In automotive manufacturing industry, maintaining consistent mold dimensions and controlling dimensional control throughout the entire production process for medium and large injection molded parts has always been a headache. As one of the largest injection molded parts on vehicle body, dimensional accuracy of bumper directly impacts the overall vehicle assembly quality and appearance. Using example of an automotive bumper body, this article focuses on balancing scale line design, dynamic measurement timing, and first-article inspection.
�� Underlying logic of material properties and dimensional control
PP+EPDM-TD20 is mainstream material used for automotive bumpers. Controlling its dimensional stability is a typical challenge in injection molding process. Understanding root causes of dimensional fluctuations based on material formulation is crucial to providing a basis for mass production control.
�� Analysis of Shrinkage Characteristics of PP+EPDM-TD20
This material consists of a polypropylene (PP) base, an ethylene propylene diene monomer (EPDM) elastomer, and 20% talc (TD20). Formulation reduces shrinkage from 1.5%-2% for pure PP to 1.1%-1.3%. However, this narrow range also places higher demands on injection molding process:
Fluctuations in mold temperature and holding pressure can cause product dimensions to exceed critical shrinkage limits.
While talc reduces overall shrinkage, difference in shrinkage between horizontal (MD) and vertical (TD) melt flow directions for large curved parts can still reach 0.3%, easily leading to uneven assembly gaps.
Industry test data shows that maximum shrinkage deviation in longitudinal direction of unoptimized bumpers can reach ±2.5mm, far exceeding assembly tolerance requirements.
Thermoplastics expand when heated and contract when cooled. Naturally, they also shrink when pressurized. During injection molding process, molten plastic is first injected into mold cavity. After filling, melt cools and solidifies, and shrinkage occurs when part is removed from mold. This shrinkage is called molding shrinkage.
During period from mold removal to stabilization, part's dimensions may still undergo slight changes. One type of change is continued shrinkage, known as post-shrinkage.
Another type of change is expansion of certain hygroscopic plastics due to moisture absorption. For example, at a 3% moisture content, nylon 6 increases in size by 2%, while at a 40% moisture content, glass-fiber-reinforced nylon 66 increases in size by 0.3%. However, molding shrinkage is primary factor in determining shrinkage rate of various plastics (molding shrinkage + post-shrinkage), generally following German national standard DIN 16901. This is calculated as difference between mold cavity dimensions at 23℃ ± 0.1℃ and corresponding part dimensions measured 24 hours after molding at 23℃ and 50 ± 5% relative humidity. Shrinkage rate S is expressed by following formula: S = {(D-M)/D} × 100% (1). Where: S is shrinkage rate; D is mold size; M is plastic part size. If mold cavity is calculated based on known part dimensions and material shrinkage, D = M / (1-S). To simplify calculations in mold design, following formula is generally used to calculate mold dimensions: D = M + MS.
�� Design Value of 1000mm Scale Mark on Inner Surface
In practice, to facilitate more intuitive measurement of dimensional changes, 1000mm dimensional scale marks are designed on inner surface of bumper. This serves as a "visualization tool" for dimensional control, its design logic stems from requirements of engineering metrology:
Visualizing Shrinkage: According to formula "Actual Shrinkage = Nominal Length × Shrinkage Rate," 1000mm length corresponds to an 11-13mm dimensional change for a 1.1%-1.3% shrinkage rate. Inspectors can quickly determine whether product's dimensional shrinkage complies with regulations using a caliper.
Ensuring Datum Stability: Laser marking achieves a 0.1mm depth accuracy for scale marks, eliminating wear issues associated with traditional markings and ensuring long-term measurement datum reliability.
Process Traceability: Scale marks can be traced every 200mm. By setting a single measurement point, a complete shrinkage distribution curve can be drawn. In one case, curve revealed that shrinkage in 300-500mm range on the left side of bumper was 1.2mm greater than average. This was ultimately traced to a blockage in cooling water channel in that area of mold.
