What causes cracking in plastic injection molded parts

cracking injection molding defects

Cracking is a commonly seen defect in plastic injection molded products, of which the main causes are stress and deformation, mainly including residual stress, external stress and product deformation caused by external environment.
(I) Cracking caused by residue stress
The residual stress is mainly generated in the following three scenarios, i.e., overfilling, product ejection and metal inserts. The cracking generated in the case of overfilling can be solved from the following aspects:
(1) Since the direct gate is able to minimize pressure loss, if cracking is mainly generated around the direct gate, you can opt for the multi-point gate, the side gate or the tab gate.
(2) On condition that the resin material does not decompose or deteriorate, appropriately increasing the resin temperature is able to reduce melt viscosity and improve fluidity, while reducing the injection pressure, so as to lower the stress.
(3) Usually, stress easily occurs when mold temperature is low, so the temperature should be appropriately increased. However, when the injection speed is high enough, stress can also be reduced even though the mold temperature is relatively low.
(4) Stress will also occur when the injection and pressure holding time is too long. It would be better to reduce the time or perform pressure switch appropriately.
(5) It should be noted that amorphous resins, such as AS, ABS and PMMA, are more prone to residual stress than crystalline resins, such as PE and POM.
During product ejection, stress is generated due to the high ejection force caused by the small draft angle, rough mold core / cavity. Sometimes, even ejector marks or cracking occurs around the ejector pin. Just look closely at the location of cracking to identify the cause.
Stress is the most likely to occur when metal inserts are involved in the plastic injection molding process. And, the cracking usually occurs after a period of time, so it is extremely harmful. The stress is mainly caused by the large difference in thermal expansion coefficient between the metal and the resin materials. As time goes by, the stress exceeds the strength of the resin material that gradually deteriorates, so cracking occurs. In order to prevent the cracking thus caused, as a rule of thumb, the general-purpose PS is basically not suitable for inserts, while the impact of the inserts on the nylon material is minimal. Thanks to the small thermal expansion coefficient, the glass fiber reinforced resin is more suitable for the application of inserts.
In addition, a better effect can be achieved if you pre-heat the metal insert before injection molding.
(II) Cracking caused by external stress
The external stress mainly refers to the stress concentration caused by inappropriate design, especially the sharp corners.
(III) Cracking caused by external environment
Chemicals, water degradation caused by moisture absorption, and excessive application of recycled materials will degrade the physical properties, and thus cause crackin

What causes black specks/black streaks in injection molding parts?

black specks streaks

Black specks/streaks and impurities are the most important factors that lead to rejects in the normal production process. It mainly affects the appearance of the product and leads to scrap. Most of the impurities and black specks are usually caused by foreign substances, i.e., they have nothing to do with the raw material itself. Only a small portion of the black specks and impurities are caused by the raw material itself.

Black specks and impurities are characterized by small dark brown particles, which generally do not reflect light. When the particles are large, the impurities are layered, brittle, fragile and porous after being broken. Some of them are irregularly scattered on the whole, some are scattered randomly in local parts, and some only occasionally appear in a local area.

Impurities are divided into two major categories, the ones formed before and during the molding process:

Black Specks & Impurities Formed before Molding:

1. Due to various reasons, foreign substances are not cleaned during the processing of the raw material, leading to black specks on the raw material;

2. Impure granulation causes black specks;

3. The raw material is mixed with color masterbatch or speckled crushed blocks and scraps;

4. Impure material – low melting-point material is mixed with high melting-point material;

5. Impurities may be mixed in during packaging, transportation and storage, of which, the obvious feature is that after the raw materials are unpacked, you can see that there are foreign matters and impurities on the surface of the material pellets if you observe carefully;

6. Impurities and foreign substances during material feeding;     

7. Carbonization of raw materials

With regard to carbonization of raw materials, the black specks are generally larger in volume, and the largest ones may reach 1-2mm in diameter. Most of the “black specks” are thicker, but there are also thinner ones consisting of one or two layers. This is usually caused by long-term raw material storage, or that the raw material is locally overheated, leading to decomposition, coking, and carbonization into agglomerates. It is formed after the material is crushed by shearing when flowing through the screw or the nozzle.

Causes of Material Carbonization:

1. The melt temperature is too high. If the temperature of the material is too high, it will cause decomposition and form carbides. Especially for some heat-sensitive materials with a very narrow temperature range, the temperature of the barrel must be kept moderate.

2. Material coking: If the molten plastic stays in a certain place for too long a time, coking will occur, thus causing black speck. The areas that may cause material retention include joints between the nozzle and the barrel, the barrel wall, the melt ring, the connection between the nozzle and the gate, the corner of the hot runner, and the dead corner in the sprue, etc.

