Mold Adhesion (sticking) in Injection Molding Process

 Mold adhesion, also known as sticking sprues or parts, is caused by improper contact of the nozzle tip with the sprue bushing, which does not allow sprue or part release and good filling. Usually, the diameter of the main runner needs to be big enough, so that the sprues are not completely solidified when injection molding

Material sticking in the mold can be a production killer, whether it is the sprue or the formed part of the cavity. The root causes of such sticking can be traced to one of several types of equipment or injection molding process issues.

 

Causes and Solutions for mold Adhesion

 

Mold Failure

There are a lot of possible causes for sticking sprues or parts, but mold failure is one of the main causes, of which the reason and solution are stated as below:

 

1)Rough mold cavity surface. If surface defects, such as grooves, nicks, cuts and/or dents exist in the mold cavity/runner, the molded parts tend to stick in the mold, leading to release failure.

 

As a result, one of the main tasks is to improve the degree of the polish for the mold cavity and runner. It is better than the inner surface of the mold cavity is chrome plated. Also, in the polishing process, the movement direction of polishing tools should be aligned with the direction of the melt flow.

 

2)Mold Abrasion & Scrapes or Too Large Gap between inserts

When melt flow flash occurs at the scraped mold parts or in the insert gap, this will also cause release failure. To solve this problem, it is suggested that the scrapes are fixed, and the insert gap reduced.

 

3)Insufficent Mold hardness

If the mold refuses to open in the very early stage of injection, it means the mold is experiencing deformation under the effect of injection pressure due to insufficient rigidity. And, if the deformation exceeds the elastic limits of the mold, it will not be able to return to the original shape for further applications. Even if the deformation is kept within the elastic limits of the mold, the melt flow is cooled and solidified under the extreme conditions in the mold cavity. When the injection pressure is removed and the mold shape is restored, the molded part will be held in position by the resilience force, so the mold still will not open. As a result, when designing a mold, the rigidity and strength must be guaranteed.

 

During a plastic injection mold test, it is better to install a dial gauge onto the mold, so as to check whether the mold cavity and mold frame will deform during the feeding process. Start with a relatively low injection pressure, and then observe the amount of deformation while gradually increasing the injection pressure, to keep the amount of deformation within a certain range.

 

When a mold cannot open due to a too great resilience force, increasing mold opening force alone is not a solution, but the mold should be disassembled immediately and then take the molded part out after heating and softening. For molds with insufficient rigidity, a frame may be attached on the outside to improve the rigidity.

 

4)Insufficient Mold Release Slope or Poor Parallelism Between the Moveable & Fixed Platens

When designing and manufacturing plastic injection molds, a sufficient mold release slope must be guaranteed, or it will not be easy to release the molded parts. If ejected by force, the parts will be warped, leaving cracks or white marks at the force application point. The moveable and fixed platens of the mold must be kept in parallel with each other, or cavity displacement will be caused, leading to poor mold release.

 

5)Improper Gating System Design

All the following situations will cause sticking sprues or parts: the runner is too long or too small, or the connection between the primary runner and the secondary runner are not strong enough, or there is no cold-slug well in the primary runner/bad sprue balance, or the diameter of the primary runner does not match nozzle diameter, or spruce bushing does not mate with the nozzle sphere. Therefore, runner length should be appropriately shortened, and its section area appropriately increased, while strengthening the connection between the primary runner and the secondary runner and setting a cold-slug well in the primary runner.

 

When determining the location of the sprue, we can add more auxiliary sprues to balance the feeding efficiency of a multi-cavity mold and at the same time reduce the pressure in the cavities. Usually, the small end of the primary runner should be 0.5 – 1mm larger than the nozzle diameter, and the concave sprue bushing should be 1- 2mm (radius) larger than the nozzle sphere.

 

6)Improper Design or Mis-operation of the Ejector Mechanism

If the ejector mechanism is designed with insufficient travel, imbalanced ejecting force or unsatisfactory ejector plate movement, sticking sprues or parts will be caused. When allowed, the ejection area of the ejector pin should be maximized, so as to guarantee a sufficient ejecting travel, while the ejection speed of the molded parts should be kept in a proper scope, neither too fast nor too slow. Unsatisfactory ejector plate movement is mainly caused by stickiness among different sliding components. For example, when the ejector plate pushes to move the sliding core since there is no cooling mechanism in the sliding core, the temperature here is higher than other cores. During continuous operation, the gap between the column itself and the sliding core is too small, so the stickiness thus caused will not allow for desirable core pulling movements. Another example is that if the ejector pinhole is not in good parallel with the guide pin of the ejector plate, or the ejector pin is curved, ejector plate will not move in a desirable manner. If a retaining pin is not set in the ejector mechanism, when foreign materials appear between the ejector plate and the mounting plate, the ejector plate will incline, causing unsatisfactory ejector plate movement. For a large-size mold, if there is only one ejector pin, the ejector plate will not be able to push in a balanced way, which will also result in unsatisfactory movements.

