Relationship between general forming defects and location of occurrence

 

Cause
When gate direction, resin flow direction, and molded product direction are consistent, molten resin passing through gate will directly enter cavity without resistance, and start filling from inside after hitting wall on opposite side of gate. Since fusion shape of resin that enters cavity first and resin that enters cavity later is very poor, a snake-like flow mark is generated and a mark is left on the surface after cooling
Countermeasures
1. Increase mold temperature to improve fusion state,
2. Increase resin temperature of heating cylinder.
3. Increase temperature of nozzle.
4. Reduce injection speed (near gate)
5. Increase gate diameter.
6. It will be relatively better if gate is changed to a handle-shaped gate
7. Consider whether gate position should be changed.

 

Causes: Viscosity of resin flowing into mold cavity will increase, reducing fluidity of resin. However, resin flowing in from back will continue to push resin to flow, and after this repetition, resin is filled. In this case, wavy resin flow marks like tree rings will be left on the surface of molded product. Surface gloss of this part of molded product becomes very poor. Therefore, mold must be filled before viscosity of resin entering mold increases. In addition, changes in wall thickness of surface layer and changes in flow pressure caused by changes in injection speed will also cause flow mark problems.
Countermeasures
1. To improve fluidity of resin, you can increase mold temperature.
2. To improve fluidity of resin, you can increase temperature of heating cylinder resin
3. Increase temperature of nozzle.
4. Increase injection speed. When there are jumping flow marks, reduce injection speed before it. When there is a change in wall thickness, injection speed and injection pressure may change, so adjust injection speed.
5. Increase nozzle diameter.

 

Mechanism of flow marks 1
When injection speed is slow

 

During filling process, resin (at the front of flow) begins to solidify and form a solidified layer as soon as it contacts mold wall. If balance between flow speed and solidification speed is broken, resin will move forward in a wave-like manner, which will produce flow marks.
Mechanism of flow marks 2
2 A caused by surface misalignment

 

When wall thickness of molded product is different, surface B is filled later than surface A, so surface is formed later
When injection speed is fast, solidification time of surface B and surface A is not synchronized, so jumping flow marks will appear
A occurs near gate
Mechanism of flow marks 3
2 Case B caused by surface misalignment

 

Causes
In molded product, if injection pressure is too high locally (especially near gate) or product is overfilled, injection pressure is maintained in this state, resin shrinks in volume when it cools, and pressurized resin is forcibly injected into mold cavity. After resin cools in this state, density of molded product will be higher and weight will increase, but at the same time, internal stress caused by overfilling will also be left in molded product. After molded product is completely cooled, it will be distorted, product will be partially cracked, or it will break. We call this phenomenon cracking.
In addition, residual internal stress changes over time and environment changes, and stress is released, leaving a very fine fracture line on the surface. We call it fragmentation.
In addition, when product is ejected, due to poor demolding, local stress around ejector pin is too high, and fracture lines or whitening will appear. These phenomena are more likely to occur with a larger gate cross-sectional area (straight gate).

 

Countermeasures
1. Reduce injection (holding) pressure
2. Shorten injection holding time.
3. Increase mold temperature.
4. Increase resin temperature of heating tube.
5. Increase injection speed.
6. Avoid direct gate. (PS)
7. When forming with metal inserts, preheating is required.

Causes
Silver glittering flow marks appear on the surface of molded products due to insufficient material drying, poor degassing in heating cylinder (moisture, air, volatile components), stagnation and burning, decomposition during heating, air being drawn into mold (including changes in product wall thickness, etc.). Location where silver streaks appear is not necessarily limited to vicinity of gate.
Mechanism of silver streaks
Silver streaks are a phenomenon in which bubbles generated in resin are stretched and become filamentous.

 

Cause
Resin is overheated and decomposed due to shearing heat when passing through gate. It is more likely to occur when mold temperature is too low, spray temperature is too low, gate is too small, and injection speed is too fast.

 

Countermeasures
1. Increase mold temperature.
2. Increase temperature of nozzle.
3. Increase gate diameter.
4. Reduce injection speed.
5. Thicken gate.
PPO, HIPS, ABS, PC and other materials are more likely to appear.

Cause
It is easy to occur when stroke setting of pressure relief after metering is too large. When stroke is too large, air will be sucked from nozzle and brought into mold by resin during next molding cycle, forming silver threads. In addition, if it is a highly viscous resin, screw will retreat due to screw effect of resin when screw starts to rotate. Air may also be drawn into nozzle. Especially when screw is rotated at the same time as high-pressure injection is completed, coupled with elastic rebound effect of resin, this phenomenon is more obvious.