3. The barrel clearance is too large – the gap between the barrel and the screw is too large, which will cause the material to stay in the barrel, and the retained material will decompose after long-term overheating, thus leading to black specks.

4. Additives discolor due to degradation and decomposition. Additives include antistatic agents, violet / infrared absorbers and general dyes. Their properties are generally more active than the raw material. Under the action of the shearing force at the processing temperature, they have already been decomposed into dark, brown, and even black colors, presented as black specks and impurities after plastic injection molding.

Black specks and impurities due to external causes are very common in production, and they are very stubborn after occurring.

1. The mold material is not good, with iron powders falling off the parting surface, the molding surface or the kiss-off surface, thus causing black specks.

2. The ejector pin is rough and easy to burn, causing iron powders to fall off and thereby the black specks.

3. The slider generates iron powders, leading to black speck.

4. The slider rusts or generates other stains due to water leakage inside the slider, which are thrown out by slider movement, and black speck are caused when they fall onto the product.

Distinguish the Black Specks:

If the black speck appear on both the entire product surface and in the depth of the part, they should be formed before injection molding; if the black specks only appear on the surface, they should be formed during injection molding. If at the same time they are only distributed in a specific area of the surface, it is undoubtedly that they are black specks formed during the injection molding process; if the black speck are large (generally 0.5- 1mm), it should be caused by material carbonization; if they are also dark, brittle and porous, it can be confirmed that they are caused by carbonization: if the black specks are particularly dense and the raw material is checked for no obvious impurities, it is usually caused because the previous material is not immediately cleaned before the material change, otherwise the material should be looked into to identify the reason.


1. For impurities in a molded product, black specks are caused due to foreign substances mixed in the raw material, so the cleanliness of all the links, including production, packaging, storage, transportation, unpacking, material mixing and the barrel, must be strictly controlled.

2. For black specks caused by carbonization, the injection temperature should be strictly controlled.

3. Usually, for the black speck caused by additive degradation or the ones caused by the existing carbonized materials deposited on the screw surface and the screw barrel wall, they can be removed from the original location into the molten material under various strong shearing effects in the screw barrel during the production process. The “removal” is a process in which black specks and impurities appear in the product. When changing materials or colors, if this impurity “removal” process is inevitable, we must try our best to minimize it, i.e., “cleaning.”

What causes parts sticking or sprue sticking during injection molding?

parts sticking or sprue sticking

The causes of parts sticking and sprue sticking are multifaceted, among which mold failure is one of the main reasons. The reasons and corresponding solutions are described as follows:

1. The surface of the mold cavity and sprue bush is rough. If there are surface defects such as gouges, nicks, scars, and dents in the mold cavity and sprue bush, the plastic parts and sprue will easily stick to the mold, resulting in difficulty in ejection. Therefore, the surface finish of the cavity and sprue should be improved as much as possible. The inner surface of the cavity should preferably be chrome-plated. When polishing, the movement direction of the polishing tool should be the same as the direction of mold filling.

2. The mold wears, is scratched or the insert clearance is too large. When the molten material produces flashing in the scratched part of the mold or in the clearances between the inserts, it can also cause difficulty in product ejection. In response, the damaged mold area should be repaired and the clearances between the inserts reduced. 

3. The mold rigidity is insufficient. If the mold cannot be opened at the beginning of injection, it means that the mold is deformed under injection pressure due to the insufficient rigidity. If the deformation exceeds the elastic limit, the mold cannot return to its original state and thus cannot be used anymore. Even though the deformation does not exceed the elastic limit of the mold, the molten material is cooled and solidified under high conditions in the mold cavity. After the injection pressure is removed and the mold recovers from deformation, the plastic part is clamped by the rebound force, so the mold still cannot be opened.

As a result, when designing the mold, sufficient rigidity and strength must be ensured. When trying the mold, it is the best to install a dial indicator on the mold to check whether the mold cavity and the mold base are deformed during the filling process. During mold trial, the initial injection pressure should not be too high. You should observe the deformation, while slowly increasing the injection pressure, so as to keep the amount of deformation within a certain range.

When mold clamping failure occurs due to the too-high rebound force, it is not enough to increase the mold opening force alone. But instead, the mold should be disassembled immediately, and the plastic part should be heated and softened before being taken out. For molds with insufficient rigidity, a frame can be mounted on the outside of the mold to increase its rigidity. 

4. The draft angle is not big enough, or the parallelism between the core and cavity plates is poor. When designing and making the mold, a sufficient draft angle should be ensured, otherwise it will be difficult to eject the plastic product. Forceful ejection often causes the plastic part to warp, leading to ejection marks or cracks. The core and cavity plates of the mold must be relatively parallel, or it will cause the cavity to deviate, thus resulting in parts sticking. 

