering occurs, gas sometimes gets trapped in the thin-wall sections of the part where the gas is unable to fully vent. These trapped gas can cuase bubbles that will still be in the gas core after the mold is opened.
Hesitation lines appear on the surfac of a part produced by GAIM when the short shot of resin stops in the cavity, when starts moving again as the gas completes the fill.
Burning of the resin can appear on either the outer surface of the part or within the gas channel itself. Burning if the part surface can be caused by gas pressure that is too high or by insuffucient venting of mold. Burning the resin within the hollow sections of the part is also possible. Burning within the gas channel can cause gas injection pins to become plugged.
On thin-walled parts, molded in certain resins, a witness line, or gloss-level change, can occur over the gas channel. Excessive gas pressure can also cause witness lines over gas channels.
When gas is injection through the molding machine nozzle, cold slugs resin many occur on the part surface. A cold slug is caused when a samll amount unmeleted resin is injection into the part.
Gas blowout occurs when there is not enough resin in the cavity to hold the gas inside the part. If the part is short, gas will migrate to the non-filled area of the cavity and blow through. When blowout occurs, the part will sometimes look like a short shot.
Most case of defects are produced by the interface of the gas and the melt. There problems can be overcome by internal water cooling between the interface of the gas and the melt.

All the above the described details are come from the production experience of plastic injection molding process of Wuxi Glory Plastics Co. Ltd, All rights reserved.

All processes have their disadvantages, but those of GAIM and GAMIC ( Gas-assisted injection molding with internal-water cooling) appear relatively minor compared with their significant advantages.

1. Large hollow sections
GAIM is not well suited for thin-walled hollow parts such as bottles or tanks. However, the thin-wall part has also tried out for some specific applications.

2. Vent hole
The gas must be vented prior to opening the mold, leaving a hole somewhere on the part. Normally this can be placed in a non-visible location, but if appearance or function ia affected or secondary operations are required, it may be necessary to seal the hole.

3. Mold temperature control
Since wall thickness along the gas flow channel is a function of cooling rate, consistent wall thickness requires precise mold temperature control.

4 Surface blush
The gas channel may leave surface blush, with arises from differences in surface gloss levels. The tendency for blush is a function of processing conditions and types of plastics.

5. Unique design
The unique part design and mold design required is most cases to fully utilize that GAIM might to be considered by some to be a disadvantages. The gas part design takes a relatively longer time than with the conventional injection molding process.

6. Extra cost of controller
In order to control the gas injection, the process requires extra equipment. Gas-assisited injection molding with internal cooling requires a system for controlling the gas and the water, and expense not required with traditional injection molding process.

Gas injection provides a solution to a number of problems that occur in conventional injection molding.

1. Reducing stress and warpage
With gas, the pressure is equal everywhere throughout the continuous network of hollow channels. When designed properly, these provide an internal runner system within the part, enabling the applied pressure, and therefore the internal stress gradients, to be reduced markedly. This reduces a part’s tendency to warp.

2. Elimination of sink marks
Sink marks resulting from ribs ot bosses on the back side of a part have long been a problem. These surface marks result from the volume contraction of the melt during cooling. Sink mark can be minimized or eliminated if a hollow gas channel can be directed between the front surface of the part and the back side detail. With a channel as the base of a rib, material shrinka are away from the inside surface of the channel as the molded part cools because the material is the hottest at the center. Therefore, no sink mark occours on the outside surface as the part shrinks during cooling.

3. Smooth surface
Unlike structural foam, gas injection permits lighter weight and saves material in a structurally rigid part. With gas holding, a good surface quality can be achieved.

4. Reduced clamp tonnage
In conventional injection, the highest pressure occurs during the packing phase. The maximum injection pressure is significantly lower in GAIM and a controlled gas pressure through a network hollow channels is used to fill out the mold. This means that clamp tonnage requirements can be reduced by as much as 90%.

