4 Stages of Quality Control in Plastic Injection Molding

There are generally 4 different stages of quality control that are performed in plastic injection molding. The first stage is the design stage, followed by Mold and Pre-Production Inspection and In Process Quality Control and Final Inspection. This is followed system certification and production quality control.

Design Review

The first stage of quality control is design review. This review should take place with both internal and external designers or engineers. The purpose of this stage is to make sure that all designs are reviewed for accuracy before moving forward into production. You can use 3D printing as a way to check your design before committing resources to producing a full-sized model. The benefits of using 3D printing are being able to see what your product will look like before investing in manufacturing equipment and materials, as well as having a true sense of scale for final production models once they’re complete.


This stage involves creating a detailed plan of how to manufacture your product. The plan includes the following:

Product design

The design of your product will determine how much material is needed for each part, as well as how many parts you’ll need for each product batch. If you’re using more than one material for your product, then this stage also includes deciding which materials will be most efficient for use during injection molding.

MaterialsPlastic manufacturers often select a standard grade of plastic for a similar application or based on supplier recommendations. However, these resins may not be optimal. In plastic selection, there are many factors to consider, including:
Heat: The stress created by normal and extreme conditions of use and during the assembly, finishing, and shipping processes.
Chemical resistance is a property affecting part performance when solids, liquids, or gases are in contact.
Agency approvals: Standards developed by the government or the private sector for properties like heat resistance, flammability, and mechanical and electrical performance.
Assemblage: During the assembly process at plastic factory, the plastic is bonded, mechanically fastened, and welded.
Finish: Ability of the material to come out of the mold with the desired appearance values, such as gloss and smoothness.
Price: The price of resin, costs of manufacturing, maintenance, assembly, disassembly, and other costs to reduce labor, finishing, and tools.
Access: The availability of resin from the point of view of the amount required for production of plastic manufacturer.
DraftA draft angle makes it easier to remove a cooled, finished part from a mold . Draft angles are an essential component of injection molding. Minimizing friction during the part release process can achieve a uniform surface finish and reduced wear and tear on the mold at plastic factory.
An angle of the draft is measured according to the direction of pull. Draft angles of at least 0.5° for the cavity and 1.0° for the core are suggested by most design engineers for parts with sufficient draft. The tool must also be designed with more draft if a textured surface is desired and steel shut-off surfaces.
Wall ThicknessThe wall thickness of injection molded parts is also an important consideration. An injection molded part from plastic products supplier with a proper and uniform wall thickness is less prone to structural and cosmetic problems.
Most resins have a typical wall thickness ranging from .04 – .150. Yet, it is recommended that you obtain thickness specifications for your material(s) of choice by consulting with an injection molder/design engineer and plastic manufacturer.
Wall thickness should be analyzed during the design process to ensure that parts don’t sink, warp, or become non-functional.
RibsAs ribs are used to reinforce the walls of your injection molded parts without increasing their thickness, they are a valuable component in injection molded parts. Rib design should reduce mold flow length when designing complex parts and ensure that the ribs are appropriately connected to increase the part’s strength.
Ribs should not exceed 2/3 of the wall thickness, depending on the material used. WIDE ribs may create sinking and design problems. It is typical for a design engineer and plastic manufacturer to core out some fabric to reduce shrinkage and keep the strength.
If the height of the ribs exceeds 3 times the wall thickness, this could result in the part being short/unable to be filled. Rib placement, thickness, and length are critical factors in determining the viability of a part in its early design phases.
GateIn a mold part, a gate is a point at which liquid plastic flows into it. Injection molded parts have at least one gate, but they are often produced with multiple gates. Runner and gate locations influence polymer molecules’ orientation and how the part shrinks during cooling. As a result, gate location affects your part’s design and functionality.
The gate should be placed at the end of a long and narrow part if it must be straight. It is recommended to have a gate positioned in the centre of parts that must be perfectly round.
With the input of your plastic manufacturer team, you will be able to make optimal decisions regarding gate placement and injection points.
Ejector PinMold ejector pins (located on the B-side/core of the mold) are used to release plastic parts from a mold after being molded. The design and positioning of ejector pins should be considered as early in the process as possible by plastic manufacturers. This is even though they are usually a relatively minor concern in the early design phases. Indentations and marks can result from improperly placed ejector pins, so proper placement should be considered in the early phases.
Ejector pins are typically located at the bottom of side walls, depending on the draft, texture, depth, and type of material. You might be able to confirm that your initial ejector pin placement was correct by reviewing the design. In addition, you may be able to make further changes to improve production outcomes.
SinkSink marks can appear on the injection molded plastic part during injection moulding when the material shrinks more in thicker areas such as ribs and bosses. In this case, the sink mark is caused by thicker areas cooling slower than thin ones, and the different cooling rates lead to a depression on the adjoining wall.
Sink marks are formed due to several factors including the processing method, the geometry of the part, the material selection, and the tooling design. The geometry and material selection of the part may not be able to be adjusted based on its specifications, but there are several options to eliminate sink areas.
Sinking can be influenced by tooling design (e.g., cooling channel design, gate type and gate size), depending on the part and its application. The manipulation of process conditions (for example, packing pressure, time, phase of packing, and conditions) can also reduce sink. Further, minor tool modifications (e.g., foaming or gas assist) can reduce sink. It is best to consult your injection molder and plastic manufacturer regarding the most effective method to minimize sink in injection-molded parts.
Parting LinesFor more complex parts and/or complex shapes, it is important to note where the parting line is located.
Having your design shared with your injection molder can greatly influence your finished product’s production and functionality since designers and molders tend to evaluate parts differently. The challenge of parting lines can be addressed in several ways.
It’s important to be aware of the importance of the parting line when designing your initial concept, but you are not limited to that. You may be able to locate other possible locations using CAD software and mold flow analysis. When you work with an injection molder, they keep your part end use in mind and help you determine where the parting lines should be placed.
Special FeaturesIt is essential to design plastic parts so that mold tools can open them and eject them without difficulty. Injection molds release parts by separating the two sides in opposite directions. A side action may be necessary in some instances, where special features such as holes, undercuts, or shoulders prevent the release from occurring.
Coring is pulled in a direction opposite that of mold separation as a side action. In some cases, costs may increase due to this flexibility in part design.
When designing and developing a product, you (plastic manufacturers )were having the right injection molder, and engineer on your side is essential. You can avoid many issues by working with them. In integrating these elements into your product design process and working with a plastics engineer who has experience with these materials, your goal will be to get your product to market as quickly and cost-effectively as possible.