5. The design of the gating system is inappropriate. Part sticking and sprue sticking will be caused, when the runner is too long and too small in size, the connection between the main runner and the branch runner is not strong enough, there is no cold slug well in the main runner, the gate is poorly balanced, the diameters of the main runner and the nozzle are not properly matched, or the gate sleeve and the spherical nozzle surface do not match. Therefore, the length and the cross-sectional area of the runner should be appropriately reduced, and the connecting strength between the main runner and the branch runner increased, with a cold slug well provided in the main runner.

When determining the gate location, the filling rate of each cavity in a multi-cavity mold should be balanced and the pressure in each cavity reduced by adding auxiliary gates. In general, the diameter of the small end of the main runner should be 0.5-1mm larger than the nozzle diameter, and the concave radius of the gate sleeve should be 1-2mm larger than the nozzle radius.

6. The ejection mechanism is not appropriately designed or operated. If insufficient stroke, uneven ejection or poor ejector plate movement occurs to the ejection mechanism, the plastic parts will not be able to be ejected.

When the conditions allow, the effective ejection area should be increased as much as possible to ensure a sufficient ejection stroke. The ejection speed of the plastic part should be kept within a suitable range, neither too fast nor too slow. The main reason for the undesired movement of the ejector plate is the stickiness between the sliders.

What Causes Scratches in Injection Molded Products?

scratches scrape marks injection molding defects

The scratches (scrape marks)on the appearance of an injection molded product usually occur on such deep-cavity products as chargers, routers, and chassis, etc. It is also a particularly common problem during the injection molding process, known as one of the 10 most challenging problems in the mold and injection molding industry. Glossy products, as well as products with textured sides are easily scratched. What on earth has caused it? When it comes to the scratches on the textured surface, the first thing that comes to the mind of an engineer is that the draft angle on the side of the product is not large enough, but why the scratch still exist after the draft angle is increased? So, solving the occurrence of scratches is not just a matter of the draft angle. In fact, it is not so simple at all. Now, let’s thoroughly analyze and solve the problem of surface scratches.

The causes of surface scratches mainly include the following factors:

I. Draft Angle

The key issue with the product is that the draft angle of the product is too small, resulting in scratches on the side of the product. To respond to the different texture requirements on the side of the product, we have already gained relevant experience – apply suitable draft angles for texture surfaces of different specifications. To solve this problem, we must try to avoid it in the early stage of product evaluation. If the customer’s product does not allow a sufficient draft angle, we must consider using other mold mechanisms or other technical means to avoid it.

II. Mold Design

In addition to problems with the product, to avoid scratches, mold design also plays a very important role, so you must consider it carefully. Here are some examples to explain how to avoid product side scratches through mold design.

1.To handle the fillet of the parting surface, product designers often directly put in the fillet when designing the product. In fact, this is an unreasonable design, because the draft angle at the conjunction between the fillet and the surface is close to zero, which is easy to cause scratches.

2. Mold Structure: Improper mold matching design will also cause the core and the cavity to be misaligned during the mold opening process, thus causing product scratches.

3. When designing the mold gate, you should consider the gate design so as not to cause too much pressure on the product and cause side scratches on the product. The gate should be as far away as possible from the textured side surface, because the pressure at the gate is high also with a long pressure holding time, so the high clamping force will make the product to be prone to scratches.

4. The product sticks to the mold cavity: We must ensure that the product 100% sticks to the mold core when the mold is open. If it sticks to the mold cavity, local product warpage may be caused, and thus scratches on the side surface. When necessary, a product fixing structure must be deployed in the mold core to ensure that the product stays with the core.

III. Mold Machining

1. Although the draft angle is designed to be big enough, it is easy to cause errors during mold machining, because it is difficult to measure the angle accurately. This point must be taken into consideration.

2. It is the easiest to create undercut on the side wall of the mold cavity during mold polishing, because the mold polishers are often afraid to polish the parting surface into a fillet. The mold polishing workload close to the bottom is higher and the one close to the parting surface is lower, which is nothing less than reducing the draft angle of the product, so it is also a possible factor that causes product scratches.

IV. Molding Machine Adjustment

1. Excessive injection pressure and holding pressure are also important causes of scratches on product side walls. Excessive injection pressure creates stress that causes product warpage in the mold. The product may be locally pressed on the side wall, so it is easy to scratch the product side surface during mold ejection.

