Injection Molding Process

mold or mould :

Mold is the American spelling for all senses of the word meaning, among other things,
(1) a frame for shaping something,
(2) to shape in a mold, and
(3) any of various fungi that commonly grow on organic matter and are often associated with decay.

Mould is the British spelling. American English has no mould, and British English has no mold.
Australian and Canadian English favour the British spelling, though mold is fairly common in Canadian publications.
The distinction extends to all derivative words, including molding (American) and moulding (British).

Casting

 

• Casting is a basic molding process as it requires the least amount of complex technology. Plastic is simply heated so it turns into a fluid, and then transferred into a mold. It is left to cool and the mold is removed. This process can be used for intricate shapes and performed under a low pressure. However, it is a common process used for making plastic sheeting, starting from 0.5 inches thick and greater.

Injection Molding

 

 

• Injection molding is used for creating high-quality three-dimensional objects, that can be commercially reproduced. The molding process begins by melting plastic in a hopper. Then the plastic is injected into a tightly closed, chilled mold. The plastic quickly takes the shape of the surrounding mold. Once it has completely set, the mold is opened and the plastic object is released. Yogurt pots, butter tubs, toys and bottle caps are made using this process.

Blow Molding

 

• Blow molding is a process used for making piping and milk bottles. Plastic is heated until molten. Then it is injected into a cold mold. The mold has a tube set within it, which has a particular shape when inflated. So, while the plastic is molten, air is blown into the tube and the plastic is formed around the tubing. It is then left to cool and removed from the mold.

Compression Molding

 

• The most labor-intensive type of molding process is compression molding. Therefore, it is only used for large-scale production purposes, and not for mass production. For example, boat hulls and car tires are made using this method. Molten plastic is poured into a mold. Then a second mold is pressed into it. This squeezes the plastic into the desired shape before being left to cool and removed from the mold.

Rotational Molding

 

• Toys, shipping drums, storage tanks and items of consumer furniture are made using rotational molding. Each object is made by coating a mold from the inside. A mold is held in place between two mechanical arms. Then, the arms rotate the mold constantly at the same level, while molten plastic is placed inside. As it turns, the plastic coats the inside of the mold to create a new hollow, plastic object.
  
  
  
  

  Injection Molding:

  

   

   
  
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  Injection molding has been one of the most important fabrication tools for the plastics industry since the reciprocating screw machine was patented in 1956. Today, it's almost impossible to do anything without using injection molded parts. They are used in automotive interior parts, electronic housings, housewares, medical equipment, compact discs, and even doghouses. Injection molding is used to fabricate pallets, toys, crates, and pails, thin-wall food containers, promotional drink cups, lids, and milk bottle caps.
  

  
  The injection molding process involves melting the plastic in an extruder and using the extruder screw to inject the plastic into a mold, where it is cooled. Speed and consistency are vital keys to running a successful injection molding operation, since profit margins are normally below 10 percent.
  
  Speed:
  
  A molder will maximize output by minimizing cycle time which is the amount of time that is taken to melt the plastic, inject it into the mold, cool, and eject a finished part.
  Using larger molds that produce more than one part each time the machine performs a cycle can also increase output. These molds are known as multiple cavity molds.
  
  Consistency:
  
  Consistency, or elimination of scrap and downtime, is just as important as output in a successful molding operation. The most consistent processing results from careful control of plastic temperature, plastic pressure as it fills the mold, the rate at which the plastic fills the mold, and the cooling conditions. These four primary molding variables are interdependent and can often be used to understand process changes and solve problems. While the variables apply to almost all injection molding processes, the process will be slightly different in each shop, depending on the application, the plastic being used, and the molder's preferences.
  
  Fill rate:
  
  In thin wall applications, the material must be injected into the mold as quickly as possible to prevent the plastic from freezing before the part has been completely filled. The newest resin and machine technologies in this area almost always focus on faster, easier fills. In addition to minimizing cycle time through better filling ability, the molder could realize resin cost savings through the ability to fill thinner molds or achieve higher outputs by using larger, higher cavity molds.
  Thin wall molding is accomplished using machines that inject the material in less than one second and are big enough to support large, multiple cavity molds. Thin wall lids and containers tend to be small, so molds may be used to fabricate over 100 small lids at a time.

 Defects in injection molding:

Black Spots, Brown streaks: Description Black spots and brown streaks appear as dark spots or streaks in the molded part and are usually caused by thermal damage to the melt. Possible Solutions Check the material for contamination. Decrease the melt temperature. Decrease the overall cycle time. Purge and/or clean the screw and barrel. Decrease the screw speed. High screw speeds may cause the material to degrade. Material may have too much regrind content. Material may be overdried. Decrease drying time/temperature. Refer to drying instructions provided by the material supplier. Material may be prone to thermal degradation. It may be necessary to use a more thermally stable material. Dead spots may be occurring, ensure that the alignment between the machine nozzle and mold sprue is correct. Residence time may be too long, or the shot size may be too small for the machine. It may be necessary to move the mold to a machine with less injection capacity.