Part layout

Part layout refers to how you arrange all of the parts on your product when it comes time to manufacture it. The arrangement of parts can affect how quickly they’re made and what materials are used during injection molding. For example, if you have several parts that could be molded using two different materials, then choosing one over another might save time and money during production since there will be less material to use.

Mold and Pre-Production Inspection

Quality control is the process of ensuring that a product meets all requirements. It’s an essential part of any manufacturing process, and it can be broken down into four stages: mold design, pre-production inspection, production inspection and post-production inspection.

Mold Design

This is where you prepare for the production of your product by designing the mold that will be used to produce it. The mold is then made using a variety of materials, depending on what you’re making. For example, molds used in injection molding are usually made from aluminum or steel alloys, while some other processes use ceramic materials or even sandstone molds instead.

injection molding

Pre-Production Inspection

This stage involves inspecting the parts produced during the injection-molding process before they’re shipped out. It’s also known as “first article inspection.” This ensures that each part meets specifications and any defects are identified before they reach customers’ hands. Inspectors perform visual checks on each part and use measuring equipment to ensure that sizes, weights and tolerances are correct according to your specifications.

They also test functionality by testing whether parts fit together properly and whether they seem strong enough for use in their intended application (they may not always work perfectly straight out of the mold, after all). In addition to visual inspection and testing, inspectors may also take x-ray scans of parts or perform other types of non-destructive testing (NDT) to ensure that they’re safe and functional.

The main types of defects that can occur during plastic injection molding are:

  • Non-uniformity in color and texture
  • Misaligned or missing holes
  • Incorrect size of holes, thickness and other dimensions
  • Incomplete filling of cavities (flash or short shot)

In Process Quality Control

The quality control process begins with raw materials that must be checked to ensure they meet specifications before they are used in the injection-molding process.

The second step is to check the injection-molding machine’s temperature and pressure controls, as well as all of its other components, so they are working properly. This step ensures that the material will be properly melted before it enters the molding machine, which helps eliminate defects such as air bubbles in finished parts or poor surface finish on molded parts.

The next step involves checking for particulates (foreign materials) in the molten plastic that could cause problems during injection molding. Once particulates have been removed from the plastic, it can be injected into a mold.