 

Countermeasures
1. Set stroke of pressure relief after metering to minimum limit.
2. Use a low speed for screw rotation.
3. Increase back pressure setting value.
4. Delay screw start rotation time.
5. Let injection holding pressure drop slowly.

Reason
Due to insufficient drying, water in resin is explosively dispersed; or resin is decomposed by adding water, resulting in appearance of silver streaks all over product. This phenomenon will occur when all resins that need to be dried are not dried enough. In addition, it is worth noting that if machine body at material inlet is overcooled, water droplets formed inside will mix with resin and will also produce silver streaks.

 

Reason
When temperature at rear of heating cylinder is too high, resin is retained. At this time, a small part of resin adheres to the front area of compression part of screw. When these resins are separated from screw and metered along with other resins, these resins with water will form silver streaks in mold. In addition, silver streaks will also be produced when resin that is not compressed (containing air) in reverse flow is melted and injected into mold. In this case, screw will be accompanied by idling. (Poor coiling) Feed volume of DM model is relatively small, and this phenomenon may also occur when relative temperature is relatively high.

 

④-5 Molded product has irregularities and some silver threads

 

Cause
It occurs when back pressure is too low and air is drawn into resin during metering. It is especially easy to occur with high-viscosity resins (PMMA, PC). In addition, it also occurs when screw speed is too fast and resin is not completely melted before molding.
Countermeasures
1. Increase back pressure.
2. Increase temperature of heating cylinder
3. Reduce screw speed.
4. In the case of DM machines, increase speed of feeding screw.
2-6 A small amount of silver filaments occurs at the front end of fusion part

 

Cause
Resin flow rate at fusion part is fast, and gas is retained. Gas is squeezed out and forms silver filaments.
HIPS and ABS resins are prone to this problem.
Countermeasure
Adjust injection speed and select a speed that does not involve air.
2-7 Silver streaks occur on opposite side of raised part of molded product

 

Cause
Resin passes through raised part too fast, so air is not discharged well. After gas is squeezed out, silver streaks are formed.
Countermeasure
Adjust injection speed and select a speed that does not involve air.

Reasons
Reasons for deformation of molded products include:
1) Molding shrinkage caused by shape
2) Internal residual stress caused by molding conditions (overfilling, resin temperature, mold temperature, injection pressure, etc.)
3) And external stress when molded product is demolded
When using a harder resin (higher rigidity), although there is internal residual stress, it will not be reflected on product. Softer resins such as PP and PE have a larger shrinkage rate and are very conspicuous. In addition, molding shrinkage is different in flow direction of resin and right angle direction. Therefore, it will also cause a large deformation. There are various deformations of molded products. Generally, there are warping, bending, twisting deformation, etc.

 

Cause
In the case of box-shaped molded products, if temperature of male mold is too high, molded product will be pulled inward and shrink. Because four corners are reinforced, structurally weakest side wall is pulled inward, forming a bow-shaped warp

 

Countermeasures
1. In the case of box-shaped molded products, male mold temperature is generally lower than female mold temperature
2. In the case of box-shaped molded products, male mold temperature must be set lower than female mold temperature

Cause
When mold temperature is relatively high, resin is cooled slowly; when it is relatively low, resin is cooled rapidly. When cooling slowly, crystallization gradually proceeds and shrinkage rate increases; when cooling rapidly, situation is opposite. Therefore, in plate-shaped molded product, side with a higher temperature will shrink more than side with a lower temperature, and product will warp toward side with a higher temperature. (Due to difference in cooling speed)
Countermeasure
Change mold temperature as for box-shaped molded products. (Lower temperature on warping side)

 

Cause
Although rib is not necessarily cause of warping, it may also occur depending on thickness and height of rib. When rib is thinner and taller than wall thickness of main body, it will cool faster than main body, and size of rib will become longer than main body, thus causing warping in direction opposite to rib.
In addition, when rib is thicker and lower, warping in direction of rib will occur. These situations can also be easily inferred from relationship between thickness and molding shrinkage. Based on this, while paying attention to mold cooling, wall thickness and height of rib should also be corrected.

 

Countermeasures
1. Perform mold temperature control at different temperatures. (Male and female molds)
2. Study pressurization method for holding pressure
3. Correct wall thickness balance of the entire molded product according to situation

Cause
This is common in molded products with straight gates. When thick-walled molded products are molded in a state close to underfilling, gate and opposite side are relatively smooth, so product with obvious unevenness on gate side will be obtained. This means that injection pressure is strongly transmitted to gate and opposite side. It can be inferred that this tendency will not change even if it is fully filled. In other words, due to injection pressure, resin on opposite side of gate is pressed very densely, while resin on gate side is relatively loose. Reason is that gate side warps due to excessive injection pressure. (Reason for density difference)

 

Countermeasures
1. Change mold temperature as with box-shaped molded products.
2. Adjust injection speed.
3. Either reduce injection pressure or shorten holding time

Cause
Poor resin fusion occurs when there is some kind of obstacle in cavity and mold is formed using a multi-point gate. Resin merges again. There will be a linear pattern on the surface of molded product, which is not good in appearance and product strength will be weakened.
In addition, wall thickness change of molded product is also one of reasons for fusion line.

