Gas-assisted injection mold design should consider impact on process parameters

 Gas-assisted injection mold design should consider impact on process parameters

Technical characteristics of gas-assisted injection moulding

(1) Design of mold cavity should try to ensure flow balance to reduce uneven penetration of gas. To ensure flow balance is also a design principle of ordinary injection molding molds, but it is more important for gas-assisted injection molding products.

(2) Mold design should consider impact on process parameters, because gas-assisted injection molding is much more sensitive to process parameters than ordinary molding. In injection molding, slight differences in mold wall temperature or injection volume will cause asymmetry in gas penetration in symmetrical parts.

Gas-assisted injection molding equipment

(1) Ordinary injection molding machine (a higher accuracy of material counting is better).

(2) Nitrogen control system, including self-enclosed gas-assisted nozzle.

(3) High-pressure nitrogen generator.

(4) Industrial nitrogen cylinders and air compressors that provide supercharged power.

(5) Moulds designed and manufactured for gas-assisted injection.

(6) Gas-assisted injection molding gas-assisted nozzle.

Nozzle air inlet method, that is, using a dedicated self-enclosed gas-assisted nozzle. After injection of plastic, high-pressure gas is directly entered into plastic through nozzle, an extended enclosed space-air cavity is formed according to airway, and a certain pressure is maintained at a certain pressure until it is cooled. Before mold is opened, nozzle is forcibly separated from forehearth of product by retreating seat, so that gas is discharged from product.

(7) Air needle

Air-needle air intake method is to install an exhaust device—air-needle in a specific position of mold. When plastic is injected into cavity, air needle is wrapped in plastic; at this time, high-pressure gas is discharged, gas needle forms an extended enclosed space in plastic according to air passage—gas cavity, maintains a certain pressure until it cools. Before mold is opened, gas in air cavity is discharged from plastic by control device by air needle.

Gas-assisted injection molding process can be divided into four stages:

First stage of gas-assisted injection molding: plastic injection. Melt enters cavity and encounters lower temperature mold wall, forming a thinner solidified layer.

Second stage of gas-assisted injection molding: gas incident. Inert gas enters molten plastic, pushing unsolidified plastic in the center into cavity that has not yet been filled.

Third stage of gas-assisted injection molding: end of gas incidence. Gas continues to push plastic melt to flow until melt fills entire cavity.

Fourth stage of gas-assisted injection molding: gas pressure keeping. In pressure-holding state, gas in gas channel compresses melt and feeds to ensure appearance of part.

Basic principles of gas-assisted product and mold design

(1) When designing, first consider which wall thicknesses need to be hollowed out and which surface sink marks need to be eliminated, then consider how to connect these parts to become airways.

(2) Large structural parts: fully thinned, and partially thickened to form airways.

(3) Air passage should be evenly distributed to entire cavity according to main material flow direction, and closed-circuit air passages should be avoided at the same time.

(4) Cross-sectional shape of air passage should be close to a circle to make gas flow smoothly; cross-sectional size of air passage should be appropriate, too small an air passage may cause gas permeation, too large an air passage may cause weld marks or cavitation.

(5) Airway should extend to final filling area (usually on non-appearance surface), but does not need to extend to edge of cavity.

(6) Main airway should be as simple as possible, branch airway length should be as equal as possible, and end of bronchus can be gradually reduced to prevent gas from accelerating.

(7) Airway can be straight but not bend (the fewer bends, the better), and a larger fillet radius should be used at the corners of airway.

(8) For multi-cavity molds, each cavity needs to be supplied by an independent gas nozzle.

(9) If possible, there is a second option not to let gas advance.

(10) Gas should be confined to airway and penetrate to end of airway.

(11) Precise cavity size is very important.

(12) Uniform cooling of each part of product is very important.

(13) When gate is used for air intake, balance of flow is very important for uniform gas penetration.

(14) Accurate injection volume of melt is very important, and error of each injection volume should not exceed 0.5%.

(15) Setting up an overflow well at final filling position can promote gas penetration, increase airway hollowing rate, eliminate hysteresis marks, and stabilize product quality. Addition of a valve gate between cavity and overflow well can ensure that final filling occurs in overflow well.

(16) When gas nozzle is in air, small gate can prevent gas from flowing back into runner.

(17) Gate can be placed on a thin wall, a distance of more than 30mm from air inlet should be kept to avoid gas penetration and backflow.

(18) Gas nozzle should be placed at thick wall and at place farthest from final filling.

(19) Direction of air outlet of gas nozzle should be as consistent as material flow direction.

(20) Keep melt flow front advancing at a balanced speed while avoiding formation of a V-shaped melt flow front.

(21) When short-material injection is used, volume of unfilled cavity before air intake shall not exceed half of total volume of airway.

(22) When using full-material injection, refer to plastic pressure, specific volume and temperature relationship diagram, so that half of total volume of airway is approximately equal to volume shrinkage of plastic in cavity.

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