Blisters (Air Entrapment): Description Blisters are hollows created on or in the molded part. In contrast to a void (vacuum) this entrapped gas can also appear near the walls. Possible Solutions Decrease melt temperature. Decrease screw speed. Dry material. Increase back pressure. Increase mold temperature. Ensure regrind is not too coarse. Provide additional mold vents. Relocate gate.

Brittleness: Description Brittleness is a condition where the part cracks or breaks at a much lower stress level than would normally be expected based on the virgin material properties. Possible Solutions Check for material contamination. Decrease amount of regrind use. Decrease back pressure. Decrease injection pressure. Decrease screw speed. Increase melt temperature. Dry material. Refer to the drying instructions provided by the material supplier.

Bubbles: Description Bubbles are similar to blisters in that there is air entrapped in the molded part. Possible Solutions Decrease injection speed. Decrease injection temperature. Dry material further. Increase injection pressure. Increase number and/or size of vents. Increase shot size.

Burn Marks, Dieseling: Description Burn Marks or Dieseling show up on the finish molded parts as charred or dark plastic caused by trapped gas and is usually accompanied by a distinctive burnt smell. Note: If this problem is allowed to continue without fixing the root cause it will very quickly cause damage to the molding surface. Possible Solutions Alter gate position and/or increase gate size. Check for heater malfunction. Decrease booster time. Decrease injection pressure. Decrease injection speed. Decrease melt and/or mold temperature. Improve mold cavity venting. Vents may become smaller over time due to wear and they will need to be brought back to their original depth. Reduce clamp force to improve venting. Vents may become smaller because they are being crushed by the clamping force. If it is possible to reduce the clamping force without causing flash then this should be done. Note: This is always good practice to minimize wear on the mold and machine. Improve venting at the burn location. Burn marks often occur on deep ribs that have no venting. If possible it may be helpful to put an ejector pin or sleeve at the burnt area to allow the trapped gas to escape to atmosphere.

Cracking, Crazing: Description Cracking or Crazing is caused by high internal molded in stress or by an external force imposed upon the part. They can also be caused by an incompatible external chemical being applied to the finished parts The cracks often don't appear until days or weeks after the parts have been molded. Possible Solutions Decrease injection pressure. Dry material. Increase cylinder temperature. Increase mold temperature. Increase nozzle temperature. Modify injection speed. If the material is partially crystalline then it may help to reduce the mold and/or melt temperature. If the material is amorphous then it may help to increase the mold and/or melt temperature.

Delamination: Description Delamination occurs when single surface layers start flaking off the molded part. Possible Solutions Adjust injection speed. Check for material contamination. Incompatible resins or colorants may have been accidently mixed causing this condition to be seen. Dry material. Increase melt temperature. Increase mold temperature. Insufficient Blending. Check melt homogeneity and plasticizing performance.

Discoloration: Description Discoloration is similar to burn marks or brown streaks but generally not as dark or severe. It may cause the part to be a darker shade than the virgin pellets and is often found nearest the gate area, however it can also appear as dark streaks throughout the part. Possible Solutions Check hopper and feed zone for contamination. Decrease back pressure. Decrease melt temperature. Decrease nozzle temperature. Move mold to smaller shot-size press. Provide additional vents in mold. Purge heating cylinder. Shorten overall cycle.

Excessive Flash: Description Excessive Flash is often seen near sealing faces, out of vent grooves, or down ejector pins. It appears as thin or sometimes thick sections of plastic where it would not be on a normal part. Note: Flash can very quickly (within a few cycles) damage the parting line surfaces. Possible Solutions Decrease back pressure. Decrease cylinder temperature. Decrease injection hold time. Decrease injection pressure. Decrease injection speed. Decrease mold temperature. Increase clamp pressure. Check mold venting. Vents may have been ground too deep for the material being used. Check sealing surfaces to ensure that they seal off properly by "blueing" them in under clamp tonnage. Check ejector pin bore diameter to pin diameter tolerances. The tolerances may be too large allowing plastic to flash down the opening. The tolerances may be too large for the material being used and can occur due to wear over time.

Flow, Halo, Blush Marks: Description Flow, Halo, Blush Marks are marks seen on the part due to flow of the molten plastic across the molding surface. Possible Solutions Decrease injection speed. Increase cold slug area in size or number. Increase injection pressure. Increase melt temperature. Increase mold temperature. Increase nozzle temperature. Increase size of sprue/runner/gate.