2. Excessive material temperature will also cause the product surface to be easily scratched.

3. Insufficient product cooling is also one of the causes of product scratches.

4. The bulging of the parting surface also easily causes product scratches. The parting surface bulging caused by excessive clamping pressure, too-soft mold material, and resin powder, etc. may also cause the product side surface to be scratched, so the mold parting surface and texture surface must be kept clean.

5. The injection molding machine is aging, so the parallelism between the front and rear plates is poor. The vibration during mold opening may also be one of the factors that cause scratches on the product side surface.

All in all, the main factors that cause scratches on the side wall and textured surface of a product are comprehensively considered above. Only by carefully following the steps one by one can we avoid the troublesome scratches on plastic injection molded products.

What Causes Weld Lines in Plastic Injection Molding Parts?

weld lines plastic injection molding defects

The definition of weld lines: When multiple plastic flow fronts meet in the cavity after encountering inserts, holes, or areas with inconsistent flow rates or interrupted melt flows, or in the case of gate injection filling, the inability of two or more flow fronts to fully “knit” together will cause the linear weld marks. The appearance of weld lines greatly reduces the mechanical strength of the product. The solution to the weld lines is basically the same as the method for reducing product sink marks.

I. Equipment

To respond to poor plasticization and uneven melt temperature, the molding cycle can be extended to make plasticization more complete, and select the molding machine with a higher plasticizing capacity when necessary.

II. Mold

(1) When the mold temperature is too low, raise the mold temperature appropriately or purposefully raise the local temperature at the weld seam.

(2) When the runner is too small, narrow or shallow, and the cold slug well is too small – Increase the dimensions and efficiency of the runner, as well as the volume of the cold slug well.

(3) Increase or reduce the gate section and change the gate location. The gate should be designed to prevent the melt from flowing around the inserts and holes. For the gate where injection filling occurs, we should try to correct, relocate or add a block buffer to it. Try to avoid the application of multiple gates.

(4) Poor venting or no vents. Open, expand or smoothen venting passages, including venting via insert and ejector pin clearances.

III. Molding Process

(1) Increase injection pressure and extend injection time.

(2) Properly adjust the injection speed: a high speed allows the melt reach the meeting point before it is cooled down, while a slow speed allows enough time for the air in the cavity to be vented.

(3) Properly adjust the temperature of the barrel and the nozzle: the higher the temperature, the smaller the viscosity of the plastic, the smoother the flow, and the thinner the weld line is; When the temperature is low, the decomposition of gaseous substances is reduced.

(4) The release agent should be used as little as possible, especially the silicone release agents, otherwise the flow fronts will fail to weld.

(5) Lower the clamping force to facilitate venting.

(6) Increase the screw speed to reduce the viscosity of the plastic; increase the backpressure to improve the density of the plastic.

IV. Raw Material

(1) Dry the raw material and minimize liquid additives in the formulation.

(2) Appropriately add lubricants and stabilizers to the plastic that features a poor fluidity or high heat sensitivity. When necessary, select a plastic that has a better fluidity or higher heat resistance.

V. Product Design

(1) When the wall thickness is small, the part should be thickened to avoid premature solidification.

(2) Make adjustments when the insert is not properly located.

What Causes Burn Marks in Plastic Injection Molding?


Factor 1: Melt fracture causes burn marks

When the melt is injected into a large-sized mold cavity under high-speed and high-pressure conditions, melt fracture will be easily caused. At this point of time, the surface of the melt is laterally fractured, with the fracture area coarsely mixed to form burn marks on the surface of the plastic part. In particular, when a small amount of molten plastic is directly injected into an oversized mold cavity, melt fracture will be more serious, leaving larger burn marks on the product surface.

In essence, melt fracture is caused due to the elastic behavior of the polymer melt. When the melt flows into the barrel, the melt close to the barrel is rubbed by the wall of the barrel, resulting in higher stress and lower melt flow rate. The moment the melt is injected from the nozzle, the stress from the pipe wall disappears. The melt flow rate in the center of the barrel is extremely high, so the melt close to the wall of the barrel is accelerated by the melt in the center. Since the flow of the melt is relatively continuous, the melt flow velocity in the center and close to the wall will be rearranged, tending to the average speed.

During this process, a sharp stress change in the melt will cause strain. Since the injection speed is extremely fast, the stress is particularly high, much higher than the strain capacity of the melt, thus resulting in melt fracture.

When the melt encounters a sudden shape change in the runner, such as diameter shrinkage, expansion, and dead angle, the melt stays and circulates at the dead corner, so its stress is different from that of the normal melt, leading to larger shear deformation. When injected after being mixed with the normal flow, the recovery after deformation of the two is inconsistent and cannot be bridged. If the disparity is large, the fracture will be caused, i.e., melt fracture.

