Quality issues and solutions of the TPR EVA plastic parts injection molding

Posted on : Sep 30 , 2022 By  GREFEE

There is an asymmetry between the quality of the molded TPR EVA parts and the qualified standard, due to which it cannot process promptly, which is the quality issue of the TPR EVA. To under the cause for these defects and minimize it to the lowest level, we should start from the following aspects: mold, raw materials, process parameters, equipment, environment and personnel.

Common defects of TPR EVA are following:  

1.Coloring difference:

TPR EVA the color of the injection parts is different from the The single standard color sample is viewed with the naked eye, which is seen as the color difference.

Cause:

Raw material,toner replacement, plastic material brand change, and setting agent replacement.

Different types of raw materials:

For instance, PP material and ABS material or PC material require the same color, but there is slight color difference due to different types of materials, but a limit is allowed.

Equipment process reason:

A. Temperature; B. Pressure; C melting time and other technological factors.

Environmental factors:

The barrel is not clean, the drying hopper is dusty, and the mold is greasy.

Factors of toner itself:

Some toners are not affected by temperature, and products are easily changed by temperature changes.

2. Shot shots (underfeeding):

TPR EVA injection parts is not full, with bubbles, voids, shrinkage holes, and others. It doesn’t meet the requirements of standard plate, which is called underfeeding.

Cause:

Mold:

A: Improper pouring system, the spouting system is that the melt enters into the pipes of mold cavity, which is associated with the quality of molding of plastic parts. The gates are not paralleled and the positions are not at the thick wall area. B: poor venting system. C: the impurities or cold slugs in impurities block the runners D: mold temperature does not meet the requirement

Rraw material aspects：

A: Large water content in raw material B: the volatile substances in the raw material exceeding the standard C: insufficient raw material supply, such as barrel blocked, the nozzle material affects blanking  

Aspects of injection molding machines :

A:Insufficient injection amount,  manufacture the 180T products with the 150T machine, B: the nozzles are blacked by external matter, the nozzle holes are too small C: insufficient raw material supply: such as blockage of barrel, the nozzle material affects blanking; D. Check valve failure; E. Insufficient injection stroke.

Molding operation:

A: Mold temperature is too low; B. Injection pressure is too low; C. The pressure maintaining time is too short D, the injection speed is too slow; E. The melt temperature is too low.

3. Warpage:

TPR EVA plastic parts will be warping and deform in a period of time after mold releasing.

Cause:

Mold: mainly due to the improper mold design aspects.

Molding operation :

A: High injection pressure, there is a large difference between the molecular orientation of the fluid direction and the vertical flow direction. The plastic tries to recover the original curl state, so the shrinkage in the fluid flow direction is greater than that in the vertical flow direction. B: high temperature of melt.  C: high holding pressure. When the pressure is high, the internal pressure inside the plastic will raise, so the internal stress will lead to warping and deformation of parts after mold releasing D: slow melt flowing speed E: the temperature of tempering is too high or the time is too long  

Aspects of raw material: PP/PA is easy to deform.

4. Welding marks (lines):

The lines on the TPR EVA plastic parts, which are welding marks, usually are straight lines.

Cause:

Mold：

A: Multiple feeding points, small cross section area of feeding point  B: no cold slug well or incorrect cold slug well positions. C: improper design of cooling system, the melt cools quickly but unevenly

Raw material:

A: Too much mold releasing agent, or unqualified mold releasing agent B: poor flow performance, easy to generate welding marks C:  high water content or volatile substances in raw material

Molding operation aspect:

A: The temperature of melt is high, the low temperature melt has poor shunting and confluence performance. B: the injection pressure of melt is too low, resulting in slow injection speed. The temperature of melts in the cavity is different, so the welding marks would be produced easily at the time of diversion and confluence.

5. Flow marks:

The surface of TPR EVA injection molding parts is characterized by spiral or cloud like undulations, or the inside of transparent products is wavy.

Cause:

Molds:

Similar to welding marks, but need to stress that the cold material has the greatest impact on the flow marks.

