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How do we deal with the undercut in injection molding?

What is undercut? I believe that those engaged in the injection molding industry know, so I will not repeat it. The undercut is not a complex problem in injection molding, but it is common in almost all injection molding factories. The reason for this is that there are too many types of burrs, and the factors that affect them are various. It is not difficult to solve as long as the burrs are classified and then one by one. Today we will follow Min-Hui to discuss the problem of burrs in injection molding.

1. How to determine the burrs caused by mold problems

First, remove the machine’s pressure-holding, V/P switch position up to play short shot samples because when playing the quick shot, the pressure of the mold cavity is the smallest. 

If the short shot samples have burrs in any local position, emphasize the local, it must be the mold problem. We need to check the mold to confirm. For the burrs against the broken surface, ensure whether there are collapsed corners and collapsed edges on the mold kernel, whether there are injured corners when polishing or sunburst, whether there is deformation on the mold kernel, whether there is deformation on the template, whether there is backing off on the slider, whether there is not a good fit when installing the mold, whether there are loose inserts or not in place. For the burrs on the ejector pin or inserts, it is necessary to confirm whether the gap is mainly due to wear. Mold problems caused by the undercut are more intuitive. Find the problem immediately to repair the mold.

The key point to add here is that the burrs caused by mold problems are closely related to daily production management, except for the new mold. Pressing the mold is the first killer that causes the mold to run burrs, low-pressure protection setting is crucial, sensitive machine low-pressure protection setting is reasonable, the pressure of a hair will be alarmed, it is this, some technicians who are not responsible for reducing the frequency of the machine alarm, so that they are easy, deliberately set the machine low-pressure protection pressure is vast, the consequences of this practice is very likely to be catastrophic. 

Once the product or water spout is not taken out of the press, the possibility of mold damage is extremely high, even if it can be repaired by patch welding. Still, the life of the mold will be significantly reduced, so you must keep a close eye on the low-pressure protection parameters of the machine.

In addition, to injection check the robot, some robot sensitivity is not enough, not taken out of the product will not alarm, resulting in the consequences of the pressed mold, encounter this type of problem, must immediately stop to repair the robot. Daily maintenance is not done in place. It will also lead to mold gradually pressure injury. 

Every two hours to clean up the mold parting surface, confirm whether there are foreign bodies or rubber wire, blow the mold cavity with a gas pipe, and clean up the rubber powder, these maintenance requirements must form a work instruction, and long-term implementation can significantly reduce the mold damage brought about by the burr problem. 

Production row also avoids the phenomenon of small molds on the big machine. It is easy to collapse the decay or pressure deformation, and production capacity burrs. The mold opening assessment should not be ignored to reduce the mold problems caused by burrs.

  • Whether the hardness of the die material meets.
  • Whether the die stress is sufficient.
  • Whether the mold structure is reasonable.
  • Whether the support head is balanced or not.

If these factors are neglected during the mold opening evaluation, it will be challenging to overcome the undercut problem in later production.

2. How to determine the undercut caused by the machine problem

For a short shot or shrunken sample, if the whole parting surface is running burr, you should also check the machine to prevent mold. When you open and close the mold manually, observe the pressure gauge first to confirm whether the mold thickness adjustment meets the requirement of clamping force and if it is not enough, readjust the mold thickness.

Add, in the process of adjusting the machine, if the mold temperature has a big vice up and down change, must reconfirm the clamping force because the mold thermal expansion and contraction affect the change of mold thickness. Still, the direct pressure machine will not have this problem only for the crankshaft type machine.

If there is still a burr on the parting surface after the mold thickness adjustment, it is necessary to recheck whether the clamping force is not enough because the tonnage of the machine is too small. 

Regarding the calculation of the clamping energy required by the mold theory, one method is to refer to the data of mold flow analysis; another way is to use the empirical formula and follow up the calculation of the projected area of the product in the vertical mold opening direction.

If there is no problem with the clamping force, the burr on the parting surface may be caused by the unbalanced machine strand, in which case the burr will be fixed on the same side of the machine and the engine needs to be overhauled and corrected.