 

7)Unsmooth mold ventilation or no air inlet in the mold core will also cause sticking sprues or parts, so mold ventilation should be improved, and equip the mold core with air inlets.

 

8)Improper Mold Temperature Control or Inappropriate Cooling Time

If it is hard to release the molded parts at the molding joint, we can appropriately increase mold temperature and shorten cooling time. If it is hard to release the molded parts from the cavity surface, we can appropriately lower mold temperature and increase cooling time. In addition, a too high temperature of the fixed mold will also cause sticking sprues or parts. Die adhesion will occur if the mold cavity features a soft porous material. For this situation, hard steel or surface electroplating should be applied.

 

9)Unsatisfactory runner pulling, or no sprue pulling mechanism

Situations like indentation below the mold joint, and cavity edge line exceeding sprue line will all affect parts release to some extent. As a result, we need to pay attention to it and make a certain adjustment.

 

Improper Control of Process Conditions

If the injection molding machine is too large in size, with a high-speed screw, great injection pressure and very long injection & pressure holding time, overfill will be caused, making molding shrinkage smaller than expected, thus leading to sticking sprues or parts. If the temperature of the barrel and the melt flow is too high and the injection pressure is too great, the hot melt flow will probably get into the gaps between mold inserts to cause flash, and subsequently sticking sprues or parts.

 

In addition, sticking sprues or parts will also be caused by low nozzle temperature, short cooling time and interrupted feeding. As a result, when solving die adhesion, or sticking sprues or parts problems, we need to appropriately reduce injection pressure, shorten injection time, lower barrel & melt flow temperature, increase cooling time and prevent interrupted feeding, etc.

 

  1. Raw Materials Not Compliant with Operational Requirements

If the raw materials are mixed with impurities during the packing and transportation process, or raw materials of different grades are mixed during the pre-drying and pre-heating process, or foreign materials get into the barrel and/or hopper, sticking sprues or parts will also occur. In addition, uneven or too large material granules affect die adhesion, too. As a result, the materials for injection molding should be carefully screened and purified.

 

  1. Improper Application of Mold Release Agent

The purpose of applying the mold release agent is to reduce the adhesive force between the surfaces of the molded part and the mold cavity, so as to prevent them from stick to each other, thus shortening the injection molding cycle and improving the surface finishing of the molded parts. However, the effect of the mold release agent is influenced by both the chemical action and the physical conditions. Also, the injection molding raw materials and their processing conditions vary a lot, so the right mold release agent selection and dosage are subject to specific cases. If improperly applied, satisfactory mold release result will not be achieved.

 

With regard to injection molding temperature, the effective working temperature for the release agents containing fatty acids is usually below 150°C, so it is not suitable for high-temperature injection molding. The working temperature for the silicone-based or the metallic soap formed release agents are usually between 150°C and 250°C. The working temperature for the PTEF release agents is as high as 260°C and above, thus known as the best mold release agent for high-temperature injection molding.

 

With regard to varieties of raw materials, it is harder to release a soft polymer part than releasing a hard polymer part. With regard to application methods, release agent pastes are brushed, and release agent sprays are applied using spraying devices. Due to the fact that it is not easy to form an even and regular agent layer when using the paste, so the released parts will have a wavy surface, the sprays should be used whenever applicable.

 

  1. Overfill

If the injection pressure is too high, the molding shrinkage will be smaller than expected, making it hard to release the molded part. At this time, we can facilitate release by reducing injection pressure, shortening injection time and lowering melt flow & mold temperatures. In such circumstances, the mold release agent that reduces the friction between the part and the mold will be the most effective. As for molds, the most effective methods would be improving the degree of polish, eliminating the bumps on the sidewall, grinding and increasing the number of ejector pins. When molding a deep part, it will be easier to release the part if you blow compressed air between the part and the mold (please refer to the overfilling in “crazing, cracks, fissures and white marks”).