So, it is necessary to solve the melt fracture to eliminate burn marks:

  1. Eliminate the dead angle in the runner and make it as streamlined as possible;
  2. Appropriately raise the material temperature, reduce the melt relaxation time, and make it easy to recover from deformation and bridge the inconsistency;
  3. Add low molecular-weight polymers to the raw material, because the lower the molecular weight of the melt, the wider the distribution, and the more helpful to reduce the elastic effect;
  4. Properly control the injection and the screw speeds;
  5. It is very important to select the gate location and the gate type reasonably. It is proved that the enlarged pinpoint gate and the submarine gate (tunnel gate) are the ideal choices. With regard to gate location, it is preferable to ensure that the molten material is first injected into a transition cavity and then the large-sized cavity. Do not allow the melt to flow into the large cavity directly.

Factor 2: Molding conditions are not appropriately controlled

This is another important cause of charring and burns marks on the surface of a plastic product. In particular, the injection speed has a great influence on it. When the melt is slowly injected into the cavity, the flow state of the melt is laminar; when the injection speed gradually increases to a certain value, the flow state gradually becomes turbulent.

Generally speaking, the surface of the plastic part formed by laminar flow is relatively glossy and flat, while that formed under turbulent flow conditions are prone to burn marks on the surface, as well as pores inside. As a result, the injection speed should not be too high, and the flow should be controlled in a laminar state.

If the melt temperature is too high, the melt is easy to decompose and coke, resulting in burn marks on the surface of the plastic product. Usually, the screw speed of the injection molding machine should be lower than 90rpm, and the backpressure lower than 2mpa, so as to avoid excessive frictional heat generated inside the barrel.

If excessive frictional heat is generated due to the excessively long rotation time when the screw returns during the plastic injection molding process, the screw speed can be appropriately increased, the molding cycle can be appropriately extended, the screw back pressure can be appropriately lowered, the temperature of the barrel feeding section can be appropriately increased, and the raw materials with poor lubricity can be used to solve the issue.

During the plastic injection molding process, the melt will depolymerize and decompose if the melt reflows too much along the screw channel, with resin staying at the anti-reverse ring. In this regard, the resin with a higher viscosity should be selected, the injection pressure should be appropriately lowered, and the injection molding machine with a relatively long diameter should be used. The anti-reverse ring commonly used in an injection molding machine is relatively easier to cause retention, causing the melt to decompose and discolor. When the decomposed and discolored molten material is injected into the cavity, brown or black burn marks is formed. In response, the nozzle-centric screw system should be cleaned regularly.

Factor 3: Mold failure

When the mold vents are blocked by the release agents and the solids deposited by the raw material, or the mold venting capacity or location are not properly designed, or the mold filling speed is too fast, the gas trapped in the mold will generate high temperature to burn the resin due to adiabatic compression. In response, the obstruction should be removed, the clamping force should be lowered, and mold venting improved.

The type and location of the mold gate are also very important considerations. The flow state of the melt and the venting performance of the mold should be fully considered in the design stage.

In addition, use an appropriate dose of release agent, and maintain a high glossiness of the cavity surface.

Factor 4: The raw material doesn’t meet requirements

If the raw material contains too much moisture and volatile contents, or the melt index is too high, or excessive lubricant is employed, charring and burn marks will be very likely to occur.

In response, treat the raw material with the hopper dryer or other pre-drying methods, select the resin with a smaller melt index, or reduce the amount of lubricant.

How Do You Operate and Maintain Plastic Injection Molds?

China plastic injection mold maintenance

When compared with other tools, the plastic injection mold is more complicated and precise, with higher requirements for operation and maintenance. As a result, throughout the entire production process, their correct use and meticulous mold maintenance are of great significance to maintain the normal production and improve the efficiency of a company.

1. Select the right molding equipment, and determine the reasonable process conditions. If the injection molding machine is too small, it will not meet the requirements; if too large, it will waste resources. Also, it may damage the mold or the mold plate due to the improper adjustment of the clamping force, while at the same time affecting the production efficiency.

When selecting the injection molding machine, check the maximum injection volume, the effective distance of the tie rod, the mold installation dimensions on the plate, the maximum mold thickness, the minimum mold thickness, the plate stroke, the ejection type, the ejection stroke, the injection pressure, and the clamping force, etc. and choose the one that meets the requirements. The reasonable determination of the process conditions is also one of the factors that ensure the correct use of the mold. If the clamping force, the injection pressure, the injection speed and the mold temperature, etc. are set to be too high, the service life of the mold will be affected.

