Dent (collapse pits, flat shape)

Plastic cooling caused due to hardening shrinkage depressions, mainly in the thick position, ribs, chassis, insert nut on the back places.

Equipment:

(1) feed shortage. Screw or plunger wear serious, melt leakage occurs during injection and holding pressure, reduce the filling pressure and quantity, resulting in insufficient melt.

(2) the nozzle hole is too big or too small. Too small, easy to plug the feed channel, the radio will force small is too large, filling difficult.

Molds:

(1) gate is too small or too narrow runner or too shallow, low flow channel efficiency, resistance, melt prematurely cooling. The gate can not be too big, or lose the shear rate, high viscosity material, the same can not make the full article. Gates should be set up in the thick portion of the article. Set up the necessary flow path has sufficient capacity cold well can exclude cold material into the cavity so that the mold filling continues. Point gate, gate gate length of the needle must be controlled in 1mm or less, otherwise the plastic solidification in the gate fast, pressure is passed; increase the number of point gate or gate position to meet the actual needs as necessary; when the flow When the road is long and thick, it should be set in the exhaust gas flow path groove edge, reducing the barrier effect of the stream of air.

(2) Multi-runner mold to adjust the gate speed filling, preferably symmetrical open gate.

(3) The key parts of the mold cooling channels should be effectively provided to ensure cooling of the mold to eliminate or reduce shrinkage play very good results.

(4) The entire mold should be without a glitch and reliable clamping sealing, can withstand high pressure, high speed, low viscosity melt filling.

Technological aspects:

(1) increasing injection pressure, holding pressure, injection time extended. For mobility plastic, high pressure will produce flash caused the collapse pit material should be appropriately reduced temperature, lower front barrel and nozzle temperature, melt into the cavity to reduce the volume change, easy coldset; high viscosity for plastics, should increase barrel temperature, make filling easy. Contraction in the gate area should extend the dwell time.

(2) increase the injection speed can more easily make the full article and to eliminate most of the contraction.

(3) thin-walled parts mold temperature should be increased to ensure the smooth flow of material; thick parts mold temperature should be reduced in order to accelerate the curing of skin stereotypes.

(4) in the mold parts to extend the residence time of the cooling, to maintain a uniform production cycle, increase back pressure, screw front retain a certain cushion, etc. are conducive to reducing shrinkage.

(5) low-precision products should be allowed to die early out slowly cooled in air or hot water, it can not affect the use of depression gentle contraction. Raw materials: raw materials too soft prone to depression, an effective method is to add a nucleating agent in plastics to accelerate the crystallization.

Product design: product design should be uniform thickness, to avoid changes in wall thickness, such as polypropylene plastic shrink greatly, when the thickness variation exceeds 50%, instead of the best of the area with thick ribs.

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