 

  1. Parts Stick in the cavity side

There are 2 reasons for this – the nozzle is stuck to the cavity at some point, or the release resistance of cavity side is larger than that of the Core side, so the part sticks to the cavity. The circumstances in which the part sticks in the cavity due to the resistance between the nozzle and the cavity include: The nozzle radius R is greater than the corresponding corner radius R in the mold, so when fixing the mold, the nozzle and the mold are not concentric, or there are plastics leaking from between the nozzle and the mold. Either of the cases will cause the part to stick in cavity side. To prevent this from happening, the mold should be installed properly. The release resistance of cavity side is too high because the degree of polish is too low or there are bumps on the sidewall. At this time, a Z-shaped pulling rod should be installed at the core side, so as to pull the molded part. Therefore, during mold design, sufficient consideration should be made to keep this from happening. It works even though there is a temperature difference between cavity side and the core side.

 

(1) The mold is overfilled with plastics – reduce injection molding pressure; Lower the too high temperature of the ejection cylinder

(2) The injection molding pressure is held for too long – reduce the time for the mold screw to move forward

(3) Mold surface scrapes, multi-holes or scratches – get rid of the stains and polish the mold surface

(4) Insufficient mold release angle – apply the minimum mold release angle of 0.5° on each side (the larger the angle, the more effortless the ejection and the faster the operation are)

(5) Improper undercut design – make sure there is no sharp angle in the undercut

(6) The injection molded part sticks to the highly polished mold surface – use discharge valve to empty the vacuum generated when the molded part is forced out through the highly polished mold surface

(7) Improper ejection mechanism – increase the number of ejector pins or change for a different system

(8) Insufficient plastic lubrication – use release agent whenever possible; Increase external lubricant, e.g. zinc stearate

 

Cause and Solution for Main Runner Adhesion:

(1) Cooling time is too short, so the main runner is not solidified yet.

(2) Insufficient primary runner slope(draft angle 3 – 5°), mold release slope should be enlarged.

 

Why the injection molding become so popular in plastic product manufacturing?

Definitely, if one of the spare parts on your machine or equipment does not function properly, you will need to order a new one for a replacement. However, if your equipment is custom-made, you will have to produce or purchase a customized spare part. But, this process will take a long time, depending on how difficult the part is manufactured and what kind of materials are used. However, if you already have a plastic injection mold which produces parts, it will be quite easy for you to get your part ready in only a very short period of time. Injection molding offers a fast way for you to get the parts you need in time, and at a very affordable price.

When scientist Alexander Parkes created the first piece of synthetic plastic in 1855, the manufacturing industry had no idea how the plastic mold would revolutionize the way factory and company thought about and produced parts. Though featuring quite a low cost, plastic injection molding is a fast, easy and the best manufacturing process. It is very possible that you are now reading this article on your computer. But you know what, your computer represents a perfect example of injection molding. It is likely that your keyboard, mouse and computer casing are made out of plastic. Generally speaking, injection molding is often used for any parts that can be mass-produced, but it is also an affordable way to create prototypes, almost as fast as you want them, and the plastic material is very easy to use and amazingly durable, too. That is why the application of plastic materials becomes so well accepted in the manufacturing industry. It is an ideal solution that can meet a lot of your manufacturing needs.

Plastic injection molding is an easy to use process. As a matter of fact, it is very simple yet effective. First, the granularized plastics are loaded into the injection chamber of the injection molding machine through a hopper. Then, there is a reciprocating screw inside the injection chamber that ensures the plastic granules flow into the machine body steadily and evenly. Next, the granules are moved through the heater which melts the plastic into liquids. After that, the liquefied plastic flows into the mold cavity through a nozzle, where it is injected into the plastic mold itself. Inside the mold, a moveable plate will apply certain pressure to ensure that the plastic is hardened.

By virtue of its ease of use and the fact that plastic is low priced and is able to be molded into almost any shape, the application of plastic injection molding has gained great popularity in the manufacturing industry. If you are a user of plastic parts, what you need to do is only to keep some extra parts at hand, so that you do not have to waste any of your time on producing the parts again in case of parts wear and tear. Since there is already a plastic mold in your desired shape, your production activity will be made more time-saving and money-saving.

Mold Trial Instruction

First, introduction
When we receive a new mold test mould proofing needs, we are always keen to try out an earlier result of prayer request and smooth so as not to waste hours of work and cause trouble.
But in this we have to remind two points: first, the mold designers and manufacturing engineers sometimes an error in our test model, fails to alert small error may arise due to large damage. Second, the test model results is to ensure the successful future production. If the tryout process did not follow reasonable steps and make the appropriate records, that is no way to guarantee the smooth progress of mass production. We also emphasize that “the use of smooth, then mold will quickly increase the profits of the recovery, otherwise the cost of damage caused by even more than the mold itself will cost.”