2. After installed with a mold, the injection molding machine must first be operated with the empty mold, to observe whether the operation of each part is smooth, whether there is any anomaly, whether the ejection and opening strokes are proper, whether the parting surface is tightly matched when the mold is closed, and whether the pressure plate screw is tightly fastened.

3. When the mold is being used, a normal temperature should be maintained to help extend the service life of the mold.

4. Sliding parts on the mold, such as the guide pin, the return pin, the push rod and the core, etc., should be inspected, cleaned and lubricated regularly. Especially in summer when the temperature is high, add lubricants at least two times per shift, so as to ensure that these sliding parts are smooth enough to prevent seizures.

5. Each time before closing the mold, check whether the mold cavity is cleaned or not – no residual products or any other foreign objects are allowed. It is strictly forbidden to use hard tools to clean the cavity, so as to prevent the cavity surface from being damaged.

6. As for molds with special requirements for the cavity surface, the surface roughness (Ra) should be no higher than 0.2cm. It is absolutely not allowed to wipe it with hand or cotton fabrics. Instead, it should be cleaned with a compressed air blower, or gently wiped with high-grade alcohol napkins or absorbent cottons.

7. The cavity surface should be cleaned regularly. During the injection molding process, the mold will produce low molecular compound to corrode the mold cavity, so that the surface of the glossy cavity gradually becomes matted, thus affecting product quality. Therefore, it needs to be cleaned regularly by using alcohol or ketone preparation before being dried in a timely manner.

8. When the machine needs to be temporarily stopped, the mold should be closed to prevent the cavity and the core from being exposed, which might create accidental damage. If the downtime is expected to exceed 24 hours, the surface of the core and the cavity should be sprayed with anti-rust oil or release agent. Especially in moist areas and rainy seasons, they should be rust-proofed even if the downtime is very short. The moisture in the air will affect the surface quality of the cavity and the molded product. Before the mold is put into operation again, the oil on the mold should be removed and wiped clean. When cleaning a mold with mirror surface requirement, first use compressed air and then hot air to dry it, or the oil will be oozing out to cause product defect during the molding process.

What Causes Short Shot in Plastic Injection Molding?

short shot plastic injection molding defects

Short shot refers to incomplete filling of a mold cavity which results in the production of an incomplete part due to the insufficient plastic fluidity. The main causes are the inappropriate injection pressure and speed (including excessive pressure loss caused by resistance), which are mainly affected by the following aspects:

I. Injection Molding Machine:

(1) Insufficient injection molding capacity – This is caused by an overestimation of the capacity of the injection molding machine, which may also occur due to insufficient plasticizing capacity or insufficient injection volume. Among them, the insufficient plasticizing capability can be increased by extending the heating time, increasing the screw speed and raising the backpressure. If the injection volume is not high enough, the injection machine with a larger injection capacity can be used to solved the problem.

II. Inappropriate Mold Design

(1) Local short shot caused by the flow imbalance in each cavity of a multi-cavity mold – When the injection molding capacity of the injection molding machine is sufficient, this defect is mainly caused by the uneven flow in each gate, i.e., the mold cavities are not distributed in a balanced way.

(2) The flow range of the melt is too long, and the flow resistance is too large – The parts that hinder the flow of the melt include the nozzle, the sprue, the runner, the gate, and the thin walls of a product. The flow resistance of the nozzle can be reduced by increasing the nozzle diameter / temperature, and using a nozzle with a small flow resistance.

(3) Poor venting. When filling the cavity, air is trapped to cause counter pressure. When the melt is injected into the cavity, the cavity is often closed by the melt in the very beginning, with air trapped in the unfilled local areas. Also, because the filling speed is too fast, sometimes there is not enough time for the air to be vented through the parting surface but compressed instead, resulting in partial unfilled areas in the cavity, thus short shot in the molded part.

III. Improper Molding Process

(1) Improper injection molding process – The low barrel temperature, slow injection speed, short injection time, and insufficient backpressure lead to a shortage of plastic.

(2) Oversupply of plastic – If too much plastic enters the barrel, the injection pressure is lost due to the compression of the pellets, thus reducing the pressure required to inject the melt from the nozzle, which is necessary for injection molding, resulting in insufficient injection pressure. The solution is to adjust the amount of feed, i.e., the amount of melt, and make the amount just right for the molding.

(3) Unstable production cycle – Frequent machine shutdown, and production inconsistent with the normal cycle make some plastics stay in the barrel for too long with reduced density and viscosity, thus resulting in plastic underfill.

(4) Inappropriate mold temperature leads to lower injection speed and plastic underfill.