Gate Stringing, Drooling: Description The part does not break cleanly from the gate area. Possible Solutions Insufficient cooling time during the cycle. Excessive heat in the gate area. Check thermocouple in the nozzle or decrease the temperature of the hot runner manifold and nozzle. Increase cooling at the gate area.Ensure that you have controllable turbulent flow in the gate area.

Gels: Description Gels are bubbles, or blisters seen on or in the part due to poor melt quality. Possible Solutions Change screw speed. Increase back pressure. Increase cylinder temperature. Increase overall cycle time. Increase plasticating capacity of machine or use machine with large plasticating capacity.

Jetting: Description Jetting is caused by an undeveloped frontal flow of melt in the cavity. The uninterrupted plastic flows or "snakes" into the cavity and cools off enough so that it does not fuse homogeneously with the material that follows. Possible Solutions Decrease injection speed. Change the melt temperature, up or down. Use higher compression screw. Increase the gate diameter. Move the gate so that when the plastic first enters the cavity it hits an obstruction such as a rib or wall.

Material Leakage: Description Material Leakage is usually caused by material forces overcoming the structural strength of the mold. NOTE: One sign that indicates that material has leaked is that the manifold reaches processing temperature very slowly. Possible Solutions Manifold locator is oversize. Processing temperature may be too low causing increased pressure in the manifold. Manifold locator may be hobbed into the mold. Decrease the force applied to the nozzle pad by the machine then repair the damaged area, then check and if necessary replace the locator. Insufficient number of mold assembly screws. Ensure that the quantity, type of screw, and the location of the screws correspond to the general assembly drawing. Nozzle may have overheated causing damage to the seal or gate. Check/replace the thermocouple in the nozzle, then check and if necessary repair the nozzle well area. Manifold may have overheated. Check and replace if necessary the following components; nozzle well area, thermocouple, valve disks, sprue disks, or pressure disks.

Oversized Part: Description Part is too large when compared to the drawing specifications. Possible Solutions Decrease booster time. Decrease cylinder temperature. Decrease holding pressure. Decrease injection pressure. Decrease injection speed. Decrease overall cycle time. Increase gate size and/or change gate location. Increase mold temperature.

Part Sticking: Description Part is getting not pulling out of the cavity and in rarer circumstances cannot be ejected off the core. Possible Solutions Check mold for undercuts and/or insufficient draft. Decrease booster time. Decrease cylinder and nozzle temperature. Decrease injection pressure. Decrease injection-hold. Decrease mold cavity temperature. Increase clamp pressure. Increase mold-close time. Texturing on part is too deep. The parts may stick in the cavity if a new texture or a retexturing has been performed on the cavity half of the mold. If possible add undercuts to the core to allow the part to pull out of the cavity.

Short Shot (Incomplete Filled Parts): Description Short Shots occur when the part does not completely fill. Possible Solutions: Increase back pressure. Increase injection pressure. Increase injection speed. Increase melt temperature. Increase mold temperature. Increase nozzle temperature. Ensure that the manifold and nozzles have reached the set temperature. Increase shot size and confirm cushion. Make sure mold is vented correctly and vents are clear. Confirm that the non-return valve used is not leaking excessively. Increase the switch over pressure, distance, or time (whichever method is being used) point from fill to hold so the fill stage is used longer. Change part design. Thin areas of the mold may not fill completely, especially if there is a thick to thin transition, or there is a long rib that cannot be vented very well. If the part design allows it, change in these areas can improve the situation.

Sink Marks: Description Sink Marks occur during the cooling process if certain areas of the part are not cooled sufficiently causing them to contract. Possible Solutions: Decrease amount of regrind use. Decrease back pressure. Confirm that the non-return valve being used is not leaking excessively. Decrease melt temperature. Do this if the sink marks are near the gate or thick walled areas. Decrease mold temperature. Do this if the sink marks are near the gate or thick walled areas. Decrease injection rate. Do this if the sink marks are near the gate or thick walled areas. Dry material. Increase injection pressure. Do this if the sink marks are away from the gate or in thin walled areas. Increase injection speed. Do this if the sink marks are away from the gate or in thin walled areas. Increase mold temperature. Do this if the sink marks are away from the gate or in thin walled areas. Increase injection-hold. Increase shot size and confirm that the a cushion is being maintained. Increase size of sprue and/or runners and/or gates. Relocate gates on or as near as possible to thick sections. Increase cooling time. If possible change the mold design to maintain an even wall thickness throughout the part.

