General knowledge about plastic molds

Sink marks

一、Features of injection molding defects Generally related to marks on the surface and resulted from coming off from the mold surface due to shrinking.

二、Possible causes

• Too high or too low melt temperature.
• Insufficient material in the cavity
• The contact surface of mold with the plastic is too hot during the cooling procedure.
• Improper runner design, too small gate section.
• Mold temperature unfit for plastic’s properties.
• Improper product structure (The rib stiffeners are too high, too thick and significantly inconsistent in thickness.)
• Poor cooling result. The product continues to shrink after being ejected.

三、Remedies

• Vary the temperature in the barrel
• Vary the velocity of screw to obtain the proper screw surface velocity.
• Increase the injection amount.
• Ensure to use proper padding; Increase the forward time of screw; Increase the injection pressure; Increase the injection speed.
• Check the stop valve to be sure it’s properly mounted.
• Decrease the temperature at the surface of mold.
• Correct the runner to avoid too much loss of pressure. Increase the section size accordingly depending on actual demands.
• Control the mold temperature applicably depending on the properties of plastic and structure of product.
• Improve the structure of product if allowed.
• Manage to let the product cool sufficiently.

Envelope        Back to top

一、Features of injection molding defects

二、Possible Causes

• Insufficient injection of materials into the mold.
• Stop valve malfunctions
• The materials have not been dried sufficiently.
• Too high preplasticizing and injection speed.
• Where some special materials are processed by special equipment

三、Remedies

• Increase the injection amount
• Increase the injection pressure
• Increase the forward time of screw.
• Decrease the melt temperature.
• Decrease or increase the injection speed. (e.g. for uncrystalline plastic, the speed shall be increased by 45%)
• Check the stop valve to see whether it has been broken or unserviceable.
• The drying condition shall be improved depending on the feature of plastic to ensure thorough drying .
• Properly decrease the screw speed and increase the back pressure , or decrease the injection speed.

Poor precision in molding size　Back to top

Features of injection molding defects
一﹐The precision in weight and size required for injection molding goes beyond the combined capability of mold, injection machine and plastic.

二、Possible Causes

• Uniform materials in the barrel.
• Too large fluctuation of barrel temperature.
• Too small injection machine capacity.
• Non-consistent injection pressure.
• Non-consistent screw resetting
• Non-consistent running time change or melt viscosity
• Non-consistent injection speed (flow controlling)
• Plastic variety not fit for this mold has been used.
• Considering the influence of mold temperature, injection pressure, speed , time and holding pressure to the product.

三、Remedies

• Check whether sufficient cooling water is available at the throat of hopper to ensure proper temperature.
• Check whether the thermocouple comes off or is of poor quality.
• Check whether the thermocouple is of proper type or not?
• Check the injection amount and plasticizing capacity of injection machine and compare it with the actual injection amount and hourly material consumption.
• Check whether consistent melts produced in each run?
• Inspect the check valve for leakage. If yes, replace it.
• Check whether the material feeding parameter is proper or not?
• Ensure the screw remains at the same position after each running and resetting, i.e. not more than 0.4mm of change.
• Check for inconsistency in running time.
• Use backing pressure.
• Check the hydraulic system to be sure it’s normal. Check the oil temperate to be sure it’s within specified range (25-60℃)
• Select a plastic variety fit for this mold (Shrinkage rate and mechanical strength are major concerns )
• Vary the whole production process accordingly.

Ripples or silver wires are found on the surface of product　Back to top

Possible Causes:

• The plastic materials contain moisture or other volatiles.
• Too high or too low material temperature.
• Too low injection pressure.
• Too large runner and gate size
• Core inserts have not been heated in advance or the temperature is too low.
• Too large internal stress

Gate is caught

一、Features of injection molding defects
The sprue is caught by the bushing

二、Possible Causes

• The sprue bushing has notbeen aligned properly with the nozzle.
• The sprue bushing has been over-packed with plastic.
• Too low temperature at the nozzle.
• Plastic has still not cured completely in the sprue, particularly for large sprues.
• The sprue bushing didn’t match with the nozzle.
• Insufficient draft for nozzle

三、Remedies

• Re-align the nozzle with the sprue bushing.
• Reduce the injection pressure.
• Reduce the forward time of screw
• Increase the nozzle temperature or heat the nozzle with a separate temperature controller.
• Increase cooling time.But it’s better to replace the sprue bushing with a smaller one.
• Correct the fitting faces of sprue bushing and nozzle.
• Increase the draft of runner.

Warpage of the molding　Back to top

一﹐Features of injection molding defects
The molding is similar to cavity in shape ,but distorted.