IV. Choice of Plastic

(1) Poor plastic fluidity – If the plastic material doesn’t feature a great fluidity, it will solidify before reaching the furthest end of the cavity or flowing to the overflow tank, which often results in a short shot.

In order to eliminate such defects, the melt / mold temperature, and the injection pressure / speed can be increased to allow the melt to reach the end of the cavity before solidifying. In such a case, a great plastic fluidity is of particular importance, so selecting the plastic with a better fluidity is also a solution. If the flow coverage is too long and the plastic cannot properly fill the injection molded part, it is recommended to change the gate location to reduce the plastic flow length.

(2) Try to evenly mix the recycled material and the raw material. Because the recycled pellets, as well as their density are relatively larger than the raw material, if the mixing is not uniform, the amount of the molten material is easily reduced, resulting in reduced injection volume accordingly.

V. Inappropriate Product Design

During the product design process, the principle of uniform wall thickness must be followed. If it is impossible to maintain a uniform wall thickness, please change the mold design in a timely manner. Increase the number of runners or ribs in the area where the feeding is difficult, so as to avoid the defect of underfill caused by too-thin wall thickness.

All in all, the reasons for short shot are multifaceted, which are also mutually restrictive and mutually influential. To reduce and correct such a defect, we need to make adjustments through an overall consideration of the relationship between these aspects, while making constant practice and accumulating experience continuously to quickly identify the causes of underfill, thereby reducing resource waste and improving product yield.

VDI 3400 Mold Texture and Drafting Angle

VDI 3400 texture for plastic injection mold

Bayer Plastics has some good Information about surfaces and injection molding available on their web- Site.
According to a (German) Document the VDI 3400 surfaces correspond to the following Ra / Rz Values and require the following drafting angles:

120.40 1.50 0.5 1.0 0.5
150.56 2.40 0.5 1.0 0.5
180.80 3.30 0.5 1.0 0.5
211.12 4.70 0.5 1.0 0.5
241.60 6.50 0.5 1.5 1.0
272.24 10.50 1.0 2.0 1.5
303.15 12.50 1.5 2.0 2.0
334.50 17.50 2.0 3.0 2.5
366.30 24.00 2.5 4.0 3.0
399.00 34.00 3.0 5.0 4.0
4212.50 48.00 4.0 6.0 5.0
4518.00 69.00 5.0 7.0 6.0

DA-PA = Drafting Angle for Polyamide
DA-PC = Drafting Angle for Polycarbonate
DA-ABS = Drafting Angle for Acrilnitrile-Butadiene-Styrol

glass reinforced materials require more drafting.

VDI 3400 Table
In the following, you can determine the roughness for your application according to VDI 3400 table.

VDI3400  0-45µm µinchN3-N10ISO1302 Rt µm
00.10 4N3 
10.11 4.4  
20.12 4.8  
30.14 5.6  
40.16 6.4  
50.18 7.2N4 
60.20 8  
70.22 8.8  
80.25 10  
90.28 11.2  
100.32 12.8  
110.35 14N5 
120.40 16 1.6
130.45 18  
140.50 20  
150.56 22.4 3.2
160.63 25.2  
170.70 28  
180.80 32N65
190.90 36  
201.00 40  
211.12 44.8  
221.26 50.4  
231.40 56  
241.62 63 12
251.80 72N7 
262.00 80  
272.20 88 16
282.50 100  
292.80 112  
303.20 125N820
313.50 140  
324.00 160  
334.50 180 25
345.00 200  
355.60 224  
366.30 250N937
377.00 280  
388.00 320  
399.00 360 46
4010.00 400  
4111.20 448  
4212.60 500N1060
4314.00 560  
4416.00 640  
4518.00 760 85

What Causes Flow Lines in Plastic Injection Molding?

Definition of Flow Lines:                                               
Flow lines, also known as flow marks Linear marks on the surface of a molded product, which indicates the flow direction of the molten plastic.

flow lines marks

Injection Molding process

1.Insufficient Pressure / Holding Pressure                                               
The injection pressure and the holding pressure are not high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

Increase the injection pressure and the holding pressure, to press the solidified layer against the mold surface until the product is molded, so as to prevent the occurrence of flow lines.

2.Improper Residence Time                                      
The plastic material stays in the barrel for a too short period of time, while the melt temperature is low. Even if the cavity is barely filled, the plastic cannot be compacted during pressure holding, thus leaving flow lines along the melt flow direction.

