Solutions to plastic product mold opening problems (1)

For this series, please refer to Solutions to plastic product mold opening problems (1), (2), (3).

This article mainly introduces problems that arise during mold opening, mold trial and mass production processes after design of plastic parts is completed. It also provides a general understanding of how to solve them. Details are as follows, mainly following points:
1. Insufficient filling
2. Overflow
3.Shrink marks

1. Insufficient filling

Underfilling manifests itself as an incomplete injection molded part or incomplete details, and is caused by plastic not completely filling mold cavity. This is a frequently encountered problem, but it is relatively easy to solve. When it is true that problem cannot be solved by technological means, improvements can be made from perspective of mold design and manufacturing, generally problem can be solved. There are many reasons for insufficient filling, and factors involved may include injection molding machines, molds, processes, raw material products, etc.

(1) Injection molding machine

①Plasticizing capacity of injection molding machine is small and cannot provide sufficient material volume. It is necessary to replace machine with a larger plasticizing capacity. When product quality exceeds actual maximum injection quality of injection molding machine, it is obvious that material supply cannot make ends meet.
②Temperature displayed by thermometer is not true. It appears to be high but is actually low, causing material temperature to be too low.
③Nozzle is partially blocked. Check nozzle hole for foreign matter or unmelted material. Diameter of inner hole of the nozzle may be too large or too small. If it is too small, flow diameter will be small and specific volume of material strip will increase, which will easily cause refrigeration, blocking feed channel or consuming injection pressure; if it is too large, flow cross-sectional area will be large and pressure per unit area of plastic entering mold will be low, resulting in a low injection force.
④ Check amount of material in hopper and whether mouth of hopper is blocked, and whether plastic frit blocks feeding channel.
⑤Put cold material from nozzle into mold. Injection molding machines usually only have straight-through nozzles due to concerns about pressure loss. However, if temperature of front end of barrel and nozzle is too high, or there is too much material stored in the front end of barrel under high pressure, “casting” will occur, causing plastic to accidentally enter main channel entrance before injection starts and mold is open, will harden under cooling effect of mold plate, which will prevent molten material from smoothly entering mold cavity. At this time, temperature of front end of barrel and nozzle should be lowered, storage volume of barrel should be reduced, and back pressure should be reduced to avoid excessive melt density at the front end of barrel.
⑥ Injection molding cycle is too short. Due to short cycle, material temperature cannot catch up in time, which will also cause material shortage, especially when voltage fluctuation is large. Cycle should be adjusted accordingly according to supply voltage. Injection and pressure holding time are generally not considered when adjusting. Main consideration is to adjust period from completion of pressure holding to screw retraction. This will not affect filling and molding conditions, but also extend or shorten the preheating time of pellets in barrel.

(2) Mold

① Is it affected by insufficient exhaust? Check whether exhaust holes are blocked or add multiple exhaust positions.
② If water inlet is too small or flow channel is too slender, water inlet needs to be enlarged accordingly, flow channel should be thickened, and corner should be an arc transition.
③ Partial section of plastic part is too thin and needs to be structurally improved to add multiple water inlets.
④ For multiple cavities, whether position of water inlet is appropriate should be adjusted according to situation.

(3) Injection molding process

① If feed adjustment is improper, there is lack of material or too much material, back pressure should be increased appropriately. Inaccurate feeding measurement or abnormal operation of feeding control system, abnormal injection cycle due to limitations of injection molding machine or mold or operating conditions, low pre-molding back pressure or low density of material particles in barrel may cause material shortage.
② If injection pressure is too low, injection time is short, and plunger or screw returns too early, injection pressure and injection time need to be increased. Molten plastic has higher viscosity and poor fluidity at low operating temperatures, so higher pressure and speed should be used for injection.
③ Injection speed is slow and injection speed needs to be increased. Injection speed is of great significance for some products with complex shapes, large thickness changes, long processes, and plastics with high viscosity such as toughened ABS. When product cannot be fully filled using high pressure, high-speed injection should be considered to overcome problem of insufficient filling.
④Material temperature is too low, increase melt temperature. Temperature of front end of barrel is low, viscosity of molten material entering cavity rises prematurely to point where it is difficult to flow due to cooling effect of mold, hindering filling of remote end; temperature in rear section of barrel is low, and it is difficult for plastics with high viscosity to flow, which hinders forward movement of screw. As a result, it seems that pressure displayed by pressure gauge is sufficient, but in fact, molten material enters mold cavity at low pressure and low speed; If nozzle temperature is low, it may be that nozzle has been in contact with cold mold for a long time during fixed feeding and lost heat, or nozzle heating ring has insufficient heating or poor contact, causing material temperature to be low, which may block feeding channel of mold; if mold does not have a cold material well, use a self-locking nozzle and adopt a post-feeding procedure so that nozzle can maintain necessary temperature. When nozzle is too cold when it is first turned on, sometimes a flame gun can be used for external heating to speed up nozzle temperature rise.

