Surface roughness -injection molding defects

Surface roughness means there is a lack of precision in the surface of a molded part, or an inconsistent level of precision all over the surface, e.g. some areas are glossier than the rest. The main reasons for surface roughness might lie in the injection molding machine, the mold, the process or the material.

plastic injection molding company in China

(1)Injection Molding Machine

①Insufficient material supply;

②Material barrel is not thoroughly cleaned when changing material;

③Check for cold melt leakage at the nozzle;

 

(2)Mold

①The gate is too small, or the runner is too thin – increase the gate size, or adopt the film gate;

②The surface of the mold cavity is rough – polish the cavity surface and check if there’s any leakage;

③Poor venting – add vents in the location with poor glossiness for an improvement;

④Increase the size of or add a cold slug well to collect the cold material;

⑤Mold temperature is too low – raise mold temperature;

⑥Might be affected by mold release agents – stop the application of mold release agents;

 

(3) The Injection Molding Process

①Increase melt temperature (check whether the heating zone of the material barrel is in disorder – non-uniform temperature, local temperature too high or too low, etc.);

②Increase nozzle temperature;

③Over shear caused by over-speed injection – lower injection speed appropriately;

④Increase injection pressure;

⑤Increase material injection time;

 

(4) Raw Materials

①Raw material is not dried;

②Too much recycled materials are added, or repeatedly recycled materials are decomposed;

③Excessive application of mold release agents.

Dimensional instability in plastic injection molding process

Dimensional instability means the dimensional changes of a molded part exceed the tolerance limit, for which the reasons might lie in the injection molding machine, the mold or the injection molding process.

(1)Injection Molding Machine

①The application of different types of injection molding machine leads to the differences in injection molding conditions;

②Melt temperature fluctuation – check for failure of the thermocouple or the temperature controller;

③Instable screw speed – check the hydraulic system;

④Instable injection pressure – check whether there’s consistent buffering during each cycle, or whether the plastic non return valve leaks, repair or change it when necessary;

 

(2)Mold

①Check whether the gate is blocked by residue;

②Check whether the mold temperature is stable, whether the coolant flows smoothly, and whether the mold cooling circuit is correctly connected;

③Check for failure of molding components;

 

(3)Injection Molding Process

①Non-uniform mold temperature or unreasonable cooling circuit leads to improper mold temperature control – make adjustments accordingly;

②Injection pressure is too low – raise injection pressure;

③Appropriately increase injection time and holding time;

④Material barrel temperature or nozzle temperature is too high – make adjustments accordingly;

⑤Filling speed is too low – increase injection speed, or adopt multistage injection.

Injection molding machine operation instruction and good habit

injection molding factory in China

1.A great habit of injection molding machine operation is of enormous benefit to both machine service life, as well as production safety.

(1)Before the Machine Starts

①Check to see whether there is any water or oil inside the electrical control box. Don’t turn it on if the machine is affected by moisture. It can only be turned on after the parts are dried by the service staff.

②Check whether the voltage meets the requirements, which is usually kept within the range of ±15%.

③Check the effectiveness of the EMERGENCY STOP button, as well as switch of the front and back safety doors. Verify whether the rotating direction of the electric motor and the oil pump are consistent.

④Check to see whether each cooling channel is smooth and free from any blockage, and let coolant into the oil cooler and the water jacket at the machine barrel end.

⑤Check to see if all joints are lubricated, and add lubricant/grease when necessary.

⑥Turn on the electric heat control system to heat each section of the machine barrel. Hold the temperature for a while after each section is heated up to the required temperature, so as to obtain a relatively stable machine temperature. Temperature holding time varies with different equipment and plastic materials.

⑦Feed sufficient plastic materials into the hopper. According to different plastic injection molding requirements, some materials need to be dried beforehand.

⑧Keep the machine barrel covered with a heat shield, so as to save power, and at the same time increase the service life of the electric heater and the contactor.

(2)During the Operation Process

①Do not ignore the importance of the safety doors for convenience.

②Always pay attention to the temperature of the hydraulic oil, to keep it within the allowed range. The ideal working temperature of the hydraulic oil should be kept between 45 and 50℃; usually, the proper temperature ranges from 35 to 60℃.

③Don‘t forget to adjust each travel distance limit switch, to avoid collision during machine operation.

(3)After the Operation

①Before turn the machine off, thoroughly clean the machine barrel, so as to prevent the residue materials from oxidation or decomposition caused by long-time heating.

②Open the mold to keep the elbow-bar mechanism locked.

③The workshop needs to hoist all the equipment. Carefully handle the heavy mold parts and components during the assembly/disassembly process, to ensure production safety.

 

  1. Instructions on Injection Molding Machine Debugging

(1) Purpose

Meet product quality requirement with the fastest speed, the lowest cost and the optimal molding cycle.

 

(2) Scope of Application

Applicable for machine debugging and work process adjustment of the Injection Molding Dept.

 

(3)Work Preparation

①No matter it is a new or an old product, production process documents, product samples, material performance documents, as well as the documents of product weight and mold structure need to be available. As for an old product, find out its production process document and input it to a computer, and then adjust the mold and equipment to the required status.