5. Elimination of external runners
One of the best features of gas injection is that flow runners can be bulit right into the part. Frequently, all external runners can be eliminated, even on a larger and complex part. These benefits include the reduced tooling costs, the lower quantities of regrind from runners, and the improvement of temperature control over the plastic melt. Ofter the internal runners can be improve the flow pattern in the mold and eliminate or control knit-line locations resulting from multiple injection gates. In addition to serving as flow channels, the ribs and thick sections can provide structural rigidity when required.

6. Permitting different wall thickness
A constant wall thickness is maintained in the plastic parts, With gas injection, this design rule is flexible. Different wall thickness are possible if has channels are designed into the part at the transition point. This permits uniform material flow in the mold and avoids the high stress and warpage that normally results from this sort of geometry.

7. Cycle time Reduction
Compared with structural foam, gas-injection parts do not have the same inherent insulating characteristics, so that cycle time are faster-reportedly even faster than would be conventional injection of the same part with no hollow sections.

8. Resin saving
Gas asssit plays a direct role in part-weight saving in the conventional of current tools. The main factor is reducing weight is that the part cavity is never completely filled. Another major contributor to resin saving is scrap reduction. With proper tool design, gas assisted allows scrap-free startups amd production runs.

Write by David at 2nd, Oct. More details about plastic injection molding please visit our site: http://www.plastics-china.com
Thanks!

Injection molding is a very popular operation for production of commercial plastic parts with it’s sophisticated control and superior surface details. However, it has limitations, such as long cycle time for parts parts with thick section due to slow cooling. Also packing of thick section can produce sink makrs on the part surface. Large thin parts can have warpage because the residual stress and strain induced during filling and packing. Thus traditional injection molding can ne modified to solve these kinds of problems, also to improve the quality of the part and lower the cost of the production.

Currently, gas-assisted injection molding is in use and being developed worldwide. In the US, the process is knowns as Gas- Assisted Injection Molding (GAIM); it is also called Gas Injection Technique (GIT) in Europe. This process is developed for the hollow plastic parts with separate internal channels. It is unique because it combines the advantage of conventional injection molding and blow molding while differing from both. GAIM offers a cost effective means of producing large, smooth surfaced and regid parts using lower clamping pressure with little or no finishing. By introducing the gas before complete filling, numerous problems such as warpage, sink marks, and high filling pressure are mostly overcome. Moreover, the process gives great benefits in terms of higher stiffness-to-weight ratio tha the solid parts whith the same overall dimensions due to the elimination of material placed inefficiently near the neutral axis of the corss section, thus increasing the freedom of part design.

In comparsion with coventional injection molding, the gas-assisted process is more critical in terms of process control, especially for multi-cavity applications. The quality of the part is determined by both tool and process variable such as degree of under-fill, gas injection condictions, and mold temperature, thus indicating the importance of process control. The process is attracting many molders due to the demand for high auotomated production of gas-assisted injection molded parts. For more details please visit: http://www.plastics-china.com thanks!

Hot Runner Systems
Hot runners are cassifed according to the ways they are heated: insulated-runner sys tems ( It is not described in this artice) and genuine hot-runner systems.
The later can be further sub-cassified according to the types of heating: internal heating and external heating.
Heating is basicaly perormed electricaly by cartridge heaters, heating rods, band heaters, heating pipes and coils， etc. To ensure uniform fow and distribution of the melt, usualy a relatively eaborate control system comprising several heating circuits and an appropriate number of sensors is needed. The operating voltage is usualy 220 V to 240 V, but smal nozzles fequently have a low volage of 5 V, and also 15 V and 24 V operating voltage.
Runner systems in conventional molds have the same temperature leve as the rest of the mold because they are in the same mold block. If, however, the runner system is located in a special manifold that is heated to the temperature of the melt, al the advantages listed below accrue. Runner manifolds heated to mel temperature have the task of distributing the mel as far as the gates without damage. They are used for al injection molded thermoplastics as wel as for crosslnking plastics, such as elastomers and thermosets.
In the case of thermoplastics, these manifolds are usualy refered to as the hot-runner system, the hot manifold, or simply as hot runners. For crosslinking plastics, they are known as cold runners.