Splay Marks, Silver Streaks: Description Splay Marks, Silver Streaks are usually caused by water vapor blisters at the flow front burst and freeze on the wall of the molding surface. Possible Solutions: Check for contamination. Decrease melt temperature. Decrease nozzle temperature. Dry resin pellets before use. As per the manufacturers recommendations. Incorrect storage of pellets. Moisture on the pellets could be transferred into the melt, especially if the resin is not normally pre-dried. Raise mold temperature. This will prevent condensation on the mold walls from being carried into the melt. Ensure the mold is not leaking water onto the cores or cavities. Again this will prevent condensation on the mold walls from being carried into the melt. Relocate gates on or as near as possible to thick sections. Shorten overall cycle.

Sprue Sticking: Description Sprue Sticking generally occurs in a cold runner mold when the sprue is staying in the mold. Possible Solutions: Check mold for undercuts and/or insufficient draft. Decrease booster time. Decrease injection pressure. Decrease injection speed. Decrease injection-hold. Decrease mold close time. Decrease nozzle temperature. Increase core temperature. Open the gates. Ensure that the correct design of nozzle tip for the material is being used.

Surface Finish (Low Gloss): Description Surface Finish (Low Gloss). Gloss is the appearance of the surface of the molded part when light is reflected off of it. Molds that are textured or resins that are filled have an inherently reduced level of gloss when compared to highly polished mold surfaces. Possible Solutions: Clean mold surface. If the part design allows increase the polish of the molding surface. Increase cylinder temperature. This applies to molds that have a polished surface. Increase injection pressure. This applies to molds that have a polished surface. Increase injection speed. This applies to molds that have a polished surface. Increase mold temperature. This applies to molds that have a polished surface. Decrease cylinder temperature. This applies to molds that have a textured surface. Decrease injection pressure. This applies to molds that have a textured surface. Decrease injection speed. This applies to molds that have a textured surface. Decrease mold temperature. This applies to molds that have a textured surface. Increase melt temperature. Make sure venting is adequate.

Surface Finish (Scars, Wrinkles): Description Surface Finish (Scars, Wrinkles). Is the appearance of the ripples or wrinkles on the surface of the molded part. Possible Solutions: Decrease back pressure. Decrease nozzle temperature. Increase booster time. Increase the melt temperature. Increase injection pressure. Increase injection speed. Increase overall cycle time. Increase shot size. Inspect mold for surface defects.

Undersized Part: Description Part is too small when compared to the drawing specifications. Possible Solutions: Decrease mold temperature. Increase booster time. Increase cylinder temperature. Increase hold-time. Increase holding pressure. Increase injection pressure. Increase injection speed. Inspect mold for surface defects.

Valve Pin Does Not Close: Description Valve pin does not close properly. This will leave the gate protruding from the part. This may also occur if the valve pin is too hot, the material may stick to the valve pin. Possible Solutions: Valve pin is too short. Check and replace if necessary. Valve pin fit. Ensure that the valve pin is lapped to the gate steel when appropriate. Damaged gate. Check if valve pin is too long, rework if necessary. Also check to ensure that the valve pin is concentric with the gate, if not replace it. Hydraulic / Pneumatic seals may be worn. Replace as necessary. Insufficient pin/land area in the gate area of the mold. Increase the gate area cooling, or increase the valve pin land contact. Insufficient hydraulic or air pressure. Increase the pressure up to but not beyond the maximum rating of the unit being used. Excessive hold time. Decrease the hold time.

Voids: Description Voids are hollows created in the part. They are normally found in thick sectioned parts caused by material being pulled away from the hot center section towards cold mold walls leaving a void in the center. Possible Solutions: Clean vents. Decrease injection speed. Decrease melt temperature. Dry material. Increase injection pressure. Increase injection-hold. Increase mold temperature. Increase shot length. Increase size of gate. Increase size of sprue and/or runners and/or gates.

Warping, Part Distortion: Description Warping, Part Distortion is shows up as parts being bowed, warped, bent or twisted beyond the normal specification outlined on the drawing. Possible Solutions: Adjust melt Temperature (increase to relieve molded-in stress, decrease to avoid overpacking). stress, decrease to avoid over packing). stress, decrease to avoid over packing). Check gates for proper location and adequate size. Check mold knockout mechanism for proper design and operation. Equalize/balance mold temperature of both halves. Increase injection-hold. Increase mold cooling time. Relocate gates on or as near as possible to thick sections. Try increasing or decreasing injection pressure.

Weld Lines: Description Weld Lines are created when two or more melt flow fronts meet possibly causing a cosmetically visible line. It can also create a weakened area in the finished molded part especially with filled resins. Possible Solutions: Increase injection pressure. Increase injection speed. Increase injection hold. Increase melt temperature. Increase mold temperature. Make sure part contains no sharp variation in cross-sections. Vent cavity in the weld area.