(4) Inspection

Material flowability is poor. Plastics plants often use recycled scrap, which often reflects a tendency to increase in viscosity. Experiments pointed out that: since unit volume density of molecular chain breaks generated by oxidative cracking increases, this increases viscosity of flow in barrel and cavity. Recycled scrap promote production of more gaseous substances, which increases injection pressure loss and causes mold filling difficulties. In order to improve fluidity of plastics, you should consider adding external lubricants such as stearic acid or other salts, preferably silicone oil (viscosity 300~600cm2/s). Addition of lubricant not only improves fluidity and stability of plastics, but also reduces air resistance of gaseous substances.

2. Overflow

Flash appears as excess flash, overflow, edges or edges on plastic parts, and usually appears on the parting surface of mold or at model joining line or hole location. If flash is not resolved in time, it will further expand, causing local collapse of imprinting mold and causing permanent damage. There are many reasons for overflow, and factors involved may include injection molding machines, molds, processes, raw material products, etc.

(1) Injection molding machine

① Mold clamping force of injection molding machine is insufficient and clamping force is too small. It is necessary to replace machine with a larger clamping force. When selecting an injection molding machine, rated mold clamping force of machine must be higher than tension formed by longitudinal projected area of injection molded product during injection, otherwise mold will expand and flash will appear.
② Mold closing device is poorly adjusted and toggle mechanism does not extend, resulting in uneven mold closing on the left and right or up and down, and mold parallelism cannot be achieved, resulting in a situation where one side of mold is tightly closed but the other side is not tightly closed, and flash will appear during injection.
③ Parallelism of mold itself is not good, or installation is not parallel, or mold plate is not parallel, or force distribution and deformation of tie rods are uneven, which will cause mold to be not tightly closed and produce flash.
④ Check ring is severely worn; spring nozzle spring fails; barrel or screw is excessively worn; inlet cooling system fails, causing a “bridging” phenomenon; injection amount set in barrel is insufficient, buffer pad is too small, etc. As a result, flash appears repeatedly, and parts must be repaired or replaced in time.
⑤ Clamping force of machine is unstable.

(2) Mold

① Precision of mold parting surface is poor. Movable mold plate (such as middle plate) is deformed and warped; foreign matter is stained on parting surface or there are protruding skid burrs around mold frame; old mold causes fatigue collapse around cavity due to earlier flash extrusion.
② Mold design is unreasonable. If opening position of mold cavity is too deviated, it will cause tension on one side of mold during injection, causing flash; plastics with too good fluidity, such as polyethylene, polypropylene, nylon, etc., have very low viscosity in molten state and are easy to move or a fixed gap, which requires higher manufacturing precision of mold; it should be placed on the center of mass symmetry as much as possible without affecting integrity of product. Feeding material into thick part of product can prevent shortage of material and flash edges at the same time; when there is a molding hole in the center of product or near it, a side gate is usually opened on the hole. Under a large injection pressure, if mold clamping force is insufficient, supporting force of this part of mold is insufficient, slight warping occurs, causing flash. If there are movable components on the side of mold, projected area of side is also affected by molding pressure. If support force is not enough, flash will also occur; fitting accuracy of sliding core is poor or installation position of fixed core and cavity is offset, resulting in flash; poor exhaust of mold cavity, no exhaust groove on parting surface of mold, or exhaust groove is too shallow, too deep or too large, or blocked by foreign matter will cause flash; for multi-cavity molds, attention should be paid to reasonable design of each runner and gate, otherwise uneven mold filling force will result in flash.
③ Foreign objects adhere to parting surface, resulting in loose mold clamping.
④ Mold and material are not matched accurately and require targeted maintenance.

(3) Injection molding process

① Injection pressure is too high or injection speed is too fast. It is necessary to reduce injection pressure or switch from injection pressure to holding pressure in advance. Due to high pressure and high speed, opening force on mold increases, which may cause overflow. Injection speed and injection time should be adjusted according to thickness of product. Thin products should be filled at high speed and no more injection after fullness; thick products should be filled at low speed and skin should be roughly fixed before reaching final pressure.
② If amount of material added is too large, flash will be caused. Appropriately reduce amount of glue injected and lower temperature of melt. It is worth noting that you should not inject too much molten material to prevent dents. In this case, dents may not be “filled up”, but flash will appear. This situation should be solved by extending injection time or holding pressure time.
③ If temperature of barrel or nozzle is too high or temperature of mold is too high, viscosity of plastic will decrease, fluidity will increase, and flash will occur when mold is smoothly fed. Melt temperature should be appropriately lowered.
④ Reduce forward time of screw and reduce injection speed.