二、Possible Causes

• Too much internal stress inside the molding leads to bending.
• Too low filling speed.
• Insufficient plastic inside the cavity
• Too low or inconsistent plastic temperature
• Too hot molding upon being ejected.
• Insufficient coolingor inconsistent temperature of fixed/movable mold platen.
• Improper structure of injection molding (e.g. the rib stiffeners gather at one side but too far away from each other)

三、Remedies

• Decrease the injection pressure.
• Reduce the forward time of screw
• Increase the cycle time (particularly the cooling time) . Immediately immerse in warm water (38℃) for further cooling after being ejected out of mold , particularly for those thick section moldings.
• Increase the injection speed.
• Increase the plastic temperature.
• Employ a cooling equipment.
• Appropriately increase the cooling time or improve cooling condition. Try to keep consistence between temperature of fixed and movable half.
• Improve the structure of molding according to actual condition.

Misrun of plastic molding

Features of injection molding defects
Incomplete injection process. The cavity has not been filled completely or some steps missed in the injection process.

二、Possible Causes

• Insufficient injection speed
• Lack of plastic
• No screw padding at the end of screw stroke.
• Running time changed.
• Too low barrel temperature.
• Insufficient injection pressure.
• The nozzle was blocked.
• The heater around the nozzle or barrel failed to work.
• Too short injection time
• Plastic adhered to the throat wall of hopper.
• Too small injection machine capacity (i.e. injection weight or plasticizing capacity)
• Too low mold temperature.
• Failed to remove all the anti-rust oil on the mold.
• The check ring was damaged ,resulting in reverse flowing of melts.

三、 Remedies

• Increase the injection speed
• Check the plastic amount in the hopper.
• Check whether the injection stroke has been properly set. If necessary, conduct anamend.
• Check the stop valve for wearing or breaking.
• Check to be sure the running is consistent.
• Increase the melt temperature.
• Increase the backing pressure.
• Increase the injection speed.
• Check the nozzle for any foreign matter or unplasticized material.
• Check all the heatersand test their outputs with anamperemeter.
• Increase the forward time of screw
• Increase the cooling capacity at the throat of hopper, or decrease the temperature at the rear area of barrel.
• Chose a larger injection machine.
• Increase the mold temperature appropriately.
• Remove all the anti-rust oil inside the mold.
• Check and replace the check ring.

Flush　Back to top

Possible Causes:

• Too high temperature at barrel, nozzle and mold.
• Too high injection pressure and too small clamping force
• The mold has not been closed completely. Foreignmatter mixedinor the platen has distorted.
• Poor ventilation of cavity
• Flow fault of plastic

weld mark　Back to top

Possible Causes

• Too low material temperature. Flow fault of plastsic.
• Too low injection pressure.
• Too low injection speed.
• Too low mold temperature.
• Poor ventilation of cavity
• Plastic was polluted

Sidewall sink mark　Back to top

Sink marks on plastic product are resulted from partial shrinkage due to short injection, which has been an old problem accompanying plastic injection molding. Sink marks usually come with the partially increasing of shrinkage rate due to the increasing of plastics wall thickness, possibly found near outer tips or at the place where the wall thickness suddenly changes, e.g. a protrude , rib stiffeners, or the back of base, and sometimes also found at other parts. The essential cause for sink marks is the nature of plastic that it always expands when heated and contracts when cooled. The thermal expansion coefficient of thermoplastics is very high. The extent of expanding and contracting depends on a variety of factors including plastic’s property , temperature range and holding pressure, which are the most essential, in addition to the size and shape of plastics, the cooling speed and the evenness of materials, etc.

   The expanding and contracting amount of plastic during the molding process bears a reference to the thermal expansion coefficient of plastic. With the contracting of molded plastic part due to cooling, the plastic separate from the cooling surface of cavity which leads to the decreasing of cooling efficiency. With the further cooling of plastic part, it will contract further and the contract amount depends on the integrated influence of various factors. The sharp corners of part have the fastest cooling rate and cure earlier than any other areas. The thick section near the center of part will be the last to release heat due to its great distance from the cavity surface. After the curing of materials at the edge and corner, the molding will keep shrinking with the cooling of melt near the center. The cool contracting of plastic at the center leads to inwardly drawing of weak surface between the already curing sharp corner and partially curing section, resulting in a Sink mark on the surface of molding. The occurring of Sink mark indicates the shrinkage rate at this place is higher than that of other areas. Under some situations, the Sink mark can be avoided by varying the processing conditions, e.g. inject additional materials to the cavity to compensate the shrinkage. Under most situations , the gate has cured while the whole molding is still very hot and contracting because the gate part is usually thinner than other area. After curing of gate, the holding pressure will have no influence on the molding inside the cavity.

   Molding of semicrystallized plastic has a high shrinkage rate, contributing to serious Sink mark problem. Molding of uncrystalline material has a fairly low shrinkage rate and results in minimum sink mark. When the materials are reinforced, the shrinkage rate will be lower and the possibility to form sink marks is lesser too. The thick section molding will result in significant shrinkage due to its long cooling time. Big thickness is the essential cause for sink mark.. So thick section molding shall be avoided in the design. If thick section molding is required, it shall be designed with hollow cavity. The Sink mark can be eliminated or reduced to the greatest extent by replacing thick section with normal thickness, and replacing sharp corner with large radius.