Second, try this before you mold on the machines
1. Relevant information about mold:
Can achieve the best mould design drawings, detailed to the analysis, and sometime have mold technician to participate in the work.

2. The first stage in the work of the mechanical inspection with the action:
Should pay attention to whether the cuts, missing pieces and loose phenomena, mode to the slide action is really, waterway and airway joints with or without leakage, mold opening process, then if restrictions should also be marked in the model. If these actions before the model was done in the hanging, that would be found to avoid problems linked to mold, mold removal go to waste hours of work have taken place.
3. When the proper action after determining mould ministries, we must select the appropriate test mold injection molding machine, the choice should pay attention to

  •  Injection capacity
  • the width of guide bar
  • the largest open range
  • are complete and other accessories

All confirmed that no problems were hanging next step is to mold, hanging in the lock should pay attention to all the templates folder and mold do not remove the fish before the hanging, in order to avoid loosening or breakage of the template folder so that die fall.
Mold should be carefully re-installed after inspection mold all the parts of mechanical movement, such as skateboarding, thimble, back tooth structure and limit switches of action really. And pay attention to Shoot mouth is aligned with the inlet. The next step is to pay attention to clamping action, then the pressure should be lowered off mode, in manual and low clamping action in attention and listen to see whether there is any different from time to smooth movement and sound and so on.

4. Raise the mold temperature:
Based on raw materials used in product performance and die size of the choice of appropriate mold temperature control unit will die when the temperature increased to produce the required temperature. Once the mold temperature increased again after the view to the various parts of the action, because the steel due to thermal expansion, may lead to card mode phenomenon, it should be noted that the sliding ministries in order to avoid strain and vibration generation.

5. If the pilot scheme is not implemented within the factory rules, we propose to adjust the conditions of a test mode a condition can only be adjusted in order to distinguish one effect of changes in the terms of the finished product.

6. According to different raw materials, on the appropriate use of the original Tu-made baking.

7. Tryout with the future production using the same raw materials as possible.

8. Do not totally inferior race tryout material, if the color needs to be arranged test color.

9. Internal stress and other issues often affect the secondary processing, should be finished after the tryout to be stable after secondary processing of slow closed mold, the mold should be fine off the pressure and movement several times to see whether the co- mold pressure inequality and to avoid distortion product generated flash and mold.

After checking the above steps are then die off die off speed and the pressure reduced, and the security buckle ejection rod and set a good trip, and then transferred to a normal off mode and off mode speed. If the trip involves the maximum limit switch should be adjusted to shorter mold stroke, in which cut off the mold before the maximum travel speed mold action. This is because the put on the mold during the trip among the high-speed movements who travel longer than the low speed of it. Mechanical machine in the plastic top of the bar must also be adjusted after the full opening-mode action role, so that thimble board or peel board Stress and deformation.

In the first model for injection before you re-check the following:

  • whether the feeding schedule is too long or less
  • the pressure is too high or too low
  • whether the filling too fast or too slow speed
  • processing cycle is too long or too short

To prevent the finished short shot, broken, deformed, or even hurt mold flash. If the processing cycle is too short to wear a thimble to the top ring Jishang finished products or peeling. Such circumstances may cause you to spend 23 hours to remove the finished product. If the processing cycle is too long, then the mold core of the thin rubber parts may tighten and broken, of course, you can not expect tryout process might happen, but do take account of prior and timely measures will help you avoid serious and costly damage.

Third, the main steps tryout
In order to avoid unnecessary waste of production time and when disturbed, indeed, necessary to pay the patience to adjust and control various processing conditions, and find the best temperature and pressure conditions, and develop standard procedures for test mode and available and in establish a regular working methods.

1. View the barrel of the plastic material is correct, and whether in accordance with the provisions of baking, (test mode and production of different raw materials if it might yield different results).

2. Feed tube to ensure thorough cleaning to prevent bad solution, or miscellaneous material rubber injection mold, plastic materials and hybrid solution because of inferior materials may be mold stuck. Test tube temperature and mold temperature of raw materials for processing.

3. To adjust pressure and injection volume in order to produce a satisfactory finished appearance, but can not run flash in particular, there are some not yet fully solidified cavity when finished, before the adjustment of various controlled conditions should think about, because the filling rate slightly changes may lead to changes in very large filling.

4. To be patient until the conditions of machines and molds to stabilize, that is, medium-sized machines may have to wait more than 30 minutes. Can use this time to see the finished product problems that might arise.

