Analysis and solution of defects in thermoplastic products

One. three indicators for evaluating the quality of plastic products

1. appearance quality: integrity, color and luster;
2. size and relative position: in line with product drawings and assembly requirements;
3. mechanical properties and chemical properties related to use: tensile testing, impact testing, environmental testing, etc.

Two. causes of product defects:

1. plastic problem: including plastic quality ingredients and drying materials.
2. adjustment problem: injection pressure, temperature and cycle.
3. mold problems: mold design, manufacturing and wear.

Mold problem is the most common and the most difficult problem to solve.

Three. products common defects analysis and Solutions

1. Insufficient filling (beer dissatisfaction, short shot): the beer rubber parts incomplete, or the cavity of the individual cavity beer dissatisfaction.
(1) improper feed adjustment, lack of material or material.
(2) injection pressure, temperature and time are not enough.
(3) material temperature is not enough.
(4) unreasonable mold design: the runner is too long, turning many; exhaust system is not good; runner cold well is not enough; multi-cavity mold cavity number is too much, misalignment, gate position, form is wrong or insufficient. The design of water transportation is unreasonable.

2. Flaping (flash): extra thin wings appear at the edges of plastic products, insert joints and thimble positions.
(1) poor accuracy of mold manufacturing.
(2) the mold design is not reasonable.
The position is unreasonable and the pressure is not balanced.
The location of the gate is improper, causing one side to lose material and one side to pull out the front.
The lock is not enough.
The mold exhaust is bad, and the parting surface is stretched under high pressure.
(3) the locking force of the beer machine is not enough.
(4) improper adjustment: too much pressure, too high temperature and too much feed.

3. silver veins (commonly known as spray silver streak): the surface bubbles are broken. When the plastic is filled with excessive water vapor, decomposition gas, solvent gas and air, the surface of the product along the direction of the flow of material to form a series of silver lustre, such as needle or mica patches.
(1) water vapor.
(2) decomposition gas: high material temperature, poor plastic quality and excessive shearing force.
(3) plastic is not pure.

4, shrink dent (shrinkage, sink mark): the surface of the plastic part is concave due to internal contraction. The main reasons are poor pressure keeping, uneven cooling, too thick wall or width of bone position, and no preheating of inserts. Terms of settlement:
(1) wall thickness as uniform as possible.
(2) strengthen cooling on thick wall.
(3) gating system design should be reasonable.
The size of A. gating system should be in line with the size, thickness and complexity of the plastic parts.
The B. gate should cope with the shrinkage of wall thickness.
(4) adjustment
A. increased injection pressure, holding time and injection time.
B. increase injection speed

5. Weld line: When melt splitting and confluence, because of the material temperature drop, lead to bad weld, trace or strength reduction. The way to reduce or eliminate weld marks is basically the same as shrinkage dents.
When the weld marks can not be eliminated,
1., the gate location and the wall thickness of the parts can be changed, and the weld marks are guided to the position that does not affect the appearance.
2. open the overflow well near the weld line.

6. transparent parts defects:
(1) spot, craze and crack:
1) eliminate pollution including water >
2) reduce material temperature and segment adjustment.
3) increase injection pressure;
4) improve flow passage and cavity exhaust;
5) shorten the molding cycle.
6) if it has been generated, it can be eliminated by annealing.
(2) poor surface gloss:
A. cavity core polishing is not good.
B. melt hardening and premature cooling can not completely replicate the fine finish of mold surface.
(3) ripple ripple >
A. increase barrel temperature, especially nozzle temperature, increase injection pressure and speed;
B. increase mold temperature;
C. improves exhaust flow and improves material flow rate.
(4) whitening and fog halo: eliminate water vapor; increase material temperature and mold temperature; increase injection pressure and back pressure.

7. warpage (Warpage):
1> product design problems: uneven wall thickness and unreasonable structure.
2> mold design time:
(1) the size and form of the gate should be reasonable.
(2) all parts of the mold should be cooled evenly.
3 > Variety of plastics: the shrinkage of crystalline plastics (PE, PP, PA, POM, etc.) is much larger than that of non-crystalline plastics, which solidifies quickly and the parts are easy to warp and deform.
4> tuning:
(1) material temperature should not be too low or too high.
(2) increase injection pressure, time and speed;
(3) guarantee the holding time;
(4) after mold release, use jig to set or temper.

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