Note: Examples of shrink line marking specifications:
1) For molds where customer requires shrink line markings, shrink line must be engraved on mold core according to customer's specified pattern. For molds where customer requires shrink line markings but does not provide a specific pattern, shrink line marking pattern must be requested from customer. For molds where customer specifically specifies shrink line markings according to "Hitech Shrink Line Marking Specifications," shrink lines are designed according to following standards.
2) Two parallel lines are engraved on the side of mold core: maximum depth 0.2mm, maximum width 1mm. Lines should be spaced as far apart as possible, a multiple of 50, and distance must match theoretical value after adding shrinkage factor. That is, actual mold size is: distance value * shrinkage ratio, as shown in figure below. Measure product after cooling.
3) Large molds such as air ducts, instrument panels, bumpers, and door panels require a "1-meter line" engraved. For products less than 1 meter in length, engrave in multiples of 100.
4) Shrink line engraving is in reverse font on mold and appears in forward font on product. Concave characters are engraved on mold and convex characters are displayed on product.
5) After shrink line engraving design is completed, it must be confirmed by customer before it can be shipped.
✂️ Scale Selection for Small and Medium-Sized Parts: 500mm and 300mm Suitable Applications
For small and medium-sized injection molded parts (such as automotive door panel accessories, center consoles, and pillars), 1000mm scale may not be suitable due to oversize limitations. Select appropriate scale based on part size:
700mm scale: Suitable for medium-sized parts (e.g., injection molded parts 500-1000mm in length). This corresponds to a dimensional variation of 5.5-6.5mm with a shrinkage rate of 1.1%-1.3%, ensuring both measurement accuracy and ease of use.
500mm scale: Suitable for small parts (e.g., injection molded parts 300-700mm in length). This corresponds to a shrinkage of 3.3-3.9mm, allowing for accurate measurement of shrinkage deviations in small parts.
Accuracy Guarantee: Scale lines are laser engraved on mold end to ensure a depth tolerance of ±0.05mm and a linearity tolerance of ±0.05mm. ±0.1mm; During product inspection, first perform a quick initial measurement with a large vernier caliper (accuracy 0.02mm), then recheck key scale points with a three-dimensional coordinate measuring machine to double-check dimensional data.
⏱️ Dynamic Measurement System: Time-Based Dimensional Stability Control
Dimensional change in injection molded parts is phase transition from molten to glassy state. PP+EPDM-TD20 material exhibits a "step-by-step stability" characteristic, necessitating a scientific measurement system based on time points.
�� Immediately after mold removal (0 minutes): Reference for mold correction
State Characteristics: Product temperature is 80-90℃, in the early stages of transition from highly elastic to glassy state. Dimensions are 4%-5% larger than final stable value (a 1000mm length corresponds to a 40-50mm deviation).
Core Value: While this data cannot directly determine compliance, it can be used to correct mold cavities.
�� 5 Minutes After Mold Release: Rapid Process Parameter Assessment
Status Characteristics: Product temperature drops to 50-60℃, 60%-70% shrinkage is achieved, and dimensional deviation at 1000mm mark stabilizes at ±1.5mm.
Control Logic: This stage is a process parameter "warning point"—if dimension exceeds the ±2mm warning value, subsequent cooling will make it difficult to bring it back within acceptable range, requiring immediate shutdown and adjustment of parameters such as holding pressure and mold temperature.
Efficiency Improvement: Some factories have reduced batch scrap risk warning time from traditional 2 hours to 15 minutes through a 5-minute rapid detection mechanism.
30 minutes after demolding: Golden window for first-article acceptance
Status characteristics: Product temperature is close to room temperature (25±3℃), shrinkage is over 90% complete, and 1000mm length deviation is controlled within ±0.8mm, with a 0.3% error from final dimensions.
Compliance: Meets automotive assembly requirements (0.5-1mm clearance between bumper and side panel, 3-5mm clearance with headlights).
Balanced advantage: Ensures measurement accuracy while keeping first-article acceptance within production schedule, avoiding delays to mass production.
�� Complete room temperature stability (24 hours): Foundation for long-term process optimization
Dimensional characteristics: Variation ≤ 0.3mm/1000mm, primarily used to establish process databases;
Environmental impact: Llinear expansion coefficient of PP+EPDM-TD20 is 8.682×10⁻⁵/℃. A 1000mm length will vary by approximately 4.3mm at a 50℃ temperature difference. This is why vehicles in cold regions require special dimensional compensation.