 

Shrinkage (voids)
Cause
A defect that often occurs in thick-walled molded products. Because cooling time of thick part is too long, surface that cools first will shrink and pull resin to surface. When this shrinkage is concentrated in the center, shrinkage occurs.

 

Countermeasures
1. Increase injection pressure and increase injection holding pressure.
2. Increase mold temperature.
3. Reduce resin temperature
4. Reduce injection speed.
5. Increase injection holding time.
(Note) Not limited to thick-walled molded products, when fine bubbles appear in the entire product
1. Air is drawn in because back pressure is too low.
2. Resin temperature is too high.
4. Depression (dent)
Cause
Depression occurs on the surface of molded products due to shrinkage in thick-walled products. Center of cross section of molded product is slowly cooled, while outside is rapidly cooled and shrinks first. Material is pulled outward, and the overall shrinkage is concentrated in the center, resulting in a hollow phenomenon in the center. If this phenomenon occurs on the surface, it is depression.

 

Countermeasures
1. Increase injection pressure.
2. Is mold temperature too high or too low? If it is too high, surface will be depressed; if it is too low, bubbles will appear.
3. Lengthen injection holding time.
4. Lower resin temperature.
5. Lengthen buffer stroke.
6. Check backflow of resin.

Cause
During filling process, viscosity of resin increases due to influence of mold temperature, resin temperature and injection speed in mold, and flow stops. And even if pressure and speed are increased, filling will still be insufficient. It must be noted that when air in mold cannot be discharged, filling may also be insufficient.

 

Cause
When viscosity of resin is relatively low, especially due to influence of mold temperature, resin temperature, and injection speed, when joints of two sides of mold are poor, molded product will have flash.
In addition, it is worth noting that flash will also appear when clamping force is insufficient compared to projected area.

 

Flash caused by poor joints
Flash caused by insufficient clamping force
Countermeasures
1. Reduce mold temperature
2. Reduce resin temperature
3. Reduce injection speed.
4. Reduce injection holding pressure.
5 When there are multiple points in one mold and there is flash on runner, use low speed injection.

Cause
Reason for gas burning is that air in mold is not completely discharged during process of filling resin into mold cavity, temperature rises and burns after being compressed by pressure.
Burning position will appear in same place if molding conditions are same. In addition, burning will occur at last position of filling.

 

Countermeasures
1. Reduce injection speed before filling is completed.
2. Set exhaust plug and exhaust duct at appropriate position like mold in figure.
3. While reducing injection speed, adjust mold temperature and resin temperature to make resin flow smoothly.
4. Do not use too much release agent.
Depth of venting channel without flash

Black is a phenomenon of black streaks produced by decomposition of resin on the surface or inside (transparent products) of molded product.
Cause
1 Resin retained in part of heating cylinder has decomposed.
2 When screw rotates, resin entrained in air is injected and compressed by heat insulation, resulting in a black appearance (often accompanied by silver threads)
3 When a problem occurs when exchanging materials.
4 When check valve (ring valve or ball valve) or spiral part of screw is bitten.
5 When there is resin retention or burning in hot runner.

Cause
During injection filling process, viscosity of resin decreases, and it is often a defect when filling with pressure. It is somewhat different from flow mark that appears near gate, and there is almost no copying of mold. If mold has a pattern, this part may become a bare plate.

 

Countermeasures
1. Increase mold temperature.
2. Increase resin temperature.
3. Increase injection speed.
4. Increase injection holding pressure.
5. Extend injection holding time.

Reasons
If mold has a hook structure, there will be scratches during ejection. In addition, it may also occur if taper of hook is too small. Other reasons such as overfilling, too long cooling time, too small resin shrinkage rate, etc. will increase possibility of these defects.
Insufficient mold strength is also one of reasons.
Countermeasures
1. Do not add useless pressure.
Reduce injection pressure.
Shorten injection holding time.
2. Replace other resins.
Reconsider shrinkage rate.
3. Increase mold temperature.
4. Increase resin temperature
5. Increase injection speed.

 

Causes
When molded product tends to be overfilled, molded product becomes convex and whitens after ejection.
Countermeasures
1. Reduce ejection speed
2. Reduce injection pressure.
3. Shorten injection holding time
③ Color mixing, discoloration, and surface cloudiness