5. Screw forward time can not be shorter than the plastic gate solidification time, otherwise the product will reduce the loss of weight and performance of finished products. And when the mold is heated screw forward time for compaction should also be extended discretion of finished products.

6. Reasonable adjustments to reduce the overall processing cycle.

7. Redeployment of the conditions of the new operation at least 30 minutes, stabilize, and then continuous production of at least a dozen products all look in their bowl marked date, number, and press cavity were placed in order to test the stability of the exact operation and export the reasonable control of tolerance. (Particularly valuable for multi-point die).

8. Will continuously measure and record the significant sample size (such samples should be cooled to room temperature and then volume).

9. The amount of each product may look like the size to make a comparison, should be noted:

  • the stability of size.
  • whether there are certain size has increased or decreased while still showing a change in processing conditions, such as poor temperature control or hydraulic control.
  • whether movements in size within the tolerance range.

10. If not quite finished size of changes in processing conditions are normal, you need to see if each cavity of the finished product its quality can be accepted, its size can be within the tolerance allowed. To measure out more or less continuous average cavity number down to check whether the correct size of the mold.

Record and analyze data in order to modify the tooling and production as conditions require, and for the future reference for mass production.

  1. To make a longer processing time of operation to stabilize the melt temperature and hydraulic oil temperature.
  2. By all the finished size is too large or too small to adjust the machine conditions, if the shrinkage appears to shoot too much and finished less than expected, also available for reference in order to increase the gate size.
  3. The cavity size is too large or too small to be amended, if the cavity size and the door is still correct, then the candidate to change the machine conditions, such as filling rate, mold temperature and pressure, ministries, and view some of the mold Point is filling slowly.
  4. In accordance with the finished product of the cavity mold core shift with circumstances or be modified individually, perhaps try transfer mold filling rate and temperature, in order to improve its uniformity
  5. Check and modify the injection machine malfunction, such as pumps, oil valves, temperature controllers, etc. will lead to adverse changes in processing conditions, even more perfection in the maintenance of the mold can not be bad to play a highly efficient machine. In reviewing the records of all values, the retention of a sample compared to proofread the sample after it has been amended to improve.

Fourth, important issues
Keep all samples in the tryout process inspection records, including the processing cycle, all kinds of pressure, melt and mold temperature, tube temperature, injection movement time, screw feeding time, etc., in short, should help to save all future was able to successfully create the same conditions of data processing in order to obtain the products meet quality standards.

The plant is often overlooked when testing mold mold temperature, and in the short-term production test mode and in the future when the mold temperature is the most difficult to master, rather than the correct sample of the mold temperature can affect the size, brightness, shrinkage, flow lines, and less material phenomena If no mold temperature controller to hold in the future when production problems occur.

Industrial innnovation:plastic injection molding

OEMs make their own products from molds that allow them to create their own original product designs. However, as a matter of fact, OEMs are not the real originals. All in all, even OEMs need to outsource injection molds from factories which will build the molds for them.

 

Currently, there are factories which make injection mold tooling for plastic products. For a diversity of industries, plastic injection molding is of great importance, and the more quickly and more effectively they can mold these products, the faster and more efficient their production can be. A great number of industries, ranging from medical care to food service, are dependent on machines with plastic injection products, so as to get their jobs well done on a day-to-day basis. What is critically important for those plastic molding factories is that they have to think about what the specific needs for each specific part are. For example, they have to consider not only the hardness of the plastic material, but also the ideal temperature for melting the plastics, as well as the type of plastic that is used for every individual end product. Only then will the parts be manufactured with the highest standards.

 

During the injection molding process, plastic granules are fed into the injection chamber through a hopper. A reciprocating screw is used inside the chamber to guarantee the even distribution of these plastic granules, which are then moved into a heater and melted, so that it is able to flow into the clamping chamber. The plastic material is evenly fed first into the mold cavity, and then into the mold, where a moveable plate clamps it into the mold. Under certain temperature and pressure conditions, the plastic is hardened and molded into the desired parts.

 

When plastic injection molding is mentioned in a conversation, not all products are produced in an equal manner. There are some tasks that are designed to mold smaller parts, while some others are carried out to mold larger parts. The appearance of plastic injection molding has brought about a revolution to the world industry. Being able to use plastics to produce parts hard enough for machines has made it a lot easier to manufacture parts more quickly and more efficiently than ever before. In some particular cases, plastic parts even perform better than the metal ones. Whether the plastic molding process is the right solution for your business? Well, it is dependent on whether you think it would be appropriate for your application. At least, it would be good for you to gather a little more information about the process, so that you will get to know how to manufacture an up-to-standard quality part.