Product defects

This phenomenon is commonly referred to as "short shot", which is the result of improper cavity filling during the injection molding process. This means that the molten plastic cannot completely fill all the space in the mold, resulting in an inadequate product.

Some possible causes:

• The equipment is not suitable (the cylinder is not large enough for the volume required by the product).

• The shot size (screw stroke) of one cycle is not properly set according to the product volume.

• The runner is blocked by solidified material.

• Melt temperature is too low.

• Mold temperature is too low.

• There are no venting channels in the mold or they are improperly placed.

In the process of injection molding, it can happen that the tool is poorly made or not good tempered, or that the injection parameters are inadequate, which can lead to improper plastic shrinkage during cooling. This can additionally strain areas where the shrinkage is slower, causing depressions on the surface. Depressions occur when the plastic is pulled inward due to these tensions, because the surface is not solid and the melt is not completely cooled. They typically appear in areas where the product's wall is thicker, for example, at the edge between the rib, the bearer, and the product's surface.

Some possible causes:

• Improper material temperature (too high or too low),

• Insufficient cooling time,

• Improper tool temperature (too high or too low),

• Too thick walls at ribs,

• Insufficient charge mass filling the tool,

• Improper gate placement or improper gate size,

• Slow injection process.

During the injection molding process, it sometimes happens that a small amount of plastic flows onto the surface from the mold cavity and creates a small film, which is called flash. Flash is undesirable in injection molding as it can cause damage to the mold inserts at the joint if part of the plastic remains on the tool and is not immediately cleaned from the mold surface. Such damage can later result in flash on all subsequent products. Therefore, it is extremely important to watch for signs of flash during the injection molding process, ensure that the parameters are properly set, and use appropriate tolerances on the machine.

Some possible causes:

• Excessive injection pressure or injection speed,

• Poor fit of mold inserts,

• Insufficient clamping force (especially if flash appears around the entire perimeter of the product or at the joint),

• Damage to mold inserts or cores due to the use of soft or unsuitable material,

• Damage to mold inserts or cores due to wear,

• Damage to ejectors or ejector inserts,

• Excessive material or mold temperature.

Understanding shrinkage and the potential for warpage is crucial in addressing issues such as the occurrence of warped or twisted products. This phenomenon is complex and depends on various factors, such as the type of processing material, the geometry of the product itself, the construction of the mold, and the temperature conditions of the mold and the material. It is important that the product and the mold are properly designed and adapted to the specific characteristics of the material and the production process to prevent unwanted effects, such as a twisted or warped final product.

Some possible causes:

• The finished product was not sufficiently hardened and cooled when ejected from the mold,

• The holding pressure or time or a combination of both were too high, which can cause internal stress in the product and the product to warp either immediately or later on after some time, when the product is reheated to a certain temperature...

• Insufficient added mass in the mold can lead to shrinkage and deformation of the product,

• Gates are not balanced according to the product,

• The ejection system is not balanced according to the product,

• The mold temperature is not evenly distributed throughout the entire cavity,

• The temperature of the front and back of the mold is not optimal (the temperature difference is too high or inadequate),

• The material's shrinkage effect is excessive in all directions, and the material may not be suitable for your application.

The jetting issue trace occurs when the material is injected into the mold cavity at high speed without encountering any obstacle before it, which would retain the jet and enable the spread of the material over a wider area. This jetted plastics freezes in the mold cavity. The incoming material fills it, but the shape of the jetted material remains, causing a visual defect and, in this part, a mechanical weakening, resembling a "snake" or "spaghetti."

Some possible causes:

• The injection speed is too high (at the entry gate),

• Improper placement of the entry gate (direct injection into the mold cavity or into an empty space without obstacles),

• The entry gate is too small or of unsuitable shape for the final product,

• The material or mold temperature is too high.

Diesel effect or trapped air in the plastic injection process can pose an issue, as could appearance like black or brown marks / stains on the final product. These marks occur due to the overheating of air or other gases trapped in the mold cavities, mainly on the opposite side of the gate or in areas of the product where there are enclosed spaces without the possibility of natural plastic flow (such as connectors, pins, screw preps, etc.). It is important to understand that this phenomenon can result from various factors, such as improperly prepared and dried material, uneven heating of the material, inadequate tooling design, or improper venting. Properly designed and installed venting channels, along with controlled, uniform injection molding, can help reduce this occurrence and ensure better final products. Additionally, regular tool maintenance, cleaning of venting channels, and monitoring of injection molding process parameters are important.

Some possible causes:

• Injection speed is too fast,

• Injection pressures is too high,

• Poor venting on mould,

• Gate is too small or in the wrong place,

• Clamping force is too high (vent openings become smaller),

• Vents are blocked with plastic or residue from gases.

Injection molding error associated with differences in glossiness occurs when the plastic product loses its original gloss or has a blurred surface after the injection molding process is completed. These differences in glossiness are often due to the condition of the surface on the mold inserts and the set temperatures. If the mold surface is not properly polished or if the temperature is not correctly adjusted, the final product may lose the desired gloss. Excessive use of release agents can also affect the surface gloss. The presence of moist materials or impurities in the material can also cause differences in gloss.