Advanced applications: Some smart factories automatically collect dimensional coordinate measurement data. Combined with AI models, they can predict final dimensions 24 hours after a 30-minute measurement.

 

�� First Article Inspection and Release: Art of Balancing Quality and Efficiency
First Article Inspection is "safety valve" of mass production. It requires clear scope, processes, and criteria, striking a balance between strict control and rapid release.
�� Triggering Scenarios and Quantity Standards for First Article Inspection
Scenarios requiring first article inspection include:
After mold changes or repairs, or when changing material grades;
Major process parameter adjustments;
Resuming production after a 12-hour downtime;
Quantity Standard: For critical bumper parts, the first three consecutive units must be inspected to eliminate any unforeseen factors.
�� Step-by-Step Inspection Process (Complete within 30 Minutes)
A three-stage process of "appearance → precision measurement → simulated assembly" is employed to ensure comprehensive and efficient inspections:
Full Appearance Inspection: Focuses on detecting surface defects such as sink marks, weld marks, and flash to prevent appearance issues from impacting subsequent assembly.
Critical Dimension Measurement: Critical control points, including mounting hole locations and edge contours, are inspected using a three-dimensional coordinate measuring machine.
Simulated Assembly Test: Specialized inspection fixtures are used to verify clearances with adjacent components (such as fenders and grilles) to ensure compliance with vehicle assembly requirements.
�� "Dual-Track" Judgment Criteria
Balancing strictness and flexibility, specific logic is as follows:
Hard Requirements: Critical dimensions measured within 30 minutes must 100% conform to drawing requirements. Minor deviations (e.g., within ±0.3mm) that do not affect assembly are permitted for non-critical dimensions.
Dynamic Adjustment: Record dimensional change curve from "0 min → 5 min → 30 min → 2 hours." If curve shows a convergence trend (subsequent measurement point is closer to target value), early judgment criteria may be appropriately relaxed.
�� Abnormal Handling and Process Control
Defective Product Handling Process:
Dimensional deviations: Prioritize checking holding pressure and holding time.
Uneven shrinkage: Check for temperature variations in mold cooling water lines (temperature should be controlled within ±2℃).
Data Support: Industry "5 Why" analysis cases show that 70% of bumper dimensional issues are due to improper holding parameters, and 18% are related to uneven mold cooling. Process Control After First-Part Acceptance:
Routine Sampling: Sample one product every hour and re-check critical dimensions along 1000mm scale mark with a large vernier caliper.
Process Monitoring: Use X-R charts to monitor process capability. If CPK value consistently exceeds 1.33, sampling can be extended to every two hours.
Abnormal Response: When dimensional fluctuations occur, immediately initiate a temporary tightening plan (sampling every 30 minutes) until process stabilizes.
�� Key Lessons
Scale Mark Design: Excellent quality control requires "visualization, measurability, and traceability." 1000mm/500mm/300mm scale marks are key tools for achieving this.
Time Dimension: Injection molded part dimensions are a function of time. Discussing accuracy without considering time is meaningless. A dynamic measurement system based on a "0 minute → 5 minutes → 30 minutes → 24 hours" cycle is required.
Balanced Logic: Quality and efficiency are not mutually exclusive. A step-by-step process and dual-track decision-making system can achieve a harmonious balance between the two. We need to understand principles of post-mold shrinkage. While rapid cooling of parts using a refrigerator, cold air, or soaking in water may lead to assumption that testing can begin after cooling to room temperature, we actually need to wait for post-mold shrinkage to complete. A post-mold shrinkage study should be conducted on product's maximum dimensions to determine stabilization time.
Also, a "hot standard" can be employed in production: specifying dimensional standards one or two hours after molding as the initial inspection and release criteria. If deviation between these dimensions and the final acceptable dimensions determined by shrinkage study is minimal, and process is stable, shrinkage curves of subsequent batches will be consistent, ensuring acceptable final dimensions.