Raw material：

A: Poor flow performance is the main cause of flow marks, such as: PMMA, PC, AS and clear plastic products. B: When the ABS material is a modified copolymer polymer material, for instance, high processing temperature will cause the generation of volatile gas from resin and lubricant, which producing flow marks on the surface of plastic parts

Molding operation ：

A: Low injection speed  B: high flow speed  C: low mold temperature  D: short holding pressure term  E: low nozzle temperature

6. Burrs:

Thin (fringing) rubber appears around the TPR EVA injection molding part along the parting line or on the sealing surface of the mold, which are burrs.

Cause:

Mold:

The biggest reason is caused by burrs

 A: The parting surface of the mold is rough; B. The sliding parts of the cavity and core pulling part are excessively worn.

Raw material:

Material with good flow performance, for example: PP, PA, PS

Molding operation:

A: High injection pressure  B: high melts temperature  C: uneven distribution of injection pressure and mold filling speed D: large injection volume, resulting in large pressure in mold cavity

7 .Silver lines:

On the surface of TPR EVA injection molding parts, it is a long, needle like silver white fine grain like frost, with the opening direction along the material flow direction. Where the plastic part is not fully filled, the front end of the fluid is rough, which is called silver thread (silver thread).

Cause:

Mold and injection molding machine

Raw material ：

A: The water in raw material is the reason of silver lines  B: Raw materials are degraded by high temperature.  C: small amount of volatile gas produced by mold releasing agent  

Molding operation:

A: High temperature of melt B: melt stay for a long time under high temperature  C: melt stay under the mold cavity for a long time D fast injection speed.

8. Uneven color (mixed color):

The color on the surface of TPR EVA injection molding parts. Some are dark and some are different, which is called mixed color.

Cause:

Mold and injection machine

Raw material：

A: Poor thermal stability of coloring agent B: the dispersion effect is not ideal C: the amount of toner is too large, for example, a bag of 25kg toner is more than 300g. D: Wave fiber products are easy to have floating fibers, resulting in uneven raw materials and color mixing. E: too much impurities in raw material, making the surface color of the products different

Molding operation:

A: The temperature of barrel is high, making the melt decompose in the barrel B: poor plasticizing, the melt cannot be completely fused together C: the melt stays for a long term in the barrel. D: the time of holding pressure and injection is too long, and the back pressure is high.

9. Poor gloss (dark color):

The surface of the TPR EVA injection molding parts is gloomy or uneven is called poor gloss.

Cause:

Mold and injection molding machine.

Aspect of raw material:

A poor flow performance of melt, resulting in the surface is not compact  B: too much recycled raw material C: Poor dispersion of additives in raw materials D: water or volatile substance in raw material is high E: problems of the raw material itself

Molding operation:

A: Insufficient cooling B: slow injection speed, low pressure  C: short time of holding pressure D: poor flow performance of melt E: too much filling polynosic, such as PA+30%GF.

10. Poor demouldng (demoulding deformation):

Similar to warpage, the TPR EVA injection molding parts cannot be released smoothly from the molds, leading to deformation, tensile crack, strain, etc. are called poor demoulding.

Cause:

Mold:

Mainly due to poor design of mold, accounting for over 90%.

Raw material:

A: Foreign matters mixed in raw materials; B. Poor effect of release agent; C. Soft plastic parts are more difficult to demould than hard plastic parts.

Molding operation ：

A: High injection pressure, high temperature of melt, poor flow performance B: burrs C: the temperature of nozzles is low, cooling time is short D: the time of injection molding and holding pressure is too long.

11. Cracks and cracks:

Cracks and damage caused by voids on the surface of TPR EVA plastic parts.  

Cause:

Raw materials

A: High water absorption and are easy to decompose and embrittle after heating  B: more recycled materials in the raw material C: two immiscible components are mixed together D: the rigidity of material is low or the rigidity is strong and there is internal stress

Molding operation:

A: High injection pressure, resulting in increase of residual stress B: long holding pressure C: the internal stress is not eliminated

12. Burnt spots:

During the TPR EVA injection molding process, there are many dark black stripes or black spots on the surface or inside of the parts, which are called burnt spots or burnt spots.

Causes:

Molds and molding machines

Raw materials:

A:High content of water and volatile substances in raw materials.

