Analysis and Solutions of Defects in Thermoplastic Products

Time:2021-08-02 17:36:52 / Popularity: / Source:

Analysis and Solutions of Defects in Thermoplastic Products 

1. Three indicators for evaluating quality of plastic products

1. Appearance quality: including completeness, color and gloss;
2. Size and relative position: in line with product drawings and assembly requirements;
3. Mechanical properties and chemical properties related to the use: such as tensile test, impact test, environmental resistance test, etc.

2. Reasons for product defects

1. Plastic issues: including plastic quality ingredients and baking materials, etc.;
2. Machine adjustment problems: including injection pressure, temperature and cycle, etc.;
3. Mold problems: including mold design, manufacturing and wear.
Among them, mold problems are the most common and the most difficult to solve.

3. Analysis of common defects in products and solutions

1. Insufficient filling (short shot): plastic parts are incomplete, or individual cavity beers are dissatisfied with multiple cavities.

(1) Improper feed adjustment, lack of material or excessive material;
(2) Insufficient injection pressure, temperature and time;
(3) Material temperature is not enough;
(4) Mold design is unreasonable: runner is too long, there are many turns; exhaust system is poor; runner is not cold enough; number of cavities in multi-cavity mold is too much, alignment is not correct, gate position, form or quantity is wrong insufficient. Water transportation design is unreasonable.

2. Flash (flash): Excessive thin wings appear on the edge of plastic product, joint of insert and thimble position.

(1) Poor mold manufacturing accuracy;
(2) Unreasonable mold design:
--Unreasonable ranking and unbalanced pressure;
--Improper gate position, resulting in a lack of material on one side, and a front line on the other side;
--Locking force of slider is not enough.
--Poor mold exhaust, parting surface is stretched under high pressure, etc.
(3) Clamping force of injection machine is not enough;
(4) Improper adjustment of machine: too much pressure, too high temperature and too much feed volume, etc.

3. Silver streak (commonly known as water splash, silver streak): surface bubbles are broken. When plastic is filled with too much water vapor, decomposition gas, solvent gas and air, a series of silvery luster, such as needle strips or mica flakes, will be formed on the surface of product along material flow direction.

(1) Water vapor.
(2) Decomposition gas: high material temperature, poor plastic quality and excessive shearing force.
(3) Plastic is impure.

4. Sink mark (shrinkage, sink mark): Surface of plastic part produces pits due to internal shrinkage. Main reasons are poor pressure holding, uneven cooling, excessive wall thickness or bone width, and inserts are not preheated. Solution:

(1) Wall thickness is as uniform as possible.
(2) Enhanced cooling of thick-walled parts.
(3) Gating system design should be reasonable:
A. Size of gating system should be compatible with size, wall thickness and complexity of plastic parts;
B. Gate should correspond to part where wall thickness is easy to shrink.
(4) Regarding machine adjustment
A. Increase injection pressure, pressure holding time and injection time;
B. Increase injection speed

5. Weld line (weld line): Temperature of material drops when melt is diverted and merged, resulting in poor welding, traces or reduced strength at the joint. Method of reducing or eliminating weld marks is basically same as shrinking dents.

When weld line cannot be eliminated,
1. Gate position and wall thickness of part can be changed to guide weld mark to a position that does not affect appearance.
2. Open an overflow well near weld line.

6. Defects of transparent parts:

(1) Shimmering spots, silver streaks and cracks:
1) Eliminate pollution <including moisture>,
2) Reduce material temperature and adjust in sections,
3) Increase injection pressure;
4) Improve exhaust of runner and cavity;
5) Shorten molding cycle;
6) If it has occurred, it can be eliminated by annealing.
(2) Poor surface gloss:
A. Cavity and core are not well polished;
B. Melt material cools and hardens prematurely, which cannot fully replicate excellent finish of mold surface;
(3) Shock pattern <corrugation>:
A. Increase barrel temperature, especially nozzle temperature, increase injection pressure and speed;
B. Increase mold temperature;
C. Improve exhaust gas and increase material flow speed.
(4) Whitening and haze: eliminate water vapor; increase material temperature and mold temperature; increase injection pressure and back pressure.

7. Warpage:

1>Product design problem: uneven wall thickness and unreasonable structure.
2>When designing mold:
(1) Size and form of gate should be reasonable;
(2) Cooling of all parts of mold should be even.
3>Plastic variety problem: crystalline plastics (PE, PP, PA, POM, etc.) have a much larger shrinkage rate than non-crystalline plastics, solidify faster, parts are easy to warp and deform;
4> Tuning:
(1) Material temperature cannot be too low or too high;
(2) Increase injection pressure, time and speed;
(3) Guarantee pressure holding time;
(4) After demoulding, use clamps to shape or temper.

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