A. Hot-Runner Systems
Hot-runner systems have more or less become established for highly-automated production of molded plastic parts that are produced in large numbers. The decision to use them is almost always based on economics, i. e. production size. Quality considerations, which played a major role in the past, are very rare now because thermoplastics employed to  day are almost al so stable that they can be processed without dificulty with hot-runner sys tems that have been adapted accordingly.
Hot-runner systems are available as standard units and it is hardly worthwhile having them made. The relevant suppliers ofer not only proven parts but also complete systems tailored to specifc needs. The choice of individual parts is large.
B. Economi Advantages and Disadvantages of Hot-Runner Systems
1 Economic Advantages
Savings in materials and costs for regrind.
Shorter cycles; coolng time no longer determined by the slowly solidifying runners; no nozzle retraction required.
Machines can be smaler because the shot volume - around the runners一is reduced, and the camping forces are smaler because the runners do not generate reactive forces since the blocks and the manifold block are cosed.

2. Economic Disadvantages
Much more complicated and considerably more expensive.
More work involved in running the mold for the first time.
More susceptible to breakdowns, higher maintenance costs (leakage, failure of heating elements, and wear caused by filed materials) .
3. Technological Advantages
Process can be automated (demolding) because runners do not need to be demolded. Gates at the best position; thanks to uniform, precisely controled coolng of the gate system, long flow paths are possible.
Pressure losses minimized， since the diameter of the runners is not restricted.
Artificial balancing of the gate system; balancing can be perormed during running production by means of temperature control or special mechanical system (e. g. adjustment of the gap in a ring-shaped die or use of plates in fow channel Natural balancing is beter). Selective infuencing of mold filing; needle valve nozzles and selective actuation of them pave the way for new technology (cascade gate system: avoidance of fow lines, in-mold dec- oration) .
Shorter opening strke needed compared with competing, conventional threeplaten molds. Longer holding pressure, which leads to less shrinkage.
4. Technological Disadvantages
Risk of thermal damage to sensitive materials because of long fow paths and dwel times, especialy on long cyces.
Elaborate temperature control required because non-uniform temperature control would cause diferent melt temperatures and thus non-uniform filling.
C. Design of a Hot-Runner System and is Components
Hot-runner molds are ambitious systems in a technological sense that involve high tech  nical and fnancial outay for meeting their main function of conveying melt to the gate with  out damage to the material Such a design is demonstrated.
D. Externaly /Internaly Heated Systems
The major advantages and disadvantages of the two types are immediatey apparent from reaserches.
E. Externaly Heated System
1 Advantage
Large fow channels cause low fow rate and uniform temperature distribution.
2. Disadvantage
The temperatures required for external heating have to be very much higher. For PA 66， for example， the mold temperature is approximately 100°C and the manifold temperature is at least 270 °C ; this means there is a temperature diference of approximately 170°C from the mold block, which means:
Special measures required for fxing the hot - runner nozzles to the gates because of the considerable thermal expansions.

1. The hopper

Thermoplastic material is supplied to molders in the form of samll pellets. The hopper on the injection molding machine holds these pellets. The pellets are gravity-fed from the hopper throat into the barrel and screw assembly.

2. The barrel

The barrel of the injection molding machine supports the reciprocating plasticizing screw. It is heated by the electric heater bands.

3. The reciprocating screw

The recipprocating screw is used to compress, melt, and convey the material. The reciprocating screw consists of three zones: the feeding zone; the compressing zone; the melting zone.

While the outside diameter of the screw remains contrast, the depth of the flights on the reciprocating screw decreases from the feed zone to the begainning of the meleting zone. These flights compress the material against the inside diameter of the barrel, which creates viscous heat. This shear heat is mainly responsible for meleting the material. The heat bands outside the barrel help maintain the material in the molten state. Typically, a molding machine can be have three or more heater bands or zones with different temperature settings.

For thermoplastics, the injection molding machine converts granular or pelleted raw plastic into final molded parts via a melt, injection, pack, and cool cycle. Atypical injection molding machine consists of the following major components:

 A. The Injection System

The injection system consists of a hopper, a reciprocating screw and barrel assembly, and an injection nozzle. This system confines and transports the plastic as it progresses through the feeding, compressing, degassing, melting, injection, and packing stage.