B: Large melt index of raw materials and too much lubricant is used.

C: Raw materials need high temperature to be plasticized, such as PBT, PPS, etc.

Molding operation:

A. The injection speed is too high. B. The temperature of the melt is high. C. High injection pressure.

13. Dimensional discrepancy:

During the TPR EVA injection molding process, the original decided dimensional accuracy cannot be maintained, which is called dimensional discrepancy.

Cause:

Molds and molding

Raw materials

A: Different shrinkage rate of different materials, resulting in poor stability, so the design of mold includes PP/ABS/PPS/PS molds B: the adding amount of nozzle material and styling agent

Molding operation

A: Low injection pressure or low injection speed B: short time for mold filling and holding pressure C: low mold temperature.

14. Bubbles and dark bubbles:

There are pores in TPR EVA injection molding parts. The bubbles are the defects with small volume or series of pores formed the product after molding. Dark bubbles are vacuum holes generated in the plastic.

Causes:

Mold and molding machine.

Raw materials

A: High water content, short baking time and low drying temperature. B: large shrinkage rate of raw material C: too much recycled material

Molding operation

A: Fast injection speed, short time and the gas cannot be voided immediately.

B: insufficient time of holding pressure.

C: uneven cooling or insufficient time for cooling.

D: the temperature of melt and mold is high, leading to the generation volatiles and bubbles.

15. Turbid surface:

The surface of TPR EVA injection molded parts shows a matte, whitish and turbid appearance, which is called mixed corrosion.

Cause: The same as warpage deformation.

16. Depressions:

The surface of TPR EVA plastic parts surface is not even, smooth, and shallow pits or pits are formed inwards.

Cause:

Mold and injection molding aspects

Raw material:

A: The shrinkage rate of melt (resin) is large, and without adding styling agent B: poor flow performance, short on lubricates C: the filler is short  

Molding operation

A: Low injection pressure and slow speed B: short time of injection molding and holding pressure  C: the temperature and mold temperature of melt is high D: some places of part are too thick, especially for thread barrel needle position.

17. Cold slugs:

TPR EVA: plastic with different color, performance and body formed by cold slugs on the surface of injection molded parts.

Cause:

Molds and injection molding

Raw material:

Materials with high rigidity are easy to cause cold slugs. The rubber wire at the nozzle is easy to stretch and adhere to the mold after breaking, resulting in cold glue in the second PCs product during the de-moulding.

Molding operation:

A: Low temperature of melt, poor plasticizing B: the mold temperature is low and the flow performance is poor C: the cold slugs often appear simultaneously with color mixing phenomenon.

18. Stress mark:

The surface of TPR EVA plastic part obvious becomes white or higher than original plane.

Cause:

Molds and injection molding

Raw material:

Mainly relate to the raw material  

Molding operation:

A: High injection pressure  B:the temperate of melt is high, low mold temperate, the temperature between the two is large C: cooling time is short  D: the holing pressure is high and the time of holding pressure is long.

19. White spots:

There are white spots inside the TPR EVA injection molding parts, which are especially obvious among transparent products.

Cause:

Raw material:

A. There is a big different between the raw material particles, resulting in uneven or hard plasticizing B. The raw materials are mixed with different materials or immiscible raw materials. C. Cause by the properties of raw material itself, for instance, too much clear plastic

Molding operation:

A. The temperature of barrel is low. B. The screw bar rotates fast, short cycle.  C. Low back pressure.

20. Insufficient strength (embrittlement):

The strength of TPR EVA injection molding parts is lower than that of expected strength, resulting in the plastics injection molding parts cannot bear the already fixed negative judgement.

Cause:

Raw material:

A. The raw material itself is brittle (poor strength of material) B. Too much recycled material.  C. Mixing of different types of materials; D. Too many fillers; E. The proportion of wave fiber materials is large.

Molding operation:

A; The temperature of barrel is too high, the melt dissolve or decompose.  B. Mold temperature is too low. The molding performance of plastic parts is damaged and the strength is decreased. C. Injection pressure is too low and the flow performance of melt is poor. D. The wall of workpiece is thin, and the force is nonuniform.

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