The final stage of quality control involves checking for voids or other defects in molded parts, ensuring that each part comes out of an injection-molding machine properly formed and free of defects.

1. Short shot The plastic parts made by plastic injection molding companies have irregular and incomplete edges. It usually occurs at the farthest point of the gate, the roots of thin and long ribs.
2, Shrinkage Occasionally, a place 1. when the material thickness is uneven, 2. thick section of injection molded part, 3. boss and rib section. Due to shrinkage, the plastic parts develop dent, uneven, and wavy under low light
3, Flash A part edge with an extra thin layer of plastic is often found in the following locations 1. Parting line; 2. Moving core; 3. Ejector pin position, boss position, hole position, snap location
4. Bubbles The bubbles on the plastic surface are of a different color from the surrounding colors and are typical 1. bubbles caused by gas, air, and water-gas that are not released in time. 2. bubbles caused by shrinkage. The bubbles in the transparent part are particularly distinct.
5. Weld line When multiple molten flow fronts coincide with each other, a deep weld mark appears on the surface of a plastic part. This most commonly occurs at the confluence of multiple molten flow fronts.
6, Burning it is not a flat surface. It is usually dark or black spots; usually, they are found in sections where it is difficult to fill and trap the gas easily.
7, Black spots black impurities are evident on the surface of the plastic parts, mainly caused by the use of mixed materials.
8. Discoloration The slight difference in color between the actual plastic part and its required color is very apparent, and it is generally because the pigment is incorrect, the mix ratio is incorrect, or the mold is set at the wrong temperature.
9. Wrinkles The plastic parts show wavy lines on the surface caused by the cooling of the resin flowing.
10. Deformation The plastic parts have distortions, undulations, curves, and this is particularly common in the bosses, ribs, and round-shaped injection parts. These are especially common to PP injection molding.
11. Wrong materials In contrast to the specified materials, the one here can be identified by checking out the packaging label, the density, the burning continuity, the color of the flame, the color of the smoke, and the length of flames.
12. Stick to the mold Incomplete plastic part staying in the cavity, in contrast to short-shot, or due to the lack of designing the mold ejector system, the plastic part is hard to be ejected from the cavity, generally in thin-walled areas, bosses, and snaps.
13. Scratch The surface of the plastic parts rub against the surface of the cavity during the ejection so that a lot of them have scratches on the surface.
14, Over-flow It is caused generally by cavity damage and occurs on the active section, bosses, ejector pins, and parting surface.

Quality control ensures that all parts coming out of the injection mold are of high quality and meet customer requirements.

There are four main stages of quality control in plastic injection molding:

  • Inspection – The first stage involves inspecting all parts coming out of the injection molding machine using a visual inspection, weight measurement and dimensional check (using a micrometer).
  • Testing – In this stage, samples are taken from each batch for testing. The test results are used to determine if there are any defects in the material or tooling.
  • Repairs – If there are any defects detected during testing, they will be repaired before continuing with mass production. Repairs can be done by replacing defective parts or modifying them to make them work as required by customers.
  • Documentation – The final stage involves documenting all repairs made so that it can be referred back to when needed later on during future production runs.

It provides the following benefits:

  • Ensures that all products meet standards or specifications
  • Identifies defective parts and materials
  • Helps identify opportunities for improvement
  • Tracks product manufacturing costs

Final Inspection

1) Final Inspection – This involves checking each part for defects before it leaves the factory. Each piece is inspected by hand before being packaged and shipped out so that only perfect pieces make their way into customers’ hands. This process ensures that the product is of high quality and is free from defects,including scratches, bubbles or other imperfections. If any problems are found, they’re corrected before packaging and shipping.

The inspection is carried out by highly trained individuals who are able to identify any problems with the parts and materials used in production. This means that only perfect products are shipped out, ensuring that customers receive only the best quality goods.

2) Package – The package must be strong enough to withstand any damage during shipping. It should also include instructions on how to install or use your product safely in case something goes wrong on your end. If possible, consider using recyclable packaging materials whenever possible because they are better for both your environment and ours!

Final words

It is also possible to automate two or more of these stages so that certain quality tests are done automatically, immediately, and repeatedly. The number of quality checks depends on your equipment and is subject to change according to the number of production runs in each area. However, a system like this would improve overall plastic injection molding manufacturer efficiency.

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