Injection Molding Defect: Flash

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Injection Molding Defect: Flash

Flash is a more common phenomenon in injection molding production. If the flash is stuck on the parting surface of the mold without cleaning and the mold is locked directly, the parting surface of the mold will be damaged, and the damaged part will lead to new raw edge.

Therefore, the injection process must pay special attention to whether there will be a burring phenomenon.

What is Injection Molding Flash Problem?

injection molding flash

The plastic melt is extruded out of the mold cavity from the mold parting surface, and the thin sheet produced at the edge of the product is called the flash, and other claims are batch front, Flash, overflow edge, burr, flash, etc.,

Injection Molding Defect Cause And Solution

Plastic Injection Molding Flash

Raw edge is a common phenomenon in injection production. If the raw edge is stuck on the parting surface of the mold without cleaning and the mold is locked directly, the parting surface of the mold will be damaged, and the damaged part will lead to new raw edge.

Therefore, the injection process must pay special attention to whether there will be a burring phenomenon.

In the process of injection production, there are many reasons for rough edges, such as too much injection pressure, too fast end injection speed, insufficient clamping force, wear of the top pinhole or slider, uneven closing surface (there is a gap), and too low viscosity of plastic.

Solutions for raw edges of plastic Injection Molding Products

Analysis of the causes of raw edges is based on the premise that the clamping force is sufficient. When the clamping force is insufficient, it is easy to misjudge the problem of raw edges. The following analysis is based on the identification and confirmation of raw edges:

(1) Gradually increase the injection pressure, with the increase of injection pressure, the raw edge also increases correspondingly, and the raw edge is mainly generated on the parting surface of the mold, which indicates that the clamping force is not enough.

(2) Gradually increase the clamping force of the injection machine, when the clamping force reaches a certain value, the raw edge on the parting surface disappears, or when the injection pressure is increased, the raw edge on the parting surface will no longer increase. This value of mode-locking force is considered to be sufficient.

(3) In the case of low temperature, low speed and low pressure, when the product is not satisfied with the glue, the raw edge has been produced. The main reason is that the mold manufacturing precision is not enough, and the cooperation gap is too large.

(4) Plastic products are just full, local shrinkage phenomenon, no raw edge; When the injection pressure or pressure holding pressure is increased to improve the local shrinkage of the product, the raw edge is produced. Possible causes are:

  • (a) Material temperature is too high. The material temperature is too high, the viscosity of the melt is low, the fluidity is good, and the stronger the ability of the melt to drill into the mold clearance, it will lead to the generation of rough edges,

  • (b) the injection speed is too fast and the injection pressure is too large (resulting in supersaturation of the fill). Too fast speed, too large injection pressure, especially too large injection pressure, will enhance the ability of the melt to drill into the mold clearance, resulting in burr.

  • (c) The fluidity of plastics is too high. The better the fluidity of the plastic, the lower the viscosity of the melt, and the stronger the ability of the melt to drill into the mold with the gap, it is easy to produce burring. When the mold production has been completed, the depth of the exhaust groove of the mold and the cooperation gap of the mold have been finalized, and another kind of plastic with good fluidity will produce rough edges.

  • (d) Insufficient strength of the mold. When the design strength of the mold is insufficient, the mold cavity will be deformed and expanded after the pressure of the plastic melt, and the colloid will drill into the gap of the mold, resulting in rough edges.

  • (e) Unreasonable design of plastic products. The local glue site of the product is too thick, and too much contraction during injection will lead to local shrinkage. In order to adjust the problem of local shrinkage of the product, it is often necessary to use a higher injection pressure and a longer injection time to fill and maintain pressure, which results in insufficient strength deformation of the mold and rough edges.

  • (f) Mold temperature is too high. High mold temperature can not only make the plastic maintain good fluidity, small pressure loss, but also reduce the strength of the mold, which will also lead to the generation of rough edges.

The above are the most common problems encountered in injection production, which can not be solved by all process means, and the biggest trouble for injection technicians. For this situation, the most important means is to fix the mold.

The solutions are:

(a) Local glue reduction of plastic products. The local shrinkage of the product is reduced, and after the glue level is reduced, the product shrinkage problem can be improved, the injection pressure will be reduced, the mold deformation will be small, and the raw edge can be suppressed. This is the most effective and commonly used method.

