Plastic Injection Molding

4 Steps to Achieve Proper Tolerance for Superior Product Performance in plastic injection molding

Tolerance is an acceptable limit of variation in a physical property or dimension.Defining a tolerance will identify boundaries for acceptable variations that will not affect the function of a part or system.
With the injection molded process, major factors that impact tight tolerance include part design and complexity, material selection, tooling, and process design and control. They can make it difficult to hold dimensions to the exact value. That is why an acceptable tolerance is necessary.
If tolerance is not done correctly, parts and products will underperform or possibly fail, resulting in customer dissatisfaction and a tooling and/or process overhaul,delays in production and unexpected additional costs. Below are 4 aspects that play key roles in aligning proper tolerance for superior performance.

1. Part Design and Complexity

The first step to controlling tolerance is to establish the tolerance zone during the design phase. Designing the part is the biggest factor in controlling tolerances. Making improvements during the design phase will not only achieve repeatable tight tolerances, but also improve manufacturability, quality, and customer satisfaction, all while reducing costs.

  • Shrink rates can vary depending on the material selection and product wall thickness. Thicker sections may shrink at a different rate than thinner sections. Varying shrink rates can affect the ability to hold tight tolerances.
  • Part size can affect shrink rates as well. Small components have smaller shrinkage and are easier to maintain tighter dimensional control. However, large parts can be challenging to hold tighter tolerance as they shrink over a greater distance.
  • Analyzing mold flow before production can be beneficial. Accurately predicting gate location, fill speed, shear stress, packing, cooling and shrinkage will help define the tolerance zone.
  • Avoid tight tolerance in areas that are prone to warp, shrinkage or distortion.

2. Material Selection

Material selection is another decision that must be made early in the design process. Different resins can produce different tolerances for the same part, so sometimes a tradeoff must be made between tolerance expectations and the physical properties of the resin.

  • A components environment can affect plastic properties and can result in a deviation from the tolerance. Plastics have larger thermal expansion than many other materials. Testing and measuring tolerance along with thermal expansion and part shrinkage in the components normal operating environment is crucial to controlling tolerance, but most importantly tight tolerance. Climate-controlled environments hold tighter tolerances well. On the other hand, tight tolerances are not always necessary in environments where temperatures are uncontrollable or where plastic expansion or contraction occurs.
  • As mentioned in the design phase, shrinkage can affect tolerances. Part geometry can cause different shrink rates. Different resins also have different shrink rates. It is important to know resin properties in combination with part complexity to understand shrink rates to align proper tolerance zones.

3. Tooling

Tool design, tool material, and cavitation all impact tolerance. The need to heat and cool tools, and the number of cavities in the mold, can make holding tolerances more of a challenge. If tooling is not designed to provide consistent, repeatable cooling, shrink rates will vary and tolerances will be harder to achieve.

  • Complex parts require complex tools. As a tool becomes more complex and additional components and features are added, it is important to make sure all components are transferring heat properly, allowing for acceptable resin and part cooling.
  • Monitoring pressure will identify resin viscosity, fill time and many other production processes that can affect resin properties and result in deviations and application failures. Utilizing in-mold pressure sensors will help track the proper pressure at each stage of the molding process.
  • Identifying an adequate gate location is important to achieve optimal material flow. This will contribute to avoiding unexpected shrinkage and warping.

4. Process Design and Control

Setting up the ideal process for the part, and being able to repeat it, is the key to molding tight tolerance parts. Proper process and document control from the beginning of production will ensure the component does not experience unnecessary pressure, stress, heat, or additional factors that can disrupt resin properties or application function.

  • Many parameters and variables must be carefully controlled during injection molding to achieve tight tolerances. Proper process control and process development ensure the part does not experience unnecessary pressure or stress during the molding process. Matching pressure curves versus simply using machine parameters such as time, temperature, and pressure help eliminate the lot-to-lot variation that is common in the industry. Conducting injection-molding operations in a climate-controlled facility also reduces process variation.
  • Understanding end user intent, life cycle expectations and environmental encounters are important to share and discuss with your plastic manufacturing partners. Keeping lines of communication open with your plastic manufacturer will aid the success of your application.
  • Repeatable processes can be achieved by conducting a Rheology test and a gate test. These tests will show you how repeatable your process and machines are, as well as, aid the optimization of your process and cycle time.

Jingwei industry has served the injection molding industry for over 15 years and has a team dedicated to providing you with the most cost effective solutions for design and manufacturing your new products.To know more,Please visit us at :jweimolding.com or via info@jweimolding.com.

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