②Check whether the status of equipment, mold and material allows debugging, e.g. whether materials are dried, whether the mold is cleansed and functions well, whether barrel temperature reaches the molding requirements, and whether the cooling system is on, etc.

 

(4)Machine Debugging

For an old product, input process parameters before performing regular operation, and start massive production after quality inspection. If it is a new product, the operation steps are as follows:

①Set material barrel temperature to regular molding temperature. Set melt and injection travel distance on basis of product weight.

②Identify injection pressure and speed based on the injection molding process. Usually, medium pressure (50~80MPa) and speed (30~60mm/s) are applied.

③Identify pressure holding time based on gate types and sizes. Pinpoint gate, 6 – 8s; side gate and direct gate, 8 – 10s.

④Identify cooling time based on cooling channel layout and product thickness, preliminarily set it to 15 – 20s.

⑤Start the injection molding process manually; raise or lower relevant data based on the defects of the molded product, till product quality meets the requirements and allows continuous operation.

⑥Switch the machine from manual mode to semi-automatic and adjust the parameters during production, so as to achieve the best injection molding cycle. Injection Molding Cycle = mold open/close time + injection time + melting time + cooling time, all of which need to be minimized. Generally speaking, for a 100g injection molding machine, if a 2-plate mold is applied, the mold open/close time is kept at about 2s, or appropriately longer if there are sliders. Injection time should be reduced progressively by 0.5s till defects occur. Then stop adjustment and return to the previous data. With regard to melting time, try to reduce back pressure and quicken up melting speed, to avoid air mixture and air bubbles, etc. The adjustment of cooling time is similar to that of injection time: reduce progressively by 1s, and then compare the molded product with the standard sample. If the two share the same quality, it is deemed the best cooling time.

 

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Difference Between two plate mold and three plate mold

Three plate mold VS Two plate Mold

Appearance difference Between two plate mold and three plate Mold

With regard to appearance, the biggest difference between the two plate mold and the three plate mold is that there is a stripper plate between the upper clamping plate and the cavity plate of the three-plate mold.

The appearance difference between three plate mold and two plate mold

Structures difference Between 2-plate  mold and 3-plate Mold

The main difference in the structure of the molds lies in the location and orientation of the guide pin, and that the three-plate mold has a small tie bar and a nylon lock on parting surface or latch on the side of the mold to control mold opening sequence and stroke.

The structure difference between 3 plates mold and 2 plates mold

Applications difference Between 2-plate mold and 3-plate Mold

2-Plate Mold 3-Plate Mold
Mold Size Average Larger
Mold Production Cost Low High
Molding Machine Capacity Requirements Low High
Structural Strength Strong Weaker
Automatic Production Semi-automatic / Automatic Semi-automatic / Automatic
Molding Cycle Short Long
Molding Pressure Low Higher
Runner weight Small Larger
Gate Trimming Dependent on Gate Type Unnecessary

Comparison of the Two Mold StructuresGate location in two plates mold

We can clearly see from the comparison in the above table that a two-plate mold is better than a three plates mold in most aspects, but why is there a three-plate mold in mold making and design? How to make a choice during design?

A three plates mold mainly exists in response to the appearance and production requirements of plastic products. For the finished product on the drawing, if the design is implemented through a two plates mold, the following two filling methods are available:

 

  1. The first filling method is to directly inject material through the large points on the finished product surface.
  2. The second method is to fill material from the non-appearance surface of the finished product (there is a location for consideration).

We can clearly see in the three mold flow analysis diagrams on the right that there are such problems as uneven flow, uneven pressure and short shot. There is no need to explain the short shot. Under uneven plastic flow and uneven pressure, though the mold is completely filled, the finished product will be severely deformed after molding. Next, it is necessary to consider the trimming problems of the finished product after molding (the trimming of the gate and the finished product) and whether the residue gate interferes with other matching components due to protruding from the product surface, which in turn affects assembly.

moldflow analysis on two plate mold

As can be clearly seen from the drawing, mold filling is directly carried out from the side of the finished product. At this time, in addition to obvious short shot, there is still a situation in which the flow is uneven.short shot on 2 plate mold

During mold trial, the molding factory will increase the injection pressure and accelerate the injection speed in order to fully fill the finished product. At this time, the excessive injection pressure will most likely cause the mold to overflow. If so, this will greatly affect the service life of mold production.

In addition, because of the lateral feeding, the finished product must be placed in a location away from the mold core; in order to reduce the size, the mold must be designed into an eccentric one; as a result, in addition to mold cost rise, it may cause an increase in the capacity of the molding machine, which indirectly increases the production cost of the molding factory.

In the three diagrams on the right, we can clearly see the difference in its injection molding conditions with that of the two-plate mold. The material flow is also obviously more even than the single point filling, and the short shot phenomenon is gone. Although pressure distribution is still not even enough, we can adjust the pressure distribution by controlling the size of the filling point.

In terms of molding pressure, the injection pressure of the three-plate mold is not much higher than that of the two-plate mold under the situation of complete filling. The main reason is that for the finished product, single-point filling is more difficult for plastic feeding.

moldflow analysis on three plate mold