(b) Add glue points. Increasing the pouring point can reduce the injection process, reduce the injection pressure, the pressure on the mold cavity will be reduced, and the generation of raw edges can be effectively solved.

Increasing the injection point, especially in the shrinking position of the product, can have an immediate effect on reducing the injection pressure of the mold cavity. It is also one of the more commonly used means.

(c) Local reinforcement of the mold. Sometimes the deformation of the formwork can be strengthened by adding a support column between the male formwork and the thimble plate.

What are the main causes of rough edges in plastic products? What are the solutions?

When the raw edge of the injection product is filled, the melt spills from the parting surface of the mold and other mating surfaces, and is formed after cooling. Thin rough edges are sometimes not obvious, but large areas of thicker rough edges can spill more than a few centimeters from the edge of the plastic.

The existence of raw edges of plastic products affects the appearance and dimensional accuracy of plastic parts, and increases the work of removing raw edges, which will seriously affect the release of plastic parts, damage the mold, etc., therefore, injection forming should be avoided as far as possible. Rough edges are often produced in die parting lines, near holes, and at exhaust or thimble locations.

(1) Causes of rough edges

  • (a) The plastic viscosity is too low or the plastic temperature is too high. Because the viscosity of the plastic itself is too low or the plastic temperature is too high during molding, the melt viscosity is low. The molten material flows into the matching gap of the mold and produces a rough edge.
  • (b) the injection pressure and holding pressure are too large or the clamping force is insufficient. When the injection pressure and pressure holding pressure are too large or the closing force is insufficient, the injection filling will cause the mold parting surface to be poorly closed, so that the molten material will overflow from the gap of the mold parting surface and form a rough edge.
  • (c) The injection pressure is not evenly distributed in the mold cavity, or the filling speed is too large or the filling speed is not uniform, which will lead to local overfilling of the product and burr.
  • (d) If the injection time is too long or the dosage is too large, the mold cavity will be overfilled and there will be rough edges.
  • (e) Excessive use of lubricant in the plastic will make the fluidity of the plastic too good, resulting in the melting of the mold into the mold gap, resulting in the generation of rough edges.
  • (f) Mold defects. The deformation of the template or the uneven surface of the plate (FIG. 5), the parting surface can not be closely fitted, the wear gap of the moving parts of the mold cavity and the mold core is too poor, there is adhesion or falling into the foreign body on the parting surface, and the exhaust slot is too large and too deep, which can easily lead to the overflow of molten material from the mold during the filling of the mold.

(2) Solution Measures

When the raw edge is produced, it is generally necessary to start from the troubleshooting of the mold, because of the raw edge and change the forming conditions or raw material formula, often have an adverse effect on other aspects, and easily lead to other forming defects.

  • (a) It is necessary to carefully check the mold and verify the parting surface, so that the male mold and the female mold are aligned, and check whether the parting surface is close and fit, whether the sliding parts of the male mold and the female mold are out of tolerance, whether there is adhesion or foreign matter on the parting surface, whether there is parallel between the templates, whether there is bending deformation, and whether the opening distance of the template is adjusted to the correct position according to the thickness of the mold.
  • Whether there is damage to the surface of the guide pin, whether the pull rod (three-plate die) has uneven deformation, and whether the exhaust slot is too large and too deep. According to the results of the above item-by-item inspection, the errors generated can be eliminated by mechanical machining (mold repair).
  • (b) Appropriately reduce the temperature of the pipe, nozzle and mold to shorten the injection cycle. For plastics with low viscosity such as PA, precision machining and mold repair should be done as far as possible to reduce the mold gap while appropriately reducing the material temperature.
  • (c) It should be checked whether the supercharger is overpressurized, and it should be confirmed whether the product of the projected area of the plastic part and the molding pressure exceeds the clamping force of the equipment.
  • If this is exceeded, the number of cavities should be reduced and the injection machine with large closing tonnage should be considered. Check whether the mold adjustment of the mold closing device is in place, whether the mold is closed (closed), whether the elbow is straight, and whether the mold closing force is enough.
  • (d) The injection pressure or holding pressure should be reduced or the gate section area should be reduced, the holding time and pressurization time can also be shortened, and the screw formation can be reduced.
  • (e) Adjust the formula of the raw material to reduce the flow of the melt by choosing a plastic grease with higher viscosity or reducing the amount of lubricants or release agents, etc.


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.