Some possible causes include:

• Mold surface is contaminated with water or oil,

• Inner surface on the inserts is not smooth,

• The process has set low back pressure,

• Material degrades due to overheating or remains in the cylinder for too long,

• Material cools too quickly due to a small gate opening,

• Injection speed is too low,

• Mold or material temperature is too low or not suitable for processing the material,

• Cylinder is not cleaned (solution: use cleaning granules like Ultra Purge),

• Excessive amount of release agent on the mold,

• Material is not properly prepared or dried enough.

If different materials are mixed during the material preparation process or if metal particles are added to the material, or if the material itself already contains metal particles, it can significantly affect the entire process and the final product. Visual and mechanical defects are common and may manifest as uneven color, surface roughness, and reduced mechanical properties in the final product. Below is a CT scan image showing granules with impurities or metal content, indicating material contamination. This can lead to nozzle clogging in the hot runner system, which in turn results in a large number of product rejects and loss of time due to corrective maintenance and cleaning of the injection molding system.

Some possible causes:

• Insufficient material preparation, leading to the mixing of different materials (cleaning of the transport path, cleaning of storage tanks, and cleaning of drying devices),

• The original material may contain impurities or metal,

• A high-speed mill near the machine that is damaged or has ground unwanted metal particles (clips, screws, nuts ...).

Weld lines are visible traces on products that occur when molten plastic is merged during the injection molding process. These traces can lead to aesthetic and mechanical deficiencies, such as uneven color, surface roughness, or weakening of the product's strength. Several types of weld lines can occur, including those due to cold slugs, flow stoppage, air entrapment and burn marks, as well as spots on colored or filled materials. Their appearance can affect product quality and may require adjustments to injection molding parameters or tooling changes.

Some possible causes:

• Insufficiently melted material,

• Moist material,

• Material impurities or contamination,

• Tool temperature is too low,

• Gate is too small or poorly located,

• Inadequate tool venting,

• Main runner and sub-runner are too thin or too long,

• Injection speed is too low,

• Injection pressure is too low.

On transparent, white, or light-colored plastic products, black specks often appear during the injection molding process. These specks can negatively affect the appearance of the products, increase production waste, and escalate manufacturing costs. The main cause of their occurrence is the degradation of materials at high temperatures, which are then "carried" out of the cylinder onto the products. Black specks can vary in size and cause, from small specks due to pitting on the cylinder and screw to larger specks from material sticking to the cylinder surface. To address the issue, regular cleaning, adherence to material manufacturer's instructions, and proper maintenance and cleanliness control of the production environment are crucial.

Some possible causes include:

• Foreign matter mixed with the original material or insufficient cleaning of the drying hopper,

• Overheating of the original material in the cylinder, causing the melt to adhere to the screw and cylinder walls,

• Insufficiently sized tooling gate and excessively high injection speed leading to material degradation,

• Residues of previous material in the cylinder causing further degradation and the appearance of black specks,

• Clogging or narrow openings in the hot runner nozzle system,

• Inadequately cleaned grinder/mill/dispenser.

White and silver lines, better known as silver or white marks, form on the surface of the product along the direction of melt flow during injection molding. The occurrence of silver or white spots is typically due to a rapid start of injection, preventing air from being expelled quickly enough from the melt and cavity of the mold. However, if the spots are present immediately from the gate towards the surface of the product, there is a high probability that it is due to moist material. Silver or white traces not only affect the appearance of plastic parts but also reduce their mechanical properties. It is necessary to check the dryness of the input material or adjust the parameters.

Some possible causes:

• Foreign substances or impurities in the original material,

• Overheating of the original material in the cylinder, causing the melt to adhere to the walls of the screw and cylinder,

• Residues of the previous material in the cylinder, causing further degradation and the appearance of white and silver spots,

• Use of ground material that is not sufficiently clean and contains a large amount of dusty particles,

• Moisture in the material, causing the formation of white or silver spots due to steam bubbles forming during injection.

The effect of splay marks is a pattern of waviness on the product's surface after the injection process. It typically occurs on plastic parts with smooth or glossy surfaces. These marks are a result of rapid cooling of the melt, creating a wavy shape. They can be remedied by increasing the melt and mold temperatures, accelerating the injection speed, and extending the holding time. Circular shapes are formed due to increasing resistance during the flow of material through narrow sections of the mold, often with more viscous materials such as PC. Increasing the injection speed, raising the temperatures of the tool and melt, and enlarging the gate opening can help prevent this issue. Light-dark spots resembling tree rings are caused by increased resistance during the melt flow, inadequate speed, and low temperatures. The issue is resolved similarly to addressing grooved plate patterns.

Some possible causes:

• Poor melting and plasticization of virgin material,

• Low melt flow (material viscosity is low),

• Tool temperature or set melt temperature is too low,

• Gate too narrow/small or in an inappropriate position,

• In the case of silver streaks appearing, injection speed is too low,

• Injection pressure too low or injection time too short.

Delamination, also known as surface peeling, is a flaw in plastic injection molding where the surface layers of the formed part can separate. This phenomenon often occurs at the gate area but can also occur on any other part of the product. Delamination typically happens when two different materials are mixed that cannot bond during melting. Consequently, the product made from such a mixture turns out to be multilayered, with layers that can peel off, especially pronounced at the gate location due to high shear stresses during injection molding. The only effective measure to prevent delamination is to avoid mixing with incompatible material, additives, or pigments.