3. How to analyze and solve the burrs caused by improper setting of molding parameters?

Almost all the five elements of molding parameters (temperature, pressure, time, speed, measurement) can cause burrs, sometimes caused by a single factor, sometimes caused by a combination of factors. When the injection pressure exceeds the machine’s clamping force or the mold’s local bearing strength, it will produce burrs.

The next cavity pressure curve explains the relationship between pressure and burr.

Under reasonable molding parameters, from the beginning to the end of pressure-holding, the process of mold cavity pressure rises to the peak and then falls into a smooth curve. This conclusion is the use of pressure sensors to monitor the pressure of the mold cavity. Many trials concluded as early as 10 years ago. 

The industry is promoting using pressure sensors to detect whether the product has phi, deformation, or not complete. Based on this curve, set the upper and lower limits to achieve.

The above pressure curve has a very high chance of cloak around the inlet gate, the parting surface, and the hole with insufficient strength, which are very likely to produce burrs. The reasons for this situation are as follows.

(1) V / P switch too late.

Generally, in the injection glue just run full product all structure, switch pressure is the most ideal, switch late, the pressure of the cavity rose sharply, excess stress, it is easy to appear burr, and even cause product deformation, size is large, sticky mold, head injury and a series of problems.

(2) The injection speed is too fast.

The injection process is the same as the opening and locking of the machine. Speed settings generally follow the principle of “slow-fast – slow,” especially at the end of the injection speed, too fast speed impact, so that the pressure of the cavity quickly reaches the peak, the mold gap is too large, or clamping force is insufficient, there will be burrs.

(3) The holding pressure is too significant, or the holding speed is too fast.

Excessive holding pressure or fast holding speed will squeeze the plastic into the mold cavity rapidly in the holding process, which will lead to a steep rise in the pressure of the mold cavity and excess pressure, resulting in burrs around the gate or even the whole analysis surface, and more seriously will lead to mold expansion. 

If the product structure allows, set a very little pressure (less than 10%) and a brief holding time (0.5 seconds for products with thickness above 1.5mm) in the first section of holding pressure to buffer the transition of V/P switching, which can play an excellent effect of preventing burrs. 

The setting of the holding pressure generally follows a principle: the location of the low temperature of the plastic melt in the mold cavity first, the place of the high temperature after the full. Common temperature location is generally distributed at the end of the filling, the site of the thin plastic thickness, and the parting surface, so the front pressure should usually not be set too high. 

Try to use the back pressure to solve the problem of local shrinkage or size problems. When adjusting the machine, the pressure setting gradually increases from low to high. It is recommended that each cycle plus increased by 5%-10% way until a reasonable pressure value is found.

4. Plastic melt temperature is too high caused by the burr

When the plastic melt temperature is too high, its viscosity will gradually decline, fluidity becomes better, and the unit area required clamping force will also increase with the local mold gap is large or clamping force is insufficient, there will be burrs. A variety of reasons also cause the causes of the actual temperature of the plastic melt. The material tube temperature is set too high. The melt measurement is too large, the screw is too thick, the cycle is too long, the back pressure is too enormous, and the speed is too fast, which will lead to the temperature of the plastic rise or directly lead to viscosity decline. When the product appears burrs, these parameters should be checked in time.

5. Plastic characteristics lead to burrs

PA, PP, and POM this crystalline plastic. Its characteristics determine that they have ultra-high mobility, and the mold gap and machine clamping force requirements are much higher than the general non-crystalline plastic. To prevent the production of burrs in the mold opening, assessment, machining, and mold allocation should be fully considered, control the mold gap; machine tonnage selection, but also do a preliminary estimate or based on mold flow analysis of the clamping force value to do For reference.

Regarding the burr problem, two more points should be added.

(1) In the preliminary evaluation of the mold opening, thoroughly evaluate the distribution of the gate, as far as possible with the lowest injection pressure to make qualified products; combined with the characteristics of plastic materials, thoroughly evaluate the structure and strength of the mold parts, reasonable control of the mold gap, in the mold opening stage to do an excellent job in advance to prevent burrs, will undoubtedly be more straightforward than the mold open out and then to repair.

(2) In the production process, any reason caused by the burrs must be resolved promptly. The waste of human resources approach is a small matter. Causing more significant damage to the mold and then to repair will only cause a vicious circle.

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