(4) Raw materials

① If viscosity of plastic is too high or too low, flash may occur. For plastics with low viscosity such as nylon, polyethylene, polypropylene, etc., mold clamping force should be increased; plastics with strong water absorption or water-sensitive plastics will greatly reduce flow viscosity at high temperatures and increase possibility of flash. These plastics must be thoroughly dried; viscosity of plastics that incorporate too much recycled material will also decrease, retained components must be replenished if necessary. If viscosity of plastic is too high, flow resistance will increase, resulting in a large back pressure that increases mold cavity pressure, resulting in insufficient mold clamping force and flash.
② When particle size of plastic raw materials is uneven, amount of feed added will vary, and finished parts may be unsatisfied or have flash edges.

3. Shrink marks

Shrink marks usually appear as dents (collapses, flat shapes) caused by shrinkage and lack of material on plastic surface as it cools, hardens, and mainly appears on thick walls, ribs, casings, and the backs of nut inserts, etc. There are many reasons for shrinkage marks, and factors involved may include injection molding machines, molds, processes, raw materials, product design, etc.

(1) Injection molding machine

① Screw or plunger is seriously worn, melt leakage occurs during injection and pressure holding, which reduces filling pressure and material volume, resulting in insufficient melt.
② Nozzle hole is too large or too small. If it is too small, it will easily block feed channel; if it is too large, injection force will be small and mold filling will be difficult.
③ Increase amount of material added and reduce thickness of cushion pad so that injection pressure can be effective without excessive consumption.

(2) Mold

① Gate is too small or runner is too narrow or shallow, runner efficiency is low, resistance is large, and melt cools prematurely. Increase gate appropriately to increase amount of glue; increase injection port or shorten runner to reduce pressure loss, making material flow smoothly. Gate should not be too large, otherwise it will reduce shear rate, viscosity of rubber will be high, and product will not be full. Gate should be opened in thick-walled part of product to facilitate feeding. Setting up a necessary cold material well with sufficient capacity in runner can prevent cold material from entering mold cavity so that mold filling can continue. Gate length of point gates and needle gates must be controlled below 1mm, otherwise plastic will solidify quickly at gate, affecting pressure transmission; if necessary, number of point gates or gate positions can be increased to meet actual needs; when runner is long and thick, an exhaust groove should be set at the edge of runner to reduce blocking effect of air on material flow.
② For multi-gate molds, filling speed of each gate must be adjusted. It is best to open gates symmetrically.
③ Balance filling of melt in each cavity by changing gate position and runner size.
④ The entire mold should be free of burrs, have reliable mold closing and sealing properties, and can withstand filling of high-pressure, high-speed, low-viscosity melt materials.
⑤ There are local high-temperature areas on mold to improve location of cooling water; cooling water channels should be effectively set up in key parts of mold to ensure that cooling of mold has a good effect on eliminating or reducing shrinkage.
⑥ Mold temperature is too low and material is incompletely filled, so mold temperature needs to be increased.

(3) Injection molding process

① Increase injection pressure, holding pressure, and extend injection time. For plastics with high fluidity, high pressure will produce flash and cause collapse. Material temperature should be appropriately lowered, as well as temperature of front section of barrel and nozzle, so that volume change of melt entering cavity is reduced, it is easy to cool and solidify; for high-viscosity plastics, barrel temperature should be increased to make mold filling easier. When shrinkage occurs in gate area, holding time should be extended.
② Increasing injection speed can more easily fill part and eliminate most of shrinkage.
③ For appropriate parts, mold temperature should be increased to ensure smooth material flow; for thick-walled parts, mold temperature should be reduced to accelerate curing and shaping of skin.
④ Increase cooling time appropriately to reduce thermal shrinkage. Extending cooling residence time of parts in mold, maintaining a uniform production cycle, increasing back pressure, and retaining a certain buffer pad in the front section of screw are all beneficial to reducing shrinkage.
⑤ Low-precision products should be removed from mold as soon as possible and allowed to cool slowly in air or hot water, so that shrinkage and depression can be gentle without affecting use.
⑥ If temperature is too high or too low, pressure maintaining and compressing effect will not be achieved.
⑦ Amount of glue injected is insufficient, increase amount of glue injected.

(4) Raw materials

Raw materials that are too soft are prone to dents. An effective method is to add nucleating agents to plastic to speed up crystallization.

(5) Product design

Product design should make wall thickness uniform and try to avoid changes in wall thickness. For plastics such as polypropylene that shrink greatly, when thickness changes by more than 50%, it is best to use ribs instead of thickened parts.

For further reading, please refer to Solutions to plastic product mold opening problems (2).