The Shot-to-Barrel Ratio should be kept between 1/1.5 and 1/4.                                      
3.Improper Cycle Time                                      
When the cycle time is too short, the plastic is not sufficiently heated in the material barrel, and the temperature of the melt is low. Even though the cavity is barely filled, the plastic cannot be compacted during pressure holding, thus leaving flow lines along the melt flow direction

The cycle time is extended until the plastic is fully melted, and the temperature of the melt is high enough to prevent flow lines along the melt flow direction.                                      
4.Barrel Temperature Too Low                      

When the barrel temperature is too low, the melt temperature will be low, and the injection pressure and holding pressure will not be high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

Increase the material temperature, injection pressure and holding pressure to press the solidified layer against the mold surface until the product is molded, so as to prevent the occurrence of flow lines. The material temperature can be set by reference to material supplier’s recommendations.

The material barrel is divided into four zones: Rear, Center, Front and Nozzle. The material temperature settings should be gradually raised as it moves forward. Increase by 6°C with every zone forward.

When necessary, the temperature of the Nozzle and/or the Front are sometimes set to be the same as the Center temperature.                                       
5.Nozzle Temperature Too Low

After absorbing the heat released by the heating band, as well as the frictional heat generated by the relative movement of the plastic molecules caused by the rotation of the screw, the plastic in the barrel undergoes gradual temperature rises.

The last heating zone in the barrel is the Nozzle, where the melt should reach the desired temperature, but it must be moderately heated to maintain the optimal conditions.

If the nozzle temperature is not set high enough, due to too much heat is taken away through the contact between the nozzle and the mold, the material temperature will decrease, so that the injection pressure and the holding pressure will not be high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

Raise the Nozzle temperature. The nozzle temperature is usually set to be 6°C higher than the Front temperature.                                       
1.Mold Temperature Too Low                                     
If the mold temperature is too low, the material temperature will drop very fast, so the injection pressure and the holding pressure will not be high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

Raise the mold temperature, maintain a high material temperature, as well as high injection pressure and holding pressure to press the solidified layer against the mold surface until the product is molded, so as to prevent the occurrence of flow lines.

The mold temperature can be set from the recommended values of the material supplier, with an increment of 6°C at each adjustment. Then perform 10 shots, and after the injection molding is stable, decide whether further adjustment is necessary according to the result.

2. Sizes of the Sprue, the Runner and/or the Gate

If the sprue, the runner, and/or the gate are too small, the flow resistance will be increased. And, if the injection pressure is not high enough, the advancement of the melt front will become slower and slower, and the plastic will become colder and colder, so that the insufficient injection pressure and holding pressure will not be able to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

It is a feasible way to simulate and analyze the filling status of the different melt transfer systems (including the sprue, the runner and the gate) on a computer with CAE (such as, C-MOLD), to find out the ideal sprue, runner, and gate sizes (including length and section related dimensions, such as diameter, etc.) 

3. Insufficient Venting

Insufficient venting will cause the melt filling to be blocked, and the melt front will not be able to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction.

Start venting at the end of each runner section, which removes a large amount of gas before filling the cavity.

The cavity venting should not be neglected. Consider adding vents on the parting surface opposite to the gate. Correspondingly, consider adding venting ejector pins at the end of the product blind hole.

Simulate melt filling through CAE (such as, C-MOLD), which helps us quickly find out all possible last filled areas, i.e., the areas where vents must be added. The addition of a vacuum system for air extracting before and during filling is an effective venting method.

For some textured products, this may be the only way of venting.                                      
Plastic Material

1.Poor Fluidity

The mold cavity with a large flow length to thickness ratio must be filled with the plastic that features great fluidity. If the fluidity of the plastic is not good enough, the melt will be flowing slower and slower, colder and colder, so that the injection pressure and the holding pressure are not high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction. Material suppliers are able to offer professional recommendations according to specific designs:

The most flowable plastic is selected on condition that no flashing is caused.

2.Improper Application of Molding Lubricant

Usually, the lubricant content is below 1%. When the flow length to wall thickness ratio is large, the lubricant content must be moderately increased to ensure that the solidified layer is pressed against the mold surface until the product is molded, so as to prevent the occurrence of flow lines. The lubricant must be increased upon agreement with the material supplier.


1. Bad Habits

Inconsistent molding result will occur if the operator switches the door of the injection molding machine too early or too late. When the barrel heater tries to replenish heat in time due to irregular heat loss, the plastic temperature will not be uniform, thus causing the cold spot. It is not easy for the injection pressure and the holding pressure to press the solidified layer around the cold spot against the mold surface, thus leaving flow lines along the melt flow direction. Usually, the operator should be constantly educated to let everyone know the troubles caused by inconsistent molding cycles and recognize the importance of maintaining best molding practices. Appropriate work shifts are able to prevent operators from making mistakes due to exhaustless or distraction. Automated production with robots or the like is a way to maintain a consistent molding cycle.